WorldWideScience

Sample records for metal-poor dwarf stars

  1. Carbon-enhanced metal-poor stars in dwarf galaxies

    NARCIS (Netherlands)

    Salvadori, Stefania; Skúladóttir, Ása; Tolstoy, Eline

    2015-01-01

    We investigate the frequency and origin of carbon-enhanced metal-poor (CEMP) stars in Local Group dwarf galaxies by means of a statistical, data-calibrated cosmological model for the hierarchical build-up of the Milky Way and its dwarf satellites. The model self-consistently explains the variation

  2. Carbon-enhanced metal-poor stars in dwarf galaxies

    OpenAIRE

    Salvadori, Stefania; Skuladottir, Asa; Tolstoy, Eline

    2015-01-01

    We investigate the frequency and origin of carbon-enhanced metal-poor (CEMP) stars in Local Group dwarf galaxies by means of a statistical, data-calibrated cosmological model for the hierarchical build-up of the Milky Way and its dwarf satellites. The model self-consistently explains the variation with dwarf galaxy luminosity of the observed: i) frequency and [Fe/H] range of CEMP stars; ii) metallicity distribution functions; iii) star formation histories. We show that if primordial faint sup...

  3. Chemical Abundances of Metal-poor stars in Dwarf Galaxies

    NARCIS (Netherlands)

    Venn, Kim A.; Jablonka, Pascale; Hill, Vanessa; Starkenburg, Else; Lemasle, Bertrand; Shetrone, Matthew; Irwin, Mike; Norris, John; Yong, David; Gilmore, Gerry; Salvadori, Stephania; Skuladottir, Asa; Tolstoy, Eline; Bragaglia, A.; Arnaboldi, M.; Rejkuba, M.; Romano, D.

    2016-01-01

    Stars in low-mass dwarf galaxies show a larger range in their chemical properties than those in the Milky Way halo. The slower star formation efficiency make dwarf galaxies ideal systems for testing nucleosynthetic yields. Not only are alpha-poor stars found at lower metallicities, and a higher

  4. Chemical composition of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy

    OpenAIRE

    Aoki, W.; Arimoto, N.; Sadakane, K.; Tolstoy, E.; Battaglia, G.; Jablonka, P.; Shetrone, M.; Letarte, B.; Irwin, M.; Hill, V.; Francois, P.; Venn, K.; Primas, F.; Helmi, A.; Kaufer, A.

    2009-01-01

    Context. Individual stars in dwarf spheroidal galaxies around the Milky Way Galaxy have been studied both photometrically and spectroscopically. Extremely metal-poor stars among them are very valuable because they should record the early enrichment in the Local Group. However, our understanding of these stars is very limited because detailed chemical abundance measurements are needed from high resolution spectroscopy. Aims. To constrain the formation and chemical evolution of dwarf galaxi...

  5. Chemical composition of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy

    NARCIS (Netherlands)

    Aoki, W.; Arimoto, N.; Sadakane, K.; Tolstoy, E.; Battaglia, G.; Jablonka, P.; Shetrone, M.; Letarte, B.; Irwin, M.; Hill, V.; Francois, P.; Venn, K.; Primas, F.; Helmi, A.; Kaufer, A.; Tafelmeyer, M.; Szeifert, T.; Babusiaux, C.

    Context. Individual stars in dwarf spheroidal galaxies around the Milky Way Galaxy have been studied both photometrically and spectroscopically. Extremely metal-poor stars among them are very valuable because they should record the early enrichment in the Local Group. However, our understanding of

  6. DETAILED ABUNDANCES OF TWO VERY METAL-POOR STARS IN DWARF GALAXIES

    Energy Technology Data Exchange (ETDEWEB)

    Kirby, Evan N.; Cohen, Judith G. [Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA 91125 (United States)

    2012-12-01

    The most metal-poor stars in dwarf spheroidal galaxies (dSphs) can show the nucleosynthetic patterns of one or a few supernovae (SNe). These SNe could have zero metallicity, making metal-poor dSph stars the closest surviving links to Population III stars. Metal-poor dSph stars also help to reveal the formation mechanism of the Milky Way (MW) halo. We present the detailed abundances from Keck/HIRES spectroscopy for two very metal-poor stars in two MW dSphs. One star, in the Sculptor dSph, has [Fe I/H] = -2.40. The other star, in the Ursa Minor dSph, has [Fe I/H] = -3.16. Both stars fall in the previously discovered low-metallicity, high-[{alpha}/Fe] plateau. Most abundance ratios of very metal-poor stars in these two dSphs are largely consistent with very metal-poor halo stars. However, the abundances of Na and some r-process elements lie at the lower end of the envelope defined by inner halo stars of similar metallicity. We propose that the metallicity dependence of SN yields is the cause. The earliest SNe in low-mass dSphs have less gas to pollute than the earliest SNe in massive halo progenitors. As a result, dSph stars at -3 < [Fe/H] < -2 sample SNe with [Fe/H] << -3, whereas halo stars in the same metallicity range sample SNe with [Fe/H] {approx} -3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo.

  7. Using photometrically selected metal-poor stars to study dwarf galaxies and the Galactic stellar halo

    Science.gov (United States)

    Youakim, Kris; Starkenburg, Else; Martin, Nicolas; Pristine Team

    2018-06-01

    The Pristine survey is a narrow-band photometric survey designed to efficiently search for extremely metal-poor (EMP) stars. In the first three years of the survey, it has demonstrated great efficiency at finding EMP stars, and also great promise for increasing the current, small sample of the most metal-poor stars. The present sky coverage is ~2500 square degrees in the Northern Galactic Halo, including several individual fields targeting dwarf galaxies. By efficiently identifying member stars in the outskirts of known faint dwarf galaxies, the dynamical histories and chemical abundance patterns of these systems can be understood in greater detail. Additionally, with reliable photometric metallicities over a large sky coverage it is possible to perform a large scale clustering analysis in the Milky Way halo, and investigate the characteristic scale of substructure at different metallicities. This can reveal important details about the process of building up the halo through dwarf galaxy accretion, and offer insight into the connection between dwarf galaxies and the Milky Way halo. In this talk I will outline our results on the search for the most pristine stars, with a focus on how we are using this information to advance our understanding of dwarf galaxies and their contribution to the formation of the Galactic stellar halo.

  8. Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor.

    Science.gov (United States)

    Frebel, Anna; Kirby, Evan N; Simon, Joshua D

    2010-03-04

    Current cosmological models indicate that the Milky Way's stellar halo was assembled from many smaller systems. On the basis of the apparent absence of the most metal-poor stars in present-day dwarf galaxies, recent studies claimed that the true Galactic building blocks must have been vastly different from the surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt on this conclusion. Verification of the iron-deficiency, however, and measurements of additional elements, such as the alpha-element Mg, are necessary to demonstrate that the same type of stars produced the metals found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars be conclusively linked to early stellar halo assembly. Here we report high-resolution spectroscopic abundances for 11 elements in S1020549, confirming its iron abundance of less than 1/4,000th that of the Sun, and showing that the overall abundance pattern follows that seen in low-metallicity halo stars, including the alpha-elements. Such chemical similarity indicates that the systems destroyed to form the halo billions of years ago were not fundamentally different from the progenitors of present-day dwarfs, and suggests that the early chemical enrichment of all galaxies may be nearly identical.

  9. Ages and Heavy Element Abundances from Very Metal-poor Stars in the Sagittarius Dwarf Galaxy

    Science.gov (United States)

    Hansen, Camilla Juul; El-Souri, Mariam; Monaco, Lorenzo; Villanova, Sandro; Bonifacio, Piercarlo; Caffau, Elisabetta; Sbordone, Luca

    2018-03-01

    Sagittarius (Sgr) is a massive disrupted dwarf spheroidal galaxy in the Milky Way halo that has undergone several stripping events. Previous chemical studies were restricted mainly to a few, metal-rich ([Fe/H] \\gtrapprox -1) stars that suggested a top-light initial mass function (IMF). Here we present the first high-resolution, very metal-poor ([Fe/H] =‑1 to ‑3) sample of 13 giant stars in the main body of Sgr. We derive abundances of 13 elements, namely C, Ca, Co, Fe, Sr, Ba, La, Ce, Nd, Eu, Dy, Pb, and Th, that challenge the interpretation based on previous studies. Our abundances from Sgr mimic those of the metal-poor halo, and our most metal-poor star ([Fe/H] ∼ -3) indicates a pure r-process pollution. Abundances of Sr, Pb, and Th are presented for the first time in Sgr, allowing for age determination using nuclear cosmochronology. We calculate ages of 9+/- 2.5 {Gyr}. Most of the sample stars have been enriched by a range of asymptotic giant branch (AGB) stars with masses between 1.3 and 5 M ⊙. Sgr J190651.47–320147.23 shows a large overabundance of Pb (2.05 dex) and a peculiar abundance pattern best fit by a 3 M ⊙ AGB star. Based on star-to-star scatter and observed abundance patterns, a mixture of low- and high-mass AGB stars and supernovae (15–25 M ⊙) is necessary to explain these patterns. The high level (0.29 ± 0.05 dex) of Ca indicates that massive supernovae must have existed and polluted the early ISM of Sgr before it lost its gas. This result is in contrast with a top-light IMF with no massive stars polluting Sgr. Based on data obtained UVES/VLT ID: 083.B-0774, 075.B-0127.

  10. Detection of a Population of Carbon-enhanced Metal-poor Stars in the Sculptor Dwarf Spheroidal Galaxy

    Science.gov (United States)

    Chiti, Anirudh; Simon, Joshua D.; Frebel, Anna; Thompson, Ian B.; Shectman, Stephen A.; Mateo, Mario; Bailey, John I., III; Crane, Jeffrey D.; Walker, Matthew

    2018-04-01

    The study of the chemical abundances of metal-poor stars in dwarf galaxies provides a venue to constrain paradigms of chemical enrichment and galaxy formation. Here we present metallicity and carbon abundance measurements of 100 stars in Sculptor from medium-resolution (R ∼ 2000) spectra taken with the Magellan/Michigan Fiber System mounted on the Magellan-Clay 6.5 m telescope at Las Campanas Observatory. We identify 24 extremely metal-poor star candidates ([Fe/H] 1.0). The existence of a large number of CEMP stars both in the halo and in Sculptor suggests that some halo CEMP stars may have originated from accreted early analogs of dwarf galaxies. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  11. J0811+4730: the most metal-poor star-forming dwarf galaxy known

    Science.gov (United States)

    Izotov, Y. I.; Thuan, T. X.; Guseva, N. G.; Liss, S. E.

    2018-01-01

    We report the discovery of the most metal-poor dwarf star-forming galaxy (SFG) known to date, J0811+4730. This galaxy, at a redshift z = 0.04444, has a Sloan Digital Sky Survey (SDSS) g-band absolute magnitude Mg = -15.41 mag. It was selected by inspecting the spectroscopic data base in the Data Release 13 (DR13) of the SDSS. Large Binocular Telescope/Multi-Object Double spectrograph (LBT/MODS) spectroscopic observations reveal its oxygen abundance to be 12 + log O/H = 6.98 ± 0.02, the lowest ever observed for an SFG. J0811+4730 strongly deviates from the main sequence defined by SFGs in the emission line diagnostic diagrams and the metallicity-luminosity diagram. These differences are caused mainly by the extremely low oxygen abundance in J0811+4730, which is ∼10 times lower than that in main-sequence SFGs with similar luminosities. By fitting the spectral energy distributions of the SDSS and LBT spectra, we derive a stellar mass of M⋆ = 106.24-106.29 M⊙, and we find that a considerable fraction of the galaxy stellar mass was formed during the most recent burst of star formation.

  12. Extremely metal-poor stars in classical dwarf spheroidal galaxies : Fornax, Sculptor, and Sextans

    NARCIS (Netherlands)

    Tafelmeyer, M.; Jablonka, P.; Hill, V.; Shetrone, M.; Tolstoy, E.; Irwin, M. J.; Battaglia, G.; Helmi, A.; Starkenburg, E.; Venn, K. A.; Abel, T.; Francois, P.; Kaufer, A.; North, P.; Primas, F.; Szeifert, T.

    2010-01-01

    We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor, and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to

  13. Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

    NARCIS (Netherlands)

    Tafelmeyer, M.; Jablonka, P.; Hill, V.; Shetrone, M.; Tolstoy, E.; Irwin, M. J.; Battaglia, G.; Helmi, A.; Starkenburg, E.; Venn, K. A.; Abel, T.; Francois, P.; Kaufer, A.; North, P.; Primas, F.; Szeifert, T.

    2010-01-01

    We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor, and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to

  14. Dwarf carbon stars are likely metal-poor binaries and unlikely hosts to carbon planets

    Science.gov (United States)

    Whitehouse, Lewis J.; Farihi, J.; Green, P. J.; Wilson, T. G.; Subasavage, J. P.

    2018-06-01

    Dwarf carbon stars make up the largest fraction of carbon stars in the Galaxy with ≈1200 candidates known to date primarily from the Sloan Digital Sky Survey. They either possess primordial carbon-enhancements, or are polluted by mass transfer from an evolved companion such that C/O is enhanced beyond unity. To directly test the binary hypothesis, a radial velocity monitoring survey has been carried out on 28 dwarf carbon stars, resulting in the detection of variations in 21 targets. Using Monte Carlo simulations,this detection fraction is found to be consistent with a 100% binary population and orbital periods on the order of hundreds of days. This result supports the post-mass transfer nature of dwarf carbon stars, and implies they are not likely hosts to carbon planets.

  15. The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A

    Science.gov (United States)

    Schulte-Ladbeck, Regina E.; Hopp, Ulrich; Drozdovsky, Igor O.; Greggio, Laura; Crone, Mary M.

    2002-08-01

    We present deep Hubble Space Telescope (HST) single-star photometry of Leo A in B, V, and I. Our new field of view is offset from the centrally located field observed by Tolstoy et al. in order to expose the halo population of this galaxy. We report the detection of metal-poor red horizontal branch stars, which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as least as old as metal-poor Galactic Globular Clusters that exhibit red horizontal branches and are considered to have a minimum age of about 9 Gyr. We discuss the distance to Leo A and perform an extensive comparison of the data with stellar isochrones. For a distance modulus of 24.5, the data are better than 50% complete down to absolute magnitudes of 2 or more. We can easily identify stars with metallicities between 0.0001 and 0.0004, and ages between about 5 and 10 Gyr, in their post-main-sequence phases, but we lack the detection of main-sequence turnoffs that would provide unambiguous proof of ancient (>10 Gyr) stellar generations. Blue horizontal branch stars are above the detection limits but difficult to distinguish from young stars with similar colors and magnitudes. Synthetic color-magnitude diagrams show it is possible to populate the blue horizontal branch in the halo of Leo A. The models also suggest ~50% of the total astrated mass in our pointing to be attributed to an ancient (>10 Gyr) stellar population. We conclude that Leo A started to form stars at least about 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, only 3% of solar, in its ionized interstellar medium. The existence of old stars in this very oxygen-deficient galaxy illustrates that a low oxygen abundance does not preclude a history of early star formation. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

  16. Ultracool Subdwarfs: Metal-poor Stars and Brown Dwarfs Extending into the Late-type M, L and T Dwarf Regimes

    OpenAIRE

    Burgasser, Adam J.; Kirkpatrick, J. Davy; Lepine, Sebastien

    2004-01-01

    Recent discoveries from red optical proper motion and wide-field near-infrared surveys have uncovered a new population of ultracool subdwarfs -- metal-poor stars and brown dwarfs extending into the late-type M, L and possibly T spectral classes. These objects are among the first low-mass stars and brown dwarfs formed in the Galaxy, and are valuable tracers of metallicity effects in low-temperature atmospheres. Here we review the spectral, photometric, and kinematic properties of recent discov...

  17. DISCOVERY OF MIRA VARIABLE STARS IN THE METAL-POOR SEXTANS DWARF SPHEROIDAL GALAXY

    Energy Technology Data Exchange (ETDEWEB)

    Sakamoto, Tsuyoshi [Japan Spaceguard Association, 1716-3 Ookura, Bisei, Ibara, Okayama 714-1411 (Japan); Matsunaga, Noriyuki; Nakada, Yoshikazu [Kiso Observatory, Institute of Astronomy, School of Science, University of Tokyo, 10762-30 Mitake, Kiso-machi, Kiso-gun, Nagano 397-0101 (Japan); Hasegawa, Takashi, E-mail: sakamoto@spaceguard.or.jp [Gunma Astronomical Observatory, 6860-86 Nakayama, Takayama, Agatsuma, Gunma 377-0702 (Japan)

    2012-12-10

    We report the discovery of two Mira variable stars (Miras) toward the Sextans dwarf spheroidal galaxy (dSph). We performed optical long-term monitoring observations for two red stars in the Sextans dSph. The light curves of both stars in the I{sub c} band show large-amplitude (3.7 and 0.9 mag) and long-period (326 {+-} 15 and 122 {+-} 5 days) variations, suggesting that they are Miras. We combine our own infrared data with previously published data to estimate the mean infrared magnitudes. The distances obtained from the period-luminosity relation of the Miras (75.3{sup +12.8}{sub -10.9} and 79.8{sup +11.5}{sub -9.9} kpc, respectively), together with the radial velocities available, support memberships of the Sextans dSph (90.0 {+-} 10.0 kpc). These are the first Miras found in a stellar system with a metallicity as low as [Fe/H] {approx} -1.9 than any other known system with Miras.

  18. BOO-1137-AN EXTREMELY METAL-POOR STAR IN THE ULTRA-FAINT DWARF SPHEROIDAL GALAXY BOOeTES I

    International Nuclear Information System (INIS)

    Norris, John E.; Yong, David; Gilmore, Gerard; Wyse, Rosemary F. G.

    2010-01-01

    We present high-resolution (R ∼ 40,000), high-signal-to-noise ratio (20-90) spectra of an extremely metal-poor giant star Boo-1137 in the 'ultra-faint' dwarf spheroidal galaxy (dSph) Booetes I, absolute magnitude M V ∼ -6.3. We derive an iron abundance of [Fe/H] = -3.7, making this the most metal-poor star as yet identified in an ultra-faint dSph. Our derived effective temperature and gravity are consistent with its identification as a red giant in Booetes I. Abundances for a further 15 elements have also been determined. Comparison of the relative abundances, [X/Fe], with those of the extremely metal-poor red giants of the Galactic halo shows that Boo-1137 is 'normal' with respect to C and N, the odd-Z elements Na and Al, the iron-peak elements, and the neutron-capture elements Sr and Ba, in comparison with the bulk of the Milky Way halo population having [Fe/H] ∼<-3.0. The α-elements Mg, Si, Ca, and Ti are all higher by Δ[X/Fe] ∼ 0.2 than the average halo values. Monte Carlo analysis indicates that Δ[α/Fe] values this large are expected with a probability ∼0.02. The elemental abundance pattern in Boo-1137 suggests inhomogeneous chemical evolution, consistent with the wide internal spread in iron abundances we previously reported. The similarity of most of the Boo-1137 relative abundances with respect to halo values, and the fact that the α-elements are all offset by a similar small amount from the halo averages, points to the same underlying galaxy-scale stellar initial mass function, but that Boo-1137 likely originated in a star-forming region where the abundances reflect either poor mixing of supernova (SN) ejecta, or poor sampling of the SN progenitor mass range, or both.

  19. J0023+0307: A Mega Metal-poor Dwarf Star from SDSS/BOSS

    Science.gov (United States)

    Aguado, David S.; Allende Prieto, Carlos; González Hernández, Jonay I.; Rebolo, Rafael

    2018-02-01

    Only a handful of stars have been identified with an iron abundance [Fe/H] support from theoretical modeling, as the result of a top-heavy initial mass function. With zero or very low metal abundance limiting radiative cooling, the formation of low-mass stars could be inhibited. Currently, the star SDSS J1029+1729 sets the potential metallicity threshold for the formation of low-mass stars at {log}Z/{Z}ȯ ∼ -5. In our quest to push down the metallicity threshold we have identified SDSS J0023+0307, a primitive star with T eff = 6188 ± 84 K, and {log}g=4.9+/- 0.5, an upper limit [Fe/H] < ‑6.6, and a carbon abundance A(C) < 6.3. We find J0023+0307 to be one of the two most iron-poor stars known, and it exhibits less carbon that most of the stars at [Fe/H] < ‑5. Based on observations made with William Herschel Telescope (WHT) and the Gran Telescopio de Canarias (GTC), at the Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in La Palma.

  20. Lithium in Very Metal-poor Dwarf Stars -- Problems for Standard Big Bang Nucleosynthesis?

    International Nuclear Information System (INIS)

    Lambert, David L.

    2004-01-01

    The standard model of primordial nucleosynthesis by the Big Bang as selected by the WMAP-based estimate of the baryon density (Ωbh2) predicts an abundance of 7Li that is a factor of three greater than the generally reported abundance for stars on the Spite plateau, and an abundance of 6Li that is about a thousand times less than is found for some stars on the plateau. This review discusses and examines these two discrepancies. They can likely be resolved without major surgery on the standard model of the Big Bang. In particular, stars on the Spite plateau may have depleted their surface lithium abundance over their long lifetime from the WMAP-based predicted abundances down to presently observed abundances, and synthesis of 6Li (and 7Li) via α + α fusion reactions may have occurred in the early Galaxy. Yet, there remain fascinating ways in which to remove the two discrepancies involving aspects of a new cosmology, particularly through the introduction of exotic particles

  1. Stellar Archaeology -- Exploring the Universe with Metal-Poor Stars

    OpenAIRE

    Frebel, Anna

    2010-01-01

    The abundance patterns of the most metal-poor stars in the Galactic halo and small dwarf galaxies provide us with a wealth of information about the early Universe. In particular, these old survivors allow us to study the nature of the first stars and supernovae, the relevant nucleosynthesis processes responsible for the formation and evolution of the elements, early star- and galaxy formation processes, as well as the assembly process of the stellar halo from dwarf galaxies a long time ago. T...

  2. Chemical Abundance Analysis of Three α-poor, Metal-poor Stars in the Ultrafaint Dwarf Galaxy Horologium I

    Science.gov (United States)

    Nagasawa, D. Q.; Marshall, J. L.; Li, T. S.; Hansen, T. T.; Simon, J. D.; Bernstein, R. A.; Balbinot, E.; Drlica-Wagner, A.; Pace, A. B.; Strigari, L. E.; Pellegrino, C. M.; DePoy, D. L.; Suntzeff, N. B.; Bechtol, K.; Walker, A. R.; Abbott, T. M. C.; Abdalla, F. B.; Allam, S.; Annis, J.; Benoit-Lévy, A.; Bertin, E.; Brooks, D.; Carnero Rosell, A.; Carrasco Kind, M.; Carretero, J.; Cunha, C. E.; D’Andrea, C. B.; da Costa, L. N.; Davis, C.; Desai, S.; Doel, P.; Eifler, T. F.; Flaugher, B.; Fosalba, P.; Frieman, J.; García-Bellido, J.; Gaztanaga, E.; Gerdes, D. W.; Gruen, D.; Gruendl, R. A.; Gschwend, J.; Gutierrez, G.; Hartley, W. G.; Honscheid, K.; James, D. J.; Jeltema, T.; Krause, E.; Kuehn, K.; Kuhlmann, S.; Kuropatkin, N.; March, M.; Miquel, R.; Nord, B.; Roodman, A.; Sanchez, E.; Santiago, B.; Scarpine, V.; Schindler, R.; Schubnell, M.; Sevilla-Noarbe, I.; Smith, M.; Smith, R. C.; Soares-Santos, M.; Sobreira, F.; Suchyta, E.; Tarle, G.; Thomas, D.; Tucker, D. L.; Wechsler, R. H.; Wolf, R. C.; Yanny, B.

    2018-01-01

    We present chemical abundance measurements of three stars in the ultrafaint dwarf galaxy Horologium I, a Milky Way satellite discovered by the Dark Energy Survey. Using high-resolution spectroscopic observations, we measure the metallicity of the three stars, as well as abundance ratios of several α-elements, iron-peak elements, and neutron-capture elements. The abundance pattern is relatively consistent among all three stars, which have a low average metallicity of [Fe/H] ∼ ‑2.6 and are not α-enhanced ([α/Fe] ∼ 0.0). This result is unexpected when compared to other low-metallicity stars in the Galactic halo and other ultrafaint dwarfs and suggests the possibility of a different mechanism for the enrichment of Hor I compared to other satellites. We discuss possible scenarios that could lead to this observed nucleosynthetic signature, including extended star formation, enrichment by a Population III supernova, and or an association with the Large Magellanic Cloud. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. This paper also includes data based on observations made with the ESO Very Large Telescope at Paranal Observatory, Chile (ID 096.D-0967(B); PI: E. Balbinot).

  3. Detecting metal-poor gas accretion in the star-forming dwarf galaxies UM 461 and Mrk 600

    Science.gov (United States)

    Lagos, P.; Scott, T. C.; Nigoche-Netro, A.; Demarco, R.; Humphrey, A.; Papaderos, P.

    2018-03-01

    Using VIMOS-IFU observations, we study the interstellar medium (ISM) of two star-forming dwarf galaxies, UM 461 and Mrk 600. Our aim was to search for the existence of metallicity inhomogeneities that might arise from infall of nearly pristine gas feeding ongoing localized star-formation. The IFU data allowed us to study the impact of external gas accretion on the chemical evolution as well as the ionised gas kinematics and morphologies of these galaxies. Both systems show signs of morphological distortions, including cometary-like morphologies. We analysed the spatial variation of 12 + log(O/H) abundances within both galaxies using the direct method (Te), the widely applied HII-CHI-mistry code, as well as by employing different standard calibrations. For UM 461 our results show that the ISM is fairly well mixed, at large scales, however we find an off-centre and low-metallicity region with 12 + log(O/H) ISM in our analysed galaxies are consistent with these systems being at different evolutionary stages.

  4. Oxygen and iron abundances in two metal-poor dwarfs

    Science.gov (United States)

    Spiesman, William J.; Wallerstein, George

    1991-11-01

    Oxygen abundances from the O I line at 6300 A in two metal-poor K dwarfs, HD 25329 and HD 134440, are derived. The spectra were obtained with the KPNO 4-m echelle spectrograph and long camera, yielding a resolution of 32,000 and an S/N of about 125. Model atmospheres with Te of 4770 were appropriate to both stars, whose metallicities were found to be -1.74 and -1.43 for HD 25329 and HD 134440, respectively. These oxygen abundances are 0.3 and 0.4 for the two stars. From the resolution an S/N a 3(sigma) upper limit of 0.8 is derived for each star, which may be combined into an upper limit of O/Fe of 0.6 for a generic K dwarf with Fe/H of 1.6. These values are more in line with O/Fe as seen in similarly metal-poor red giant than those reported in metal-poor subdwarfs by Abia and Rebolo (1989).

  5. A New View of the Dwarf Spheroidal Satellites of the Milky Way From VLT/FLAMES: Where are the Very Metal Poor Stars?

    Energy Technology Data Exchange (ETDEWEB)

    Helmi, Amina; Irwin, M.J.; Tolstoy, E.; Battaglia, G.; Hill, V.; Jablonka, P.; Venn, K.; Shetrone, M.; Letarte, B.; Arimoto, N.; Abel, T.; Francois, P.; Kaufer, A.; Primas, F.; Sadakane, K.; Szeifert, T.; /Kapteyn Astron. Inst., Groningen /Cambridge U., Inst. of Astron. /Meudon Observ. /LASTRO Observ. /Victoria U. /Texas U., McDonald Observ.

    2006-11-20

    As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) Programme, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph): Sculptor, Sextans, Fornax and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early Universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below [Fe/H] {approx} -3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSph appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.

  6. A new view of the Dwarf spheroidal satellites of the Milky Way from VLT FLAMES : Where are the very metal-poor stars?

    NARCIS (Netherlands)

    Helmi, Amina; Irwin, M. J.; Tolstoy, E.; Battaglia, G.; Hill, V.; Jablonka, P.; Venn, K.; Shetrone, M.; Letarte, B.; Arimoto, N.; Abel, T.; Francois, P.; Kaufer, A.; Primas, F.; Sadakane, K.; Szeifert, T.

    2006-01-01

    As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) program, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph's): Sculptor, Sextans, Fornax, and Carina. The low mean metal abundances and the presence of very old

  7. Chemical Abundance Analysis of Three α -poor, Metal-poor Stars in the Ultrafaint Dwarf Galaxy Horologium I

    Energy Technology Data Exchange (ETDEWEB)

    Nagasawa, D. Q.; Marshall, J. L.; Li, T. S.; Hansen, T. T.; Simon, J. D.; Bernstein, R. A.; Balbinot, E.; Drlica-Wagner, A.; Pace, A. B.; Strigari, L. E.; Pellegrino, C. M.; DePoy, D. L.; Suntzeff, N. B.; Bechtol, K.; Walker, A. R.; Abbott, T. M. C.; Abdalla, F. B.; Allam, S.; Annis, J.; Benoit-Lévy, A.; Bertin, E.; Brooks, D.; Rosell, A. Carnero; Kind, M. Carrasco; Carretero, J.; Cunha, C. E.; D’Andrea, C. B.; da Costa, L. N.; Davis, C.; Desai, S.; Doel, P.; Eifler, T. F.; Flaugher, B.; Fosalba, P.; Frieman, J.; García-Bellido, J.; Gaztanaga, E.; Gerdes, D. W.; Gruen, D.; Gruendl, R. A.; Gschwend, J.; Gutierrez, G.; Hartley, W. G.; Honscheid, K.; James, D. J.; Jeltema, T.; Krause, E.; Kuehn, K.; Kuhlmann, S.; Kuropatkin, N.; March, M.; Miquel, R.; Nord, B.; Roodman, A.; Sanchez, E.; Santiago, B.; Scarpine, V.; Schindler, R.; Schubnell, M.; Sevilla-Noarbe, I.; Smith, M.; Smith, R. C.; Soares-Santos, M.; Sobreira, F.; Suchyta, E.; Tarle, G.; Thomas, D.; Tucker, D. L.; Wechsler, R. H.; Wolf, R. C.; Yanny, B.

    2018-01-10

    We present chemical abundance measurements of three stars in the ultra-faintdwarf galaxy Horologium I, a Milky Way satellite discovered by the Dark EnergySurvey. Using high resolution spectroscopic observations we measure themetallicity of the three stars as well as abundance ratios of several$\\alpha$-elements, iron-peak elements, and neutron-capture elements. Theabundance pattern is relatively consistent among all three stars, which have alow average metallicity of [Fe/H] $\\sim -2.6$ and are not $\\alpha$-enhanced([$\\alpha$/Fe] $\\sim 0.0$). This result is unexpected when compared to otherlow-metallicity stars in the Galactic halo and other ultra-faint dwarfs andhints at an entirely different mechanism for the enrichment of Hor I comparedto other satellites. We discuss possible scenarios that could lead to thisobserved nucleosynthetic signature including extended star formation, aPopulation III supernova, and a possible association with the Large MagellanicCloud.

  8. THE STAR FORMATION HISTORY OF THE VERY METAL-POOR BLUE COMPACT DWARF I Zw 18 FROM HST/ACS DATA

    Energy Technology Data Exchange (ETDEWEB)

    Annibali, F.; Cignoni, M.; Tosi, M.; Clementini, G.; Contreras Ramos, R.; Fiorentino, G. [INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna (Italy); Van der Marel, R. P.; Aloisi, A. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Marconi, M.; Musella, I., E-mail: francesca.annibali@oabo.inaf.it [INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, I-80131 Napoli (Italy)

    2013-12-01

    We have derived the star formation history (SFH) of the blue compact dwarf galaxy I Zw 18 through comparison of deep HST/ACS data with synthetic color-magnitude diagrams (CMDs). A statistical analysis was implemented for the identification of the best-fit SFH and relative uncertainties. We confirm that I Zw 18 is not a truly young galaxy, having started forming stars earlier than ∼1 Gyr ago, and possibly at epochs as old as a Hubble time. In I Zw 18's main body we infer a lower limit of ≈2 × 10{sup 6} M {sub ☉} for the mass locked up in old stars. I Zw 18's main body has been forming stars very actively during the last ∼10 Myr, with an average star formation rate (SFR) as high as ≈1 M {sub ☉} yr{sup –1} (or ≈2 × 10{sup –5} M {sub ☉} yr{sup –1} pc{sup –2}). On the other hand, the secondary body was much less active at these epochs, in agreement with the absence of significant nebular emission. The high current SFR can explain the very blue colors and the high ionized gas content in I Zw 18, resembling primeval galaxies in the early universe. Detailed chemical evolution models are required to quantitatively check whether the SFH from the synthetic CMDs can explain the low measured element abundances, or if galactic winds with loss of metals are needed.

  9. The Most Metal-poor Stars in the Large Magellanic Cloud

    Science.gov (United States)

    Schlaufman, Kevin C.

    2018-06-01

    The chemical abundances of the most metal-poor stars in a galaxy can be used to investigate the earliest stages of its formation and chemical evolution. Differences between the abundances of the most metal-poor stars in the Milky Way and in its satellite dwarf galaxies have been noted and provide the strongest available constraints on the earliest stages of general galactic chemical evolution models. However, the masses of the Milky Way and its satellite dwarf galaxies differ by four orders of magnitude, leaving a gap in our knowledge of the early chemical evolution of intermediate mass galaxies like the Magellanic Clouds. To close that gap, we have initiated a survey of the metal-poor stellar populations of the Magellanic Clouds using the mid-infrared metal-poor star selection of Schlaufman & Casey (2014). We have discovered the three most metal-poor giant stars known in the Large Magellanic Cloud (LMC) and reobserved the previous record holder. The stars have metallicities in the range -2.70 < [Fe/H] < -2.00 and three show r-process enhancement: one has [Eu II/Fe] = +1.65 and two others have [Eu II/Fe] = +0.65. The probability that four randomly selected very metal-poor stars in the halo of the Milky Way are as r-process enhanced is 0.0002. For that reason, the early chemical enrichment of the heaviest elements in the LMC and Milky Way were qualitatively different. It is also suggestive of a possible chemical link between the LMC and the ultra-faint dwarf galaxies nearby with evidence of r-process enhancement (e.g., Reticulum II and Tucana III). Like Reticulum II, the most metal-poor star in our LMC sample is the only one not enhanced in r-process elements.

  10. Carbon-enhanced metal-poor stars and thermohaline mixing

    NARCIS (Netherlands)

    Stancliffe, R.J.; Glebbeek, E.; Izzard, R.G.; Pols, O.R.

    2007-01-01

    One possible scenario for the formation of carbon-enhanced metal-poor stars is the accretion of carbon-rich material from a binary companion which may no longer visible. It is generally assumed that the accreted material remains on the surface of the star and does not mix with the interior until

  11. Abundances in very metal-poor stars

    Science.gov (United States)

    Johnson, Jennifer Anne

    We measured the abundances of 35 elements in 22 field red giants and a red giant in the globular cluster M92. We found the [Zn/Fe] ratio increases with decreasing [Fe/H], reaching ~0.3 at [Fe/H] = -3.0. While this is a larger [Zn/Fe] than found by previous investigators, it is not sufficient to account for the [Zn/Fe] observed in the damped Lyα systems. We test different models for the production of the s-process elements by comparing our [Y/Zr] values, which have been produced by the r- process, to predictions of what the s-process does not produce. We find that the models of Arlandini et al. (1999), which calculate s-process production in a model AGB star, agree the best. We then look at the r-process abundances across a wide range in mass. The [Y/Ba] values for most of our stars cluster around -0.30, but there are three outliers with [Y/Ba] values up to 1 dex higher. Thus the heavy element abundances do not show the same pattern from Z = 39 to Z = 56. However, our abundances ratios from Pd (Z = 46) to Yb (Z = 70) are consistent with a scaled solar system r- process pattern, arguing that at least the heavy r- process elements are made in a universal pattern. If we assume that this same pattern hold through thorium, we can determine the ages of our stars from the present abundance of radioactive thorium and an initial thorium abundance based on the abundance of stable heavy elements. Our results for five stars are consistent with those stars being the same age. Our mean age is 10.8 +/- 2 Gyr. However that result depends critically on the assumed Th/stable ratio, which we adopt from models of the r-process. For an average age of 15 Gyrs, the initial Th/Eu ratio we would need is 0.590. Finally, the [element/Fe] ratios for elements in the iron group and lower do not show any dispersion, unlike for the r- process elements such as Y and Ba. Therefore the individual contributions of supernovae have been erased for the lighter elements.

  12. The best and brightest metal-poor stars

    Energy Technology Data Exchange (ETDEWEB)

    Schlaufman, Kevin C.; Casey, Andrew R., E-mail: kschlauf@mit.edu, E-mail: arc@ast.cam.ac.uk [Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)

    2014-12-10

    The chemical abundances of large samples of extremely metal-poor (EMP) stars can be used to investigate metal-free stellar populations, supernovae, and nucleosynthesis as well as the formation and galactic chemical evolution of the Milky Way and its progenitor halos. However, current progress on the study of EMP stars is being limited by their faint apparent magnitudes. The acquisition of high signal-to-noise spectra for faint EMP stars requires a major telescope time commitment, making the construction of large samples of EMP star abundances prohibitively expensive. We have developed a new, efficient selection that uses only public, all-sky APASS optical, 2MASS near-infrared, and WISE mid-infrared photometry to identify bright metal-poor star candidates through their lack of molecular absorption near 4.6 microns. We have used our selection to identify 11,916 metal-poor star candidates with V < 14, increasing the number of publicly available candidates by more than a factor of five in this magnitude range. Their bright apparent magnitudes have greatly eased high-resolution follow-up observations that have identified seven previously unknown stars with [Fe/H] ≲ –3.0. Our follow-up campaign has revealed that 3.8{sub −1.1}{sup +1.3}% of our candidates have [Fe/H] ≲ –3.0 and 32.5{sub −2.9}{sup +3.0}% have –3.0 ≲ [Fe/H] ≲ –2.0. The bulge is the most likely location of any existing Galactic Population III stars, and an infrared-only variant of our selection is well suited to the identification of metal-poor stars in the bulge. Indeed, two of our confirmed metal-poor stars with [Fe/H] ≲ –2.7 are within about 2 kpc of the Galactic center. They are among the most metal-poor stars known in the bulge.

  13. AN EXTREMELY CARBON-RICH, EXTREMELY METAL-POOR STAR IN THE SEGUE 1 SYSTEM

    International Nuclear Information System (INIS)

    Norris, John E.; Yong, David; Gilmore, Gerard; Wyse, Rosemary F. G.; Frebel, Anna

    2010-01-01

    We report the analysis of high-resolution, high signal-to-noise ratio, spectra of an extremely metal-poor, extremely C-rich red giant, Seg 1-7, in Segue 1-described in the literature alternatively as an unusually extended globular cluster or an ultra-faint dwarf galaxy. The radial velocity of Seg 1-7 coincides precisely with the systemic velocity of Segue 1, and its chemical abundance signature of [Fe/H] = -3.52, [C/Fe] = +2.3, [N/Fe] = +0.8, [Na/Fe] = +0.53, [Mg/Fe] = +0.94, [Al/Fe] = +0.23, and [Ba/Fe] < -1.0 is similar to that of the rare and enigmatic class of Galactic halo objects designated CEMP-no (carbon-rich, extremely metal-poor with no enhancement (over solar ratios) of heavy neutron-capture elements). This is the first star in a Milky Way 'satellite' that unambiguously lies on the metal-poor, C-rich branch of the Aoki et al. bimodal distribution of field halo stars in the ([C/Fe], [Fe/H])-plane. Available data permit us only to identify Seg 1-7 as a member of an ultra-faint dwarf galaxy or as debris from the Sgr dwarf spheroidal galaxy. In either case, this demonstrates that at extremely low abundance, [Fe/H ] <-3.0, star formation and associated chemical evolution proceeded similarly in the progenitors of both the field halo and satellite systems. By extension, this is consistent with other recent suggestions that the most metal-poor dwarf spheroidal and ultra-faint dwarf satellites were the building blocks of the Galaxy's outer halo.

  14. Impact of Lyman alpha pressure on metal-poor dwarf galaxies

    Science.gov (United States)

    Kimm, Taysun; Haehnelt, Martin; Blaizot, Jérémy; Katz, Harley; Michel-Dansac, Léo; Garel, Thibault; Rosdahl, Joakim; Teyssier, Romain

    2018-04-01

    Understanding the origin of strong galactic outflows and the suppression of star formation in dwarf galaxies is a key problem in galaxy formation. Using a set of radiation-hydrodynamic simulations of an isolated dwarf galaxy embedded in a 1010 M⊙ halo, we show that the momentum transferred from resonantly scattered Lyman-α (Lyα) photons is an important source of stellar feedback which can shape the evolution of galaxies. We find that Lyα feedback suppresses star formation by a factor of two in metal-poor galaxies by regulating the dynamics of star-forming clouds before the onset of supernova explosions (SNe). This is possible because each Lyα photon resonantly scatters and imparts ˜10-300 times greater momentum than in the single scattering limit. Consequently, the number of star clusters predicted in the simulations is reduced by a factor of ˜5, compared to the model without the early feedback. More importantly, we find that galactic outflows become weaker in the presence of strong Lyα radiation feedback, as star formation and associated SNe become less bursty. We also examine a model in which radiation field is arbitrarily enhanced by a factor of up to 10, and reach the same conclusion. The typical mass-loading factors in our metal-poor dwarf system are estimated to be ˜5-10 near the mid-plane, while it is reduced to ˜1 at larger radii. Finally, we find that the escape of ionizing radiation and hence the reionization history of the Universe is unlikely to be strongly affected by Lyα feedback.

  15. Infrared colours and inferred masses of metal-poor giant stars in the Keplerfield

    Science.gov (United States)

    Casey, A. R.; Kennedy, G. M.; Hartle, T. R.; Schlaufman, Kevin C.

    2018-05-01

    Intrinsically luminous giant stars in the Milky Way are the only potential volume-complete tracers of the distant disk, bulge, and halo. The chemical abundances of metal-poor giants also reflect the compositions of the earliest star-forming regions, providing the initial conditions for the chemical evolution of the Galaxy. However, the intrinsic rarity of metal-poor giants combined with the difficulty of efficiently identifying them with broad-band optical photometry has made it difficult to exploit them for studies of the Milky Way. One long-standing problem is that photometric selections for giant and/or metal-poor stars frequently include a large fraction of metal-rich dwarf contaminants. We re-derive a giant star photometric selection using existing public g-band and narrow-band DDO51photometry obtained in the Keplerfield. Our selection is simple and yields a contamination rate of main-sequence stars of ≲1% and a completeness of about 80 % for giant stars with Teff ≲ 5250 K - subject to the selection function of the spectroscopic surveys used to estimate these rates, and the magnitude range considered (11 ≲ g ≲ 15). While the DDO51filter is known to be sensitive to stellar surface gravity, we further show that the mid-infrared colours of DDO51-selected giants are strongly correlated with spectroscopic metallicity. This extends the infrared metal-poor selection developed by Schlaufman & Casey, demonstrating that the principal contaminants in their selection can be efficiently removed by the photometric separation of dwarfs and giants. This implies that any similarly efficient dwarf/giant discriminant (e.g., Gaiaparallaxes) can be used in conjunction with WISEcolours to select samples of giant stars with high completeness and low contamination. We employ our photometric selection to identify three metal-poor giant candidates in the Keplerfield with global asteroseismic parameters and find that masses inferred for these three stars using standard

  16. LITHIUM ABUNDANCES OF EXTREMELY METAL-POOR TURNOFF STARS

    International Nuclear Information System (INIS)

    Aoki, Wako; Inoue, Susumu; Barklem, Paul S.; Beers, Timothy C.; Christlieb, Norbert; Perez, Ana E. GarcIa; Norris, John E.; Carollo, Daniela

    2009-01-01

    We have determined Li abundances for eleven metal-poor turnoff stars, among which eight have [Fe/H] <-3, based on LTE analyses of high-resolution spectra obtained with the High Dispersion Spectrograph on the Subaru Telescope. The Li abundances for four of these eight stars are determined for the first time by this study. Effective temperatures are determined by a profile analysis of Hα and Hβ. While seven stars have Li abundances as high as the Spite Plateau value, the remaining four objects with [Fe/H] <-3 have A(Li) =log (Li/H)+ 12 ∼< 2.0, confirming the existence of extremely metal-poor (EMP) turnoff stars having low Li abundances, as reported by previous work. The average of the Li abundances for stars with [Fe/H]<-3 is lower by 0.2 dex than that of the stars with higher metallicity. No clear constraint on the metallicity dependence or scatter of the Li abundances is derived from our measurements for the stars with [Fe/H]<-3. Correlations of the Li abundance with effective temperatures, with abundances of Na, Mg, and Sr, and with the kinematical properties are investigated, but no clear correlation is seen in the EMP star sample.

  17. Three-dimensional models of metal-poor stars

    OpenAIRE

    Collet, R.

    2008-01-01

    I present here the main results of recent realistic, 3D, hydrodynamical simulations of convection at the surface of metal-poor red giant stars. I discuss the application of these convection simulations as time-dependent, 3D, hydrodynamical model atmospheres to spectral line formation calculations and abundance analyses. The impact of 3D models on derived elemental abundances is investigated by means of a differential comparison of the line strengths predicted in 3D under the assumption of loc...

  18. Three-dimensional models of metal-poor stars

    International Nuclear Information System (INIS)

    Collet, R

    2008-01-01

    I present here the main results of recent realistic, three-dimensional (3D), hydrodynamical simulations of convection at the surface of metal-poor red giant stars. I discuss the application of these convection simulations as time-dependent, 3D, hydrodynamical model atmospheres to spectral line formation calculations and abundance analyses. The impact of 3D models on derived elemental abundances is investigated by means of a differential comparison of the line strengths predicted in 3D under the assumption of local thermodynamic equilibrium (LTE) with the results of analogous line formation calculations performed with classical, 1D, hydrostatic model atmospheres. The low surface temperatures encountered in the upper photospheric layers of 3D model atmospheres of very metal-poor stars cause spectral lines of neutral metals and molecules to appear stronger in 3D than in 1D calculations. Hence, 3D elemental abundances derived from such lines are significantly lower than estimated by analyses with 1D models. In particular, differential 3D-1D LTE abundances for C, N and O derived from CH, NH and OH lines are found to be in the range -0.5 to - 1 dex. Large negative differential 3D-1D corrections to the Fe abundance are also computed for weak low-excitation Fe i lines. The application of metal-poor 3D models to the spectroscopic analysis of extremely iron-poor halo stars is discussed.

  19. Heavy elements abundances in metal-poor stars

    International Nuclear Information System (INIS)

    Magain, P.; Jehin, E.; Neuforge, C.; Noels, A.

    1998-01-01

    A sample of 21 metal-poor stars have been analysed on the basis of high resolution and high signal-to-noise spectra. Correlations between relative abundances of 16 elements have been studied, with a special emphasis on the neutron-capture ones. This analysis reveals the existence of two sub-populations of field halo stars, namely Pop IIa and Pop IIb. They differ by the behaviour of the s-process elements versus the α and r-process elements. We suggest a scenario of formation of these stars, which closely relates the field halo stars to the evolution of globular clusters. The two sub-populations would have evaporated the clusters during two different stages of their chemical evolution

  20. THE KENNICUTT–SCHMIDT RELATION IN EXTREMELY METAL-POOR DWARF GALAXIES

    Energy Technology Data Exchange (ETDEWEB)

    Filho, M. E.; Almeida, J. Sánchez; Muñoz-Tuñón, C. [Instituto Astrofísica de Canarias, E-38200 La Laguna, Tenerife (Spain); Amorín, R. [National Institute for Astrophysics, Astronomical Observatory of Rome, Via Frascati 33, I-00040 Monteporzio Catone (Rome) (Italy); Elmegreen, B. G. [IBM, T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598 (United States); Elmegreen, D. M., E-mail: mfilho@astro.up.pt [Department of Physics and Astronomy, Vassar College, Poughkeepsie, NY 12604 (United States)

    2016-04-01

    The Kennicutt–Schmidt (KS) relation between the gas mass and star formation rate (SFR) describes the star formation regulation in disk galaxies. It is a function of gas metallicity, but the low-metallicity regime of the KS diagram is poorly sampled. We have analyzed data for a representative set of extremely metal-poor galaxies (XMPs), as well as auxiliary data, and compared these to empirical and theoretical predictions. The majority of the XMPs possess high specific SFRs, similar to high-redshift star-forming galaxies. On the KS plot, the XMP H i data occupy the same region as dwarfs and extend the relation for low surface brightness galaxies. Considering the H i gas alone, a considerable fraction of the XMPs already fall off the KS law. Significant quantities of “dark” H{sub 2} mass (i.e., not traced by CO) would imply that XMPs possess low star formation efficiencies (SFE{sub gas}). Low SFE{sub gas} in XMPs may be the result of the metal-poor nature of the H i gas. Alternatively, the H i reservoir may be largely inert, the star formation being dominated by cosmological accretion. Time lags between gas accretion and star formation may also reduce the apparent SFE{sub gas}, as may galaxy winds, which can expel most of the gas into the intergalactic medium. Hence, on global scales, XMPs could be H i-dominated, high-specific-SFR (≳10{sup −10} yr{sup −1}), low-SFE{sub gas} (≲10{sup −9} yr{sup −1}) systems, in which the total H i mass is likely not a good predictor of the total H{sub 2} mass, nor of the SFR.

  1. Searching for dust around hyper metal poor stars

    International Nuclear Information System (INIS)

    Venn, Kim A.; Divell, Mike; Starkenburg, Else; Puzia, Thomas H.; Côté, Stephanie; Lambert, David L.

    2014-01-01

    We examine the mid-infrared fluxes and spectral energy distributions for stars with iron abundances [Fe/H] <–5, and other metal-poor stars, to eliminate the possibility that their low metallicities are related to the depletion of elements onto dust grains in the formation of a debris disk. Six out of seven stars examined here show no mid-IR excesses. These non-detections rule out many types of circumstellar disks, e.g., a warm debris disk (T ≤ 290 K), or debris disks with inner radii ≤1 AU, such as those associated with the chemically peculiar post-asymptotic giant branch spectroscopic binaries and RV Tau variables. However, we cannot rule out cooler debris disks, nor those with lower flux ratios to their host stars due to, e.g., a smaller disk mass, a larger inner disk radius, an absence of small grains, or even a multicomponent structure, as often found with the chemically peculiar Lambda Bootis stars. The only exception is HE0107-5240, for which a small mid-IR excess near 10 μm is detected at the 2σ level; if the excess is real and associated with this star, it may indicate the presence of (recent) dust-gas winnowing or a binary system.

  2. Searching for dust around hyper metal poor stars

    Energy Technology Data Exchange (ETDEWEB)

    Venn, Kim A.; Divell, Mike; Starkenburg, Else [Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 5C2 (Canada); Puzia, Thomas H. [Institute of Astrophysics, Pontificia Universidad Catolica de Chile, Av. Vicuna Mackenna 4860, 7820436 Macul, Santiago (Chile); Côté, Stephanie [NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7 (Canada); Lambert, David L., E-mail: kvenn@uvic.ca [McDonald Observatory and the Department of Astronomy, University of Texas at Austin, RLM 15.308, Austin, TX 78712 (United States)

    2014-08-20

    We examine the mid-infrared fluxes and spectral energy distributions for stars with iron abundances [Fe/H] <–5, and other metal-poor stars, to eliminate the possibility that their low metallicities are related to the depletion of elements onto dust grains in the formation of a debris disk. Six out of seven stars examined here show no mid-IR excesses. These non-detections rule out many types of circumstellar disks, e.g., a warm debris disk (T ≤ 290 K), or debris disks with inner radii ≤1 AU, such as those associated with the chemically peculiar post-asymptotic giant branch spectroscopic binaries and RV Tau variables. However, we cannot rule out cooler debris disks, nor those with lower flux ratios to their host stars due to, e.g., a smaller disk mass, a larger inner disk radius, an absence of small grains, or even a multicomponent structure, as often found with the chemically peculiar Lambda Bootis stars. The only exception is HE0107-5240, for which a small mid-IR excess near 10 μm is detected at the 2σ level; if the excess is real and associated with this star, it may indicate the presence of (recent) dust-gas winnowing or a binary system.

  3. THE MOST METAL-POOR DAMPED Lyα SYSTEMS: AN INSIGHT INTO DWARF GALAXIES AT HIGH-REDSHIFT

    International Nuclear Information System (INIS)

    Cooke, Ryan J.; Pettini, Max; Jorgenson, Regina A.

    2015-01-01

    In this paper we analyze the kinematics, chemistry, and physical properties of a sample of the most metal-poor damped Lyα systems (DLAs), to uncover their links to modern-day galaxies. We present evidence that the DLA population as a whole exhibits a ''knee'' in the relative abundances of the α-capture and Fe-peak elements when the metallicity is [Fe/H] ≅ –2.0, assuming that Zn traces the buildup of Fe-peak elements. In this respect, the chemical evolution of DLAs is clearly different from that experienced by Milky Way halo stars, but resembles that of dwarf spheroidal galaxies in the Local Group. We also find a close correspondence between the kinematics of Local Group dwarf galaxies and of high-redshift metal-poor DLAs, which further strengthens this connection. On the basis of such similarities, we propose that the most metal-poor DLAs provide us with a unique opportunity to directly study the dwarf galaxy population more than ten billion years in the past, at a time when many dwarf galaxies were forming the bulk of their stars. To this end, we have measured some of the key physical properties of the DLA gas, including their neutral gas mass, size, kinetic temperature, density, and turbulence. We find that metal-poor DLAs contain a warm neutral medium with T gas ≅ 9600 K predominantly held up by thermal pressure. Furthermore, all of the DLAs in our sample exhibit a subsonic turbulent Mach number, implying that the gas distribution is largely smooth. These results are among the first empirical descriptions of the environments where the first few generations of stars may have formed in the universe

  4. Oxygen abundance in metal-poor dwarfs, derived from the forbidden line

    Science.gov (United States)

    Spite, M.; Spite, F.

    1991-12-01

    The oxygen abundance is redetermined in a few metal-poor dwarfs, using the oxygen forbidden line at 630 nm rather than the oxygen triplet at 777 nm previously used by Abia and Rebolo (1989). The ratios form O/Fe are clearly lower than the previous ones and are in agreement with the ratios found in the metal-poor red giants, suggesting that no real difference exists between dwarfs and giants. Finally, it can be argued that, pending the acquisition of additional information, the oxygen abundances derived from the forbidden line are more reliable than the abundances found from the triplet.

  5. Possible evidence for metal accretion onto the surfaces of metal-poor main-sequence stars

    Energy Technology Data Exchange (ETDEWEB)

    Hattori, Kohei; Yoshii, Yuzuru [Institute of Astronomy, School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015 (Japan); Beers, Timothy C. [National Optical Astronomy Observatories, Tucson, AZ 85719 (United States); Carollo, Daniela [Department of Physics and Astronomy, Macquarie University, Sydney, 2109 NSW (Australia); Lee, Young Sun, E-mail: khattori@ioa.s.u-tokyo.ac.jp [Department of Astronomy, New Mexico State University, Las Cruces, NM 88003 (United States)

    2014-04-01

    The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by position, velocity, and elemental abundance ratios. In particular, the chemo-dynamical distribution functions for low-mass stars (e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the Milky Way's formation, since their main-sequence lifetimes approach or exceed the age of the universe. A basic tenet of essentially all previous analyses is that the stellar metallicity, usually parameterized as [Fe/H], is conserved over time for main-sequence stars (at least those that have not been polluted due to mass transfer from binary companions). If this holds true, any correlations between metallicity and kinematics for long-lived main-sequence stars of different masses, effective temperatures, or spectral types must strictly be the same, since they reflect the same mass-assembly and star-formation histories. By analyzing a sample of nearby metal-poor halo and thick-disk stars on the main sequence, taken from Data Release 8 of the Sloan Digital Sky Survey, we find that the median metallicity of G-type dwarfs is systematically higher (by about 0.2 dex) than that of K-type dwarfs having the same median rotational velocity about the Galactic center. If it can be confirmed, this finding may invalidate the long-accepted assumption that the atmospheric metallicities of long-lived stars are conserved over time.

  6. HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: BOÖTES II

    International Nuclear Information System (INIS)

    Ji, Alexander P.; Frebel, Anna; Simon, Joshua D.; Geha, Marla

    2016-01-01

    We present high-resolution Magellan/MIKE spectra of the four brightest confirmed red giant stars in the ultra-faint dwarf galaxy Boötes II (Boo II). These stars all inhabit the metal-poor tail of the Boo II metallicity distribution function. The chemical abundance pattern of all detectable elements in these stars is consistent with that of the Galactic halo. However, all four stars have undetectable amounts of neutron-capture elements Sr and Ba, with upper limits comparable to the lowest ever detected in the halo or in other dwarf galaxies. One star exhibits significant radial velocity variations over time, suggesting it to be in a binary system. Its variable velocity has likely increased past determinations of the Boo II velocity dispersion. Our four stars span a limited metallicity range, but their enhanced α-abundances and low neutron-capture abundances are consistent with the interpretation that Boo II has been enriched by very few generations of stars. The chemical abundance pattern in Boo II confirms the emerging trend that the faintest dwarf galaxies have neutron-capture abundances distinct from the halo, suggesting the dominant source of neutron-capture elements in halo stars may be different than in ultra-faint dwarfs

  7. Quantitative spectroscopy of blue supergiants in metal-poor dwarf galaxy NGC 3109

    International Nuclear Information System (INIS)

    Hosek, Matthew W. Jr.; Kudritzki, Rolf-Peter; Bresolin, Fabio; Urbaneja, Miguel A.; Przybilla, Norbert; Evans, Christopher J.; Pietrzyński, Grzegorz; Gieren, Wolfgang; Carraro, Giovanni

    2014-01-01

    We present a quantitative analysis of the low-resolution (∼4.5 Å) spectra of 12 late-B and early-A blue supergiants (BSGs) in the metal-poor dwarf galaxy NGC 3109. A modified method of analysis is presented which does not require use of the Balmer jump as an independent T eff indicator, as used in previous studies. We determine stellar effective temperatures, gravities, metallicities, reddening, and luminosities, and combine our sample with the early-B-type BSGs analyzed by Evans et al. to derive the distance to NGC 3109 using the flux-weighted gravity-luminosity relation (FGLR). Using primarily Fe-group elements, we find an average metallicity of [ Z-bar ] = –0.67 ± 0.13, and no evidence of a metallicity gradient in the galaxy. Our metallicities are higher than those found by Evans et al. based on the oxygen abundances of early-B supergiants ([ Z-bar ] = –0.93 ± 0.07), suggesting a low α/Fe ratio for the galaxy. We adjust the position of NGC 3109 on the BSG-determined galaxy mass-metallicity relation accordingly and compare it to metallicity studies of H II regions in star-forming galaxies. We derive an FGLR distance modulus of 25.55 ± 0.09 (1.27 Mpc) that compares well with Cepheid and tip of the red giant branch distances. The FGLR itself is consistent with those found in other galaxies, demonstrating the reliability of this method as a measure of extragalactic distances.

  8. Quantitative spectroscopy of blue supergiants in metal-poor dwarf galaxy NGC 3109

    Energy Technology Data Exchange (ETDEWEB)

    Hosek, Matthew W. Jr.; Kudritzki, Rolf-Peter; Bresolin, Fabio [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Urbaneja, Miguel A.; Przybilla, Norbert [Institute for Astro and Particle Physics, A-6020 Innsbruck University (Austria); Evans, Christopher J. [UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh (United Kingdom); Pietrzyński, Grzegorz; Gieren, Wolfgang [Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción (Chile); Carraro, Giovanni, E-mail: mwhosek@ifa.hawaii.edu, E-mail: kud@ifa.hawaii.edu, E-mail: bresolin@ifa.hawaii.edu, E-mail: Miguel.Urbaneja-Perez@uibk.ac.at, E-mail: Norbert.Przybilla@uibk.ac.at, E-mail: chris.evans@stfc.ac.uk, E-mail: pietrzyn@astrouw.edu.pl, E-mail: wgieren@astro-udec.cl, E-mail: gcarraro@eso.org [European Southern Observatory, La Silla Paranal Observatory (Chile)

    2014-04-20

    We present a quantitative analysis of the low-resolution (∼4.5 Å) spectra of 12 late-B and early-A blue supergiants (BSGs) in the metal-poor dwarf galaxy NGC 3109. A modified method of analysis is presented which does not require use of the Balmer jump as an independent T {sub eff} indicator, as used in previous studies. We determine stellar effective temperatures, gravities, metallicities, reddening, and luminosities, and combine our sample with the early-B-type BSGs analyzed by Evans et al. to derive the distance to NGC 3109 using the flux-weighted gravity-luminosity relation (FGLR). Using primarily Fe-group elements, we find an average metallicity of [ Z-bar ] = –0.67 ± 0.13, and no evidence of a metallicity gradient in the galaxy. Our metallicities are higher than those found by Evans et al. based on the oxygen abundances of early-B supergiants ([ Z-bar ] = –0.93 ± 0.07), suggesting a low α/Fe ratio for the galaxy. We adjust the position of NGC 3109 on the BSG-determined galaxy mass-metallicity relation accordingly and compare it to metallicity studies of H II regions in star-forming galaxies. We derive an FGLR distance modulus of 25.55 ± 0.09 (1.27 Mpc) that compares well with Cepheid and tip of the red giant branch distances. The FGLR itself is consistent with those found in other galaxies, demonstrating the reliability of this method as a measure of extragalactic distances.

  9. The metal-poor knee in the Fornax dwarf spheroidal galaxy

    Energy Technology Data Exchange (ETDEWEB)

    Hendricks, Benjamin; Koch, Andreas [Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117, Heidelberg (Germany); Lanfranchi, Gustavo A. [Núcleo de Astrofísica Teórica, Universidade Cruzeiro do Sul, R. Galvão Bueno 868, Liberdade, 01506-000, São Paulo, SP (Brazil); Boeche, Corrado [Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, D-69120, Heidelberg (Germany); Walker, Matthew [McWilliams Center for Cosmology, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213 (United States); Johnson, Christian I. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-15, Cambridge, MA 02138 (United States); Peñarrubia, Jorge [Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom); Gilmore, Gerard, E-mail: ben.hendricks@lsw.uni-heidelberg.de [Institute of Astronomy, Cambridge University, Madingley Rd, Cambridge CB3 OHA (United Kingdom)

    2014-04-20

    We present α-element abundances of Mg, Si, and Ti for a large sample of field stars in two outer fields of the Fornax dwarf spheroidal (dSph) galaxy, obtained with Very Large Telescope/GIRAFFE (R ∼ 16, 000). Due to the large fraction of metal-poor (MP) stars in our sample, we are able to follow the α-element evolution from [Fe/H] ≈ –2.5 continuously to [Fe/H] ≈ –0.7. For the first time we are able to resolve the turnover from the Type II supernovae (SNe) dominated, α-enhanced plateau down to subsolar [α/Fe] values, due to the onset of SNe Ia, and thus to trace the chemical enrichment efficiency of the galaxy. Our data support the general concept of an α-enhanced plateau at early epochs, followed by a well-defined 'knee' caused by the onset of SNe Ia, and finally a second plateau with sub-solar [α/Fe] values. We find the position of this knee to be at [Fe/H] ≈ –1.9 and therefore significantly more MP than expected from comparison with other dSphs and standard evolutionary models. Surprisingly, this value is rather comparable to the knee in Sculptor, a dSph ∼10 times less luminous than Fornax. Using chemical evolution models, we find that the position of the knee and the subsequent plateau at the sub-solar level can hardly be explained unless the galaxy experienced several discrete star formation (SF) events with a drastic variation in SF efficiency, while a uniform SF can be ruled out. One possible evolutionary scenario is that Fornax experienced one or several major accretion events from gas-rich systems in the past, so that its current stellar mass is not indicative of the chemical evolution environment at ancient times. If Fornax is the product of several smaller buildings blocks, this may also have implications for the understanding of the formation process of dSphs in general.

  10. New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy

    Science.gov (United States)

    Aguado, D. S.; Allende Prieto, C.; González Hernández, J. I.; Rebolo, R.; Caffau, E.

    2017-07-01

    Context. The first generation of stars formed in the Galaxy left behind the chemical signatures of their nucleosynthesis in the interstellar medium, visible today in the atmospheres of low-mass stars that formed afterwards. Sampling the chemistry of those low-mass provides insight into the first stars. Aims: We aim to increase the samples of stars with extremely low metal abundances, identifying ultra metal-poor stars from spectra with modest spectral resolution and signal-to-noise ratio (S/N). Achieving this goal involves deriving reliable metallicities and carbon abundances from such spectra. Methods: We carry out follow-up observations of faint, V > 19, metal-poor candidates selected from SDSS spectroscopy and observed with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at GTC. The SDSS and follow-up OSIRIS spectra were analyzed using the FERRE code to derive effective temperatures, surface gravities, metallicities and carbon abundances. In addition, a well-known extremely metal-poor star has been included in our sample to calibrate the analysis methodology. Results: We observed and analyzed five metal-poor candidates from modest-quality SDSS spectra. All stars in our sample have been confirmed as extremely metal-poor stars, in the [Fe/H] Palma. Programme ID GTC2E-16A and ID GTC65-16B.

  11. SPECTROSCOPIC STUDIES OF EXTREMELY METAL-POOR STARS WITH THE SUBARU HIGH DISPERSION SPECTROGRAPH. V. THE Zn-ENHANCED METAL-POOR STAR BS 16920-017

    International Nuclear Information System (INIS)

    Honda, Satoshi; Aoki, Wako; Beers, Timothy C.; Takada-Hidai, Masahide

    2011-01-01

    We report Zn abundances for 18 very metal-poor stars studied in our previous work, covering the metallicity range -3.2< [Fe/H] <-2.5. The [Zn/Fe] values of most stars show an increasing trend with decreasing [Fe/H] in this metallicity range, confirming the results found by previous studies. However, the extremely metal-poor star BS 16920-017([Fe/H] =-3.2) exhibits a significantly high [Zn/Fe] ratio ([Zn/Fe] = +1.0). Comparison of the chemical abundances of this object with HD 4306, which has similar atmospheric parameters to BS 16920-017, clearly demonstrates a deficiency of α elements and neutron-capture elements in this star, along with enhancements of Mn and Ni, as well as Zn. The association with a hypernova explosion that has been proposed to explain the high Zn abundance ratios found in extremely metal-poor stars is a possible explanation, although further studies are required to fully interpret the abundance pattern of this object.

  12. The s-Process Nucleosynthesis in Extremely Metal-Poor Stars as the Generating Mechanism of Carbon Enhanced Metal-Poor Stars

    Science.gov (United States)

    Suda, Takuma; Yamada, Shimako; Fujimoto, Masayuki Y.

    The origin of carbon-enhanced metal-poor (CEMP) stars plays a key role in characterising the formation and evolution of the first stars and the Galaxy since the extremely-metal-poor (EMP) stars with [Fe/H] ≤ -2.5 share the common features of carbon enhancement in their surface chemical compositions. The origin of these stars is not yet established due to the controversy of the origin of CEMP stars without the enhancement of s-process element abundances, i.e., so called CEMP-no stars. In this paper, we elaborate the s-process nucleosynthesis in the EMP AGB stars and explore the origin of CEMP stars. We find that the efficiency of the s-process is controlled by O rather than Fe at [Fe/H] ≲ -2. We demonstrate that the relative abundances of Sr, Ba, Pb to C are explained in terms of the wind accretion from AGB stars in binary systems.

  13. Rotational mixing in carbon-enhanced metal-poor stars with s-process enrichment

    Science.gov (United States)

    Matrozis, E.; Stancliffe, R. J.

    2017-10-01

    Carbon-enhanced metal-poor (CEMP) stars with s-process enrichment (CEMP-s) are believed to be the products of mass transfer from an asymptotic giant branch (AGB) companion, which has long since become a white dwarf. The surface abundances of CEMP-s stars are thus commonly assumed to reflect the nucleosynthesis output of the first AGB stars. We have previously shown that, for this to be the case, some physical mechanism must counter atomic diffusion (gravitational settling and radiative levitation) in these nearly fully radiative stars, which otherwise leads to surface abundance anomalies clearly inconsistent with observations. Here we take into account angular momentum accretion by these stars. We compute in detail the evolution of typical CEMP-s stars from the zero-age main sequence, through the mass accretion, and up the red giant branch for a wide range of specific angular momentum ja of the accreted material, corresponding to surface rotation velocities, vrot, between about 0.3 and 300 kms-1. We find that only for ja ≳ 1017 cm2s-1 (vrot > 20 kms-1, depending on mass accreted) angular momentum accretion directly causes chemical dilution of the accreted material. This could nevertheless be relevant to CEMP-s stars, which are observed to rotate more slowly, if they undergo continuous angular momentum loss akin to solar-like stars. In models with rotation velocities characteristic of CEMP-s stars, rotational mixing primarily serves to inhibit atomic diffusion, such that the maximal surface abundance variations (with respect to the composition of the accreted material) prior to first dredge-up remain within about 0.4 dex without thermohaline mixing or about 0.5-1.5 dex with thermohaline mixing. Even in models with the lowest rotation velocities (vrot ≲ 1 kms-1), rotational mixing is able to severely inhibit atomic diffusion, compared to non-rotating models. We thus conclude that it offers a natural solution to the problem posed by atomic diffusion and cannot be

  14. White Dwarf Stars

    OpenAIRE

    Kepler, S. O.; Romero, Alejandra Daniela; Pelisoli, Ingrid; Ourique, Gustavo

    2017-01-01

    White dwarf stars are the final stage of most stars, born single or in multiple systems. We discuss the identification, magnetic fields, and mass distribution for white dwarfs detected from spectra obtained by the Sloan Digital Sky Survey up to Data Release 13 in 2016, which lead to the increase in the number of spectroscopically identified white dwarf stars from 5000 to 39000. This number includes only white dwarf stars with log g >= 6.5 stars, i.e., excluding the Extremely Low Mass white dw...

  15. Metal-Poor Stars and the Chemical Enrichment of the Universe

    OpenAIRE

    Frebel, Anna; Norris, John E.

    2011-01-01

    Metal-poor stars hold the key to our understanding of the origin of the elements and the chemical evolution of the Universe. This chapter describes the process of discovery of these rare stars, the manner in which their surface abundances (produced in supernovae and other evolved stars) are determined from the analysis of their spectra, and the interpretation of their abundance patterns to elucidate questions of origin and evolution. More generally, studies of these stars contribute to other ...

  16. Formation and Evolution of Carbon-Enhanced Metal-Poor Stars

    NARCIS (Netherlands)

    Abate, C.; Pols, O.R.; Izzard, R.G.

    2010-01-01

    Very metal-poor stars observed in the Galactic halo constitute a window on the primordial conditions under which the Milky Way was formed. A large fraction of these stars show a great enhancement in the abundance of carbon and other heavy elements. One explanation of this observation is that these

  17. Metal-poor star formation triggered by the feedback effects from Pop III stars

    Science.gov (United States)

    Chiaki, Gen; Susa, Hajime; Hirano, Shingo

    2018-04-01

    Metal enrichment by first-generation (Pop III) stars is the very first step of the matter cycle in structure formation and it is followed by the formation of extremely metal-poor (EMP) stars. To investigate the enrichment process by Pop III stars, we carry out a series of numerical simulations including the feedback effects of photoionization and supernovae (SNe) of Pop III stars with a range of masses of minihaloes (MHs), Mhalo, and Pop III stars, MPopIII. We find that the metal-rich ejecta reach neighbouring haloes and external enrichment (EE) occurs when the H II region expands before the SN explosion. The neighbouring haloes are only superficially enriched, and the metallicity of the clouds is [Fe/H] < -5. Otherwise, the SN ejecta fall back and recollapse to form an enriched cloud, i.e. an internal-enrichment (IE) process takes place. In the case where a Pop III star explodes as a core-collapse SN (CCSN), the MH undergoes IE, and the metallicity in the recollapsing region is -5 ≲ [Fe/H] ≲ -3 in most cases. We conclude that IE from a single CCSN can explain the formation of EMP stars. For pair-instability SNe (PISNe), EE takes place for all relevant mass ranges of MHs, consistent with the lack of observational signs of PISNe among EMP stars.

  18. Lithium-rich very metal-poor stars discovered with LAMOST and Subaru

    Science.gov (United States)

    Aoki, Wako; Li, Haining; Matsuno, Tadafumi; Kumar, Yerra Bharat; Shi, Jianrong; Suda, Takuma; Zhao, Gang

    2018-04-01

    Lithium is a unique element that is produced in the Big Bang nucleosynthesis but is destroyed by nuclear reactions inside stars. As a result, almost constant lithium abundance is found in unevolved main-sequence metal-poor stars, although the value is systematically lower than that expected from the standard Big Bang nucleosynthesis models, whereas lithium abundances of red giants are more than one order of magnitudes lower than those of unevolved stars. There are, however, a small fraction of metal-poor stars that show extremely high lithium abundances, which is not explained by standard stellar evolution models. We have discovered 12 new very metal-poor stars that have enhancement of lithium by more than 10 times compared with typical metal-poor stars at similar evolutionary stages by the large-scale spectroscopic survey with LAMOST and the follow-up high-resolution spectroscopy with the Subaru Telescope. The sample shows a wide distribution of evolutionary stages from subgiants to red giants with the metallicity of -3.3 <[Fe/H]< -1.6. The chemical abundance ratios of other elements have been obtained by our spectroscopic study, and an estimate of the binary frequency by radial velocity monitoring is ongoing. The observational results provide new constraints on the scenarios to explain lithium-rich metal-poor stars, such as extra mixing during the evolution along the red giant branch, mass-transfer from a companion AGB star, and engulfment of planet-like objects. These explanations are very unlikely for at least some of lithium-rich objects in our sample, suggesting a new mechanism that enhances lithium during the low-mass star evolution.

  19. SPECTROSCOPIC ANALYSIS OF METAL-POOR STARS FROM LAMOST: EARLY RESULTS

    International Nuclear Information System (INIS)

    Li, Hai-Ning; Zhao, Gang; Wang, Liang; Wang, Wei; Yuan, Hailong; Christlieb, Norbert; Zhang, Yong; Hou, Yonghui

    2015-01-01

    We report on early results from a pilot program searching for metal-poor stars with LAMOST and follow-up high-resolution observation acquired with the MIKE spectrograph attached to the Magellan II telescope. We performed detailed abundance analysis for eight objects with iron abundances [Fe/H] < -2.0, including five extremely metal-poor (EMP; [Fe/H] < -3.0) stars with two having [Fe/H] < -3.5. Among these objects, three are newly discovered EMP stars, one of which is confirmed for the first time with high-resolution spectral observations. Three program stars are regarded as carbon-enhanced metal-poor (CEMP) stars, including two stars with no enhancement in their neutron-capture elements, which thus possibly belong to the class of CEMP-no stars; one of these objects also exhibits significant enhancement in nitrogen, and is thus a potential carbon and nitrogen-enhanced metal-poor star. The [X/Fe] ratios of the sample stars generally agree with those reported in the literature for other metal-poor stars in the same [Fe/H] range. We also compared the abundance patterns of individual program stars with the average abundance pattern of metal-poor stars and find only one chemically peculiar object with abundances of at least two elements (other than C and N) showing deviations larger than 0.5 dex. The distribution of [Sr/Ba] versus [Ba/H] agrees that an additional nucleosynthesis mechanism is needed aside from a single r-process. Two program stars with extremely low abundances of Sr and Ba support the prospect that both main and weak r-processes may have operated during the early phase of Galactic chemical evolution. The distribution of [C/N] shows that there are two groups of carbon-normal giants with different degrees of mixing. However, it is difficult to explain the observed behavior of the [C/N] of the nitrogen-enhanced unevolved stars based on current data

  20. Investigation of a sample of carbon-enhanced metal-poor stars observed with FORS and GMOS

    Science.gov (United States)

    Caffau, E.; Gallagher, A. J.; Bonifacio, P.; Spite, M.; Duffau, S.; Spite, F.; Monaco, L.; Sbordone, L.

    2018-06-01

    Aims: Carbon-enhanced metal-poor (CEMP) stars represent a sizeable fraction of all known metal-poor stars in the Galaxy. Their formation and composition remains a significant topic of investigation within the stellar astrophysics community. Methods: We analysed a sample of low-resolution spectra of 30 dwarf stars, obtained using the visual and near UV FOcal Reducer and low dispersion Spectrograph for the Very Large Telescope (FORS/VLT) of the European Southern Observatory (ESO) and the Gemini Multi-Object Spectrographs (GMOS) at the GEMINI telescope, to derive their metallicity and carbon abundance. Results: We derived C and Ca from all spectra, and Fe and Ba from the majority of the stars. Conclusions: We have extended the population statistics of CEMP stars and have confirmed that in general, stars with a high C abundance belonging to the high C band show a high Ba-content (CEMP-s or -r/s), while stars with a normal C abundance or that are C-rich, but belong to the low C band, are normal in Ba (CEMP-no). Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 099.D-0791.Based on observations obtained at the Gemini Observatory (processed using the Gemini IRAF package), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil).Tables 1 and 2 are also available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/614/A68

  1. HIERARCHICAL FORMATION OF THE GALACTIC HALO AND THE ORIGIN OF HYPER METAL-POOR STARS

    International Nuclear Information System (INIS)

    Komiya, Yutaka; Habe, Asao; Suda, Takuma; Fujimoto, Masayuki Y.

    2009-01-01

    Extremely metal-poor (EMP) stars in the Galactic halo are unique probes into the early universe and the first stars. We construct a new program to calculate the formation history of EMP stars in the early universe with the chemical evolution, based on the merging history of the Galaxy. We show that the hierarchical structure formation model reproduces the observed metallicity distribution function and also the total number of observed EMP stars, when we take into account the high-mass initial mass function and the contribution of binaries, as proposed by Komiya et al. The low-mass survivors divide into two groups of those born before and after the mini-halos are polluted by their own first supernovae. The former has observational counterparts in the hyper metal-poor (HMP) stars below [Fe/H] - 4. In this Letter, we focus on the origin of the extremely small iron abundances of HMP stars. We compute the change in the surface abundances of individual stars through the accretion of the metal-enriched interstellar gas along with the dynamical and chemical evolution of the Galaxy, to demonstrate that after-birth pollution of Population III stars is sufficiently effective to explain the observed abundances of HMP stars. Metal pre-enrichment by possible pair instability supernovae is also discussed, to derive constraints on their roles and on the formation of the first low-mass stars.

  2. VERY METAL-POOR STARS IN THE OUTER GALACTIC BULGE FOUND BY THE APOGEE SURVEY

    International Nuclear Information System (INIS)

    García Pérez, Ana E.; Majewski, Steven R.; Hearty, Fred R.; Cunha, Katia; Shetrone, Matthew; Johnson, Jennifer A.; Zasowski, Gail; Smith, Verne V.; Beers, Timothy C.; Schiavon, Ricardo P.; Holtzman, Jon; Nidever, David; Allende Prieto, Carlos; Bizyaev, Dmitry; Ebelke, Garrett; Malanushenko, Elena; Malanushenko, Viktor; Eisenstein, Daniel J.; Frinchaboy, Peter M.; Girardi, Léo

    2013-01-01

    Despite its importance for understanding the nature of early stellar generations and for constraining Galactic bulge formation models, at present little is known about the metal-poor stellar content of the central Milky Way. This is a consequence of the great distances involved and intervening dust obscuration, which challenge optical studies. However, the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber, high-resolution spectroscopic survey within Sloan Digital Sky Survey III, is exploring the chemistry of all Galactic stellar populations at infrared wavelengths, with particular emphasis on the disk and the bulge. An automated spectral analysis of data on 2403 giant stars in 12 fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity ([Fe/H] ≤ –1.7), including two that are very metal-poor [Fe/H] ∼ –2.1 by bulge standards. Luminosity-based distance estimates place the 5 stars within the outer bulge, where 1246 of the other analyzed stars may reside. A manual reanalysis of the spectra verifies the low metallicities, and finds these stars to be enhanced in the α-elements O, Mg, and Si without significant α-pattern differences with other local halo or metal-weak thick-disk stars of similar metallicity, or even with other more metal-rich bulge stars. While neither the kinematics nor chemistry of these stars can yet definitively determine which, if any, are truly bulge members, rather than denizens of other populations co-located with the bulge, the newly identified stars reveal that the chemistry of metal-poor stars in the central Galaxy resembles that of metal-weak thick-disk stars at similar metallicity.

  3. VERY METAL-POOR STARS IN THE OUTER GALACTIC BULGE FOUND BY THE APOGEE SURVEY

    Energy Technology Data Exchange (ETDEWEB)

    Garcia Perez, Ana E.; Majewski, Steven R.; Hearty, Fred R. [Department of Astronomy, University of Virginia, Charlottesville, VA 22904-4325 (United States); Cunha, Katia [Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States); Shetrone, Matthew [McDonald Observatory, University of Texas at Austin, Fort Davis, TX 79734 (United States); Johnson, Jennifer A.; Zasowski, Gail [Department of Astronomy, The Ohio State University, Columbus, OH 43210 (United States); Smith, Verne V.; Beers, Timothy C. [National Optical Astronomy Observatories, Tucson, AZ 85719 (United States); Schiavon, Ricardo P. [Gemini Observatory, 670 N. A' Ohoku Place, Hilo, HI 96720 (United States); Holtzman, Jon [Department of Astronomy, MSC 4500, New Mexico State University, P.O. Box 30001, Las Cruces, NM 88003 (United States); Nidever, David [Department of Astronomy, University of Michigan, Ann Arbor, MI 48109 (United States); Allende Prieto, Carlos [Departamento de Astrofisica, Universidad de La Laguna, E-38206 La Laguna, Tenerife (Spain); Bizyaev, Dmitry; Ebelke, Garrett; Malanushenko, Elena; Malanushenko, Viktor [Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349-0059 (United States); Eisenstein, Daniel J. [Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Frinchaboy, Peter M. [Department of Physics and Astronomy, Texas Christian University, 2800 South University Drive, Fort Worth, TX 76129 (United States); Girardi, Leo [Laboratorio Interinstitucional de e-Astronomia - LIneA, Rua Gal. Jose Cristino 77, Rio de Janeiro, RJ - 20921-400 (Brazil); and others

    2013-04-10

    Despite its importance for understanding the nature of early stellar generations and for constraining Galactic bulge formation models, at present little is known about the metal-poor stellar content of the central Milky Way. This is a consequence of the great distances involved and intervening dust obscuration, which challenge optical studies. However, the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber, high-resolution spectroscopic survey within Sloan Digital Sky Survey III, is exploring the chemistry of all Galactic stellar populations at infrared wavelengths, with particular emphasis on the disk and the bulge. An automated spectral analysis of data on 2403 giant stars in 12 fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity ([Fe/H] {<=} -1.7), including two that are very metal-poor [Fe/H] {approx} -2.1 by bulge standards. Luminosity-based distance estimates place the 5 stars within the outer bulge, where 1246 of the other analyzed stars may reside. A manual reanalysis of the spectra verifies the low metallicities, and finds these stars to be enhanced in the {alpha}-elements O, Mg, and Si without significant {alpha}-pattern differences with other local halo or metal-weak thick-disk stars of similar metallicity, or even with other more metal-rich bulge stars. While neither the kinematics nor chemistry of these stars can yet definitively determine which, if any, are truly bulge members, rather than denizens of other populations co-located with the bulge, the newly identified stars reveal that the chemistry of metal-poor stars in the central Galaxy resembles that of metal-weak thick-disk stars at similar metallicity.

  4. A SEARCH FOR UNRECOGNIZED CARBON-ENHANCED METAL-POOR STARS IN THE GALAXY

    International Nuclear Information System (INIS)

    Placco, Vinicius M.; Rossi, Silvia; Kennedy, Catherine R.; Beers, Timothy C.; Lee, Young Sun; Christlieb, Norbert; Sivarani, Thirupathi; Reimers, Dieter; Wisotzki, Lutz

    2010-01-01

    We have developed a new procedure to search for carbon-enhanced metal-poor (CEMP) stars from the Hamburg/ESO (HES) prism-survey plates. This method employs an extended line index for the CH G band, which we demonstrate to have superior performance when compared to the narrower G-band index formerly employed to estimate G-band strengths for these spectra. Although CEMP stars have been found previously among candidate metal-poor stars selected from the HES, the selection on metallicity undersamples the population of intermediate-metallicity CEMP stars (-2.5 ≤ [Fe/H] ≤ -1.0); such stars are of importance for constraining the onset of the s-process in metal-deficient asymptotic giant branch stars (thought to be associated with the origin of carbon for roughly 80% of CEMP stars). The new candidates also include substantial numbers of warmer carbon-enhanced stars, which were missed in previous HES searches for carbon stars due to selection criteria that emphasized cooler stars. A first subsample, biased toward brighter stars (B< 15.5), has been extracted from the scanned HES plates. After visual inspection (to eliminate spectra compromised by plate defects, overlapping spectra, etc., and to carry out rough spectral classifications), a list of 669 previously unidentified candidate CEMP stars was compiled. Follow-up spectroscopy for a pilot sample of 132 candidates was obtained with the Goodman spectrograph on the SOAR 4.1 m telescope. Our results show that most of the observed stars lie in the targeted metallicity range, and possess prominent carbon absorption features at 4300 A. The success rate for the identification of new CEMP stars is 43% (13 out of 30) for [Fe/H] < -2.0. For stars with [Fe/H] < -2.5, the ratio increases to 80% (four out of five objects), including one star with [Fe/H] < -3.0.

  5. A KECK HIRES DOPPLER SEARCH FOR PLANETS ORBITING METAL-POOR DWARFS. II. ON THE FREQUENCY OF GIANT PLANETS IN THE METAL-POOR REGIME

    International Nuclear Information System (INIS)

    Sozzetti, Alessandro; Torres, Guillermo; Latham, David W.; Stefanik, Robert P.; Korzennik, Sylvain G.; Boss, Alan P.; Carney, Bruce W.; Laird, John B.

    2009-01-01

    We present an analysis of three years of precision radial velocity (RV) measurements of 160 metal-poor stars observed with HIRES on the Keck 1 telescope. We report on variability and long-term velocity trends for each star in our sample. We identify several long-term, low-amplitude RV variables worthy of followup with direct imaging techniques. We place lower limits on the detectable companion mass as a function of orbital period. Our survey would have detected, with a 99.5% confidence level, over 95% of all companions on low-eccentricity orbits with velocity semiamplitude K ∼> 100 m s -1 , or M p sin i ∼> 3.0 M J (P/yr) (1/3) , for orbital periods P ∼ p p ≅ 1%. Our results can usefully inform theoretical studies of the process of giant-planet formation across two orders of magnitude in metallicity.

  6. Are the Formation and Abundances of Metal-poor Stars the Result of Dust Dynamics?

    Energy Technology Data Exchange (ETDEWEB)

    Hopkins, Philip F. [TAPIR, Mailcode 350-17, California Institute of Technology, Pasadena, CA 91125 (United States); Conroy, Charlie, E-mail: phopkins@caltech.edu [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

    2017-02-01

    Large dust grains can fluctuate dramatically in their local density, relative to the gas, in neutral turbulent disks. Small, high-redshift galaxies (before reionization) represent ideal environments for this process. We show via simple arguments and simulations that order-of-magnitude fluctuations are expected in local abundances of large grains (>100 Å) under these conditions. This can have important consequences for star formation and stellar metal abundances in extremely metal-poor stars. Low-mass stars can form in dust-enhanced regions almost immediately after some dust forms even if the galaxy-average metallicity is too low for fragmentation to occur. We argue that the metal abundances of these “promoted” stars may contain interesting signatures as the CNO abundances (concentrated in large carbonaceous grains and ices) and Mg and Si (in large silicate grains) can be enhanced and/or fluctuate almost independently. Remarkably, the otherwise puzzling abundance patterns of some metal-poor stars can be well fit by standard IMF-averaged core-collapse SNe yields if we allow for fluctuating local dust-to-gas ratios. We also show that the observed log-normal distribution of enhancements in pure SNe yields, shows very large enhancements and variations up to factors of ≳100 as expected in the dust-promoted model, preferentially in the [C/Fe]-enhanced metal-poor stars. Together, this suggests that (1) dust exists in second-generation star formation, (2) local dust-to-gas ratio fluctuations occur in protogalaxies and can be important for star formation, and (3) the light element abundances of these stars may be affected by the local chemistry of dust where they formed, rather than directly tracing nucleosynthesis from earlier populations.

  7. Are the Formation and Abundances of Metal-poor Stars the Result of Dust Dynamics?

    International Nuclear Information System (INIS)

    Hopkins, Philip F.; Conroy, Charlie

    2017-01-01

    Large dust grains can fluctuate dramatically in their local density, relative to the gas, in neutral turbulent disks. Small, high-redshift galaxies (before reionization) represent ideal environments for this process. We show via simple arguments and simulations that order-of-magnitude fluctuations are expected in local abundances of large grains (>100 Å) under these conditions. This can have important consequences for star formation and stellar metal abundances in extremely metal-poor stars. Low-mass stars can form in dust-enhanced regions almost immediately after some dust forms even if the galaxy-average metallicity is too low for fragmentation to occur. We argue that the metal abundances of these “promoted” stars may contain interesting signatures as the CNO abundances (concentrated in large carbonaceous grains and ices) and Mg and Si (in large silicate grains) can be enhanced and/or fluctuate almost independently. Remarkably, the otherwise puzzling abundance patterns of some metal-poor stars can be well fit by standard IMF-averaged core-collapse SNe yields if we allow for fluctuating local dust-to-gas ratios. We also show that the observed log-normal distribution of enhancements in pure SNe yields, shows very large enhancements and variations up to factors of ≳100 as expected in the dust-promoted model, preferentially in the [C/Fe]-enhanced metal-poor stars. Together, this suggests that (1) dust exists in second-generation star formation, (2) local dust-to-gas ratio fluctuations occur in protogalaxies and can be important for star formation, and (3) the light element abundances of these stars may be affected by the local chemistry of dust where they formed, rather than directly tracing nucleosynthesis from earlier populations.

  8. Lithium isotopic abundances in metal-poor stars: a problem for standard big bang nucleosynthesis?

    International Nuclear Information System (INIS)

    Nissen, P.E.; Asplund, M.; Lambert, D.L.; Primas, F.; Smith, V.V.

    2005-01-01

    Spectral obtained with VLT/UVES suggest the existence of the 6 Li isotope in several metal-poor stars at a level that challenges ideas about its synthesis. The 7 Li abundance is, on the other hand, a factor of three lower than predicted by standard Big Bang nucleosynthesis theory. Both problems may be explained if decaying suppersymmetric particles affect the synthesis of light elements in the Big Bang. (orig.)

  9. Lithium evolution in metal-poor stars: from Pre-Main Sequence to the Spite plateau

    OpenAIRE

    Fu, Xiaoting; Bressan, Alessandro; Molaro, Paolo; Marigo, Paola

    2015-01-01

    Lithium abundance derived in metal-poor main sequence stars is about three times lower than the value of primordial Li predicted by the standard Big Bang nucleosynthesis when the baryon density is taken from the CMB or the deuterium measurements. This disagreement is generally referred as the lithium problem. We here reconsider the stellar Li evolution from the pre-main sequence to the end of the main sequence phase by introducing the effects of convective overshooting and residual mass accre...

  10. Empirical Determination of Dark Matter Velocities Using Metal-Poor Stars.

    Science.gov (United States)

    Herzog-Arbeitman, Jonah; Lisanti, Mariangela; Madau, Piero; Necib, Lina

    2018-01-26

    The Milky Way dark matter halo is formed from the accretion of smaller subhalos. These sub-units also harbor stars-typically old and metal-poor-that are deposited in the Galactic inner regions by disruption events. In this Letter, we show that the dark matter and metal-poor stars in the Solar neighborhood share similar kinematics due to their common origin. Using the high-resolution eris simulation, which traces the evolution of both the dark matter and baryons in a realistic Milky Way analog galaxy, we demonstrate that metal-poor stars are indeed effective tracers for the local, virialized dark matter velocity distribution. The local dark matter velocities can therefore be inferred from observations of the stellar halo made by the Sloan Digital Sky Survey within 4 kpc of the Sun. This empirical distribution differs from the standard halo model in important ways and suggests that the bounds on the spin-independent scattering cross section may be weakened for dark matter masses below ∼10  GeV. Data from Gaia will allow us to further refine the expected distribution for the smooth dark matter component, and to test for the presence of local substructure.

  11. The Pristine survey - I. Mining the Galaxy for the most metal-poor stars

    Science.gov (United States)

    Starkenburg, Else; Martin, Nicolas; Youakim, Kris; Aguado, David S.; Allende Prieto, Carlos; Arentsen, Anke; Bernard, Edouard J.; Bonifacio, Piercarlo; Caffau, Elisabetta; Carlberg, Raymond G.; Côté, Patrick; Fouesneau, Morgan; François, Patrick; Franke, Oliver; González Hernández, Jonay I.; Gwyn, Stephen D. J.; Hill, Vanessa; Ibata, Rodrigo A.; Jablonka, Pascale; Longeard, Nicolas; McConnachie, Alan W.; Navarro, Julio F.; Sánchez-Janssen, Rubén; Tolstoy, Eline; Venn, Kim A.

    2017-11-01

    We present the Pristine survey, a new narrow-band photometric survey focused on the metallicity-sensitive Ca H&K lines and conducted in the Northern hemisphere with the wide-field imager MegaCam on the Canada-France-Hawaii Telescope. This paper reviews our overall survey strategy and discusses the data processing and metallicity calibration. Additionally we review the application of these data to the main aims of the survey, which are to gather a large sample of the most metal-poor stars in the Galaxy, to further characterize the faintest Milky Way satellites, and to map the (metal-poor) substructure in the Galactic halo. The current Pristine footprint comprises over 1000 deg2 in the Galactic halo ranging from b ˜ 30° to ˜78° and covers many known stellar substructures. We demonstrate that, for Sloan Digital Sky Survey (SDSS) stellar objects, we can calibrate the photometry at the 0.02-mag level. The comparison with existing spectroscopic metallicities from SDSS/Sloan Extension for Galactic Understanding and Exploration (SEGUE) and Large Sky Area Multi-Object Fiber Spectroscopic Telescope shows that, when combined with SDSS broad-band g and I photometry, we can use the CaHK photometry to infer photometric metallicities with an accuracy of ˜0.2 dex from [Fe/H] = -0.5 down to the extremely metal-poor regime ([Fe/H] < -3.0). After the removal of various contaminants, we can efficiently select metal-poor stars and build a very complete sample with high purity. The success rate of uncovering [Fe/H]SEGUE < -3.0 stars among [Fe/H]Pristine < -3.0 selected stars is 24 per cent, and 85 per cent of the remaining candidates are still very metal poor ([Fe/H]<-2.0). We further demonstrate that Pristine is well suited to identify the very rare and pristine Galactic stars with [Fe/H] < -4.0, which can teach us valuable lessons about the early Universe.

  12. THE SYNTHETIC-OVERSAMPLING METHOD: USING PHOTOMETRIC COLORS TO DISCOVER EXTREMELY METAL-POOR STARS

    Energy Technology Data Exchange (ETDEWEB)

    Miller, A. A., E-mail: amiller@astro.caltech.edu [Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 169-506, Pasadena, CA 91109 (United States)

    2015-09-20

    Extremely metal-poor (EMP) stars ([Fe/H] ≤ −3.0 dex) provide a unique window into understanding the first generation of stars and early chemical enrichment of the universe. EMP stars are exceptionally rare, however, and the relatively small number of confirmed discoveries limits our ability to exploit these near-field probes of the first ∼500 Myr after the Big Bang. Here, a new method to photometrically estimate [Fe/H] from only broadband photometric colors is presented. I show that the method, which utilizes machine-learning algorithms and a training set of ∼170,000 stars with spectroscopically measured [Fe/H], produces a typical scatter of ∼0.29 dex. This performance is similar to what is achievable via low-resolution spectroscopy, and outperforms other photometric techniques, while also being more general. I further show that a slight alteration to the model, wherein synthetic EMP stars are added to the training set, yields the robust identification of EMP candidates. In particular, this synthetic-oversampling method recovers ∼20% of the EMP stars in the training set, at a precision of ∼0.05. Furthermore, ∼65% of the false positives from the model are very metal-poor stars ([Fe/H] ≤ −2.0 dex). The synthetic-oversampling method is biased toward the discovery of warm (∼F-type) stars, a consequence of the targeting bias from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding survey. This EMP selection method represents a significant improvement over alternative broadband optical selection techniques. The models are applied to >12 million stars, with an expected yield of ∼600 new EMP stars, which promises to open new avenues for exploring the early universe.

  13. Searching for chemical classes among metal-poor stars using medium-resolution spectroscopy

    Science.gov (United States)

    Cruz, Monique A.; Cogo-Moreira, Hugo; Rossi, Silvia

    2018-04-01

    Astronomy is in the era of large spectroscopy surveys, with the spectra of hundreds of thousands of stars in the Galaxy being collected. Although most of these surveys have low or medium resolution, which makes precise abundance measurements not possible, there is still important information to be extracted from the available data. Our aim is to identify chemically distinct classes among metal-poor stars, observed by the Sloan Digital Sky Survey, using line indices. The present work focused on carbon-enhanced metal-poor (CEMP) stars and their subclasses. We applied the latent profile analysis technique to line indices for carbon, barium, iron and europium, in order to separate the sample into classes with similar chemical signatures. This technique provides not only the number of possible groups but also the probability of each object to belong to each class. The method was able to distinguish at least two classes among the observed sample, with one of them being probable CEMP stars enriched in s-process elements. However, it was not able to separate CEMP-no stars from the rest of the sample. Latent profile analysis is a powerful model-based tool to be used in the identification of patterns in astrophysics. Our tests show the potential of the technique for the attainment of additional chemical information from `poor' data.

  14. A NON-LOCAL THERMODYNAMIC EQUILIBRIUM ANALYSIS OF BORON ABUNDANCES IN METAL-POOR STARS

    International Nuclear Information System (INIS)

    Tan Kefeng; Shi Jianrong; Zhao Gang

    2010-01-01

    The non-local thermodynamic equilibrium (NLTE) line formation of neutral boron in the atmospheres of cool stars are investigated. Our results confirm that NLTE effects for the B I resonance lines, which are due to a combination of overionization and optical pumping effects, are most important for hot, metal-poor, and low-gravity stars; however, the amplitude of departures from local thermodynamic equilibrium (LTE) found by this work is smaller than that of previous studies. In addition, our calculation shows that the line formation of B I will get closer to LTE if the strength of collisions with neutral hydrogen increases, which is contrary to the result of previous studies. The NLTE line formation results are applied to the determination of boron abundances for a sample of 16 metal-poor stars with the method of spectrum synthesis of the B I 2497 A resonance lines using the archived HST/GHRS spectra. Beryllium and oxygen abundances are also determined for these stars with the published equivalent widths of the Be II 3131 A resonance and O I 7774 A triplet lines, respectively. The abundances of the nine stars which are not depleted in Be or B show that, no matter what the strength of collisions with neutral hydrogen may be, both Be and B increase with O quasilinearly in the logarithmic plane, which confirms the conclusions that Be and B are mainly produced by the primary process in the early Galaxy. The most noteworthy result of this work is that B increases with Fe or O at a very similar speed as, or a bit faster than, Be does, which is in accord with the theoretical models. The B/Be ratios remain almost constant over the metallicity range investigated here. Our average B/Be ratio falls in the interval [13 ± 4, 17 ± 4], which is consistent with the predictions of the spallation process. The contribution of B from the ν-process may be required if the 11 B/ 10 B isotopic ratios in metal-poor stars are the same as the meteoric value. An accurate measurement of the

  15. Looking for imprints of the first stellar generations in metal-poor bulge field stars

    Science.gov (United States)

    Siqueira-Mello, C.; Chiappini, C.; Barbuy, B.; Freeman, K.; Ness, M.; Depagne, E.; Cantelli, E.; Pignatari, M.; Hirschi, R.; Frischknecht, U.; Meynet, G.; Maeder, A.

    2016-09-01

    Context. Efforts to look for signatures of the first stars have concentrated on metal-poor halo objects. However, the low end of the bulge metallicity distribution has been shown to host some of the oldest objects in the Milky Way and hence this Galactic component potentially offers interesting targets to look at imprints of the first stellar generations. As a pilot project, we selected bulge field stars already identified in the ARGOS survey as having [Fe/H] ≈-1 and oversolar [α/Fe] ratios, and we used FLAMES-UVES to obtain detailed abundances of key elements that are believed to reveal imprints of the first stellar generations. Aims: The main purpose of this study is to analyse selected ARGOS stars using new high-resolution (R ~ 45 000) and high-signal-to-noise (S/N> 100) spectra. We aim to derive their stellar parameters and elemental ratios, in particular the abundances of C, N, the α-elements O, Mg, Si, Ca, and Ti, the odd-Z elements Na and Al, the neutron-capture s-process dominated elements Y, Zr, La, and Ba, and the r-element Eu. Methods: High-resolution spectra of five field giant stars were obtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. Spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe I and Fe II. The abundance analysis was performed with a MARCS LTE spherical model atmosphere grid and the Turbospectrum spectrum synthesis code. Results: We confirm that the analysed stars are moderately metal-poor (-1.04 ≤ [Fe/H] ≤-0.43), non-carbon-enhanced (non-CEMP) with [C/Fe] ≤ + 0.2, and α-enhanced. We find that our three most metal-poor stars are nitrogen enhanced. The α-enhancement suggests that these stars were formed from a gas enriched by core-collapse supernovae, and that the values are in agreement with results in the literature for bulge stars in the same metallicity range. No abundance anomalies (Na - O, Al - O, Al - Mg anti-correlations) were

  16. Pristine Survey : High-Resolution Spectral Analyses of New Metal-poor Stars

    Science.gov (United States)

    Venn, Kim; Starkenburg, Else; Martin, Nicolas; Kielty, Collin; Youakim, Kris; Arnetsen, Anke

    2018-06-01

    The Pristine survey (Starkenburg et al. 2017) is a new and very successful metal-poor star survey. Combining high-quality narrow-band CaHK CFHT/MegaCam photometry with existing broadband photometry from SDSS, then very metal-poor stars have been found as confirmed from low-resolution spectroscopy (Youakim et al. 2017). Furthermore, we have extended this survey towards the Galactic bulge in a pilot program to test the capabilities in the highly crowded and (inhomogeneously) extincted bulge (Arentsen et al. 2018). High resolution spectral follow-up analyses have been initiated at the CFHT with Espadons (Vevolution or changes in the IMF, e.g., carbon enrichment, high [alpha/Fe] ratios vs alpha-challenged stars, and details in the neutron capture element ratios. While these early studies are being carried out using classical model atmospheres and synthetic spectral fitting (Venn et al. 2017, 2018), we are also exploring the use of a neural network for the fast, efficient, and precise determination of these stellar parameters and chemical abundances (e.g., StarNet, Fabbro et al. 2018).

  17. The origin of light neutron-capture elements in very metal-poor stars

    International Nuclear Information System (INIS)

    Honda, S.; Aoki, W.; Kajino, T.; Ando, H.; Beers, T.C.

    2005-01-01

    We obtained high resolution spectra of 40 very metal-poor stars, and measured the abundances of heavy elements. The abundance pattern of the heavy neutron-capture elements (56=< Z=<70) in r-process-enhanced, metal-poor stars are quite similar to that of the r-process component in solar-system material. In contrast, the abundance ratios of the light neutron-capture elements (38=< Z=<40) to heavier ones show a large dispersion. We investigated the correlation between Sr(Z=38) and Ba(Z=56) abundances, and obtained two clear results: (1) Ba-enhanced stars also show large excess of Sr (there is no object which is Ba-rich and Sr-poor); (2) stars with low Ba abundance show large scatter in Sr abundance. This trend is naturally explained by hypothesizing the existence of two processes, one that produces Sr without Ba and the other that produces Sr and Ba in similar proportions

  18. On the temperatures, colours, and ages of metal-poor stars predicted by stellar models

    International Nuclear Information System (INIS)

    Van den Berg, D A

    2008-01-01

    Most (but not all) of the investigations that have derived the effective temperatures of metal-poor, solar-neighbourhood field stars, from analyses of their spectra or from the infrared flux method, favour a T eff scale that is ∼100-120 K cooler than that given by stellar evolutionary models. This seems to be at odds with photometric results, given that the application of current colour-T eff relations to the observed subdwarf colours suggests a preference for hotter temperatures. Moreover, the predicted temperatures for main-sequence stars at the lowest metallicities ([Fe/H] eff for them unless some fundamental modification is made to the adopted physics. No such problems are found if the temperatures of metal-poor field stars are ∼100-120 K warmer than most determinations. In this case, stellar models would appear to provide consistent interpretations of both field and globular cluster (GC) stars of low metallicity. However, this would imply, e.g. that M 92 has an [Fe/H] value of approximately - 2.2, which is obtained from analyses of Fe I lines, instead of approximately equal to - 2.4, as derived from Fe II lines (and favoured by studies of three-dimensional model atmospheres). Finally, the age of the local, Population II subgiant HD 140283 (and GCs having similar metal abundances) is estimated to be ∼13 Gyr, if diffusive processes are taken into account.

  19. THE CHEMICAL ABUNDANCES OF STARS IN THE HALO (CASH) PROJECT. II. A SAMPLE OF 14 EXTREMELY METAL-POOR STARS ,

    International Nuclear Information System (INIS)

    Hollek, Julie K.; Sneden, Christopher; Shetrone, Matthew; Frebel, Anna; Roederer, Ian U.; Beers, Timothy C.; Kang, Sung-ju; Thom, Christopher

    2011-01-01

    We present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the Chemical Abundances of Stars in the Halo (CASH) project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R ∼15, 000) and corresponding high-resolution (R ∼35, 000) Magellan Inamori Kyocera Echelle spectra. The stars span a metallicity range from [Fe/H] from –2.9 to –3.9, including four new stars with [Fe/H] < –3.7. We find four stars to be carbon-enhanced metal-poor (CEMP) stars, confirming the trend of increasing [C/Fe] abundance ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]< – 3. We also find four neutron-capture-enhanced stars in the sample, one of which has [Eu/Fe] of 0.8 with clear r-process signatures. These pilot sample stars are the most metal-poor ([Fe/H] ∼< –3.0) of the brightest stars included in CASH and are used to calibrate a newly developed, automated stellar parameter and abundance determination pipeline. This code will be used for the entire ∼500 star CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum.

  20. Synthetic Stromgren photometry for F dwarf stars

    Science.gov (United States)

    Bell, R. A.

    1988-01-01

    Recent synthetic spectrum and color calculations for cool dwarf star models are tested by comparison with observation. The accuracy of the computed dependence of the thermal colors B-V and b-y on effective temperature is examined, and H-beta indices are presented and compared with observed values. The accuracy of the predictions of the Stromgren uvby system metal-abundance indicator m1 and luminosity indicator c1 are tested. A new calibration of the c1, b-y diagram in terms of absolute magnitudes is given, making use of recent calculations of stellar isochrones. Observations of very metal-poor subdwarfs are used to study the accuracy of the isochrones. The c1, b-y diagram of the subdwarfs is compared with that of the turnoff-region stars in the very metal-poor globular cluster NGC 6397.

  1. TESTING THE ASTEROSEISMIC MASS SCALE USING METAL-POOR STARS CHARACTERIZED WITH APOGEE AND KEPLER

    Energy Technology Data Exchange (ETDEWEB)

    Epstein, Courtney R.; Johnson, Jennifer A.; Tayar, Jamie; Pinsonneault, Marc [Department of Astronomy, Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210 (United States); Elsworth, Yvonne P.; Chaplin, William J. [School of Physics and Astronomy, University of Birmingham, Edgbaston Park Road, West Midlands, Birmingham B15 2TT (United Kingdom); Shetrone, Matthew [McDonald Observatory, The University of Texas at Austin, 1 University Station, C1400, Austin, TX 78712-0259 (United States); Mosser, Benoît [LESIA, CNRS, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, F-92195 Meudon Cedex (France); Hekker, Saskia [Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D-37077 Göttingen (Germany); Harding, Paul [Department of Astronomy, Case Western Reserve University, Cleveland, OH 44106-7215 (United States); Silva Aguirre, Víctor [Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C (Denmark); Basu, Sarbani [Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520-8101 (United States); Beers, Timothy C. [National Optical Astronomy Observatory, Tucson, AZ 85719, USA and JINA: Joint Institute for Nuclear Astrophysics (United States); Bizyaev, Dmitry [Apache Point Observatory, Sunspot, NM 88349 (United States); Bedding, Timothy R. [Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006 (Australia); Frinchaboy, Peter M. [Department of Physics and Astronomy, Texas Christian University, TCU Box 298840, Fort Worth, TX 76129 (United States); García, Rafael A. [Laboratoire AIM, CEA/DSM-CNRS, Universit Paris 7 Diderot, IRFU/SAp, Centre de Saclay, F-91191, Gif-sur-Yvette (France); Pérez, Ana E. García; Hearty, Fred R., E-mail: epstein@astronomy.ohio-state.edu [Department of Astronomy, University of Virginia, Charlottesville, VA 22904 (United States); and others

    2014-04-20

    Fundamental stellar properties, such as mass, radius, and age, can be inferred using asteroseismology. Cool stars with convective envelopes have turbulent motions that can stochastically drive and damp pulsations. The properties of the oscillation frequency power spectrum can be tied to mass and radius through solar-scaled asteroseismic relations. Stellar properties derived using these scaling relations need verification over a range of metallicities. Because the age and mass of halo stars are well-constrained by astrophysical priors, they provide an independent, empirical check on asteroseismic mass estimates in the low-metallicity regime. We identify nine metal-poor red giants (including six stars that are kinematically associated with the halo) from a sample observed by both the Kepler space telescope and the Sloan Digital Sky Survey-III APOGEE spectroscopic survey. We compare masses inferred using asteroseismology to those expected for halo and thick-disk stars. Although our sample is small, standard scaling relations, combined with asteroseismic parameters from the APOKASC Catalog, produce masses that are systematically higher (<ΔM > =0.17 ± 0.05 M {sub ☉}) than astrophysical expectations. The magnitude of the mass discrepancy is reduced by known theoretical corrections to the measured large frequency separation scaling relationship. Using alternative methods for measuring asteroseismic parameters induces systematic shifts at the 0.04 M {sub ☉} level. We also compare published asteroseismic analyses with scaling relationship masses to examine the impact of using the frequency of maximum power as a constraint. Upcoming APOKASC observations will provide a larger sample of ∼100 metal-poor stars, important for detailed asteroseismic characterization of Galactic stellar populations.

  2. DETECTION OF THE SECOND r-PROCESS PEAK ELEMENT TELLURIUM IN METAL-POOR STARS ,

    International Nuclear Information System (INIS)

    Roederer, Ian U.; Lawler, James E.; Cowan, John J.; Beers, Timothy C.; Frebel, Anna; Ivans, Inese I.; Schatz, Hendrik; Sobeck, Jennifer S.; Sneden, Christopher

    2012-01-01

    Using near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, we detect neutral tellurium in three metal-poor stars enriched by products of r-process nucleosynthesis, BD +17 3248, HD 108317, and HD 128279. Tellurium (Te, Z = 52) is found at the second r-process peak (A ≈ 130) associated with the N = 82 neutron shell closure, and it has not been detected previously in Galactic halo stars. The derived tellurium abundances match the scaled solar system r-process distribution within the uncertainties, confirming the predicted second peak r-process residuals. These results suggest that tellurium is predominantly produced in the main component of the r-process, along with the rare earth elements.

  3. Constraining cosmic scatter in the Galactic halo through a differential analysis of metal-poor stars

    Science.gov (United States)

    Reggiani, Henrique; Meléndez, Jorge; Kobayashi, Chiaki; Karakas, Amanda; Placco, Vinicius

    2017-12-01

    Context. The chemical abundances of metal-poor halo stars are important to understanding key aspects of Galactic formation and evolution. Aims: We aim to constrain Galactic chemical evolution with precise chemical abundances of metal-poor stars (-2.8 ≤ [Fe/H] ≤ -1.5). Methods: Using high resolution and high S/N UVES spectra of 23 stars and employing the differential analysis technique we estimated stellar parameters and obtained precise LTE chemical abundances. Results: We present the abundances of Li, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Zn, Sr, Y, Zr, and Ba. The differential technique allowed us to obtain an unprecedented low level of scatter in our analysis, with standard deviations as low as 0.05 dex, and mean errors as low as 0.05 dex for [X/Fe]. Conclusions: By expanding our metallicity range with precise abundances from other works, we were able to precisely constrain Galactic chemical evolution models in a wide metallicity range (-3.6 ≤ [Fe/H] ≤ -0.4). The agreements and discrepancies found are key for further improvement of both models and observations. We also show that the LTE analysis of Cr II is a much more reliable source of abundance for chromium, as Cr I has important NLTE effects. These effects can be clearly seen when we compare the observed abundances of Cr I and Cr II with GCE models. While Cr I has a clear disagreement between model and observations, Cr II is very well modeled. We confirm tight increasing trends of Co and Zn toward lower metallicities, and a tight flat evolution of Ni relative to Fe. Our results strongly suggest inhomogeneous enrichment from hypernovae. Our precise stellar parameters results in a low star-to-star scatter (0.04 dex) in the Li abundances of our sample, with a mean value about 0.4 dex lower than the prediction from standard Big Bang nucleosynthesis; we also study the relation between lithium depletion and stellar mass, but it is difficult to assess a correlation due to the limited mass range. We

  4. Observational Constraints on First-Star Nucleosynthesis. II. Spectroscopy of an Ultra metal-poor CEMP-no Star

    Science.gov (United States)

    Placco, Vinicius M.; Frebel, Anna; Beers, Timothy C.; Yoon, Jinmi; Chiti, Anirudh; Heger, Alexander; Chan, Conrad; Casey, Andrew R.; Christlieb, Norbert

    2016-12-01

    We report on the first high-resolution spectroscopic analysis of HE 0020-1741, a bright (V = 12.9), ultra metal-poor ([{Fe}/{{H}}] = -4.1), carbon-enhanced ([{{C}}/{Fe}] = +1.7) star selected from the Hamburg/ESO Survey. This star exhibits low abundances of neutron-capture elements ([{Ba}/{Fe}] = -1.1) and an absolute carbon abundance A(C) = 6.1 based on either criterion, HE 0020-1741 is subclassified as a carbon-enhanced metal-poor star without enhancements in neutron-capture elements (CEMP-no). We show that the light-element abundance pattern of HE 0020-1741 is consistent with predicted yields from a massive (M = 21.5 {M}⊙ ), primordial-composition, supernova (SN) progenitor. We also compare the abundance patterns of other ultra metal-poor stars from the literature with available measures of C, N, Na, Mg, and Fe abundances with an extensive grid of SN models (covering the mass range 10{--}100 {M}⊙ ), in order to probe the nature of their likely stellar progenitors. Our results suggest that at least two classes of progenitors are required at [{Fe}/{{H}}] \\lt -4.0, as the abundance patterns for more than half of the sample studied in this work (7 out of 12 stars) cannot be easily reproduced by the predicted yields. Based on observations gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile, and the New Technology Telescope (NTT) of the European Southern Observatory (088.D-0344A), La Silla, Chile.

  5. CHEMICAL ANALYSIS OF A CARBON-ENHANCED VERY METAL-POOR STAR: CD-27 14351

    Energy Technology Data Exchange (ETDEWEB)

    Karinkuzhi, Drisya; Goswami, Aruna [Indian Institute of Astrophysics, Koramangala, Bangalore 560034 (India); Masseron, Thomas [Institute of Astronomy, Madingley Road, Cambridge CB3 0HA (United Kingdom)

    2017-01-01

    We present, for the first time, an abundance analysis of a very metal-poor carbon-enhanced star CD-27 14351 based on a high-resolution ( R  ∼ 48,000) FEROS spectrum. Our abundance analysis performed using local thermodynamic equilibrium model atmospheres shows that the object is a cool star with stellar atmospheric parameters, effective temperature T {sub eff} = 4335 K, surface gravity log g  = 0.5, microturbulence ξ  = 2.42 km s{sup −1}, and metallicity [Fe/H] = −2.6. The star exhibits high carbon and nitrogen abundances with [C/Fe] = 2.89 and [N/Fe] = 1.89. Overabundances of neutron-capture elements are evident in Ba, La, Ce, and Nd, with estimated [X/Fe] > 1, the largest enhancement being seen in Ce with [Ce/Fe] = 2.63. While the first peak s -process elements Sr and Y are found to be enhanced with respect to Fe, ([Sr/Fe] = 1.73 and [Y/Fe] = 1.91), the third peak s -process element Pb could not be detected in our spectrum at the given resolution. Europium, primarily an r -process element also shows an enhancement with [Eu/Fe] = 1.65. With [Ba/Eu] = 0.12, the object CD-27 14351 satisfies the classification criterion for a CEMP-r/s star. The elemental abundance distributions observed in this star are discussed in light of the chemical abundances observed in other CEMP stars in the literature.

  6. THE INTERMEDIATE NEUTRON-CAPTURE PROCESS AND CARBON-ENHANCED METAL-POOR STARS

    Energy Technology Data Exchange (ETDEWEB)

    Hampel, Melanie [Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117 Heidelberg (Germany); Stancliffe, Richard J. [Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, D-53121 Bonn (Germany); Lugaro, Maria [Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, H-1121 Budapest (Hungary); Meyer, Bradley S., E-mail: mhampel@lsw.uni-heidelberg.de [Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978 (United States)

    2016-11-10

    Carbon-enhanced metal-poor (CEMP) stars in the Galactic Halo display enrichments in heavy elements associated with either the s (slow) or the r (rapid) neutron-capture process (e.g., barium and europium, respectively), and in some cases they display evidence of both. The abundance patterns of these CEMP- s / r stars, which show both Ba and Eu enrichment, are particularly puzzling, since the s and the r processes require neutron densities that are more than ten orders of magnitude apart and, hence, are thought to occur in very different stellar sites with very different physical conditions. We investigate whether the abundance patterns of CEMP- s / r stars can arise from the nucleosynthesis of the intermediate neutron-capture process (the i process), which is characterized by neutron densities between those of the s and the r processes. Using nuclear network calculations, we study neutron capture nucleosynthesis at different constant neutron densities n ranging from 10{sup 7}–10{sup 15} cm{sup -3}. With respect to the classical s process resulting from neutron densities on the lowest side of this range, neutron densities on the highest side result in abundance patterns, which show an increased production of heavy s -process and r -process elements, but similar abundances of the light s -process elements. Such high values of n may occur in the thermal pulses of asymptotic giant branch stars due to proton ingestion episodes. Comparison to the surface abundances of 20 CEMP- s / r stars shows that our modeled i -process abundances successfully reproduce observed abundance patterns, which could not be previously explained by s -process nucleosynthesis. Because the i -process models fit the abundances of CEMP- s / r stars so well, we propose that this class should be renamed as CEMP- i .

  7. WASP-36b: A NEW TRANSITING PLANET AROUND A METAL-POOR G-DWARF, AND AN INVESTIGATION INTO ANALYSES BASED ON A SINGLE TRANSIT LIGHT CURVE

    Energy Technology Data Exchange (ETDEWEB)

    Smith, A. M. S.; Anderson, D. R.; Hellier, C.; Maxted, P. F. L.; Smalley, B.; Southworth, J. [Astrophysics Group, Keele University, Staffordshire, ST5 5BG (United Kingdom); Collier Cameron, A. [SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, Fife, KY16 9SS (United Kingdom); Gillon, M.; Jehin, E. [Institut d' Astrophysique et de Geophysique, Universite de Liege, Allee du 6 Aout, 17 Bat. B5C, Liege 1 (Belgium); Lendl, M.; Queloz, D.; Triaud, A. H. M. J.; Pepe, F.; Segransan, D.; Udry, S. [Observatoire de Geneve, Universite de Geneve, 51 Chemin des Maillettes, 1290 Sauverny (Switzerland); West, R. G. [Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH (United Kingdom); Barros, S. C. C.; Pollacco, D. [Astrophysics Research Centre, School of Mathematics and Physics, Queen' s University, University Road, Belfast, BT7 1NN (United Kingdom); Street, R. A., E-mail: amss@astro.keele.ac.uk [Las Cumbres Observatory, 6740 Cortona Drive Suite 102, Goleta, CA 93117 (United States)

    2012-04-15

    We report the discovery, from WASP and CORALIE, of a transiting exoplanet in a 1.54 day orbit. The host star, WASP-36, is a magnitude V = 12.7, metal-poor G2 dwarf (T{sub eff} = 5959 {+-} 134 K), with [Fe/H] =-0.26 {+-} 0.10. We determine the planet to have mass and radius, respectively, 2.30 {+-} 0.07 and 1.28 {+-} 0.03 times that of Jupiter. We have eight partial or complete transit light curves, from four different observatories, which allow us to investigate the potential effects on the fitted system parameters of using only a single light curve. We find that the solutions obtained by analyzing each of these light curves independently are consistent with our global fit to all the data, despite the apparent presence of correlated noise in at least two of the light curves.

  8. A search for stars of very low metal abundance. VI. Detailed abundances of 313 metal-poor stars

    International Nuclear Information System (INIS)

    Roederer, Ian U.; Preston, George W.; Thompson, Ian B.; Shectman, Stephen A.; Burley, Gregory S.; Kelson, Daniel D.; Sneden, Christopher

    2014-01-01

    We present radial velocities, equivalent widths, model atmosphere parameters, and abundances or upper limits for 53 species of 48 elements derived from high resolution optical spectroscopy of 313 metal-poor stars. A majority of these stars were selected from the metal-poor candidates of the HK Survey of Beers, Preston, and Shectman. We derive detailed abundances for 61% of these stars for the first time. Spectra were obtained during a 10 yr observing campaign using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coudé Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform a standard LTE abundance analysis using MARCS model atmospheres, and we apply line-by-line statistical corrections to minimize systematic abundance differences arising when different sets of lines are available for analysis. We identify several abundance correlations with effective temperature. A comparison with previous abundance analyses reveals significant differences in stellar parameters, which we investigate in detail. Our metallicities are, on average, lower by ≈0.25 dex for red giants and ≈0.04 dex for subgiants. Our sample contains 19 stars with [Fe/H] ≤–3.5, 84 stars with [Fe/H] ≤–3.0, and 210 stars with [Fe/H] ≤–2.5. Detailed abundances are presented here or elsewhere for 91% of the 209 stars with [Fe/H] ≤–2.5 as estimated from medium resolution spectroscopy by Beers, Preston, and Shectman. We will discuss the interpretation of these abundances in subsequent papers.

  9. White Dwarf Stars

    Science.gov (United States)

    1999-01-01

    Peering deep inside a cluster of several hundred thousand stars, NASA's Hubble Space Telescope has uncovered the oldest burned-out stars in our Milky Way Galaxy, giving astronomers a fresh reading on the age of the universe. Located in the globular cluster M4, these small, burned-out stars -- called white dwarfs -- are about 12 to 13 billion years old. By adding the one billion years it took the cluster to form after the Big Bang, astronomers found that the age of the white dwarfs agrees with previous estimates that the universe is 13 to 14 billion years old. The images, including some taken by Hubble's Wide Field and Planetary Camera 2, are available online at http://oposite.stsci.edu/pubinfo/pr/2002/10/ or http://www.jpl.nasa.gov/images/wfpc . The camera was designed and built by NASA's Jet Propulsion Laboratory, Pasadena, Calif. In the top panel, a ground-based observatory snapped a panoramic view of the entire cluster, which contains several hundred thousand stars within a volume of 10 to 30 light-years across. The Kitt Peak National Observatory's .9-meter telescope took this picture in March 1995. The box at left indicates the region observed by the Hubble telescope. The Hubble telescope studied a small region of the cluster. A section of that region is seen in the picture at bottom left. A sampling of an even smaller region is shown at bottom right. This region is only about one light-year across. In this smaller region, Hubble pinpointed a number of faint white dwarfs. The blue circles indicate the dwarfs. It took nearly eight days of exposure time over a 67-day period to find these extremely faint stars. Globular clusters are among the oldest clusters of stars in the universe. The faintest and coolest white dwarfs within globular clusters can yield a globular cluster's age. Earlier Hubble observations showed that the first stars formed less than 1 billion years after the universe's birth in the big bang. So, finding the oldest stars puts astronomers within

  10. Asteroseismology of White Dwarf Stars

    Science.gov (United States)

    Hansen, Carl J.

    1997-01-01

    The primary purpose of this investigation has been to study various aspects of multimode pulsations in variable white dwarfs. In particular, nonlinear interactions among pulsation modes in white dwarfs (and, to some extent, in other variable stars), analysis of recent observations where such interactions are important, and preliminary work on the effects of crystallization in cool white dwarfs are reported.

  11. K2-111 b - a short period super-Earth transiting a metal poor, evolved old star

    Science.gov (United States)

    Fridlund, Malcolm; Gaidos, Eric; Barragán, Oscar; Persson, Carina M.; Gandolfi, Davide; Cabrera, Juan; Hirano, Teruyuki; Kuzuhara, Masayuki; Csizmadia, Sz.; Nowak, Grzegorz; Endl, Michael; Grziwa, Sascha; Korth, Judith; Pfaff, Jeremias; Bitsch, Bertram; Johansen, Anders; Mustill, Alexander J.; Davies, Melvyn B.; Deeg, Hans J.; Palle, Enric; Cochran, William D.; Eigmüller, Philipp; Erikson, Anders; Guenther, Eike; Hatzes, Artie P.; Kiilerich, Amanda; Kudo, Tomoyuki; MacQueen, Phillip; Narita, Norio; Nespral, David; Pätzold, Martin; Prieto-Arranz, Jorge; Rauer, Heike; Van Eylen, Vincent

    2017-07-01

    Context. From a light curve acquired through the K2 space mission, the star K2-111(EPIC 210894022) has been identified as possibly orbited by a transiting planet. Aims: Our aim is to confirm the planetary nature of the object and derive its fundamental parameters. Methods: We analyse the light curve variations during the planetary transit using packages developed specifically for exoplanetary transits. Reconnaissance spectroscopy and radial velocity observations have been obtained using three separate telescope and spectrograph combinations. The spectroscopic synthesis package SME has been used to derive the stellar photospheric parameters that were used as input to various stellar evolutionary tracks in order to derive the parameters of the system. The planetary transit was also validated to occur on the assumed host star through adaptive imaging and statistical analysis. Results: The star is found to be located in the background of the Hyades cluster at a distance at least 4 times further away from Earth than the cluster itself. The spectrum and the space velocities of K2-111 strongly suggest it to be a member of the thick disk population. The co-added high-resolution spectra show that that it is a metal poor ([Fe/H] = - 0.53 ± 0.05 dex) and α-rich somewhat evolved solar-like star of spectral type G3. We find Teff = 5730 ± 50 K, log g⋆ = 4.15 ± 0.1 cgs, and derive a radius of R⋆ = 1.3 ± 0.1 R⊙ and a mass of M⋆ = 0.88 ± 0.02 M⊙. The currently available radial velocity data confirms a super-Earth class planet with a mass of 8.6 ± 3.9 M⊕ and a radius of 1.9 ± 0.2 R⊕. A second more massive object with a period longer than about 120 days is indicated by a long-term radial velocity drift. Conclusions: The radial velocity detection together with the imaging confirms with a high level of significance that the transit signature is caused by a planet orbiting the star K2-111. This planet is also confirmed in the radial velocity data. A second more

  12. Testing Gravity Using Dwarf Stars

    OpenAIRE

    Sakstein, Jeremy

    2015-01-01

    Generic scalar-tensor theories of gravity predict deviations from Newtonian physics inside astrophysical bodies. In this paper, we point out that low mass stellar objects, red and brown dwarf stars, are excellent probes of these theories. We calculate two important and potentially observable quantities: the radius of brown dwarfs and the minimum mass for hydrogen burning in red dwarfs. The brown dwarf radius can differ significantly from the GR prediction and upcoming surveys that probe the m...

  13. Abundance patterns of the light neutron-capture elements in very and extremely metal-poor stars

    Science.gov (United States)

    Spite, F.; Spite, M.; Barbuy, B.; Bonifacio, P.; Caffau, E.; François, P.

    2018-03-01

    Aims: The abundance patterns of the neutron-capture elements in metal-poor stars provide a unique record of the nucleosynthesis products of the earlier massive primitive objects. Methods: We measured new abundances of so-called light neutron-capture of first peak elements using local thermodynamic equilibrium (LTE) 1D analysis; this analysis resulted in a sample of 11 very metal-poor stars, from [Fe/H] = -2.5 to [Fe/H] = -3.4, and one carbon-rich star, CS 22949-037 with [Fe/H] = -4.0. The abundances were compared to those observed in two classical metal-poor stars: the typical r-rich star CS 31082-001 ([Eu/Fe] > +1.0) and the r-poor star HD 122563 ([Eu/Fe] < 0.0), which are known to present a strong enrichment of the first peak neutron-capture elements relative to the second peak. Results: Within the first peak, the abundances are well correlated in analogy to the well-known correlation inside the abundances of the second-peak elements. In contrast, there is no correlation between any first peak element with any second peak element. We show that the scatter of the ratio of the first peak abundance over second peak abundance increases when the mean abundance of the second peak elements decreases from r-rich to r-poor stars. We found two new r-poor stars that are very similar to HD 122563. A third r-poor star, CS 22897-008, is even more extreme; this star shows the most extreme example of first peak elements enrichment to date. On the contrary, another r-poor star (BD-18 5550) has a pattern of first peak elements that is similar to the typical r-rich stars CS 31082-001, however this star has some Mo enrichment. Conclusions: The distribution of the neutron-capture elements in our very metal-poor stars can be understood as the combination of at least two mechanisms: one that enriches the forming stars cloud homogeneously through the main r-process and leads to an element pattern similar to the r-rich stars, such as CS 31082-001; and another that forms mainly lighter

  14. Asteroseismology of white dwarf stars

    OpenAIRE

    Córsico, A. H.

    2014-01-01

    Most of low- and intermediate-mass stars that populate the Universe will end their lives as white dwarf stars. These ancient stellar remnants have encrypted inside a precious record of the evolutionary history of the progenitor stars, providing a wealth of information about the evolution of stars, star formation, and the age of a variety of stellar populations, such as our Galaxy and open and globular clusters. While some information like surface chemical composition, temperature and gravity ...

  15. Evolution of White Dwarf Stars

    OpenAIRE

    L. G. Althaus

    2001-01-01

    This paper is aimed at presenting the main results we have obtained for the study of the evoution of white dwarf stars. The calculations are carried out by means of a detailed evolutionary code based on an updated physical description. In particular, we briefly discuss the results for the evolution of white dwarfs of different stellar masses and chemical composition, and the evolution of whit e dwarfs in the framework of a varying gravitational constant G scenario as well.

  16. The Chemical Abundances of Stars in the Halo (CASH) Project. II. New Extremely Metal-poor Stars

    Science.gov (United States)

    Krugler, Julie A.; Frebel, A.; Roederer, I. U.; Sneden, C.; Shetrone, M.; Beers, T.; Christlieb, N.

    2011-01-01

    We present new abundance results from the Chemical Abundances of Stars in the Halo (CASH) project. The 500 CASH spectra were observed using the Hobby-Eberly Telescope in "snapshot" mode and are analyzed using an automated stellar parameter and abundance pipeline called CASHCODE. For the 20 most metal-poor stars of the CASH sample we have obtained high resolution spectra using the Magellan Telescope in order to test the uncertainties and systematic errors associated with the snapshot quality (i.e., R 15,000 and S/N 65) HET spectra and to calibrate the newly developed CASHCODE by making a detailed comparison between the stellar parameters and abundances determined from the high resolution and snapshot spectra. We find that the CASHCODE stellar parameters (effective temperature, surface gravity, metallicity, and microturbulence) agree well with the results of the manual analysis of the high resolution spectra. We present the abundances of three newly discovered stars with [Fe/H] ratios with alpha-enhancement and Fe-peak depletion and a range of n-capture elements. The full CASH sample will be used to derive statistically robust abundance trends and frequencies (e.g. carbon and n-capture), as well as placing constraints on nucleosynthetic processes that occurred in the early universe.

  17. SEARCHES FOR METAL-POOR STARS FROM THE HAMBURG/ESO SURVEY USING THE CH G BAND

    Energy Technology Data Exchange (ETDEWEB)

    Placco, Vinicius M.; Rossi, Silvia [Departamento de Astronomia-Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Universidade de Sao Paulo, Sao Paulo, SP 05508-090 (Brazil); Kennedy, Catherine R.; Beers, Timothy C.; Lee, Young Sun [Department of Physics and Astronomy and JINA (Joint Institute for Nuclear Astrophysics), Michigan State University, East Lansing, MI 48824 (United States); Christlieb, Norbert [Zentrum fuer Astronomie der Universitaet Heidelberg, Landessternwarte, Koenigstuhl 12, 69117 Heidelberg (Germany); Sivarani, Thirupathi [Indian Institute of Astrophysics, 2nd Block, Koramangala, Bangalore 560034 (India); Reimers, Dieter [Hamburger Sternwarte, Universitaet Hamburg, Gojenbergsweg 112, 21029 Hamburg (Germany); Wisotzki, Lutz, E-mail: vmplacco@astro.iag.usp.br [Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam (Germany)

    2011-12-15

    We describe a new method to search for metal-poor candidates from the Hamburg/ESO objective-prism survey (HES) based on identifying stars with apparently strong CH G-band strengths for their colors. The hypothesis we exploit is that large overabundances of carbon are common among metal-poor stars, as has been found by numerous studies over the past two decades. The selection was made by considering two line indices in the 4300 A region, applied directly to the low-resolution prism spectra. This work also extends a previously published method by adding bright sources to the sample. The spectra of these stars suffer from saturation effects, compromising the index calculations and leading to an undersampling of the brighter candidates. A simple numerical procedure, based on available photometry, was developed to correct the line indices and overcome this limitation. Visual inspection and classification of the spectra from the HES plates yielded a list of 5288 new metal-poor (and by selection, carbon-rich) candidates, which are presently being used as targets for medium-resolution spectroscopic follow-up. Estimates of the stellar atmospheric parameters, as well as carbon abundances, are now available for 117 of the first candidates, based on follow-up medium-resolution spectra obtained with the SOAR 4.1 m and Gemini 8 m telescopes. We demonstrate that our new method improves the metal-poor star fractions found by our pilot study by up to a factor of three in the same magnitude range, as compared with our pilot study based on only one CH G-band index. Our selection scheme obtained roughly a 40% success rate for identification of stars with [Fe/H] <-1.0; the primary contaminant is late-type stars with near-solar abundances and, often, emission line cores that filled in the Ca II K line on the prism spectrum. Because the selection is based on carbon, we greatly increase the numbers of known carbon-enhanced metal-poor stars from the HES with intermediate metallicities -2

  18. Distances of Dwarf Carbon Stars

    Science.gov (United States)

    Harris, Hugh C.; Dahn, Conard C.; Subasavage, John P.; Munn, Jeffrey A.; Canzian, Blaise J.; Levine, Stephen E.; Monet, Alice B.; Pier, Jeffrey R.; Stone, Ronald C.; Tilleman, Trudy M.; Hartkopf, William I.

    2018-06-01

    Parallaxes are presented for a sample of 20 nearby dwarf carbon stars. The inferred luminosities cover almost two orders of magnitude. Their absolute magnitudes and tangential velocities confirm prior expectations that some originate in the Galactic disk, although more than half of this sample are halo stars. Three stars are found to be astrometric binaries, and orbital elements are determined; their semimajor axes are 1–3 au, consistent with the size of an AGB mass-transfer donor star.

  19. Rotation of White Dwarf Stars

    OpenAIRE

    Kawaler, Steven D.

    2014-01-01

    I discuss and consider the status of observational determinations of the rotation velocities of white dwarf stars via asteroseismology and spectroscopy. While these observations have important implications on our understanding of the angular momentum evolution of stars in their late stages of evolution, more direct methods are sorely needed to disentangle ambiguities.

  20. NEW RARE EARTH ELEMENT ABUNDANCE DISTRIBUTIONS FOR THE SUN AND FIVE r-PROCESS-RICH VERY METAL-POOR STARS

    International Nuclear Information System (INIS)

    Sneden, Christopher; Lawler, James E.; Den Hartog, Elizabeth A.; Cowan, John J.; Ivans, Inese I.

    2009-01-01

    We have derived new abundances of the rare earth elements Pr, Dy, Tm, Yb, and Lu for the solar photosphere and for five very metal-poor, neutron-capture r-process-rich giant stars. The photospheric values for all five elements are in good agreement with meteoritic abundances. For the low-metallicity sample, these abundances have been combined with new Ce abundances from a companion paper, and reconsideration of a few other elements in individual stars, to produce internally consistent Ba, rare earth, and Hf (56 ≤ Z ≤ 72) element distributions. These have been used in a critical comparison between stellar and solar r-process abundance mixes.

  1. The Little Cub: Discovery of an Extremely Metal-poor Star-forming Galaxy in the Local Universe

    Energy Technology Data Exchange (ETDEWEB)

    Hsyu, Tiffany; Prochaska, J. Xavier; Bolte, Michael [Department of Astronomy and Astrophysics, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95060 (United States); Cooke, Ryan J. [Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE (United Kingdom)

    2017-08-20

    We report the discovery of the Little Cub, an extremely metal-poor star-forming galaxy in the local universe, found in the constellation Ursa Major (a.k.a. the Great Bear). We first identified the Little Cub as a candidate metal-poor galaxy based on its Sloan Digital Sky Survey photometric colors, combined with spectroscopy using the Kast spectrograph on the Shane 3 m telescope at Lick Observatory. In this Letter, we present high-quality spectroscopic data taken with the Low Resolution Imaging Spectrometer at Keck Observatory, which confirm the extremely metal-poor nature of this galaxy. Based on the weak [O iii] λ 4363 Å emission line, we estimate a direct oxygen abundance of 12 + log(O/H) = 7.13 ± 0.08, making the Little Cub one of the lowest-metallicity star-forming galaxies currently known in the local universe. The Little Cub appears to be a companion of the spiral galaxy NGC 3359 and shows evidence of gas stripping. We may therefore be witnessing the quenching of a near-pristine galaxy as it makes its first passage about a Milky Way–like galaxy.

  2. The Little Cub: Discovery of an Extremely Metal-poor Star-forming Galaxy in the Local Universe

    Science.gov (United States)

    Hsyu, Tiffany; Cooke, Ryan J.; Prochaska, J. Xavier; Bolte, Michael

    2017-08-01

    We report the discovery of the Little Cub, an extremely metal-poor star-forming galaxy in the local universe, found in the constellation Ursa Major (a.k.a. the Great Bear). We first identified the Little Cub as a candidate metal-poor galaxy based on its Sloan Digital Sky Survey photometric colors, combined with spectroscopy using the Kast spectrograph on the Shane 3 m telescope at Lick Observatory. In this Letter, we present high-quality spectroscopic data taken with the Low Resolution Imaging Spectrometer at Keck Observatory, which confirm the extremely metal-poor nature of this galaxy. Based on the weak [O III] λ4363 Å emission line, we estimate a direct oxygen abundance of 12 + log(O/H) = 7.13 ± 0.08, making the Little Cub one of the lowest-metallicity star-forming galaxies currently known in the local universe. The Little Cub appears to be a companion of the spiral galaxy NGC 3359 and shows evidence of gas stripping. We may therefore be witnessing the quenching of a near-pristine galaxy as it makes its first passage about a Milky Way-like galaxy.

  3. The Little Cub: Discovery of an Extremely Metal-poor Star-forming Galaxy in the Local Universe

    International Nuclear Information System (INIS)

    Hsyu, Tiffany; Prochaska, J. Xavier; Bolte, Michael; Cooke, Ryan J.

    2017-01-01

    We report the discovery of the Little Cub, an extremely metal-poor star-forming galaxy in the local universe, found in the constellation Ursa Major (a.k.a. the Great Bear). We first identified the Little Cub as a candidate metal-poor galaxy based on its Sloan Digital Sky Survey photometric colors, combined with spectroscopy using the Kast spectrograph on the Shane 3 m telescope at Lick Observatory. In this Letter, we present high-quality spectroscopic data taken with the Low Resolution Imaging Spectrometer at Keck Observatory, which confirm the extremely metal-poor nature of this galaxy. Based on the weak [O iii] λ 4363 Å emission line, we estimate a direct oxygen abundance of 12 + log(O/H) = 7.13 ± 0.08, making the Little Cub one of the lowest-metallicity star-forming galaxies currently known in the local universe. The Little Cub appears to be a companion of the spiral galaxy NGC 3359 and shows evidence of gas stripping. We may therefore be witnessing the quenching of a near-pristine galaxy as it makes its first passage about a Milky Way–like galaxy.

  4. Star Formation in Dwarf-Dwarf Mergers: Fueling Hierarchical Assembly

    Science.gov (United States)

    Stierwalt, Sabrina; Johnson, K. E.; Kallivayalil, N.; Patton, D. R.; Putman, M. E.; Besla, G.; Geha, M. C.

    2014-01-01

    We present early results from the first systematic study a sample of isolated interacting dwarf pairs and the mechanisms governing their star formation. Low mass dwarf galaxies are ubiquitous in the local universe, yet the efficiency of gas removal and the enhancement of star formation in dwarfs via pre-processing (i.e. dwarf-dwarf interactions occurring before the accretion by a massive host) are currently unconstrained. Studies of Local Group dwarfs credit stochastic internal processes for their complicated star formation histories, but a few intriguing examples suggest interactions among dwarfs may produce enhanced star formation. We combine archival UV imaging from GALEX with deep optical broad- and narrow-band (Halpha) imaging taken with the pre- One Degree Imager (pODI) on the WIYN 3.5-m telescope and with the 2.3-m Bok telescope at Steward Observatory to confirm the presence of stellar bridges and tidal tails and to determine whether dwarf-dwarf interactions alone can trigger significant levels of star formation. We investigate star formation rates and global galaxy colors as a function of dwarf pair separation (i.e. the dwarf merger sequence) and dwarf-dwarf mass ratio. This project is a precursor to an ongoing effort to obtain high spatial resolution HI imaging to assess the importance of sequential triggering caused by dwarf-dwarf interactions and the subsequent affect on the more massive hosts that later accrete the low mass systems.

  5. Atmospheric parameters and magnesium and calcium NLTE abundances for a sample of 16 ultra metal-poor stars

    Science.gov (United States)

    Sitnova, Tatyana; Mashonkina, Lyudmila; Ezzeddine, Rana; Frebel, Anna

    2018-06-01

    The most metal-poor stars provide important observational clues to the astrophysical objects that enriched the primordial gas with heavy elements. Accurate atmospheric parameters is a prerequisite of determination of accurate abundances. We present atmospheric parameters and abundances of calcium and magnesium for a sample of 16 ultra-metal poor (UMP) stars. In spectra of UMP stars, iron is represented only by lines of Fe I, while calcium is represented with lines of Ca I and Ca II, which can be used for determination/checking of effective temperature and surface gravity. Accurate calculations of synthetic spectra of UMP stars require non-local thermodynamic equilibrium (NLTE) treatment of line formation, since deviations from LTE grow with metallicity decreasing. The method of atmospheric parameter determination is based on NLTE analysis of lines of Ca I and Ca II, multi-band photometry, and isochrones. The method was tested in advance with the ultra metal-poor giant CD-38 245, where, in addition, trigonometric parallax measurements from Gaia DR1 and lines of Fe I and Fe II are available. Using photometric Teff = 4900 K and distance based log g = 2.0 for CD-38 245, we derived consistent within error bars NLTE abundances from Fe I and Fe II and Ca I and Ca II, while LTE leads to a discrepancy of 0.6 dex between Ca I and Ca II. We determined NLTE and LTE abundances of magnesium and calcium in 16 stars of the sample. For the majority of stars, as expected, [Ca/Mg] NLTE abundance ratios are close to 0, while LTE leads to systematically higher [Ca/Mg], by up to 0.3 dex, and larger spread of [Ca/Mg] for different stars. Three stars of our sample are strongly enhanced in magnesium, with [Mg/Ca] of 1.3 dex. It is worth noting that, for these three stars, we got very similar [Mg/Ca] of 1.30, 1.45, and 1.29, in contrast to the data from the literature, where, for the same stars, [Mg/Ca] vary from 0.7 to 1.4. Very similar [Mg/Ca] abundance ratios of these stars argue that

  6. FORMATION OF CARBON-ENHANCED METAL-POOR STARS IN THE PRESENCE OF FAR-ULTRAVIOLET RADIATION

    Energy Technology Data Exchange (ETDEWEB)

    Bovino, S.; Schleicher, D. R. G.; Latif, M. A. [Institut für Astrophysik Georg-August-Universität, Friedrich-Hund Platz 1, 37077 Göttingen (Germany); Grassi, T., E-mail: sbovino@astro.physik.uni-goettingen.de [Centre for Star and Planet Formation, Natural History Museum of Denmark, Øster Voldgade 5-7, 1350 Copenhagen (Denmark)

    2014-08-01

    Recent discoveries of carbon-enhanced metal-poor stars like SMSS J031300.36–670839.3 provide increasing observational insights into the formation conditions of the first second-generation stars in the universe, reflecting the chemical conditions after the first supernova explosion. Here, we present the first cosmological simulations with a detailed chemical network including primordial species as well as C, C{sup +}, O, O{sup +}, Si, Si{sup +}, and Si{sup 2+} following the formation of carbon-enhanced metal-poor stars. The presence of background UV flux delays the collapse from z = 21 to z = 15 and cool the gas down to the cosmic microwave background temperature for a metallicity of Z/Z {sub ☉} = 10{sup –3}. This can potentially lead to the formation of lower-mass stars. Overall, we find that the metals have a stronger effect on the collapse than the radiation, yielding a comparable thermal structure for large variations in the radiative background. We further find that radiative backgrounds are not able to delay the collapse for Z/Z {sub ☉} = 10{sup –2} or a carbon abundance as in SMSS J031300.36–670839.3.

  7. Accurate effective temperatures of the metal-poor benchmark stars HD 140283, HD 122563, and HD 103095 from CHARA interferometry

    Science.gov (United States)

    Karovicova, I.; White, T. R.; Nordlander, T.; Lind, K.; Casagrande, L.; Ireland, M. J.; Huber, D.; Creevey, O.; Mourard, D.; Schaefer, G. H.; Gilmore, G.; Chiavassa, A.; Wittkowski, M.; Jofré, P.; Heiter, U.; Thévenin, F.; Asplund, M.

    2018-03-01

    Large stellar surveys of the Milky Way require validation with reference to a set of `benchmark' stars whose fundamental properties are well determined. For metal-poor benchmark stars, disagreement between spectroscopic and interferometric effective temperatures has called the reliability of the temperature scale into question. We present new interferometric measurements of three metal-poor benchmark stars, HD 140283, HD 122563, and HD 103095, from which we determine their effective temperatures. The angular sizes of all the stars were determined from observations with the PAVO beam combiner at visible wavelengths at the CHARA array, with additional observations of HD 103095 made with the VEGA instrument, also at the CHARA array. Together with photometrically derived bolometric fluxes, the angular diameters give a direct measurement of the effective temperature. For HD 140283, we find θLD = 0.324 ± 0.005 mas, Teff = 5787 ± 48 K; for HD 122563, θLD = 0.926 ± 0.011 mas, Teff = 4636 ± 37 K; and for HD 103095, θLD = 0.595 ± 0.007 mas, Teff = 5140 ± 49 K. Our temperatures for HD 140283 and HD 103095 are hotter than the previous interferometric measurements by 253 and 322 K, respectively. We find good agreement between our temperatures and recent spectroscopic and photometric estimates. We conclude some previous interferometric measurements have been affected by systematic uncertainties larger than their quoted errors.

  8. Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. I. Carbon Abundance Estimation and Frequency of CEMP Stars

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Young Sun [NMSU, Las Cruces; Beers, Timothy C. [Michigan State U., JINA; Masseron, Thomas [Brussels U.; Plez, Bertrand [U. Montpellier 2, LUPM; Rockosi, Constance M. [Lick Observ.; Sobeck, Jennifer [Chicago U.; Yanny, Brian [Fermilab; Lucatello, Sara [Padua Observ.; Sivarani, Thirupathi [Bangalore, Indian Inst. Astrophys.; Placco, Vinicius M. [Sao Paulo U., IAG; Carollo, Daniela [Macquarie U.

    2013-10-17

    We describe a method for the determination of stellar [C/Fe] abundance ratios using low-resolution (R = 2000) stellar spectra from the SDSS and SEGUE. By means of a star-by-star comparison with a set of SDSS/SEGUE spectra with available estimates of [C/Fe] based on published high-resolution analyses, we demonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N > 15 to a precision better than 0.35 dex. Using the measured carbon-to-iron abundance ratios obtained by this technique, we derive the frequency of carbon-enhanced stars ([C/Fe] > +0.7) as a function of [Fe/H], for both the SDSS/SEGUE stars and other samples from the literature. We find that the differential frequency slowly rises from almost zero to about 14% at [Fe/H] ~ -2.4, followed by a sudden increase, by about a factor of three, to 39% from [Fe/H] ~ -2.4 to [Fe/H] ~ -3.7. We also examine how the cumulative frequency of CEMP stars varies across different luminosity classes. The giant sample exhibits a cumulative CEMP frequency of 32% for [Fe/H] < -2.5, 31% for [Fe/H] < -3.0, and 33% for [Fe/H] < -3.5. For the main-sequence turnoff stars, we obtain a lower cumulative CEMP frequency, around 10% for [Fe/H] < -2.5. The dwarf population displays a large change in the cumulative frequency for CEMP stars below [Fe/H] = -2.5, jumping from 15% for [Fe/H] < -2.5 to about 75% for [Fe/H] < -3.0. When we impose a restriction with respect to distance from the Galactic mid-plane (|Z| < 5 kpc), the frequency of the CEMP giants does not increase at low metallicity ([Fe/H] < -2.5), but rather, decreases, due to the dilution of C-rich material in stars that have undergone mixing with CNO-processed material from their interiors. The frequency of CEMP stars near the main-sequence turnoff, which are not expected to have experienced mixing, increases for [Fe/H] < -3.0. [abridged

  9. Innocent Bystanders and Smoking Guns: Dwarf Carbon Stars

    Science.gov (United States)

    Green, Paul J.

    2014-01-01

    As far as we know, most carbon throughout the Universe is created and dispersed by AGB stars. So it was at first surprising to find that the carbon stars most prevalent in the Galaxy are in fact dwarfs. We suspect that dC stars are most likely innocent bystanders in post-mass transfer binaries, and may be predominantly metal-poor. Among 1200 C stars found in the SDSS (Green 2013), we confirm 724 dCs, of which a dozen are DA/dC stars in composite spectrum binaries, quadrupling the total sample of these "smoking guns" for AGB binary mass transfer. The dCs likely span absolute magnitudes M_i from about 6.5 to 10.5. G-type dC stars with weak CN and relatively blue colors are probably the most massive dCs still cool enough to show C_2 bands. Eleven very red C stars with strong red CN bands appear to be N-type AGB stars at large Galactocentric distances, one likely a new discovery in the dIrr galaxy Le A. Two such stars within 30arcmin of each other may trace a previously unidentified dwarf galaxy or tidal stream at ~40 kpc. We describe follow-up projects to study the spatial, kinematic, and binary properties of these C-enriched dwarfs.

  10. Chances for earth-like planets and life around metal-poor stars

    OpenAIRE

    Zinnecker, Hans

    2003-01-01

    We discuss the difficulties of forming earth-like planets in metal-poor environments, such as those prevailing in the Galactic halo (Pop II), the Magellanic Clouds, and the early universe. We suggest that, with less heavy elements available, terrestrial planets will be smaller size and lower mass than in our solar system (solar metallicity). Such planets may not be able to sustain life as we know it. Therefore, the chances of very old lifeforms in the universe are slim, and a threshold metall...

  11. Non-LTE line formation of Fe in late-type stars - III. 3D non-LTE analysis of metal-poor stars

    DEFF Research Database (Denmark)

    Amarsi, A. M.; Lind, K.; Asplund, M.

    2016-01-01

    As one of the most important elements in astronomy, iron abundance determinations need to be as accurate as possible. We investigate the accuracy of spectroscopic iron abundance analyses using archetypal metal-poor stars. We perform detailed 3D non-LTE radiative transfer calculations based on 3D...

  12. An Unusual Transient in the Extremely Metal-Poor Galaxy SDSS J094332.35+332657.6 (Leoncino Dwarf)

    Science.gov (United States)

    Filho, Mercedes E.; Sánchez Almeida, J.

    2018-05-01

    We have serendipitously discovered that Leoncino Dwarf, an ultra-faint, low-metallicity record-holder dwarf galaxy, may have hosted a transient source, and possibly exhibited a change in morphology, a shift in the center of brightness, and peak variability of the main (host) source in images taken approximately 40 yr apart; it is highly likely that these phenomena are related. Scenarios involving a Solar System object, a stellar cluster, dust enshrouding, and accretion variability have been considered, and discarded, as the origin of the transient. Although a combination of time-varying strong and weak lensing effects, induced by an intermediate mass black hole (104 - 5 × 105 M⊙) moving within the Milky Way halo (0.1 - 4 kpc), can conceivably explain all of the observed variable galaxy properties, it is statistically highly unlikely according to current theoretical predictions, and, therefore, also discarded. A cataclysmic event such as a supernova/hypernova could have occurred, as long as the event was observed towards the later/late-stage descent of the light curve, but this scenario fails to explain the absence of a post-explosion source and/or host HII region in recent optical images. An episode related to the giant eruption of a luminous blue variable star, a stellar merger or a nova, observed at, or near, peak magnitude may explain the transient source and possibly the change in morphology/center of brightness, but can not justify the main source peak variability, unless stellar variability is evoked.

  13. First stars X. The nature of three unevolved carbon-enhanced metal-poor stars

    DEFF Research Database (Denmark)

    Sivarani, T.; Beers, T.C.; Bonifacio, P.

    2006-01-01

    Stars: abundances, stars: population II, Galaxy: abundances, stars: AGB and post-AGB Udgivelsesdato: Nov.......Stars: abundances, stars: population II, Galaxy: abundances, stars: AGB and post-AGB Udgivelsesdato: Nov....

  14. Evidence for dwarf stars at D of about 100 kiloparsecs near the Sextans dwarf spheroidal galaxy

    Science.gov (United States)

    Gould, Andrew; Guhathakurta, Puragra; Richstone, Douglas; Flynn, Chris

    1992-01-01

    A method is presented for detecting individual, metal-poor, dwarf stars at distances less than about 150 kpc - a method specifically designed to filter out stars from among the much more numerous faint background field galaxies on the basis of broad-band colors. This technique is applied to two fields at high Galactic latitude, for which there are deep CCD data in four bands ranging from 3600 to 9000 A. The field in Sextans probably contains more than about five dwarf stars with BJ not greater than 25.5. These are consistent with being at a common distance about 100 kpc and lie about 1.7 deg from the newly discovered dwarf galaxy in Sextans whose distance is about 85 +/- 10 kpc. The stars lie near the major axis of the galaxy and are near or beyond the tidal radius. The second field, toward the south Galactic pole, may contain up to about five extra-Galactic stars, but these show no evidence for being at a common distance. Possible applications of this type technique are discussed, and it is shown that even very low surface brightness star clusters or dwarf galaxies may be detected at distances less than about 1 Mpc.

  15. The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

    Energy Technology Data Exchange (ETDEWEB)

    Brown, Warren R. [Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 (United States); Kilic, Mukremin; Gianninas, A., E-mail: wbrown@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK, 73019 (United States)

    2017-04-10

    We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ∼1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M {sub ⊙} main sequence stars with ≃0.8 M {sub ⊙} companions. While WDs must exist at sdA temperatures, only ∼1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A–F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.

  16. The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

    Science.gov (United States)

    Brown, Warren R.; Kilic, Mukremin; Gianninas, A.

    2017-04-01

    We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ˜1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M ⊙ main sequence stars with ≃0.8 M ⊙ companions. While WDs must exist at sdA temperatures, only ˜1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A-F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.

  17. The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

    International Nuclear Information System (INIS)

    Brown, Warren R.; Kilic, Mukremin; Gianninas, A.

    2017-01-01

    We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ∼1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M ⊙ main sequence stars with ≃0.8 M ⊙ companions. While WDs must exist at sdA temperatures, only ∼1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A–F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.

  18. K2-155: A Bright Metal-poor M Dwarf with Three Transiting Super-Earths

    Science.gov (United States)

    Hirano, Teruyuki; Dai, Fei; Livingston, John H.; Fujii, Yuka; Cochran, William D.; Endl, Michael; Gandolfi, Davide; Redfield, Seth; Winn, Joshua N.; Guenther, Eike W.; Prieto-Arranz, Jorge; Albrecht, Simon; Barragan, Oscar; Cabrera, Juan; Cauley, P. Wilson; Csizmadia, Szilard; Deeg, Hans; Eigmüller, Philipp; Erikson, Anders; Fridlund, Malcolm; Fukui, Akihiko; Grziwa, Sascha; Hatzes, Artie P.; Korth, Judith; Narita, Norio; Nespral, David; Niraula, Prajwal; Nowak, Grzegorz; Pätzold, Martin; Palle, Enric; Persson, Carina M.; Rauer, Heike; Ribas, Ignasi; Smith, Alexis M. S.; Van Eylen, Vincent

    2018-03-01

    We report on the discovery of three transiting super-Earths around K2-155 (EPIC 210897587), a relatively bright early M dwarf (V = 12.81 mag) observed during Campaign 13 of the NASA K2 mission. To characterize the system and validate the planet candidates, we conducted speckle imaging and high-dispersion optical spectroscopy, including radial velocity measurements. Based on the K2 light curve and the spectroscopic characterization of the host star, the planet sizes and orbital periods are {1.55}-0.17+0.20 {R}\\oplus and 6.34365 ± 0.00028 days for the inner planet; {1.95}-0.22+0.27 {R}\\oplus and 13.85402 ± 0.00088 days for the middle planet; and {1.64}-0.17+0.18 {R}\\oplus and 40.6835 ± 0.0031 days for the outer planet. The outer planet (K2-155d) is near the habitable zone, with an insolation 1.67 ± 0.38 times that of the Earth. The planet’s radius falls within the range between that of smaller rocky planets and larger gas-rich planets. To assess the habitability of this planet, we present a series of three-dimensional global climate simulations, assuming that K2-155d is tidally locked and has an Earth-like composition and atmosphere. We find that the planet can maintain a moderate surface temperature if the insolation proves to be smaller than ∼1.5 times that of the Earth. Doppler mass measurements, transit spectroscopy, and other follow-up observations should be rewarding, as K2-155 is one of the optically brightest M dwarfs known to harbor transiting planets.

  19. VLT/UVES abundances of individual stars in the Fornax dwarf spheroidal globular clusters

    NARCIS (Netherlands)

    Letarte, B.; Hill, V.; Jablonka, P.; Tolstoy, E.; Randich, S; Pasquini, L

    2006-01-01

    We present high resolution abundance analysis of nine stars belonging to three of the five globular clusters (GCs) of the Fornax dwarf galaxy. The spectra were taken with UVES at a resolution of 43 000. We find them to be slightly more metal-poor than what was previously calculated with other

  20. Habitability of planets around red dwarf stars.

    Science.gov (United States)

    Heath, M J; Doyle, L R; Joshi, M M; Haberle, R M

    1999-08-01

    Recent models indicate that relatively moderate climates could exist on Earth-sized planets in synchronous rotation around red dwarf stars. Investigation of the global water cycle, availability of photosynthetically active radiation in red dwarf sunlight, and the biological implications of stellar flares, which can be frequent for red dwarfs, suggests that higher plant habitability of red dwarf planets may be possible.

  1. CARBON-ENHANCED METAL-POOR STARS IN THE INNER AND OUTER HALO COMPONENTS OF THE MILKY WAY

    International Nuclear Information System (INIS)

    Carollo, Daniela; Norris, John E.; Freeman, Ken C.; Beers, Timothy C.; Lee, Young Sun; Kennedy, Catherine R.; Bovy, Jo; Sivarani, Thirupathi; Aoki, Wako

    2012-01-01

    Carbon-enhanced metal-poor (CEMP) stars in the halo components of the Milky Way are explored, based on accurate determinations of the carbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities for over 30,000 calibration stars from the Sloan Digital Sky Survey. Using our present criterion that low-metallicity stars exhibiting [C/Fe] ratios ( c arbonicity ) in excess of [C/Fe] =+0.7 are considered CEMP stars, the global frequency of CEMP stars in the halo system for [Fe/H] 5 kpc, the CarDF exhibits a strong tail toward high values, up to [C/Fe] > +3.0. We also find a clear increase in the CEMP frequency with |Z|. For stars with –2.0 < [Fe/H] <–1.5, the frequency grows from 5% at |Z| ∼2 kpc to 10% at |Z| ∼10 kpc. For stars with [Fe/H] <–2.0, the frequency grows from 8% at |Z| ∼2 kpc to 25% at |Z| ∼10 kpc. For stars with –2.0 < [Fe/H] <–1.5, the mean carbonicity is ([C/Fe]) ∼+1.0 for 0 kpc < |Z| < 10 kpc, with little dependence on |Z|; for [Fe/H] <–2.0, ([C/Fe]) ∼+1.5, again roughly independent of |Z|. Based on a statistical separation of the halo components in velocity space, we find evidence for a significant contrast in the frequency of CEMP stars between the inner- and outer-halo components—the outer halo possesses roughly twice the fraction of CEMP stars as the inner halo. The carbonicity distribution also differs between the inner-halo and outer-halo components—the inner halo has a greater portion of stars with modest carbon enhancement ([C/Fe] ∼+0.5]); the outer halo has a greater portion of stars with large enhancements ([C/Fe] ∼+2.0), although considerable overlap still exists. We interpret these results as due to the possible presence of additional astrophysical sources of carbon production associated with outer-halo stars, beyond the asymptotic giant-branch source that may dominate for inner-halo stars, with implications for the progenitors of these populations.

  2. TOPoS. IV. Chemical abundances from high-resolution observations of seven extremely metal-poor stars

    Science.gov (United States)

    Bonifacio, P.; Caffau, E.; Spite, M.; Spite, F.; Sbordone, L.; Monaco, L.; François, P.; Plez, B.; Molaro, P.; Gallagher, A. J.; Cayrel, R.; Christlieb, N.; Klessen, R. S.; Koch, A.; Ludwig, H.-G.; Steffen, M.; Zaggia, S.; Abate, C.

    2018-04-01

    Context. Extremely metal-poor (EMP) stars provide us with indirect information on the first generations of massive stars. The TOPoS survey has been designed to increase the census of these stars and to provide a chemical inventory that is as detailed as possible. Aims: Seven of the most iron-poor stars have been observed with the UVES spectrograph at the ESO VLT Kueyen 8.2 m telescope to refine their chemical composition. Methods: We analysed the spectra based on 1D LTE model atmospheres, but also used 3D hydrodynamical simulations of stellar atmospheres. Results: We measured carbon in six of the seven stars: all are carbon-enhanced and belong to the low-carbon band, defined in the TOPoS II paper. We measured lithium (A(Li) = 1.9) in the most iron-poor star (SDSS J1035+0641, [Fe/H] measure Li in three stars at [Fe/H] -4.0, two of which lie on the Spite plateau. We confirm that SDSS J1349+1407 is extremely rich in Mg, but not in Ca. It is also very rich in Na. Several of our stars are characterised by low α-to-iron ratios. Conclusions: The lack of high-carbon band stars at low metallicity can be understood in terms of evolutionary timescales of binary systems. The detection of Li in SDSS J1035+0641 places a strong constraint on theories that aim at solving the cosmological lithium problem. The Li abundance of the two warmer stars at [Fe/H] -4.0 places them on the Spite plateau, while the third, cooler star, lies below. We argue that this suggests that the temperature at which Li depletion begins increases with decreasing [Fe/H]. SDSS J1349+1407 may belong to a class of Mg-rich EMP stars. We cannot assess if there is a scatter in α-to-iron ratios among the EMP stars or if there are several discrete populations. However, the existence of stars with low α-to-iron ratios is supported by our observations. Based on observations obtained at ESO Paranal Observatory, Programmes 189.D-0165,090.D-0306, 093.D-0136, and 096.D-0468.

  3. Spectroscopic Comparison of Metal-rich RRab Stars of the Galactic Field with their Metal-poor Counterparts

    Energy Technology Data Exchange (ETDEWEB)

    Chadid, Merieme [Université Nice Sophia–Antipolis, Observatoire de la Côte dAzur, UMR 7293, Parc Valrose, F-06108, Nice Cedex 02 (France); Sneden, Christopher [Department of Astronomy and McDonald Observatory, The University of Texas, Austin, TX 78712 (United States); Preston, George W., E-mail: chadid@unice.fr, E-mail: chris@verdi.as.utexas.edu, E-mail: gwp@obs.carnegiescience.edu [Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)

    2017-02-01

    We investigate atmospheric properties of 35 stable RRab stars that possess the full ranges of period, light amplitude, and metal abundance found in Galactic RR Lyrae stars. Our results are derived from several thousand echelle spectra obtained over several years with the du Pont telescope of Las Campanas Observatory. Radial velocities of metal lines and the H α line were used to construct curves of radial velocity versus pulsation phase. From these we estimated radial velocity amplitudes for metal lines (formed near the photosphere) and H α Doppler cores (formed at small optical depths). We also measured H α emission fluxes when they appear during primary light rises. Spectra shifted to rest wavelengths, binned into small phase intervals, and co-added were used to perform model atmospheric and abundance analyses. The derived metallicities and those of some previous spectroscopic surveys were combined to produce a new calibration of the Layden abundance scale. We then divided our RRab sample into metal-rich (disk) and metal-poor (halo) groups at [Fe/H] = −1.0; the atmospheres of RRab families, so defined, differ with respect to (a) peak strength of H α emission flux, (b) H α radial velocity amplitude, (c) dynamical gravity, (d) stellar radius variation, (e) secondary acceleration during the photometric bump that precedes minimum light, and (f) duration of H α line-doubling. We also detected H α line-doubling during the “bump” in the metal-poor family, but not in the metal-rich one. Although all RRab probably are core helium-burning horizontal branch stars, the metal-rich group appears to be a species sui generis.

  4. From strange stars to strange dwarfs

    International Nuclear Information System (INIS)

    Glendenning, N.K.; Kettner, C.; Weber, F.

    1995-01-01

    We determine all possible equilibrium sequences of compact strange-matter stars with nuclear crusts, which range from massive strange stars to strange white dwarf endash like objects (strange dwarfs). The properties of such stars are compared with those of their nonstrange counterparts emdash neutron stars and ordinary white dwarfs. The main emphasis of this paper is on strange dwarfs, which we divide into two distinct categories. The first one consists of a core of strange matter enveloped within ordinary white dwarf matter. Such stars are hydrostatically stable with or without the strange core and are therefore referred to as open-quote open-quote trivial close-quote close-quote strange dwarfs. This is different for the second category which forms an entirely new class of dwarf stars that contain nuclear material up to 4x10 4 times denser than in ordinary white dwarfs of average mass, M∼0.6 M circle-dot , and still about 400 times denser than in the densest white dwarfs. The entire family of such dwarfs, denoted dense strange dwarfs, owes its hydrostatic stability to the strange core. A striking features of strange dwarfs is that the entire sequence from the maximum-mass strange star to the maximum-mass strange dwarf is stable to radial oscillations. The minimum-mass star is only conditionally stable, and the sequences on both sides are stable. Such a stable, continuous connection does not exist between ordinary white dwarfs and neutron stars, which are known to be separated by a broad range of unstable stars. We find an expansive range of very low mass (planetary-like) strange-matter stars (masses even below 10 -4 M circle-dot are possible) that arise as natural dark-matter candidates, which if abundant enough in our Galaxy, should be seen in the gravitational microlensing searches that are presently being performed. copyright 1995 The American Astronomical Society

  5. VizieR Online Data Catalog: Carbon-enhanced metal-poor stars sample (Caffau+, 2018)

    Science.gov (United States)

    Caffau, E.; Gallagher, A. J.; Bonifacio, P.; Spite, M.; Duffau, S.; Spite, F.; Monaco, L.; Sbordone, L.

    2018-06-01

    We selected a sample of turn-off stars from the Sloan Digital Sky Survey (SDSS York et al. 2000AJ....120.1579Y; Yanny et al. 2009, Cat. J/AJ/137/4377) that were bright enough (gGMOS spectra were acquired in service mode on the nights of 21/07/2017 and 25/07/2017. Table 1 lists the stars we examined here, along with their coordinates, g-mag, and metallicities derived from Fe abundances computed using SDSS and FORS/GMOS spectra. (2 data files).

  6. Star Formation Histories of Nearby Dwarf Galaxies

    OpenAIRE

    Grebel, Eva K.

    2000-01-01

    Properties of nearby dwarf galaxies are briefly discussed. Dwarf galaxies vary widely in their star formation histories, the ages of their subpopulations, and in their enrichment history. Furthermore, many dwarf galaxies show evidence for spatial variations in their star formation history; often in the form of very extended old populations and radial gradients in age and metallicity. Determining factors in dwarf galaxy evolution appear to be both galaxy mass and environment. We may be observi...

  7. VizieR Online Data Catalog: Very metal poor stars in MW halo (Mashonkina+, 2017)

    Science.gov (United States)

    Mashonkina, L.; Jablonka, P.; Sitnova, T.; Pakhomov, Yu; North, P.

    2017-10-01

    Tables 3 and 4 from the article are presented. They include the LTE and NLTE abundances from individual lines and average abundances of the investigated stars in the dSphs Sculptor (Scl), Ursa Minor (UMi), Fornax (Fnx), Sextans (Sex), Bootes I (Boo), UMa II, and Leo IV and the Milky Way (MW) halo. (3 data files).

  8. Massive stars in the Sagittarius Dwarf Irregular Galaxy

    Science.gov (United States)

    Garcia, Miriam

    2018-02-01

    Low metallicity massive stars hold the key to interpret numerous processes in the past Universe including re-ionization, starburst galaxies, high-redshift supernovae, and γ-ray bursts. The Sagittarius Dwarf Irregular Galaxy [SagDIG, 12+log(O/H) = 7.37] represents an important landmark in the quest for analogues accessible with 10-m class telescopes. This Letter presents low-resolution spectroscopy executed with the Gran Telescopio Canarias that confirms that SagDIG hosts massive stars. The observations unveiled three OBA-type stars and one red supergiant candidate. Pending confirmation from high-resolution follow-up studies, these could be the most metal-poor massive stars of the Local Group.

  9. Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies

    OpenAIRE

    François, P.; Monaco, L.; Villanova, S.; Catelan, M.; Bonifacio, P.; Bellazzini, M.; Bidin, C. Moni; Marconi, G.; Geisler, D.; Sbordone, L.

    2012-01-01

    We report preliminary results concerning the detailed chemical composition of metal poor stars belonging to close ultra-faint dwarf galaxies (hereafter UfDSphs). The abundances have been determined thanks to spectra obtained with X-Shooter, a high efficiency spectrograph installed on one of the ESO VLT units. The sample of ultra-faint dwarf spheroidal stars have abundance ratios slightly lower to what is measured in field halo star of the same metallicity.We did not find extreme abundances in...

  10. Oxygen abundances in unevolved metal-poor stars - Interpretation and consequences

    International Nuclear Information System (INIS)

    Abia, C.; Rebolo, R.

    1989-01-01

    The oxygen abundance has been determined by analysis of the O I infrared triplet in 30 unevolved field stars of metallicities in the range Fe/H abundance ratio between -0.2 and -3.5. The data show that the O/Fe abundance ratio increases monotonically as metallicity decreases from solar, reaching values in the range 1.0-1.2 at an Fe/H abundance ratio of about -2. The results, when compared with those already published for metal-deficient red giants, suggest that oxygen could have been depleted in the latter. A discussion of the O/Fe abundance ratios in connection with the chemical evolution of the Galaxy is also presented. 83 refs

  11. Discovery of a Metal-poor, Luminous Post-AGB Star that Failed the Third Dredge-up

    Energy Technology Data Exchange (ETDEWEB)

    Kamath, D.; Winckel, H. Van [Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200D bus 2401, B-3001 Leuven (Belgium); Wood, P. R.; Asplund, M.; Karakas, A. I. [Research School of Astronomy and Astrophysics, Australian National University, Canberra ACT 2611 (Australia); Lattanzio, J. C. [Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800 (Australia)

    2017-02-10

    Post-asymptotic giant branch (post-AGB) stars are known to be chemically diverse. In this paper we present the first observational evidence of a star that has failed the third dredge-up (TDU). J005252.87-722842.9 is an A-type ( T {sub eff} = 8250 ± 250 K) luminous (8200 ± 700 L {sub ⊙}) metal-poor ([Fe/H] = −1.18 ± 0.10) low-mass ( M {sub initial} ≈ 1.5–2.0 M {sub ⊙}) post-AGB star in the Small Magellanic Cloud. Through a systematic abundance study, using high-resolution optical spectra from UVES, we found that this likely post-AGB object shows an intriguing photospheric composition with no confirmed carbon-enhancement (upper limit of [C/Fe] < 0.50) nor enrichment of s -process elements. We derived an oxygen abundance of [O/Fe] = 0.29 ± 0.1. For Fe and O, we took the effects of nonlocal thermodynamic equilibrium into account. We could not derive an upper limit for the nitrogen abundance as there are no useful nitrogen lines within our spectral coverage. The chemical pattern displayed by this object has not been observed in single or binary post-AGBs. Based on its derived stellar parameters and inferred evolutionary state, single-star nucleosynthesis models predict that this star should have undergone TDU episodes while on the AGB, and it should be carbon enriched. However, our observations are in contrast with these predictions. We identify two possible Galactic analogs that are likely to be post-AGB stars, but the lack of accurate distances (hence luminosities) to these objects does not allow us to confirm their post-AGB status. If they have low luminosities, then they are likely to be dusty post-RGB stars. The discovery of J005252.87-722842.9 reveals a new stellar evolutionary channel whereby a star evolves without any TDU episodes.

  12. White Dwarfs, Neutron Stars and Black Holes

    Science.gov (United States)

    Szekeres, P.

    1977-01-01

    The three possible fates of burned-out stars: white dwarfs, neutron stars and black holes, are described in elementary terms. Characteristics of these celestial bodies, as provided by Einstein's work, are described. (CP)

  13. Physics of white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Koester, D.; Chanmugam, G. (Louisiana State Univ., New Orleans, LA (USA))

    1990-07-01

    White dwarf stars, compact objects with extremely high interior densities, are the most common end product in the evolution of stars. In this paper we review the history of their discovery, and of the realisation that their structure is determined by the physics of the degenerate electron gas. Spectral types and surface chemical composition show a complicated pattern dominated by diffusion processes and their interaction with accretion, convection and mass loss. While this interaction is not completely understood in all its detail at present, the study may ultimately lead to important constraints on the theory of stellar evolution in general. Variability, caused by non-radial oscillations of the star, is a common phenomenon and is shown to be a powerful probe of the structure of deeper layers that are not directly accessible to observation. Very strong magnetic fields detected in a small fraction of white dwarfs offer a unique opportunity to study the behaviour of atoms under conditions that cannot be simulated in terrestrial laboratories. (author).

  14. Rare White dwarf stars with carbon atmospheres

    OpenAIRE

    Dufour, P.; Liebert, James; Fontaine, G.; Behara, N.

    2007-01-01

    White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 msun and 8-10 msun, where msun is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for ~80 per cent of known white dwarfs, by an additional hydrogen layer. All white dwarfs...

  15. Stars at Low Metallicity in Dwarf Galaxies

    NARCIS (Netherlands)

    Tolstoy, Eline; Battaglia, Giuseppina; Cole, Andrew; Hunt, LK; Madden, S; Schneider, R

    2008-01-01

    Dwarf galaxies offer an opportunity to understand the properties of low metallicity star formation both today and at the earliest times at the, epoch of the formation of the first stars. Here we concentrate on two galaxies in the Local Group: the dwarf irregular galaxy Leo A, which has been the

  16. METAL-POOR STARS OBSERVED WITH THE MAGELLAN TELESCOPE. I. CONSTRAINTS ON PROGENITOR MASS AND METALLICITY OF AGB STARS UNDERGOING s-PROCESS NUCLEOSYNTHESIS

    Energy Technology Data Exchange (ETDEWEB)

    Placco, Vinicius M.; Rossi, Silvia [Departamento de Astronomia-Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Universidade de Sao Paulo, Sao Paulo, SP 05508-900 (Brazil); Frebel, Anna [Massachusetts Institute of Technology and Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139 (United States); Beers, Timothy C. [National Optical Astronomy Observatory, Tucson, AZ 85719 (United States); Karakas, Amanda I.; Kennedy, Catherine R. [Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston, ACT 2611 (Australia); Christlieb, Norbert [Zentrum fuer Astronomie der Universitaet Heidelberg, Landessternwarte, Koenigstuhl 12, D-69117 Heidelberg (Germany); Stancliffe, Richard J. [Argelander-Institut fuer Astronomie der Universitaet Bonn, Auf dem Huegel 71, D-53121 Bonn (Germany)

    2013-06-20

    We present a comprehensive abundance analysis of two newly discovered carbon-enhanced metal-poor (CEMP) stars. HE 2138-3336 is a s-process-rich star with [Fe/H] = -2.79, and has the highest [Pb/Fe] abundance ratio measured thus far, if non-local thermodynamic equilibrium corrections are included ([Pb/Fe] = +3.84). HE 2258-6358, with [Fe/H] = -2.67, exhibits enrichments in both s- and r-process elements. These stars were selected from a sample of candidate metal-poor stars from the Hamburg/ESO objective-prism survey, and followed up with medium-resolution (R {approx} 2000) spectroscopy with GEMINI/GMOS. We report here on derived abundances (or limits) for a total of 34 elements in each star, based on high-resolution (R {approx} 30, 000) spectroscopy obtained with Magellan-Clay/MIKE. Our results are compared to predictions from new theoretical asymptotic giant branch (AGB) nucleosynthesis models of 1.3 M{sub Sun} with [Fe/H] = -2.5 and -2.8, as well as to a set of AGB models of 1.0 to 6.0 M{sub Sun} at [Fe/H] = -2.3. The agreement with the model predictions suggests that the neutron-capture material in HE 2138-3336 originated from mass transfer from a binary companion star that previously went through the AGB phase, whereas for HE 2258-6358, an additional process has to be taken into account to explain its abundance pattern. We find that a narrow range of progenitor masses (1.0 {<=} M(M{sub Sun }) {<=} 1.3) and metallicities (-2.8 {<=} [Fe/H] {<=}-2.5) yield the best agreement with our observed elemental abundance patterns.

  17. Evidence for a vanishing 6Li/7Li isotopic signature in the metal-poor halo star HD84937

    DEFF Research Database (Denmark)

    Lind, K.; Asplund, M.; Collet, Remo

    2012-01-01

    The claimed detections of 6Li in the atmospheres of some metal-poor halo stars have lead to speculative additions to the standard model of Big Bang nucleosynthesis and the early Universe, as the inferred abundances cannot be explained by Galactic cosmic ray production. A prominent example of a so...

  18. An overview of white dwarf stars

    OpenAIRE

    Fontaine, Gilles; Brassard, Pierre; Charpinet, Stéphane; Randall, Suzanna K.; Van Grootel, Valérie

    2013-01-01

    We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of stellar evolution for the vast majority of stars and, as such, bear the signatures of past events (such as mass loss, mixing phases, loss and redistribution of angular momentum, and thermonuclear burning) that are of essential importance in the evolution of stars in general. In addition, whit...

  19. The Chemical Abundances of Stars in the Halo (CASH) Project. II. A Sample of 14 Extremely Metal-poor Stars

    Science.gov (United States)

    Hollek, Julie K.; Frebel, Anna; Roederer, Ian U.; Sneden, Christopher; Shetrone, Matthew; Beers, Timothy C.; Kang, Sung-ju; Thom, Christopher

    2011-11-01

    We present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the Chemical Abundances of Stars in the Halo (CASH) project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R ~15, 000) and corresponding high-resolution (R ~35, 000) Magellan Inamori Kyocera Echelle spectra. The stars span a metallicity range from [Fe/H] from -2.9 to -3.9, including four new stars with [Fe/H] ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]CASH and are used to calibrate a newly developed, automated stellar parameter and abundance determination pipeline. This code will be used for the entire ~500 star CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum. Based on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen. Based on observations gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  20. Metal-Poor, Strongly Star-Forming Galaxies in the DEEP2 Survey: The Relationship Between Stellar Mass, Temperature-Based Metallicity, and Star Formation Rate

    Science.gov (United States)

    Ly, Chun; Rigby, Jane R.; Cooper, Michael; Yan, Renbin

    2015-01-01

    We report on the discovery of 28 redshift (z) approximately equal to 0.8 metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [O (sub III)] lambda 4363 emission line, which provides a "direct" measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFR with stellar mass estimates from modeling the optical photometry. We find that these galaxies are 1.05 plus or minus 0.61 dex above the redshift (z) approximately 1 stellar mass-SFR relation and 0.23 plus or minus 0.23 dex below the local mass-metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 dex, but significant dispersion remains dex with 0.16 dex due to measurement uncertainties). This dispersion suggests that gas accretion, star formation, and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of approximately equal to 100 (sup plus 310) (sub minus 75) million years which suggests stochastic star formation. Combining our sample with other redshift (z) of approximately 1 metal-poor galaxies, we find a weak positive SFR-metallicity dependence (at fixed stellar mass) that is significant at 94.4 percent confidence. We interpret this positive correlation as recent star formation that has enriched the gas but has not had time to drive the metal-enriched gas out with feedback mechanisms.

  1. A window on first-stars models from studies of dwarf galaxies and galactic halo stars

    Science.gov (United States)

    Venkatesan, Aparna

    2018-06-01

    Dwarf galaxies dominate the local universe by number and are predicted to be even more dominant at early times, with many having large star formation rates per unit mass. The cosmological role of dwarf galaxies in the metal enrichment and the reionization of the universe is an important but unresolved problem at present. Nearby low-mass galaxies are much more accessible observationally for detailed study and may be local analogs of the types of galaxies that hosted the first-light sources relevant for reionization. I will share recent results on UV studies of the escaping radiation from nearby low-mass starforming galaxies, as well as the tantalizing similarities in element abundance patterns between local dwarf galaxies and the latest data compilations on extremely metal-poor stars in galactic halos. I will highlight trends of interest in a variety of individual elements at values of [Fe/H] between -7 and -3, including alpha-elements, elements originating mostly in intermediate-mass stars, lithium, titanium, and r-process elements. These trends constrain not only models of the first stars and their supernovae, but provide a window into the physical conditions in early galaxies and when metal-free star formation may have ceased in the early universe.This work was supported by the University of San Francisco Faculty Development Fund, and NSF grant AST-1637339. We thank the Aspen Center for Physics, where some of this work was conducted, and which is supported by National Science Foundation grant PHY-1607611.

  2. Photometry of Southern Hemisphere red dwarf stars

    Science.gov (United States)

    Weistrop, D.

    1980-01-01

    Results are presented for a photometric investigation of a spectroscopically selected sample of red dwarf stars in the Southern Hemisphere. Absolute magnitudes and distances for the stars are estimated from broadband red colors. Three stars which may be subluminous are identified, as are several stars which may be within 25 pc. The tangential velocity and velocity dispersion of the sample are similar to values found in other studies of nearby late-type stars.

  3. Metal-poor dwarf galaxies in the SIGRID galaxy sample. I. H II region observations and chemical abundances

    International Nuclear Information System (INIS)

    Nicholls, David C.; Dopita, Michael A.; Sutherland, Ralph S.; Jerjen, Helmut; Kewley, Lisa J.; Basurah, Hassan

    2014-01-01

    In this paper we present the results of observations of 17 H II regions in thirteen galaxies from the SIGRID sample of isolated gas-rich irregular dwarf galaxies. The spectra of all but one of the galaxies exhibit the auroral [O III] 4363 Å line, from which we calculate the electron temperature, T e , and gas-phase oxygen abundance. Five of the objects are blue compact dwarf galaxies, of which four have not previously been analyzed spectroscopically. We include one unusual galaxy which exhibits no evidence of the [N II] λλ 6548,6584 Å lines, suggesting a particularly low metallicity (< Z ☉ /30). We compare the electron temperature based abundances with those derived using eight of the new strong-line diagnostics presented by Dopita et al. Using a method derived from first principles for calculating total oxygen abundance, we show that the discrepancy between the T e -based and strong-line gas-phase abundances have now been reduced to within ∼0.07 dex. The chemical abundances are consistent with what is expected from the luminosity-metallicity relation. We derive estimates of the electron densities and find them to be between ∼5 and ∼100 cm –3 . We find no evidence for a nitrogen plateau for objects in this sample with metallicities 0.5 > Z ☉ > 0.15.

  4. Metal-poor dwarf galaxies in the SIGRID galaxy sample. I. H II region observations and chemical abundances

    Energy Technology Data Exchange (ETDEWEB)

    Nicholls, David C.; Dopita, Michael A.; Sutherland, Ralph S.; Jerjen, Helmut; Kewley, Lisa J. [Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston ACT 2611 (Australia); Basurah, Hassan, E-mail: David.Nicholls@anu.edu.au [Astronomy Department, King Abdulaziz University, P.O. Box 80203 Jeddah (Saudi Arabia)

    2014-05-10

    In this paper we present the results of observations of 17 H II regions in thirteen galaxies from the SIGRID sample of isolated gas-rich irregular dwarf galaxies. The spectra of all but one of the galaxies exhibit the auroral [O III] 4363 Å line, from which we calculate the electron temperature, T{sub e} , and gas-phase oxygen abundance. Five of the objects are blue compact dwarf galaxies, of which four have not previously been analyzed spectroscopically. We include one unusual galaxy which exhibits no evidence of the [N II] λλ 6548,6584 Å lines, suggesting a particularly low metallicity (< Z {sub ☉}/30). We compare the electron temperature based abundances with those derived using eight of the new strong-line diagnostics presented by Dopita et al. Using a method derived from first principles for calculating total oxygen abundance, we show that the discrepancy between the T{sub e} -based and strong-line gas-phase abundances have now been reduced to within ∼0.07 dex. The chemical abundances are consistent with what is expected from the luminosity-metallicity relation. We derive estimates of the electron densities and find them to be between ∼5 and ∼100 cm{sup –3}. We find no evidence for a nitrogen plateau for objects in this sample with metallicities 0.5 > Z {sub ☉} > 0.15.

  5. THE EATING HABITS OF MILKY WAY-MASS HALOS: DESTROYED DWARF SATELLITES AND THE METALLICITY DISTRIBUTION OF ACCRETED STARS

    International Nuclear Information System (INIS)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-01-01

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M vir  ∼ 10 12.1 M ⊙ ) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M star  ∼ 10 8 –10 10 M ⊙ . Halos with more quiescent accretion histories tend to have lower mass progenitors (10 8 –10 9 M ⊙ ), and lower overall accreted stellar masses. Ultra-faint mass (M star  < 10 5 M ⊙ ) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%–5%) of the very metal-poor stars with [Fe/H] < −2. Dwarfs with masses 10 5  < M star /M ⊙  < 10 8 provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with M star  > 10 8 M ⊙ can contribute a considerable fraction (∼20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil”; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo

  6. VLT/UVES spectroscopy of individual stars in three globular clusters in the Fornax dwarf spheroidal galaxy

    NARCIS (Netherlands)

    Letarte, B; Hill, [No Value; Jablonka, P; Tolstoy, E; Francois, P; Meylan, G

    We present a high resolution ( R similar to 43 000) abundance analysis of a total of nine stars in three of the five globular clusters associated with the nearby Fornax dwarf spheroidal galaxy. These three clusters ( 1, 2 and 3) trace the oldest, most metal-poor stellar populations in Fornax. We

  7. White Dwarf Stars as Polytropic Gas Spheres

    OpenAIRE

    Nouh, M. I.; Saad, A. S.; Elkhateeb, M. M.; Korany, B.

    2014-01-01

    Due to the highly degeneracy of electrons in white dwarf stars, we expect that the relativistic effects play very important role in these stars. In the present article, we study the properties of the condensed matter in white dwarfs using Newtonian and relativistic polytropic fluid sphere. Two polytropic indices (namely n=3 and n=1.5) are proposed to investigate the physical characteristics of the models. We solve the Lane-Emden equations numerically.. The results show that the relativistic e...

  8. New Fe i Level Energies and Line Identifications from Stellar Spectra. II. Initial Results from New Ultraviolet Spectra of Metal-poor Stars

    Energy Technology Data Exchange (ETDEWEB)

    Peterson, Ruth C. [SETI Institute and Astrophysical Advances, 607 Marion Place, Palo Alto, CA 94301 (United States); Kurucz, Robert L. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Ayres, Thomas R., E-mail: peterson@ucolick.org [Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO 80309-0389 (United States)

    2017-04-01

    The Fe i spectrum is critical to many areas of astrophysics, yet many of the high-lying levels remain uncharacterized. To remedy this deficiency, Peterson and Kurucz identified Fe i lines in archival ultraviolet and optical spectra of metal-poor stars, whose warm temperatures favor moderate Fe i excitation. Sixty-five new levels were recovered, with 1500 detectable lines, including several bound levels in the ionization continuum of Fe i. Here, we extend the previous work by identifying 59 additional levels, with 1400 detectable lines, by incorporating new high-resolution UV spectra of warm metal-poor stars recently obtained by the Hubble Space Telescope Imaging Spectrograph. We provide gf values for these transitions, both computed as well as adjusted to fit the stellar spectra. We also expand our spectral calculations to the infrared, confirming three levels by matching high-quality spectra of the Sun and two cool stars in the H -band. The predicted gf values suggest that an additional 3700 Fe i lines should be detectable in existing solar infrared spectra. Extending the empirical line identification work to the infrared would help confirm additional Fe i levels, as would new high-resolution UV spectra of metal-poor turnoff stars below 1900 Å.

  9. General Relativistic Calculations for White Dwarf Stars

    OpenAIRE

    Mathew, Arun; Nandy, Malay K.

    2014-01-01

    The mass-radius relations for white dwarf stars are investigated by solving the Newtonian as well as Tolman-Oppenheimer-Volkoff (TOV) equations for hydrostatic equilibrium assuming the electron gas to be non-interacting. We find that the Newtonian limiting mass of $1.4562M_\\odot$ is modified to $1.4166M_\\odot$ in the general relativistic case for $^4_2$He (and $^{12}_{\\ 6}$C) white dwarf stars. Using the same general relativistic treatment, the critical mass for $^{56}_{26}$Fe white dwarf is ...

  10. Quantum liquid signatures in dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Gabadadze, Gregory; Pirtskhalava, David, E-mail: gg32@nyu.edu, E-mail: dmp371@nyu.edu [Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY 10003 (United States)

    2009-05-15

    We develop further the proposal of arXiv:0806.3692 that a new state of matter - charged condensate of spin-0 nuclei - may exist in helium-core dwarf stars. The charged condensate and its fluctuations are described by an effective field theory Lagrangian. The spectrum of bosonic fluctuations is gapped, while electrons, at temperatures of interest, give rise to gapless excitations near the Fermi surface. These properties determine the evolution of the dwarfs with condensed cores. In particular, we show that such dwarf stars would cool significantly faster than their crystallized counterparts. As a result, the luminosity function for the helium-core dwarfs will have a sharp drop-off after the condensation. It is tempting to interpret the recently discovered abrupt termination of a sequence of 24 helium-core dwarf candidates in NGC 6397 as a signature of the charged condensation.

  11. An overview of white dwarf stars

    Directory of Open Access Journals (Sweden)

    Charpinet S.

    2013-03-01

    Full Text Available We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of stellar evolution for the vast majority of stars and, as such, bear the signatures of past events (such as mass loss, mixing phases, loss and redistribution of angular momentum, and thermonuclear burning that are of essential importance in the evolution of stars in general. In addition, white dwarf stars represent ideal testbeds for our understanding of matter under extreme conditions, and work on their constitutive physics (neutrino production rates, conductive and radiative opacities, interior liquid/solid equations of state, partially ionized and partially degenerate envelope equations of state, diffusion coefficients, line broadening mechanisms is still being actively pursued. Given a set of constitutive physics, cooling white dwarfs can be used advantageously as cosmochronometers. Moreover, the field has been blessed by the existence of four distinct families of pulsating white dwarfs, each mapping a different evolutionary phase, and this allows the application of the asteroseismological method to probe and test their internal structure and evolutionary state. We set the stage for the reviews that follow on cooling white dwarfs as cosmochronometers and physics laboratories, as well as on the properties of pulsating white dwarfs and the asteroseismological results that can be inferred.

  12. Carbon-enhanced metal-poor stars in SDSS/Segue. II. Comparison of CEMP-star frequencies with binary population-synthesis models

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Young Sun [Department of Astronomy, New Mexico State University, Las Cruces, NM 88003 (United States); Suda, Takuma [National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588 (Japan); Beers, Timothy C. [National Optical Astronomy Observatory, Tucson, AZ 85719 (United States); Stancliffe, Richard J., E-mail: yslee@nmsu.edu [Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn (Germany)

    2014-06-20

    We present a comparison of the frequencies of carbon-enhanced metal-poor (CEMP) giant and main-sequence turnoff (MSTO) stars with predictions from binary population-synthesis models involving asymptotic giant-branch (AGB) mass transfer. The giant and MSTO stars are selected from the Sloan Digital Sky Survey and the Sloan Extension for Galactic Understanding and Exploration. We consider two initial mass functions (IMFs)—a Salpeter IMF, and a mass function with a characteristic mass of 10 M {sub ☉}. For giant stars, the comparison indicates a good agreement between the observed CEMP frequencies and the AGB binary model using a Salpeter IMF for [Fe/H] > – 1.5, and a characteristic mass of 10 M {sub ☉} for [Fe/H] < – 2.5. This result suggests that the IMF shifted from high- to low-mass dominated in the early history of the Milky Way, which appears to have occurred at a 'chemical time' between [Fe/H] =–2.5 and [Fe/H] =–1.5. The CEMP frequency for the turnoff stars with [Fe/H] < – 3.0 is much higher than the AGB model prediction from the high-mass IMF, supporting the previous assertion that one or more additional mechanisms, not associated with AGB stars, are required for the production of carbon-rich material below [Fe/H] =–3.0. We also discuss possible effects of first dredge-up and extra mixing in red giants and internal mixing in turnoff stars on the derived CEMP frequencies.

  13. EXTREMELY METAL-POOR GALAXIES: THE ENVIRONMENT

    Energy Technology Data Exchange (ETDEWEB)

    Filho, M. E. [Universidad de Las Palmas de Gran Canaria–Universidad de La Laguna, CIE Canarias: Tri-Continental Atlantic Campus, Canary Islands (Spain); Almeida, J. Sánchez; Muñoz-Tuñón, C. [Instituto Astrofísica de Canarias, E-38200 La Laguna, Tenerife (Spain); Nuza, S. E.; Kitaura, F.; Heß, S., E-mail: mfilho@astro.up.pt [Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam (Germany)

    2015-04-01

    We have analyzed bibliographical observational data and theoretical predictions, in order to probe the environment in which extremely metal-poor dwarf galaxies (XMPs) reside. We have assessed the H i component and its relation to the optical galaxy, the cosmic web type (voids, sheets, filaments and knots), the overdensity parameter and analyzed the nearest galaxy neighbors. The aim is to understand the role of interactions and cosmological accretion flows in the XMP observational properties, particularly the triggering and feeding of the star formation. We find that XMPs behave similarly to Blue Compact Dwarfs; they preferably populate low-density environments in the local universe: ∼60% occupy underdense regions, and ∼75% reside in voids and sheets. This is more extreme than the distribution of irregular galaxies, and in contrast to those regions preferred by elliptical galaxies (knots and filaments). We further find results consistent with previous observations; while the environment does determine the fraction of a certain galaxy type, it does not determine the overall observational properties. With the exception of five documented cases (four sources with companions and one recent merger), XMPs do not generally show signatures of major mergers and interactions; we find only one XMP with a companion galaxy within a distance of 100 kpc, and the H i gas in XMPs is typically well-behaved, demonstrating asymmetries mostly in the outskirts. We conclude that metal-poor accretion flows may be driving the XMP evolution. Such cosmological accretion could explain all the major XMP observational properties: isolation, lack of interaction/merger signatures, asymmetric optical morphology, large amounts of unsettled, metal-poor H i gas, metallicity inhomogeneities, and large specific star formation.

  14. Gaia Confirms that SDSS J102915+172927 is a Dwarf Star

    Science.gov (United States)

    Bonifacio, P.; Caffau, E.; Spite, M.; Spite, F.; François, P.; Zaggia, S.; Arenou, F.; Haigron, R.; Leclerc, N.; Marchal, O.; Panuzzo, P.; Plum, G.; Sartoretti, P.

    2018-05-01

    The Gaia Data Release 2 provides a parallax of 0.734+/-0.073 mas for SDSS J102915+172927, currently the most metal-poor known object. This parallax implies that it is dwarf star, ruling out the scenario that it is a subgiant. The subgiant scenario had as a corollary that the star had been formed in a medium highly enriched in C, thus making line cooling efficient during the collapse, that was also highly enriched in Fe by Type Ia SNe. This scenario can also now be ruled out for this star, reinforcing the need of dust cooling and fragmentation to explain its formation.

  15. New Light on Dark Stars Red Dwarfs, Low-Mass Stars, Brown Dwarfs

    CERN Document Server

    Reid, I. Neill

    2005-01-01

    There has been very considerable progress in research into low-mass stars, brown dwarfs and extrasolar planets during the past few years, particularly since the fist edtion of this book was published in 2000. In this new edtion the authors present a comprehensive review of both the astrophysical nature of individual red dwarf and brown dwarf stars and their collective statistical properties as an important Galactic stellar population. Chapters dealing with the observational properies of low-mass dwarfs, the stellar mass function and extrasolar planets have been completely revised. Other chapters have been significantly revised and updated as appropriate, including important new material on observational techniques, stellar acivity, the Galactic halo and field star surveys. The authors detail the many discoveries of new brown dwarfs and extrasolar planets made since publication of the first edition of the book and provide a state-of-the-art review of our current knowledge of very low-mass stars, brown dwarfs a...

  16. OBSERVATIONS OF BINARY STARS WITH THE DIFFERENTIAL SPECKLE SURVEY INSTRUMENT. V. TOWARD AN EMPIRICAL METAL-POOR MASS–LUMINOSITY RELATION

    International Nuclear Information System (INIS)

    Horch, Elliott P.; Van Altena, William F.; Demarque, Pierre; Howell, Steve B.; Everett, Mark E.; Ciardi, David R.; Teske, Johanna K.; Henry, Todd J.; Winters, Jennifer G.

    2015-01-01

    In an effort to better understand the details of the stellar structure and evolution of metal-poor stars, the Gemini North telescope was used on two occasions to take speckle imaging data of a sample of known spectroscopic binary stars and other nearby stars in order to search for and resolve close companions. The observations were obtained using the Differential Speckle Survey Instrument, which takes data in two filters simultaneously. The results presented here are of 90 observations of 23 systems in which one or more companions was detected, and six stars where no companion was detected to the limit of the camera capabilities at Gemini. In the case of the binary and multiple stars, these results are then further analyzed to make first orbit determinations in five cases, and orbit refinements in four other cases. The mass information is derived, and since the systems span a range in metallicity, a study is presented that compares our results with the expected trend in total mass as derived from the most recent Yale isochrones as a function of metal abundance. These data suggest that metal-poor main-sequence stars are less massive at a given color than their solar-metallicity analogues in a manner consistent with that predicted from the theory

  17. White dwarf stars with carbon atmospheres.

    Science.gov (United States)

    Dufour, P; Liebert, J; Fontaine, G; Behara, N

    2007-11-22

    White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 and 8-10, where is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for approximately 80 per cent of known white dwarfs, by an additional hydrogen layer. All white dwarfs therefore have been traditionally found to belong to one of two categories: those with a hydrogen-rich atmosphere (the DA spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we report the discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution, although these objects might be the cooler counterpart of the unique and extensively studied PG 1159 star H1504+65 (refs 4-7). These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follows the asymptotic giant branch.

  18. Pulsations in white dwarf stars

    OpenAIRE

    Van Grootel, Valérie; Fontaine, Gilles; Brassard, Pierre; Dupret, Marc-Antoine

    2017-01-01

    I will present a description of the six distinct families of pulsating white dwarfs that are currently known. Pulsations are present at various stages of the evolution (from hot, pre-white dwarfs to cool white dwarfs), at various stellar masses, and for various atmospheric compositions. In all of them, a mechanism linked to opacity changes along the evolution drives the oscillations. The existence of these oscillations offers the opportunity to apply asteroseismology for constraining physics ...

  19. Abundance analysis of SDSS J134338.67+484426.6; an extremely metal-poor star from the MARVELS pre-survey

    Science.gov (United States)

    Susmitha Rani, A.; Sivarani, T.; Beers, T. C.; Fleming, S.; Mahadevan, S.; Ge, J.

    2016-05-01

    We present an elemental-abundance analysis of an extremely metal-poor (EMP; [Fe/H] <-3.0) star, SDSS J134338.67+484426.6, identified during the course of the Multi-object Apache Point Observatory Radial Velocity Exoplanet Large-area Survey spectroscopic pre-survey of some 20 000 stars to identify suitable candidates for exoplanet searches. This star, with an apparent magnitude V = 12.14, is the lowest metallicity star found in the pre-survey, and is one of only ˜20 known EMP stars that are this bright or brighter. Our high-resolution spectroscopic analysis shows that this star is a subgiant with [Fe/H] = -3.42, having `normal' carbon and no enhancement of neutron-capture abundances. Strontium is underabundant, [Sr/Fe] = -0.47, but the derived lower limit on [Sr/Ba] indicates that Sr is likely enhanced relative to Ba. This star belongs to the sparsely populated class of α-poor EMP stars that exhibit low ratios of [Mg/Fe], [Si/Fe], and [Ca/Fe] compared to typical halo stars at similar metallicity. The observed variations in radial velocity from several epochs of (low- and high-resolution) spectroscopic follow-up indicate that SDSS J134338.67+484426.6 is a possible long-period binary. We also discuss the abundance trends in EMP stars for r-process elements, and compare with other magnesium-poor stars.

  20. White dwarfs: connection with masses of the parent stars

    International Nuclear Information System (INIS)

    Amnuel', P.R.; Guseinov, O.Kh.; Novruzova, Kh.I.; Rustamov, Yu.S.

    1988-01-01

    A relationship between the mass of a white dwarf and the mass of the parent star on the main sequence is established. The white dwarf birth-rate matches the birth-rate (death-rate) of main sequence stars

  1. Flaring red dwarf stars: news from Crimea

    International Nuclear Information System (INIS)

    Gershberg, Roald E

    1998-01-01

    Important phenomena are briefly described which have recently been discovered in the Crimean studies of flaring red dwarf stars believed to be the most common type of variable stars in the Galaxy. These phenomena include (i) long-lived radiation from a blueshifted component in the ionized-helium λ 4686 A emission line in the active state of one such star, (ii) a long-lived absorption component in the stellar flare light curves with a lifetime exceeding that of the conventional flare emission, and (iii) solarcycle-like activity periodicity of the star EV Lac, whose mass is only 0.3 solar masses. In theoretical terms, a red dwarf star spot model is constructed which, in contrast to the commonly accepted model, agrees well with the solar spot picture. (physics of our days)

  2. Flaring red dwarf stars: news from Crimea

    Energy Technology Data Exchange (ETDEWEB)

    Gershberg, Roald E [Crimean Astrophysical Observatory, Nauchnyi, Crimea (Ukraine)

    1998-08-31

    Important phenomena are briefly described which have recently been discovered in the Crimean studies of flaring red dwarf stars believed to be the most common type of variable stars in the Galaxy. These phenomena include (i) long-lived radiation from a blueshifted component in the ionized-helium {lambda} 4686 A emission line in the active state of one such star, (ii) a long-lived absorption component in the stellar flare light curves with a lifetime exceeding that of the conventional flare emission, and (iii) solarcycle-like activity periodicity of the star EV Lac, whose mass is only 0.3 solar masses. In theoretical terms, a red dwarf star spot model is constructed which, in contrast to the commonly accepted model, agrees well with the solar spot picture. (physics of our days)

  3. Stellar model chromospheres. XIII - M dwarf stars

    Science.gov (United States)

    Giampapa, M. S.; Worden, S. P.; Linsky, J. L.

    1982-01-01

    Single-component, homogeneous model chromospheres that are consistent with high-resolution profiles of the Ca II K line calibrated in surface flux units for three dMe and 2 dM stars observed at quiescent times are constructed. The models reveal several systematic trends. Large values of the ratio of T(min) to T(eff) are derived, indicating a large amount of nonradiative heating present in the upper photospheres of M dwarf stars. It is also found that the lower chromospheric temperature gradient is similar for all the M dwarf stars. Since for the models here the chromospheric K line emission strength is most sensitive to the total amount of chromospheric material present within the approximate temperature range T(min)-6000 K, increasing the emission strength is not simply due to increasing chromospheric temperature gradients. It is also found that both the electron density and electron temperature at one thermalization length in the K line below the top of the chromospheres are greater in the dMe stars than in the dM stars. The M dwarf models here have microturbulent velocities between 1 and 2 km/sec, which are much smaller than for solar chromosphere models.

  4. Accreting neutron stars, black holes, and degenerate dwarf stars.

    Science.gov (United States)

    Pines, D

    1980-02-08

    During the past 8 years, extended temporal and broadband spectroscopic studies carried out by x-ray astronomical satellites have led to the identification of specific compact x-ray sources as accreting neutron stars, black holes, and degenerate dwarf stars in close binary systems. Such sources provide a unique opportunity to study matter under extreme conditions not accessible in the terrestrial laboratory. Quantitative theoretical models have been developed which demonstrate that detailed studies of these sources will lead to a greatly increased understanding of dense and superdense hadron matter, hadron superfluidity, high-temperature plasma in superstrong magnetic fields, and physical processes in strong gravitational fields. Through a combination of theory and observation such studies will make possible the determination of the mass, radius, magnetic field, and structure of neutron stars and degenerate dwarf stars and the identification of further candidate black holes, and will contribute appreciably to our understanding of the physics of accretion by compact astronomical objects.

  5. Gas, Stars, and Star Formation in Alfalfa Dwarf Galaxies

    Science.gov (United States)

    Huang, Shan; Haynes, Martha P.; Giovanelli, Riccardo; Brinchmann, Jarle; Stierwalt, Sabrina; Neff, Susan G.

    2012-01-01

    We examine the global properties of the stellar and Hi components of 229 low H i mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H i masses ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M* obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M* approximately less than10(exp 8)M(sub 0) is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper Hi mass limit yields the selection of a sample with lower gas fractions for their M* than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H i depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that Hi disks are more extended than stellar ones.

  6. Unlocking the secrets of white dwarf stars

    CERN Document Server

    Van Horn, Hugh M

    2015-01-01

    White dwarfs, each containing about as much mass as our Sun but packed into a volume about the size of Earth, are the endpoints of evolution for most stars. Thousands of these faint objects have now been discovered, though only a century ago only three were known. They are among the most common stars in the Milky Way Galaxy, and they have become important tools in understanding the universe. Yet a century ago only three white dwarfs were known.   The existence of these stars completely baffled the scientists of the day, and solving the mysteries of these strange objects required revolutionary advances in science and technology, including the development of quantum physics, the construction and utilization of large telescopes, the invention of the digital computer, and the ability to make astronomical observations from space.   This book tells the story of the growth in our understanding of white dwarf stars, set within the context of the relevant scientific and technological advances. Part popular science, ...

  7. Charged condensate and helium dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Gabadadze, Gregory; Rosen, Rachel A, E-mail: gg32@nyu.edu, E-mail: rar339@nyu.edu [Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY 10003 (United States)

    2008-10-15

    White dwarf stars composed of carbon, oxygen and heavier elements are expected to crystallize as they cool down below certain temperatures. Yet, simple arguments suggest that the helium white dwarf cores may not solidify, mostly because of zero-point oscillations of the helium ions that would dissolve the crystalline structure. We argue that the interior of the helium dwarfs may instead form a macroscopic quantum state in which the charged helium-4 nuclei are in a Bose-Einstein condensate, while the relativistic electrons form a neutralizing degenerate Fermi liquid. We discuss the electric charge screening, and the spectrum of this substance, showing that the bosonic long-wavelength fluctuations exhibit a mass gap. Hence, there is a suppression at low temperatures of the boson contribution to the specific heat-the latter being dominated by the specific heat of the electrons near the Fermi surface. This state of matter may have observational signatures.

  8. The Diverse Origins of Neutron-capture Elements in the Metal-poor Star HD 94028: Possible Detection of Products of I-Process Nucleosynthesis

    Science.gov (United States)

    Roederer, Ian U.; Karakas, Amanda I.; Pignatari, Marco; Herwig, Falk

    2016-04-01

    We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = -1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = -0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (I process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the I process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the I process may have been common in the early Galaxy. These data are associated with Program 072.B-0585(A), PI. Silva. Some data presented in this paper were obtained from the Barbara A. Mikulski Archive for Space Telescopes (MAST). The Space Telescope Science Institute is

  9. HAT-P-12b: A LOW-DENSITY SUB-SATURN MASS PLANET TRANSITING A METAL-POOR K DWARF

    International Nuclear Information System (INIS)

    Hartman, J. D.; Bakos, G. A.; Torres, G.; Noyes, R. W.; Pal, A.; Latham, D. W.; Sipocz, B.; Esquerdo, G. A.; Sasselov, D. D.; Kovacs, Gabor; Stefanik, R. P.; Fernandez, J. M.; Kovacs, Geza; Fischer, D. A.; Johnson, J. A.; Marcy, G. W.; Howard, A. W.; Butler, R. P.; Lazar, J.; Papp, I.

    2009-01-01

    We report on the discovery of HAT-P-12b, a transiting extrasolar planet orbiting the moderately bright V ∼ 12.8 K4 dwarf GSC 03033 - 00706, with a period P = 3.2130598 ± 0.0000021 d, transit epoch T c = 2454419.19556 ± 0.00020 (BJD), and transit duration 0.0974 ± 0.0006 d. The host star has a mass of 0.73 ± 0.02 M sun , radius of 0.70 +0.02 -0.01 R sun , effective temperature 4650 ± 60 K, and metallicity [Fe/H] = -0.29 ± 0.05. We find a slight correlation between the observed spectral line bisector spans and the radial velocity, so we consider, and rule out, various blend configurations including a blend with a background eclipsing binary, and hierarchical triple systems where the eclipsing body is a star or a planet. We conclude that a model consisting of a single star with a transiting planet best fits the observations, and show that a likely explanation for the apparent correlation is contamination from scattered moonlight. Based on this model, the planetary companion has a mass of 0.211 ± 0.012 M J and radius of 0.959 +0.029 -0.021 R J yielding a mean density of 0.295 ± 0.025 g cm -3 . Comparing these observations with recent theoretical models, we find that HAT-P-12b is consistent with a ∼1-4.5 Gyr, mildly irradiated, H/He-dominated planet with a core mass M C ∼ + . HAT-P-12b is thus the least massive H/He-dominated gas giant planet found to date. This record was previously held by Saturn.

  10. Global and photospheric physical parameters of active dwarf stars

    International Nuclear Information System (INIS)

    Pettersen, B.R.

    1983-01-01

    Physical parameters (temperature, luminosity, radius, mass and chemical abundance) of the photospheres of red dwarf flare stars and spotted stars are determined for quiescent conditions. The interrelations between these quantities are compared to the results of theoretical investigation for low mass stars. The evolutionary state of flare stars is discussed. Observational results from spectroscopic and photometric methods to determine the rotation of active dwarfs are reviewed. The possibilities of global oscillations in dwarf stars are considered and preliminary results of a photometric search for oscillation in red dwarf luminosities are presented. (orig.)

  11. Asteroseismology of DAV White Dwarf Stars

    Energy Technology Data Exchange (ETDEWEB)

    Bradley, Paul A.

    1997-12-31

    The author reviews the seismological structural determinations of ZZ Ceti stars done to date, and supplement these with additional preliminary determinations of his own. He compares the constraints on the hydrogen layer mass to see what trends emerge and also determines if the observed hydrogen layer masses are consistent with proposed theories. He then looks ahead to the prospects of further DAV white dwarf seismology.

  12. Lessons for Asteroseismology from White Dwarf Stars

    OpenAIRE

    Metcalfe, Travis S.

    2005-01-01

    The interpretation of pulsation data for Sun-like stars is currently facing challenges quite similar to those faced by white dwarf modelers ten years ago. The observational requirements for uninterrupted long-term monitoring are beginning to be satisfied by successful multi-site campaigns and dedicated satellite missions. But exploration of the most important physical parameters in theoretical models has been fairly limited, making it difficult to establish a detailed best-fit model for a par...

  13. A systematic search for brown dwarfs orbiting nearby stars

    International Nuclear Information System (INIS)

    Henry, T.J.; Mccarthy, D.W. Jr.

    1990-01-01

    Survey data for brown dwarf and stellar companions relative to known M dwarf stars within 5 pc north of -30 deg are analyzed. A region 0.2 to 5 arcsec in radius around 27 stars at the IR H and K bands are examined using IR speckle interferometry. The frequency of binary versus single M dwarfs in the solar neighborhood is examined. The IR mass-magnitude relations and mass-luminosity-age relation are studied. The data reveal that there are 19 single M dwarfs, 8 M dwarf binaries, 1 M dwarf triple system, and 1 M dwarf in a triple system for M dwarfs within 5 pc north of -30 deg. Also of the 27 M dwarfs studied none was found to have a brown dwarf companion. 64 refs

  14. Pulsations in M dwarf stars

    OpenAIRE

    Rodríguez-López, C.; MacDonald, J.; Moya, A.

    2011-01-01

    We present the results of the first theoretical non-radial non-adiabatic pulsational study of M dwarf stellar models with masses in the range 0.1 to 0.5M_solar. We find the fundamental radial mode to be unstable due to an \\epsilon mechanism caused by deuterium (D-) burning for the young 0.1 and 0.2M_solar models, by non-equilibrium He^3 burning for the 0.2 and 0.25M_solar models of 10^4Myr, and by a flux blocking mechanism for the partially convective 0.4 and 0.5M_solar models once they reach...

  15. Star Formation Histories of Dwarf Irregular Galaxies

    Science.gov (United States)

    Skillman, Evan

    1995-07-01

    We propose to obtain deep WFPC2 `BVI' color-magnitude diagrams {CMDs} for the dwarf irregular {dI} Local Group galaxies GR 8, Leo A, Pegasus, and Sextans A. In addition to resolved stars, we will use star clusters, and especially any globulars, to probe the history of intense star formation. These data will allow us to map the Pop I and Pop II stellar components, and thereby construct the first detailed star formation histories for non-interacting dI galaxies. Our results will bear on a variety of astrophysical problems, including the evolution of small galaxies, distances in the Local Group, age-metallicity distributions in small galaxies, ages of dIs, and the physics of star formation. The four target galaxies are typical dI systems in terms of luminosity, gas content, and H II region abundance, and represent a range in current star forming activity. They are sufficiently near to allow us to reach to stars at M_V = 0, have 0.1 of the luminosity of the SMC and 0.25 of its oxygen abundance. Unlike the SMC, these dIs are not near giant galaxies. This project will allow the extension of our knowledge of stellar populations in star forming galaxies from the spirals in the Local Group down to its smallest members. We plan to take maximum advantage of the unique data which this project will provide. Our investigator team brings extensive and varied experience in studies of dwarf galaxies, stellar populations, imaging photometry, and stellar evolution to this project.

  16. GAS, STARS, AND STAR FORMATION IN ALFALFA DWARF GALAXIES

    Energy Technology Data Exchange (ETDEWEB)

    Huang Shan; Haynes, Martha P.; Giovanelli, Riccardo [Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853 (United States); Brinchmann, Jarle [Sterrewacht Leiden, Leiden University, NL-2300 RA Leiden (Netherlands); Stierwalt, Sabrina [Spitzer Science Center, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 (United States); Neff, Susan G., E-mail: shan@astro.cornell.edu, E-mail: haynes@astro.cornell.edu, E-mail: riccardo@astro.cornell.edu, E-mail: jarle@strw.leidenuniv.nl, E-mail: sabrina@ipac.caltech.edu, E-mail: susan.g.neff@nasa.gov [NASA GSFC, Code 665, Observational Cosmology Lab, Greenbelt, MD 20771 (United States)

    2012-06-15

    We examine the global properties of the stellar and H I components of 229 low H I mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H I masses <10{sup 7.7} M{sub Sun} and H I line widths <80 km s{sup -1}. Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M{sub *}) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M{sub *} obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M{sub *} {approx}< 10{sup 8} M{sub Sun} is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper H I mass limit yields the selection of a sample with lower gas fractions for their M{sub *} than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H I depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that H I disks are more extended than stellar ones.

  17. GAS, STARS, AND STAR FORMATION IN ALFALFA DWARF GALAXIES

    International Nuclear Information System (INIS)

    Huang Shan; Haynes, Martha P.; Giovanelli, Riccardo; Brinchmann, Jarle; Stierwalt, Sabrina; Neff, Susan G.

    2012-01-01

    We examine the global properties of the stellar and H I components of 229 low H I mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H I masses 7.7 M ☉ and H I line widths –1 . Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M * ) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M * obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M * ∼ 8 M ☉ is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper H I mass limit yields the selection of a sample with lower gas fractions for their M * than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H I depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that H I disks are more extended than stellar ones.

  18. THE EATING HABITS OF MILKY WAY-MASS HALOS: DESTROYED DWARF SATELLITES AND THE METALLICITY DISTRIBUTION OF ACCRETED STARS

    Energy Technology Data Exchange (ETDEWEB)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H., E-mail: adeason@stanford.edu [Kavli Institute for Particle Astrophysics and Cosmology and Physics Department, Stanford University, Stanford, CA 94305 (United States)

    2016-04-10

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M{sub vir} ∼ 10{sup 12.1} M{sub ⊙}) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M{sub star} ∼ 10{sup 8}–10{sup 10}M{sub ⊙}. Halos with more quiescent accretion histories tend to have lower mass progenitors (10{sup 8}–10{sup 9} M{sub ⊙}), and lower overall accreted stellar masses. Ultra-faint mass (M{sub star} < 10{sup 5} M{sub ⊙}) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%–5%) of the very metal-poor stars with [Fe/H] < −2. Dwarfs with masses 10{sup 5} < M{sub star}/M{sub ⊙} < 10{sup 8} provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with M{sub star} > 10{sup 8} M{sub ⊙} can contribute a considerable fraction (∼20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil”; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

  19. Classification of extremely metal-poor stars: absent region in A(C)-[Fe/H] plane and the role of dust cooling

    Science.gov (United States)

    Chiaki, Gen; Tominaga, Nozomu; Nozawa, Takaya

    2017-11-01

    Extremely metal-poor (EMP) stars are the living fossils with records of chemical enrichment history at the early epoch of galaxy formation. By the recent large observation campaigns, statistical samples of EMP stars have been obtained. This motivates us to reconsider their classification and formation conditions. From the observed lower limits of carbon and iron abundances of Acr(C) ∼ 6 and [Fe/H]cr ∼ -5 for C-enhanced EMP (CE-EMP) and C-normal EMP (CN-EMP) stars, we confirm that gas cooling by dust thermal emission is indispensable for the fragmentation of their parent clouds to form such low mass, i.e. long-lived stars, and that the dominant grain species are carbon and silicate, respectively. We constrain the grain radius r_i^cool of a species i and condensation efficiency fij of a key element j as r_C^cool / f_C,C = 10 {μ m} and r_Sil^cool / f_Sil,Mg = 0.1 {μ m} to reproduce Acr(C) and [Fe/H]cr, which give a universal condition 10[C/H] - 2.30 + 10[Fe/H] > 10-5.07 for the formation of every EMP star. Instead of the conventional boundary [C/Fe] = 0.7 between CE-EMP and CN-EMP stars, this condition suggests a physically meaningful boundary [C/Fe]b = 2.30 above and below which carbon and silicate grains are dominant coolants, respectively.

  20. An unsuccessful search for brown dwarf companions to white dwarf stars

    Science.gov (United States)

    Shipman, Harry L.

    1986-01-01

    The results of a survey to detect excess infrared emission from white dwarf stars which would be attributable to a low mass companion are reviewed. Neither a simple comparison of spectroscopically identified white dwarf stars with the IRAS Point Source Catalog nor the coadding of IRAS survey data resulted in a detection of a brown dwarf. The seven nearest stars where the most stringent limits to the presence of a brown dwarf were obtained are listed, and an effort to detect brown dwarfs in the solar neighborhood is discussed.

  1. Activity-induced radial velocity variation of M dwarf stars

    DEFF Research Database (Denmark)

    Andersen, Jan Marie; Korhonen, Heidi Helena

    2012-01-01

    that can drown out a planetary signature. Cool, low-mass M dwarf stars can be highly active, which can make detection of potentially habitable planets around these stars difficult. We investigate radial velocity variations caused by different activity (spot) patterns on M dwarf stars in order to determine...... the limits of detectability for small planets orbiting active M dwarfs. We report on our progress toward the aim of answering the following questions: What types of spot patterns are realistic for M dwarf stars? What effect will spots have on M dwarf RV measurements? Can jitter from M dwarf spots mimic...... planetary signals? What is the ideal observing wavelength to reduce M dwarf jitter?...

  2. Dwarf Star Erupts in Giant Flare

    Science.gov (United States)

    2005-01-01

    This movie taken by NASA'S Galaxy Evolution Explorer shows one of the largest flares, or star eruptions, ever recorded at ultraviolet wavelengths. The star, called GJ 3685A, just happened to be in the Galaxy Evolution Explorer's field of view while the telescope was busy observing galaxies. As the movie demonstrates, the seemingly serene star suddenly exploded once, then even more intensely a second time, pouring out in total about one million times more energy than a typical flare from our Sun. The second blast of light constituted an increase in brightness by a factor of at least 10,000. Flares are huge explosions of energy stemming from a single location on a star's surface. They are caused by the brief destruction of a star's magnetic fields. Many types of stars experience them, though old, small, rapidly rotating 'red dwarfs' like GJ 3685A tend to flare more frequently and dramatically. These stars, called flare stars, can experience powerful eruptions as often as every few hours. Younger stars, in general, also erupt more often. One of the reasons astronomers study flare stars is to gain a better picture and history of flare events taking place on the Sun. A preliminary analysis of the GJ 3685A flare shows that the mechanisms underlying stellar eruptions may be more complex than previously believed. Evidence for the two most popular flare theories was found. Though this movie has been sped up (the actual flare lasted about 20 minutes), time-resolved data exist for each one-hundredth of a second. These observations were taken at 2 p.m. Pacific time, April 24, 2004. In the still image, the time sequence starts in the upper left panel, continues in the upper right, then moves to the lower left and ends in the lower right. The circular and linear features that appear below and to the right of GJ 3685A during the flare event are detector artifacts caused by the extreme brightness of the flare.

  3. IRAS observations of chromospherically active dwarf stars

    Science.gov (United States)

    Tsikoudi, Vassiliki

    1989-01-01

    Far-infrared observations of chromospherically active, spotted, and plage stars in the dF7-dk7 spectral range are examined. Most (75 percent) of the stars have detectable 12-micron fluxes, and 50 percent of them have 25-micron emission. The 12-micron luminosity, L(12), is found to be in the range of 1.5-13 x 10 to the 30th ergs/s and to comprise only 0.2-0.5 percent of the star's total luminosity, L(bol). The present work extends to earlier spectral types and higher stellar luminosities the L(12) vs L(bol) relationship noted previously for late-type active dwarfs (K5-M5).

  4. IRAS observations of chromospherically active dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Tsikoudi, V. (Ioannina Univ. (Greece))

    1989-07-01

    Far-infrared observations of chromospherically active, spotted, and plage stars in the dF7-dk7 spectral range are examined. Most (75 percent) of the stars have detectable 12-micron fluxes, and 50 percent of them have 25-micron emission. The 12-micron luminosity, L(12), is found to be in the range of 1.5-13 x 10 to the 30th ergs/s and to comprise only 0.2-0.5 percent of the star's total luminosity, L(bol). The present work extends to earlier spectral types and higher stellar luminosities the L(12) vs L(bol) relationship noted previously for late-type active dwarfs (K5-M5). 17 refs.

  5. CEMP Stars in the Halo and Their Origin in Ultra-Faint Dwarf Galaxies

    Science.gov (United States)

    Beers, Timothy C.

    2018-06-01

    The very metal-poor (VMP; [Fe/H] 3.0) stars provide a direct view of Galactic chemical and dynamical evolution; detailed spectroscopic studies of these objects are the best way to identify and distinguish between various scenarios for the enrichment of early star-forming gas clouds soon after the Big Bang. It has been recognized that a large fraction of VMP (15-20%) and EMP stars (30-40%) possess significant over-abundances of carbon relative to iron, [C/Fe] > +0.7. This fraction rises to at least 80% for stars with [Fe/H] 3.0 belong to the CEMP-no sub-class, characterized by the lack of strong enhancements in the neutron-capture elements (e.g., [Ba/Fe] < 0.0). The CEMP-no abundance signature is commonly observed among stars ultra-faint dwarf spheroidal galaxies such as SEGUE-1. In addition, kinematic studies of CEMP-no stars strongly suggest an association with the outer-halo population of the Galaxy, which was likely formed from the accretion of low-mass mini-halos. These observations, and other lines of evidence, indicate that the CEMP-no stars of the Milky Way were born in low-mass dwarf galaxies, and later subsumed into the halo.

  6. On the necessity of composition-dependent low-temperature opacity in models of metal-poor asymptotic giant branch stars

    Energy Technology Data Exchange (ETDEWEB)

    Constantino, Thomas; Campbell, Simon; Lattanzio, John [Monash Centre for Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800 (Australia); Gil-Pons, Pilar, E-mail: thomas.constantino@monash.edu [Department of Applied Physics, Polytechnic University of Catalonia, 08860 Barcelona (Spain)

    2014-03-20

    The vital importance of composition-dependent low-temperature opacity in low-mass (M ≤ 3 M {sub ☉}) asymptotic giant branch (AGB) stellar models of metallicity Z ≥ 0.001 has recently been demonstrated. Its significance to more metal-poor, intermediate-mass (M ≥ 2.5 M {sub ☉}) models has yet to be investigated. We show that its inclusion in lower-metallicity models ([Fe/H] ≤–2) is essential and that there exists no threshold metallicity below which composition-dependent molecular opacity may be neglected. We find it to be crucial in all intermediate-mass models investigated ([Fe/H] ≤–2 and 2.5 ≤ M/M {sub ☉} ≤ 5), because of the evolution of the surface chemistry, including the orders of magnitude increase in the abundance of molecule-forming species. Its effect on these models mirrors that previously reported for higher-metallicity models—increase in radius, decrease in T {sub eff}, faster mass loss, shorter thermally pulsing AGB lifetime, reduced enrichment in third dredge-up products (by a factor of 3-10), and an increase in the mass limit for hot bottom burning. We show that the evolution of low-metallicity models with composition-dependent low-temperature opacity is relatively independent of initial metal abundance because its contribution to the opacity is far outweighed by changes resulting from dredge-up. Our results imply a significant reduction in the expected number of nitrogen-enhanced metal-poor stars, which may help explain their observed paucity. We note that these findings are partially a product of the macrophysics adopted in our models, in particular, the Vassiliadis and Wood mass loss rate which is strongly dependent on radius.

  7. VLA observations of dwarf M flare stars and magnetic stars

    Science.gov (United States)

    Willson, R. F.; Lang, K. R.; Foster, P.

    1988-01-01

    The VLA has been used to search for 6 cm emission from 16 nearby dwarf M stars, leading to the detection of only one of them - Gliese 735. The dwarf M flare stars AD Leonis and YZ Canis Minoris were also monitored at 6 cm and 20 cm wavelength in order to study variability. Successive oppositely circularly polarized bursts were detected from AD Leo at 6 cm, suggesting the presence of magnetic fields of both magnetic polarities. An impulsive 20-cm burst from YZ CMi preceded slowly varying 6-cm emission. The VLA was also used, unsuccessfully, to search for 6-cm emission from 13 magnetic Ap stars, all of which exhibit kG magnetic fields. Although the Ap magnetic stars have strong dipolar magnetic fields, the failure to detect gyroresonant radiation suggests that these stars do not have hot, dense coronae. The quiescent microwave emission from GL 735 is probably due to nonthermal radiation, since unusually high (H = 50 kG or greater) surface magnetic fields are inferred under the assumption that the 6-cm radiation is the gyroresonant radiation of thermal electrons.

  8. Variable Stars in the M31 Dwarf Spheroidal Companion Cassiopeia

    Science.gov (United States)

    Pritzl, Barton J.; Armandroff, T. E.; Jacoby, G. H.; Da Costa, G. S.

    2007-12-01

    Dwarf spheroidal galaxies show very diverse star formation histories. For the Galactic dwarf spheroidal galaxies, a correlation exists between Galactocentric distance and the prominence of intermediate-age ( 2 - 10 Gyr) populations. To test whether this correlation exists for the M31 dwarf spheroidal galaxies, we observed the Cassiopeia (And VII) dwarf galaxy, which is one of the most distant M31 dwarf spheroidal galaxies. We will present the results of a variable star search using HST/ACS data, along with a preliminary color-magnitude diagram. From the RR Lyrae stars we can obtain an independent distance and metallicity estimate for the dwarf galaxy. These results will be compared to those found for the other M31 dwarf spheroidal galaxies.This research is supported in part by NASA through grant number GO-11081.11 from the Space Telescope Science Institute.

  9. METALLICITY GRADIENTS OF THICK DISK DWARF STARS

    Energy Technology Data Exchange (ETDEWEB)

    Carrell, Kenneth; Chen Yuqin; Zhao Gang, E-mail: carrell@nao.cas.cn [Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China)

    2012-12-01

    We examine the metallicity distribution of the Galactic thick disk using F, G, and K dwarf stars selected from the Sloan Digital Sky Survey, Data Release 8. Using the large sample of dwarf stars with proper motions and spectroscopically determined stellar parameters, metallicity gradients in the radial direction for various heights above the Galactic plane and in the vertical direction for various radial distances from the Galaxy center have been found. In particular, we find a vertical metallicity gradient of -0.113 {+-} 0.010 (-0.125 {+-} 0.008) dex kpc{sup -1} using an isochrone (photometric) distance determination in the range 1 kpc <|Z| < 3 kpc, which is the vertical height range most consistent with the thick disk of our Galaxy. In the radial direction, we find metallicity gradients between +0.02 and +0.03 dex kpc{sup -1} for bins in the vertical direction between 1 kpc <|Z| < 3 kpc. Both of these results agree with similar values determined from other populations of stars, but this is the first time a radial metallicity gradient for the thick disk has been found at these vertical heights. We are also able to separate thin and thick disk stars based on kinematic and spatial probabilities in the vertical height range where there is significant overlap of these two populations. This should aid further studies of the metallicity gradients of the disk for vertical heights lower than those studied here but above the solar neighborhood. Metallicity gradients in the thin and thick disks are important probes into possible formation scenarios for our Galaxy and a consistent picture is beginning to emerge from results using large spectroscopic surveys, such as the ones presented here.

  10. The Ital-FLAMES survey of the Sagittarius dwarf Spheroidal galaxy. I. Chemical abundances of bright RGB stars

    OpenAIRE

    Monaco, L.; Bellazzini, M.; Bonifacio, P.; Ferraro, F. R.; Marconi, G.; Pancino, E.; Sbordone, L.; Zaggia, S.

    2005-01-01

    We present iron and $\\alpha$ element (Mg, Ca, Ti) abundances for a sample of 15 Red Giant Branch stars belonging to the main body of the Sagittarius dwarf Spheroidal galaxy. Abundances have been obtained from spectra collected using the high resolution spectrograph FLAMES-UVES mounted at the VLT. Stars of our sample have a mean metallicity of [Fe/H]=-0.41$\\pm$0.20 with a metal poor tail extending to [Fe/H]=-1.52. The $\\alpha$ element abundance ratios are slightly subsolar for metallicities hi...

  11. Chemical Abundances of New Member Stars in the Tucana II Dwarf Galaxy

    Science.gov (United States)

    Chiti, Anirudh; Frebel, Anna; Ji, Alexander P.; Jerjen, Helmut; Kim, Dongwon; Norris, John E.

    2018-04-01

    We present chemical abundance measurements for seven stars with metallicities ranging from Fe/H] = ‑3.3 to [Fe/H] = ‑2.4 in the Tucana II ultra-faint dwarf galaxy (UFD), based on high-resolution spectra obtained with the MIKE spectrograph on the 6.5 m Magellan-Clay Telescope. For three stars, we present detailed chemical abundances for the first time. Of those, two stars are newly discovered members of Tucana II and were selected as probable members from deep narrowband photometry of the Tucana II UFD taken with the SkyMapper telescope. This result demonstrates the potential for photometrically identifying members of dwarf galaxy systems based on chemical composition. One new star was selected from the membership catalog of Walker et al. The other four stars in our sample have been reanalyzed, following additional observations. Overall, six stars have chemical abundances that are characteristic of the UFD stellar population. The seventh star shows chemical abundances that are discrepant from the other Tucana II members and an atypical, higher strontium abundance than what is expected for typical UFD stars. While unlikely, its strontium abundance raises the possibility that it may be a foreground metal-poor halo star with the same systemic velocity as Tucana II. If we were to exclude this star, Tucana II would satisfy the criteria to be a surviving first galaxy. Otherwise, this star implies that Tucana II has likely experienced somewhat extended chemical evolution. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  12. A heating mechanism for the chromospheres of M dwarf stars

    Science.gov (United States)

    Giampapa, M. S.; Golub, L.; Rosner, R.; Vaiana, G.; Linsky, J. L.; Worden, S. P.

    1981-01-01

    The atmospheric structure of the dwarf M-stars which is especially important to the general field of stellar chromospheres and coronae was investigated. The M-dwarf stars constitute a class of objects for which the discrepancy between the predictions of the acoustic wave chromospheric/coronal heating hypothesis and the observations is most vivid. It is assumed that they represent a class of stars where alternative atmospheric heating mechanisms, presumably magnetically related, are most clearly manifested. Ascertainment of the validity of a hypothesis to account for the origin of the chromospheric and transition region line emission in M-dwarf stars is proposed.

  13. On the Stability of Strange Dwarf Hybrid Stars

    Energy Technology Data Exchange (ETDEWEB)

    Alford, Mark G.; Harris, Steven P. [Physics Department, Washington University, St. Louis, MO 63130 (United States); Sachdeva, Pratik S., E-mail: harrissp@wustl.edu [Department of Physics, University of California, Berkeley, CA 94720 (United States)

    2017-10-01

    We investigate the stability of “strange dwarfs”: white-dwarf-sized stars with a density discontinuity between a small dense core of quark matter and a thick low-density mantle of degenerate electrons. Previous work on strange dwarfs suggested that such a discontinuity could stabilize stars that would have been classified as unstable by the conventional criteria based on extrema in the mass–radius relation. We investigate the stability of such stars by numerically solving the Sturm–Liouville equations for the lowest-energy modes of the star. We find that the conventional criteria are correct, and strange dwarfs are not stable.

  14. Star Formation in Dwarf Galaxies: Life in a Rough Neighborhood

    Energy Technology Data Exchange (ETDEWEB)

    Murray, S

    2003-10-16

    Star formation within dwarf galaxies is governed by several factors. Many of these factors are external, including ram-pressure stripping, tidal stripping, and heating by external UV radiation. The latter, in particular, may prevent star formation in the smallest systems. Internal factors include negative feedback in the form of UV radiation, winds and supernovae from massive stars. These act to reduce the star formation efficiency within dwarf systems, which may, in turn, solve several theoretical and observational problems associated with galaxy formation. In this contribution, we discuss our recent work being done to examine the importance of the many factors in the evolution of dwarf galaxies.

  15. Extremely Low-Metallicity Stars in the Classical Dwarf Galaxies

    NARCIS (Netherlands)

    Starkenburg, E.; DART Team, [Unknown; Aoki, W; Ishigaki, M; Suda, T; Tsujimoto, T; Arimoto, N

    After careful re-analysis of Ca II triplet calibration at low-metallicity, the classical satellites around the Milky Way are found not to be devoided of extremely low-metallicity stars and their (extremely) metal-poor tails are predicted to be much more in agreement with the Milky Way halo. A first

  16. A low-temperature companion to a white dwarf star

    Science.gov (United States)

    Becklin, E. E.; Zuckerman, B.

    1988-01-01

    An infrared object located about 120 AU from the white dwarf GD165 has been discovered. With the exception of the possible brown dwarf companion to Giclas 29-38 reported last year, the companion to GD165 is the coolest (2100 K) dwarf star ever reported and, according to some theoretical models, it should be a substellar brown dwarf with a mass between 0.06 and 0.08 solar mass. These results, together with newly discovered low-mass stellar companions to white dwarfs, change the investigation of very low-mass stars from the study of a few chance objects to that of a statistical distribution. In particular, it appears that very low-mass stars and perhaps even brown dwarfs could be quite common in the Galaxy.

  17. Ages of M Dwarf Stars from their Alpha Enhancement

    Science.gov (United States)

    Muirhead, Philip Steven; Veyette, Mark

    2018-01-01

    M dwarf stars dominate stellar populations, and recent results from NASA's Kepler Mission suggest rocky planets are abundant around M dwarf stars. With so many planets orbiting M dwarfs, exoplanet scientists can now turn to questions about their history and evolution. Unfortunately, measuring fundamental properties of M dwarfs is challenging for a variety of reasons. I will discuss the importance of near-infrared spectroscopy in this effort. With high-resolution near-infrared spectroscopy covering Y to K band, we can measure detailed fundamental properties of low-mass stars. With new techniques to measure stellar alpha and iron abundances, we can begin to measure the most challenging fundamental property of M dwarfs: their age. These efforts are even more exciting in the coming years, when the TESS spacecraft is expected to discover five times as many planets orbiting low-mass stars as Kepler.

  18. Localized thermonuclear runaways and volcanoes on degenerate dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Shara, M.M.

    1982-10-15

    Practically all studies to date of thermonuclear runaways on degenerate dwarf stars in binary systems have considered only spherically symmetric eruptions. We emphasize that even slightly non-spherically symmetric accretion leads to transverse temperature gradients in the dwarfs' accreted envelopes. Over a rather broad range of parameter space, thermalization time scales in accreted envelopes are much longer than thermonuclear runaway time scales. Thus localized thermonuclear runaways (i.e., runaways much smaller than the host degenerate star) rather than spherically symmetric global eruptions are likely to occur on many degenerate dwarfs. Localized runaways are more likely to occur on more massive and/or hotter dwarfs.

  19. A radio-pulsing white dwarf binary star.

    Science.gov (United States)

    Marsh, T R; Gänsicke, B T; Hümmerich, S; Hambsch, F-J; Bernhard, K; Lloyd, C; Breedt, E; Stanway, E R; Steeghs, D T; Parsons, S G; Toloza, O; Schreiber, M R; Jonker, P G; van Roestel, J; Kupfer, T; Pala, A F; Dhillon, V S; Hardy, L K; Littlefair, S P; Aungwerojwit, A; Arjyotha, S; Koester, D; Bochinski, J J; Haswell, C A; Frank, P; Wheatley, P J

    2016-09-15

    White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 10 7 -year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.

  20. Diffusion of neon in white dwarf stars.

    Science.gov (United States)

    Hughto, J; Schneider, A S; Horowitz, C J; Berry, D K

    2010-12-01

    Sedimentation of the neutron rich isotope 22Ne may be an important source of gravitational energy during the cooling of white dwarf stars. This depends on the diffusion constant for 22Ne in strongly coupled plasma mixtures. We calculate self-diffusion constants D(i) from molecular dynamics simulations of carbon, oxygen, and neon mixtures. We find that D(i) in a mixture does not differ greatly from earlier one component plasma results. For strong coupling (coulomb parameter Γ> few), D(i) has a modest dependence on the charge Z(i) of the ion species, D(i)∝Z(i)(-2/3). However, D(i) depends more strongly on Z(i) for weak coupling (smaller Γ). We conclude that the self-diffusion constant D(Ne) for 22Ne in carbon, oxygen, and neon plasma mixtures is accurately known so that uncertainties in D(Ne) should be unimportant for simulations of white dwarf cooling.

  1. Halo carbon stars associated with dwarf spheroidal galaxies

    Energy Technology Data Exchange (ETDEWEB)

    Van Den Bergh, S.; Lafontaine, A.

    1984-11-01

    Star counts have been performed for rings centered on the carbon star at 1 69 degrees, b + 55 degrees at a distance of 60 kpc. The counts were performed in order to determine whether halo carbon stars might be situated in dwarf spheroidal galaxies which are too star-poor to have been recognized as galaxies. The counts were made on a IIIa-J plate baked in forming gas that was exposed for 40 minutes through a 2C filter with the Palomar 1.2-m Schmidt telescope. It is shown that the carbon star is not situated in a dwarf spheroidal galaxy brighter than M(V) 5.7.

  2. Luminosities and temperatures of M dwarf stars from infrared photometry

    Science.gov (United States)

    Veeder, G. J.

    1974-01-01

    Bolometric magnitudes for a large number of M type dwarf stars, obtained by broadband infrared photometry at 1.65, 2.2, and 3.5 microns, are reviewed. The data obtained indicate that one parameter is sufficient to describe the blanketing in all of the UBVRI bands for all types of M dwarfs. In general, late M dwarfs seem to have lower effective temperatures than are predicted by theoretical models.

  3. Infrared spectrum of an extremely cool white-dwarf star

    Science.gov (United States)

    Hodgkin; Oppenheimer; Hambly; Jameson; Smartt; Steele

    2000-01-06

    White dwarfs are the remnant cores of stars that initially had masses of less than 8 solar masses. They cool gradually over billions of years, and have been suggested to make up much of the 'dark matter' in the halo of the Milky Way. But extremely cool white dwarfs have proved difficult to detect, owing to both their faintness and their anticipated similarity in colour to other classes of dwarf stars. Recent improved models indicate that white dwarfs are much more blue than previously supposed, suggesting that the earlier searches may have been looking for the wrong kinds of objects. Here we report an infrared spectrum of an extremely cool white dwarf that is consistent with the new models. We determine the star's temperature to be 3,500 +/- 200 K, making it the coolest known white dwarf. The kinematics of this star indicate that it is in the halo of the Milky Way, and the density of such objects implied by the serendipitous discovery of this star is consistent with white dwarfs dominating the dark matter in the halo.

  4. The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    Science.gov (United States)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ˜ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ˜ 108-1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108-109 M⊙), and lower overall accreted stellar masses. Ultra-faint mass (Mstar 108 M⊙ can contribute a considerable fraction (˜20%-60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

  5. Do some x-ray stars have white dwarf companions

    Science.gov (United States)

    Mccollum, Bruce

    1995-01-01

    Some Be stars which are intermittent X-ray sources may have white dwarf companions rather than neutron stars. It is not possible to prove or rule out the existence of Be + WD systems using X-ray or optical data. However, the presence of a white dwarf could be established by the detection of its EUV continuum shortward of the Be star's continuum turnover at 100 A. Either the detection or the nondetection of Be + WD systems would have implications for models of Be star variability, models of Be binary system formation and evolution, and models of wind-fed accretion.

  6. New light on dark stars red dwarfs, low-mass stars, brown dwarfs

    CERN Document Server

    Reid, I Neill

    2000-01-01

    Perhaps the most common question that a child asks when he or she sees the night sky from a dark site for the first time is: 'How many stars are there?' This happens to be a question which has exercised the intellectual skills of many astronomers over the course of most of the last century, including, for the last two decades, one of the authors of this text. Until recently, the most accurate answer was 'We are not certain, but there is a good chance that almost all of them are M dwarfs. ' Within the last three years, results from new sky-surveys - particularly the first deep surveys at near­ infrared wavelengths - have provided a breakthrough in this subject, solidifying our census of the lowest-mass stars and identifying large numbers of the hitherto almost mythical substellar-mass brown dwarfs. These extremely low-luminosity objects are the central subjects of this book, and the subtitle should be interpreted accordingly. The expression 'low-mass stars' carries a wide range of meanings in the astronomical...

  7. A Formation Timescale of the Galactic Halo from Mg Isotopes in Dwarf Stars

    Science.gov (United States)

    Carlos, Marília; Karakas, Amanda I.; Cohen, Judith G.; Kobayashi, Chiaki; Meléndez, Jorge

    2018-04-01

    We determine magnesium isotopic abundances of metal-poor dwarf stars from the galactic halo, to shed light on the onset of asymptotic giant branch (AGB) star nucleosynthesis in the galactic halo and constrain the timescale of its formation. We observed a sample of eight new halo K dwarfs in a metallicity range of ‑1.9 ‑1.4 are somewhat higher (1–3σ) than previous chemical evolution model predictions, indicating perhaps higher yields of the neutron-rich isotopes. Our results using only AGB star enrichment suggest a timescale for formation for the galactic halo of about 0.3 Gyr, but considering also supernova enrichment, the upper limit for the timescale formation is about 1.5 Gyr. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  8. Explosive helium burning in white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Khokhlov, A.M. (AN SSSR, Moscow. Astronomicheskij Sovet)

    1984-04-01

    Helium burning kinetics in white dwarfs has been considered at constant temperatures T >= 10/sup 9/ K and densities rho >10/sup 5/ g/cm/sup 3/. It is found, that helium detonation in white dwarfs does not lead to formation of light (A < 56) elements. Thus, helium white dwarf model for supernova 1 is inconsistent with observations.

  9. He stars and He-accreting CO white dwarfs

    International Nuclear Information System (INIS)

    Limongi, M.; Tornambe, A.

    1991-01-01

    He star models in the mass range 0.4-1.0 solar mass have been evolved until the red giant phase or, depending on their mass, until crystallization on the white-dwarf cooling sequence. Some of the degenerate structures obtained in these computations have been successively accreted at various He accretion rates in order to better define the fate of the accreting dwarf versus its mass and accretion rate for a fixed degeneracy level of the accreting dwarf. He stars have been further induced to transfer mass to a degenerate companion through Roche lobe overflow, in conditions of large gravitational wave radiation by the system. CO dwarfs in binary systems with He stars are found to experience a thermal behavior whose effects are such to locate the structure on the verge of obtaining a strong SN-like explosive event. 22 refs

  10. Speckle Interferometry of Red Dwarf Stars

    Science.gov (United States)

    Mason, Brian D.; Hartkopf, William I.; Miles, Korie N.; Subasavage, John P.; Raghavan, Deepak; Henry, Todd J.

    2018-05-01

    We report high-resolution optical speckle observations of 336 M dwarfs, which results in 113 measurements of the relative position of 80 systems and 256 other stars with no indications of duplicity. These are the first measurements for two of the systems. We also present the earliest measurements of relative position for 17 others. We include orbits for six of the systems, two revised and four reported for the first time. For one of the systems with a new orbit, G 161-7, we determine masses of 0.156 ± 0.011 and 0.1175+/- 0.0079 {{ \\mathcal M }}ȯ for the A and B components, respectively. All six of these new calculated orbits have short periods (between five and 38 years) and hold the promise of deriving accurate masses in the near future. For many other pairs we can establish their nature as physical or chance alignment, depending on their relative motion. Of the 80 systems, 32 have calculated orbits, 25 others are physical pairs, four are optical pairs, and 19 are currently unknown.

  11. Discovery of a Metal-Poor Little Cub

    Science.gov (United States)

    Kohler, Susanna

    2017-09-01

    The discovery of an extremely metal-poor star-forming galaxy in our local universe, dubbed Little Cub, is providing astronomers with front-row seats to the quenching of a near-pristine galaxy.SDSS image of NGC 3359 (left) and Little Cub (right), with overlying contours displaying the location of hydrogen gas. Little Cubs (also shown in the inset) stellar mass lies in the blue contour of the right-hand side. The outer white contours show the extended gas of the galaxy, likely dragged out as a tidal tail by Little Cubs interaction with NGC 3359. [Hsyu et al. 2017]The Hunt for Metal-Poor GalaxiesLow-metallicity, star-forming galaxies can show us the conditions under which the first stars formed. The galaxies with the lowest metallicities, however, also tend to be those with the lowest luminosities making them difficult to detect. Though we know that there should be many low-mass, low-luminosity, low-metallicity galaxies in the universe, weve detected very few of them nearby.In an effort to track down more of these metal-poor galaxies, a team of scientists led by Tiffany Hsyu (University of California Santa Cruz) searched through Sloan Digital Sky Survey data, looking for small galaxies with the correct photometric color to qualify a candidate blue compact dwarfs, a type of small, low-luminosity, star-forming galaxy that is often low-metallicity.Hsyu and collaborators identified more than 2,500 candidate blue compact dwarfs, and next set about obtaining follow-up spectroscopy for many of the candidates from the Keck and Lick Observatories. Though this project is still underway, around 100 new blue compact dwarfs have already been identified via the spectroscopy, including one of particular interest: the Little Cub.Little CubThis tiny star-forming galaxy gained its nickname from its location in the constellation Ursa Major. Little Cub is perhaps 50 or 60 million light-years away, and Hsyu and collaborators find it to be one of the lowest-metallicity star

  12. Metal-poor dwarf galaxies in the SIGRID galaxy sample. II. The electron temperature-abundance calibration and the parameters that affect it

    Energy Technology Data Exchange (ETDEWEB)

    Nicholls, David C.; Dopita, Michael A.; Sutherland, Ralph S.; Jerjen, Helmut; Kewley, Lisa J., E-mail: David.Nicholls@anu.edu.au [Research School of Astronomy and Astrophysics, Australian National University, Cotter Rd., Weston ACT 2611 (Australia)

    2014-07-20

    In this paper, we use the Mappings photoionization code to explore the physical parameters that impact on the measurement of electron temperature and abundance in H II regions. In our previous paper, we presented observations and measurements of physical properties from the spectra of 17 H II regions in 14 isolated dwarf irregular galaxies from the SIGRID sample. Here, we analyze these observations further, together with three additional published data sets. We explore the effects of optical thickness, electron density, ionization parameter, ionization source, and non-equilibrium effects on the relation between electron temperature and metallicity. We present a standard model that fits the observed data remarkably well at metallicities between one-tenth and 1 solar. We investigate the effects of optically thin H II regions, and show that they can have a considerable effect on the measured electron temperature, and that there is evidence that some of the observed objects are optically thin. We look at the role of the ionization parameter and find that lower ionization parameter values give better fits at higher oxygen abundance. We show that higher pressures combined with low optical depth, and also κ electron energy distributions at low κ values, can generate the apparent high electron temperatures in low-metallicity H II regions, and that the former provides the better fit to observations. We examine the effects of these parameters on the strong line diagnostic methods. We extend this to three-dimensional diagnostic grids to confirm how well the observations are described by the grids.

  13. Elemental abundances in the Galactic bulge from microlensed dwarf stars

    NARCIS (Netherlands)

    Bensby, T.; Feltzing, S.; Johnson, J.A.; Gould, A.; Sana, H.; Gal-Yam, A.; Asplund, M.; Lucatello, S.; Melendez, J.; Udalski, A.; Kubas, D.; James, G.; Adén, D.; Simmerer, J.

    2010-01-01

    We present elemental abundances of 13 microlensed dwarf and subgiant stars in the Galactic bulge, which constitute the largest sample to date. We show that these stars span the full range of metallicity from Fe/H= −0.8 to +0.4, and that they follow well-defined abundance trends, coincident with

  14. Bose-Einstein condensation in helium white dwarf stars. I

    Energy Technology Data Exchange (ETDEWEB)

    Mosquera, M.E. [Faculty of Astronomy and Geophysics, University of La Plata, Paseo del Bosque s.n., La Plata (Argentina); Department of Physics, University of La Plata, c.c. 67 1900, La Plata (Argentina); Civitarese, O., E-mail: osvaldo.civitarese@fisica.unlp.edu.a [Department of Physics, University of La Plata, c.c. 67 1900, La Plata (Argentina); Benvenuto, O.G.; De Vito, M.A. [Faculty of Astronomy and Geophysics, University of La Plata, Paseo del Bosque s.n., La Plata (Argentina); Instituto de Astrofisica La Plata, CCT (Argentina)

    2010-01-18

    The formation of a Bose-Einstein condensate in the interior of helium white dwarfs stars is discussed. Following the proposal made by Gabadadze and Rosen, we have explored the consequences of such a mechanism by calculating the cooling time of the stars. We have found that it is shorter than the value predicted by the standard model.

  15. Inhomogeneous structure in the chromospheres of dwarf M stars

    Science.gov (United States)

    Turner, N. J.; Cram, L. E.; Robinson, R. D.

    1991-01-01

    Linear combinations of observed spectra of the H-alpha and Ca-II resonance and IR lines from the chromospheres of a quiet (Gl 1) and an active (Gl 735) dwarf-M star are compared with the corresponding spectra from a star of intermediate activity (Gl 887). It is shown that the intermediate spectra cannot be explained as a simple juxtaposition of the extreme chromospheric states. It is concluded that the range of observed strengths of chromospheric activity indicators in dwarf-M stars is due, at least in part, to changes in the radial structure of the chromospheric heating function and not to changes in the area filling factor.

  16. New white dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 12

    OpenAIRE

    Kepler, S. O.; Pelisoli, Ingrid; Koester, Detlev; Ourique, Gustavo; Romero, Alejandra Daniela; Reindl, Nicole; Kleinman, Scot J.; Eisenstein, Daniel J.; Valois, A. Dean M.; Amaral, Larissa A.

    2015-01-01

    We report the discovery of 6576 new spectroscopically confirmed white dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 12. We obtain Teff, log g and mass for hydrogen atmospherewhite dwarf stars (DAs) and helium atmospherewhite dwarf stars (DBs), estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon-dominated spectra (DQs). We found one central star of a planetary nebula, one ultracompact helium binary (AM ...

  17. Abundance Survey of M and K Dwarf Stars

    Energy Technology Data Exchange (ETDEWEB)

    Woolf, Vincent M. [Astronomy Department, University of Washington, Seattle, WA 98133 (United States); Wallerstein, George [Astronomy Department, University of Washington, Seattle, WA 98133 (United States)

    2005-07-25

    We report the measurement of chemical abundances in 35 low-mass main sequence (M and K dwarf) stars. We have measured the abundance of 12 elements in Kapteyn's Star, a nearby halo M subdwarf. The abundances indicate an iron abundance of [Fe/H] = -0.98, which is about 0.5 dex smaller than that measured in the only previous published measurement using atomic absorption lines. We have measured Fe and Ti abundances in 35 M and K dwarfs with -2.39 [Fe/H] +0.21 using atomic absorption lines, mostly in the 8000A <{lambda} < 8850A range. These will be used to calibrate photometric and low-resolution spectrum metallicity indices for low mass dwarfs, which will make metallicity estimates for these stars more certain. We also describe some difficulties encountered which are not normally necessary to consider when studying warmer stars.

  18. The habitability of planets orbiting M-dwarf stars

    Science.gov (United States)

    Shields, Aomawa L.; Ballard, Sarah; Johnson, John Asher

    2016-12-01

    The prospects for the habitability of M-dwarf planets have long been debated, due to key differences between the unique stellar and planetary environments around these low-mass stars, as compared to hotter, more luminous Sun-like stars. Over the past decade, significant progress has been made by both space- and ground-based observatories to measure the likelihood of small planets to orbit in the habitable zones of M-dwarf stars. We now know that most M dwarfs are hosts to closely-packed planetary systems characterized by a paucity of Jupiter-mass planets and the presence of multiple rocky planets, with roughly a third of these rocky M-dwarf planets orbiting within the habitable zone, where they have the potential to support liquid water on their surfaces. Theoretical studies have also quantified the effect on climate and habitability of the interaction between the spectral energy distribution of M-dwarf stars and the atmospheres and surfaces of their planets. These and other recent results fill in knowledge gaps that existed at the time of the previous overview papers published nearly a decade ago by Tarter et al. (2007) and Scalo et al. (2007). In this review we provide a comprehensive picture of the current knowledge of M-dwarf planet occurrence and habitability based on work done in this area over the past decade, and summarize future directions planned in this quickly evolving field.

  19. The Universality of the Rapid Neutron-capture Process Revealed by a Possible Disrupted Dwarf Galaxy Star

    Science.gov (United States)

    Casey, Andrew R.; Schlaufman, Kevin C.

    2017-12-01

    The rapid neutron-capture or r-process is thought to produce the majority of the heavy elements (Z> 30) in extremely metal-poor stars. The same process is also responsible for a significant fraction of the heavy elements in the Sun. This universality of the r-process is one of its characteristic features, as well as one of the most important clues to its astrophysical origin. We report the discovery of an extremely metal-poor field giant with [{Sr},{Ba}/{{H}}]≈ -6.0 and [{Sr},{Ba}/{Fe}]≈ -3.0, the lowest abundances of strontium and barium relative to iron ever observed. Despite its low abundances, the star 2MASS J151113.24-213003.0 has [{Sr}/{Ba}]=-0.11+/- 0.14, therefore its neutron-capture abundances are consistent with the main solar r-process pattern that has [{Sr}/{Ba}]=-0.25. It has been suggested that extremely low neutron-capture abundances are a characteristic of dwarf galaxies, and we find that this star is on a highly eccentric orbit with an apocenter ≳100 kpc that lies in the disk of satellites in the halo of the Milky Way. We show that other extremely metal-poor stars with low [Sr, Ba/H] and [Sr, Ba/Fe] plus solar [Sr/Ba] tend to have orbits with large apocenters, consistent with a dwarf galaxy origin for this class of object. The nucleosynthesis event that produced the neutron-capture elements in 2MASS J151113.24-213003.0 must produce both strontium and barium together in the solar ratio. We exclude contributions from the s-process in intermediate-mass asymptotic giant branch or fast-rotating massive metal-poor stars, pair-instability supernovae, the weak r-process, and neutron-star mergers. We argue that the event was a Pop III or extreme Pop II core-collapse supernova explosion. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  20. THE CLOSE BINARY FRACTION OF DWARF M STARS

    International Nuclear Information System (INIS)

    Clark, Benjamin M.; Blake, Cullen H.; Knapp, Gillian R.

    2012-01-01

    We describe a search for close spectroscopic dwarf M star binaries using data from the Sloan Digital Sky Survey to address the question of the rate of occurrence of multiplicity in M dwarfs. We use a template-fitting technique to measure radial velocities from 145,888 individual spectra obtained for a magnitude-limited sample of 39,543 M dwarfs. Typically, the three or four spectra observed for each star are separated in time by less than four hours, but for ∼17% of the stars, the individual observations span more than two days. In these cases we are sensitive to large-amplitude radial velocity variations on timescales comparable to the separation between the observations. We use a control sample of objects having observations taken within a four-hour period to make an empirical estimate of the underlying radial velocity error distribution and simulate our detection efficiency for a wide range of binary star systems. We find the frequency of binaries among the dwarf M stars with a < 0.4 AU to be 3%-4%. Comparison with other samples of binary stars demonstrates that the close binary fraction, like the total binary fraction, is an increasing function of primary mass.

  1. THE CLOSE BINARY FRACTION OF DWARF M STARS

    Energy Technology Data Exchange (ETDEWEB)

    Clark, Benjamin M. [Penn Manor High School, 100 East Cottage Avenue, Millersville, PA 17551 (United States); Blake, Cullen H.; Knapp, Gillian R. [Princeton University, Department of Astrophysical Sciences, Peyton Hall, Ivy Lane, Princeton, NJ 08544 (United States)

    2012-01-10

    We describe a search for close spectroscopic dwarf M star binaries using data from the Sloan Digital Sky Survey to address the question of the rate of occurrence of multiplicity in M dwarfs. We use a template-fitting technique to measure radial velocities from 145,888 individual spectra obtained for a magnitude-limited sample of 39,543 M dwarfs. Typically, the three or four spectra observed for each star are separated in time by less than four hours, but for {approx}17% of the stars, the individual observations span more than two days. In these cases we are sensitive to large-amplitude radial velocity variations on timescales comparable to the separation between the observations. We use a control sample of objects having observations taken within a four-hour period to make an empirical estimate of the underlying radial velocity error distribution and simulate our detection efficiency for a wide range of binary star systems. We find the frequency of binaries among the dwarf M stars with a < 0.4 AU to be 3%-4%. Comparison with other samples of binary stars demonstrates that the close binary fraction, like the total binary fraction, is an increasing function of primary mass.

  2. EXTENDED STAR CLUSTERS IN THE REMOTE HALO OF THE INTRIGUING DWARF GALAXY NGC 6822

    International Nuclear Information System (INIS)

    Hwang, Narae; Lee, Myung Gyoon; Lee, Jong Chul; Park, Hong Soo; Park, Won-Kee; Kim, Sang Chul; Park, Jang-Hyun

    2011-01-01

    We present a study on four new star clusters discovered in the halo of the intriguing dwarf irregular galaxy NGC 6822 from a wide-field survey covering 3 0 x 3 0 area carried out with MegaCam at the Canada-France-Hawaii Telescope. The star clusters have extended structures with half-light radii R h ∼ 7.5-14.0 pc, larger than typical Galactic globular clusters and other known globular clusters in NGC 6822. The integrated colors and color-magnitude diagrams of resolved stars suggest that the new star clusters are 2-10 Gyr old and relatively metal poor with Z = 0.0001-0.004 based on the comparison with theoretical models. The projected distance of each star cluster from the galaxy center ranges from 10.'7 (∼1.5 kpc) to 77' (∼11 kpc), far beyond the optical body of the galaxy. Interestingly, the new star clusters are aligned along the elongated old stellar halo of NGC 6822, which is almost perpendicular to the H I gas distribution where young stellar populations exist. We also find that the colors and half-light radii of the new clusters are correlated with the galactocentric distance: clusters farther from the galaxy center are larger and bluer than those closer to the galaxy center. We discuss the stellar structure and evolution of NGC 6822 implied by these new extended star clusters in the halo. We also discuss the current status of observational and theoretical understandings regarding the origin of extended star clusters in NGC 6822 and other galaxies.

  3. Star Formation History of Dwarf Galaxies in Cosmological Hydrodynamic Simulations

    Directory of Open Access Journals (Sweden)

    Kentaro Nagamine

    2010-01-01

    Full Text Available We examine the past and current work on the star formation (SF histories of dwarf galaxies in cosmological hydrodynamic simulations. The results obtained from different numerical methods are still somewhat mixed, but the differences are understandable if we consider the numerical and resolution effects. It remains a challenge to simulate the episodic nature of SF history in dwarf galaxies at late times within the cosmological context of a cold dark matter model. More work is needed to solve the mysteries of SF history of dwarf galaxies employing large-scale hydrodynamic simulations on the next generation of supercomputers.

  4. Star's death and rebirth. White dwarfs, supernovae, pulsars, black holes

    Energy Technology Data Exchange (ETDEWEB)

    Otzen Petersen, J [Copenhagen Univ. (Denmark)

    1975-01-01

    The evolution of a star from a main sequence star of approximately solar mass, first to a red giant, thereafter to a white dwarf is described in detail. The evolution of more massive stars to supernovae, neutron stars and pulsars is then discussed with special reference to the Crab Nebula. Black holes and X-ray sources are also discussed, in this case with reference to the Cygnus X-1 system. In conclusion, it is pointed out that after their active phase white dwarfs, neutron stars and black holes may exist as dead bodies in space, and only be observeable through their gravitational fields. It is possible that a great number of such bodies may exist, and contribute to the stability of galaxies, also possibly facilitating the explanation of the galaxies' red shifts by means of simple universe models.

  5. Ageing in old degenerates: asteroseismology of white dwarf stars

    International Nuclear Information System (INIS)

    O'Donoghue, D.

    1988-01-01

    Recent results on the use of pulsations in white dwarf stars as seismic probes of their structure are reviewed. The evolution of stars to the white dwarf stage is first described, followed by a discussion of their structure as expected from the theory of stellar evolution. A summary of the salient points of stellar pulsation theory is given and then compared with observations of pulsating white dwarfs: the pulsations are non-radial 'g-mode' pulsations and occur in all white dwarfs as they cool through the temperature ranges defining each of the four 'instability strips' on the white dwarf cooling curve. The presence of only some of the possible pulsation modes in any given star suggest that a filter mechanism to select these modes is at work, possibly the chemical stratification of the star. The pulsation periods can be measured very accurately so that period changes, due to evolutionary cooling, can be detected over relatively short intervals (2 - 30 years). The detection of such period changes can be used to place interesting limits on the age of the Galaxy and ultimately the age of the Universe. 17 refs., 10 figs., 2 tabs

  6. Properties of O dwarf stars in 30 Doradus

    Science.gov (United States)

    Sabín-Sanjulián, Carolina; VFTS Collaboration

    2017-11-01

    We perform a quantitative spectroscopic analysis of 105 presumably single O dwarf stars in 30 Doradus, located within the Large Magellanic Cloud. We use mid-to-high resolution multi-epoch optical spectroscopic data obtained within the VLT-FLAMES Tarantula Survey. Stellar and wind parameters are derived by means of the automatic tool iacob-gbat, which is based on a large grid of fastwind models. We also benefit from the Bayesian tool bonnsai to estimate evolutionary masses. We provide a spectral calibration for the effective temperature of O dwarf stars in the LMC, deal with the mass discrepancy problem and investigate the wind properties of the sample.

  7. Space distribution and physical properties of cool dwarf stars

    International Nuclear Information System (INIS)

    Staller, R.F.A.

    1979-01-01

    A new study of the space density of red dwarfs based on a sample of red dwarfs in a field of 238 square degrees towards the South Galactic Pole is presented. A blink survey using red and blue copies of Mount Palomar Sky Survey plates of a six square degrees field centered on the South Galactic Pole was performed and the results (approximately 2500 red objects) and the discussion of these results are presented. The time that elapsed before a black dwarf becomes invisible is estimated and is suggested that low-velocity red dwarfs could be explained by contracting black dwarfs. Based on theoretical considerations it can be shown that the existence of a large number of low-velocity stars is in serious conflict with criteria for the stability of the galactic disk. It is shown that if one also takes into account all generations of black dwarfs that are already invisible and therfore old, the mean velocity of all black dwarfs is much higher so that there is no conflict with theory. Luminosity functions of red and black dwarfs in several photometric passbands are calculated. (Auth.)

  8. Observational diagnostics of accretion on young stars and brown dwarfs

    Science.gov (United States)

    Stelzer, Beate; Argiroffi, Costanza

    I present a summary of recent observational constraints on the accretion properties of young stars and brown dwarfs with focus on the high-energy emission. In their T Tauri phase young stars assemble a few percent of their mass by accretion from a disk. Various observational signatures of disks around pre-main sequence stars and the ensuing accretion process are found in the IR and optical regime: e.g. excess emission above the stellar photosphere, strong and broad emission lines, optical veiling. At high energies evidence for accretion is less obvious, and the X-ray emission from stars has historically been ascribed to magnetically confined coronal plasmas. While being true for the bulk of the emission, new insight obtained from XMM-Newton and Chandra observations has unveiled contributions from accretion and outflow processes to the X-ray emission from young stars. Their smaller siblings, the brown dwarfs, have been shown to undergo a T Tauri phase on the basis of optical/IR observations of disks and measurements of accretion rates. Most re-cently, first evidence was found for X-rays produced by accretion in a young brown dwarf, complementing the suspected analogy between stars and substellar objects.

  9. White dwarf stars with chemically stratified atmospheres

    Science.gov (United States)

    Muchmore, D.

    1982-01-01

    Recent observations and theory suggest that some white dwarfs may have chemically stratified atmospheres - thin layers of hydrogen lying above helium-rich envelopes. Models of such atmospheres show that a discontinuous temperature inversion can occur at the boundary between the layers. Model spectra for layered atmospheres at 30,000 K and 50,000 K tend to have smaller decrements at 912 A, 504 A, and 228 A than uniform atmospheres would have. On the basis of their continuous extreme ultraviolet spectra, it is possible to distinguish observationally between uniform and layered atmospheres for hot white dwarfs.

  10. Positions and proper motions of dwarf carbon stars

    Science.gov (United States)

    Deutsch, Eric W.

    1994-01-01

    Recent-epochs positions and proper motions of nine dwarf carbon star candidates are presented along with finding charts for each object. Measurements are obtained from digitized Palomar Observatory Sky Survey (POSS) and Quik V plate archives at the Space Telescope Science Institute, and from recent CCD images.

  11. Optical photometry and spectroscopy for five dwarf M stars

    Energy Technology Data Exchange (ETDEWEB)

    Doyle, J.G.; Byrne, P.B.; Menzies, J.W.

    1986-05-01

    A search for flaring and BY Dra (spotted) variations on five M-dwarf stars, Gliese 1, 461, 825, 899 and 908 is reported. The results for Gl 461 and 825 are compared with predictions of activity levels based on the measured quiescent X-ray flux. The chromospheric radiative loss rate in the Ca H and K lines are determined for Gl 461.

  12. Lessons for Asteroseismology from White Dwarf Stars Travis S ...

    Indian Academy of Sciences (India)

    are evenly spaced, but chemical stratification and variations in other relevant physical ... In white dwarf models, the mean period spacing is related to the total stellar mass, .... estimate the mean density of the star (see Kjeldsen et al. 1995 ... in any stellar population (the Galactic disk and halo, open and globular clusters) to.

  13. Chemo-orbital evidence from SDSS/SEGUE G dwarf stars for a mixed origin of the Galactic thick disk

    Directory of Open Access Journals (Sweden)

    van de Ven G.

    2012-02-01

    Full Text Available About 13,000 G dwarf within 7stars and smoothly extends into a thick disk with α-enhanced stars, consistent with an in-situ formation through radial migration. On the other hand, the metal-poor population with enhanced α-abundance, higher scale height, and disperse kinematical properties, is difficult to explain with radial migration but might have originated from gas-rich mergers. The thick disk of the Milky Way seems to have a mixed origin.

  14. Absence of young white dwarf companions to five technetium stars

    Energy Technology Data Exchange (ETDEWEB)

    Smith, V.V.; Lambert, D.L.

    1987-10-01

    A search for hot companions to five stars of type MS and S has been carried out using the IUE satellite. No hot companions were detected for the MS stars HR 85, 4647, 6702, and 8062, and the S star HR 8714. Limits on the luminosities of possible white dwarf companions provide lower limits of 2-5x10 to the 8th yr to the ages of any degenerate companions. All five stars exhibit strong Tc I lines, and the presence of technetium, with a half-life of 2.1x10 to the 5th yr, signifies recent nucleosynthesis. The limits on the ages of possible white dwarf companions that are equal to or greater than 1000 half-lives of Tc exclude the possibility that the s-process elemental enhancement seen in these MS and S stars resulted from mass transfer from a more highly evolved companion (as is probably the mechanism by which barium stars are created). These MS and S stars represent a sample of true thermally pulsing asymptotic giant-branch stars. 41 references.

  15. Variable stars in the Pegasus dwarf galaxy (DDO 216)

    Energy Technology Data Exchange (ETDEWEB)

    Hoessel, J.G.; Abbott, M.J.; Saha, A.; Mossman, A.E.; Danielson, G.E. (Washburn Observatory, Madison, WI (USA) Space Telescope Science Institute, Baltimore, MD (USA) Palomar Observatory, Pasadena, CA (USA))

    1990-10-01

    Observations obtained over a period of five years of the resolved stars in the Pegasus dwarf irregular galaxy (DDO 216) have been searched for variable stars. Thirty-one variables were found, and periods established for 12. Two of these variable stars are clearly eclipsing variables, seven are very likely Cepheid variables, and the remaining three are probable Cepheids. The period-luminosity relation for the Cepheids indicates a distance modulus for Pegasus of m - M = 26.22 + or - 0.20. This places Pegasus very near the zero-velocity surface of the Local Group. 25 refs.

  16. Effective field theory for quantum liquid in dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Gabadadze, Gregory [Center for Cosmology and Particle Physics, Department of Physics, New York University, 4 Washington Place, New York, NY 10003 (United States); Rosen, Rachel A., E-mail: gg32@nyu.edu, E-mail: rarosen@physik.su.se [Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, AlbaNova, Roslagstullsbacken 21, SE - 106 91, Stockholm (Sweden)

    2010-04-01

    An effective field theory approach is used to describe quantum matter at greater-than-atomic but less-than-nuclear densities which are encountered in white dwarf stars. We focus on the density and temperature regime for which charged spin-0 nuclei form an interacting charged Bose-Einstein condensate, while the neutralizing electrons form a degenerate fermi gas. After a brief introductory review, we summarize distinctive properties of the charged condensate, such as a mass gap in the bosonic sector as well as gapless fermionic excitations. Charged impurities placed in the condensate are screened with great efficiency, greater than in an equivalent uncondensed plasma. We discuss a generalization of the Friedel potential which takes into account bosonic collective excitations in addition to the fermionic excitations. We argue that the charged condensate could exist in helium-core white dwarf stars and discuss the evolution of these dwarfs. Condensation would lead to a significantly faster rate of cooling than that of carbon- or oxygen-core dwarfs with crystallized cores. This prediction can be tested observationally: signatures of charged condensation may have already been seen in the recently discovered sequence of helium-core dwarfs in the nearby globular cluster NGC 6397. Sufficiently strong magnetic fields can penetrate the condensate within Abrikosov-like vortices. We find approximate analytic vortex solutions and calculate the values of the lower and upper critical magnetic fields at which vortices are formed and destroyed respectively. The lower critical field is within the range of fields observed in white dwarfs, but tends toward the higher end of this interval. This suggests that for a significant fraction of helium-core dwarfs, magnetic fields are entirely expelled within the core.

  17. Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

    OpenAIRE

    Bensby, T.; Johnson, J. A.; Cohen, J.; Feltzing, S.; Udalski, A.; Gould, A.; Huang, W.; Thompson, I.; Simmerer, J.; Adén, D.

    2009-01-01

    Aims. Our aims are twofold. First we aim to evaluate the robustness and accuracy of stellar parameters and detailed elemental abundances that can be derived from high-resolution spectroscopic observations of microlensed dwarf and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to investigate the abundance structure and chemical evolution of the Milky Way Bulge. Contrary to the cool giant stars, with their extremely crowded spectra, the dwarf stars are hotter, their spe...

  18. Compact objects for everyone: I. White dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Jackson, C B; Taruna, J; Pouliot, S L; Ellison, B W; Lee, D D; Piekarewicz, J [Department of Physics, Florida State University, Tallahassee, FL 32306 (United States)

    2005-09-01

    Based upon previous discussions on the structure of compact stars geared towards undergraduate physics students, a real experiment involving two upper-level undergraduate physics students, a beginning physics graduate and two advanced graduate students was conducted. A recent addition to the physics curriculum at Florida State University, The Physics of Stars, sparked quite a few students' interests in the subject matter involving stellar structure. This, coupled with Stars and statistical physics by Balian and Blaizot (1999 Am. J. Phys. 67 1189) and Neutron stars for undergraduates by Silbar and Reddy (2004 Am. J. Phys. 72 892), is the cornerstone of this small research group who tackled solving the structure equations for compact objects in the summer of 2004. Through the use of a simple finite-difference algorithm coupled to Microsoft Excel and Maple, solutions to the equations for stellar structure are presented in the Newtonian regime appropriate to the physics of white dwarf stars.

  19. Compact objects for everyone: I. White dwarf stars

    International Nuclear Information System (INIS)

    Jackson, C B; Taruna, J; Pouliot, S L; Ellison, B W; Lee, D D; Piekarewicz, J

    2005-01-01

    Based upon previous discussions on the structure of compact stars geared towards undergraduate physics students, a real experiment involving two upper-level undergraduate physics students, a beginning physics graduate and two advanced graduate students was conducted. A recent addition to the physics curriculum at Florida State University, The Physics of Stars, sparked quite a few students' interests in the subject matter involving stellar structure. This, coupled with Stars and statistical physics by Balian and Blaizot (1999 Am. J. Phys. 67 1189) and Neutron stars for undergraduates by Silbar and Reddy (2004 Am. J. Phys. 72 892), is the cornerstone of this small research group who tackled solving the structure equations for compact objects in the summer of 2004. Through the use of a simple finite-difference algorithm coupled to Microsoft Excel and Maple, solutions to the equations for stellar structure are presented in the Newtonian regime appropriate to the physics of white dwarf stars

  20. SEARCH FOR RED DWARF STARS IN GLOBULAR CLUSTER NGC 6397

    Science.gov (United States)

    2002-01-01

    Left A NASA Hubble Space Telescope image of a small region (1.4 light-years across) in the globular star cluster NGC 6397. Simulated stars (diamonds) have been added to this view of the same region of the cluster to illustrate what astronomers would have expected to see if faint red dwarf stars were abundant in the Milky Way Galaxy. The field would then contain 500 stars, according to theoretical calculations. Right The unmodified HST image shows far fewer stars than would be expected, according to popular theories of star formation. HST resolves about 200 stars. The stellar density is so low that HST can literally see right through the cluster and resolve far more distant background galaxies. From this observation, scientists have identified the surprising cutoff point below which nature apparently doesn't make many stars smaller that 1/5 the mass of our Sun. These HST findings provide new insights into star formation in our Galaxy. Technical detail:The globular cluster NGC 6397, one of the nearest and densest agglomerations of stars, is located 7,200 light-years away in the southern constellation Ara. This visible-light picture was taken on March 3, 1994 with the Wide Field Planetary Camera 2, as part the HST parallel observing program. Credit: F. Paresce, ST ScI and ESA and NASA

  1. Delayed star formation in isolated dwarf galaxies: Hubble space telescope star formation history of the Aquarius dwarf irregular

    Energy Technology Data Exchange (ETDEWEB)

    Cole, Andrew A. [School of Physical Sciences, University of Tasmania, Private Bag 37, Hobart, Tasmania, 7001 Australia (Australia); Weisz, Daniel R. [Department of Astronomy, University of California at Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 (United States); Dolphin, Andrew E. [Raytheon, 1151 East Hermans Road, Tucson, AZ 85706 (United States); Skillman, Evan D. [Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, MN 55441 (United States); McConnachie, Alan W. [NRC Herzberg Institute of Astrophysics, Dominion Astrophysical Observatory, Victoria, BC, V9E 2E7 Canada (Canada); Brooks, Alyson M. [Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854 (United States); Leaman, Ryan, E-mail: andrew.cole@utas.edu.au, E-mail: drw@ucsc.edu, E-mail: adolphin@raytheon.com, E-mail: skillman@astro.umn.edu, E-mail: alan.mcconnachie@nrc-cnrc.gc.ca, E-mail: abrooks@physics.rutgers.edu, E-mail: rleaman@iac.es [Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife (Spain)

    2014-11-01

    We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope Advanced Camera for Surveys. The resulting color-magnitude diagram (CMD) reaches more than a magnitude below the oldest main-sequence turnoff, allowing us to derive the star formation history (SFH) over the entire lifetime of the galaxy with a timing precision of ≈10% of the lookback time. Using a maximum likelihood fit to the CMD we find that only ≈10% of all star formation in Aquarius took place more than 10 Gyr ago (lookback time equivalent to redshift z ≈ 2). The star formation rate increased dramatically ≈6-8 Gyr ago (z ≈ 0.7-1.1) and then declined until the present time. The only known galaxy with a more extreme confirmed delay in star formation is Leo A, a galaxy of similar M {sub H} {sub I}/M {sub *}, dynamical mass, mean metallicity, and degree of isolation. The delayed stellar mass growth in these galaxies does not track the mean dark matter accretion rate from CDM simulations. The similarities between Leo A and Aquarius suggest that if gas is not removed from dwarf galaxies by interactions or feedback, it can linger for several gigayears without cooling in sufficient quantity to form stars efficiently. We discuss possible causes for the delay in star formation including suppression by reionization and late-time mergers. We find reasonable agreement between our measured SFHs and select cosmological simulations of isolated dwarfs. Because star formation and merger processes are both stochastic in nature, delayed star formation in various degrees is predicted to be a characteristic (but not a universal) feature of isolated small galaxies.

  2. Abundances in field dwarf stars. II. Carbon and nitrogen abundances

    Energy Technology Data Exchange (ETDEWEB)

    Laird, J.B.

    1985-02-15

    Intermediate-dispersion spectra of 116 field dwarf stars, plus 10 faint field giants and 3 Hyades dwarfs, have been used to derive carbon and nitrogen abundances relative to iron. The program sample includes both disk and halo stars, spanning a range in (Fe/H) of +0.50 to -2.45. Synthetic spectra of CH and NH bands have been used to determine carbon and nitrogen abundances. The C/Fe ratio is solar over the range of metallicity studied, with an estimated intrinsic scatter of 0.10 dex. Down to (Fe/H)roughly-equal-1.8, below which the nitrogen abundance could not be measured, the N/Fe ratio is also constant for the majority of stars, indicating that nitrogen production is largely primary. Four halo stars are found to be enhanced in nitrogen relative to iron, by factors between 5 and 50, although their carbon abundances appear to be normal. These results are discussed in connection with the chemical evolution of the Galaxy and the sites of C, N, and Fe nucleosynthesis. The results require that C, N, and Fe be produced in stars of similar mass. Our current understanding of N production, then, implies that most Type I supernovae have intermediate-mass progenitors. The nitrogen in the N-enhanced halo stars is very probably primordial, indicating that the interstellar medium at early epochs contained substantial inhomogeneities.

  3. Abundances in field dwarf stars. II. Carbon and nitrogen abundances

    International Nuclear Information System (INIS)

    Laird, J.B.

    1985-01-01

    Intermediate-dispersion spectra of 116 field dwarf stars, plus 10 faint field giants and 3 Hyades dwarfs, have been used to derive carbon and nitrogen abundances relative to iron. The program sample includes both disk and halo stars, spanning a range in [Fe/H] of +0.50 to -2.45. Synthetic spectra of CH and NH bands have been used to determine carbon and nitrogen abundances. The C/Fe ratio is solar over the range of metallicity studied, with an estimated intrinsic scatter of 0.10 dex. Down to [Fe/H]roughly-equal-1.8, below which the nitrogen abundance could not be measured, the N/Fe ratio is also constant for the majority of stars, indicating that nitrogen production is largely primary. Four halo stars are found to be enhanced in nitrogen relative to iron, by factors between 5 and 50, although their carbon abundances appear to be normal. These results are discussed in connection with the chemical evolution of the Galaxy and the sites of C, N, and Fe nucleosynthesis. The results require that C, N, and Fe be produced in stars of similar mass. Our current understanding of N production, then, implies that most Type I supernovae have intermediate-mass progenitors. The nitrogen in the N-enhanced halo stars is very probably primordial, indicating that the interstellar medium at early epochs contained substantial inhomogeneities

  4. RUNAWAY DWARF CARBON STARS AS CANDIDATE SUPERNOVA EJECTA

    Energy Technology Data Exchange (ETDEWEB)

    Plant, Kathryn A.; Margon, Bruce; Guhathakurta, Puragra; Cunningham, Emily C.; Toloba, Elisa [Department of Astronomy and Astrophysics and University of California Observatories, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 (United States); Munn, Jeffrey A., E-mail: kaplant@ucsc.edu [US Naval Observatory, Flagstaff Station, 10391 West Naval Observatory Road, Flagstaff, AZ 86005-8521 (United States)

    2016-12-20

    The dwarf carbon (dC) star SDSS J112801.67+004034.6 has an unusually high radial velocity, 531 ± 4 km s{sup −1}. We present proper motion and new spectroscopic observations which imply a large Galactic rest frame velocity, 425 ± 9 km s{sup −1}. Several other SDSS dC stars are also inferred to have very high galactocentric velocities, again each based on both high heliocentric radial velocity and also confidently detected proper motions. Extreme velocities and the presence of C {sub 2} bands in the spectra of dwarf stars are both rare. Passage near the Galactic center can accelerate stars to such extreme velocities, but the large orbital angular momentum of SDSS J1128 precludes this explanation. Ejection from a supernova in a binary system or disruption of a binary by other stars are possibilities, particularly as dC stars are thought to obtain their photospheric C {sub 2} via mass transfer from an evolved companion.

  5. Neutron star formation in theoretical supernovae. Low mass stars and white dwarfs

    International Nuclear Information System (INIS)

    Nomoto, K.

    1986-01-01

    The presupernova evolution of stars that form semi-degenerate or strongly degenerate O + Ne + Mg cores is discussed. For the 10 to 13 Msub solar stars, behavior of off-center neon flashes is crucial. The 8 to 10 m/sub solar stars do not ignite neon and eventually collapse due to electron captures. Properties of supernova explosions and neutron stars expected from these low mass progenitors are compared with the Crab nebula. The conditions for which neutron stars form from accretion-induced collapse of white dwarfs in clsoe binary systems is also examined

  6. ABOUT EXOBIOLOGY: THE CASE FOR DWARF K STARS

    Energy Technology Data Exchange (ETDEWEB)

    Cuntz, M. [Department of Physics, University of Texas at Arlington, Arlington, TX 76019 (United States); Guinan, E. F., E-mail: cuntz@uta.edu, E-mail: edward.guinan@villanova.edu [Department of Astrophysics and Planetary Science, Villanova University, Villanova, PA 19085 (United States)

    2016-08-10

    One of the most fundamental topics of exobiology concerns the identification of stars with environments consistent with life. Although it is believed that most types of main-sequence stars might be able to support life, particularly extremophiles, special requirements appear to be necessary for the development and sustainability of advanced life forms. From our study, orange main-sequence stars, ranging from spectral type late-G to mid-K (with a maximum at early K), are most promising. Our analysis considers a variety of aspects, including (1) the frequency of the various types of stars, (2) the speed of stellar evolution in their lifetimes, (3) the size of the stellar climatological habitable zones (CLI-HZs), (4) the strengths and persistence of their magnetic-dynamo-generated X-ray–UV emissions, and (5) the frequency and severity of flares, including superflares; both (4) and (5) greatly reduce the suitability of red dwarfs to host life-bearing planets. The various phenomena show pronounced dependencies on the stellar key parameters such as effective temperature and mass, permitting the assessment of the astrobiological significance of various types of stars. Thus, we developed a “Habitable-Planetary-Real-Estate Parameter” (HabPREP) that provides a measure for stars that are most suitable for planets with life. Early K stars are found to have the highest HabPREP values, indicating that they may be “Goldilocks” stars for life-hosting planets. Red dwarfs are numerous, with long lifetimes, but their narrow CLI-HZs and hazards from magnetic activity make them less suitable for hosting exolife. Moreover, we provide X-ray–far-UV irradiances for G0 V–M5 V stars over a wide range of ages.

  7. ABOUT EXOBIOLOGY: THE CASE FOR DWARF K STARS

    International Nuclear Information System (INIS)

    Cuntz, M.; Guinan, E. F.

    2016-01-01

    One of the most fundamental topics of exobiology concerns the identification of stars with environments consistent with life. Although it is believed that most types of main-sequence stars might be able to support life, particularly extremophiles, special requirements appear to be necessary for the development and sustainability of advanced life forms. From our study, orange main-sequence stars, ranging from spectral type late-G to mid-K (with a maximum at early K), are most promising. Our analysis considers a variety of aspects, including (1) the frequency of the various types of stars, (2) the speed of stellar evolution in their lifetimes, (3) the size of the stellar climatological habitable zones (CLI-HZs), (4) the strengths and persistence of their magnetic-dynamo-generated X-ray–UV emissions, and (5) the frequency and severity of flares, including superflares; both (4) and (5) greatly reduce the suitability of red dwarfs to host life-bearing planets. The various phenomena show pronounced dependencies on the stellar key parameters such as effective temperature and mass, permitting the assessment of the astrobiological significance of various types of stars. Thus, we developed a “Habitable-Planetary-Real-Estate Parameter” (HabPREP) that provides a measure for stars that are most suitable for planets with life. Early K stars are found to have the highest HabPREP values, indicating that they may be “Goldilocks” stars for life-hosting planets. Red dwarfs are numerous, with long lifetimes, but their narrow CLI-HZs and hazards from magnetic activity make them less suitable for hosting exolife. Moreover, we provide X-ray–far-UV irradiances for G0 V–M5 V stars over a wide range of ages.

  8. White dwarf stars as strange quark matter detectors

    Energy Technology Data Exchange (ETDEWEB)

    Benvenuto, O G [Departamento de AstronomIa y AstroFisica, Pontificia Universidad Catolica, Vicuna Mackenna 4860, Casilla 306, Santiago (Chile); Facultad de Ciencias Astronomicas y GeoFisicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, B1900FWA, La Plata (Argentina)

    2005-11-01

    We show that the presence of a strange matter core inside a white dwarf (WD) star produces a drastic change in the spectrum of non-radial oscillations in the range of periods corresponding to gravity modes. The distinctive, observable signal for such a core is a very short period spacing between consecutive modes, far shorter than in the case of pulsating WDs without any compact core. (letter to the editor)

  9. Variable stars in the Leo A dwarf galaxy (DDO 69)

    Science.gov (United States)

    Hoessel, John G.; Saha, A.; Krist, John; Danielson, G. Edward

    1994-01-01

    Observations of the Leo A dwarf galaxy, obtained over the period from 1980 to 1991 are reported. Forty two separate Charge Coupled Devices (CCD) frames were searched for variable stars. A total of 14 suspected variables were found, 9 had sufficient coverage for period determination, and 5 had Cepheid light curves. Four of these stars fit well on a P-L relation and yield a distance modulus, after correction for Galactic foreground extinction, of m-M = 26.74. This corresponds to a distance of 2.2 Mpc, placing Leo A near the Local Group zero-velocity surface.

  10. White dwarf stars: cosmic chronometers and dark matter probes

    Science.gov (United States)

    Salaris, Maurizio; Cassisi, Santi

    2018-04-01

    White dwarfs (WD) are the endpoint of the evolution of the large majority of stars formed in our galaxy. In the last two decades observations and theory have improved to a level that makes it possible to employ WD for determining ages of the stellar populations in the disk of the Milky Way and in the nearest star clusters, and constrain the existence and properties of dark matter (DM) candidates. This review is centred on WD models, age-dating, and DM identification methods, recent results and future developments of the field.

  11. THE STAR FORMATION HISTORY OF ISOLATED DWARF UGC 4879

    International Nuclear Information System (INIS)

    Jacobs, Bradley A.; Tully, R. Brent; Rizzi, Luca; Karachentsev, Igor D.; Chiboucas, Kristin; Held, Enrico V.

    2011-01-01

    Recent observations of UGC 4879 with the Advanced Camera for Surveys on the Hubble Space Telescope confirm that it is a nearby isolated dwarf irregular galaxy. We measure a distance of 1.36 ± 0.03 Mpc using the tip of the red giant branch method. This distance puts UGC 4879 beyond the radius of first turnaround of the Local Group and ∼700 kpc from its nearest neighbor Leo A. This isolation makes this galaxy an ideal laboratory for studying pristine star formation uncomplicated by interactions with other galaxies. We present the star formation history of UGC 4879 derived from simulated color-magnitude diagrams.

  12. Theoretical colours for F and G dwarf stars.

    Science.gov (United States)

    Bell, R. A.

    1971-01-01

    Synthetic spectra have been computed for F and G dwarf stars, using a number of values of chemical abundance, Doppler broadening velocity, and damping constant. Metal abundances for a number of such stars have been obtained using computed and observed m(sub 1) and 40-52 colors. These abundances are in good agreement with spectroscopically determined ones. The c(sub 1) colors of such stars with exactly known trigonometric parallaxes have been used in order to determine how accurately absolute magnitudes can be predicted from the colors. Generally reasonable agreement can be obtained between observed and predicted absolute magnitudes for certain of these stars. The effects of interstellar reddening on the colors of the models are examined.

  13. J0815+4729: A Chemically Primitive Dwarf Star in the Galactic Halo Observed with Gran Telescopio Canarias

    Science.gov (United States)

    Aguado, David S.; González Hernández, Jonay I.; Allende Prieto, Carlos; Rebolo, Rafael

    2018-01-01

    We report the discovery of the carbon-rich hyper metal-poor unevolved star J0815+4729. This dwarf star was selected from SDSS/BOSS as a metal-poor candidate and follow-up spectroscopic observations at medium resolution were obtained with the Intermediate dispersion Spectrograph and Imaging System (ISIS) at William Herschel Telescope and the Optical System for Imaging and low-intermediate-Resolution Integrated Spectroscopy (OSIRIS) at Gran Telescopio de Canarias. We use the FERRE code to derive the main stellar parameters, {T}{eff}=6215+/- 82 K, and {log}g=4.7+/- 0.5, an upper limit to the metallicity of [Fe/H] ≤ ‑5.8, and a carbon abundance of [C/Fe] ≥ +5.0, while [α /{Fe}]=0.4 is assumed. The metallicity upper limit is based on the Ca II K line, which at the resolving power of the OSIRIS spectrograph cannot be resolved from possible interstellar calcium. The star could be the most iron-poor unevolved star known and also be among the ones with the largest overabundances of carbon. High-resolution spectroscopy of J0815+4729 will certainly help to derive other important elemental abundances, possibly providing new fundamental constraints on the early stages of the universe, the formation of the first stars, and the properties of the first supernovae. Based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. Program ID GTC90-15B and the Discretionary Director Time GTC03-16ADDT and also based on observations made with the William Herschel Telescope (WHT).

  14. The Productivity of Oxygenic Photosynthesis around Cool, M Dwarf Stars

    Science.gov (United States)

    Lehmer, Owen R.; Catling, David C.; Parenteau, Mary N.; Hoehler, Tori M.

    2018-06-01

    In the search for life around cool stars, the presence of atmospheric oxygen is a prominent biosignature, as it may indicate oxygenic photosynthesis (OP) on the planetary surface. On Earth, most oxygenic photosynthesizing organisms (OPOs) use photons between 400 and 750 nm, which have sufficient energy to drive the photosynthetic reaction that generates O2 from H2O and CO2. OPOs around cool stars may evolve similar biological machinery capable of producing oxygen from water. However, in the habitable zones (HZs) of the coolest M dwarf stars, the flux of 400–750 nm photons may be just a few percent that of Earth’s. We show that the reduced flux of 400–750 nm photons around M dwarf stars could result in Earth-like planets being growth limited by light, unlike the terrestrial biosphere, which is limited by nutrient availability. We consider stars with photospheric temperatures between 2300 and 4200 K and show that such light-limited worlds could occur at the outer edge of the HZ around TRAPPIST-1-like stars. We find that even if OP can use photons longer than 750 nm, there would still be insufficient energy to sustain the Earth’s extant biosphere throughout the HZ of the coolest stars. This is because such stars emit largely in the infrared and near-infrared, which provide sufficient energy to make the planet habitable, but limits the energy available for OP. TRAPPIST-1f and g may fall into this category. Biospheres on such planets, potentially limited by photon availability, may generate small biogenic signals, which could be difficult for future observations to detect.

  15. The Ultraviolet Radiation Environment around M Dwarf Exoplanet Host Stars

    Science.gov (United States)

    France, Kevin; Froning, Cynthia S.; Linsky, Jeffrey L.; Roberge, Aki; Stocke, John T.; Tian, Feng; Bushinsky, Rachel; Desert, Jean-Michel; Mauas, Pablo; Mauas, Pablo; hide

    2013-01-01

    The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. Ultraviolet photons influence the atmospheric temperature profiles and production of potential biomarkers on Earth-like planets around these stars. At present, little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both far-UV (FUV) and near-UV (NUV) wavelengths. The combined FUV+NUV spectra are publicly available in machine-readable format. We find that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region UV emission. No "UV-quiet" M dwarfs are observed. The bright stellar Lyman-alpha emission lines are reconstructed, and we find that the Lyman-alpha line fluxes comprise approximately 37%-75% of the total 1150-3100 A flux from most M dwarfs; approximately greater than 10(exp3) times the solar value. We develop an empirical scaling relation between Lyman-alpha and Mg II emission, to be used when interstellar H I attenuation precludes the direct observation of Lyman-alpha. The intrinsic unreddened flux ratio is F(Lyman-alpha)/F(Mg II) = 10(exp3). The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O2 and O3, is shown to be approximately 0.5-3 for all M dwarfs in our sample, greather than 10(exp3) times the solar ratio. For the four stars with moderate signal-to-noise Cosmic Origins Spectrograph time-resolved spectra, we find UV emission line variability with amplitudes of 50%.500% on 10(exp2)-10(exp3) s timescales. This effect should be taken

  16. THE ULTRAVIOLET RADIATION ENVIRONMENT AROUND M DWARF EXOPLANET HOST STARS

    Energy Technology Data Exchange (ETDEWEB)

    France, Kevin; Froning, Cynthia S.; Stocke, John T.; Bushinsky, Rachel [Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO 80309 (United States); Linsky, Jeffrey L. [JILA, University of Colorado and NIST, 440 UCB, Boulder, CO 80309 (United States); Roberge, Aki [Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States); Tian, Feng [Center for Earth System Sciences, Tsinghua University, Beijing 100084 (China); Desert, Jean-Michel [Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 (United States); Mauas, Pablo; Vieytes, Mariela [Instituto de Astronomsica del Espacio (CONICET-UBA), C.C. 67 Sucursal 28, 1428 Buenos Aires (Argentina); Walkowicz, Lucianne M., E-mail: kevin.france@colorado.edu [Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)

    2013-02-15

    The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. Ultraviolet photons influence the atmospheric temperature profiles and production of potential biomarkers on Earth-like planets around these stars. At present, little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both far-UV (FUV) and near-UV (NUV) wavelengths. The combined FUV+NUV spectra are publicly available in machine-readable format. We find that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region UV emission. No 'UV-quiet' M dwarfs are observed. The bright stellar Ly{alpha} emission lines are reconstructed, and we find that the Ly{alpha} line fluxes comprise {approx}37%-75% of the total 1150-3100 A flux from most M dwarfs; {approx}>10{sup 3} times the solar value. We develop an empirical scaling relation between Ly{alpha} and Mg II emission, to be used when interstellar H I attenuation precludes the direct observation of Ly{alpha}. The intrinsic unreddened flux ratio is F(Ly{alpha})/F(Mg II) = 10 {+-} 3. The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O{sub 2} and O{sub 3}, is shown to be {approx}0.5-3 for all M dwarfs in our sample, >10{sup 3} times the solar ratio. For the four stars with moderate signal-to-noise Cosmic Origins Spectrograph time-resolved spectra, we find UV emission line variability with amplitudes of 50%-500% on 10{sup 2}-10{sup 3} s timescales. This effect should be taken into account in future UV

  17. Exploration of the brown dwarf regime around solar-like stars by CoRoT

    OpenAIRE

    Csizmadia, Szilárd

    2016-01-01

    Aims. A summary of the CoRoT brown dwarf investigations are presented. Methods. Transiting brown dwarfs around solar like stars were studied by using the photometric time-series of CoRoT, and ground based radial velocity measurements. Results. CoRoT detected three transiting brown dwarfs around F and G dwarf stars. The occurence rate of brown dwarfs was found to be 0.20 +/- 0.15% around solar-like stars which is compatible with the value obtained by Kepler-data.

  18. Atmospheric activity in red dwarf stars

    International Nuclear Information System (INIS)

    Pettersen, B.R.

    1986-01-01

    Active and inactive stars of similar mass and luminosity have similar physical conditions in their photospheres, outside of magnetically disturbed regions. Such field structures give rise to stellar activity, which manifests itself at all heights of the atmosphere. Observations of uneven distributions of flux across the stellar disc have led to the disovery of photospheric starspots, chromospheric plage areas, and coronal holes. Localized transient behavior has been identified in both thermal and non-thermal sources, such as flares, shock waves and particle acceleration. The common element to all active regions is the presence of strong magnetic field structures connecting the violently turbulent deep layers in the convection zones of stars with the tenuous outer atmospheres. Transport and dissipation of energy into the chromospheric and coronal regions are still much debated topics

  19. The Variable Stars of the Draco Dwarf Spheroidal Galaxy: Revisited

    Science.gov (United States)

    Kinemuchi, K.; Harris, H. C.; Smith, Horace A.; Silbermann, N. A.; Snyder, L. A.; La Cluyzé, A. P.; Clark, C. L.

    2008-11-01

    We present a CCD survey of variable stars in the Draco dwarf spheroidal galaxy. This survey, which has the largest areal coverage since the original variable star survey by Baade & Swope, includes photometry for 270 RR Lyrae (RRL) stars, 9 anomalous Cepheids (ACs), 2 eclipsing binaries, and 12 slow, irregular red variables, as well as 30 background QSOs. Twenty-six probable double-mode RRL stars were identified. Observed parameters, including mean V and I magnitudes, V amplitudes, and periods, have been derived. Photometric metallicities of the ab-type RRL stars were calculated according to the method of Jurcsik & Kovacs, yielding a mean metallicity of lang[Fe/H]rang = -2.19 ± 0.03. The well-known Oosterhoff intermediate nature of the RRL stars in Draco is reconfirmed, although the double-mode RRL stars, with one exception, have properties similar to those found in Oosterhoff type II globular clusters. The period-luminosity relation of the ACs is rediscussed with the addition of the new Draco ACs.

  20. Search for white dwarf companions of cool stars with peculiar element abundances

    Energy Technology Data Exchange (ETDEWEB)

    Boehm-Vitense, E.

    1984-01-01

    A search for a white dwarf companions of cool stars with peculiar element abundances was undertaken. One additional star the xi Cet, was found with a white dwarf companion. It was found that HR 1016, 56Uma, 16 Ser, have high excitation emission lines which indicate a high temperature object in the system. It is suggested that since these indications for high temperature companions were seen for all nearby Ba stars, it is highly probable that all Ba stars have white dwarf companions, and that the peculiar element abundances seen in the Ba stars are due to mass transfer. Observations, arguments and conclusions are presented. White dwarf companions were not found. Together with the Li and Be abundances and the chromospheric emission line spectra in these stars were studied. No white dwarf companions were seen for subgiant CH stars.

  1. Search for white dwarf companions of cool stars with peculiar element abundances

    International Nuclear Information System (INIS)

    Boehm-Vitense, E.

    1984-01-01

    A search for a white dwarf companions of cool stars with peculiar element abundances was undertaken. One additional star the xi Cet, was found with a white dwarf companion. It was found that HR 1016, 56Uma, 16 Ser, have high excitation emission lines which indicate a high temperature object in the system. It is suggested that since these indications for high temperature companions were seen for all nearby Ba stars, it is highly probable that all Ba stars have white dwarf companions, and that the peculiar element abundances seen in the Ba stars are due to mass transfer. Observations, arguments and conclusions are presented. White dwarf companions were not found. Together with the Li and Be abundances and the chromospheric emission line spectra in these stars were studied. No white dwarf companions were seen for subgiant CH stars

  2. UVES Abundances of Stars in Nearby Dwarf Spheroidal Galaxies

    Science.gov (United States)

    Tolstoy, Eline; Venn, Kim; Shetrone, Matt; Primas, Francesca; Hill, Vanessa; Kaufer, Andreas; Szeifert, Thomas

    2002-07-01

    It is a truth universally acknowledged, that a galaxy in possession of a good quantity of gas must want to form stars. It is the details of how and why that baffle us all. The simplest theories either would have this process a carefully self-regulated affair, or one that goes completely out of control and is capable of wrecking the galaxy which hosts it. Of course the majority of galaxies seem to amble along somewhere between these two extremes, and the mean properties tend to favour a quiescent self-regulated evolutionary scenario. But there area variety of observations which require us to invoke transitory ‘bursts’ of star-formation at one time or another in most galaxy types. Several nearby dwarf spheroidal galaxies have clearly determined star-formation histories with apparent periods of zero star formation followed by periods of fairly active star formation. If we are able to understand what separated these bursts we would understand several important phenomena in galaxy evolution. Were these galaxies able to clear out their gas reservoir in a burst of star formation? How did this gas return? or did it? Have these galaxies receieved gas from the IGM instead? Could stars from these types of galaxy contribute significantly to the halo population in our Galaxy? To answer these questions we need to combine accurate stellar photometry and Colour-Magnitude Diagram interpretation with detailed metal abundances to combine a star-formation rate versus time with a range of element abundances with time. Different elements trace different evolutionary process (e.g., relative contributions of type I and II supernovae). We often aren't even sure of the abundance spread in these galaxies. We have collected detailed high resolution UVES spectra of four nearby dwarf spheroidal galaxies (Sculptor, Fornax, Leo I & Carina) to begin to answer these questions. This is a precursor study to a more complete study with FLAMES. We presented at this meeting the initial results for

  3. The chemical composition of TS 01, the most oxygen-deficient planetary nebula. AGB nucleosynthesis in a metal-poor binary star

    Science.gov (United States)

    Stasińska, G.; Morisset, C.; Tovmassian, G.; Rauch, T.; Richer, M. G.; Peña, M.; Szczerba, R.; Decressin, T.; Charbonnel, C.; Yungelson, L.; Napiwotzki, R.; Simón-Díaz, S.; Jamet, L.

    2010-02-01

    The planetary nebula TS 01 (also called PN G 135.9+55.9 or SBS 1150+599A) with its record-holding low oxygen abundance and its double degenerate close binary core (period 3.9 h) is an exceptional object located in the Galactic halo. We have secured observational data in a complete wavelength range to pin down the abundances of half a dozen elements in the nebula. The abundances are obtained via detailed photoionization modelling which takes into account all the observational constraints (including geometry and aperture effects) using the pseudo-3D photoionization code Cloudy_3D. The spectral energy distribution of the ionizing radiation is taken from appropriate model atmospheres. Incidentally we find from the new observational constraints that both stellar components contribute to the ionization: the “cool” one provides the bulk of hydrogen ionization, while the “hot” one is responsible for the presence of the most highly charged ions, which explains why previous attempts to model the nebula experienced difficulties. The nebular abundances of C, N, O, and Ne are found to be 1/3.5, 1/4.2, 1/70, and 1/11 of the solar value respectively, with uncertainties of a factor 2. Thus the extreme O deficiency of this object is confirmed. The abundances of S and Ar are less than 1/30 of solar. The abundance of He relative to H is 0.089 ± 0.009. Standard models of stellar evolution and nucleosynthesis cannot explain the abundance pattern observed in the nebula. To obtain an extreme oxygen deficiency in a star whose progenitor has an initial mass of about 1 M⊙ requires an additional mixing process, which can be induced by stellar rotation and/or by the presence of the close companion. We have computed a stellar model with an initial mass of 1 M⊙, appropriate metallicity, and initial rotation of 100 km s-1, and find that rotation greatly improves the agreement between the predicted and observed abundances. Based on observations obtained at the Canada

  4. Thomson scattering in magnetic fields. [of white dwarf stars

    Science.gov (United States)

    Whitney, Barbara

    1989-01-01

    The equation of transfer in Thomson scattering atmospheres with magnetic fields is solved using Monte Carlo methods. Two cases, a plane parallel atmosphere with a magnetic field perpendicular to the atmosphere, and a dipole star, are investigated. The wavelength dependence of polarization from plane-parallel atmosphere is qualitatively similar to that observed in the magnetic white dwarf Grw+70 deg 8247, and the field strength determined by the calculation, 320 MG, is quantitatively similar to that determined from the line spectrum. The dipole model does not resemble the data as well as the single plane-parallel atmosphere.

  5. Microlensing of unresolved stars as a brown dwarf detection method

    CERN Document Server

    Bouquet, Alain; Melchior, Anne-Laure; Giraud-Heraud, Yannick; Baillon, Paul

    1993-01-01

    We describe a project of brown dwarf detection in the dark halo of a galaxy using the microlensing effect. We argue that monitoring pixels instead of stars could provide an enhancement in the number of detectable events. We estimate the detection efficiency with a Monte-Carlo simulation. We expect a ten-fold increase with respect to current experiments. To assess the feasibility of this method we have determined the photometric precision of a pixel by comparing several pictures of a same field in the LMC. To be published in the Proceeding of the workshop 'The dark side of the universe...', Roma, Juin 1993,

  6. Spot temperatures and area coverages on active dwarf stars

    Science.gov (United States)

    Sarr, Steven H.; Neff, James E.

    1990-01-01

    Two active K dwarfs are examined to determine the temperatures of the stars and to estimate the locations and sizes of cool spots on the stellar surfaces. Two wavelength regions with TiO absorption bands at different temperature sensitivities are modeled simultaneously using the method developed by Huenemoerder and Ramsey (1987). The spectrum of BD +26deg730 shows excess absorption in the TiO band, and the absence of the 8860 A band in HD 82558 indicates that its spots are warmer than those of BD +26deg730.

  7. White dwarf stars and the age of the Galactic disk

    Science.gov (United States)

    Wood, M. A.

    1990-01-01

    The history of the Galaxy is written in its oldest stars, the white dwarf (WD) stars. Significant limits can be placed on both the Galactic age and star formation history. A wide range of input WD model sequences is used to derive the current limits to the age estimates suggested by fitting to the observed falloff in the WD luminosity function. The results suggest that the star formation rate over the history of the Galaxy has been relatively constant, and that the disk age lies in the range 6-12 billion years, depending upon the assumed structure of WD stars, and in particular on the core composition and surface helium layer mass. Using plausible mixed C/O core input models, the estimates for the disk age range from 8-10.5 Gyr, i.e.,sustantially younger than most age estimates for the halo globular clusters. After speculating on the significance of the results, expected observational and theoretical refinements which will further enhance the reliability of the method are discussed.

  8. Variable Stars in (Not Only) Dwarf Galaxies : Key Tools to Constrain Distances and Stellar Content

    NARCIS (Netherlands)

    Fiorentino, G.; Koleva, M; Prugniel, P; Vauglin,

    2011-01-01

    The important role of Cepheid and RR Lyrae variable stars and what they teach us about dwarf galaxies is discussed. Despite ever improving star formation histories of Local Group dwarf galaxies uncertainties remain, in particular in the identification and characterisation of the oldest stellar

  9. Crystallization of carbon-oxygen mixtures in white dwarf stars.

    Science.gov (United States)

    Horowitz, C J; Schneider, A S; Berry, D K

    2010-06-11

    We determine the phase diagram for dense carbon-oxygen mixtures in white dwarf (WD) star interiors using molecular dynamics simulations involving liquid and solid phases. Our phase diagram agrees well with predictions from Ogata et al. and from Medin and Cumming and gives lower melting temperatures than Segretain et al. Observations of WD crystallization in the globular cluster NGC 6397 by Winget et al. suggest that the melting temperature of WD cores is close to that for pure carbon. If this is true, our phase diagram implies that the central oxygen abundance in these stars is less than about 60%. This constraint, along with assumptions about convection in stellar evolution models, limits the effective S factor for the 12C(α,γ)16O reaction to S(300)≤170  keV b.

  10. RCB stars from double degenerate white dwarf mergers

    Science.gov (United States)

    Staff, Jan; Wiggins, Brandon K.; Marcello, Dominic; Motl, Patrick; Clayton, Geoffrey C.

    2018-01-01

    We have conducted grid based and SPH based hydrodynamic simulations of white dwarf mergers, to investigate the role of dredge-up and mixing during the merger. The goal is to test if sufficiently little 16O can be brought up to the surface to explain the observed 16O to 18O ratio of order unity found in RCB stars. In all simulations, the total mass is ~< 1 M⊙. By initializing both the grid based and the SPH simulations with the same setup, we can compare the results from these different methods. In most of the simulations, more than 0.01 M⊙ of 16O is brought up to the surface. Hence a similar mass of 18O must be produced in order to explain the observed oxygen ratio. However,in SPH simulations where the accretor is a hybrid He/CO white dwarf, much less 16O is brought to the surface, making this an excellent candidate for the progenitor of RCB stars.

  11. CALIBRATING UV STAR FORMATION RATES FOR DWARF GALAXIES FROM STARBIRDS

    Energy Technology Data Exchange (ETDEWEB)

    McQuinn, Kristen B. W.; Skillman, Evan D.; Mitchell, Noah P. [Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street, S.E., Minneapolis, MN 55455 (United States); Dolphin, Andrew E., E-mail: kmcquinn@astro.umn.edu [Raytheon Company, 1151 E. Hermans Road, Tucson, AZ 85756 (United States)

    2015-08-01

    Integrating our knowledge of star formation (SF) traced by observations at different wavelengths is essential for correctly interpreting and comparing SF activity in a variety of systems and environments. This study compares extinction corrected integrated ultraviolet (UV) emission from resolved galaxies with color–magnitude diagram (CMD) based star formation rates (SFRs) derived from resolved stellar populations and CMD fitting techniques in 19 nearby starburst and post-starburst dwarf galaxies. The data sets are from the panchromatic Starburst Irregular Dwarf Survey and include deep legacy GALEX UV imaging, Hubble Space Telescope optical imaging, and Spitzer MIPS imaging. For the majority of the sample, the integrated near-UV fluxes predicted from the CMD-based SFRs—using four different models—agree with the measured, extinction corrected, integrated near-UV fluxes from GALEX images, but the far-UV (FUV) predicted fluxes do not. Furthermore, we find a systematic deviation between the SFRs based on integrated FUV luminosities and existing scaling relations, and the SFRs based on the resolved stellar populations. This offset is not driven by different SF timescales, variations in SFRs, UV attenuation, nor stochastic effects. This first comparison between CMD-based SFRs and an integrated FUV emission SFR indicator suggests that the most likely cause of the discrepancy is the theoretical FUV–SFR calibration from stellar evolutionary libraries and/or stellar atmospheric models. We present an empirical calibration of the FUV-based SFR relation for dwarf galaxies, with uncertainties, which is ∼53% larger than previous relations.

  12. Adaptive Optics Observations of Exoplanets, Brown Dwarfs, and Binary Stars

    Science.gov (United States)

    Hinkley, Sasha

    2012-04-01

    The current direct observations of brown dwarfs and exoplanets have been obtained using instruments not specifically designed for overcoming the large contrast ratio between the host star and any wide-separation faint companions. However, we are about to witness the birth of several new dedicated observing platforms specifically geared towards high contrast imaging of these objects. The Gemini Planet Imager, VLT-SPHERE, Subaru HiCIAO, and Project 1640 at the Palomar 5m telescope will return images of numerous exoplanets and brown dwarfs over hundreds of observing nights in the next five years. Along with diffraction-limited coronagraphs and high-order adaptive optics, these instruments also will return spectral and polarimetric information on any discovered targets, giving clues to their atmospheric compositions and characteristics. Such spectral characterization will be key to forming a detailed theory of comparative exoplanetary science which will be widely applicable to both exoplanets and brown dwarfs. Further, the prevalence of aperture masking interferometry in the field of high contrast imaging is also allowing observers to sense massive, young planets at solar system scales (~3-30 AU)- separations out of reach to conventional direct imaging techniques. Such observations can provide snapshots at the earliest phases of planet formation-information essential for constraining formation mechanisms as well as evolutionary models of planetary mass companions. As a demonstration of the power of this technique, I briefly review recent aperture masking observations of the HR 8799 system. Moreover, all of the aforementioned techniques are already extremely adept at detecting low-mass stellar companions to their target stars, and I present some recent highlights.

  13. THE ACS NEARBY GALAXY SURVEY TREASURY. IX. CONSTRAINING ASYMPTOTIC GIANT BRANCH EVOLUTION WITH OLD METAL-POOR GALAXIES

    International Nuclear Information System (INIS)

    Girardi, Leo; Williams, Benjamin F.; Gilbert, Karoline M.; Rosenfield, Philip; Dalcanton, Julianne J.; Marigo, Paola; Boyer, Martha L.; Dolphin, Andrew; Weisz, Daniel R.; Skillman, Evan; Melbourne, Jason; Olsen, Knut A. G.; Seth, Anil C.

    2010-01-01

    In an attempt to constrain evolutionary models of the asymptotic giant branch (AGB) phase at the limit of low masses and low metallicities, we have examined the luminosity functions and number ratios between AGB and red giant branch (RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy Survey Treasury. This database provides Hubble Space Telescope optical photometry together with maps of completeness, photometric errors, and star formation histories for dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by predominantly metal-poor populations as indicated by a very steep and blue RGB, and which do not present any indication of recent star formation in their color-magnitude diagrams. Thousands of AGB stars brighter than the tip of the RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence, the Poisson noise has little impact in our measurements of the AGB/RGB ratio. We model the photometric data with a few sets of thermally pulsing AGB (TP-AGB) evolutionary models with different prescriptions for the mass loss. This technique allows us to set stringent constraints on the TP-AGB models of low-mass, metal-poor stars (with M sun , [Fe/H]∼ sun . This is also in good agreement with recent observations of white dwarf masses in the M4 old globular cluster. These constraints can be added to those already derived from Magellanic Cloud star clusters as important mileposts in the arduous process of calibrating AGB evolutionary models.

  14. Comparing the asteroseismic properties of pulsating extremely low-mass pre-white dwarf stars and δ Scuti stars

    Directory of Open Access Journals (Sweden)

    Arias J.P.Sánchez

    2017-01-01

    Full Text Available We present the first results of a detailed comparison between the pulsation properties of pulsating Extremely Low-Mass pre-white dwarf stars (the pre-ELMV variable stars and δ Scuti stars. The instability domains of these very different kinds of stars nearly overlap in the log Teff vs. log g diagram, leading to a degeneracy in the classification of the stars. Our aim is to provide asteroseismic tools for their correct classification.

  15. Prospects of the "WSO-UV" Project for Star Formation Study in Nearby Dwarf Galaxies

    Science.gov (United States)

    Makarova, L. N.; Makarov, D. I.

    2017-12-01

    In the present work we consider the questions of star formation and evolution of nearby dwarf galaxies. We describe the method of star formation history determination based on multicolor photometry of resolved stars and models of color-magnitude diagrams of the galaxies. We present the results of star formation rate determination and its dependence on age and metallicity for dwarf irregular and dwarf spheroidal galaxies in the two nearby galaxy groups M81 and Cen A. Similar age of the last episode of star formation in the central part of the M81 group and also unusually high level of metal enrichment in the several galaxies of the Cen A group are mentioned. We pay special attention to the consideration of perspectives of star formation study in nearby dwarf galaxies with he new WSO-UV observatory.

  16. Survival of a brown dwarf after engulfment by a red giant star.

    Science.gov (United States)

    Maxted, P F L; Napiwotzki, R; Dobbie, P D; Burleigh, M R

    2006-08-03

    Many sub-stellar companions (usually planets but also some brown dwarfs) orbit solar-type stars. These stars can engulf their sub-stellar companions when they become red giants. This interaction may explain several outstanding problems in astrophysics but it is unclear under what conditions a low mass companion will evaporate, survive the interaction unchanged or gain mass. Observational tests of models for this interaction have been hampered by a lack of positively identified remnants-that is, white dwarf stars with close, sub-stellar companions. The companion to the pre-white dwarf AA Doradus may be a brown dwarf, but the uncertain history of this star and the extreme luminosity difference between the components make it difficult to interpret the observations or to put strong constraints on the models. The magnetic white dwarf SDSS J121209.31 + 013627.7 may have a close brown dwarf companion but little is known about this binary at present. Here we report the discovery of a brown dwarf in a short period orbit around a white dwarf. The properties of both stars in this binary can be directly observed and show that the brown dwarf was engulfed by a red giant but that this had little effect on it.

  17. Stars of type MS with evidence of white dwarf companions. [IUE, Main Sequence (MS)

    Science.gov (United States)

    Peery, Benjamin F., Jr.

    1986-01-01

    A search for white dwarf companions of MS-type stars was conducted, using IUE. The overendowments of these stars in typical S-process nuclides suggest that they, like the Ba II stars, may owe their peculiar compositions to earlier mass transfer. Short-wavelength IUE spectra show striking emission line variability in HD35155, HD61913, and 4 Ori; HD35155 and 4 Ori show evidence of white dwarf companions.

  18. Environmental effects on star formation in dwarf galaxies and star clusters

    Science.gov (United States)

    Pasetto, Stefano; Cropper, Mark; fujita, Yutaka; Chiosi, Cesare; Grebel, Eva K.

    2015-08-01

    We investigate the competitive role of the different dissipative phenomena acting on the onset of star formation history of gravitationally bound system in an external environment.Ram pressure, Kelvin-Helmholtz instability, Rayleigh-Taylor, and tidal forces are accounted separately in an analytical framework and compared in their role in influencing the star forming regions. The two-fluids instability at the interface between a stellar system and its surrounding hotter and less dense environment is related to the star formation processes through a set of differential equations. We present an analytical criterion to elucidate the dependence of star formation in a spherical stellar system on its surrounding environment useful in theoretical interpretations of numerical results as well as observational applications. We show how spherical coordinates naturally enlighten the interpretation of the two-fluids instability in a geometry that directly applies to astrophysical case. Finally, we consider the different signatures of these phenomena in synthetically realized colour-magnitude diagrams of the orbiting system thus investigating the detectability limits of these different effects for future observational projects and their relevance.The theoretical framework developed has direct applications to the cases of dwarf galaxies in galaxy clusters and dwarf galaxies orbiting our Milky Way system, as well as any primordial gas-rich cluster of stars orbiting within its host galaxy.

  19. The quiescent chromospheres and transition regions of active dwarf stars - What are we learning from recent observations and models?

    Science.gov (United States)

    Linsky, J. L.

    1983-01-01

    Progress in understanding active dwarf stars based on recent IUE, Einstein, and ground-based observations is reviewed. The extent of magnetic field control over nonflare phenomena in active dwarf stars is considered, and the spatial homogeneity and time variability of active dwarf atmospheres is discussed. The possibility that solar like flux tubes can explain enhanced heating in active dwarf stars in examined, and the roles of systematic flows in active dwarf star atmospheres are considered. The relation between heating rates in different layers of active dwarf stars is summarized, and the mechanism of chromosphere and transition region heating in these stars are discussed. The results of one-component and two-component models of active dwarf stars are addressed.

  20. Narrow-band radio flares from red dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    White, S.M.; Kundu, M.R.; Jackson, P.D.

    1986-12-01

    VLA observations of narrow-band behavior in 20 cm flares from two red dwarf stars, L726 - 8A and AD Leo, are reported. The flare on L726 - 8A was observed at 1415 and 1515 MHz; the flux and the evolution differed significantly at the two frequencies. The flare on AD Leo lasted for 2 hr at 1415 MHz but did not appear at 1515 MHz. The AD Leo flare appears to rule out a source drifting through the stellar corona and is unlikely to be due to plasma emission. In the cyclotron maser model the narrow-band behavior reflects the range of magnetic fields present within the source. The apparent constancy of this field for 2 hr is difficult to understand if magnetic reconnection is the source of energy for the flare. The consistent polarization exhibited by red dwarf flares at 20 cm may be related to stellar activity cycles, and changes in this polarization will permit measuring the length of these cycles. 22 references.

  1. Narrow-band radio flares from red dwarf stars

    Science.gov (United States)

    White, Stephen M.; Kundu, Mukul R.; Jackson, Peter D.

    1986-01-01

    VLA observations of narrow-band behavior in 20 cm flares from two red dwarf stars, L726 - 8A and AD Leo, are reported. The flare on L726 - 8A was observed at 1415 and 1515 MHz; the flux and the evolution differed significantly at the two frequencies. The flare on AD Leo lasted for 2 hr at 1415 MHz but did not appear at 1515 MHz. The AD Leo flare appears to rule out a source drifting through the stellar corona and is unlikely to be due to plasma emission. In the cyclotron maser model the narrow-band behavior reflects the range of magnetic fields present within the source. The apparent constancy of this field for 2 hr is difficult to understand if magnetic reconnection is the source of energy for the flare. The consistent polarization exhibited by red dwarf flares at 20 cm may be related to stellar activity cycles, and changes in this polarization will permit measuring the length of these cycles.

  2. Star formation rate in Holmberg IX dwarf galaxy

    Directory of Open Access Journals (Sweden)

    Anđelić M.M.

    2011-01-01

    Full Text Available In this paper we use previously determined Hα fluxes for dwarf galaxy Holmberg IX (Arbutina et al. 2009 to calculate star formation rate (SFR in this galaxy. We discuss possible contaminations of Hα flux and, for the first time, we take into account optical emission from supernova remnants (SNRs as a possible source of contamination of Hα flux. Derived SFR for Holmberg IX is 3:4 x 10-4M.yr-1. Our value is lower then in previous studies, due to luminous shock-heated source M&H 9-10, possible hypernova remnant, which we excluded from the total Hα flux in our calculation of SFR.

  3. The ACS LCID project. IX. Imprints of the early universe in the radial variation of the star formation history of dwarf galaxies

    International Nuclear Information System (INIS)

    Hidalgo, Sebastian L.; Monelli, Matteo; Aparicio, Antonio; Gallart, Carme

    2013-01-01

    Based on Hubble Space Telescope observations from the Local Cosmology from Isolated Dwarfs project, we present the star formation histories, as a function of galactocentric radius, of four isolated Local Group dwarf galaxies: two dSph galaxies, Cetus and Tucana, and two transition galaxies (dTrs), LGS-3 and Phoenix. The oldest stellar populations of the dSphs and dTrs are, within the uncertainties, coeval (∼13 Gyr) at all galactocentric radii. We find that there are no significative differences between the four galaxies in the fundamental properties (such as the normalized star formation rate or age-metallicity relation) of their outer regions (radii greater than four exponential scale lengths); at large radii, these galaxies consist exclusively of old (≳ 10.5 Gyr) metal-poor stars. The duration of star formation in the inner regions varies from galaxy to galaxy, and the extended central star formation in the dTrs produces the dichotomy between dSph and dTr galaxy types. The dTr galaxies show prominent radial stellar population gradients: The centers of these galaxies host young (≲ 1 Gyr) populations, while the age of the last formation event increases smoothly with increasing radius. This contrasts with the two dSph galaxies. Tucana shows a similar, but milder, gradient, but no gradient in age is detected Cetus. For the three galaxies with significant stellar population gradients, the exponential scale length decreases with time. These results are in agreement with outside-in scenarios of dwarf galaxy evolution, in which a quenching of the star formation toward the center occurs as the galaxy runs out of gas in the outskirts.

  4. The ACS LCID project. IX. Imprints of the early universe in the radial variation of the star formation history of dwarf galaxies

    Energy Technology Data Exchange (ETDEWEB)

    Hidalgo, Sebastian L.; Monelli, Matteo; Aparicio, Antonio; Gallart, Carme, E-mail: shidalgo@iac.es, E-mail: monelli@iac.es, E-mail: aparicio@iac.es, E-mail: carme@iac.es [Instituto de Astrofísica de Canarias, Vía Láctea s/n, E-38200 La Laguna, Tenerife, Canary Islands (Spain); and others

    2013-12-01

    Based on Hubble Space Telescope observations from the Local Cosmology from Isolated Dwarfs project, we present the star formation histories, as a function of galactocentric radius, of four isolated Local Group dwarf galaxies: two dSph galaxies, Cetus and Tucana, and two transition galaxies (dTrs), LGS-3 and Phoenix. The oldest stellar populations of the dSphs and dTrs are, within the uncertainties, coeval (∼13 Gyr) at all galactocentric radii. We find that there are no significative differences between the four galaxies in the fundamental properties (such as the normalized star formation rate or age-metallicity relation) of their outer regions (radii greater than four exponential scale lengths); at large radii, these galaxies consist exclusively of old (≳ 10.5 Gyr) metal-poor stars. The duration of star formation in the inner regions varies from galaxy to galaxy, and the extended central star formation in the dTrs produces the dichotomy between dSph and dTr galaxy types. The dTr galaxies show prominent radial stellar population gradients: The centers of these galaxies host young (≲ 1 Gyr) populations, while the age of the last formation event increases smoothly with increasing radius. This contrasts with the two dSph galaxies. Tucana shows a similar, but milder, gradient, but no gradient in age is detected Cetus. For the three galaxies with significant stellar population gradients, the exponential scale length decreases with time. These results are in agreement with outside-in scenarios of dwarf galaxy evolution, in which a quenching of the star formation toward the center occurs as the galaxy runs out of gas in the outskirts.

  5. VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy

    NARCIS (Netherlands)

    Lemasle, B.; de Boer, T.J.L.; Hill, V.; Tolstoy, E.; Irwin, M.J.; Jablonka, P.; Venn, K.; Battaglia, G.; Starkenburg, E.; Shetrone, M.; Letarte, B.; François, P.; Helmi, A.; Primas, F.; Kaufer, A.; Szeifert, T.

    2014-01-01

    Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history. It has been proposed that Fornax experienced a minor merger event. Aims.

  6. Star-Formation Histories, Abundances, and Kinematics of Dwarf Galaxies in the Local Group

    NARCIS (Netherlands)

    Tolstoy, Eline; Hill, Vanessa; Tosi, Monica; Blandford, R; Kormendy, J; VanDishoeck, E

    2009-01-01

    Within the Local Universe galaxies can be studied in great detail star by star, and here we review the results of quantitative studies in nearby dwarf galaxies. The color-magnitude diagram synthesis method is well established as the most accurate way to determine star-formation histories of galaxies

  7. Origin of the DA and non-DA white dwarf stars

    Science.gov (United States)

    Shipman, Harry L.

    1989-01-01

    Various proposals for the bifurcation of the white dwarf cooling sequence are reviewed. 'Primordial' theories, in which the basic bifurcation of the white dwarf sequence is rooted in events predating the white dwarf stage of stellar evolution, are discussed, along with the competing 'mixing' theories in which processes occurring during the white dwarf stage are responsible for the existence of DA or non-DA stars. A new proposal is suggested, representing a two-channel scenario. In the DA channel, some process reduces the hydrogen layer mass to the value of less than 10 to the -7th. The non-DA channel is similar to that in the primordial scenario. These considerations suggest that some mechanism operates in both channels to reduce the thickness of the outermost layer of the white dwarf. It is also noted that accretion from the interstellar medium has little to do with whether a particular white dwarf becomes a DA or a non-DA star.

  8. An historical perspective - Brown is not a color. [astrophysics of infrared dwarf stars

    Science.gov (United States)

    Tarter, J. C.

    1986-01-01

    Major shifts in theoretical understanding of the star formation process and the possible components of the local mass density are reviewed. Those aspects of brown dwarf structure and evolution that are still not well enough understood are outlined, and the types of observations that might force the modification of current theories to accommodate the existence of brown dwarfs are suggested. The appropriateness of the name 'brown dwarf' is defended.

  9. Evolution models of helium white dwarf-main-sequence star merger remnants: the mass distribution of single low-mass white dwarfs

    Science.gov (United States)

    Zhang, Xianfei; Hall, Philip D.; Jeffery, C. Simon; Bi, Shaolan

    2018-02-01

    It is not known how single white dwarfs with masses less than 0.5Msolar -- low-mass white dwarfs -- are formed. One way in which such a white dwarf might be formed is after the merger of a helium-core white dwarf with a main-sequence star that produces a red giant branch star and fails to ignite helium. We use a stellar-evolution code to compute models of the remnants of these mergers and find a relation between the pre-merger masses and the final white dwarf mass. Combining our results with a model population, we predict that the mass distribution of single low-mass white dwarfs formed through this channel spans the range 0.37 to 0.5Msolar and peaks between 0.45 and 0.46Msolar. Helium white dwarf--main-sequence star mergers can also lead to the formation of single helium white dwarfs with masses up to 0.51Msolar. In our model the Galactic formation rate of single low-mass white dwarfs through this channel is about 8.7X10^-3yr^-1. Comparing our models with observations, we find that the majority of single low-mass white dwarfs (<0.5Msolar) are formed from helium white dwarf--main-sequence star mergers, at a rate which is about $2$ per cent of the total white dwarf formation rate.

  10. Brown dwarfs: at last filling the gap between stars and planets.

    Science.gov (United States)

    Zuckerman, B

    2000-02-01

    Until the mid-1990s a person could not point to any celestial object and say with assurance that "here is a brown dwarf." Now dozens are known, and the study of brown dwarfs has come of age, touching upon major issues in astrophysics, including the nature of dark matter, the properties of substellar objects, and the origin of binary stars and planetary systems.

  11. Evolution models of helium white dwarf--main-sequence star merger remnants: the mass distribution of single low-mass white dwarfs

    OpenAIRE

    Zhang, Xianfei; Hall, Philip D.; Jeffery, C. Simon; Bi, Shaolan

    2017-01-01

    It is not known how single white dwarfs with masses less than 0.5Msolar -- low-mass white dwarfs -- are formed. One way in which such a white dwarf might be formed is after the merger of a helium-core white dwarf with a main-sequence star that produces a red giant branch star and fails to ignite helium. We use a stellar-evolution code to compute models of the remnants of these mergers and find a relation between the pre-merger masses and the final white dwarf mass. Combining our results with ...

  12. Special and general relativity with applications to white dwarfs, neutron stars and black holes

    CERN Document Server

    Glendenning, Norman K

    2007-01-01

    Special and General Relativity are concisely developed together with essential aspects of nuclear and particle physics. Problem sets are provided for many chapters, making the book ideal for a course on the physics of white dwarf and neutron star interiors.

  13. Ages of evolved low mass stars: Central stars of planetary nebulae and white dwarfs

    Directory of Open Access Journals (Sweden)

    Costa R.D.D.

    2013-03-01

    Full Text Available We have developed several methods to estimate the ages of central stars of planetary nebulae (CSPN, which are based either on observed nebular properties or on data from the stars themselves. Our goal is to derive the age distribution of these stars and compare the results with empirical distributions for CSPN and white dwarfs. We have initially developed three methods based on nebular abundances, using (i an age-metallicity relation which is also a function of the galactocentric distance; (ii an age-metallicity relation obtained for the galactic disk, and (iii the central star masses derived from the observed nitrogen abundances. In this work we present two new, more accurate methods, which are based on kinematic properties: (I in this method, the expected rotation velocities of the nebulae around the galactic centre at their galactocentric distances are compared with the predicted values for the galactic rotation curve, and the differences are attributed to the different ages of the evolved stars; (II we determine directly the U, V, W, velocity components of the stars, as well as the velocity dispersions, and use the dispersion-age relation by the Geneva-Copenhagen survey. These methods were applied to two large samples of galactic CSPN. We conclude that most CSPN in the galactic disk have ages under 5 Gyr, and that the age distribution is peaked around 1 to 3 Gyr.

  14. A statistical analysis of IUE spectra of dwarf novae and nova-like stars

    Science.gov (United States)

    Ladous, Constanze

    1990-01-01

    First results of a statistical analysis of the IUE International Ultraviolet Explorer archive on dwarf novae and nova like stars are presented. The archive contains approximately 2000 low resolution spectra of somewhat over 100 dwarf novae and nova like stars. Many of these were looked at individually, but so far the collective information content of this set of data has not been explored. The first results of work are reported.

  15. Rapid Evolution of the Gaseous Exoplanetary Debris Around the White Dwarf Star HE 1349--2305

    OpenAIRE

    Dennihy, E.; Clemens, J. C.; Dunlap, B. H.; Fanale, S. M.; Fuchs, J. T.; Hermes, J. J.

    2018-01-01

    Observations of heavy metal pollution in white dwarf stars indicate that metal-rich planetesimals are frequently scattered into star-grazing orbits, tidally disrupted, and accreted onto the white dwarf surface, offering direct insight into the dynamical evolution of post-main-sequence exoplanetary systems. Emission lines from the gaseous debris in the accretion disks of some of these systems show variations on timescales of decades, and have been interpreted as the general relativistic preces...

  16. Star formation in globular clusters and dwarf galaxies and implications for the early evolution of galaxies

    Science.gov (United States)

    Lin, Douglas N. C.; Murray, Stephen D.

    1991-01-01

    Based upon the observed properties of globular clusters and dwarf galaxies in the Local Group, we present important theoretical constraints on star formation in these systems. These constraints indicate that protoglobular cluster clouds had long dormant periods and a brief epoch of violent star formation. Collisions between protocluster clouds triggered fragmentation into individual stars. Most protocluster clouds dispersed into the Galactic halo during the star formation epoch. In contrast, the large spread in stellar metallicity in dwarf galaxies suggests that star formation in their pregenitors was self-regulated: we propose the protocluster clouds formed from thermal instability in the protogalactic clouds and show that a population of massive stars is needed to provide sufficient UV flux to prevent the collapsing protogalactic clouds from fragmenting into individual stars. Based upon these constraints, we propose a unified scenario to describe the early epochs of star formation in the Galactic halo as well as the thick and thin components of the Galactic disk.

  17. A Deep Study of the Dwarf Satellites Andromeda XXVIII & Andromeda XXIX

    OpenAIRE

    Slater, Colin T.; Bell, Eric F.; Martin, Nicolas F.; Tollerud, Erik J.; Ho, Nhung

    2015-01-01

    We present the results of a deep study of the isolated dwarf galaxies Andromeda XXVIII and Andromeda XXIX with Gemini/GMOS and Keck/DEIMOS. Both galaxies are shown to host old, metal-poor stellar populations with no detectable recent star formation, conclusively identifying both of them as dwarf spheroidal galaxies (dSphs). And XXVIII exhibits a complex horizontal branch morphology, which is suggestive of metallicity enrichment and thus an extended period of star formation in the past. Decomp...

  18. Low-Metallicity Blue Compact Dwarfs as Templates for Primordial Star Formation

    OpenAIRE

    Hunt, L. K.; Hirashita, H.; Thuan, T. X.; Izotov, Y. I.; Vanzi, L.

    2003-01-01

    Understanding how galaxies formed their first stars is a vital cosmological question, but the study of high-redshift objects, caught in the act of forming their first stars, is difficult. Here we argue that two extremely low-metallicity Blue Compact Dwarf galaxies (BCDs), IZw18 and SBS0335-052, could be local templates for primordial star formation, since both lack evolved ($> $1 Gyr) stellar populations; but they form stars differently.

  19. SOLAR-LIKE OSCILLATIONS IN A METAL-POOR GLOBULAR CLUSTER WITH THE HUBBLE SPACE TELESCOPE

    International Nuclear Information System (INIS)

    Stello, Dennis; Gilliland, Ronald L.

    2009-01-01

    We present analyses of variability in the red giant stars in the metal-poor globular cluster NGC 6397, based on data obtained with the Hubble Space Telescope. We use a nonstandard data reduction approach to turn a 23 day observing run originally aimed at imaging the white dwarf population, into time-series photometry of the cluster's highly saturated red giant stars. With this technique we obtain noise levels in the final power spectra down to 50 parts per million, which allows us to search for low-amplitude solar-like oscillations. We compare the observed excess power seen in the power spectra with estimates of the typical frequency range, frequency spacing, and amplitude from scaling the solar oscillations. We see evidence that the detected variability is consistent with solar-like oscillations in at least one and perhaps up to four stars. With metallicities 2 orders of magnitude lower than those of the Sun, these stars present so far the best evidence of solar-like oscillations in such a low-metallicity environment.

  20. Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. V. Asteroseismology of ELMV white dwarf stars

    Science.gov (United States)

    Calcaferro, Leila M.; Córsico, Alejandro H.; Althaus, Leandro G.

    2017-11-01

    Context. Many pulsating low-mass white dwarf stars have been detected in the past years in the field of our Galaxy. Some of them exhibit multiperiodic brightness variation, therefore it is possible to probe their interiors through asteroseismology. Aims: We present a detailed asteroseismological study of all the known low-mass variable white dwarf stars based on a complete set of fully evolutionary models that are representative of low-mass He-core white dwarf stars. Methods: We employed adiabatic radial and nonradial pulsation periods for low-mass white dwarf models with stellar masses ranging from 0.1554 to 0.4352 M⊙ that were derived by simulating the nonconservative evolution of a binary system consisting of an initially 1 M⊙ zero-age main-sequence (ZAMS) star and a 1.4 M⊙ neutron star companion. We estimated the mean period spacing for the stars under study (where this was possible), and then we constrained the stellar mass by comparing the observed period spacing with the average of the computed period spacings for our grid of models. We also employed the individual observed periods of every known pulsating low-mass white dwarf star to search for a representative seismological model. Results: We found that even though the stars under analysis exhibit few periods and the period fits show multiplicity of solutions, it is possible to find seismological models whose mass and effective temperature are in agreement with the values given by spectroscopy for most of the cases. Unfortunately, we were not able to constrain the stellar masses by employing the observed period spacing because, in general, only few periods are exhibited by these stars. In the two cases where we were able to extract the period spacing from the set of observed periods, this method led to stellar mass values that were substantially higher than expected for this type of stars. Conclusions: The results presented in this work show the need for further photometric searches, on the one hand

  1. Rejuvenation of the Innocent Bystander: Testing Spin-Up in a Dwarf Carbon Star Sample

    Science.gov (United States)

    Green, Paul

    2014-09-01

    Carbon stars (C>O) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. We now know that dwarf carbon (dC) stars are actually far more common than C giants. These dC stars are hypothesized to have accreted C-rich envelope material from an AGB companion, in systems that have likely undergone a planetary nebula phase, eventually yielding a white dwarf and a dC star that has gained both significant mass and angular momentum. To test whether the X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo, we propose a Chandra pilot study of dCs selected from the SDSS; some have hot white dwarf companions (indicating more recent mass transfer), and all show Balmer emission lines (a sign of activity).

  2. Determination of the upper mass limit for stars producing white-dwarf remnants

    International Nuclear Information System (INIS)

    Romanishin, W.; Angel, J.R.P.

    1980-01-01

    We have searched ultraviolet and red plates of four open clusters (NGC 2168, 2287, 2422, and 6633) for faint blue objects which might be white dwarf members of the clusters. The most massive stars in these clusters range from 3 to 6 M/sub sun/. We find a definite concentration of faint blue objects in the clusters. This fact, plus initial photoelectric photometry, provides strong support for the identification of many of these objects as cluster white dwarfs. By modeling the expected number of possible white dwarfs in each cluster, we are able to put some limits on m/sub w/, the upper stellar mass limit for formation of white dwarfs. Our data require that some stars of at least 5 M/sub sun/ have evolved into white dwarfs and give a most probable value of 7 M/sub sun/ for m/sub w/

  3. The Metal-poor non-Sagittarius (?) Globular Cluster NGC 5053: Orbit and Mg, Al, and Si Abundances

    Science.gov (United States)

    Tang, Baitian; Fernández-Trincado, J. G.; Geisler, Doug; Zamora, Olga; Mészáros, Szabolcs; Masseron, Thomas; Cohen, Roger E.; García-Hernández, D. A.; Dell’Agli, Flavia; Beers, Timothy C.; Schiavon, Ricardo P.; Sohn, Sangmo Tony; Hasselquist, Sten; Robin, Annie C.; Shetrone, Matthew; Majewski, Steven R.; Villanova, Sandro; Schiappacasse Ulloa, Jose; Lane, Richard R.; Minnti, Dante; Roman-Lopes, Alexandre; Almeida, Andres; Moreno, E.

    2018-03-01

    Metal-poor globular clusters (GCs) exhibit intriguing Al–Mg anti-correlations and possible Si–Al correlations, which are important clues to decipher the multiple-population phenomenon. NGC 5053 is one of the most metal-poor GCs in the nearby universe and has been suggested to be associated with the Sagittarius (Sgr) dwarf galaxy, due to its similarity in location and radial velocity with one of the Sgr arms. In this work, we simulate the orbit of NGC 5053, and argue against a physical connection between Sgr and NGC 5053. On the other hand, the Mg, Al, and Si spectral lines, which are difficult to detect in the optical spectra of NGC 5053 stars, have been detected in the near-infrared APOGEE spectra. We use three different sets of stellar parameters and codes to derive the Mg, Al, and Si abundances. Regardless of which method is adopted, we see a large Al variation, and a substantial Si spread. Along with NGC 5053, metal-poor GCs exhibit different Mg, Al, and Si variations. Moreover, NGC 5053 has the lowest cluster mass among the GCs that have been identified to exhibit an observable Si spread until now.

  4. Deposition of steeply infalling debris around white dwarf stars

    Science.gov (United States)

    Brown, John C.; Veras, Dimitri; Gänsicke, Boris T.

    2017-06-01

    High-metallicity pollution is common in white dwarf (WD) stars hosting remnant planetary systems. However, they rarely have detectable debris accretion discs, possibly because much of the influx is fast steeply infalling debris in star-grazing orbits, producing a more tenuous signature than a slowly accreting disc. Processes governing such deposition between the Roche radius and photosphere have so far received little attention and we model them here analytically by extending recent work on sun-grazing comets to WD systems. We find that the evolution of cm-to-km size (a0) infallers most strongly depends on two combinations of parameters, which effectively measure sublimation rate and binding strength. We then provide an algorithm to determine the fate of infallers for any WD, and apply the algorithm to four limiting combinations of hot versus cool (young/old) WDs with snowy (weak, volatile) versus rocky (strong, refractory) infallers. We find: (I) Total sublimation above the photosphere befalls all small infallers across the entire WD temperature (TWD) range, the threshold size rising with TWD and 100× larger for rock than snow. (II) All very large objects fragment tidally regardless of TWD: for rock, a0 ≽ 105 cm; for snow, a0 ≽ 103-3 × 104 cm across all WD cooling ages. (III) A considerable range of a0 avoids fragmentation and total sublimation, yielding impacts or grazes with cold WDs. This range rapidly narrows with increasing TWD, especially for snowy bodies. Finally, we briefly discuss how the various forms of deposited debris may finally reach the photosphere surface itself.

  5. SPITZER OBSERVATIONS OF WHITE DWARFS: THE MISSING PLANETARY DEBRIS AROUND DZ STARS

    International Nuclear Information System (INIS)

    Xu, S.; Jura, M.

    2012-01-01

    We report a Spitzer/Infrared Array Camera search for infrared excesses around white dwarfs, including 14 newly observed targets and 16 unpublished archived stars. We find a substantial infrared excess around two warm white dwarfs—J220934.84+122336.5 and WD 0843+516, the latter apparently being the hottest white dwarf known to display a close-in dust disk. Extending previous studies, we find that the fraction of white dwarfs with dust disks increases as the star's temperature increases; for stars cooler than 10,000 K, even the most heavily polluted ones do not have ∼1000 K dust. There is tentative evidence that the dust disk occurrence is correlated with the volatility of the accreted material. In the Appendix, we modify a previous analysis to clarify how Poynting-Robertson drag might play an important role in transferring materials from a dust disk into a white dwarf's atmosphere.

  6. Astero-archaeology: Reading the galactic history recorded in the white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Wood, M.A.

    1990-01-01

    Galactic history is written in its oldest stars, the white dwarfs. Although still some years away from reading the details of that history, significant limits can already be placed on both the Galactic age and star formation history. The following is a complete analysis of the problem, starting with a fresh exploration of the physics of white dwarf stars. An extensive grid of numerical model sequences is presented and these are used to describe in detail the behavior of the white dwarf stars as a function of mass, core composition, surface layer masses and compositions, and uncertainties in the constitutive physics. These model sequences are used to decode the information contained in the white dwarf luminosity function. A theoretical context is established for current and future observations by presenting luminosity functions computed with differing choices for the input white dwarf evolutionary sequences, the assumed age of the local disk, the star formation rate as a function of time, and the possibility of scale height inflation of the disk with time. Finally, white dwarf cosmochronology is discussed within the context of other, conflicting, methods of cosmochronology. How this work can help resolve these conflicts and shed light on fundamental problems in galaxy formation and cosmology.

  7. Astero-archaeology: Reading the galactic history recorded in the white dwarf stars

    International Nuclear Information System (INIS)

    Wood, M.A.

    1990-01-01

    Galactic history is written in its oldest stars, the white dwarfs. Although still some years away from reading the details of that history, significant limits can already be placed on both the Galactic age and star formation history. The following is a complete analysis of the problem, starting with a fresh exploration of the physics of white dwarf stars. An extensive grid of numerical model sequences is presented and these are used to describe in detail the behavior of the white dwarf stars as a function of mass, core composition, surface layer masses and compositions, and uncertainties in the constitutive physics. These model sequences are used to decode the information contained in the white dwarf luminosity function. A theoretical context is established for current and future observations by presenting luminosity functions computed with differing choices for the input white dwarf evolutionary sequences, the assumed age of the local disk, the star formation rate as a function of time, and the possibility of scale height inflation of the disk with time. Finally, white dwarf cosmochronology is discussed within the context of other, conflicting, methods of cosmochronology. How this work can help resolve these conflicts and shed light on fundamental problems in galaxy formation and cosmology

  8. Asteroseismology of white dwarf stars. I - Adiabatic results

    Energy Technology Data Exchange (ETDEWEB)

    Bradley, P.A.; Winget, D.E. (Texas, University (USA) McDonald Observatory, Austin (USA))

    1991-02-01

    A preliminary investigation of the asteroseismological properties of chemically stratified evolutionary DA and DB white dwarf models is reported. The period and kinetic energy distributions for nonradial g-modes of spherical harmonic indices l = 1-3 are computed in the adiabatic approximation, and the effects of varying the total stellar masss and the surface layer masses on the pulsation properties are studied using an extensive grid of models. Significant resonant mode trapping due to chemical stratification is found. Modes trapped in the outer layers have much lower kinetic energies; these minima also show up as minima in the period spacing between modes of consecutive radial overtone k. Mode trapping occurs at the same or nearly the same value of k for different l-values. Thus, l-values of trapped modes may be identified on the basis of their period ratios. It is shown that observational identification of these period ratios can be used to constrain the mass of the star and its surface layer. 68 refs.

  9. PROTOPLANETARY DISK MASSES FROM STARS TO BROWN DWARFS

    International Nuclear Information System (INIS)

    Mohanty, Subhanjoy; Mortlock, Daniel; Greaves, Jane; Pascucci, Ilaria; Apai, Daniel; Scholz, Aleks; Thompson, Mark; Lodato, Giuseppe; Looper, Dagny

    2013-01-01

    We present SCUBA-2 850 μm observations of seven very low mass stars (VLMS) and brown dwarfs (BDs). Three are in Taurus and four in the TW Hydrae Association (TWA), and all are classical T Tauri (cTT) analogs. We detect two of the three Taurus disks (one only marginally), but none of the TWA ones. For standard grains in cTT disks, our 3σ limits correspond to a dust mass of 1.2 M ⊕ in Taurus and a mere 0.2 M ⊕ in the TWA (3-10× deeper than previous work). We combine our data with other submillimeter/millimeter (sub-mm/mm) surveys of Taurus, ρ Oph, and the TWA to investigate the trends in disk mass and grain growth during the cTT phase. Assuming a gas-to-dust mass ratio of 100:1 and fiducial surface density and temperature profiles guided by current data, we find the following. (1) The minimum disk outer radius required to explain the upper envelope of sub-mm/mm fluxes is ∼100 AU for intermediate-mass stars, solar types, and VLMS, and ∼20 AU for BDs. (2) While the upper envelope of apparent disk masses increases with M * from BDs to VLMS to solar-type stars, no such increase is observed from solar-type to intermediate-mass stars. We propose this is due to enhanced photoevaporation around intermediate stellar masses. (3) Many of the disks around Taurus and ρ Oph intermediate-mass and solar-type stars evince an opacity index of β ∼ 0-1, indicating significant grain growth. Of the only four VLMS/BDs in these regions with multi-wavelength measurements, three are consistent with considerable grain growth, though optically thick disks are not ruled out. (4) For the TWA VLMS (TWA 30A and B), combining our 850 μm fluxes with the known accretion rates and ages suggests substantial grain growth by 10 Myr, comparable to that in the previously studied TWA cTTs Hen 3-600A and TW Hya. The degree of grain growth in the TWA BDs (2M1207A and SSPM1102) remains largely unknown. (5) A Bayesian analysis shows that the apparent disk-to-stellar mass ratio has a roughly

  10. Flames High Resolution Spectroscopy of RGB Stars in the Carina Dwarf Spheroidal Galaxy

    NARCIS (Netherlands)

    Lemasle, B.; Hill, V.; Tolstoy, E.; Venn, K.; Koleva, M; Prugniel, P; Vauglin,

    Carina is a small and faint classical dwarf spheroidal galaxy in the halo of the Milky Way with a highly episodic star formation history (e.g., Hurley-Keller et al. 1998). Using VLT/FLAMES in high resolution mode, we significantly increase the sample of stars with abundance determinations in Carina,

  11. Black holes, white dwarfs and neutron stars: The physics of compact objects

    International Nuclear Information System (INIS)

    Shapiro, S.L.; Teukolsky, S.A.

    1983-01-01

    The contents include: Star deaths and the formation of compact objects; White dwarfs; Rotation and magnetic fields; Cold equation of state above neutron drip; Pulsars; Accretion onto black holes; Supermassive stars and black holes; Appendices; and Indexes. This book discusses one aspect, compact objects, of astronomy and provides information of astrophysics or general relativity

  12. The luminosity and mass functions of the Pleiades: low-mass stars and brown dwarfs

    International Nuclear Information System (INIS)

    Hambly, N.C.; Jameson, R.F.

    1991-01-01

    COSMOS measurements of R and I Schmidt plates are used to determine the luminosity function and hence mass function of the Pleiades open cluster. Star counts are made in the cluster and the field star contribution, measured outside the cluster, is subtracted. A lower limit of 30 brown dwarfs is found; the mass function is flat at the lowest masses. (author)

  13. Constraining the Stellar Populations and Star Formation Histories of Blue Compact Dwarf Galaxies with SED Fits

    Energy Technology Data Exchange (ETDEWEB)

    Janowiecki, Steven [International Center for Radio Astronomy Research, M468, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia, 6009 (Australia); Salzer, John J.; Zee, Liese van [Department of Astronomy, Indiana University, 727 East Third Street, Bloomington, IN 47405 (United States); Rosenberg, Jessica L. [Department of Physics and Astronomy, George Mason University, Fairfax, VA 22030 (United States); Skillman, Evan, E-mail: steven.janowiecki@uwa.edu.au [Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street, SE Minneapolis, MN, 55455 (United States)

    2017-02-10

    We discuss and test possible evolutionary connections between blue compact dwarf galaxies (BCDs) and other types of dwarf galaxies. BCDs provide ideal laboratories to study intense star formation episodes in low-mass dwarf galaxies, and have sometimes been considered a short-lived evolutionary stage between types of dwarf galaxies. To test these connections, we consider a sample of BCDs as well as a comparison sample of nearby galaxies from the Local Volume Legacy (LVL) survey for context. We fit the multi-wavelength spectral energy distributions (SED, far-ultra-violet to far-infrared) of each galaxy with a grid of theoretical models to determine their stellar masses and star formation properties. We compare our results for BCDs with the LVL galaxies to put BCDs in the context of normal galaxy evolution. The SED fits demonstrate that the star formation events currently underway in BCDs are at the extreme of the continuum of normal dwarf galaxies, both in terms of the relative mass involved and in the relative increase over previous star formation rates. Today’s BCDs are distinctive objects in a state of extreme star formation that is rapidly transforming them. This study also suggests ways to identify former BCDs whose star formation episodes have since faded.

  14. GD1212: Probing deep into the interior of a pulsating white dwarf star

    Directory of Open Access Journals (Sweden)

    Giammichele N.

    2015-01-01

    Full Text Available We present the first self-consistent seismic analysis of a white dwarf star, GD 1212, in the Kepler2 field. We precisely establish the fundamental parameters of the star using the forward method based on physically sound models. We unravel the internal structure as well as the rotation profile of GD1212 deeper than in any other ZZCeti stars studied so far. This opens up interesting prospects for future analyses of the white dwarf pulsators monitored in the Kepler and Kepler2 fields.

  15. Observations on the variability of linear polarization in late-type dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Huovelin, J.; Linnaluoto, S.; Tuominen, I.; Virtanen, H.

    1989-04-01

    Broadband (UBV) linear polarimetric observations of a sample of late-type (F7-K5) dwarfs are reported. The observations include ten stars and extend over a maximum of 20 nights. Seven stars show significant temporal variability of polarization, which could be interpreted as rotational modulation due to slowly varying magnetic regions. Magnetic intensification in saturated Zeeman sensitive absorption lines is suggested as the dominant effect connecting linear polarization with magnetic activity in the most active single late-type dwarfs, while the wavelength dependence in the less active stars could also be due to a combination of Rayleigh and Thomson scattering.

  16. Detection of a white dwarf companion to the Hyades stars HD 27483

    Science.gov (United States)

    Boehm-Vitense, Erika

    1993-01-01

    We observed with IUE a white dwarf (WD) companion to the Hyades F6 V binary stars HD 27483. This system is known to be a close binary of two nearly equal stars with an orbital period of 3.05 days. Our IUE observations revealed the presence of a third star, a white dwarf with an effective temperature of 23,000 +/- 1000 K and a mass of approximately 0.6 solar mass. Its presence in the Hyades cluster with a known age permits me to derive the mass of its progenitor, which must have been about 2.3 solar masses. The presence of the white dwarf in a binary system opens the possibility that some of the envelope material, which was expelled by the WD progenitor, may have been collected by the F6 stars. We may thus be able to study abundance anomalies of the WD progenitor with known mass on the surface of the F6 companions.

  17. Model stars with degenerate dwarf cores and helium-burning shells - A stationary-burning approximation

    Energy Technology Data Exchange (ETDEWEB)

    Iben, I. Jr.; Tutukov, A.V. (Illinois Univ., Urbana (USA); Astronomicheskii Sovet, Moscow (USSR))

    1989-07-01

    The characteristics of model stars consisting of a degenerate dwarf core and an envelope which is burning a nuclear fuel or fuels in its interior are explored. The models are relevant to stars which are accreting matter from a companion, to single stars in late stages of evolution, to stripped noninteracting remnants of binary star evolution, and to merging and merged degenerate dwarfs. For any given mass and choice of nuclear fuels, a sequence of models is constructed which differ with respect to the mass of the degenerate core and the envelope characteristics. Each sequence has at least three distinct branches: a degenerate dwarf branch along which envelope mass increases with decreasing luminosity, a plateau branch characterized by a very small envelope mass and by a nearly constant luminosity which reaches the maximum achievable value for the sequence, and an asymptotic giant branch which is at the lowest temperatures achievable and along which envelope mass decreases with increasing luminosity. 78 refs.

  18. Model stars with degenerate dwarf cores and helium-burning shells - A stationary-burning approximation

    International Nuclear Information System (INIS)

    Iben, I. Jr.; Tutukov, A.V.

    1989-01-01

    The characteristics of model stars consisting of a degenerate dwarf core and an envelope which is burning a nuclear fuel or fuels in its interior are explored. The models are relevant to stars which are accreting matter from a companion, to single stars in late stages of evolution, to stripped noninteracting remnants of binary star evolution, and to merging and merged degenerate dwarfs. For any given mass and choice of nuclear fuels, a sequence of models is constructed which differ with respect to the mass of the degenerate core and the envelope characteristics. Each sequence has at least three distinct branches: a degenerate dwarf branch along which envelope mass increases with decreasing luminosity, a plateau branch characterized by a very small envelope mass and by a nearly constant luminosity which reaches the maximum achievable value for the sequence, and an asymptotic giant branch which is at the lowest temperatures achievable and along which envelope mass decreases with increasing luminosity. 78 refs

  19. Suppression of cooling by strong magnetic fields in white dwarf stars.

    Science.gov (United States)

    Valyavin, G; Shulyak, D; Wade, G A; Antonyuk, K; Zharikov, S V; Galazutdinov, G A; Plachinda, S; Bagnulo, S; Machado, L Fox; Alvarez, M; Clark, D M; Lopez, J M; Hiriart, D; Han, Inwoo; Jeon, Young-Beom; Zurita, C; Mujica, R; Burlakova, T; Szeifert, T; Burenkov, A

    2014-11-06

    Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

  20. Brown dwarf accretion: Nonconventional star formation over very long timescales

    Directory of Open Access Journals (Sweden)

    Ćirković Milan M.

    2005-01-01

    Full Text Available We investigate the process of accretion of interstellar gas by the Galactic population of brown dwarfs over very long timescales typical for physical eschatology. In particular, we use the classical Hoyle-Lyttleton-Bondi accretion model to investigate the rate at which brown dwarfs collect enough additional mass to become red dwarfs, accretion-induced changes in the mass function of the low- mass objects, and the corresponding accretion heating of brown dwarfs. In addition, we show how we can make the definition of the final mass function for stellar objects more precise.

  1. Fundmental Parameters of Low-Mass Stars, Brown Dwarfs, and Planets

    Science.gov (United States)

    Montet, Benjamin; Johnson, John A.; Bowler, Brendan; Shkolnik, Evgenya

    2016-01-01

    Despite advances in evolutionary models of low-mass stars and brown dwarfs, these models remain poorly constrained by observations. In order to test these predictions directly, masses of individual stars must be measured and combined with broadband photometry and medium-resolution spectroscopy to probe stellar atmospheres. I will present results from an astrometric and spectroscopic survey of low-mass pre-main sequence binary stars to measure individual dynamical masses and compare to model predictions. This is the first systematic test of a large number of stellar systems of intermediate age between young star-forming regions and old field stars. Stars in our sample are members of the Tuc-Hor, AB Doradus, and beta Pictoris moving groups, the last of which includes GJ 3305 AB, the wide binary companion to the imaged exoplanet host 51 Eri. I will also present results of Spitzer observations of secondary eclipses of LHS 6343 C, a T dwarf transiting one member of an M+M binary in the Kepler field. By combining these data with Kepler photometry and radial velocity observations, we can measure the luminosity, mass, and radius of the brown dwarf. This is the first non-inflated brown dwarf for which all three of these parameters have been measured, providing the first benchmark to test model predictions of the masses and radii of field T dwarfs. I will discuss these results in the context of K2 and TESS, which will find additional benchmark transiting brown dwarfs over the course of their missions, including a description of the first planet catalog developed from K2 data and a program to search for transiting planets around mid-M dwarfs.

  2. Star Formation Histories of Local Group Dwarf Galaxies. (Ludwig Biermann Award Lecture 1996)

    Science.gov (United States)

    Grebel, E. K.

    The star formation histories of dwarf galaxies in the Local Group are reviewed. First the question of Local Group membership is considered based on various criteria. The properties of 31 (36) galaxies are consistent with likely (potential) Local Group membership. To study the star formation histories of these galaxies, a multi-parameter problem needs to be solved: Ages, metallicities, population fractions, and spatial variations must be determined, which depend crucially on the knowledge of reddening and distance. The basic methods for studying resolvable stellar populations are summarized. One method is demonstrated using the Fornax dwarf spheroidal galaxy. A comprehensive compilation of the star formation histories of dwarf irregulars, dwarf ellipticals, and dwarf spheroidals in the Local Group is presented and visualized through Hodge's population boxes. All galaxies appear to have differing fractions of old and intermediate-age populations, and those sufficiently massive and undisturbed to retain and recycle their gas are still forming stars today. Star formation has occurred either in distinct episodes or continuously over long periods of time. Metallicities and enrichment vary widely. Constraints on merger and remnant scenarios are discussed, and a unified picture based on the current knowledge is presented. Primary goals for future observations are: accurate age determinations based on turnoff photometry, detection of subpopulations distinct in age, metallicity, and/or spatial distribution; improved distances; and astrometric studies to derive orbits and constrain past and future interactions.

  3. Binary Star Orbits. V. The Nearby White Dwarf/Red Dwarf Pair 40 Eri BC

    Science.gov (United States)

    Mason, Brian D.; Hartkopf, William I.; Miles, Korie N.

    2017-11-01

    A new relative orbit solution with new dynamical masses is determined for the nearby white dwarf-red dwarf pair 40 Eri BC. The period is 230.09 ± 0.68 years. It is predicted to close slowly over the next half-century, getting as close as 1.″32 in early 2066. We determine masses of 0.575 ± 0.018 {{ M }}⊙ for the white dwarf and 0.2041 ± 0.0064 {{ M }}⊙ for the red dwarf companion. The inconsistency of the masses determined by gravitational redshift and dynamical techniques, due to a premature orbit calculation, no longer exists.

  4. Chromospheric and Transition Region Emission Properties of G, K, and M dwarf Exoplanet Host Stars

    Science.gov (United States)

    France, Kevin; Arulanantham, Nicole; Fossati, Luca; Lanza, A. F.; Linsky, Jeffrey L.; Redfield, Seth; Loyd, Robert; Schneider, Christian

    2018-01-01

    Exoplanet magnetic fields have proven notoriously hard to detect, despite theoretical predictions of substantial magnetic field strengths on close-in extrasolar giant planets. It has been suggested that stellar and planetary magnetic field interactions can manifest as enhanced stellar activity relative to nominal age-rotation-activity relationships for main sequence stars or enhanced activity on stars hosting short-period massive planets. In a recent study of M and K dwarf exoplanet host stars, we demonstrated a significant correlation between the relative luminosity in high-temperature stellar emission lines (L(ion)/L_Bol) and the “star-planet interaction strength”, M_plan/a_plan. Here, we expand on that work with a survey of G, K, and M dwarf exoplanet host stars obtained in two recent far-ultraviolet spectroscopic programs with the Hubble Space Telescope. We have measured the relative luminosities of stellar lines C II, Si III, Si IV, and N V (formation temperatures from 30,000 – 150,000 K) in a sample of ~60 exoplanet host stars and an additional ~40 dwarf stars without known planets. We present results on star-planet interaction signals as a function of spectral type and line formation temperature, as well as a statistical comparison of stars with and without planets.

  5. ALFALFA DISCOVERY OF THE MOST METAL-POOR GAS-RICH GALAXY KNOWN: AGC 198691

    Energy Technology Data Exchange (ETDEWEB)

    Hirschauer, Alec S.; Salzer, John J.; Rhode, Katherine L., E-mail: ash@astro.indiana.edu, E-mail: slaz@astro.indiana.edu, E-mail: krhode@indiana.edu [Department of Astronomy, Indiana University, 727 East Third Street, Bloomington, IN 47405 (United States); and others

    2016-05-10

    We present spectroscopic observations of the nearby dwarf galaxy AGC 198691. This object is part of the Survey of H i in Extremely Low-Mass Dwarfs project, which is a multi-wavelength study of galaxies with H i masses in the range of 10{sup 6}–10{sup 7.2} M {sub ⊙}, discovered by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We have obtained spectra of the lone H ii region in AGC 198691 with the new high-throughput KPNO Ohio State Multi-Object Spectrograph on the Mayall 4 m, as well as with the Blue Channel spectrograph on the MMT 6.5 m telescope. These observations enable the measurement of the temperature-sensitive [O iii] λ 4363 line and hence the determination of a “direct” oxygen abundance for AGC 198691. We find this system to be an extremely metal-deficient (XMD) system with an oxygen abundance of 12+log(O/H) = 7.02 ± 0.03, making AGC 198691 the lowest-abundance star-forming galaxy known in the local universe. Two of the five lowest-abundance galaxies known have been discovered by the ALFALFA blind H i survey; this high yield of XMD galaxies represents a paradigm shift in the search for extremely metal-poor galaxies.

  6. 3He, red dwarf stars and future trillion years of the Galaxy

    International Nuclear Information System (INIS)

    Solpiter, Eh.Eh.

    1986-01-01

    Certain problems of red dwarf evolution are considered. On the basis of the observed upper limit of 3 He content in interstellar medium the evaluation of the limit of average rate of a dwarf star mass loss is given. For a typical dwarf of the age approximately equal to half of the Galaxy age the upper limit for the average part of the star mass, which has been lost in 10 10 years by means of stellar wind, constitutes g≤0.04. If g is near its upper limit, equal to 0.04, energy inflow to the interstellar medium from dwarfs is small, as compared with supernovae in the Galaxy, but not negligible

  7. A Population Study of Wide-Separation Brown Dwarf Companions to Main Sequence Stars

    Science.gov (United States)

    Smith, Jeffrey J.

    2005-01-01

    Increased interest in infrared astronomy has opened the frontier to study cooler objects that shed significant light on the formation of planetary systems. Brown dwarf research provides a wealth of information useful for sorting through a myriad of proposed formation theories. Our study combines observational data from 2MASS with rigorous computer simulations to estimate the true population of long-range (greater than 1000 AU) brown dwarf companions in the solar neighborhood (less than 25 pc from Earth). Expanding on Gizis et al. (2001), we have found the margin of error in previous estimates to be significantly underestimated after we included orbit eccentricity, longitude of pericenter, angle of inclination, field star density, and primary and secondary luminosities as parameters influencing the companion systems in observational studies. We apply our simulation results to current L- and T-dwarf catalogs to provide updated estimates on the frequency of wide-separation brown dwarf companions to main sequence stars.

  8. K-band spectroscopic metallicities and temperatures of M-dwarf stars

    Directory of Open Access Journals (Sweden)

    Rojas-Ayala Bárbara

    2013-04-01

    Full Text Available I present the metallicity and effective temperature techniques developed for M dwarf stars by Rojas-Ayala et al. (2010, 2012. These techniques are based on absorption features present in the modest resolution K-band spectra (R∼2700 of M dwarfs and have been calibrated using FGK+M dwarf pairs and synthetic atmosphere models. The H2O-K2 index seems to overestimate the effective temperatures of M dwarfs when compared to interferometric measurements. The metallicity distribution of the M dwarf host candidates by the Kepler Mission hints that jovian-size planets form preferentially around solar and super-solar metallicity environments, while small rocky planet host exhibit a wide range of metallicities, just like in their solar-type counterparts.

  9. The critical binary star separation for a planetary system origin of white dwarf pollution

    Science.gov (United States)

    Veras, Dimitri; Xu, Siyi; Rebassa-Mansergas, Alberto

    2018-01-01

    The atmospheres of between one quarter and one half of observed single white dwarfs in the Milky Way contain heavy element pollution from planetary debris. The pollution observed in white dwarfs in binary star systems is, however, less clear, because companion star winds can generate a stream of matter which is accreted by the white dwarf. Here, we (i) discuss the necessity or lack thereof of a major planet in order to pollute a white dwarf with orbiting minor planets in both single and binary systems, and (ii) determine the critical binary separation beyond which the accretion source is from a planetary system. We hence obtain user-friendly functions relating this distance to the masses and radii of both stars, the companion wind, and the accretion rate on to the white dwarf, for a wide variety of published accretion prescriptions. We find that for the majority of white dwarfs in known binaries, if pollution is detected, then that pollution should originate from planetary material.

  10. Environmental effects on stellar populations of star clusters and dwarf galaxies

    Science.gov (United States)

    Pasetto, Stefano; Cropper, Mark; Fujita, Yutaka; Chiosi, Cesare; Grebel, Eva K.

    2017-03-01

    We investigate the competitive role of the different dissipative phenomena acting on the onset of star formation of gravitationally bound systems in an external environment. Ram pressure, Kelvin-Helmholtz and Rayleigh-Taylor instabilities, and tidal forces are accounted for separately in an analytical framework and compared in their role in influencing the star forming regions. We present an analytical criterion to elucidate the dependence of star formation in a spherical stellar system on its surrounding environment. We consider the different signatures of these phenomena in synthetically realized colour-magnitude diagrams (CMDs) of the orbiting system thus investigating the detectability limits of these different effects for future observational projects and their relevance. The developed theoretical framework has direct applications to the cases of massive star clusters, dwarf galaxies in galaxy clusters and dwarf galaxies orbiting our Milky Way system, as well as any primordial gas-rich cluster of stars orbiting within its host galaxy.

  11. The distribution of masses and radii of white-dwarf stars

    International Nuclear Information System (INIS)

    Shipman, H.L.

    1978-01-01

    The status of determinations of white dwarf radii by model atmosphere methods is reviewed. The results are that (i) the mean radius of a sample of 95 hydrogen-rich stars with parallaxes is 0.0131 R(Sun); (ii) the mean radius of a sample of 13 helium-rich stars is 0.011 R(Sun), indistinguishably different from the radius of the hydrogen-rich stars; and (iii) that the most serious limitation on our knowledge of the mean radius of white dwarfs is the influence of selection effects. An estimate of the selection effects indicates that the true mean white dwarf radius is near 0.011 R(Sun). (Auth.)

  12. Rejuvenation of the Innocent Bystander: Testing Spin-Up in Dwarf Carbon Stars

    Science.gov (United States)

    Green, Paul

    2013-09-01

    Carbon stars (C>O) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. We now know that dwarf carbon (dC) stars are actually far more common than C giants. These dCs are hypothesized to have accreted C-rich envelope material from an AGB companion, in systems that have likely undergone a planetary nebula phase, eventually yielding a white dwarf and a dC that has gained both significant mass and angular momentum. To test whether the X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo, we propose a Chandra pilot study of dCs selected from the SDSS; some have hot white dwarf companions (indicating more recent mass transfer), and all show Balmer emission lines (a sign of activity).

  13. Relativistic deflection of background starlight measures the mass of a nearby white dwarf star.

    Science.gov (United States)

    Sahu, Kailash C; Anderson, Jay; Casertano, Stefano; Bond, Howard E; Bergeron, Pierre; Nelan, Edmund P; Pueyo, Laurent; Brown, Thomas M; Bellini, Andrea; Levay, Zoltan G; Sokol, Joshua; Dominik, Martin; Calamida, Annalisa; Kains, Noé; Livio, Mario

    2017-06-09

    Gravitational deflection of starlight around the Sun during the 1919 total solar eclipse provided measurements that confirmed Einstein's general theory of relativity. We have used the Hubble Space Telescope to measure the analogous process of astrometric microlensing caused by a nearby star, the white dwarf Stein 2051 B. As Stein 2051 B passed closely in front of a background star, the background star's position was deflected. Measurement of this deflection at multiple epochs allowed us to determine the mass of Stein 2051 B-the sixth-nearest white dwarf to the Sun-as 0.675 ± 0.051 solar masses. This mass determination provides confirmation of the physics of degenerate matter and lends support to white dwarf evolutionary theory. Copyright © 2017, American Association for the Advancement of Science.

  14. Milky Way red dwarfs in the BoRG survey; galactic scale-height and the distribution of dwarf stars in WFC3 imaging

    International Nuclear Information System (INIS)

    Holwerda, B. W.; Bouwens, R.; Trenti, M.; Clarkson, W.; Sahu, K.; Bradley, L.; Stiavelli, M.; Pirzkal, N.; Ryan, R.; De Marchi, G.; Andersen, M.

    2014-01-01

    We present a tally of Milky Way late-type dwarf stars in 68 Wide Field Camera 3 (WFC3) pure-parallel fields (227 arcmin 2 ) from the Brightest of Reionizing Galaxies survey for high-redshift galaxies. Using spectroscopically identified M-dwarfs in two public surveys, the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey and the Early Release Science mosaics, we identify a morphological selection criterion using the half-light radius (r 50 ), a near-infrared J – H, G – J color region where M-dwarfs are found, and a V – J relation with M-dwarf subtype. We apply this morphological selection of stellar objects, color-color selection of M-dwarfs, and optical-near-infrared color subtyping to compile a catalog of 274 M-dwarfs belonging to the disk of the Milky Way with a limiting magnitude of m F125W < 24(AB). Based on the M-dwarf statistics, we conclude that (1) the previously identified north-south discrepancy in M-dwarf numbers persists in our sample; there are more M-dwarfs in the northern fields on average than in southern ones, (2) the Milky Way's single disk scale-height for M-dwarfs is 0.3-4 kpc, depending on subtype, (3) the scale-height depends on M-dwarf subtype with early types (M0-4) high scale-height (z 0 = 3-4 kpc) and later types M5 and above in the thin disk (z 0 = 0.3-0.5 kpc), (4) a second component is visible in the vertical distribution, with a different, much higher scale-height in the southern fields compared to the northern ones. We report the M-dwarf component of the Sagittarius stream in one of our fields with 11 confirmed M-dwarfs, seven of which are at the stream's distance. In addition to the M-dwarf catalog, we report the discovery of 1 T-dwarfs and 30 L-dwarfs from their near-infrared colors. The dwarf scale-height and the relative low incidence in our fields of L- and T-dwarfs in these fields makes it unlikely that these stars will be interlopers in great numbers in color-selected samples of high-redshift galaxies

  15. Milky Way red dwarfs in the BoRG survey; galactic scale-height and the distribution of dwarf stars in WFC3 imaging

    Energy Technology Data Exchange (ETDEWEB)

    Holwerda, B. W.; Bouwens, R. [Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden (Netherlands); Trenti, M. [Kavli Institute for Cosmology and Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA (United Kingdom); Clarkson, W. [Department of Natural Sciences College of Arts, Sciences and Letters, University of Michigan-Dearborn 4901 Evergreen Road, Dearborn, MI 48128 (United States); Sahu, K.; Bradley, L.; Stiavelli, M.; Pirzkal, N.; Ryan, R. [Space Telescope Science Institute, Baltimore, MD 21218 (United States); De Marchi, G. [European Space Agency, ESA-ESTEC, Keplerlaan 1, 2200 AG Noordwijk (Netherlands); Andersen, M., E-mail: holwerda@strw.leidenuniv.nl [UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d' Astrophysique de Grenoble (IPAG) UMR 5274, F-38041 Grenoble (France)

    2014-06-10

    We present a tally of Milky Way late-type dwarf stars in 68 Wide Field Camera 3 (WFC3) pure-parallel fields (227 arcmin{sup 2}) from the Brightest of Reionizing Galaxies survey for high-redshift galaxies. Using spectroscopically identified M-dwarfs in two public surveys, the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey and the Early Release Science mosaics, we identify a morphological selection criterion using the half-light radius (r {sub 50}), a near-infrared J – H, G – J color region where M-dwarfs are found, and a V – J relation with M-dwarf subtype. We apply this morphological selection of stellar objects, color-color selection of M-dwarfs, and optical-near-infrared color subtyping to compile a catalog of 274 M-dwarfs belonging to the disk of the Milky Way with a limiting magnitude of m {sub F125W} < 24(AB). Based on the M-dwarf statistics, we conclude that (1) the previously identified north-south discrepancy in M-dwarf numbers persists in our sample; there are more M-dwarfs in the northern fields on average than in southern ones, (2) the Milky Way's single disk scale-height for M-dwarfs is 0.3-4 kpc, depending on subtype, (3) the scale-height depends on M-dwarf subtype with early types (M0-4) high scale-height (z {sub 0} = 3-4 kpc) and later types M5 and above in the thin disk (z {sub 0} = 0.3-0.5 kpc), (4) a second component is visible in the vertical distribution, with a different, much higher scale-height in the southern fields compared to the northern ones. We report the M-dwarf component of the Sagittarius stream in one of our fields with 11 confirmed M-dwarfs, seven of which are at the stream's distance. In addition to the M-dwarf catalog, we report the discovery of 1 T-dwarfs and 30 L-dwarfs from their near-infrared colors. The dwarf scale-height and the relative low incidence in our fields of L- and T-dwarfs in these fields makes it unlikely that these stars will be interlopers in great numbers in color-selected samples of

  16. A PHOTOMETRIC VARIABILITY SURVEY OF FIELD K AND M DWARF STARS WITH HATNet

    International Nuclear Information System (INIS)

    Hartman, J. D.; Bakos, G. A.; Noyes, R. W.; Sipocz, B.; Pal, A.; Kovacs, G.; Mazeh, T.; Shporer, A.

    2011-01-01

    Using light curves from the HATNet survey for transiting extrasolar planets we investigate the optical broadband photometric variability of a sample of 27, 560 field K and M dwarfs selected by color and proper motion (V - K ∼> 3.0, μ > 30 mas yr -1 , plus additional cuts in J - H versus H - K S and on the reduced proper motion). We search the light curves for periodic variations and for large-amplitude, long-duration flare events. A total of 2120 stars exhibit potential variability, including 95 stars with eclipses and 60 stars with flares. Based on a visual inspection of these light curves and an automated blending classification, we select 1568 stars, including 78 eclipsing binaries (EBs), as secure variable star detections that are not obvious blends. We estimate that a further ∼26% of these stars may be blends with fainter variables, though most of these blends are likely to be among the hotter stars in our sample. We find that only 38 of the 1568 stars, including five of the EBs, have previously been identified as variables or are blended with previously identified variables. One of the newly identified EBs is 1RXS J154727.5+450803, a known P = 3.55 day, late M-dwarf SB2 system, for which we derive preliminary estimates for the component masses and radii of M 1 = M 2 = 0.258 ± 0.008 M sun and R 1 = R 2 = 0.289 ± 0.007 R sun . The radii of the component stars are larger than theoretical expectations if the system is older than ∼200 Myr. The majority of the variables are heavily spotted BY Dra-type stars for which we determine rotation periods. Using this sample, we investigate the relations between period, color, age, and activity measures, including optical flaring, for K and M dwarfs, finding that many of the well-established relations for F, G, and K dwarfs continue into the M dwarf regime. We find that the fraction of stars that is variable with peak-to-peak amplitudes greater than 0.01 mag increases exponentially with the V - K S color such that

  17. Sulphur in the metal poor globular cluster NGC 6397

    Science.gov (United States)

    Koch, A.; Caffau, E.

    2011-10-01

    Sulphur (S) is a non-refractory α-element that is not locked into dust grains in the interstellar medium. Thus no correction to the measured, interstellar sulphur abundance is needed and it can be readily compared to the S content in stellar photospheres. Here we present the first measurement of sulphur in the metal poor globular cluster (GC) NGC 6397, as detected in a MIKE/Magellan high signal-to-noise, high-resolution spectrum of one red giant star. While abundance ratios of sulphur are available for a larger number of Galactic stars down to an [Fe/H] of ~ -3.5 dex, no measurements in globular clusters more metal poor than -1.5 dex have been reported so far. We find aNLTE, 3-D abundance ratio of [S/Fe] = +0.52 ± 0.20 (stat.) ± 0.08 (sys.), based on theS I, Multiplet 1 line at 9212.8 Å. This value is consistent with a Galactic halo plateau as typical of other α-elements in GCs and field stars, but we cannot rule out its membership with a second branch of increasing [S/Fe] with decreasing [Fe/H], claimed in the literature, which leads to a large scatter at metallicities around - 2 dex. The [S/Mg] and [S/Ca] ratios in this star are compatible with a Solar value to within the (large) uncertainties. Despite the very large scatter in these ratios across Galactic stars between literature samples, this indicates that sulphur traces the chemical imprints of the other α-elements in metal poor GCs. Combined with its moderate sodium abundance ([S/Na]NLTE = 0.48), the [S/Fe] ratio in this GC extends a global, positive S-Na correlation that is not seen in field stars and might indicate that proton-capture reactions contributed to the production of sulphur in the (metal poor) early GC environments. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  18. The influence of H2O line blanketing on the spectra of cool dwarf stars

    Science.gov (United States)

    Allard, F.; Hauschildt, P. H.; Miller, S.; Tennyson, J.

    1994-01-01

    We present our initial results of model atmosphere calculations for cool M dwarfs using an opacity sampling method and a new list of H2O lines. We obtain significantly improved fits to the infrared spectrum of the M dwarf VB10 when compared to earlier models. H2O is by far the dominant opacity source in cool stars. To illustrate this, we show the Rosseland mean of the total extinction under various assumptions. Our calculations demonstrate the importance of a good treatment of the water opacities in cool stars and the improvements possible by using up-to-date data for the water line absorption.

  19. Dwarf Galaxies with Gentle Star Formation and the Counts of Galaxies in the Hubble Deep Field

    OpenAIRE

    Campos, Ana

    1997-01-01

    In this paper the counts and colors of the faint galaxies observed in the Hubble Deep Field are fitted by means of simple luminosity evolution models that incorporate a numerous population of fading dwarfs. The observed color distribution of the very faint galaxies now allows us to put constraints on the star formation history in dwarfs. It is shown that the star-forming activity in these small systems has to proceed in a gentle way, i.e., through episodes where each one lasts much longer tha...

  20. Theoretical models for asteroseismology of DA white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Bradley, P.A. [XTA, MS B220, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

    1996-09-01

    Because white dwarfs are the most common end state of stellar evolution, determining their internal structure will yield many clues about the final stages of stellar evolution and the physics of matter under extreme conditions. We present the results of our parametric survey of evolutionary models of compositionally stratified white dwarfs with hydrogen surface layers (DA white dwarfs) and provide a comprehensive set of theoretical {ital g}-mode pulsation periods for comparison to observations of pulsating DA white dwarfs. This survey complements the previous survey of helium atmosphere (DB) white dwarf periods of Bradley, Winget, & Wood. We show how to use the periods of low-overtone and/or trapped modes to constrain the internal structure of pulsating DA white dwarfs by utilizing their sensitivity to the total stellar mass and the location of the hydrogen/helium transition zone. We use G117-B15A as an example to demonstrate the potential of our models for asteroseismology; we suggest that G117-B15A has a mass of 0.55 {ital M}{sub {circle_dot}} and a hydrogen layer mass of {approx_equal}1.5{times}10{sup {minus}4} {ital M}{sub {asterisk}}. {copyright} {ital 1996 The American Astronomical Society.}

  1. Temperate Earth-sized planets transiting a nearby ultracool dwarf star.

    Science.gov (United States)

    Gillon, Michaël; Jehin, Emmanuël; Lederer, Susan M; Delrez, Laetitia; de Wit, Julien; Burdanov, Artem; Van Grootel, Valérie; Burgasser, Adam J; Triaud, Amaury H M J; Opitom, Cyrielle; Demory, Brice-Olivier; Sahu, Devendra K; Bardalez Gagliuffi, Daniella; Magain, Pierre; Queloz, Didier

    2016-05-12

    Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as 'ultracool dwarfs'. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks, there should be a large but hitherto undetected population of terrestrial planets orbiting them--ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system.

  2. The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni

    Science.gov (United States)

    Bitner, Martin A.; Robinson, Edward L.; Behr, Bradford B.

    2007-06-01

    The dwarf nova SS Cygni is a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg. Fits of synthetic spectra for Roche lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio, KK=162.5+/-1.0 km s-1 and q=MK/MWD=0.685+/-0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f=0.535+/-0.075 of the total flux at 5500 Å. Taking the weighted average of our results with previously published results obtained using similar techniques, we find =163.7+/-0.7 km s-1 and =0.683+/-0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations, we limit the orbital inclination to the range 45degAustin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.

  3. Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star.

    Science.gov (United States)

    Nugent, Peter E; Sullivan, Mark; Cenko, S Bradley; Thomas, Rollin C; Kasen, Daniel; Howell, D Andrew; Bersier, David; Bloom, Joshua S; Kulkarni, S R; Kandrashoff, Michael T; Filippenko, Alexei V; Silverman, Jeffrey M; Marcy, Geoffrey W; Howard, Andrew W; Isaacson, Howard T; Maguire, Kate; Suzuki, Nao; Tarlton, James E; Pan, Yen-Chen; Bildsten, Lars; Fulton, Benjamin J; Parrent, Jerod T; Sand, David; Podsiadlowski, Philipp; Bianco, Federica B; Dilday, Benjamin; Graham, Melissa L; Lyman, Joe; James, Phil; Kasliwal, Mansi M; Law, Nicholas M; Quimby, Robert M; Hook, Isobel M; Walker, Emma S; Mazzali, Paolo; Pian, Elena; Ofek, Eran O; Gal-Yam, Avishay; Poznanski, Dovi

    2011-12-14

    Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.

  4. INSIGHTS INTO PRE-ENRICHMENT OF STAR CLUSTERS AND SELF-ENRICHMENT OF DWARF GALAXIES FROM THEIR INTRINSIC METALLICITY DISPERSIONS

    International Nuclear Information System (INIS)

    Leaman, Ryan

    2012-01-01

    Star clusters are known to have smaller intrinsic metallicity spreads than dwarf galaxies due to their shorter star formation timescales. Here we use individual spectroscopic [Fe/H] measurements of stars in 19 Local Group dwarf galaxies, 13 Galactic open clusters, and 49 globular clusters to show that star cluster and dwarf galaxy linear metallicity distributions are binomial in form, with all objects showing strong correlations between their mean linear metallicity Z-bar and intrinsic spread in metallicity σ(Z) 2 . A plot of σ(Z) 2 versus Z-bar shows that the correlated relationships are offset for the dwarf galaxies from the star clusters. The common binomial nature of these linear metallicity distributions can be explained with a simple inhomogeneous chemical evolution model, where the star cluster and dwarf galaxy behavior in the σ(Z) 2 - Z-bar diagram is reproduced in terms of the number of enrichment events, covering fraction, and intrinsic size of the enriched regions. The inhomogeneity of the self-enrichment sets the slope for the observed dwarf galaxy σ(Z) 2 - Z-bar correlation. The offset of the star cluster sequence from that of the dwarf galaxies is due to pre-enrichment, and the slope of the star cluster sequence represents the remnant signature of the self-enriched history of their host galaxies. The offset can be used to separate star clusters from dwarf galaxies without a priori knowledge of their luminosity or dynamical mass. The application of the inhomogeneous model to the σ(Z) 2 - Z-bar relationship provides a numerical formalism to connect the self-enrichment and pre-enrichment between star clusters and dwarf galaxies using physically motivated chemical enrichment parameters. Therefore we suggest that the σ(Z) 2 - Z-bar relationship can provide insight into what drives the efficiency of star formation and chemical evolution in galaxies, and is an important prediction for galaxy simulation models to reproduce.

  5. Energy transport in radially accreting white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Thompson, A.M.

    1986-10-01

    Some of the non-thermal energy transport processes which may be present in a white dwarf accretion column are examined and it is determined whether these could in any way contribute to a resolution of the soft X-ray puzzle. The first two Chapters of this Thesis constitute a review of the observations and proposed models for white dwarf accretion columns. In Chapter 3 we show that in Kuijpers and Pringle's original bombardment model of white dwarf accretion columns, in which the energy of the accreting material is deposited uniformly into a static atmosphere which then radiates the energy away as optically thin bremsstrahlung/line radiation, an incorrect Coulomb collisional timescale was used. In Chapter 4 we extend the calculations of Chapter 3 to include the effect of cyclotron radiation. It is concluded that a cyclotron cooled bombardment solution for a white dwarf accretion column may exist. We extend this calculation to derive a simple piecewise uniform temperature structure for such an accretion column, incorporating the effect of thermal conduction. In Chaper 5 we examine two of the non thermal emission mechanisms that might be present in white dwarf accretion columns:- non thermal Lyman-{alpha} emission and non thermal inverse bremsstrahlung emission. It is shown that neither would actually be sufficiently large to be detectable. In Chapter 6 some possible extensions to the work presented are suggested. (author).

  6. White dwarf stars exceeding the Chandrasekhar mass limit

    Science.gov (United States)

    Tomaschitz, Roman

    2018-01-01

    The effect of nonlinear ultra-relativistic electron dispersion on the mass-radius relation of high-mass white dwarfs is studied. The dispersion is described by a permeability tensor in the Dirac equation, generated by the ionized high-density stellar matter, which constitutes the neutralizing background of the nearly degenerate electron plasma. The electron dispersion results in a stable mass-radius relation for high-mass white dwarfs, in contrast to a mass limit in the case of vacuum permeabilities. In the ultra-relativistic regime, the dispersion relation is a power law whose amplitude and scaling exponent is inferred from mass and radius estimates of two high-mass white dwarfs, Sirius B and LHS 4033. Evidence for the existence of super-Chandrasekhar mass white dwarfs is provided by several Type Ia supernovae (e.g., SN 2013cv, SN 2003fg, SN 2007if and SN 2009dc), whose mass ejecta exceed the Chandrasekhar limit by up to a factor of two. The dispersive mass-radius relation is used to estimate the radii, central densities, Fermi temperatures, bulk and compression moduli and sound velocities of their white dwarf progenitors.

  7. Stellar Archeology: What White Dwarf Stars Tell Us About the History of the Galaxy

    Directory of Open Access Journals (Sweden)

    Terry D. Oswalt

    2012-06-01

    Full Text Available White dwarf stars have played important roles in rather diverse areas of astrophysics. This paper outlines how these stellar remnants, especially those in widely separated “fragile” binaries, have provided unique leverage on difficult astrophysical problems such as the ages of stars, the structure and evolution of the Galaxy, the nature of dark matter and even the discovery of dark energy.

  8. Detection of brown dwarfs by the micro-lensing of unresolved stars

    CERN Document Server

    Baillon, Paul; Giraud-Héraud, Yannick; Kaplan, J; Baillon, Paul; Bouquet, Alain; Giraud-Héraud, Yannick; Kaplan, Jean

    1993-01-01

    The presence of brown dwarfs in the dark galactic halo could be detected through their gravitational lensing effect and experiments under way monitor about one million stars to observe a few lensing events per year. We show that if the photon flux from a galaxy is measured with a good precision, it is not necessary to resolve the stars and besides more events could be observed.

  9. Stellar Abundances for Galactic Archaeology Database. IV. Compilation of stars in dwarf galaxies

    Science.gov (United States)

    Suda, Takuma; Hidaka, Jun; Aoki, Wako; Katsuta, Yutaka; Yamada, Shimako; Fujimoto, Masayuki Y.; Ohtani, Yukari; Masuyama, Miyu; Noda, Kazuhiro; Wada, Kentaro

    2017-10-01

    We have constructed a database of stars in Local Group galaxies using the extended version of the SAGA (Stellar Abundances for Galactic Archaeology) database that contains stars in 24 dwarf spheroidal galaxies and ultra-faint dwarfs. The new version of the database includes more than 4500 stars in the Milky Way, by removing the previous metallicity criterion of [Fe/H] ≤ -2.5, and more than 6000 stars in the Local Group galaxies. We examined the validity of using a combined data set for elemental abundances. We also checked the consistency between the derived distances to individual stars and those to galaxies as given in the literature. Using the updated database, the characteristics of stars in dwarf galaxies are discussed. Our statistical analyses of α-element abundances show that the change of the slope of the [α/Fe] relative to [Fe/H] (so-called "knee") occurs at [Fe/H] = -1.0 ± 0.1 for the Milky Way. The knee positions for selected galaxies are derived by applying the same method. The star formation history of individual galaxies is explored using the slope of the cumulative metallicity distribution function. Radial gradients along the four directions are inspected in six galaxies where we find no direction-dependence of metallicity gradients along the major and minor axes. The compilation of all the available data shows a lack of CEMP-s population in dwarf galaxies, while there may be some CEMP-no stars at [Fe/H] ≲ -3 even in the very small sample. The inspection of the relationship between Eu and Ba abundances confirms an anomalously Ba-rich population in Fornax, which indicates a pre-enrichment of interstellar gas with r-process elements. We do not find any evidence of anti-correlations in O-Na and Mg-Al abundances, which characterizes the abundance trends in the Galactic globular clusters.

  10. The binary fraction of stars in dwarf galaxies: the case of Leo II

    OpenAIRE

    Spencer, Meghin; Mateo, Mario; Walker, Matthew; Olszewski, Edward; McConnachie, Alan; Kirby, Evan; Koch, Andreas

    2017-01-01

    We combine precision radial velocity data from four different published works of the stars in the Leo II dwarf spheroidal galaxy. This yields a data set that spans 19 years, has 14 different epochs of observation, and contains 372 unique red giant branch stars, 196 of which have repeat observations. Using this multi-epoch data set, we constrain the binary fraction for Leo II. We generate a suite of Monte Carlo simulations that test different binary fractions using Bayesian analysis and determ...

  11. Evidences of extragalactic origin and planet engulfment in the metal-poor twin pair HD 134439/HD 134440

    Science.gov (United States)

    Reggiani, Henrique; Meléndez, Jorge

    2018-04-01

    Recent studies of chemical abundances in metal-poor halo stars show the existence of different populations, which is important for studies of Galaxy formation and evolution. Here, we revisit the twin pair of chemically anomalous stars HD 134439 and HD 134440, using high resolution (R ˜ 72 000) and high S/N ratio (S/N ˜ 250) HDS/Subaru spectra. We compare them to the well-studied halo star HD 103095, using the line-by-line differential technique to estimate precise stellar parameters and LTE chemical abundances. We present the abundances of C, O, Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Sr, Y, Ba, La, Ce, Nd, and Sm. We compare our results to the precise abundance patterns of Nissen & Schuster (2010) and data from dwarf Spheroidal galaxies (dSphs). We show that the abundance pattern of these stars appears to be closely linked to that of dSphs with [α/Fe] knee below [Fe/H] < -1.5. We also find a systematic difference of 0.06 ± 0.01 dex between the abundances of these twin binary stars, which could be explained by the engulfment of a planet, thus suggesting that planet formation is possible at low metallicities ([Fe/H] = -1.4).

  12. A possible formation scenario for dwarf spheroidal galaxies - III. Adding star formation histories to the fiducial model

    Science.gov (United States)

    Alarcón Jara, A. G.; Fellhauer, M.; Matus Carrillo, D. R.; Assmann, P.; Urrutia Zapata, F.; Hazeldine, J.; Aravena, C. A.

    2018-02-01

    Dwarf spheroidal galaxies are regarded as the basic building blocks in the formation of larger galaxies and are the most dark matter dominated systems in the Universe, known so far. There are several models that attempt to explain their formation and evolution, but they have problems modelling the formation of isolated dwarf spheroidal galaxies. Here, we will explain a possible formation scenario in which star clusters form inside the dark matter halo of a dwarf spheroidal galaxy. These star clusters suffer from low star formation efficiency and dissolve while orbiting inside the dark matter halo. Thereby, they build the faint luminous components that we observe in dwarf spheroidal galaxies. In this paper, we study this model by adding different star formation histories to the simulations and compare the results with our previous work and observational data to show that we can explain the formation of dwarf spheroidal galaxies.

  13. REVERSAL OF FORTUNE: INCREASED STAR FORMATION EFFICIENCIES IN THE EARLY HISTORIES OF DWARF GALAXIES?

    International Nuclear Information System (INIS)

    Madau, Piero; Weisz, Daniel R.; Conroy, Charlie

    2014-01-01

    On dwarf galaxy scales, the different shapes of the galaxy stellar mass function and the dark halo mass function require a star-formation efficiency (SFE) in these systems that is currently more than 1 dex lower than that of Milky Way-size halos. Here, we argue that this trend may actually be reversed at high redshift. Specifically, by combining the resolved star-formation histories of nearby isolated dwarfs with the simulated mass-growth rates of dark matter halos, we show that the assembly of these systems occurs in two phases: (1) an early, fast halo accretion phase with a rapidly deepening potential well, characterized by a high SFE; and (2) a late, slow halo accretion phase where, perhaps as a consequence of reionization, the SFE is low. Nearby dwarfs have more old stars than predicted by assuming a constant or decreasing SFE with redshift, a behavior that appears to deviate qualitatively from the trends seen among more massive systems. Taken at face value, the data suggest that at sufficiently early epochs, dwarf galaxy halos above the atomic cooling mass limit can be among the most efficient sites of star formation in the universe

  14. Metallicity and the spectral energy distribution and spectral types of dwarf O-stars

    NARCIS (Netherlands)

    Mokiem, MR; Martin-Hernandez, NL; Lenorzer, A; de Koter, A; Tielens, AGGA

    We present a systematic study of the effect of metallicity on the stellar spectral energy distribution (SED) of 0 main sequence (dwarf) stars, focussing on the hydrogen and helium ionizing continua, and on the optical and near-IR lines used for spectral classification. The spectra are based on

  15. Metallicity and the spectral energy distribution and spectral types of dwarf O-stars

    NARCIS (Netherlands)

    Mokiem, M.R.; Martín-Hernández, N.L.; Lenorzer, A.; de Koter, A.; Tielens, A.G.G.M.

    2004-01-01

    We present a systematic study of the effect of metallicity on the stellar spectral energy distribution (SED) of O main sequence (dwarf) stars, focussing on the hydrogen and helium ionizing continua, and on the optical and near-IR lines used for spectral classification. The spectra are based on

  16. Reversal of Fortune: Increased Star Formation Efficiencies in the Early Histories of Dwarf Galaxies?

    Science.gov (United States)

    Madau, Piero; Weisz, Daniel R.; Conroy, Charlie

    2014-08-01

    On dwarf galaxy scales, the different shapes of the galaxy stellar mass function and the dark halo mass function require a star-formation efficiency (SFE) in these systems that is currently more than 1 dex lower than that of Milky Way-size halos. Here, we argue that this trend may actually be reversed at high redshift. Specifically, by combining the resolved star-formation histories of nearby isolated dwarfs with the simulated mass-growth rates of dark matter halos, we show that the assembly of these systems occurs in two phases: (1) an early, fast halo accretion phase with a rapidly deepening potential well, characterized by a high SFE; and (2) a late, slow halo accretion phase where, perhaps as a consequence of reionization, the SFE is low. Nearby dwarfs have more old stars than predicted by assuming a constant or decreasing SFE with redshift, a behavior that appears to deviate qualitatively from the trends seen among more massive systems. Taken at face value, the data suggest that at sufficiently early epochs, dwarf galaxy halos above the atomic cooling mass limit can be among the most efficient sites of star formation in the universe.

  17. Cool carbon stars in the halo and in dwarf galaxies: Hα, colours, and variability

    Science.gov (United States)

    Mauron, N.; Gigoyan, K. S.; Berlioz-Arthaud, P.; Klotz, A.

    2014-02-01

    The population of cool carbon (C) stars located far from the galactic plane is probably made of debris of small galaxies such as the Sagittarius dwarf spheroidal galaxy (Sgr), which are disrupted by the gravitational field of the Galaxy. We aim to know this population better through spectroscopy, 2MASS photometric colours, and variability data. When possible, we compared the halo results to C star populations in the Fornax dwarf spheroidal galaxy, Sgr, and the solar neighbourhood. We first present a few new discoveries of C stars in the halo and in Fornax. The number of spectra of halo C stars is now 125. Forty percent show Hα in emission. The narrow location in the JHK diagram of the halo C stars is found to differ from that of similar C stars in the above galaxies. The light curves of the Catalina and LINEAR variability databases were exploited to derive the pulsation periods of 66 halo C stars. A few supplementary periods were obtained with the TAROT telescopes. We confirm that the period distribution of the halo strongly resembles that of Fornax, and we found that it is very different from the C stars in the solar neighbourhood. There is a larger proportion of short-period Mira/SRa variables in the halo than in Sgr, but the survey for C stars in this dwarf galaxy is not complete, and the study of their variability needs to be continued to investigate the link between Sgr and the cool halo C stars. Based on observations made with the NTT and 3.6 m telescope at the European Southern Observatory (La Silla, Chile; programs 084.D-0302 and 070.D-0203), with the TAROT telescopes at La Silla and at Observatoire de la Côte d'Azur (France), and on the exploitation of the Catalina Sky Survey and the LINEAR variability databases.Appendix A is available in electronic form at http://www.aanda.org

  18. Multi-Epoch Hubble Space Telescope Observations of IZw18 : Characterization of Variable Stars at Ultra-Low Metallicities

    NARCIS (Netherlands)

    Fiorentino, G.; Ramos, R. Contreras; Clementini, G.; Marconi, M.; Musella, I.; Aloisi, A.; Annibali, F.; Saha, A.; Tosi, M.; van der Marel, R. P.

    2010-01-01

    Variable stars have been identified for the first time in the very metal-poor blue compact dwarf galaxy IZw18, using deep multi-band (F606W, F814W) time-series photometry obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope. We detected 34 candidate variable stars in the

  19. On the Number of Comets Around White Dwarf Stars: Orbit Survival During the Late Stages of Stellar Evolution

    Energy Technology Data Exchange (ETDEWEB)

    Parriott, J. [Department of Astronomy, University of Michigan, Ann Arbor, MI 48109 (United States); Alcock, C. [Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States)

    1998-07-01

    The accretion of comets onto DA white dwarfs can produce observable metal absorption lines. We show here that comet systems around the progenitor main-sequence star are vulnerable to being lost during asymptotic giant branch mass loss, if the mass loss is sufficiently asymmetric to impart modest linear momentum to the white dwarf. This may have bearing on the frequency of observation of heavy elements in white dwarf stars and on inferences regarding the frequency of comet systems, if the imparted linear velocities of white dwarfs can be estimated. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}

  20. Linear theory radial and nonradial pulsations of DA dwarf stars

    International Nuclear Information System (INIS)

    Starrfield, S.; Cox, A.N.; Hodson, S.; Pesnell, W.D.

    1982-01-01

    The Los Alamos stellar envelope and radial linear non-adiabatic computer code, along with a new Los Alamos non-radial code are used to investigate the total hydrogen mass necessary to produce the non-radial instability of DA dwarfs

  1. Spectropolarimetric Survey of Hydrogen-rich White Dwarf Stars

    Czech Academy of Sciences Publication Activity Database

    Kawka, Adela; Vennes, S.; Schmidt, G. D.; Wickramasinghe, D.T.; Koch, R.

    2007-01-01

    Roč. 654, č. 1 (2007), s. 499-520 ISSN 0004-637X R&D Projects: GA ČR GP205/05/P186 Institutional research plan: CEZ:AV0Z10030501 Keywords : magnetic fields * white dwarf s Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 6.405, year: 2007

  2. A detailed study of lithium in 107 CHEPS dwarf stars

    Science.gov (United States)

    Pavlenko, Ya. V.; Jenkins, J. S.; Ivanyuk, O. M.; Jones, H. R. A.; Kaminsky, B. M.; Lyubchik, Yu. P.; Yakovina, L. A.

    2018-03-01

    Context. We report results from lithium abundance determinations using high resolution spectral analysis of the 107 metal-rich stars from the Calan-Hertfordshire Extrasolar Planet Search programme. Aims: We aim to set out to understand the lithium distribution of the population of stars taken from this survey. Methods: The lithium abundance taking account of non-local thermodynamical equilibrium effects was determined from the fits to the Li I 6708 Å resonance doublet profiles in the observed spectra. Results: We find that a) fast rotators tend to have higher lithium abundances; b) log N(Li) is higher in more massive and hot stars; c) log N(Li) is higher in stars of lower log g; d) stars with the metallicities >0.25 dex do not show the lithium lines in their spectra; e) most of our planet hosts rotate slower; and f) a lower limit of lithium isotopic ratio is 7Li/6Li > 10 in the atmospheres of two stars with planets (SWP) and two non-SWP stars. Conclusions: Measurable lithium abundances were found in the atmospheres of 45 stars located at distances of 20-170 pc from the Sun, for the other 62 stars the upper limits of log N(Li) were computed. We found well defined dependences of lithium abundances on Teff, V sin i, and less pronounced for the log g. In case of V sin i we see two sequences of stars: with measurable lithium and with the upper limit of log N(Li). About 10% of our targets are known to host planets. Only two SWP have notable lithium abundances, so we found a lower proportion of stars with detectable Li among known planet hosts than among stars without planets. However, given the small sample size of our planet-host sample, our analysis does not show any statistically significant differences in the lithium abundance between SWP and stars without known planets.

  3. On the absence of young white dwarf companions to five technetium stars

    Science.gov (United States)

    Smith, Verne V.; Lambert, David L.

    1987-01-01

    A search for hot companions to five stars of type MS and S has been carried out using the IUE satellite. No hot companions were detected for the MS stars HR 85, 4647, 6702, and 8062, and the S star HR 8714. Limits on the luminosities of possible white dwarf companions provide lower limits of 2-5x10 to the 8th yr to the ages of any degenerate companions. All five stars exhibit strong Tc I lines, and the presence of technetium, with a half-life of 2.1x10 to the 5th yr, signifies recent nucleosynthesis. The limits on the ages of possible white dwarf companions that are equal to or greater than 1000 half-lives of Tc exclude the possibility that the s-process elemental enhancement seen in these MS and S stars resulted from mass transfer from a more highly evolved companion (as is probably the mechanism by which barium stars are created). These MS and S stars represent a sample of true thermally pulsing asymptotic giant-branch stars.

  4. White Dwarfs in Star Clusters: The Initial-Final Mass Relation for Stars from 0.85 to 8 M$_\\odot$

    Science.gov (United States)

    Cummings, Jeffrey; Kalirai, Jason; Tremblay, P.-E.; Ramírez-Ruiz, Enrico

    2018-01-01

    The spectroscopic study of white dwarfs provides both their mass, cooling age, and intrinsic photometric properties. For white dwarfs in the field of well-studied star clusters, this intrinsic photometry can be used to determine if they are members of that star cluster. Comparison of a member white dwarf's cooling age to its total cluster's age provides the evolutionary timescale of its progenitor star, and hence the mass. This is the initial-final mass relation (IFMR) for stars, which gives critical information on how a progenitor star evolves and loses mass throughout its lifetime, and how this changes with progenitor mass. Our work, for the first time, presents a uniform analysis of 85 white dwarf cluster members spanning from progenitor masses of 0.85 to 8 M$_\\odot$. Comparison of our work to theoretical IFMRs shows remarkable consistency in their shape but differences remain. We will discuss possible explanations for these differences, including the effects of stellar rotation.

  5. A reappraisal of the habitability of planets around M dwarf stars.

    Science.gov (United States)

    Tarter, Jill C; Backus, Peter R; Mancinelli, Rocco L; Aurnou, Jonathan M; Backman, Dana E; Basri, Gibor S; Boss, Alan P; Clarke, Andrew; Deming, Drake; Doyle, Laurance R; Feigelson, Eric D; Freund, Friedmann; Grinspoon, David H; Haberle, Robert M; Hauck, Steven A; Heath, Martin J; Henry, Todd J; Hollingsworth, Jeffery L; Joshi, Manoj M; Kilston, Steven; Liu, Michael C; Meikle, Eric; Reid, I Neill; Rothschild, Lynn J; Scalo, John; Segura, Antigona; Tang, Carol M; Tiedje, James M; Turnbull, Margaret C; Walkowicz, Lucianne M; Weber, Arthur L; Young, Richard E

    2007-02-01

    Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.

  6. White dwarf-red dwarf binaries in the Galaxy

    NARCIS (Netherlands)

    Besselaar, E.J.M. van den

    2007-01-01

    This PhD thesis shows several studies on white dwarf - red dwarf binaries. White dwarfs are the end products of most stars and red dwarfs are normal hydrogen burning low-mass stars. White dwarf - red dwarf binaries are both blue (white dwarf) and red (red dwarf). Together with the fact that they are

  7. Double white dwarfs as progenitors of R coronae borealis stars and type I supernovae

    International Nuclear Information System (INIS)

    Webbink, R.F.

    1984-01-01

    Close double white dwarfs should arise from the second phase of mass exchagne in close binaries which first encountered mass exchange while the more massive star was crossing the Hertzprung gap. Tidal mass transfer in these double degenerate systems is explored. The sequence of double white dwarf divides naturally into three segments. (1) Low-mass helium/helium pairs are unstable to dynamical time-scale mass transfer and probably coalesce to form helium-burning sdO stars. (2) In helium/carbon-oxygen pairs, mass transfer occurs on the time scale for gravitational radiation losses (approx.10 -4 M/sub sun/ yr -1 ); the accreted helium is quickly ignited, and the accretor expands to dimensions characteristic of R CrB stars, engulfing its companion star. (3) Carbon-oxygen/carbon-oxygen pairs are again unstable to dynamical time-scale mass transfer and, since their total masses exceed the Chandrasekhar limit, are destined to become supernovae. Inactive lifetimes in these latter systems between creation and interaction can exceed 10 10 years. Birthrates of R CrB stars and Type I supernovae by evolution of double white dwarfs are in reasonable agreement with observational estimates

  8. Microwave emission from the coronae of late-type dwarf stars

    Science.gov (United States)

    Linsky, J. L.; Gary, D. E.

    1983-01-01

    VLA microwave observations of 14 late-type dwarf and subgiant stars and binary systems are examined. In this extensive set of observations, four sources at 6 cm (Chi-1 Ori, UV Cet, YY Gem, and Wolf 630AB) were detected and low upper limits for the remaining stars were found. The microwave luminosities of the nondetected F-K dwarfs are as small as 0.01 those of the dMe stars. The detected emission is slowly variable in all cases and is consistent with gyroresonant emission from thermal electrons spiraling in magnetic fields of about 300 gauss if the source sizes are as large as R/R(asterisk) = 3-4. This would correspond to magnetic fields that are probably in the range 0.001-0.0001 gauss at the photospheric level. An alternative mechanism is gyrosynchrotron emission from a relatively small number of electrons with effective temperature.

  9. Compact Objects in Astrophysics White Dwarfs, Neutron Stars and Black Holes

    CERN Document Server

    Camenzind, Max

    2007-01-01

    Compact objects are an important class of astronomical objects in current research. Supermassive black holes play an important role in the understanding of the formation of galaxies in the early Universe. Old white dwarfs are nowadays used to calibrate the age of the Universe. Mergers of neutron stars and black holes are the sources of intense gravitational waves which will be measured in the next ten years by gravitational wave detectors. Camenzind's Compact Objects in Astrophysics gives a comprehensive introduction and up-to-date overview about the physical processes behind these objects, covering the field from the beginning to most recent results, including all relevant observations. After a presentation of the taxonomy of compact objects, the basic principles of general relativity are given. The author then discusses in detail the physics and observations of white dwarfs and neutron stars (including the most recent equations of state for neutron star matter), the gravitational field of rapidly rotating c...

  10. Understanding of variability properties in very low mass stars and brown dwarfs

    Science.gov (United States)

    Mondal, Soumen; Ghosh, Samrat; Khata, Dhrimadri; Joshi, Santosh; Das, Ramkrishna

    2018-04-01

    We report on photometric variability studies of a L3.5 brown dwarf 2MASS J00361617+1821104 (2M0036+18) in the field and of four young brown dwarfs in the star-forming region IC 348. From muti-epoch observations, we found significant periodic variability in 2M0036+18 with a period of 2.66 ± 0.55 hours on one occasion while it seemed to be non-variable on three other occasions. An evolving dust cloud might cause such a scenario. Among four young brown dwarfs of IC 348 in the spectral range M7.25 - M8, one brown dwarf 2MASS J03443921+3208138 shows significant variability. The K-band spectra (2.0-2.4 μm) of nine very low mass stars (M1 - M9 V) are used to characterize the water band index (H20-K2). We found that it is strongly correlated with the surface temperature of M dwarfs.

  11. White-dwarf rotational equilibria in magnetic cataclysmic variable stars

    Energy Technology Data Exchange (ETDEWEB)

    Warner, B. (Cape Town Univ. (South Africa). Dept. of Astronomy Australian National Univ., Canberra (Australia). Dept. of Mathematics); Wickramasinghe, D.T. (Australian National Univ., Canberra (Australia). Dept. of Mathematics)

    1991-02-01

    The magnetic cataclysmic variable stars (polars, intermediate polars and DQ Her stars) are grouped about three lines in the orbital period-spin period diagram. This segregation is shown to be the consequence of competition between braking and accretion torques when combined with the effects of cyclical variations in rate of mass transfer. (author).

  12. CARBON-TO-OXYGEN RATIOS IN M DWARFS AND SOLAR-TYPE STARS

    International Nuclear Information System (INIS)

    Nakajima, Tadashi; Sorahana, Satoko

    2016-01-01

    It has been suggested that high C/O ratios (>0.8) in circumstellar disks lead to the formation of carbon-dominated planets. Based on the expectation that elemental abundances in the stellar photospheres give the initial abundances in the circumstellar disks, the frequency distributions of C/O ratios of solar-type stars have been obtained by several groups. The results of these investigations are mixed. Some find C/O > 0.8 in more than 20% of stars, and C/O > 1.0 in more than 6%. Others find C/O > 0.8 in none of the sample stars. These works on solar-type stars are all differential abundance analyses with respect to the Sun and depend on the adopted C/O ratio in the Sun. Recently, a method of molecular line spectroscopy of M dwarfs, in which carbon and oxygen abundances are derived respectively from CO and H 2 O lines in the K band, has been developed. The resolution of the K- band spectrum is 20,000. Carbon and oxygen abundances of 46 M dwarfs have been obtained by this nondifferential abundance analysis. Carbon-to-oxygen ratios in M dwarfs derived by this method are more robust than those in solar-type stars derived from neutral carbon and oxygen lines in the visible spectra because of the difficulty in the treatment of oxygen lines. We have compared the frequency distribution of C/O distributions in M dwarfs with those of solar-type stars and have found that the low frequency of high-C/O ratios is preferred.

  13. CARBON-TO-OXYGEN RATIOS IN M DWARFS AND SOLAR-TYPE STARS

    Energy Technology Data Exchange (ETDEWEB)

    Nakajima, Tadashi [Astrobiology Center, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588 (Japan); Sorahana, Satoko, E-mail: tadashi.nakajima@nao.ac.jp, E-mail: sorahana@astron.s.u-tokyo.ac.jp [Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)

    2016-10-20

    It has been suggested that high C/O ratios (>0.8) in circumstellar disks lead to the formation of carbon-dominated planets. Based on the expectation that elemental abundances in the stellar photospheres give the initial abundances in the circumstellar disks, the frequency distributions of C/O ratios of solar-type stars have been obtained by several groups. The results of these investigations are mixed. Some find C/O > 0.8 in more than 20% of stars, and C/O > 1.0 in more than 6%. Others find C/O > 0.8 in none of the sample stars. These works on solar-type stars are all differential abundance analyses with respect to the Sun and depend on the adopted C/O ratio in the Sun. Recently, a method of molecular line spectroscopy of M dwarfs, in which carbon and oxygen abundances are derived respectively from CO and H{sub 2}O lines in the K band, has been developed. The resolution of the K- band spectrum is 20,000. Carbon and oxygen abundances of 46 M dwarfs have been obtained by this nondifferential abundance analysis. Carbon-to-oxygen ratios in M dwarfs derived by this method are more robust than those in solar-type stars derived from neutral carbon and oxygen lines in the visible spectra because of the difficulty in the treatment of oxygen lines. We have compared the frequency distribution of C/O distributions in M dwarfs with those of solar-type stars and have found that the low frequency of high-C/O ratios is preferred.

  14. Population II brown dwarfs and dark haloes

    International Nuclear Information System (INIS)

    Zinnecker, H.

    1986-01-01

    Opacity-limited fragmentation is investigated as a function of the dust-to-gas ratio and it is found that the characteristic protostellar mass Msub(*) is metallicity-dependent. This dependence is such that, for the low metallicity gas out of which the stars of Population II formed in the halo, Msub(*) is less than 0.1 M solar mass. If applicable, these theoretical considerations would predict that substellar masses have formed more frequently under the metal-poor conditions in the early Galaxy (Population II brown dwarfs). Thus the missing mass in the Galactic halo and in the dark haloes around other spirals may well reside in these metal-poor Population II brown dwarfs. (author)

  15. THE NEAR-ULTRAVIOLET LUMINOSITY FUNCTION OF YOUNG, EARLY M-TYPE DWARF STARS

    International Nuclear Information System (INIS)

    Ansdell, Megan; Baranec, Christoph; Gaidos, Eric; Mann, Andrew W.; Lépine, Sebastien; James, David; Buccino, Andrea; Mauas, Pablo; Petrucci, Romina; Law, Nicholas M.; Riddle, Reed

    2015-01-01

    Planets orbiting within the close-in habitable zones of M dwarf stars will be exposed to elevated high-energy radiation driven by strong magnetohydrodynamic dynamos during stellar youth. Near-ultraviolet (NUV) irradiation can erode and alter the chemistry of planetary atmospheres, and a quantitative description of the evolution of NUV emission from M dwarfs is needed when modeling these effects. We investigated the NUV luminosity evolution of early M-type dwarfs by cross-correlating the Lépine and Gaidos catalog of bright M dwarfs with the Galaxy Evolution Explorer (GALEX) catalog of NUV (1771-2831 Å) sources. Of the 4805 sources with GALEX counterparts, 797 have NUV emission significantly (>2.5σ) in excess of an empirical basal level. We inspected these candidate active stars using visible-wavelength spectra, high-resolution adaptive optics imaging, time-series photometry, and literature searches to identify cases where the elevated NUV emission is due to unresolved background sources or stellar companions; we estimated the overall occurrence of these ''false positives'' (FPs) as ∼16%. We constructed an NUV luminosity function that accounted for FPs, detection biases of the source catalogs, and GALEX upper limits. We found the NUV luminosity function to be inconsistent with predictions from a constant star-formation rate and simplified age-activity relation defined by a two-parameter power law

  16. HOW THE FIRST STARS SHAPED THE FAINTEST GAS-DOMINATED DWARF GALAXIES

    International Nuclear Information System (INIS)

    Verbeke, R.; Vandenbroucke, B.; Rijcke, S. De

    2015-01-01

    Low-mass dwarf galaxies are very sensitive test-beds for theories of cosmic structure formation since their weak gravitational fields allow the effects of the relevant physical processes to clearly stand out. Up to now, no unified account has existed of the sometimes seemingly conflicting properties of the faintest isolated dwarfs in and around the Local Group, such as Leo T and the recently discovered Leo P and Pisces A systems. Using new numerical simulations, we show that this serious challenge to our understanding of galaxy formation can be effectively resolved by taking into account the regulating influence of the ultraviolet radiation of the first population of stars on a dwarf’s star formation rate while otherwise staying within the standard cosmological paradigm for structure formation. These simulations produce faint, gas-dominated, star-forming dwarf galaxies that lie on the baryonic Tully–Fisher relation and that successfully reproduce a broad range of chemical, kinematical, and structural observables of real late-type dwarf galaxies. Furthermore, we stress the importance of obtaining properties of simulated galaxies in a manner as close as possible to the typically employed observational techniques

  17. HOW THE FIRST STARS SHAPED THE FAINTEST GAS-DOMINATED DWARF GALAXIES

    Energy Technology Data Exchange (ETDEWEB)

    Verbeke, R.; Vandenbroucke, B.; Rijcke, S. De, E-mail: robbert.verbeke@UGent.be [Sterrenkundig Observatorium, Ghent University, Krijgslaan 281, S9, 9000 Gent (Belgium)

    2015-12-20

    Low-mass dwarf galaxies are very sensitive test-beds for theories of cosmic structure formation since their weak gravitational fields allow the effects of the relevant physical processes to clearly stand out. Up to now, no unified account has existed of the sometimes seemingly conflicting properties of the faintest isolated dwarfs in and around the Local Group, such as Leo T and the recently discovered Leo P and Pisces A systems. Using new numerical simulations, we show that this serious challenge to our understanding of galaxy formation can be effectively resolved by taking into account the regulating influence of the ultraviolet radiation of the first population of stars on a dwarf’s star formation rate while otherwise staying within the standard cosmological paradigm for structure formation. These simulations produce faint, gas-dominated, star-forming dwarf galaxies that lie on the baryonic Tully–Fisher relation and that successfully reproduce a broad range of chemical, kinematical, and structural observables of real late-type dwarf galaxies. Furthermore, we stress the importance of obtaining properties of simulated galaxies in a manner as close as possible to the typically employed observational techniques.

  18. THE NEAR-ULTRAVIOLET LUMINOSITY FUNCTION OF YOUNG, EARLY M-TYPE DWARF STARS

    Energy Technology Data Exchange (ETDEWEB)

    Ansdell, Megan; Baranec, Christoph [Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 (United States); Gaidos, Eric [Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822 (United States); Mann, Andrew W. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Lépine, Sebastien [Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30302 (United States); James, David [Cerro Tololo Inter-American Observatory, Casilla 603 La Serena (Chile); Buccino, Andrea; Mauas, Pablo; Petrucci, Romina [Instituto de Astronomía y Física del Espacio, C1428EHA Buenos Aires (Argentina); Law, Nicholas M. [Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 (United States); Riddle, Reed [Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125 (United States)

    2015-01-01

    Planets orbiting within the close-in habitable zones of M dwarf stars will be exposed to elevated high-energy radiation driven by strong magnetohydrodynamic dynamos during stellar youth. Near-ultraviolet (NUV) irradiation can erode and alter the chemistry of planetary atmospheres, and a quantitative description of the evolution of NUV emission from M dwarfs is needed when modeling these effects. We investigated the NUV luminosity evolution of early M-type dwarfs by cross-correlating the Lépine and Gaidos catalog of bright M dwarfs with the Galaxy Evolution Explorer (GALEX) catalog of NUV (1771-2831 Å) sources. Of the 4805 sources with GALEX counterparts, 797 have NUV emission significantly (>2.5σ) in excess of an empirical basal level. We inspected these candidate active stars using visible-wavelength spectra, high-resolution adaptive optics imaging, time-series photometry, and literature searches to identify cases where the elevated NUV emission is due to unresolved background sources or stellar companions; we estimated the overall occurrence of these ''false positives'' (FPs) as ∼16%. We constructed an NUV luminosity function that accounted for FPs, detection biases of the source catalogs, and GALEX upper limits. We found the NUV luminosity function to be inconsistent with predictions from a constant star-formation rate and simplified age-activity relation defined by a two-parameter power law.

  19. Dwarf galaxies in the coma cluster: Star formation properties and evolution

    Science.gov (United States)

    Hammer, Derek M.

    The infall regions of galaxy clusters are unique laboratories for studying the impact of environment on galaxy evolution. This intermediate region links the low-density field environment and the dense core of the cluster, and is thought to host recently accreted galaxies whose star formation is being quenched by external processes associated with the cluster. In this dissertation, we measure the star formation properties of galaxies at the infall region of the nearby rich cluster of galaxies, Coma. We rely primarily on Ultraviolet (UV) data owing to its sensitivity to recent star formation and we place more emphasis on the properties of dwarf galaxies. Dwarf galaxies are good tracers of external processes in clusters but their evolution is poorly constrained as they are intrinsically faint and hence more challenging to detect. We make use of deep GALEX far-UV and near-UV observations at the infall region of the Coma cluster. This area of the cluster has supporting photometric coverage at optical and IR wavelengths in addition to optical spectroscopic data that includes deep redshift coverage of dwarf galaxies in Coma. Our GALEX observations were the deepest exposures taken for a local galaxy cluster. The depth of these images required alternative data analysis techniques to overcome systematic effects that limit the default GALEX pipeline analysis. Specifically, we used a deblending method that improved detection efficiency by a factor of ˜2 and allowed reliable photometry a few magnitudes deeper than the pipeline catalog. We performed deep measurements of the total UV galaxy counts in our field that were used to measure the source confusion limit for crowded GALEX fields. The star formation properties of Coma members were studied for galaxies that span from starbursts to passive galaxies. Star-forming galaxies in Coma tend to have lower specific star formation rates, on average, as compared to field galaxies. We show that the majority of these galaxies are likely

  20. Outer atmospheres of cool stars. XII - A survey of IUE ultraviolet emission line spectra of cool dwarf stars

    Science.gov (United States)

    Linsky, J. L.; Bornmann, P. L.; Carpenter, K. G.; Hege, E. K.; Wing, R. F.; Giampapa, M. S.; Worden, S. P.

    1982-01-01

    Quantitative information is obtained on the chromospheres and transition regions of M dwarf stars, in order to determine how the outer atmospheres of dMe stars differ from dM stars and how they compare with the outer atmospheres of quiet and active G and K type dwarfs. IUE spectra of six dMe and four dM stars, together with ground-based photometry and spectroscopy of the Balmer and Ca II H and K lines, show no evidence of flares. It is concluded, regarding the quiescent behavior of these stars, that emission-line spectra resemble that of the sun and contain emission lines formed in regions with 4000-20,000 K temperatures that are presumably analogous to the solar chromosphere, as well as regions with temperatures of 20,000-200,000 K that are presumably analogous to the solar transition region. Emission-line surface fluxes are proportional to the emission measure over the range of temperatures at which the lines are formed.

  1. Chemical abundances of giant stars in NGC 5053 and NGC 5634, two globular clusters associated with the Sagittarius dwarf spheroidal galaxy?

    Science.gov (United States)

    Sbordone, L.; Monaco, L.; Moni Bidin, C.; Bonifacio, P.; Villanova, S.; Bellazzini, M.; Ibata, R.; Chiba, M.; Geisler, D.; Caffau, E.; Duffau, S.

    2015-07-01

    Context. The tidal disruption of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) is producing the most prominent substructure in the Milky Way (MW) halo, the Sagittarius Stream. Aside from field stars, it is suspected that the Sgr dSph has lost a number of globular clusters (GC). Many Galactic GC are thought to have originated in the Sgr dSph. While for some candidates an origin in the Sgr dSph has been confirmed owing to chemical similarities, others exist whose chemical composition has never been investigated. Aims: NGC 5053 and NGC 5634 are two of these scarcely studied Sgr dSph candidate-member clusters. To characterize their composition we analyzed one giant star in NGC 5053, and two in NGC 5634. Methods: We analyze high-resolution and signal-to-noise spectra by means of the MyGIsFOS code, determining atmospheric parameters and abundances for up to 21 species between O and Eu. The abundances are compared with those of MW halo field stars, of unassociated MW halo globulars, and of the metal-poor Sgr dSph main body population. Results: We derive a metallicity of [Fe ii/H] = -2.26 ± 0.10 for NGC 5053, and of [Fe i/H] = -1.99 ± 0.075 and -1.97 ± 0.076 for the two stars in NGC 5634. This makes NGC 5053 one of the most metal-poor globular clusters in the MW. Both clusters display an α enhancement similar to the one of the halo at comparable metallicity. The two stars in NGC 5634 clearly display the Na-O anticorrelation widespread among MW globulars. Most other abundances are in good agreement with standard MW halo trends. Conclusions: The chemistry of the Sgr dSph main body populations is similar to that of the halo at low metallicity. It is thus difficult to discriminate between an origin of NGC 5053 and NGC 5634 in the Sgr dSph, and one in the MW. However, the abundances of these clusters do appear closer to that of Sgr dSph than of the halo, favoring an origin in the Sgr dSph system. Appendix A is available in electronic form at http

  2. Some evidence on the evolution of the flare mechanism in dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Skumanich, A.

    1986-10-01

    White-light flare parameters are estimated for the sun as a star. It is found that these parameters fall in the same domain as those for the dMe flare stars. In particular, it is found that the time-averaged flare power loss and quiescent coronal soft X-ray power loss at solar maximum satisfies the recently proposed flare power-coronal X-ray relation for dMe stars (Doyle and Butler; Skumanich). In addition, one finds that dM stars, which are believed to be magnetically evolved dMe stars, also satisfy the same relation. On this basis, an evolutionary scenario is suggested for the flare mechanism in which the total flare rate remains, more or less, constant but the mean flare yield decreases linearly with coronal X-ray strength. It is also suggested that the flare mechanism is universal in all magnetically active dwarfs. 48 references.

  3. First detection of nonflare microwave emissions from the coronae of single late-type dwarf stars

    Science.gov (United States)

    Gary, D. E.; Linsky, J. L.

    1981-01-01

    Results are presented of a search for nonflare microwave radiation from the coronae of nearby late-type dwarf stars comparable to the sun: single stars without evidence for either a large wind or circumstellar envelope. The observing program consisted of flux measurements of six stars over a 24-h period with the VLA in the C configuration at a wavelength of 6 cm with 50 MHz bandwidth. Positive detections at 6 cm were made for Chi 1 Ori (0.6 mJy) and the flare star UV Cet (1.55 mJy), and upper limits were obtained for the stars Pi 1 UMa, Xi Boo A, 70 Oph A and Epsilon Eri. It is suggested that Chi 1 Ori, and possibly UV Cet, represent the first detected members of a new class of radio sources which are driven by gyroresonance emission, i.e. cyclotron emission from nonrelativistic Maxwellian electrons.

  4. Two new pulsating low-mass pre-white dwarfs or SX Phoenicis stars?

    Science.gov (United States)

    Corti, M. A.; Kanaan, A.; Córsico, A. H.; Kepler, S. O.; Althaus, L. G.; Koester, D.; Sánchez Arias, J. P.

    2016-03-01

    Context. The discovery of pulsations in low-mass stars opens an opportunity to probe their interiors and determine their evolution by employing the tools of asteroseismology. Aims: We aim to analyse high-speed photometry of SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25 and discover brightness variabilities. In order to locate these stars in the Teff - log g diagram, we fit optical spectra (SDSS) with synthetic non-magnetic spectra derived from model atmospheres. Methods: To carry out this study, we used the photometric data we obtained for these stars with the 2.15 m telescope at CASLEO, Argentina. We analysed their light curves and applied the discrete Fourier transform (FT) to determine the pulsation frequencies. Finally, we compare both stars in the Teff - log g diagram, with two known pre-white dwarfs and seven pulsating pre-ELM white dwarf stars, δ Scuti, and SX Phe stars Results: We report the discovery of pulsations in SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25. We determine their effective temperature and surface gravity to be Teff = 7972 ± 200 K, log g = 4.25 ± 0.5 and Teff = 7925 ± 200 K, log g = 4.25 ± 0.5, respectively. With these parameters, these new pulsating low-mass stars can be identified with either ELM white dwarfs (with ~0.17 M⊙) or more massive SX Phe stars. We identified pulsation periods of 3278.7 and 1633.9 s for SDSS J145847.02+070754.46 and a pulsation period of 3367.1 s for SDSS J173001.94+070600.25. These two new objects, together with those of Maxted et al. (2013, 2014), indicate the possible existence of a new instability domain towards the late stages of evolution of low-mass white dwarf stars, although their identification with SX Phe stars cannot be discarded. Visiting Astronomer, Complejo Astronómico El Leoncito operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba, and San Juan.

  5. Lithium abundances in samples of dwarf stars of population II and very old population I

    International Nuclear Information System (INIS)

    Molaro, P.; Beckman, J.; Rebolo, R.

    1986-01-01

    We have used the CCD camera and Intermediate Dispersion Spectrograph of the 2.5m Isaac Newton Telescope to obtain high quality spectra of the 6708 A 7 Li resonance doublet in 22 dwarfs with metallicities ≤ -0.7. We find a mean constant value of Log N(Li)= 2.07 (±0.1) for highly metal deficient dwarfs ([Fe/H] ≤ -1.5) with atmospheric temperatures T eff > 5500 K, and a larger spread with values of Log N(Li) up to 2.4 for the warmer dwarfs with metallicities between -0.7 and -1.2. Our results agree with previous findings showing a highly uniform Li abundance near the inception of the galaxy. Li depletion appears to set in at higher temperatures for the moderately metal deficient stars than for the extremely metal deficient, consistent with metallicity-dependent depletion rates (low metals, low depletion)

  6. Probing the LHS Catalog. I. New Nearby Stars and the Coolest Subdwarf

    OpenAIRE

    Gizis, John E.; Reid, I. Neill

    1997-01-01

    We present moderate resolution spectroscopy of 112 cool dwarf stars to supplement the observations we have already presented in the Palomar/MSU Nearby-Star Spectroscopic Survey. The sample consists of 72 suspected nearby stars added to the The Preliminary Third Catalog of Nearby Stars since 1991 as well as 40 faint red stars selected from the LHS catalog. LHS 1826 is more metal-poor and cooler than the coolest previously known extreme subdwarf, LHS 1742a. LHS 2195 is a very late M dwarf of ty...

  7. RED DWARF DYNAMO RAISES PUZZLE OVER INTERIORS OF LOWEST-MASS STARS

    Science.gov (United States)

    2002-01-01

    NASA's Hubble Space Telescope has uncovered surprising evidence that powerful magnetic fields might exist around the lowest mass stars in the universe, which are near the threshold of stellar burning processes. 'New theories will have to be developed to explain how these strong fields are produced, since conventional models predict that these low mass red dwarfs should have very weak or no magnetic fields,' says Dr. Jeffrey Linsky of the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado. 'The Hubble observations provide clear evidence that very low mass red dwarf stars must have some form of dynamo to amplify their magnetic fields.' His conclusions are based upon Hubble's detection of a high-temperature outburst, called a flare, on the surface of the extremely small, cool red dwarf star Van Biesbroeck 10 (VB10) also known as Gliese 752B. Stellar flares are caused by intense, twisted magnetic fields that accelerate and contain gasses which are much hotter than a star's surface. Explosive flares are common on the Sun and expected for stars that have internal structures similar to our Sun's. Stars as small as VB10 are predicted to have a simpler internal structure than that of the Sun and so are not expected to generate the electric currents required for magnetic fields that drive flares. Besides leading to a clearer understanding of the interior structure of the smallest red dwarf stars known, these unexpected results might possibly shed light on brown dwarf stars. A brown dwarf is a long-sought class of astronomical object that is too small to shine like a star through nuclear fusion processes, but is too large to be considered a planet. 'Since VB10 is nearly a brown dwarf, it is likely brown dwarfs also have strong magnetic fields,' says Linsky. 'Additional Hubble searches for flares are needed to confirm this prediction.' A QUARTER-MILLION DEGREE TORCH The star VB10 and its companion star Gliese 752A make up a binary system located 19 light

  8. Stellar model chromospheres. IX - Chromospheric activity in dwarf stars

    Science.gov (United States)

    Kelch, W. L.; Worden, S. P.; Linsky, J. L.

    1979-01-01

    High-resolution Ca II K line profiles are used to model the upper photospheres and lower chromospheres of eight main-sequence stars ranging in spectral type from F0 to M0 and exhibiting different degrees of chromospheric activity. The model chromospheres are studied as a function of spectral type and activity for stars of similar spectral type in order to obtain evidence of enhanced nonradiative heating in the upper-photospheric models and in the ratio of minimum temperature at the base of the chromosphere to effective temperature, a correlation between activity and temperature in the lower chromospheres, and a correlation of the width at the base of the K-line emission core and at the K2 features with activity. Chromospheric radiative losses are estimated for the modelled stars and other previously analyzed main-sequence stars. The results obtained strengthen the argument that dMe flare stars exhibit fundamentally solar-type activity but on an increased scale.

  9. INVESTIGATION OF THE PUZZLING ABUNDANCE PATTERN IN THE STARS OF THE FORNAX DWARF SPHEROIDAL GALAXY

    Energy Technology Data Exchange (ETDEWEB)

    Li Hongjie; Cui Wenyuan; Zhang Bo, E-mail: zhangbo@mail.hebtu.edu.cn [Department of Physics, Hebei Normal University, No. 20 East of South 2nd Ring Road, Shijiazhuang 050024 (China)

    2013-09-20

    Many works have found unusual characteristics of elemental abundances in nearby dwarf galaxies. This implies that there is a key factor of galactic evolution that is different from that of the Milky Way (MW). The chemical abundances of the stars in the Fornax dwarf spheroidal galaxy (Fornax dSph) provide excellent information for setting constraints on the models of galactic chemical evolution. In this work, adopting the five-component approach, we fit the abundances of the Fornax dSph stars, including {alpha} elements, iron group elements, and neutron-capture elements. For most sample stars, the relative contributions from the various processes to the elemental abundances are not usually in the MW proportions. We find that the contributions from massive stars to the primary {alpha} elements and iron group elements increase monotonically with increasing [Fe/H]. This means that the effect of the galactic wind is not strong enough to halt star formation and the contributions from the massive stars to {alpha} elements did not halt for [Fe/H] {approx}< -0.5. The average contribution ratios of various processes between the dSph stars and the MW stars monotonically decrease with increasing progenitor mass. This is important evidence of a bottom-heavy initial mass function (IMF) for the Fornax dSph, compared to the MW. Considering a bottom-heavy IMF for the dSph, the observed relations of [{alpha}/Fe] versus [Fe/H], [iron group/Fe] versus [Fe/H], and [neutron-capture/Fe] versus [Fe/H] for the dSph stars can be explained.

  10. Evolution of the pulsation properties of hot pre-white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Kawaler, S.D.; Winget, D.E.; Hansen, C.J.

    1985-08-01

    After solving the equations of linear, nonradial adiabatic oscillation for evolutionary pre-white dwarf (PWD) models, calculations are made for the periods, eigenfunctions, weight functions and rates of period change for high order dipole and quadrupole gravity mode oscillations in spherical nonrotating PWD models. The results obtained place stringent upper limits on the absolute magnitude of the rates of period change expected in stars represented by this class of models. 43 references.

  11. UVES abundances of stars in nearby dwarf spheroidal galaxies

    NARCIS (Netherlands)

    Tolstoy, E; Venn, K; Shetrone, M; Primas, F; Hill, [No Value; Kaufer, A; Szeifert, T

    2002-01-01

    It is a truth universally acknowledged, that a galaxy in possession of a good quantity of gas must want to form stars. It is the details of how and why that baffle us all. The simplest theories either would have this process a carefully self-regulated affair, or one that goes completely out of

  12. Instability of g-mode oscillations in white dwarf stars

    Science.gov (United States)

    Keeley, D. A.

    1979-01-01

    A white dwarf model with M = 6 solar masses, Te = 12,000 K, and L = 1.2 x 10 to the 31st erg/sec provided by Cox has been tested for linear stability of radial oscillations. The radial mode instability first reported for this model by Cox, et al. (1979) has been confirmed. The growth rates obtained are comparable to the rates found by Cox. A sequence of l = 2 g-modes has also been found to be unstable. The e-folding times range from around 10 to the 11th periods for a 137 second mode (1 radial node) to less than 100 periods for a 629 second mode (17 nodes). It is likely that the latter rate is too high because the eigenfunction has been forced to vanish at the non-zero inner radius of the model, at which the Brunt-Vaisala frequency is barely less than the mode frequency.

  13. The temperatures, abundances and gravities of F dwarf stars.

    Science.gov (United States)

    Bell, R. A.

    1971-01-01

    Theoretical colors computed from laboratory line data and from model stellar atmospheres have been used to interpret the colors of about 150 F and early G dwarfs. Effective temperatures have been derived from the H-beta index and from R-I, abundances have been obtained from m(sub 1) and from b-y, and gravities have been obtained from c(sub 1) and from b-y. The effective temperatures and gravities are in good agreement with values obtained from spectral scans. Absolute magnitudes have been obtained from the effective temperatures and gravities, the latter being used with assumed stellar masses to yield radii. The present results provide theoretical justification of the empirical formulas given by Crawford and by Stroemgren for the determination of absolute magnitudes and abundances from uvby photometry.

  14. White Dwarfs in Cataclysmic Variable Stars: Surface Temperatures and Evolution

    Directory of Open Access Journals (Sweden)

    Edward M. Sion

    2012-06-01

    Full Text Available A summary is presented of what is currently known about the surface temperatures of accreting white dwarfs (WDs detected in non-magnetic and magnetic cataclysmic variables (CVs based upon synthetic spectral analyses of far ultraviolet data. A special focus is placed on WD temperatures above and below the CV period gap as a function of the orbital period, Porb. The principal uncertainty of the temperatures for the CV WDs in the Teff - Porb distribution, besides the distance to the CV, is the mass of the WD. Only in eclipsing CV systems, an area of eclipsing binary studies, which was so central to Robert H. Koch’s career, is it possible to know CV WD masses with high precision.

  15. The Ca II resonance lines in M dwarf stars without H-alpha emission

    Energy Technology Data Exchange (ETDEWEB)

    Giampapa, M.S.; Cram, L.E.; Wild, W.J. (National Solar Observatory, Tucson, AZ (USA) Sydney Univ. (Australia) Arizona Univ., Tucson (USA))

    1989-10-01

    Spectra of the Ca II H and K lines in a sample of 31 M dwarf stars without H-alpha emission are used to calculate chromospheric K line radiative losses, F(k), and to study the joint response of Ca II K and H-alpha to chromospheric heating in dwarf M stars. It is suggested that the poor correlation found in the equivalent width - log F(K) diagram may be due either to radial segregation of the H-alpha and K line forming regions or to lateral inhomogeneities in the chromospheres. The results confirm the existence of dM stars with weak H-alpha absorption and K line emission only slightly weaker than that of the dMe stars, and show that dM stars with weak H-alpha but kinematics and metallicities representative of the young disk population belong to a class characterized by a comparatively high degree of chromospheric activity. 32 refs.

  16. The Star-forming Main Sequence of Dwarf Low Surface Brightness Galaxies

    Science.gov (United States)

    McGaugh, Stacy S.; Schombert, James M.; Lelli, Federico

    2017-12-01

    We explore the star-forming properties of late-type, low surface brightness (LSB) galaxies. The star-forming main sequence ({SFR}-{M}* ) of LSB dwarfs has a steep slope, indistinguishable from unity (1.04 ± 0.06). They form a distinct sequence from more massive spirals, which exhibit a shallower slope. The break occurs around {M}* ≈ {10}10 {M}⊙ , and can also be seen in the gas mass—stellar mass plane. The global Kennicutt-Schmidt law ({SFR}-{M}g) has a slope of 1.47 ± 0.11 without the break seen in the main sequence. There is an ample supply of gas in LSB galaxies, which have gas depletion times well in excess of a Hubble time, and often tens of Hubble times. Only ˜ 3 % of this cold gas needs be in the form of molecular gas to sustain the observed star formation. In analogy with the faint, long-lived stars of the lower stellar main sequence, it may be appropriate to consider the main sequence of star-forming galaxies to be defined by thriving dwarfs (with {M}* {10}10 {M}⊙ ) are weary giants that constitute more of a turn-off population.

  17. Discovery of Temperate Earth-Sized Planets Transiting a Nearby Ultracool Dwarf Star

    Science.gov (United States)

    Jehin, Emmanuel; Gillon, Michael; Lederer, Susan M.; Delrez, Laetitia; De Wit, Julien; Burdanov, Artem; Van Grootel, Valerie; Burgasser, Adam; Triaud, Amaury; Demory, Brice-Olivier; hide

    2016-01-01

    We report the discovery of three short-period Earth-sized planets transiting a nearby ultracool dwarf star using data collected by the Liège TRAPPIST telescope, located in la Silla (Chile). TRAPPIST-1 is an isolated M8.0+/-0.5-type dwarf star at a distance of 12.0+/-0.4 parsecs as measured by its trigonometric parallax, with an age constrained to be > 500 Myr, and with a luminosity, mass, and radius of 0.05%, 8% and 11.5% those of the Sun, respectively. The small size of the host star, only slightly larger than Jupiter, translates into Earth-like radii for the three discovered planets, as deduced from their transit depths. The inner two planets receive four and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Several orbits remain possible for the third planet based on our current data. The infrared brightness of the host star combined with its Jupiter-like size offer the possibility of thoroughly characterizing the components of this nearby planetary system.

  18. Rapid Evolution of the Gaseous Exoplanetary Debris around the White Dwarf Star HE 1349–2305

    Science.gov (United States)

    Dennihy, E.; Clemens, J. C.; Dunlap, B. H.; Fanale, S. M.; Fuchs, J. T.; Hermes, J. J.

    2018-02-01

    Observations of heavy metal pollution in white dwarf stars indicate that metal-rich planetesimals are frequently scattered into star-grazing orbits, tidally disrupted, and accreted onto the white dwarf surface, offering direct insight into the dynamical evolution of post-main-sequence exoplanetary systems. Emission lines from the gaseous debris in the accretion disks of some of these systems show variations on timescales of decades, and have been interpreted as the general relativistic precession of a recently formed, elliptical disk. Here we present a comprehensive spectroscopic monitoring campaign of the calcium infrared triplet emission in one system, HE 1349–2305, which shows morphological emission profile variations suggestive of a precessing, asymmetric intensity pattern. The emission profiles are shown to vary on a timescale of one to two years, which is an order of magnitude shorter than what has been observed in other similar systems. We demonstrate that this timescale is likely incompatible with general relativistic precession, and consider alternative explanations for the rapid evolution, including the propagation of density waves within the gaseous debris. We conclude with recommendations for follow-up observations, and discuss how the rapid evolution of the gaseous debris in HE 1349–2305 could be leveraged to test theories of exoplanetary debris disk evolution around white dwarf stars.

  19. Rejuvenation of the Innocent Bystander: Results from a Pilot X-ray Study of Dwarf Carbon Stars

    Science.gov (United States)

    Mazzoni, Fernando; Montez, Rodolfo; Green, Paul

    2018-01-01

    We present the results of a pilot study by the Chandra X-ray Observatory of X-ray emission from dwarf Carbon (dC) stars. Carbon stars were thought to be exclusively AGB stars but main sequence dwarfs showing carbon molecular bands appear to be the dominant variety. The existence of dC stars is surprising since dwarf stars cannot intrinsically produce carbon as an AGB star can. It is hypothesized that dC stars are polluted by an evolved companion star. Evidence of past pollution can appear in X-ray emission where increased coronal activity (“spin-up”) or mass accretion via a disk can be detected. Using the Chandra X-ray Observatory we detected X-ray photons in the vicinity of all the dC stars in our a pilot sample. For each detection we characterized the X-ray emission and compared to the emission expected from potential emission scenarios. Although the process that produces the X-ray emission from dC stars is presently unclear and our pilot sample is small, our results suggest that X-ray emission might be a universal characteristic of dC stars. Further examination of the X-ray emission plus future X-ray and multiwavelength observations will help us better understand the nature of these intriguing stars.

  20. CHEMICAL SIGNATURES OF THE FIRST SUPERNOVAE IN THE SCULPTOR DWARF SPHEROIDAL GALAXY

    International Nuclear Information System (INIS)

    Simon, Joshua D.; Thompson, Ian B.; Shectman, Stephen A.; Jacobson, Heather R.; Frebel, Anna; Adams, Joshua J.

    2015-01-01

    We present a homogeneous chemical abundance analysis of five of the most metal-poor stars in the Sculptor dwarf spheroidal galaxy. We analyze new and archival high resolution spectroscopy from Magellan/MIKE and VLT/UVES and determine stellar parameters and abundances in a consistent way for each star. Two of the stars in our sample, at [Fe/H] = −3.5 and [Fe/H] = −3.8, are new discoveries from our Ca K survey of Sculptor, while the other three were known in the literature. We confirm that Scl 07-50 is the lowest metallicity star identified in an external galaxy, at [Fe/H] = −4.1. The two most metal-poor stars both have very unusual abundance patterns, with striking deficiencies of the α elements, while the other three stars resemble typical extremely metal-poor Milky Way halo stars. We show that the star-to-star scatter for several elements in Sculptor is larger than that for halo stars in the same metallicity range. This scatter and the uncommon abundance patterns of the lowest metallicity stars indicate that the oldest surviving Sculptor stars were enriched by a small number of earlier supernovae, perhaps weighted toward high-mass progenitors from the first generation of stars the galaxy formed

  1. Evidence of enrichment by individual SN from elemental abundance ratios in the very metal-poor dSph galaxy Boötes I

    Science.gov (United States)

    Feltzing, S.; Eriksson, K.; Kleyna, J.; Wilkinson, M. I.

    2009-12-01

    Aims. We establish the mean metallicity from high-resolution spectroscopy for the recently found dwarf spheroidal galaxy Boötes I and test whether it is a common feature for ultra-faint dwarf spheroidal galaxies to show signs of inhomogeneous chemical evolution (e.g. as found in the Hercules dwarf spheroidal galaxy). Methods: We analyse high-resolution, moderate signal-to-noise spectra for seven red giant stars in the Boötes I dSph galaxy using standard abundance analysis techniques. In particular, we assume local thermodynamic equilibrium and employ spherical model atmospheres and codes that take the sphericity of the star into account when calculating the elemental abundances. Results: We confirm previous determinations of the mean metallicity of the Boötes I dwarf spheroidal galaxy to be -2.3 dex. Whilst five stars are clustered around this metallicity, one is significantly more metal-poor, at -2.9 dex, and one is more metal-rich at, -1.9 dex. Additionally, we find that one of the stars, Boo-127, shows an atypically high [Mg/Ca] ratio, indicative of stochastic enrichment processes within the dSph galaxy. Similar results have previously only been found in the Hercules and Draco dSph galaxies and appear, so far, to be unique to this type of galaxy. The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

  2. Microwave emission from the coronae of late-type dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Linsky, J.L.; Gary, D.E.

    1983-11-15

    We present VLA microwave observatios of 14 late-type dwarf and subgiant stars ad binary systems. In this extensive set of observations we detected four sources at 6 cm (chi/sup 1/ Ori, UV Cet, YY Gem, and Wolf 630AB) and found low upper limits for the remaining stars. The microwave luminosities of the nondetected F--K dwarfs are as small as 10/sup -2/ those of the dMe stars. The detected emission is slowly variable in all cases and is consistent with gyroresonant emission from thermal electrons spiralig in magnetic fields of about 300 gauss if the source sizes are as large as R/R/sub asterisk/roughly-equal3--4. This would correspond to magnetic fields that are probably in the range 10/sup 3/--10/sup 4/ gauss at the photospheric level. These photospheric field strengths are somewhat larger than have been observed so far in G--K dwarfs. An alternative mechanism is gyrosynchrotron emission from a relatively small number of electrons (only 10/sup -3/ the number of ambient electrons) with effective temperature, T/sub eff/>10/sup 8/ K. This mechanism is consistent with much smaller and presumably more realistic source sizes. Observations of YY gem dMle+dMle) at a number of phase are consistent with maximum but variable microwave flux at the same phase as miximum plage and central meridian passage of a large starspot of the secondary star. If confirmed by subsequent observations, this provides the first direct evidence that the emission process is magnetic in character on dMe stars.

  3. OPTICAL–NEAR-INFRARED PHOTOMETRIC CALIBRATION OF M DWARF METALLICITY AND ITS APPLICATION

    International Nuclear Information System (INIS)

    Hejazi, N.; Robertis, M. M. De; Dawson, P. C.

    2015-01-01

    Based on a carefully constructed sample of dwarf stars, a new optical–near-infrared photometric calibration to estimate the metallicity of late-type K and early-to-mid-type M dwarfs is presented. The calibration sample has two parts; the first part includes 18 M dwarfs with metallicities determined by high-resolution spectroscopy and the second part contains 49 dwarfs with metallicities obtained through moderate-resolution spectra. By applying this calibration to a large sample of around 1.3 million M dwarfs from the Sloan Digital Sky Survey and 2MASS, the metallicity distribution of this sample is determined and compared with those of previous studies. Using photometric parallaxes, the Galactic heights of M dwarfs in the large sample are also estimated. Our results show that stars farther from the Galactic plane, on average, have lower metallicity, which can be attributed to the age–metallicity relation. A scarcity of metal-poor dwarf stars in the metallicity distribution relative to the Simple Closed Box Model indicates the existence of the “M dwarf problem,” similar to the previously known G and K dwarf problems. Several more complicated Galactic chemical evolution models which have been proposed to resolve the G and K dwarf problems are tested and it is shown that these models could, to some extent, mitigate the M dwarf problem as well

  4. OPTICAL–NEAR-INFRARED PHOTOMETRIC CALIBRATION OF M DWARF METALLICITY AND ITS APPLICATION

    Energy Technology Data Exchange (ETDEWEB)

    Hejazi, N.; Robertis, M. M. De [Physics and Astronomy Department, York University, Toronto, ON M3J 1P3 (Canada); Dawson, P. C., E-mail: nedahej@yorku.ca, E-mail: mmdr@yorku.ca, E-mail: pdawson@trentu.ca [Physics Department, Trent University, Peterborough, K9J 7B8 (Canada)

    2015-04-15

    Based on a carefully constructed sample of dwarf stars, a new optical–near-infrared photometric calibration to estimate the metallicity of late-type K and early-to-mid-type M dwarfs is presented. The calibration sample has two parts; the first part includes 18 M dwarfs with metallicities determined by high-resolution spectroscopy and the second part contains 49 dwarfs with metallicities obtained through moderate-resolution spectra. By applying this calibration to a large sample of around 1.3 million M dwarfs from the Sloan Digital Sky Survey and 2MASS, the metallicity distribution of this sample is determined and compared with those of previous studies. Using photometric parallaxes, the Galactic heights of M dwarfs in the large sample are also estimated. Our results show that stars farther from the Galactic plane, on average, have lower metallicity, which can be attributed to the age–metallicity relation. A scarcity of metal-poor dwarf stars in the metallicity distribution relative to the Simple Closed Box Model indicates the existence of the “M dwarf problem,” similar to the previously known G and K dwarf problems. Several more complicated Galactic chemical evolution models which have been proposed to resolve the G and K dwarf problems are tested and it is shown that these models could, to some extent, mitigate the M dwarf problem as well.

  5. The extent of chemically enriched gas around star-forming dwarf galaxies

    Science.gov (United States)

    Johnson, Sean

    2018-01-01

    Supernovae driven winds are often invoked to remove chemically enriched gas from galaxies to match the low metallicities of dwarf galaxies. In such shallow potential wells, outflows may produce massive amounts of enriched halo gas (circum-galactic medium or CGM) and pollute the intergalactic medium (IGM). I will present a survey of the CGM and IGM around 18 star-forming field dwarf galaxies with stellar masses of log M*/M⊙ ≈ 8 ‑ 9 at z ≈ 0.2. Eight of these have CGM probed by quasar absorption spectra at projected distances, d, less than the host virial radius, Rh. Ten are probed at d/Rh = 1 ‑ 3 to study the surrounding IGM. The absorption measurements include neutral hydrogen (H I), the dominant silicon ions for diffuse cool gas (T ∼ 104 K; Si II, Si III, and Si IV), more highly ionized carbon (C IV), and highly ionized oxygen (O VI). The metal absorption from the CGM of the dwarf galaxies is less common and ≈ 4× weaker compared to massive star-forming galaxies though O VI absorption is still common. None of the dwarfs probed at d/Rh = 1 ‑ 3 have definitive metal-line detections. Combining the available silicon ions, we estimate that the cool CGM accounts for only 2 ‑ 6% of the expected silicon budget. CGM absorption from O VI can account for ≈ 8% of the expected oxygen budget. As O VI traces an ion with expected equilibrium ion fractions of 0.2, this highly ionized phase of the CGM may represent a significant metal reservoir even for dwarf galaxies not expected to maintain gravitationally shock heated hot halos.

  6. PROBABILITY OF CME IMPACT ON EXOPLANETS ORBITING M DWARFS AND SOLAR-LIKE STARS

    Energy Technology Data Exchange (ETDEWEB)

    Kay, C. [Solar Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States); Opher, M.; Kornbleuth, M., E-mail: ckay@bu.edu [Astronomy Department, Boston University, Boston, MA 02215 (United States)

    2016-08-01

    Solar coronal mass ejections (CMEs) produce adverse space weather effects at Earth. Planets in the close habitable zone of magnetically active M dwarfs may experience more extreme space weather than at Earth, including frequent CME impacts leading to atmospheric erosion and leaving the surface exposed to extreme flare activity. Similar erosion may occur for hot Jupiters with close orbits around solar-like stars. We have developed a model, Forecasting a CME's Altered Trajectory (ForeCAT), which predicts a CME's deflection. We adapt ForeCAT to simulate CME deflections for the mid-type M dwarf V374 Peg and hot Jupiters with solar-type hosts. V374 Peg's strong magnetic fields can trap CMEs at the M dwarfs's Astrospheric Current Sheet, that is, the location of the minimum in the background magnetic field. Solar-type CMEs behave similarly, but have much smaller deflections and do not become trapped at the Astrospheric Current Sheet. The probability of planetary impact decreases with increasing inclination of the planetary orbit with respect to the Astrospheric Current Sheet: 0.5–5 CME impacts per day for M dwarf exoplanets, 0.05–0.5 CME impacts per day for solar-type hot Jupiters. We determine the minimum planetary magnetic field necessary to shield a planet's atmosphere from CME impacts. M dwarf exoplanets require values between tens and hundreds of Gauss. Hot Jupiters around a solar-type star, however, require a more reasonable <30 G. These values exceed the magnitude required to shield a planet from the stellar wind, suggesting that CMEs may be the key driver of atmospheric losses.

  7. PROBABILITY OF CME IMPACT ON EXOPLANETS ORBITING M DWARFS AND SOLAR-LIKE STARS

    International Nuclear Information System (INIS)

    Kay, C.; Opher, M.; Kornbleuth, M.

    2016-01-01

    Solar coronal mass ejections (CMEs) produce adverse space weather effects at Earth. Planets in the close habitable zone of magnetically active M dwarfs may experience more extreme space weather than at Earth, including frequent CME impacts leading to atmospheric erosion and leaving the surface exposed to extreme flare activity. Similar erosion may occur for hot Jupiters with close orbits around solar-like stars. We have developed a model, Forecasting a CME's Altered Trajectory (ForeCAT), which predicts a CME's deflection. We adapt ForeCAT to simulate CME deflections for the mid-type M dwarf V374 Peg and hot Jupiters with solar-type hosts. V374 Peg's strong magnetic fields can trap CMEs at the M dwarfs's Astrospheric Current Sheet, that is, the location of the minimum in the background magnetic field. Solar-type CMEs behave similarly, but have much smaller deflections and do not become trapped at the Astrospheric Current Sheet. The probability of planetary impact decreases with increasing inclination of the planetary orbit with respect to the Astrospheric Current Sheet: 0.5–5 CME impacts per day for M dwarf exoplanets, 0.05–0.5 CME impacts per day for solar-type hot Jupiters. We determine the minimum planetary magnetic field necessary to shield a planet's atmosphere from CME impacts. M dwarf exoplanets require values between tens and hundreds of Gauss. Hot Jupiters around a solar-type star, however, require a more reasonable <30 G. These values exceed the magnitude required to shield a planet from the stellar wind, suggesting that CMEs may be the key driver of atmospheric losses.

  8. The atomic and molecular content of disks around very low-mass stars and brown dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Pascucci, I. [Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721 (United States); Herczeg, G. [Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871 (China); Carr, J. S. [Naval Research Laboratory, Code 7211, Washington, DC 20375 (United States); Bruderer, S., E-mail: pascucci@lpl.arizona.edu [Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, D-85748 Garching (Germany)

    2013-12-20

    There is growing observational evidence that disk evolution is stellar-mass-dependent. Here, we show that these dependencies extend to the atomic and molecular content of disk atmospheres. We analyze a unique dataset of high-resolution Spitzer/IRS spectra from eight very low mass star and brown dwarf disks. We report the first detections of Ne{sup +}, H{sub 2}, CO{sub 2}, and tentative detections of H{sub 2}O toward these faint and low-mass disks. Two of our [Ne II] 12.81 μm emission lines likely trace the hot (≥5000 K) disk surface irradiated by X-ray photons from the central stellar/sub-stellar object. The H{sub 2} S(2) and S(1) fluxes are consistent with arising below the fully or partially ionized surface traced by the [Ne II] emission in gas at ∼600 K. We confirm the higher C{sub 2}H{sub 2}/HCN flux and column density ratio in brown dwarf disks previously noted from low-resolution IRS spectra. Our high-resolution spectra also show that the HCN/H{sub 2}O fluxes of brown dwarf disks are on average higher than those of T Tauri disks. Our LTE modeling hints that this difference extends to column density ratios if H{sub 2}O lines trace warm ≥600 K disk gas. These trends suggest that the inner regions of brown dwarf disks have a lower O/C ratio than those of T Tauri disks, which may result from a more efficient formation of non-migrating icy planetesimals. An O/C = 1, as inferred from our analysis, would have profound implications on the bulk composition of rocky planets that can form around very low mass stars and brown dwarfs.

  9. CHEMICAL ABUNDANCES IN NGC 5053: A VERY METAL-POOR AND DYNAMICALLY COMPLEX GLOBULAR CLUSTER

    Energy Technology Data Exchange (ETDEWEB)

    Boberg, Owen M.; Friel, Eileen D.; Vesperini, Enrico [Astronomy Department, Indiana University, Bloomington, IN 47405 (United States)

    2015-05-10

    NGC 5053 provides a rich environment to test our understanding of the complex evolution of globular clusters (GCs). Recent studies have found that this cluster has interesting morphological features beyond the typical spherical distribution of GCs, suggesting that external tidal effects have played an important role in its evolution and current properties. Additionally, simulations have shown that NGC 5053 could be a likely candidate to belong to the Sagittarius dwarf galaxy (Sgr dSph) stream. Using the Wisconsin–Indiana–Yale–NOAO–Hydra multi-object spectrograph, we have collected high quality (signal-to-noise ratio ∼ 75–90), medium-resolution spectra for red giant branch stars in NGC 5053. Using these spectra we have measured the Fe, Ca, Ti, Ni, Ba, Na, and O abundances in the cluster. We measure an average cluster [Fe/H] abundance of −2.45 with a standard deviation of 0.04 dex, making NGC 5053 one of the most metal-poor GCs in the Milky Way (MW). The [Ca/Fe], [Ti/Fe], and [Ba/Fe] we measure are consistent with the abundances of MW halo stars at a similar metallicity, with alpha-enhanced ratios and slightly depleted [Ba/Fe]. The Na and O abundances show the Na–O anti-correlation found in most GCs. From our abundance analysis it appears that NGC 5053 is at least chemically similar to other GCs found in the MW. This does not, however, rule out NGC 5053 being associated with the Sgr dSph stream.

  10. Chemical Abundances in NGC 5053: A Very Metal-poor and Dynamically Complex Globular Cluster

    Science.gov (United States)

    Boberg, Owen M.; Friel, Eileen D.; Vesperini, Enrico

    2015-05-01

    NGC 5053 provides a rich environment to test our understanding of the complex evolution of globular clusters (GCs). Recent studies have found that this cluster has interesting morphological features beyond the typical spherical distribution of GCs, suggesting that external tidal effects have played an important role in its evolution and current properties. Additionally, simulations have shown that NGC 5053 could be a likely candidate to belong to the Sagittarius dwarf galaxy (Sgr dSph) stream. Using the Wisconsin-Indiana-Yale-NOAO-Hydra multi-object spectrograph, we have collected high quality (signal-to-noise ratio ˜ 75-90), medium-resolution spectra for red giant branch stars in NGC 5053. Using these spectra we have measured the Fe, Ca, Ti, Ni, Ba, Na, and O abundances in the cluster. We measure an average cluster [Fe/H] abundance of -2.45 with a standard deviation of 0.04 dex, making NGC 5053 one of the most metal-poor GCs in the Milky Way (MW). The [Ca/Fe], [Ti/Fe], and [Ba/Fe] we measure are consistent with the abundances of MW halo stars at a similar metallicity, with alpha-enhanced ratios and slightly depleted [Ba/Fe]. The Na and O abundances show the Na-O anti-correlation found in most GCs. From our abundance analysis it appears that NGC 5053 is at least chemically similar to other GCs found in the MW. This does not, however, rule out NGC 5053 being associated with the Sgr dSph stream.

  11. Identification and characterization of low mass stars and brown dwarfs using Virtual Observatory tools

    Science.gov (United States)

    Aberasturi, Miriam

    2015-11-01

    Context: Two thirds of the stars in our galactic neighborhood (d searches. Brown dwarfs (BDs) are self-gravitating objects that do not get enough mass to maintain a sufficiently high temperature in their core for stable hydrogen fusion. They represent the link between low-mass stars and giant planets. Due to their low temperatures, BDs emit significant flux at mid-infrared wavelength which makes this range very adequate to look for this type of objects. The Virtual Observatory (VO) is an international initiative designed to help the astronomical community in the exploitation of the multi-wavelength information that resides in data archives. In the last years the Spanish Virtual Observatory is conducting a number of projects focused on the study of substellar objects taking advantage of Virtual Observatory tools for an easy data access and analysis of large area surveys. This is the framework where this thesis has been carried out. This dissertation addresses three problems in the framework of low-mass stars and brown dwarfs, namely, the search for brown dwarf candidates crossmatching catalogues (Chapter 4), the search for nearby bright M dwarfs and the subsequent spectroscopic characterization (Chapter 5), and a study of binarity in mid to late-T brown dwarfs (Chapter 6); the first two topics use Virtual Observatory tools. Aims and methodology:In the first paper we carried out a search of brown dwarfs in the sky area in common to the WISE, 2MASS Point Source and SDSS catalogues. A VO-workflow with the criteria that must accomplish our candidates was built using STILTS. The workflow returned 138 sources that were visually inspected. For the six new candidates that passed the inspection, proper motions were calculated using the positions and the different observing epochs of the catalogues previously quoted. Effective temperatures were estimated using VOSA and spectral types and distances using appropriate photometric calibrations. In the second publication we

  12. Evidence for halo kinematics among cool carbon-rich dwarfs

    Science.gov (United States)

    Farihi, J.; Arendt, A. R.; Machado, H. S.; Whitehouse, L. J.

    2018-04-01

    This paper reports preliminary yet compelling kinematical inferences for N ≳ 600 carbon-rich dwarf stars that demonstrate around 30% to 60% are members of the Galactic halo. The study uses a spectroscopically and non-kinematically selected sample of stars from the SDSS, and cross-correlates these data with three proper motion catalogs based on Gaia DR1 astrometry to generate estimates of their 3-D space velocities. The fraction of stars with halo-like kinematics is roughly 30% for distances based on a limited number of parallax measurements, with the remainder dominated by the thick disk, but close to 60% of the sample lie below an old, metal-poor disk isochrone in reduced proper motion. An ancient population is consistent with an extrinsic origin for C/O >1 in cool dwarfs, where a fixed mass of carbon pollution more readily surmounts lower oxygen abundances, and with a lack of detectable ultraviolet-blue flux from younger white dwarf companions. For an initial stellar mass function that favors low-mass stars as in the Galactic disk, the dC stars are likely to be the dominant source of carbon-enhanced, metal-poor stars in the Galaxy.

  13. Field #3 of the Palomar-Groningen Survey; 1, Variable stars at the edge of the Sagittarius dwarf galaxy

    NARCIS (Netherlands)

    Schultheis, M.

    1996-01-01

    Submitted to: Astron. Astrophys. Abstract: A catalogue is presented with variable (RR Lyrae, semiregular and Mira) stars located inside field #3 of the Palomar-Groningen Survey, at the outer edge of the Sagittarius dwarf galaxy. One of the semiregular variables is a carbon star, comparable with

  14. Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

    Science.gov (United States)

    Shipman, Harry; Barnhill, Maurice; Provencal, Judi; Roby, Scott; Bues, Irmela; Cordova, France; Hammond, Gordon; Hintzen, Paul; Koester, Detlev; Liebert, James

    1995-01-01

    Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

  15. A COMPREHENSIVE, WIDE-FIELD STUDY OF PULSATING STARS IN THE CARINA DWARF SPHEROIDAL GALAXY

    Energy Technology Data Exchange (ETDEWEB)

    Vivas, A. Katherina [Centro de Investigaciones de Astronomía (CIDA), Apartado Postal 264, Mérida 5101-A (Venezuela, Bolivarian Republic of); Mateo, Mario, E-mail: akvivas@cida.ve, E-mail: mmateo@umich.edu [Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States)

    2013-12-01

    We report the detection of 388 pulsating variable stars (and some additional miscellaneous variables) in the Carina dwarf spheroidal galaxy over an area covering the full visible extent of the galaxy and extending a few times beyond its photometric (King) tidal radius along the direction of its major axis. Included in this total are 340 newly discovered dwarf Cepheids (DCs), which are mostly located ∼2.5 mag below the horizontal branch and have very short periods (<0.1 days), typical of their class and consistent with their location on the upper part of the extended main sequence of the younger populations of the galaxy. Several extra-tidal DCs were found in our survey up to a distance of ∼1° from the center of Carina. Our sample also includes RR Lyrae stars and anomalous Cepheids, some of which were found outside the galaxy's tidal radius as well. This supports past works that suggest that Carina is undergoing tidal disruption. We use the period-luminosity relationship for DCs to estimate a distance modulus of μ{sub 0} = 20.17 ± 0.10 mag, in very good agreement with the estimate from RR Lyrae stars. We find some important differences in the properties of the DCs of Carina and those in Fornax and the LMC, the only extragalactic samples of DCs currently known. These differences may reflect a metallicity spread, depth along the line of sight, and/or different evolutionary paths of the DC stars.

  16. The Binary Dwarf Carbon Star SDSS J125017.90+252427.6

    Science.gov (United States)

    Margon, Bruce; Kupfer, Thomas; Burdge, Kevin; Prince, Thomas A.; Kulkarni, Shrinivas R.; Shupe, David L.

    2018-03-01

    Although dwarf carbon (dC) stars are universally thought to be binaries in order to explain the presence of C 2 in their spectra while still near main-sequence luminosity, direct observational evidence for their binarity is remarkably scarce. Here, we report the detection of a 2.92 day periodicity in both the photometry and radial velocity of SDSS J125017.90+252427.6, an r = 16.4 dC star. This is the first photometric binary dC, and only the second dC spectroscopic binary. The relative phase of the photometric period to the spectroscopic observations suggests that the photometric variations are a reflection effect due to heating from an unseen companion. The observed radial velocity amplitude of the dC component (K = 98.8 ± 10.7 km s‑1) is consistent with a white dwarf companion, presumably the evolved star that earlier donated the carbon to the dC, although substantial orbital evolution must have occurred. Large synoptic photometric surveys such as the Palomar Transient Factory, which was used for this work, may prove useful for identifying binaries among the shorter-period dC stars.

  17. Environmental effects on stellar populations of dwarf galaxies and star clusters

    Science.gov (United States)

    Pasetto, Stefano; Cropper, Mark; fujita, Yutaka; Chiosi, Cesare; Grebel, Eva K.

    2015-08-01

    We investigate the competitive role of the different dissipative phenomena acting on the onset of star formation history of gravitationally bound system in an external environment. Ram pressure, Kelvin-Helmholtz instability, Rayleigh-Taylor, and tidal forces are accounted separately in an analytical framework and compared in their role in influencing the star forming regions. We present an analytical criterion to elucidate the dependence of star formation in a spherical stellar system on its surrounding environment useful in observational applications as well as theoretical interpretations of numerical results.We consider the different signatures of these phenomena in synthetically realized colour-magnitude diagrams (CMDs) of the orbiting system thus investigating the detectability limits of these different effects for future observational projects and their relevance.The theoretical framework developed has direct applications to the cases of dwarf galaxies in galaxy clusters and dwarf galaxies orbiting our Milky Way system, as well as any primordial gas-rich cluster of stars orbiting within its host galaxy.

  18. Mapping the Tidal Destruction of the Hercules Dwarf: A Wide-field DECam Imaging Search for RR Lyrae Stars

    Science.gov (United States)

    Garling, Christopher; Willman, Beth; Sand, David J.; Hargis, Jonathan; Crnojević, Denija; Bechtol, Keith; Carlin, Jeffrey L.; Strader, Jay; Zou, Hu; Zhou, Xu; Nie, Jundan; Zhang, Tianmeng; Zhou, Zhimin; Peng, Xiyan

    2018-01-01

    We investigate the hypothesized tidal disruption of the Hercules ultra-faint dwarf galaxy (UFD). Previous tidal disruption studies of the Hercules UFD have been hindered by the high degree of foreground contamination in the direction of the dwarf. We bypass this issue by using RR Lyrae stars, which are standard candles with a very low field-volume density at the distance of Hercules. We use wide-field imaging from the Dark Energy Camera on CTIO to identify candidate RR Lyrae stars, supplemented with observations taken in coordination with the Beijing–Arizona Sky Survey on the Bok Telescope. Combining color, magnitude, and light-curve information, we identify three new RR Lyrae stars associated with Hercules. All three of these new RR Lyrae stars lie outside its published tidal radius. When considered with the nine RR Lyrae stars already known within the tidal radius, these results suggest that a substantial fraction of Hercules’ stellar content has been stripped. With this degree of tidal disruption, Hercules is an interesting case between a visibly disrupted dwarf (such as the Sagittarius dwarf spheroidal galaxy) and one in dynamic equilibrium. The degree of disruption also shows that we must be more careful with the ways we determine object membership when estimating dwarf masses in the future. One of the three discovered RR Lyrae stars sits along the minor axis of Hercules, but over two tidal radii away. This type of debris is consistent with recent models that suggest Hercules’ orbit is aligned with its minor axis.

  19. The Leoncino Dwarf: The Lowest Metallicity Star-Forming Galaxy in the Nearby Universe

    Science.gov (United States)

    McQuinn, Kristen

    2017-08-01

    Extremely metal-poor (XMP) galaxies are dwarf irregular galaxies with very low metallicities, traced by their gas-phase oxygen abundance. Galaxy evolution scenarios suggest three pathways to form an XMP: (1) secular evolution at low galaxy masses, (2) slow evolution in voids, or (3) dilution of measured abundances from infall of pristine gas. These scenarios have proven challenging to test because, despite concerted efforts, XMP galaxies in the nearby universe have proven hard to find. A notable exception is the recently discovered dwarf galaxy Leoncino. Leoncino has the lowest gas-phase oxygen abundance ever measured in a galaxy in the local Universe. From optical spectroscopy, the oxygen abundance is 12+log(O/H)=7.02+/-0.03, more than 40% lower than the iconic low-metallicity galaxy I Zw 18 and less than 2% Z_sun. Despite a precision oxygen abundance measurement, the evolutionary context of Leoncino remains uncertain without a secure distance. We propose HST WFC3 high-resolution optical imaging of Leoncino to accurately measure the distance to the galaxy using the tip of the red giant branch (TRGB) method. The distance will determine whether Leoncino is located in a typical field environment or in a void, and whether the galaxy is consistent with the luminosity-metallicity relation at low galaxy masses. The detailed study of Leoncino will provide benchmark results for future XMP discoveries in the nearby Universe, and an exceptionally timely comparison for studies of chemically primitive, high-redshift galaxies that will be observable in the JWST era.

  20. A GMOS-N IFU study of the central H II region in the blue compact dwarf galaxy NGC 4449: kinematics, nebular metallicity and star formation

    Science.gov (United States)

    Kumari, Nimisha; James, Bethan L.; Irwin, Mike J.

    2017-10-01

    We use integral field spectroscopic (IFS) observations from the Gemini Multi-Object Spectrograph North (GMOS-N) to study the central H II region in a nearby blue compact dwarf (BCD) galaxy NGC 4449. The IFS data enable us to explore the variation of physical and chemical conditions of the star-forming region and the surrounding gas on spatial scales as small as 5.5 pc. Our kinematical analysis shows possible signatures of shock ionization and shell structures in the surroundings of the star-forming region. The metallicity maps of the region, created using direct Te and indirect strong line methods (R23, O3N2 and N2), do not show any chemical variation. From the integrated spectrum of the central H II region, we find a metallicity of 12 + log(O/H) = 7.88 ± 0.14 ({˜ }0.15^{+0.06}_{-0.04} Z⊙) using the direct method. Comparing the central H II region metallicity derived here with those of H II regions throughout this galaxy from previous studies, we find evidence of increasing metallicity with distance from the central nucleus. Such chemical inhomogeneities can be due to several mechanisms, including gas loss via supernova blowout, galactic winds or metal-poor gas accretion. However, we find that the localized area of decreased metallicity aligns spatially with the peak of star-forming activity in the galaxy, suggesting that gas accretion may be at play here. Spatially resolved IFS data for the entire galaxy are required to confirm the metallicity inhomogeneity found in this study and determine its possible cause.

  1. NuSTAR and Swift Observations of the Dwarf Nova Z Camelpardalis in a Standstill

    Science.gov (United States)

    Mukai, Koji; Sokoloski, Jennifer; Nelson, Thomas; Luna, Gerardo Juan Manuel; Ringwald, Frederick

    2018-01-01

    Dwarf nova outbursts are dramatic increases in the optical/UV emission from the accretion disks surrounding non-magnetic, or weakly magnetic, white dwarfs, and they are believed to be caused by disk instabilities. During the optical outburst, the optically thin X-rays originating from the boundary layer between the disk and the white dwarf are known to become fainter and softer. However, during an outburst, neither the disk nor the boundary layer has the time to settle into a steady state, exhibiting clear hysteresis effects instead. The Z Cam-type dwarf novae exhibit a rare, third state called standstill, lasting several months to several years, at an optical brightness roughly one magnitude below outburst peak. A standstill is therefore an ideal opportunity to study a high-state disk while minimizing the hysteresis effects. Here we report our NuSTAR and Swift observations of the prototype, Z Cam, in late September, 2017, roughly 6 months into its most recent standstill episode. To the best of our knowledge, this is the first pointed X-ray observation of a Z Cam-type object in a standstill, and our preliminary analysis suggests Z Cam in standstill has X-ray properties broadly similar to those seen during past outbursts. We will describe these results and discuss implications for the disk physics.

  2. Modeling the Cloudy Atmospheres of Cool Stars, Brown Dwarfs and Hot Exoplanets

    DEFF Research Database (Denmark)

    Juncher, Diana

    M-dwarfs are very attractive targets when searching for new exoplanets. Unfortunately, they are also very difficult to model since their temperatures are low enough for dust clouds to form in their atmospheres. Because the properties of an exoplanet cannot be determined without knowing the proper......M-dwarfs are very attractive targets when searching for new exoplanets. Unfortunately, they are also very difficult to model since their temperatures are low enough for dust clouds to form in their atmospheres. Because the properties of an exoplanet cannot be determined without knowing......-consistent cloudy atmosphere models that can be used to properly determine the stellar parameters of cool stars. With this enhanced model atmosphere code I have created a grid of cool, dusty atmosphere models ranging in effective temperatures from Teff = 2000 − 3000 K. I have studied the formation and structure...... of their clouds and found that their synthetic spectra fit the observed spectra of mid to late type M-dwarfs and early type L-dwarfs well. With additional development into even cooler regimes, they could be used to characterize the atmospheres of exoplanets and aid us in our search for the kind of chemical...

  3. Contributions of late-type dwarf stars to the soft X-ray diffuse background

    Energy Technology Data Exchange (ETDEWEB)

    Schmitt, J.H.M.M.; Snowden, S.L. (Max-Planck-Institut fuer Extraterrestrische Physik, Garching (Germany, F.R.) Wisconsin Univ., Madison (USA))

    1990-09-01

    Comprehensive calculations of the contribution of late-type dwarf stars to the soft X-ray diffuse background are presented. The mean X-ray luminosity as derived from optically and X-ray selected samples is examined, using the Bahcall-Soneira Galaxy model to describe the spatial distribution of stars and recent results on the X-ray spectra. The model calculations are compared with the Wisconsin sky maps in the C, M1, M2, I and J bands to assess the uncertainties of the calculations. Contributions of up to 10 percent to the M2 and I band background at high Galactic latitudes are found, while at low Galactic latitudes late-type stars contribute up to 40 percent of the background. However, a Galactic ridge as well as a relatively isotropic component still remains unexplained, even with the added contribution of the extrapolated high-energy power law. 41 refs.

  4. Contributions of late-type dwarf stars to the soft X-ray diffuse background

    Science.gov (United States)

    Schmitt, J. H. M. M.; Snowden, S. L.

    1990-01-01

    Comprehensive calculations of the contribution of late-type dwarf stars to the soft X-ray diffuse background are presented. The mean X-ray luminosity as derived from optically and X-ray selected samples is examined, using the Bahcall-Soneira Galaxy model to describe the spatial distribution of stars and recent results on the X-ray spectra. The model calculations are compared with the Wisconsin sky maps in the C, M1, M2, I and J bands to assess the uncertainties of the calculations. Contributions of up to 10 percent to the M2 and I band background at high Galactic latitudes are found, while at low Galactic latitudes late-type stars contribute up to 40 percent of the background. However, a Galactic ridge as well as a relatively isotropic component still remains unexplained, even with the added contribution of the extrapolated high-energy power law.

  5. The First APOKASC Catalog of Kepler Dwarf and Subgiant Stars

    Science.gov (United States)

    Serenelli, Aldo; Johnson, Jennifer; Huber, Daniel; Pinsonneault, Marc; Ball, Warrick H.; Tayar, Jamie; Silva Aguirre, Victor; Basu, Sarbani; Troup, Nicholas; Hekker, Saskia; Kallinger, Thomas; Stello, Dennis; Davies, Guy R.; Lund, Mikkel N.; Mathur, Savita; Mosser, Benoit; Stassun, Keivan G.; Chaplin, William J.; Elsworth, Yvonne; García, Rafael A.; Handberg, Rasmus; Holtzman, Jon; Hearty, Fred; García-Hernández, D. A.; Gaulme, Patrick; Zamora, Olga

    2017-12-01

    We present the first APOKASC catalog of spectroscopic and asteroseismic data for dwarfs and subgiants. Asteroseismic data for our sample of 415 objects have been obtained by the Kepler mission in short (58.5 s) cadence, and light curves span from 30 up to more than 1000 days. The spectroscopic parameters are based on spectra taken as part of the Apache Point Observatory Galactic Evolution Experiment and correspond to Data Release 13 of the Sloan Digital Sky Survey. We analyze our data using two independent {T}{eff} scales, the spectroscopic values from DR13 and those derived from SDSS griz photometry. We use the differences in our results arising from these choices as a test of systematic temperature uncertainties and find that they can lead to significant differences in the derived stellar properties. Determinations of surface gravity ({log}g), mean density ( ), radius (R), mass (M), and age (τ) for the whole sample have been carried out by means of (stellar) grid-based modeling. We have thoroughly assessed random and systematic error sources in the spectroscopic and asteroseismic data, as well as in the grid-based modeling determination of the stellar quantities provided in the catalog. We provide stellar properties determined for each of the two {T}{eff} scales. The median combined (random and systematic) uncertainties are 2% (0.01 dex; {log}g), 3.4% ( ), 2.6% (R), 5.1% (M), and 19% (τ) for the photometric {T}{eff} scale and 2% ({log}g), 3.5% ( ), 2.7% (R), 6.3% (M), and 23% (τ) for the spectroscopic scale. We present comparisons with stellar quantities in the asteroseismic catalog by Chaplin et al. that highlight the importance of having metallicity measurements for determining stellar parameters accurately. Finally, we compare our results with those coming from a variety of sources, including stellar radii determined from TGAS parallaxes and asteroseismic analyses based on individual frequencies. We find a very good agreement for all inferred quantities

  6. PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS

    Energy Technology Data Exchange (ETDEWEB)

    Bowler, Brendan P. [California Institute of Technology, Division of Geological and Planetary Sciences, 1200 East California Boulevard, Pasadena, CA 91101 (United States); Liu, Michael C. [Institute for Astronomy, University of Hawai' i, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Shkolnik, Evgenya L. [Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001 (United States); Tamura, Motohide, E-mail: bpbowler@caltech.edu [National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)

    2015-01-01

    We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (≳1 M {sub Jup}) around 122 newly identified nearby (≲40 pc) young M dwarfs. Half of our targets are younger than 135 Myr and 90% are younger than the Hyades (620 Myr). After removing 44 close stellar binaries (implying a stellar companion fraction of >35.4% ± 4.3% within 100 AU), 27 of which are new or spatially resolved for the first time, our remaining sample of 78 single M dwarfs makes this the largest imaging search for planets around young low-mass stars (0.1-0.6 M {sub ☉}) to date. Our H- and K-band coronagraphic observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of 12-14 mag and 9-13 mag at 1'', respectively, which correspond to limiting planet masses of 0.5-10 M {sub Jup} at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 ± 6 M {sub Jup}; L0{sub −1}{sup +2}; 120 ± 20 AU), GJ 3629 B (64{sub −23}{sup +30} M {sub Jup}; M7.5 ± 0.5; 6.5 ± 0.5 AU), 1RXS J034231.8+121622 B (35 ± 8 M {sub Jup}; L0 ± 1; 19.8 ± 0.9 AU), and 2MASS J15594729+4403595 B (43 ± 9 M {sub Jup}; M8.0 ± 0.5; 190 ± 20 AU). Over 150 candidate planets were identified; we obtained follow-up imaging for 56% of these but all are consistent with background stars. Our null detection of planets enables strong statistical constraints on the occurrence rate of long-period giant planets around single M dwarfs. We infer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13 M {sub Jup} planets between 10-100 AU for hot-start and cold-start (Fortney) evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbor massive gas giants in the 5-13 M {sub Jup} range like those orbiting HR 8799 and β Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. The frequency of brown dwarf (13-75 M {sub Jup}) companions

  7. Mapping radio emitting-region on low-mass stars and brown dwarfs

    Directory of Open Access Journals (Sweden)

    Hallinan G.

    2011-07-01

    Full Text Available Strong magnetic activity in ultracool dwarfs (UCDs, spectral classes later than M7 have emerged from a number of radio observations, including the periodic beams. The highly (up to 100% circularly polarized nature of the emission point to an effective amplification mechanism of the high-frequency electromagnetic waves – the electron cyclotron maser (ECM instability. Several anisotropic velocity distibution models of electrons, including the horseshoe distribution, ring shell distribution and the loss-cone distribution, are able to generate the ECM instability. A magnetic-field-aligned electric potential would play an significant role in the ECM process. We are developing a theoretical model in order to simulate ECM and apply this model to map the radio-emitting region on low-mass stars and brown dwarfs.

  8. The BDNYC database of low-mass stars, brown dwarfs, and planetary mass companions

    Science.gov (United States)

    Cruz, Kelle; Rodriguez, David; Filippazzo, Joseph; Gonzales, Eileen; Faherty, Jacqueline K.; Rice, Emily; BDNYC

    2018-01-01

    We present a web-interface to a database of low-mass stars, brown dwarfs, and planetary mass companions. Users can send SELECT SQL queries to the database, perform searches by coordinates or name, check the database inventory on specified objects, and even plot spectra interactively. The initial version of this database contains information for 198 objects and version 2 will contain over 1000 objects. The database currently includes photometric data from 2MASS, WISE, and Spitzer and version 2 will include a significant portion of the publicly available optical and NIR spectra for brown dwarfs. The database is maintained and curated by the BDNYC research group and we welcome contributions from other researchers via GitHub.

  9. Asteroseismology of DAV white dwarf stars and G29–38

    International Nuclear Information System (INIS)

    Chen Yan-Hui; Li Yan

    2013-01-01

    Asteroseismology is a powerful tool used for detecting the inner structure of stars, which is also widely used to study white dwarfs. We discuss the asteroseismology of DAV stars. The period-to-period fitting method is discussed in detail, including its reliability in detecting the inner structure of DAV stars. If we assume that all observed modes of some DAV stars are the l = 1 cases, the errors associated with model fitting will be always large. If we assume that the observed modes are composed of l = 1 and 2 modes, the errors associated with model fitting in this case will be small. However, there will be modes identified as l = 2 that do not have observed quintuplets. G29–38 has been observed spectroscopically and photometrically for many years. Thompson et al. made l modes identifications in the star through the limb darkening effect. With 11 known l modes, we also study the asteroseismology of G29–38, which reduces the blind l fittings and is a fair choice. Unfortunately, our two best-fitting models are not in line with the previous atmospheric results. Based on factors like only a few observed modes, stability and identification of eigenmodes, identification of spherical degrees, construction of physical and realistic models and so on, detecting the inner structure of DAV stars by asteroseismology needs further development

  10. Simulating neutron star mergers as r-process sources in ultrafaint dwarf galaxies

    Science.gov (United States)

    Safarzadeh, Mohammadtaher; Scannapieco, Evan

    2017-10-01

    To explain the high observed abundances of r-process elements in local ultrafaint dwarf (UFD) galaxies, we perform cosmological zoom simulations that include r-process production from neutron star mergers (NSMs). We model star formation stochastically and simulate two different haloes with total masses ≈108 M⊙ at z = 6. We find that the final distribution of [Eu/H] versus [Fe/H] is relatively insensitive to the energy by which the r-process material is ejected into the interstellar medium, but strongly sensitive to the environment in which the NSM event occurs. In one halo, the NSM event takes place at the centre of the stellar distribution, leading to high levels of r-process enrichment such as seen in a local UFD, Reticulum II (Ret II). In a second halo, the NSM event takes place outside of the densest part of the galaxy, leading to a more extended r-process distribution. The subsequent star formation occurs in an interstellar medium with shallow levels of r-process enrichment that results in stars with low levels of [Eu/H] compared to Ret II stars even when the maximum possible r-process mass is assumed to be ejected. This suggests that the natal kicks of neutron stars may also play an important role in determining the r-process abundances in UFD galaxies, a topic that warrants further theoretical investigation.

  11. The disk averaged star formation relation for Local Volume dwarf galaxies

    Science.gov (United States)

    López-Sánchez, Á. R.; Lagos, C. D. P.; Young, T.; Jerjen, H.

    2018-05-01

    Spatially resolved H I studies of dwarf galaxies have provided a wealth of precision data. However these high-quality, resolved observations are only possible for handful of dwarf galaxies in the Local Volume. Future H I surveys are unlikely to improve the current situation. We therefore explore a method for estimating the surface density of the atomic gas from global H I parameters, which are conversely widely available. We perform empirical tests using galaxies with resolved H I maps, and find that our approximation produces values for the surface density of atomic hydrogen within typically 0.5 dex of the true value. We apply this method to a sample of 147 galaxies drawn from modern near-infrared stellar photometric surveys. With this sample we confirm a strict correlation between the atomic gas surface density and the star formation rate surface density, that is vertically offset from the Kennicutt-Schmidt relation by a factor of 10 - 30, and significantly steeper than the classical N = 1.4 of Kennicutt (1998). We further infer the molecular fraction in the sample of low surface brightness, predominantly dwarf galaxies by assuming that the star formation relationship with molecular gas observed for spiral galaxies also holds in these galaxies, finding a molecular-to-atomic gas mass fraction within the range of 5-15%. Comparison of the data to available models shows that a model in which the thermal pressure balances the vertical gravitational field captures better the shape of the ΣSFR-Σgas relationship. However, such models fail to reproduce the data completely, suggesting that thermal pressure plays an important role in the disks of dwarf galaxies.

  12. The brown dwarf kinematics project

    Science.gov (United States)

    Faherty, Jackie K.

    2010-10-01

    Brown dwarfs are a recent addition to the plethora of objects studied in Astronomy. With theoretical masses between 13 and 75 MJupiter , they lack sustained stable Hydrogen burning so they never join the stellar main sequence. They have physical properties similar to both planets and low-mass stars so studies of their population inform on both. The distances and kinematics of brown dwarfs provide key statistical constraints on their ages, moving group membership, absolute brightnesses, evolutionary trends, and multiplicity. Yet, until my thesis, fundamental measurements of parallax and proper motion were made for only a relatively small fraction of the known population. To address this deficiency, I initiated the Brown Dwarf Kinematics (BDKP). Over the past four years I have re-imaged the majority of spectroscopically confirmed field brown dwarfs (or ultracool dwarfs---UCDs) and created the largest proper motion catalog for ultracool dwarfs to date. Using new astrometric information I examined population characteristics such as ages calculated from velocity dispersions and correlations between kinematics and colors. Using proper motions, I identified several new wide co-moving companions and investigated binding energy (and hence formation) limitations as well as the frequency of hierarchical companions. Concurrently over the past four years I have been conducting a parallax survey of 84 UCDs including those showing spectral signatures of youth, metal-poor brown dwarfs, and those within 20 pc of the Sun. Using absolute magnitude relations in J,H, and K, I identified overluminous binary candidates and investigated known flux-reversal binaries. Using current evolutionary models, I compared the MK vs J-K color magnitude diagram to model predictions and found that the low-surface gravity dwarfs are significantly red-ward and underluminous of predictions and a handful of late-type T dwarfs may require thicker clouds to account for their scatter.

  13. WEAK GALACTIC HALO-DWARF SPHEROIDAL CONNECTION FROM RR LYRAE STARS

    Energy Technology Data Exchange (ETDEWEB)

    Fiorentino, Giuliana [INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna (Italy); Bono, Giuseppe [Dipartimento di Fisica, Universitá di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Roma (Italy); Monelli, Matteo; Gallart, Carme; Martínez-Vásquez, Clara E. [Instituto de Astrofísica de Canarias, Calle Via Lactea s/n, E-38205 La Laguna, Tenerife (Spain); Stetson, Peter B. [National Research Council, 5071 West Saanich Road, Victoria, BC V9E 2E7 (Canada); Tolstoy, Eline [Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen (Netherlands); Salaris, Maurizio [Astrophysics Research Institute, Liverpool John Moores University IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L35RF (United Kingdom); Bernard, Edouard J., E-mail: giuliana.fiorentino@oabo.inaf.it [SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom)

    2015-01-01

    We discuss the role that dwarf galaxies may have played in the formation of the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient stellar component. The comparison is performed using two observables (periods, luminosity amplitudes) that are reddening and distance independent. Fundamental mode RRL in 6 dwarf spheroidals (dSphs) and 11 ultra faint dwarf galaxies (∼1300) show a Gaussian period distribution well peaked around a mean period of (Pab) = 0.610 ± 0.001 days (σ = 0.03). The Halo RRL (∼15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dSphs apart from Sagittarius are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P ≲ 0.48 days and A{sub V} ≥ 0.75 mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered 18 globulars covering a broad range in metallicity (–2.3 ≲ [Fe/H] ≲ –1.1) and hosting more than 35 RRL each. The metallicity turns out to be the main parameter, since only globulars more metal-rich than [Fe/H] ∼ –1.5 host RRL in the HASP region. This finding suggests that dSphs similar to the surviving ones do not appear to be the major building-blocks of the Halo. Leading physical arguments suggest an extreme upper limit of ∼50% to their contribution. On the other hand, massive dwarfs hosting an old population with a broad metallicity distribution (Large Magellanic Cloud, Sagittarius) may have played a primary role in the formation of the Halo.

  14. MAGNETIC FIELD TOPOLOGY IN LOW-MASS STARS: SPECTROPOLARIMETRIC OBSERVATIONS OF M DWARFS

    International Nuclear Information System (INIS)

    Phan-Bao, Ngoc; Lim, Jeremy; Donati, Jean-Francois; Johns-Krull, Christopher M.; MartIn, Eduardo L.

    2009-01-01

    The magnetic field topology plays an important role in the understanding of stellar magnetic activity. While it is widely accepted that the dynamo action present in low-mass partially convective stars (e.g., the Sun) results in predominantly toroidal magnetic flux, the field topology in fully convective stars (masses below ∼0.35 M sun ) is still under debate. We report here our mapping of the magnetic field topology of the M4 dwarf G 164-31 (or Gl 490B), which is expected to be fully convective, based on time series data collected from 20 hr of observations spread over three successive nights with the ESPaDOnS spectropolarimeter. Our tomographic imaging technique applied to time series of rotationally modulated circularly polarized profiles reveals an axisymmetric large-scale poloidal magnetic field on the M4 dwarf. We then apply a synthetic spectrum fitting technique for measuring the average magnetic flux on the star. The flux measured in G 164-31 is |Bf| = 3.2 ± 0.4 kG, which is significantly greater than the average value of 0.68 kG determined from the imaging technique. The difference indicates that a significant fraction of the stellar magnetic energy is stored in small-scale structures at the surface of G 164-31. Our Hα emission light curve shows evidence for rotational modulation suggesting the presence of localized structure in the chromosphere of this M dwarf. The radius of the M4 dwarf derived from the rotational period and the projected equatorial velocity is at least 30% larger than that predicted from theoretical models. We argue that this discrepancy is likely primarily due to the young nature of G 164-31 rather than primarily due to magnetic field effects, indicating that age is an important factor which should be considered in the interpretation of this observational result. We also report here our polarimetric observations of five other M dwarfs with spectral types from M0 to M4.5, three of them showing strong Zeeman signatures.

  15. FORMING HABITABLE PLANETS AROUND DWARF STARS: APPLICATION TO OGLE-06-109L

    International Nuclear Information System (INIS)

    Wang Su; Zhou Jilin

    2011-01-01

    Dwarf stars are believed to have a small protostar disk where planets may grow up. During the planet formation stage, embryos undergoing type I migration are expected to be stalled at an inner edge of the magnetically inactive disk (a crit ∼ 0.2-0.3 AU). This mechanism makes the location around a crit a 'sweet spot' for forming planets. In dwarf stars with masses ∼0.5 M sun , a crit is roughly inside the habitable zone of the system. In this paper, we study the formation of habitable planets due to this mechanism using model system OGLE-06-109L, which has a 0.51 M sun dwarf star with two giant planets in 2.3 and 4.6 AU observed by microlensing. We model the embryos undergoing type I migration in the gas disk with a constant disk-accretion rate ( M-dot ). Giant planets in outside orbits affect the formation of habitable planets through secular perturbations at the early stage and secular resonance at the late stage. We find that the existence and the masses of the habitable planets in the OGLE-06-109L system depend on both M-dot and the speed of type I migration. If planets are formed earlier, so that M-dot is larger (∼10 -7 M sun yr -1 ), terrestrial planets cannot survive unless the type I migration rate is an order of magnitude less. If planets are formed later, so that M-dot is smaller (∼10 -8 M sun yr -1 ), single and high-mass terrestrial planets with high water contents (∼5%) will be formed by inward migration of outer planet cores. A slower-speed migration will result in several planets via collisions of embryos, and thus their water contents will be low (∼2%). Mean motion resonances or apsidal resonances among planets may be observed if multiple planets survive in the inner system.

  16. MODELS FOR METAL-POOR STARS WITH ENHANCED ABUNDANCES OF C, N, O, Ne, Na, Mg, Si, S, Ca, AND Ti, IN TURN, AT CONSTANT HELIUM AND IRON ABUNDANCES

    Energy Technology Data Exchange (ETDEWEB)

    VandenBerg, Don A.; Dotter, Aaron [Department of Physics and Astronomy, University of Victoria, P.O. Box 3055, Victoria, B.C. V8W 3P6 (Canada); Bergbusch, Peter A. [Department of Physics, University of Regina, Regina, Saskatchewan S4S 0A2 (Canada); Ferguson, Jason W. [Department of Physics, Wichita State University, Wichita, KS 67260-0032 (United States); Michaud, Georges; Richer, Jacques [Departement de Physique, Universite de Montreal, Montreal, Quebec H3C 3J7 (Canada); Proffitt, Charles R., E-mail: vandenbe@uvic.ca, E-mail: Aaron.Dotter@gmail.com, E-mail: pbergbusch@accesscomm.ca, E-mail: proffitt@stsci.edu, E-mail: Jason.Ferguson@wichita.edu, E-mail: michaudg@astro.umontreal.ca, E-mail: jacques.richer@umontreal.ca [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)

    2012-08-10

    Recent work has shown that most globular clusters have at least two chemically distinct components, as well as cluster-to-cluster differences in the mean [O/Fe], [Mg/Fe], and [Si/Fe] ratios at similar [Fe/H] values. In order to investigate the implications of variations in the abundances of these and other metals for H-R diagrams and predicted ages, grids of evolutionary sequences have been computed for scaled solar and enhanced {alpha}-element metal abundances, and for mixtures in which the assumed [m/Fe] value for each of the metals C, N, O, Ne, Na, Mg, Si, S, Ca, and Ti has been increased, in turn, by 0.4 dex at constant [Fe/H]. These tracks, together with isochrones for ages from Almost-Equal-To 5 to 14 Gyr, have been computed for -3.0 {<=} [Fe/H] {<=}-0.6, with helium abundances Y = 0.25, 0.29, and 0.33 at each [Fe/H] value, using upgraded versions of the Victoria stellar structure program and the Regina interpolation code, respectively. Turnoff luminosity versus age relations from isochrones are found to depend almost entirely on the importance of the CNO cycle, and thereby mainly on the abundance of oxygen. Since C, N, and O, as well as Ne and S, do not contribute significantly to the opacities at low temperatures and densities, variations in their abundances do not impact the predicted T{sub eff} scale of red giants. The latter is a strong function of the abundances of only Mg and Si (and Fe, possibly to a lesser extent) because they are so abundant and because they are strong sources of opacity at low temperatures. For these reasons, Mg and Si also have important effects on the temperatures of main-sequence stars. Due to their low abundances, Na, Ca, and Ti are of little consequence for stellar models. The effects of varying the adopted solar metals mixture and the helium abundance at a fixed [Fe/H] are also briefly discussed.

  17. Lyman series profiles: From laser-plasmas to white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Kielkopf, J.F. [University of Louisville, Louisville, Kentucky 40292 (United States); Allard, N.F. [Observatoire de Paris-Meudon, France and Institut d Astrophysique, Paris (France)

    1999-04-01

    The low energy interactions of neutral and ionized hydrogen atoms are fundamental processes which also have important applications to the diagnostics of laboratory and astrophysical plasmas. Satellites in the far wings of Lyman {alpha} and Lyman {beta} have been identified as ultraviolet absorption features in the spectra of white dwarf and {lambda} Bootis stars, and they are seen in the emission spectra of plasmas produced when a pulsed laser excites a target H{sub 2} gas. The observed Lyman series profiles agree with unified line shape theory which includes variation of the dipole transition moment during the radiative collision. {copyright} {ital 1999 American Institute of Physics.}

  18. Stars of the Draco Dwarf Spheroidal Galaxy Beyond its Measured Tidal Boundary

    OpenAIRE

    Piatek, Slawomir; Pryor, Carlton; Armandroff, Taft E.; Olszewski, Edward W.

    2000-01-01

    We report R- and V-band photometry derived from CCD imaging for objects in nine fields in and around the Draco dwarf spheroidal galaxy. The most distant fields are about 1.3 degrees from the center. We use these data to search for Draco stars outside of its measured tidal boundary. The search involves three methods: 1) Plotting color-magnitude diagrams for individual fields, for sections of fields, and for combined fields and sections. A color-magnitude diagram can reveal a population of Drac...

  19. A Study of Two Dwarf Irregular Galaxies with Asymmetrical Star Formation Distributions

    Science.gov (United States)

    Hunter, Deidre A.; Gallardo, Samavarti; Zhang, Hong-Xin; Adamo, Angela; Cook, David O.; Oh, Se-Heon; Elmegreen, Bruce G.; Kim, Hwihyun; Kahre, Lauren; Ubeda, Leonardo; Bright, Stacey N.; Ryon, Jenna E.; Fumagalli, Michele; Sacchi, Elena; Kennicutt, R. C.; Tosi, Monica; Dale, Daniel A.; Cignoni, Michele; Messa, Matteo; Grebel, Eva K.; Gouliermis, Dimitrios A.; Sabbi, Elena; Grasha, Kathryn; Gallagher, John S., III; Calzetti, Daniela; Lee, Janice C.

    2018-03-01

    Two dwarf irregular galaxies, DDO 187 and NGC 3738, exhibit a striking pattern of star formation: intense star formation is taking place in a large region occupying roughly half of the inner part of the optical galaxy. We use data on the H I distribution and kinematics and stellar images and colors to examine the properties of the environment in the high star formation rate (HSF) halves of the galaxies in comparison with the low star formation rate halves. We find that the pressure and gas density are higher on the HSF sides by 30%–70%. In addition we find in both galaxies that the H I velocity fields exhibit significant deviations from ordered rotation and there are large regions of high-velocity dispersion and multiple velocity components in the gas beyond the inner regions of the galaxies. The conditions in the HSF regions are likely the result of large-scale external processes affecting the internal environment of the galaxies and enabling the current star formation there.

  20. Tidal interaction, star formation and chemical evolution in blue compact dwarf galaxy Mrk 22

    Science.gov (United States)

    Paswan, A.; Omar, A.; Jaiswal, S.

    2018-02-01

    The optical spectroscopic and radio interferometric H I 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission-line features corresponding to high ionization lines of He II λ4686 and C IV λ5808 from young massive stars are detected. The ages of two prominent star-forming regions in the galaxy are estimated as ∼10 and ∼ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young starburst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission-line features. A significant N/O enrichment is seen in the fainter star-forming region. The gas-phase metallicities [12 + log(O/H)] for the bright and faint regions are estimated as 7.98±0.07 and 7.46±0.09, respectively. The galaxy has a large diffuse H I envelop. The H I images reveal disturbed gas kinematics and H I clouds outside the optical extent of the galaxy, indicating recent tidal interaction or merger in the system. The results strongly indicate that Mrk 22 is undergoing a chemical and morphological evolution due to ongoing star formation, most likely triggered by a merger.

  1. THE LEO IV DWARF SPHEROIDAL GALAXY: COLOR-MAGNITUDE DIAGRAM AND PULSATING STARS

    International Nuclear Information System (INIS)

    Moretti, Maria Ida; Dall'Ora, Massimo; Ripepi, Vincenzo

    2009-01-01

    We present the first V, B - V color-magnitude diagram of the Leo IV dwarf spheroidal galaxy, a faint Milky Way satellite recently discovered by the Sloan Digital Sky Survey. We have obtained B, V time-series photometry reaching about half a magnitude below the Leo IV turnoff, which we detect at V = 24.7 mag, and have performed the first study of the variable star population. We have identified three RR Lyrae stars (all fundamental-mode pulsators, RRab) and one SX Phoenicis variable in the galaxy. In the period-amplitude diagram the Leo IV RR Lyrae stars are located close to the loci of Oosterhoff type I systems and the evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3. However, their mean pulsation period, (Pab) = 0.655 days, would suggest an Oosterhoff type II classification for this galaxy. The RR Lyrae stars trace very well the galaxy's horizontal branch, setting its average magnitude at (V RR ) = 21.48 ± 0.03 mag (standard deviation of the mean). This leads to a distance modulus of μ 0 = 20.94 ± 0.07 mag, corresponding to a distance of 154 ± 5 kpc, by adopting for the Leo IV dSph a reddening E(B - V) = 0.04 ± 0.01 mag and a metallicity of [Fe/H] = -2.31 ± 0.10.

  2. Cleaning spectroscopic samples of stars in nearby dwarf galaxies : The use of the nIR Mg I line to weed out Milky Way contaminants

    NARCIS (Netherlands)

    Battaglia, G.; Starkenburg, E.

    Dwarf galaxies provide insight into the processes of star formation and chemical enrichment at the low end of the galaxy mass function, as well as into the clustering of dark matter on small scales. In studies of Local Group dwarf galaxies, spectroscopic samples of individual stars are used to

  3. Checking in on the Neighbors: Speckle Interferometry of Red Dwarf Stars

    Science.gov (United States)

    Mason, Brian; Hartkopf, William; Miles, Korie; Subasavage, John P.; Raghavan, Deepak; Henry, Todd

    2018-01-01

    High resolution optical speckle observations of M dwarf stars are reported which result in 113 resolved measurements of 80 systems and 256 other stars with no indications of duplicity. Nineteen of these had their first measures here, although all but two of those have later measures reported elsewhere. Included are six orbits, two revised and four reported for the first time. For one of the systems with a new orbit, G 161-7, we determine masses of 0.127+/-0.011 and 0.0961+/-0.0085 \\msun ~ for A and B, respectively. All of the orbit pairs have short periods of between five and thirty-eight years and hold the promise of accurate masses in the near future. For many other pairs we can establish their nature as physical or not depending on their relative motion. Of the 80 systems, 32 have orbits, 25 others are physical, 4 are optical and 19 are currently unknown.

  4. The formation of Dwarf Spheroidal galaxies by the dissolving star cluster model.

    Science.gov (United States)

    Alarcon, Alex; Theory and Star Formation Group

    2018-01-01

    Dwarf spheroidal (dSph) galaxies are regarded as key object in the formation of larger galaxies and are believed to be the most dark matter dominated systems known. There are several model that attempt to explain their formation, but they have problems to model the formation of isolated dSph. Here we will explain a possible formation scenario in which star clusters form in the dark matter halo of a dSph. these cluster suffer from low star formation efficiency and dissolve while orbiting inside the halo. Thereby they build the faint luminous components that we observe in dSph galaxies. Here we will show the main results of this simulations and how they would be corroborated using observational data.

  5. The Shape of Extremely Metal-Poor Galaxies

    Science.gov (United States)

    Putko, Joseph; Sánchez Almeida, Jorge; Muñoz-Tuñón, Casiana; Elmegreen, Bruce; Elmegreen, Debra

    2018-01-01

    This work is the first study on the 3D shape of starbursting extremely metal-poor galaxies (XMPs; a galaxy is said to be an XMP if its ionized gas-phase metallicity is less than 1/10 the solar value). A few hundred XMPs have been identified in the local universe primarily through mining the spectroscopic catalog of the Sloan Digital Sky Survey (SDSS), and follow-up observations have shown that metallicity drops significantly at the starburst (compared to the quiescent component of the galaxy). As the timescale for gas mixing is short, the metal-poor gas triggering the starburst must have been accreted recently. This is strong observational evidence for the cold flow accretion predicted by cosmological models of galaxy formation, and, in this respect, XMPs seem to be the best local analogs of the very first galaxies.The ellipsoidal shape of a class of galaxies can be inferred from the observed axial ratio (q) distribution (q = minor axis/major axis) of a large sample of randomly-oriented galaxies. Fitting ellipses to 200 XMPs using r-band SDSS images, we observe that the axial ratio distribution falls off at q ~0.8, and we determine that these falloffs are not due to biases in the data. The falloff at low axial ratio indicates that the XMPs are thick for their size, and the falloff at high axial ratio suggests the vast majority of XMPs are triaxial. We also observe that smaller XMPs are thicker in proportion to their size, and it is expected that for decreasing galaxy size the ratio of random to rotational motions increases, which correlates with increasing relative thickness. The XMPs are low-redshift dwarf galaxies dominated by dark matter, and our results are compatible with simulations that have shown dark matter halos to be triaxial, with triaxial stellar distributions for low-mass galaxies and with triaxiality increasing over time. We will offer precise constraints on the 3D shape of XMPs via Bayesian analysis of our observed axial ratio distribution.This work

  6. Dynamics of quadruple systems composed of two binaries: stars, white dwarfs, and implications for Ia supernovae

    Science.gov (United States)

    Fang, Xiao; Thompson, Todd A.; Hirata, Christopher M.

    2018-05-01

    We investigate the long-term secular dynamics and Lidov-Kozai (LK) eccentricity oscillations of quadruple systems composed of two binaries at quadrupole and octupole orders in the perturbing Hamiltonian. We show that the fraction of systems reaching high eccentricities is enhanced relative to triple systems, over a broader range of parameter space. We show that this fraction grows with time, unlike triple systems evolved at quadrupole order. This is fundamentally because with their additional degrees of freedom, quadruple systems do not have a maximal set of commuting constants of the motion, even in secular theory at quadrupole order. We discuss these results in the context of star-star and white dwarf-white dwarf (WD) binaries, with emphasis on WD-WD mergers and collisions relevant to the Type Ia supernova problem. For star-star systems, we find that more than 30 per cent of systems reach high eccentricity within a Hubble time, potentially forming triple systems via stellar mergers or close binaries. For WD-WD systems, taking into account general relativistic and tidal precession and dissipation, we show that the merger rate is enhanced in quadruple systems relative to triple systems by a factor of 3.5-10, and that the long-term evolution of quadruple systems leads to a delay-time distribution ˜1/t for mergers and collisions. In gravitational wave-driven mergers of compact objects, we classify the mergers by their evolutionary patterns in phase space and identify a regime in about 8 per cent of orbital shrinking mergers, where eccentricity oscillations occur on the general relativistic precession time-scale, rather than the much longer LK time-scale. Finally, we generalize previous treatments of oscillations in the inner binary eccentricity (evection) to eccentric mutual orbits. We assess the merger rate in quadruple and triple systems and the implications for their viability as progenitors of stellar mergers and Type Ia supernovae.

  7. MAPPING THE SHORES OF THE BROWN DWARF DESERT. II. MULTIPLE STAR FORMATION IN TAURUS-AURIGA

    International Nuclear Information System (INIS)

    Kraus, Adam L.; Ireland, Michael J.; Martinache, Frantz; Hillenbrand, Lynne A.

    2011-01-01

    We have conducted a high-resolution imaging study of the Taurus-Auriga star-forming region in order to characterize the primordial outcome of multiple star formation and the extent of the brown dwarf desert. Our survey identified 16 new binary companions to primary stars with masses of 0.25-2.5 M sun , raising the total number of binary pairs (including components of high-order multiples) with separations of 3-5000 AU to 90. We find that ∼2/3-3/4 of all Taurus members are multiple systems of two or more stars, while the other ∼1/4-1/3 appear to have formed as single stars; the distribution of high-order multiplicity suggests that fragmentation into a wide binary has no impact on the subsequent probability that either component will fragment again. The separation distribution for solar-type stars (0.7-2.5 M sun ) is nearly log-flat over separations of 3-5000 AU, but lower-mass stars (0.25-0.7 M sun ) show a paucity of binary companions with separations of ∼>200 AU. Across this full mass range, companion masses are well described with a linear-flat function; all system mass ratios (q = M B /M A ) are equally probable, apparently including substellar companions. Our results are broadly consistent with the two expected modes of binary formation (free-fall fragmentation on large scales and disk fragmentation on small scales), but the distributions provide some clues as to the epochs at which the companions are likely to form.

  8. Merger of white dwarf-neutron star binaries: Prelude to hydrodynamic simulations in general relativity

    International Nuclear Information System (INIS)

    Paschalidis, Vasileios; MacLeod, Morgan; Baumgarte, Thomas W.; Shapiro, Stuart L.

    2009-01-01

    White dwarf-neutron star binaries generate detectable gravitational radiation. We construct Newtonian equilibrium models of corotational white dwarf-neutron star (WDNS) binaries in circular orbit and find that these models terminate at the Roche limit. At this point the binary will undergo either stable mass transfer (SMT) and evolve on a secular time scale, or unstable mass transfer (UMT), which results in the tidal disruption of the WD. The path a given binary will follow depends primarily on its mass ratio. We analyze the fate of known WDNS binaries and use population synthesis results to estimate the number of LISA-resolved galactic binaries that will undergo either SMT or UMT. We model the quasistationary SMT epoch by solving a set of simple ordinary differential equations and compute the corresponding gravitational waveforms. Finally, we discuss in general terms the possible fate of binaries that undergo UMT and construct approximate Newtonian equilibrium configurations of merged WDNS remnants. We use these configurations to assess plausible outcomes of our future, fully relativistic simulations of these systems. If sufficient WD debris lands on the NS, the remnant may collapse, whereby the gravitational waves from the inspiral, merger, and collapse phases will sweep from LISA through LIGO frequency bands. If the debris forms a disk about the NS, it may fragment and form planets.

  9. An extended star formation history in an ultra-compact dwarf

    Science.gov (United States)

    Norris, Mark A.; Escudero, Carlos G.; Faifer, Favio R.; Kannappan, Sheila J.; Forte, Juan Carlos; van den Bosch, Remco C. E.

    2015-08-01

    There has been significant controversy over the mechanisms responsible for forming compact stellar systems like ultra-compact dwarfs (UCDs), with suggestions that UCDs are simply the high-mass extension of the globular cluster population, or alternatively, the liberated nuclei of galaxies tidally stripped by larger companions. Definitive examples of UCDs formed by either route have been difficult to find, with only a handful of persuasive examples of stripped-nucleus-type UCDs being known. In this paper, we present very deep Gemini/GMOS spectroscopic observations of the suspected stripped-nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (noise spectrum to detect a temporally extended star formation history for this UCD. We find that the UCD was forming stars since the earliest epochs until at least 1-2 Gyr ago. Taken together these observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1-2 Gyr.

  10. AN M DWARF COMPANION TO AN F-TYPE STAR IN A YOUNG MAIN-SEQUENCE BINARY

    Energy Technology Data Exchange (ETDEWEB)

    Eigmüller, Ph.; Csizmadia, Sz.; Erikson, A.; Fridlund, M.; Pasternacki, Th.; Rauer, H. [Institute of Planetary Research, German Aerospace Center Rutherfordstr. 2, D-12489 Berlin (Germany); Eislöffel, J.; Lehmann, H.; Hartmann, M.; Hatzes, A. [Thüringer Landessternwarte Tautenburg Sternwarte 5, D-07778 Tautenburg (Germany); Tkachenko, A. [Instituut voor Sterrenkunde, KU Leuven Celestijnenlaan 200D, 3001 Leuven (Belgium); Voss, H., E-mail: philipp.eigmueller@dlr.de [Universitat de Barcelona, Department of Astronomy and Meteorology Martí i Franquès, 1, E-08028 Barcelona (Spain)

    2016-03-15

    Only a few well characterized very low-mass M dwarfs are known today. Our understanding of M dwarfs is vital as these are the most common stars in our solar neighborhood. We aim to characterize the properties of a rare F+dM stellar system for a better understanding of the low-mass end of the Hertzsprung–Russel diagram. We used photometric light curves and radial velocity follow-up measurements to study the binary. Spectroscopic analysis was used in combination with isochrone fitting to characterize the primary star. The primary star is an early F-type main-sequence star with a mass of (1.493 ± 0.073) M{sub ⊙} and a radius of (1.474 ± 0.040) R{sub ⊙}. The companion is an M dwarf with a mass of (0.188 ± 0.014) M{sub ⊙} and a radius of (0.234 ± 0.009) R{sub ⊙}. The orbital period is (1.35121 ± 0.00001) days. The secondary star is among the lowest-mass M dwarfs known to date. The binary has not reached a 1:1 spin–orbit synchronization. This indicates a young main-sequence binary with an age below ∼250 Myr. The mass–radius relation of both components are in agreement with this finding.

  11. ULTRA-FAINT DWARF GALAXIES AS A TEST OF EARLY ENRICHMENT AND METALLICITY-DEPENDENT STAR FORMATION

    International Nuclear Information System (INIS)

    Tassis, Konstantinos; Gnedin, Nickolay Y.; Kravtsov, Andrey V.

    2012-01-01

    The close relation of star formation with molecular gas indicated by observations and assumed in recent models implies that the efficiency with which galaxies convert their gas into stars depends on gas metallicity. This is because abundance of molecular hydrogen is sensitive to abundance of dust, which catalyzes formation of H 2 and helps to shield it from dissociating radiation. In this study, we point out that in the absence of significant pre-enrichment by Population III stars forming out of zero metallicity gas, such H 2 -based star formation is expected to leave an imprint in the form of bi-modality in the metallicity distribution among dwarf galaxies and in the metallicity distribution of stars within individual galaxies. The bi-modality arises because when gas metallicity (and dust abundance) is low, formation of molecular gas is inefficient, the gas consumption timescale is long, and star formation and metal enrichment proceed slowly. When metallicity reaches a critical threshold value star formation and enrichment accelerate, which leads to rapid increase in both stellar mass and metallicity of galaxies. We demonstrate this process both using a simple analytical model and full cosmological simulations. In contrast, the observed metallicity distributions of dwarf galaxies or stars within them are not bi-modal. We argue that this discrepancy points to substantial early stochastic pre-enrichment by Population III stars to levels Z ∼ 10 –2 Z ☉ in dense, star-forming regions of early galaxies.

  12. Galactic Outflows, Star Formation Histories, and Timescales in Starburst Dwarf Galaxies from STARBIRDS

    Science.gov (United States)

    McQuinn, Kristen B. W.; Skillman, Evan D.; Heilman, Taryn N.; Mitchell, Noah P.; Kelley, Tyler

    2018-03-01

    Winds are predicted to be ubiquitous in low-mass, actively star-forming galaxies. Observationally, winds have been detected in relatively few local dwarf galaxies, with even fewer constraints placed on their timescales. Here, we compare galactic outflows traced by diffuse, soft X-ray emission from Chandra Space Telescope archival observations to the star formation histories derived from Hubble Space Telescope imaging of the resolved stellar populations in six starburst dwarfs. We constrain the longevity of a wind to have an upper limit of 25 Myr based on galaxies whose starburst activity has already declined, although a larger sample is needed to confirm this result. We find an average 16% efficiency for converting the mechanical energy of stellar feedback to thermal, soft X-ray emission on the 25 Myr timescale, somewhat higher than simulations predict. The outflows have likely been sustained for timescales comparable to the duration of the starbursts (i.e., 100's Myr), after taking into account the time for the development and cessation of the wind. The wind timescales imply that material is driven to larger distances in the circumgalactic medium than estimated by assuming short, 5-10 Myr starburst durations, and that less material is recycled back to the host galaxy on short timescales. In the detected outflows, the expelled hot gas shows various morphologies which are not consistent with a simple biconical outflow structure. The sample and analysis are part of a larger program, the STARBurst IRregular Dwarf Survey (STARBIRDS), aimed at understanding the lifecycle and impact of starburst activity in low-mass systems.

  13. The star formation histories of local group dwarf galaxies. II. Searching for signatures of reionization

    Energy Technology Data Exchange (ETDEWEB)

    Weisz, Daniel R. [Department of Astronomy, University of California at Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 (United States); Dolphin, Andrew E. [Raytheon Company, 1151 East Hermans Road, Tucson, AZ 85756 (United States); Skillman, Evan D. [Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455 (United States); Holtzman, Jon [Department of Astronomy, New Mexico State University, Box 30001, 1320 Frenger Street, Las Cruces, NM 88003 (United States); Gilbert, Karoline M.; Dalcanton, Julianne J.; Williams, Benjamin F., E-mail: drw@ucsc.edu [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 (United States)

    2014-07-10

    We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (10{sup 4} < M{sub *} < 10{sup 9} M{sub ☉}). The SFHs are derived from color-magnitude diagrams using archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five quenched galaxies (And V, And VI, And XIII, Leo IV, and Hercules) are consistent with forming the bulk of their stars before reionization, when full uncertainties are considered. Observations of 13 of the predicted 'true fossils' identified by Bovill and Ricotti show that only two (Hercules and Leo IV) indicate star formation quenched by reionization. However, both are within the virial radius of the Milky Way and evidence of tidal disturbance complicates this interpretation. We argue that the late-time gas capture scenario posited by Ricotti for the low mass, gas-rich, and star-forming fossil candidate Leo T is observationally indistinguishable from simple gas retention. Given the ambiguity between environmental effects and reionization, the best reionization fossil candidates are quenched low mass field galaxies (e.g., KKR 25).

  14. Relation between initial and minimum final white dwarf mass for Population I stars

    Energy Technology Data Exchange (ETDEWEB)

    Mazzitelli, I.; Dantona, F.

    1986-12-01

    The evolutionary paths for Population I stars having initial masses 1, 2.5, 3, 4, and 5 solar masses were computed from the homogeneous main sequence to the onset of the first major thermal pulse to evaluate the minimum mass and the chemical stratification of the remnant white dwarf (WD) associated with each parent mass. The helium flash phase was followed in detail for a 2.5 solar masses star, whereas for the 1 solar mass star the flash was bypassed, and the models at the beginning of the steady central helium burning phase were obtained by means of a scaling procedure upon the properly computed total and core masses. The results show that for a parent ranging between 1-3 solar masses the core mass at the first thermal pulse ranges only from 0.64-0.69 solar mass. If some very fast mass-loss mechanism is triggered in connection with the early stages of the thermal pulse phase, as suggested by the observed deficiency of asymptotic giant branch stars, the relation between final and initial mass is almost flat at least up to an initial mass of 3 solar masses, and the mass spectrum of the WDs is narrow and heavily peaked around 0.65 solar mass. 53 references.

  15. Macho project photometry of RR Lyrae stars in the Sagittarius dwarf galaxy

    Energy Technology Data Exchange (ETDEWEB)

    Alcock, C. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)]|[Center for Particle Astrophysics, University of California, Berkeley, California 94720 (United States); Allsman, R.A. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)]|[Mount Stromlo and Siding Spring Observatories, Australian National University, Weston, ACT 2611 (Australia); Alves, D.R. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)]|[Department of Physics, University of California, Davis, California 95616 (United States); Axelrod, T.S. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)]|[Mount Stromlo and Siding Spring Observatories, Australian National University, Weston, ACT 2611 (Australia); Becker, A.C. [Center for Particle Astrophysics, University of California, Berkeley, California 94720 (United States)]|[Department of Astronomy, University of Washington, Seattle, Washington 98195 (United States); Bennett, D.P.; Cook, K.H. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)]|[Center for Particle Astrophysics, University of California, Berkeley, California 94720 (United States); Freeman, K.C. [Mount Stromlo and Siding Spring Observatories, Australian National University, Weston, ACT 2611 (Australia); Griest, K. [Center for Particle Astrophysics, University of California, Berkeley, California 94720 (United States)]|[Department of Physics, University of California, San Diego, California 92093 (United States); Guern, J.A.; Lehner, M.J. [Department of Physics, University of California, San Diego, California 92093 (United States); Marshall, S.L.; Minniti, D. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States); Peterson, B.A. [Mount Stromlo and Siding Spring Observatories, Australian National University, Weston, ACT 2611 (Australia); Pratt, M.R.

    1997-01-01

    We report the discovery of 30 type a, b RR Lyrae (RRab) stars that are likely members of the Sagittarius dwarf galaxy (Sgr). Accurate positions, periods, amplitudes, and magnitudes are presented. Their distances are determined with respect to RRab stars in the Galactic bulge found also in the MACHO 1993 data. For R{sub {circle_dot}}=8kpc, the mean distance to these stars is D=22{plus_minus}1kpc, smaller than previous determinations for this galaxy. This indicates that Sgr has an elongated main body extending for more than 10 kpc, which is inclined along the line of sight, with its northern part (in Galactic coordinates) closer to us. The size and shape of Sgr give clues about the past history of this galaxy. If the shape of Sgr follows the direction of its orbit, the observed spatial orientation suggests that Sgr is moving away from the Galactic plane. Also, Sgr stars may be the sources of some of the microlensing events seen toward the bulge. {copyright} {ital 1997} {ital The American Astronomical Society}

  16. NEW BROWN DWARF COMPANIONS TO YOUNG STARS IN SCORPIUS-CENTAURUS

    Energy Technology Data Exchange (ETDEWEB)

    Janson, Markus [Department of Astrophysical Sciences, Princeton University, Princeton, NJ (United States); Jayawardhana, Ray; Bonavita, Mariangela [Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON (Canada); Girard, Julien H. [European Southern Observatory, Santiago (Chile); Lafreniere, David [Department of Physics, University of Montreal, Montreal, QC (Canada); Gizis, John [Department of Physics and Astronomy, University of Delaware, Newark, DE (United States); Brandeker, Alexis, E-mail: janson@astro.princeton.edu [Department of Astronomy, Stockholm University, Stockholm (Sweden)

    2012-10-10

    We present the discoveries of three faint companions to young stars in the Scorpius-Centaurus region, imaged with the NICI instrument on Gemini South. We have confirmed all three companions through common proper motion tests. Follow-up spectroscopy has confirmed two of them, HIP 65423 B and HIP 65517 B, to be brown dwarfs, while the third, HIP 72099 B, is more likely a very low mass star just above the hydrogen burning limit. The detection of wide companions in the mass range of {approx}40-100 M{sub jup} complements previous work in the same region, reporting detections of similarly wide companions with lower masses, in the range of {approx}10-30 M{sub jup}. Such low masses near the deuterium burning limit have raised the question of whether those objects formed like planets or stars. The existence of intermediate objects as reported here could represent a bridge between lower-mass companions and stellar companions, but in any case demonstrate that mass alone may not provide a clear-cut distinction for the formation of low-mass companions to stars.

  17. Relation between initial and minimum final white dwarf mass for Population I stars

    International Nuclear Information System (INIS)

    Mazzitelli, I.; Dantona, F.; CNR, Istituto di Astrofisica Spaziale, Frascati; Roma, Osservatorio Astronomico, Rome, Italy)

    1986-01-01

    The evolutionary paths for Population I stars having initial masses 1, 2.5, 3, 4, and 5 solar masses were computed from the homogeneous main sequence to the onset of the first major thermal pulse to evaluate the minimum mass and the chemical stratification of the remnant white dwarf (WD) associated with each parent mass. The helium flash phase was followed in detail for a 2.5 solar masses star, whereas for the 1 solar mass star the flash was bypassed, and the models at the beginning of the steady central helium burning phase were obtained by means of a scaling procedure upon the properly computed total and core masses. The results show that for a parent ranging between 1-3 solar masses the core mass at the first thermal pulse ranges only from 0.64-0.69 solar mass. If some very fast mass-loss mechanism is triggered in connection with the early stages of the thermal pulse phase, as suggested by the observed deficiency of asymptotic giant branch stars, the relation between final and initial mass is almost flat at least up to an initial mass of 3 solar masses, and the mass spectrum of the WDs is narrow and heavily peaked around 0.65 solar mass. 53 references

  18. Exploring simulated early star formation in the context of the ultrafaint dwarf galaxies

    Science.gov (United States)

    Corlies, Lauren; Johnston, Kathryn V.; Wise, John H.

    2018-04-01

    Ultrafaint dwarf galaxies (UFDs) are typically assumed to have simple, stellar populations with star formation ending at reionization. Yet as the observations of these galaxies continue to improve, their star formation histories (SFHs) are revealed to be more complicated than previously thought. In this paper, we study how star formation, chemical enrichment, and mixing proceed in small, dark matter haloes at early times using a high-resolution, cosmological, hydrodynamical simulation. The goals are to inform the future use of analytic models and to explore observable properties of the simulated haloes in the context of UFD data. Specifically, we look at analytic approaches that might inform metal enrichment within and beyond small galaxies in the early Universe. We find that simple assumptions for modelling the extent of supernova-driven winds agree with the simulation on average, whereas inhomogeneous mixing and gas flows have a large effect on the spread in simulated stellar metallicities. In the context of the UFDs, this work demonstrates that simulations can form haloes with a complex SFH and a large spread in the metallicity distribution function within a few hundred Myr in the early Universe. In particular, bursty and continuous star formation are seen in the simulation and both scenarios have been argued from the data. Spreads in the simulated metallicities, however, remain too narrow and too metal-rich when compared to the UFDs. Future work is needed to help reduce these discrepancies and advance our interpretation of the data.

  19. STAR FORMATION IN ULTRA-FAINT DWARFS: CONTINUOUS OR SINGLE-AGE BURSTS?

    International Nuclear Information System (INIS)

    Webster, David; Bland-Hawthorn, Joss; Sutherland, Ralph

    2015-01-01

    We model the chemical evolution of six ultra-faint dwarfs (UFDs): Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I based on their recently determined star formation histories. We show that two single-age bursts cannot explain the observed [α/Fe] versus [Fe/H] distribution in these galaxies and that some self-enrichment is required within the first burst. An alternative scenario is modeled, in which star formation is continuous except for short interruptions when one or more supernovae temporarily blow the dense gas out from the center of the system. This model allows for self-enrichment and can reproduce the chemical abundances of the UFDs in which the second burst is only a trace population. We conclude that the most likely star formation history is one or two extended periods of star formation, with the first burst lasting for at least 100 Myr. As found in earlier work, the observed properties of UFDs can be explained by formation at a low mass (M vir ∼10 7 M ⊙ ), rather than being stripped remnants of much larger systems

  20. STAR FORMATION IN ULTRA-FAINT DWARFS: CONTINUOUS OR SINGLE-AGE BURSTS?

    Energy Technology Data Exchange (ETDEWEB)

    Webster, David; Bland-Hawthorn, Joss [Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006 (Australia); Sutherland, Ralph, E-mail: d.webster@physics.usyd.edu.au [Research School of Astronomy and Astrophysics, Australian National University, Cotter Rd, Weston, ACT 2611 (Australia)

    2015-01-30

    We model the chemical evolution of six ultra-faint dwarfs (UFDs): Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I based on their recently determined star formation histories. We show that two single-age bursts cannot explain the observed [α/Fe] versus [Fe/H] distribution in these galaxies and that some self-enrichment is required within the first burst. An alternative scenario is modeled, in which star formation is continuous except for short interruptions when one or more supernovae temporarily blow the dense gas out from the center of the system. This model allows for self-enrichment and can reproduce the chemical abundances of the UFDs in which the second burst is only a trace population. We conclude that the most likely star formation history is one or two extended periods of star formation, with the first burst lasting for at least 100 Myr. As found in earlier work, the observed properties of UFDs can be explained by formation at a low mass (M{sub vir}∼10{sup 7} M{sub ⊙}), rather than being stripped remnants of much larger systems.

  1. A system of three transiting super-Earths in a cool dwarf star

    Science.gov (United States)

    Díez Alonso, E.; Suárez& Gómez, S. L.; González Hernández, J. I.; Suárez Mascareño, A.; González Gutiérrez, C.; Velasco, S.; Toledo-Padrón, B.; de Cos Juez, F. J.; Rebolo, R.

    2018-05-01

    We present the detection of three super-Earths transiting the cool star LP415-17, monitored by K2 mission in its 13th campaign. High-resolution spectra obtained with High Accuracy Radial velocity Planet Searcher-North/Telescopio Nazionale Galileo (HARPS-N/TNG) showed that the star is a mid-late K dwarf. Using spectral synthesis models, we infer its effective temperature, surface gravity, and metallicity, and subsequently determined from evolutionary models a stellar radius of 0.58 R⊙. The planets have radii of 1.8, 2.6, and 1.9 R⊕ and orbital periods of 6.34, 13.85, and 40.72 d. High-resolution images discard any significant contamination by an intervening star in the line of sight. The orbit of the furthest planet has radius of 0.18 au, close to the inner edge of the habitable zone. The system is suitable to improve our understanding of formation and dynamical evolution of super-Earth systems in the rocky-gaseous threshold, their atmospheres, internal structure, composition, and interactions with host stars.

  2. The episodic star formation history of the Carina dwarf spheroidal galaxy

    Science.gov (United States)

    de Boer, T. J. L.; Tolstoy, E.; Lemasle, B.; Saha, A.; Olszewski, E. W.; Mateo, M.; Irwin, M. J.; Battaglia, G.

    2014-12-01

    We present deep photometry of the Carina dwarf spheroidal galaxy in the B and V filters from CTIO/MOSAIC out to and beyond the tidal radius of rell ≈ 0.48 degrees. The accurately calibrated photometry is combined with spectroscopic metallicity distributions of red giant branch (RGB) stars to determine the detailed star formation and chemical evolution history of Carina. The star formation history (SFH) confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail as a function of radius from the centre. Two main episodes of star formation occurred at old (>8 Gyr) and intermediate (2-8 Gyr) ages, both enriching stars starting from low metallicities ([Fe/H] < - 2 dex). By dividing the SFH into two components, we determine that 60 ± 9 percent of the total number of stars formed within the intermediate-age episode. Furthermore, within the tidal radius (0.48 degrees or 888 pc) a total mass in stars of 1.07 ± 0.08 × 106 M⊙ was formed, giving Carina a stellar mass-to-light ratio of 1.8 ± 0.8. By combining the detailed SFH with spectroscopic observations of RGB stars, we determined the detailed age-metallicity relation of each episode and the timescale of α-element evolution of Carina from individual stars. The oldest episode displays a tight age-metallicity relation during ≈6 Gyr with steadily declining α-element abundances and a possible α-element "knee" visible at [Fe/H] ≈ - 2.5 dex. The intermediate-age sequence displays a more complex age-metallicity relation starting from low metallicity and a sequence in α-element abundances with a slope much steeper than observed in the old episode, starting from [Fe/H] = -1.8 dex and [Mg/Fe] ≈ 0.4 dex and declining to Mg-poor values ([Mg/Fe] ≤ - 0.5 dex). This clearly indicates that the two episodes of star formation formed from gas with different abundance patterns, which is inconsistent with simple evolution in an isolated system. Tables 1-3 are

  3. Magnetic Inflation and Stellar Mass. II. On the Radii of Single, Rapidly Rotating, Fully Convective M-Dwarf Stars

    Science.gov (United States)

    Kesseli, Aurora Y.; Muirhead, Philip S.; Mann, Andrew W.; Mace, Greg

    2018-06-01

    Main-sequence, fully convective M dwarfs in eclipsing binaries are observed to be larger than stellar evolutionary models predict by as much as 10%–15%. A proposed explanation for this discrepancy involves effects from strong magnetic fields, induced by rapid rotation via the dynamo process. Although, a handful of single, slowly rotating M dwarfs with radius measurements from interferometry also appear to be larger than models predict, suggesting that rotation or binarity specifically may not be the sole cause of the discrepancy. We test whether single, rapidly rotating, fully convective stars are also larger than expected by measuring their R\\sin i distribution. We combine photometric rotation periods from the literature with rotational broadening (v\\sin i) measurements reported in this work for a sample of 88 rapidly rotating M dwarf stars. Using a Bayesian framework, we find that stellar evolutionary models underestimate the radii by 10 % {--}15{ % }-2.5+3, but that at higher masses (0.18 theory is 13%–18%, and we argue that the discrepancy is unlikely to be due to effects from age. Furthermore, we find no statistically significant radius discrepancy between our sample and the handful of M dwarfs with interferometric radii. We conclude that neither rotation nor binarity are responsible for the inflated radii of fully convective M dwarfs, and that all fully convective M dwarfs are larger than models predict.

  4. Effects of Pop III to PopII transition on the lowest metallicity stars in dwarf galaxies

    Science.gov (United States)

    Zhang, Yimiao; Keres, Dusan; FIRE Team

    2018-01-01

    We examine the effects of the enrichments from Population III (Pop III) stars on the formation and properties of the first generation of the Population II (Pop II) stars. Pop III stars begin to transition towards Pop II stars when the metals dispersed in Pop III supernovae pollute the nearby gas. However, details of this transition are still largely unknown. We use dwarf galaxy simulations from the Feedback In Realistic Environments (FIRE) project to identify the star-forming gas that is likely to be pre-enriched by Pop III supernovae and follow the stars that form in such gas. This pre-enrichment will leave the signature in the lowest metallicity stars that can be used to better constrain the details of the Pop III-to-Pop II transition.

  5. Star Formation Histories of the LEGUS Dwarf Galaxies. II. Spatially Resolved Star Formation History of the Magellanic Irregular NGC 4449

    Science.gov (United States)

    Sacchi, E.; Cignoni, M.; Aloisi, A.; Tosi, M.; Calzetti, D.; Lee, J. C.; Adamo, A.; Annibali, F.; Dale, D. A.; Elmegreen, B. G.; Gouliermis, D. A.; Grasha, K.; Grebel, E. K.; Hunter, D. A.; Sabbi, E.; Smith, L. J.; Thilker, D. A.; Ubeda, L.; Whitmore, B. C.

    2018-04-01

    We present a detailed study of the Magellanic irregular galaxy NGC 4449 based on both archival and new photometric data from the Legacy Extragalactic UV Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3. Thanks to its proximity (D = 3.82 ± 0.27 Mpc), we reach stars 3 mag fainter than the tip of the red giant branch in the F814W filter. The recovered star formation history (SFH) spans the whole Hubble time, but due to the age–metallicity degeneracy of the red giant branch stars, it is robust only over the lookback time reached by our photometry, i.e., ∼3 Gyr. The most recent peak of star formation (SF) is around 10 Myr ago. The average surface density SF rate over the whole galaxy lifetime is 0.01 M ⊙ yr‑1 kpc‑2. From our study, it emerges that NGC 4449 has experienced a fairly continuous SF regime in the last 1 Gyr, with peaks and dips whose SF rates differ only by a factor of a few. The very complex and disturbed morphology of NGC 4449 makes it an interesting galaxy for studies of the relationship between interactions and starbursts, and our detailed and spatially resolved analysis of its SFH does indeed provide some hints on the connection between these two phenomena in this peculiar dwarf galaxy. Based on observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555.

  6. Mass Modelling of Dwarf Spheroidal Galaxies: the Effect of Unbound Stars From Tidal Tails And the Milky Way

    Energy Technology Data Exchange (ETDEWEB)

    Klimentowski, Jaroslaw; Lokas, Ewa L.; /Warsaw, Copernicus Astron. Ctr.; Kazantzidis, Stelios; /KIPAC, Menlo Park; Prada, Francisco; /IAA, Granada; Mayer, Lucio; /Zurich,; Mamon, Gary A.; /Paris, Inst. Astrophys. /Meudon Observ.

    2006-11-14

    We study the origin and properties of the population of unbound stars in the kinematic samples of dwarf spheroidal galaxies. For this purpose we have run a high resolution N- body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. In agreement with the tidal stirring scenario of Mayer et al., the dwarf is placed on a highly eccentric orbit, its initial stellar component is in the form of an exponential disk and it has a NFW-like dark matter halo. After 10 Gyrs of evolution the dwarf produces a spheroidal stellar component and is strongly tidally stripped so that mass follows light and the stars are on almost isotropic orbits. From this final state, we create mock kinematic data sets for 200 stars by observing the dwarf in different directions.We find that when the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails.We also study another source of possible contamination by adding stars from the Milky Way. We demonstrate that most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert and recently tested on simulated dark matter haloes. We model the cleaned up kinematic samples using solutions of the Jeans equation with constant mass-to-light ratio and velocity anisotropy parameter. We show that even for such strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and mass-to-light ratio of the dwarf with accuracy typically better than 25 percent and almost exactly in the case when the line of sight is perpendicular to the tidal tails. The analysis was applied to the new data for the Fornax dSph galaxy for which we find a mass-to-light ratio of 11 solar units and isotropic orbits. We demonstrate that most of the contamination in the kinematic sample of Fornax probably originates from the Milky Way.

  7. Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. Detailed abundance analysis of OGLE-2008-BLG-209S

    OpenAIRE

    Bensby, T.; Johnson, J. A.; Cohen, J.; Feltzing, S.; Udalski, A.; Gould, A.; Huang, W.; Thompson, I.; Simmerer, J.; Adén, D.

    2009-01-01

    AIMS. Our aims are twofold. First we aim to evaluate the robustness and accuracy of stellar parameters and detailed elemental abundances that can be derived from high-resolution spectroscopic observations of microlensed dwarf and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to investigate the abundance structure and chemical evolution of the Milky Way Bulge. [ABRIDGED] METHODS. We present a detailed elemental abundance analysis of OGLE-2008-BLG-209S, the source star...

  8. The quiescent chromospheres and transition regions of active dwarf stars: what are we learning from recent observations and models

    International Nuclear Information System (INIS)

    Linsky, J.L.

    1983-01-01

    The rapid progress in understanding active dwarf stars, which has been stimulated by recent IUE, Einstein and ground-based observations, is reviewed. Active phenomena in late-type dwarf stars are seen as somehow a direct consequence of strong magnetic fields. The nonflare phenomena in the chromosphere and transition regions of these stars are discussed, while some suggestions are given about the way in which magnetic fields control these phenomena. Especially, the review deals with a description and comparison of those activities which are similar in active and quiescent dwarf stars and summarizes the various roles which magnetic fields likely play in modifying the chromospheres and transition regions of active stars. Successively, the following subjects are discussed: the basic structure of the stars, the enhanced heating and solar-like flux tubes, the consequences of plasma flows, heating rates in different layers, heating mechanism of chromosphere and transition region, semi-empirical models. The author finishes with some suggestions for future work. (G.J.P.)

  9. VARIABLE STARS IN THE ULTRA-FAINT DWARF SPHEROIDAL GALAXY URSA MAJOR I

    Energy Technology Data Exchange (ETDEWEB)

    Garofalo, Alessia; Moretti, Maria Ida [Dipartimento di Astronomia, Universita di Bologna, Via Ranzani 1, I-40127 Bologna (Italy); Cusano, Felice; Clementini, Gisella [INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna (Italy); Ripepi, Vincenzo; Dall' Ora, Massimo; Coppola, Giuseppina; Musella, Ilaria; Marconi, Marcella, E-mail: alessia.garofalo@studio.unibo.it, E-mail: fcusano@na.astro.it, E-mail: gisella.clementini@oabo.inaf.it, E-mail: ripepi@na.astro.it, E-mail: dallora@na.astro.it, E-mail: imoretti@na.astro.it, E-mail: coppola@na.astro.it, E-mail: ilaria@na.astro.it, E-mail: marcella@na.astro.it [INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Napoli (Italy)

    2013-04-10

    We have performed the first study of the variable star population of Ursa Major I (UMa I), an ultra-faint dwarf satellite recently discovered around the Milky Way (MW) by the Sloan Digital Sky Survey. Combining time series observations in the B and V bands from four different telescopes, we have identified seven RR Lyrae stars in UMa I, of which five are fundamental-mode (RRab) and two are first-overtone pulsators (RRc). Our V, B - V color-magnitude diagram of UMa I reaches V {approx} 23 mag (at a signal-to-noise ratio of {approx}6) and shows features typical of a single old stellar population. The mean pulsation period of the RRab stars (P{sub ab}) = 0.628, {sigma} = 0.071 days (or (P{sub ab}) = 0.599, {sigma} = 0.032 days, if V4, the longest period and brightest variable, is discarded) and the position on the period-amplitude diagram suggest an Oosterhoff-intermediate classification for the galaxy. The RR Lyrae stars trace the galaxy horizontal branch (HB) at an average apparent magnitude of (V(RR)) = 20.43 {+-} 0.02 mag (average on six stars and discarding V4), giving in turn a distance modulus for UMa I of (m - M){sub 0} = 19.94 {+-} 0.13 mag, distance d = 97.3{sup +6.0}{sub -5.7} kpc, in the scale where the distance modulus of the Large Magellanic Cloud is 18.5 {+-} 0.1 mag. Isodensity contours of UMa I red giants and HB stars (including the RR Lyrae stars identified in this study) show that the galaxy has an S-shaped structure, which is likely caused by the tidal interaction with the MW. Photometric metallicities were derived for six of the UMa I RR Lyrae stars from the parameters of the Fourier decomposition of the V-band light curves, leading to an average metal abundance of [Fe/H] = -2.29 dex ({sigma} = 0.06 dex, average on six stars) on the Carretta et al. metallicity scale.

  10. High resolution spectroscopy of Red Giant Branch stars and the chemical evolution of the Fornax dwarf spheroidal galaxy

    NARCIS (Netherlands)

    Lemasle, B.; de Boer, T. J. L.; Hill, V.; Tolstoy, E.; Irwin, M. J.; Jablonka, P.; Venn, K.; Battaglia, G.; Starkenburg, E.; Shetrone, M.; Letarte, B.; Francois, P.; Helmi, A.; Primas, F.; Kaufer, A.; Szeifert, T.; Ballet, J.; Martins, F.; Bournaud, F.; Monier, R.; Reylé, C.

    2014-01-01

    From VLT-FLAMES high-resolution spectra, we determine the abundances of several α, iron-peak and neutron-capture elements in 47 Red Giant Branch stars in the Fornax dwarf spheroidal galaxy. We confirm that SNe Ia started to contribute to the chemical enrichment of Fornax at [Fe/H] between --2.0 and

  11. The ACS LCID project : RR Lyrae stars as tracers of old population gradients in the isolated dwarf spheroidal galaxy tucana

    NARCIS (Netherlands)

    Bernard, Edouard J.; Gallart, Carme; Monelli, Matteo; Aparicio, Antonio; Cassisi, Santi; Skillman, Evan D.; Stetson, Peter B.; Cole, Andrew A.; Drozdovsky, Igor; Hidalgo, Sebastian L.; Mateo, Mario; Tolstoy, Eline

    2008-01-01

    We present a study of the radial distribution of RR Lyrae variables, which present a range of photometric and pulsational properties, in the dwarf spheroidal galaxy Tucana. We find that the fainter RR Lyrae stars, having a shorter period, are more centrally concentrated than the more luminous,

  12. DO R CORONAE BOREALIS STARS FORM FROM DOUBLE WHITE DWARF MERGERS?

    Energy Technology Data Exchange (ETDEWEB)

    Staff, Jan. E.; Clayton, Geoffrey C.; Tohline, Joel E. [Department of Physics and Astronomy, Louisiana State University, 202 Nicholson Hall, Tower Drive, Baton Rouge, LA 70803-4001 (United States); Menon, Athira; Herwig, Falk [Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P5C2 (Canada); Even, Wesley; Fryer, Chris L. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Motl, Patrick M. [Department of Science, Mathematics and Informatics, Indiana University Kokomo, Kokomo, IN 46904-9003 (United States); Geballe, Tom [Gemini Observatory, 670 North A' ohoku Place, Hilo, HI 96720 (United States); Pignatari, Marco [Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel (Switzerland)

    2012-09-20

    A leading formation scenario for R Coronae Borealis (RCB) stars invokes the merger of degenerate He and CO white dwarfs (WDs) in a binary. The observed ratio of {sup 16}O/{sup 18}O for RCB stars is in the range of 0.3-20 much smaller than the solar value of {approx}500. In this paper, we investigate whether such a low ratio can be obtained in simulations of the merger of a CO and a He WD. We present the results of five three-dimensional hydrodynamic simulations of the merger of a double WD system where the total mass is 0.9 M{sub Sun} and the initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations with q {approx}< 0.7 a feature around the merged stars where the temperatures and densities are suitable for forming {sup 18}O. However, more {sup 16}O is being dredged up from the C- and O-rich accretor during the merger than the amount of {sup 18}O that is produced. Therefore, on the dynamical timescale over which our hydrodynamics simulation runs, an {sup 16}O/{sup 18}O ratio of {approx}2000 in the 'best' case is found. If the conditions found in the hydrodynamic simulations persist for 10{sup 6} s the oxygen ratio drops to 16 in one case studied, while in a hundred years it drops to {approx}4 in another case studied, consistent with the observed values in RCB stars. Therefore, the merger of two WDs remains a strong candidate for the formation of these enigmatic stars.

  13. On the Iron Abundance Anomaly in K-dwarf and Hyades Stars

    Energy Technology Data Exchange (ETDEWEB)

    Aleo, Patrick D.; Sobotka, Alexander C. [McDonald Observatory and Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1402, Austin, TX 78712-1205 (United States); Ramírez, Ivan [Tacoma Community College, 6501 South 19th Street, Tacoma, WA 98466-7400 (United States)

    2017-09-01

    Using standard 1D-LTE model atmosphere analysis, we provide an in-depth investigation of iron abundance as derived from neutral and singly ionization iron lines (Fe i, ii) in nearby star clusters. Specifically, we replicate the discrepancy regarding Δ[Fe/H], wherein the difference of Fe ii–Fe i increases for stars of the same cluster with decreasing T {sub eff}, reaching an astonishing 1.0 dex at T {sub eff} ∼ 4000 K. Previous studies have investigated this anomaly in the Pleiades and Hyades clusters with no concrete solution. In this analysis, we probe two samples: 63 wide binary field stars where the primary star is of Sun-like temperatures and the secondary is a K-dwarf, ranging from 4231 K ≤ T {sub eff} ≤ 6453 K, and 33 Hyades stars of temperatures 4268 K ≤ T {sub eff} ≤ 6072 K. Previous studies have found discrepancies on the order of 1.0 dex. However, we find that these studies have neglected line-blending effects of certain Fe ii lines, namely λ = (4508.29 Å, 4993.34 Å, 5197.58 Å, 5325.55 Å, 5425.26 Å, 6456.38 Å). When these lines are removed from the line-list, we find Δ[Fe/H] decreases to ∼0.6 dex in the field binaries and ∼0.3 dex in the Hyades. The reason for this remaining trend is investigated by probing NLTE effects, as well as age and activity considerations using Ca ii H+K emission and Li absorption, but these results appear to be small to negligible.

  14. On the Iron Abundance Anomaly in K-dwarf and Hyades Stars

    International Nuclear Information System (INIS)

    Aleo, Patrick D.; Sobotka, Alexander C.; Ramírez, Ivan

    2017-01-01

    Using standard 1D-LTE model atmosphere analysis, we provide an in-depth investigation of iron abundance as derived from neutral and singly ionization iron lines (Fe i, ii) in nearby star clusters. Specifically, we replicate the discrepancy regarding Δ[Fe/H], wherein the difference of Fe ii–Fe i increases for stars of the same cluster with decreasing T eff , reaching an astonishing 1.0 dex at T eff ∼ 4000 K. Previous studies have investigated this anomaly in the Pleiades and Hyades clusters with no concrete solution. In this analysis, we probe two samples: 63 wide binary field stars where the primary star is of Sun-like temperatures and the secondary is a K-dwarf, ranging from 4231 K ≤ T eff ≤ 6453 K, and 33 Hyades stars of temperatures 4268 K ≤ T eff ≤ 6072 K. Previous studies have found discrepancies on the order of 1.0 dex. However, we find that these studies have neglected line-blending effects of certain Fe ii lines, namely λ = (4508.29 Å, 4993.34 Å, 5197.58 Å, 5325.55 Å, 5425.26 Å, 6456.38 Å). When these lines are removed from the line-list, we find Δ[Fe/H] decreases to ∼0.6 dex in the field binaries and ∼0.3 dex in the Hyades. The reason for this remaining trend is investigated by probing NLTE effects, as well as age and activity considerations using Ca ii H+K emission and Li absorption, but these results appear to be small to negligible.

  15. On the Iron Abundance Anomaly in K-dwarf and Hyades Stars

    Science.gov (United States)

    Aleo, Patrick D.; Sobotka, Alexander C.; Ramírez, Ivan

    2017-09-01

    Using standard 1D-LTE model atmosphere analysis, we provide an in-depth investigation of iron abundance as derived from neutral and singly ionization iron lines (Fe I, II) in nearby star clusters. Specifically, we replicate the discrepancy regarding Δ[Fe/H], wherein the difference of Fe II-Fe I increases for stars of the same cluster with decreasing T eff, reaching an astonishing 1.0 dex at T eff ˜ 4000 K. Previous studies have investigated this anomaly in the Pleiades and Hyades clusters with no concrete solution. In this analysis, we probe two samples: 63 wide binary field stars where the primary star is of Sun-like temperatures and the secondary is a K-dwarf, ranging from 4231 K ≤ T eff ≤ 6453 K, and 33 Hyades stars of temperatures 4268 K ≤ T eff ≤ 6072 K. Previous studies have found discrepancies on the order of 1.0 dex. However, we find that these studies have neglected line-blending effects of certain Fe II lines, namely λ = {4508.29 Å, 4993.34 Å, 5197.58 Å, 5325.55 Å, 5425.26 Å, 6456.38 Å}. When these lines are removed from the line-list, we find Δ[Fe/H] decreases to ˜0.6 dex in the field binaries and ˜0.3 dex in the Hyades. The reason for this remaining trend is investigated by probing NLTE effects, as well as age and activity considerations using Ca II H+K emission and Li absorption, but these results appear to be small to negligible.

  16. Ground-based observation of emission lines from the corona of a red-dwarf star.

    Science.gov (United States)

    Schmitt, J H; Wichmann, R

    2001-08-02

    All 'solar-like' stars are surrounded by coronae, which contain magnetically confined plasma at temperatures above 106 K. (Until now, only the Sun's corona could be observed in the optical-as a shimmering envelope during a total solar eclipse.) As the underlying stellar 'surfaces'-the photospheres-are much cooler, some non-radiative process must be responsible for heating the coronae. The heating mechanism is generally thought to be magnetic in origin, but is not yet understood even for the case of the Sun. Ultraviolet emission lines first led to the discovery of the enormous temperature of the Sun's corona, but thermal emission from the coronae of other stars has hitherto been detectable only from space, at X-ray wavelengths. Here we report the detection of emission from highly ionized iron (Fe XIII at 3,388.1 A) in the corona of the red-dwarf star CN Leonis, using a ground-based telescope. The X-ray flux inferred from our data is consistent with previously measured X-ray fluxes, and the non-thermal line width of 18.4 km s-1 indicates great similarities between solar and stellar coronal heating mechanisms. The accessibility and spectral resolution (45,000) of the ground-based instrument are much better than those of X-ray satellites, so a new window to the study of stellar coronae has been opened.

  17. Spectra of late type dwarf stars of known abundance for stellar population models

    Science.gov (United States)

    Oconnell, R. W.

    1990-01-01

    The project consisted of two parts. The first was to obtain new low-dispersion, long-wavelength, high S/N IUE spectra of F-G-K dwarf stars with previously determined abundances, temperatures, and gravities. To insure high quality, the spectra are either trailed, or multiple exposures are taken within the large aperture. Second, the spectra are assembled into a library which combines the new data with existing IUE Archive data to yield mean spectral energy distributions for each important type of star. My principal responsibility is the construction and maintenance of this UV spectral library. It covers the spectral range 1200-3200A and is maintained in two parts: a version including complete wavelength coverage at the full spectral resolution of the Low Resolution cameras; and a selected bandpass version, consisting of the mean flux in pre-selected 20A bands. These bands are centered on spectral features or continuum regions of special utility - e.g. the C IV lambda 1550 or Mg II lambda 2800 feature. In the middle-UV region, special emphasis is given to those features (including continuum 'breaks') which are most useful in the study of F-G-K star spectra in the integrated light of old stellar populations.

  18. Three small transiting planets around the M-dwarf host star LP 358-499

    Science.gov (United States)

    Wells, R.; Poppenhaeger, K.; Watson, C. A.

    2018-01-01

    We report on the detection of three transiting small planets around the low-mass star LP 358-499 (K2-133), using photometric data from the Kepler-K2 mission. Using multiband photometry, we determine the host star to be an early M dwarf with an age likely older than a gigayear. The three detected planets K2-133 b, c and d have orbital periods of ca. 3, 4.9 and 11 d and transit depths of ca. 700, 1000 and 2000 ppm, respectively. We also report a planetary candidate EPIC 247887989.01 with a period of 26.6 d and a depth of ca. 1000 ppm, which may be at the inner edge of the stellar habitable zone, depending on the specific host star properties. Using the transit parameters and the stellar properties, we estimate that the innermost planet may be rocky. The system is suited for follow-up observations to measure planetary masses and JWST transmission spectra of planetary atmospheres.

  19. A STRANGE STAR SCENARIO FOR THE FORMATION OF ECCENTRIC MILLISECOND PULSAR/HELIUM WHITE DWARF BINARIES

    Energy Technology Data Exchange (ETDEWEB)

    Jiang, Long; Li, Xiang-Dong [Department of Astronomy, Nanjing University, Nanjing 210046 (China); Dey, Jishnu; Dey, Mira, E-mail: lixd@nju.edu.cn [Department of Physics, Presidency University, 86/1, College Street, Kolkata 700 073 (India)

    2015-07-01

    According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during binary evolution, as observed in most binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are the PSRs J2234+06, J1946+3417, and J1950+2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf (WD) companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive WDs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars (NSs) in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from NSs to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the NS during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X–1, which also possesses a low-field compact star in an eccentric orbit.

  20. Milky Way Tomography with K and M Dwarf Stars: The Vertical Structure of the Galactic Disk

    Energy Technology Data Exchange (ETDEWEB)

    Ferguson, Deborah; Gardner, Susan [Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055 (United States); Yanny, Brian [Fermi National Accelerator Laboratory, Batavia, IL 60510 (United States)

    2017-07-10

    We use the number density distributions of K and M dwarf stars with vertical height from the Galactic disk, determined using observations from the Sloan Digital Sky Survey, to probe the structure of the Milky Way disk across the survey’s footprint. Using photometric parallax as a distance estimator we analyze a sample of several million disk stars in matching footprints above and below the