WorldWideScience

Sample records for black dwarf stars

  1. Black hole, neutron star and white dwarf candidates from microlensing with OGLE-III★

    Science.gov (United States)

    Wyrzykowski, Ł.; Kostrzewa-Rutkowska, Z.; Skowron, J.; Rybicki, K. A.; Mróz, P.; Kozłowski, S.; Udalski, A.; Szymański, M. K.; Pietrzyński, G.; Soszyński, I.; Ulaczyk, K.; Pietrukowicz, P.; Poleski, R.; Pawlak, M.; Iłkiewicz, K.; Rattenbury, N. J.

    2016-05-01

    Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III data base of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 9.3 M⊙ and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events will be observed by the astrometric Gaia mission.

  2. Black Holes, Neutron Stars and White Dwarf Candidates from Microlensing with OGLE-III

    CERN Document Server

    Wyrzykowski, L; Skowron, J; Rybicki, K A; Mroz, P; Kozlowski, S; Udalski, A; Szymanski, M K; Pietrzynski, G; Soszynski, I; Ulaczyk, K; Pietrukowicz, P; Poleski, R; Pawlak, M; Ilkiewicz, K; Rattenbury, N J

    2015-01-01

    Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III database of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 15 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 8.3 M_Sun and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events w...

  3. White Dwarf Stars

    Science.gov (United States)

    Kawaler, Steven; Dahlstrom, Michael

    2000-12-01

    A white dwarf is a very dense star: The earth-sized remains of a Sun-like star that has burned all of its nuclear fuel. Although it's unable to carry out the workaday activities of a living star, a white dwarf is still an interesting object to astronomers. For one thing, white dwarfs experience "starquakes"—gentle pulsations that allow astronomers to deduce certain physical qualities of the star, such as its mass, rate of rotation, its structure and the strength of its magnetic field. The authors have been studying the starquakes with a global network of instruments, collectively called the Whole Earth Telescope, which provide around-the-clock observations of a white dwarf's seismic activity. Kawaler and Dahlstrom discuss what we know about white dwarfs and their significance for questions concerning the age of our Galaxy and the composition of dark matter.

  4. 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...

  5. Correlation of the Quasi-Periodic Oscillation Frequencies of White Dwarf, Neutron Star, and Black Hole Binaries

    CERN Document Server

    Mauche, C W

    2002-01-01

    Using data obtained in 1994 June/July with the Extreme Ultraviolet Explorer deep survey photometer and in 2001 January with the Chandra X-ray Observatory Low Energy Transmission Grating Spectrograph, we investigate the extreme-ultraviolet (EUV) and soft X-ray oscillations of the dwarf nova SS Cyg in outburst. We find quasi-periodic oscillations (QPOs) at nu_0 ~ 0.012 Hz and nu_1 ~ 0.13 Hz in the EUV flux and at nu_0 ~ 0.0090 Hz, nu_1 ~ 0.11 Hz, and possibly nu_2 ~ nu_0 + nu_1 ~ 0.12 Hz in the soft X-ray flux. These data, combined with the optical data of Woudt & Warner for VW Hyi, extend the Psaltis, Belloni, & van der Klis nu_high-nu_low correlation for neutron star and black hole low-mass X-ray binaries (LMXBs) nearly two orders of magnitude in frequency, with nu_low ~ 0.08 nu_high. This correlation identifies the high-frequency quasi-coherent oscillations (so-called ``dwarf nova oscillations'') of cataclysmic variables (CVs) with the kilohertz QPOs of LMXBs, and the low-frequency QPOs of CVs with t...

  6. Asteroseismology of white dwarf stars

    CERN Document Server

    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 of white dwarfs can be inferred from spectroscopy, the internal structure of these compact stars can be unveiled only by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable stars and the periods of appropriate theoretical models. In this communication, we first briefly describe the physical properties of white dwarf stars and the various families of pulsating white dwarfs known up to the present day, and then we present two recent analysis carried out by the La...

  7. 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. PMID:18033290

  8. Space distribution and physical properties of cool dwarf stars

    International Nuclear Information System (INIS)

    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.)

  9. FIRST RESULTS FROM THE SWARMS SURVEY. SDSS 1257+5428: A NEARBY, MASSIVE WHITE DWARF BINARY WITH A LIKELY NEUTRON STAR OR BLACK HOLE COMPANION

    International Nuclear Information System (INIS)

    We present the first results from the SWARMS survey, an ongoing project to identify compact white dwarf (WD) binaries in the spectroscopic catalog of the Sloan Digital Sky Survey (SDSS). The first object identified by SWARMS, SDSS 1257+5428, is a single-lined spectroscopic binary in a circular orbit with a period of 4.56 hr and a semiamplitude of 322.7 ± 6.3 km s-1. From the spectrum and photometry, we estimate a WD mass of 0.92+0.28-0.32 Msun. Together with the orbital parameters of the binary, this implies that the unseen companion must be more massive than 1.62+0.20-0.25 Msun, and is in all likelihood either a neutron star or a black hole. At an estimated distance of 48+10-19 pc, this would be the closest known stellar remnant of a supernova explosion.

  10. Interpretations for Low- and High-Frequency QPO Correlations of X-Ray Sources among White Dwarfs, Neutron Stars, and Black Holes

    Science.gov (United States)

    Zhang, C. M.; Yin, H. X.; Zhao, Y. H.

    2007-04-01

    An empirical linear relation is found to exist between the high and low frequencies (νhigh, νlow) of quasi-periodic oscillations (QPOs) for black hole candidates (BHCs), neutron stars (NSs), and white dwarfs (WDs) in binary systems, spanning 5 orders of magnitude in frequency. For the NS Z (atoll) sources, νhigh and νlow are identified as the lower kHz QPO frequency and horizontal-branch oscillation (HBO) frequency νHBO (broad noise components); for the BHCs and low-luminosity NSs, they are the QPOs and broad noise components at frequencies between 1 and 10 Hz; for WDs, they are the ``dwarf nova oscillations'' (DNOs) and QPOs of cataclysmic variables (CVs). To interpret this relation, our model ascribes νhigh to the Alfvén wave oscillation frequency at a preferred radius, and νlow to the same mechanism at another radius. We can then obtain νlow=0.08νhigh and the relation between the upper kHz QPO frequency ν2 and the HBO frequency, νHBO~=(56 Hz)(ν2/kHz)2, which are in accordance with the observed empirical relations. Furthermore, some implications of the model are discussed, including why QPO frequencies of WDs and NSs span 5 orders of magnitude.

  11. Nuclear-dominated accretion and subluminous supernovae from the merger of a white dwarf with a neutron star or black hole

    CERN Document Server

    Metzger, Brian D

    2011-01-01

    We construct one dimensional steady-state models of accretion disks produced by the tidal disruption of a white dwarf (WD) by a neutron star (NS) or stellar mass black hole (BH). At radii r 50-80 per cent of the total WD mass is unbound. The ejecta composition is predominantly O, C, Si, Mg, Ne, Fe, and S [He, C, Si, S, Ar, and Fe], in the case of C-O [He] WDs, respectively, along with a small quantity ~1e-3-1e-2 Msun of radioactive Ni56 and, potentially, a trace amount of H. We use our results to evaluate possible EM counterparts of WD-NS/BH mergers, including optical transients powered by the radioactive decay of Ni56 and radio transients powered by the interaction of the ejecta with the interstellar medium. We address whether recently discovered subluminous Type I supernovae result from WD-NS/BH mergers. Our results also have implications for accretion following the core collapse of massive stars in collapsar models for gamma-ray bursts.

  12. Charged Condensate and Helium Dwarf Stars

    OpenAIRE

    Gabadadze, Gregory; Rosen, Rachel A.

    2008-01-01

    White dwarf stars composed of carbon, oxygen or 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, whi...

  13. Gravitational Interactions of White Dwarf Double Stars

    Science.gov (United States)

    McKeough, James; Robinson, Chloe; Ortiz, Bridget; Hira, Ajit

    2016-03-01

    In the light of the possible role of White Dwarf stars as progenitors of Type Ia supernovas, we present computational simulations of some astrophysical phenomena associated with a study of gravitationally-bound binary stars, composed of at least one white dwarf star. Of particular interest to astrophysicists are the conditions inside a white dwarf star in the time frame leading up to its explosive end as a Type Ia supernova, for an understanding of the massive stellar explosions. In addition, the studies of the evolution of white dwarfs could serve as promising probes of theories of gravitation. We developed FORTRAN computer programs to implement our models for white dwarfs and other stars. These codes allow for different sizes and masses of stars. Simulations were done in the mass interval from 0.1 to 2.5 solar masses. Our goal was to obtain both atmospheric and orbital parameters. The computational results thus obtained are compared with relevant observational data. The data are further analyzed to identify trends in terms of sizes and masses of stars. We will extend our computational studies to blue giant and red giant stars in the future. Funding from National Science Foundation.

  14. A radio pulsing white dwarf binary star

    CERN Document Server

    Marsh, T R; 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

    2016-01-01

    White dwarfs are compact stars, similar in size to Earth but ~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 delta-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56 hr period close binary, pulsing in brightness on a period of 1.97 min. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 s, and they are detectable a...

  15. Correlation of the Quasi-Periodic Oscillation Frequencies of White Dwarf, Neutron Star, and Black Hole Binaries

    OpenAIRE

    Mauche, Christopher W.

    2002-01-01

    Using data obtained in 1994 June/July with the Extreme Ultraviolet Explorer deep survey photometer and in 2001 January with the Chandra X-ray Observatory Low Energy Transmission Grating Spectrograph, we investigate the extreme-ultraviolet (EUV) and soft X-ray oscillations of the dwarf nova SS Cyg in outburst. We find quasi-periodic oscillations (QPOs) at nu_0 ~ 0.012 Hz and nu_1 ~ 0.13 Hz in the EUV flux and at nu_0 ~ 0.0090 Hz, nu_1 ~ 0.11 Hz, and possibly nu_2 ~ nu_0 + nu_1 ~ 0.12 Hz in the...

  16. Pulsating White Dwarf Stars and Precision Asteroseismology

    CERN Document Server

    Winget, D E

    2008-01-01

    Galactic history is written in the white dwarf stars. Their surface properties hint at interiors composed of matter under extreme conditions. In the forty years since their discovery, pulsating white dwarf stars have moved from side-show curiosities to center stage as important tools for unraveling the deep mysteries of the Universe. Innovative observational techniques and theoretical modeling tools have breathed life into precision asteroseismology. We are just learning to use this powerful tool, confronting theoretical models with observed frequencies and their time rate-of-change. With this tool, we calibrate white dwarf cosmochronology; we explore equations of state; we measure stellar masses, rotation rates, and nuclear reaction rates; we explore the physics of interior crystallization; we study the structure of the progenitors of Type Ia supernovae, and we test models of dark matter. The white dwarf pulsations are at once the heartbeat of galactic history and a window into unexplored and exotic physics.

  17. Limits on Planets Around White Dwarf Stars

    CERN Document Server

    Mullally, F; Degennaro, Steven; Jeffery, Elizabeth; Thompson, S E; Chandler, Dean

    2008-01-01

    We present limits on planetary companions to pulsating white dwarf stars. A subset of these stars exhibit extreme stability in the period and phase of some of their pulsation modes; a planet can be detected around such a star by searching for periodic variations in the arrival time of these pulsations. We present limits on companions greater than a few Jupiter masses around a sample of 15 white dwarf stars as part of an on-going survey. One star shows a variation in arrival time consistent with a 2 M_J planet in a 4.5 year orbit. We discuss other possible explanations for the observed signal and conclude that a planet is the most plausible explanation based on the data available.

  18. Massive Star Clusters in Dwarf Galaxies

    CERN Document Server

    Larsen, Soeren S

    2015-01-01

    Dwarf galaxies can have very high globular cluster specific frequencies, and the GCs are in general significantly more metal-poor than the bulk of the field stars. In some dwarfs, such as Fornax, WLM, and IKN, the fraction of metal-poor stars that belong to GCs can be as high as 20%-25%, an order of magnitude higher than the 1%-2% typical of GCs in halos of larger galaxies. Given that chemical abundance anomalies appear to be present also in GCs in dwarf galaxies, this implies severe difficulties for self-enrichment scenarios that require GCs to have lost a large fraction of their initial masses. More generally, the number of metal-poor field stars in these galaxies is today less than what would originally have been present in the form of low-mass clusters if the initial cluster mass function was a power-law extending down to low masses. This may imply that the initial GC mass function in these dwarf galaxies was significantly more top-heavy than typically observed in present-day star forming environments.

  19. The SDSS White Dwarf - M Star Library

    CERN Document Server

    Heller, René; Østensen, Roy H

    2011-01-01

    The Sloan Digital Sky Survey (SDSS), originally targeted at quasi-stellar objects, has provided us with a wealth of astronomical byproducts through the last decade. Since then, the number of white dwarfs (WDs) with physically bound main-sequence star companions (mostly dM stars) has increased radically, allowing for fundamentally new insights into stellar physics. Different methods for the retrieval and follow-up analysis of SDSS WD-dM binaries have been applied in the literature, leading to a rising number of WD-dM catalogs. Here we present a detailed literature search, coupled with our own hunting for SDSS WD-dMs by color selection, the outcome being named the "SDSS White Dwarf - M Star Library". We also explain improvements of our automated spectral analysis method.

  20. ULXs: Neutron stars versus black holes

    Science.gov (United States)

    King, Andrew; Lasota, Jean-Pierre

    2016-05-01

    We consider ultraluminous X-ray systems (ULXs) where the accretor is a neutron star rather than a black hole. We show that the recently discovered example (M82 X-2) fits naturally into the simple picture of ULXs as beamed X-ray sources fed at super-Eddington rates, provided that its magnetic field is weaker (≃1011G) than a new-born X-ray pulsar, as expected if there has been mass gain. Continuing accretion is likely to weaken the field to the point that pulsing stops, and make the system indistinguishable from a ULX containing a black hole. Accordingly we suggest that a significant fraction of all ULXs may actually contain neutron star accretors rather than black holes, reflecting the neutron-star fraction among their X-ray binary progenitors. We emphasize that neutron-star ULXs are likely to have higher apparent luminosities than black hole ULXs for a given mass transfer rate, as their tighter beaming outweighs their lower Eddington luminosities. This further increases the likely proportion of neutron-star accretors among all ULXs. Cygnus X-2 is probably a typical descendant of neutron-star ULXs, which may therefore ultimately end as millisecond pulsar binaries with massive white dwarf companions.

  1. Magnetic White Dwarf Stars in the SDSS

    CERN Document Server

    Kepler, S O; Jordan, Stefan; Kleinman, Scot J; Kulebi, Baybars; Koester, Detlev; Peçanha, Viviane; Castanheira, Bárbara G; Nitta, Atsuko; Costa, José Eduardo da Silveira; Winget, Don Earl; Kanaan, Antonio; Fraga, Luciano

    2012-01-01

    To obtain a better statistics on the occurrence of magnetism among white dwarfs, we searched the spectra of the hydrogen atmosphere white dwarf stars (DAs) in the Data Release 7 of the Sloan Digital Sky Survey (SDSS) for Zeeman splittings and estimated the magnetic fields. We found 521 DAs with detectable Zeeman splittings, with fields in the range from around 1 MG to 733 MG, which amounts to 4% of all DAs observed. As the SDSS spectra have low signal-to-noise ratios, we carefully investigated by simulations with theoretical spectra how reliable our detection of magnetic field was.

  2. 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, ...

  3. Charged condensate and helium dwarf stars

    International Nuclear Information System (INIS)

    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

  4. Global and photospheric physical parameters of active dwarf stars

    International Nuclear Information System (INIS)

    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.)

  5. Lessons for Asteroseismology from White Dwarf Stars

    Indian Academy of Sciences (India)

    Travis S. Metcalfe

    2005-06-01

    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 particular set of oscillation frequencies. I review the past development and the current state of white dwarf asteroseismology, with an emphasis on what this can tell us about the road to success for asteroseismology of other types of stars.

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

    International Nuclear Information System (INIS)

    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.

  7. Gas, Stars and Star Formation in ALFALFA Dwarf Galaxies

    CERN Document Server

    Huang, S; Giovanelli, R; Brinchmann, J; Stierwalt, S; Neff, S G

    2012-01-01

    We examine the global properties of the stellar and HI components of 229 low HI mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with HI masses < 10^{7.7} M_sun and HI line widths < 80 km s^{-1}. SDSS data are combined with photometric properties derived from GALEX 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_* < 10^8 M_sun is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of t...

  8. An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10.

    Science.gov (United States)

    Reines, Amy E; Sivakoff, Gregory R; Johnson, Kelsey E; Brogan, Crystal L

    2011-02-01

    Supermassive black holes are now thought to lie at the heart of every giant galaxy with a spheroidal component, including our own Milky Way. The birth and growth of the first 'seed' black holes in the earlier Universe, however, is observationally unconstrained and we are only beginning to piece together a scenario for their subsequent evolution. Here we report that the nearby dwarf starburst galaxy Henize 2-10 (refs 5 and 6) contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source. From these observations, we conclude that Henize 2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. This nearby dwarf galaxy, simultaneously hosting a massive black hole and an extreme burst of star formation, is analogous in many ways to galaxies in the infant Universe during the early stages of black-hole growth and galaxy mass assembly. Our results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and that by implication so can their primordial counterparts. Moreover, the lack of a substantial spheroidal component in Henize 2-10 indicates that supermassive black-hole growth may precede the build-up of galaxy spheroids. PMID:21217688

  9. An Extended Star Formation History in an Ultra Compact Dwarf

    CERN Document Server

    Norris, Mark A; Faifer, Favio R; Kannappan, Sheila J; Forte, Juan Carlos; Bosch, Remco C E van den

    2015-01-01

    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 (GC) 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 (< 0.7"). With these data we examine the spatially resolved kinematics and star formation history of this unusual object. We find no evidence of a rise in the central velocity dispersion of the UCD, suggesting that this UCD lacks a massive central black hole like those found in some other compact stellar systems, a conclusion confirmed by detailed dynamical mod...

  10. Do all barium stars have a white dwarf companion?

    Science.gov (United States)

    Dominy, J. F.; Lambert, D. L.

    1983-01-01

    International Ultraviolet Explorer short-wavelength, low-dispersion spectra were analyzed for four barium, two mild barium, and one R-type carbon star in order to test the hypothesis that the barium and related giants are produced by mass transfer from a companion now present as a white dwarf. An earlier tentative identification of a white dwarf companion to the mild barium star Zeta Cyg is confirmed. For the other stars, no ultraviolet excess attributable to a white dwarf is seen. Limits are set on the bolometric magnitude and age of a possible white dwarf companion. Since the barium stars do not have obvious progenitors among main-sequence and subgiant stars, mass transfer must be presumed to occur when the mass-gaining star is already on the giant branch. This restriction, and the white dwarf's minimum age, which is greater than 8 x 10 to the 8th yr, determined for several stars, effectively eliminates the hypothesis that mass transfer from an asymptotic giant branch star creates a barium star. Speculations are presented on alternative methods of producing a barium star in a binary system.

  11. Abundances In Very Metal Poor Dwarf Stars

    CERN Document Server

    Cohen, J G; McWilliam, A; Shectman, S; Thompson, I; Wasserburg, G J; Ivans, I I; Dehn, M; Karlsson, T; Melendez, J; Cohen, Judith G.; Christlieb, Norbert; William, Andrew Mc; Shectman, Steve; Thompson, Ian; Ivans, Inese; Dehn, Matthias; Karlsson, Torgny

    2004-01-01

    We discuss the detailed composition of 28 extremely metal-poor dwarfs, 22 of which are from the Hamburg/ESO Survey, based on Keck Echelle spectra. Our sample has a median [Fe/H] of -2.7 dex, extends to -3.5 dex, and is somewhat less metal-poor than was expected from [Fe/H](HK,HES) determined from low resolution spectra. Our analysis supports the existence of a sharp decline in the distribution of halo stars with metallicity below [Fe/H] = -3.0 dex. So far no additional turnoff stars with [Fe/H]}<-3.5 have been identified in our follow up efforts. For the best observed elements between Mg and Ni, we find that the abundance ratios appear to have reached a plateau, i.e. [X/Fe] is approximately constant as a function of [Fe/H], except for Cr, Mn and Co, which show trends of abundance ratios varying with [Fe/H]. These abundance ratios at low metallicity correspond approximately to the yield expected from Type II SN with a narrow range in mass and explosion parameters; high mass Type II SN progenitors are requir...

  12. 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...

  13. Massive compact dwarf stars and C-field

    OpenAIRE

    Singh, L. P.; Sahu, P. K.

    1996-01-01

    The effect of C-field in high density matter has been studied. We find that the negative energy and negative pressure of the C-field helps in formation of massive compact stable neutron stars of mass $\\sim$ 0.5 solar mass which is in the range of 0.01 to 1.0 solar mass of recently observed dwarf stars.

  14. 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.

  15. Delayed Star Formation in Isolated Dwarf Galaxies: HST Star Formation History of the Aquarius Dwarf Irregular

    CERN Document Server

    Cole, Andrew A; Dolphin, Andrew E; Skillman, Evan D; McConnachie, Alan W; Brooks, Alyson M; Leaman, Ryan

    2014-01-01

    We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). 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(HI)/M(stellar), 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 CD...

  16. Can brown dwarfs survive on close orbits around convective stars?

    CERN Document Server

    Damiani, Cilia

    2016-01-01

    Brown dwarfs straddle the mass range transition from planetary to stellar objects. There is a relative paucity of brown dwarfs companions around FGKM stars compared to exoplanets for orbital periods less than a few years, but most of the short-period brown dwarf companions fully characterised by transits and radial velocities are found around F-type stars. We examine the hypothesis that brown dwarf companions could not survive on close orbit around stars with important convective envelopes because the tides and angular momentum loss through magnetic breaking should lead to a rapid orbital decay and quick engulfment of the companion. We use a classical Skumanich-type braking law, and constant time-lag tidal theory to assess the characteristic timescale for orbital decay for the brown dwarf mass range as a function of the host properties. We find that F-type stars may host massive companions for a significantly longer time than G-type stars for a given orbital period, which may explain the paucity of G-type hos...

  17. Can brown dwarfs survive on close orbits around convective stars?

    Science.gov (United States)

    Damiani, C.; Díaz, R. F.

    2016-04-01

    Context. The mass range of brown dwarfs extends across the planetary domain to stellar objects. There is a relative paucity of brown dwarfs companions around FGKM-type stars compared to exoplanets for orbital periods of less than a few years, but most of the short-period brown dwarf companions that are fully characterised by transits and radial velocities are found around F-type stars. Aims: We examine the hypothesis that brown dwarf companions could not survive on close orbit around stars with important convective envelopes because the tides and angular momentum loss, the result of magnetic braking, would lead to a rapid orbital decay with the companion being quickly engulfed. Methods: We use a classical Skumanich-type braking law and constant time-lag tidal theory to assess the characteristic timescale for orbital decay for the brown dwarf mass range as a function of the host properties. Results: We find that F-type stars may host massive companions for a significantly longer time than G-type stars for a given orbital period, which may explain the paucity of G-type hosts for brown dwarfs with an orbital period less than five days. On the other hand, we show that the small radius of early M-type stars contributes to orbital decay timescales that are only half those of F-type stars, despite their more efficient tidal dissipation and magnetic braking. For fully convective later type M-dwarfs, orbital decay timescales could be orders of magnitude greater than for F-type stars. Moreover, we find that, for a wide range of values of tidal dissipation efficiency and magnetic braking, it is safe to assume that orbital decay for massive companions can be neglected for orbital periods greater than ten days. Conclusions: For orbital periods greater than ten days, brown dwarf occurrence should largely be unaffected by tidal decay, whatever the mass of the host. On closer orbital periods, the rapid engulfment of massive companions could explain the lack of G and K-type hosts

  18. The White Dwarf Binary Pathways Survey I: A sample of FGK stars with white dwarf companions

    CERN Document Server

    Parsons, S G; Schreiber, M R; Gansicke, B T; Zorotovic, M; Ren, J J

    2016-01-01

    The number of white dwarf plus main-sequence star binaries has increased rapidly in the last decade, jumping from only ~30 in 2003 to over 3000. However, in the majority of known systems the companion to the white dwarf is a low mass M dwarf, since these are relatively easy to identify from optical colours and spectra. White dwarfs with more massive FGK type companions have remained elusive due to the large difference in optical brightness between the two stars. In this paper we identify 934 main-sequence FGK stars from the Radial Velocity Experiment (RAVE) survey in the southern hemisphere and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey in the northern hemisphere, that show excess flux at ultraviolet wavelengths which we interpret as the likely presence of a white dwarf companion. We obtained Hubble Space Telescope ultraviolet spectra for nine systems which confirmed that the excess is indeed caused, in all cases, by a hot compact companion, eight being white dwarfs and one ...

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

    International Nuclear Information System (INIS)

    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

  20. New White Dwarf Stars in the Sloan Digital Sky Survey Data Release 10

    CERN Document Server

    Kepler, S O; Koester, Detlev; Ourique, Gustavo; Kleinman, Scot J; Romero, Alejandra Daniela; Nitta, Atsuko; Eisenstein, Daniel J; Costa, José Eduardo da Silveira; Külebi, Baybars; Jordan, Stefan; Dufour, Patrick; Giommi, Paolo; Rebassa-Mansergas, Alberto

    2014-01-01

    We report the discovery of 9 088 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmosphere white dwarfs, 71 DQs, 42 hot DO/PG1159s, 171 white dwarf+main sequence star binaries, 206 magnetic DAHs, 327 continuum dominated DCs, 397 metal polluted white dwarfs, 450 helium dominated white dwarfs, 647 subdwarfs and 6887 new hydrogen dominated white dwarf stars.

  1. White-dwarf red-giant mergers, early-type R stars, J stars and lithium

    CERN Document Server

    Zhang, Xianfei

    2013-01-01

    Early-type R stars and J stars are a special type of carbon star, having enhanced nitrogen ($\\rm [N/Fe]\\approx 0.5$), lithium, a low \\iso{12}{C}/\\iso{13}{C} ratio ($<15$) and no s-element enhancements. The merger of a helium white dwarf with a red giant is regarded to be a possible model for the origin of early-type R stars, but the details of nucleosynthesis are not clear. In this paper we investigate three possible channels for helium white-dwarf + red-giant mergers, and find that, amongst the three, only a high-mass helium white dwarf subducted into a low core-mass red giant can make an early-type R star. Nucleosynthesis of elements carbon, nitrogen, oxygen and lithium correspond well with the observations. Furthermore, we find that the J stars may represent a short and luminous stage in the evolution of an early-R star.

  2. S. Chandrasekhar: White Dwarfs, $H^-$ ion,.., Black holes, Gravitational waves

    OpenAIRE

    Gupta, Patrick Das

    2011-01-01

    This is a concise review, addressed to undergraduate students, of S. Chandrasekhar's oeuvre in astrophysics, ranging from his early studies on white dwarfs using relativistic quantum statistics to topics as diverse as dynamical friction, negative hydrogen ion, fluid dynamical instabilities, black holes and gravitational waves. The exposition is based on simple physical explanations in the context of observational astronomy. Black holes and their role as central engines of active, compact, hig...

  3. Circumstellar debris and pollution at white dwarf stars

    Science.gov (United States)

    Farihi, J.

    2016-04-01

    Circumstellar disks of planetary debris are now known or suspected to closely orbit hundreds of white dwarf stars. To date, both data and theory support disks that are entirely contained within the preceding giant stellar radii, and hence must have been produced during the white dwarf phase. This picture is strengthened by the signature of material falling onto the pristine stellar surfaces; disks are always detected together with atmospheric heavy elements. The physical link between this debris and the white dwarf host abundances enables unique insight into the bulk chemistry of extrasolar planetary systems via their remnants. This review summarizes the body of evidence supporting dynamically active planetary systems at a large fraction of all white dwarfs, the remnants of first generation, main-sequence planetary systems, and hence provide insight into initial conditions as well as long-term dynamics and evolution.

  4. Circumstellar Debris and Pollution at White Dwarf Stars

    CERN Document Server

    Farihi, J

    2016-01-01

    Circumstellar disks of planetary debris are now known or suspected to closely orbit hundreds of white dwarf stars. To date, both data and theory support disks that are entirely contained within the preceding giant stellar radii, and hence must have been produced during the white dwarf phase. This picture is strengthened by the signature of material falling onto the pristine stellar surfaces; disks are always detected together with atmospheric heavy elements. The physical link between this debris and the white dwarf host abundances enables unique insight into the bulk chemistry of extrasolar planetary systems via their remnants. This review summarizes the body of evidence supporting dynamically active planetary systems at a large fraction of all white dwarfs, the remnants of first generation, main-sequence planetary systems, and hence provide insight into initial conditions as well as long-term dynamics and evolution.

  5. Lithium production in the merging of white dwarf stars

    CERN Document Server

    Longland, Richard; José, Jordi; García-Berro, Enrique; Althaus, Leandro G

    2012-01-01

    The origin of R Coronae Borealis stars has been elusive for over 200 years. Currently, two theories for their formation have been presented. These are the Final Flash scenario, in which a dying asymptotic giant branch (AGB) star throws off its atmosphere to reveal the hydrogen poor, heavily processed material underneath, and the double degenerate scenario, in which two white dwarfs merge to produce a new star with renewed vigour. Some theories predict that the temperatures reached during the latter scenario would destroy any lithium originally present in the white dwarfs. The observed lithium content of some R Coronae Borealis stars, therefore, is often interpreted as an indication that the Final Flash scenario best describes their formation. In this paper, it is shown that lithium production can, indeed, occur in the merging of a helium white dwarf with a carbon-oxygen white dwarf if their chemical composition, particularly that of 3He, is fully considered. The production mechanism is described in detail, an...

  6. Population synthesis for symbiotic stars with white dwarf accretors

    CERN Document Server

    Lu, G; Han, Z; Lu, Guoliang; Yungelson, Lev; Han, Zhanwen

    2006-01-01

    We have carried out a detailed study of symbiotic stars with white dwarf accretors by means of a population synthesis code. We estimate the total number of symbiotic stars with white dwarf accretors in the Galaxy as 1,200 - 15,000. This range is compatible with observational estimates. Two crucial physical parameters that define the birthrate and number of symbiotic stars are the efficiency of accretion by white dwarfs (which greatly depends on the separation of components after common envelope stage and stellar wind velocity) and the mass of the hydrogen layer which the white dwarf can accumulate prior to the hydrogen ignition. The theoretical estimate of the Galactic occurrence rate of symbiotic novae ranges from about 1.3 to about 13.5 per year out of which weak symbiotic novae comprise about 0.5 to 6.0 per year, depending on the model assumptions. We simulate the distributions of symbiotic stars over orbital periods, masses of components, mass-loss rates of cool components, mass-accretion rates of hot com...

  7. The Star Formation Properties of Void Dwarf Galaxies

    Science.gov (United States)

    Moorman, Crystal; Vogeley, Michael S.

    2016-01-01

    We measure the star formation properties of two large samples of galaxies from the SDSS in large-scale cosmic voids on time scales of 10 Myr and 100 Myr, using Ha emission line strengths and GALEX FUV fluxes, respectively. The first sample consists of 109,818 optically selected galaxies. We find that void galaxies in this sample have higher specific star formation rates (SSFRs; star formation rates per unit stellar mass) than similar stellar mass galaxies in denser regions. The second sample is a subset of the optically selected sample containing 8070 galaxies with reliable S/N HI detections from ALFALFA. For the HI detected sample, SSFRs are similar regardless of large-scale environment. Investigating only the HI detected dwarf galaxies reveals a trend towards higher SSFRs in voids. Furthermore, we estimate the star formation rate per unit HI mass, known as the star formation efficiency (SFE) of a galaxy, as a function of environment. For the overall HI detected population, we notice no environmental dependence. Limiting the sample to dwarf galaxies again reveals a trend towards higher SFEs in voids. These results suggest that void environments provide a nurturing environment for dwarf galaxy evolution.

  8. The Star Formation History of the Carina Dwarf Galaxy

    CERN Document Server

    Hurley-Keller, D A; Nemec, J; Hurley-Keller, Denise; Mateo, Mario; Nemec, James

    1998-01-01

    We have analyzed deep B and V photometry of the Carina dwarf spheroidal reaching below the old main-sequence turnoff to about V = 25. Using simulated color-magnitude diagrams to model a range of star formation scenarios, we have extracted a detailed, global star formation history. Carina experienced three significant episodes of star formation at about 15 Gyr, 7 Gyr, and 3 Gyr. Contrary to the generic picture of galaxy evolution, however, the bulk of star formation, at least 50%, occured during the episode 7 Gyr ago, which may have lasted as long as 2 Gyr. For unknown reasons, Carina formed only 10-20% of its stars at an ancient epoch and then remained quiescent for more than 4 Gyr. The remainder (~30%) formed relatively recently, only 3 Gyr ago. Interest in the local population of dwarf galaxies has increased lately due to their potential importance in the understanding of faint galaxy counts. We surmise that objects like Carina, which exhibits the most extreme episodic behavior of any of the dwarf spheroida...

  9. Calibrating UV Star Formation Rates for Dwarf Galaxies from STARBIRDS

    CERN Document Server

    McQuinn, Kristen B W; Dolphin, Andrew E; Mitchell, Noah P

    2015-01-01

    Integrating our knowledge of star formation traced by observations at different wavelengths is essential for correctly interpreting and comparing star formation 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 datasets are from the panchromatic STARBurst IRregular Dwarf Survey (STARBIRDS) and include deep legacy GALEX UV imaging, HST 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 predicted fluxes do not. Further, we find a systematic deviation between the SFRs based on integrated far U...

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

    International Nuclear Information System (INIS)

    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

  11. Using Narrow Band Photometry to Classify Stars and Brown Dwarfs

    CERN Document Server

    Mainzer, A K; Sievers, J L; Young, E T; Lean, Ian S. Mc

    2004-01-01

    We present a new system of narrow band filters in the near infrared that can be used to classify stars and brown dwarfs. This set of four filters, spanning the H band, can be used to identify molecular features unique to brown dwarfs, such as H2O and CH4. The four filters are centered at 1.495 um (H2O), 1.595 um (continuum), 1.66 um (CH4), and 1.75 um (H2O). Using two H2O filters allows us to solve for individual objects' reddenings. This can be accomplished by constructing a color-color-color cube and rotating it until the reddening vector disappears. We created a model of predicted color-color-color values for different spectral types by integrating filter bandpass data with spectra of known stars and brown dwarfs. We validated this model by making photometric measurements of seven known L and T dwarfs, ranging from L1 - T7.5. The photometric measurements agree with the model to within +/-0.1 mag, allowing us to create spectral indices for different spectral types. We can classify A through early M stars to...

  12. Magnetic White Dwarf Stars in the Sloan Digital Sky Survey

    OpenAIRE

    Kepler, S. O.; Pelisoli, Ingrid; Jordan, Stefan; Kleinman, Scot J.; Kulebi, Baybars; Koester, Detlev; Peçanha, Viviane; Castanheira, Bárbara G.; Nitta, Atsuko; Costa, José Eduardo da Silveira; Winget, Don Earl; Kanaan, Antonio; Fraga, Luciano

    2012-01-01

    To obtain a better statistics on the occurrence of magnetism among white dwarfs, we searched the spectra of the hydrogen atmosphere white dwarf stars (DAs) in the Data Release 7 of the Sloan Digital Sky Survey (SDSS) for Zeeman splittings and estimated the magnetic fields. We found 521 DAs with detectable Zeeman splittings, with fields in the range from around 1 MG to 733 MG, which amounts to 4% of all DAs observed. As the SDSS spectra have low signal-to-noise ratios, we carefully investigate...

  13. Adaptive Optics Observations of Exoplanets, Brown Dwarfs, & Binary Stars

    CERN Document Server

    Hinkley, Sasha

    2011-01-01

    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. Fu...

  14. An Active Black Hole in a Compact Dwarf

    Science.gov (United States)

    Kohler, Susanna

    2016-05-01

    A new type of galaxy has just been added to the galaxy zoo: a small, compact, and old elliptical galaxy that shows signs of a monster black hole actively accreting material in its center. What can this unusual discovery tell us about how compact elliptical galaxies form?A New Galactic BeastCompact elliptical galaxies are an extremely rare early-type dwarf galaxy. Consistent with their name, compact ellipticals are small, very compact collections of ancient stars; these galaxies exhibit a high surface brightness and arent actively forming stars.Optical view of the ancient compact elliptical galaxy SDSS J085431.18+173730.5 (center of image) in an SDSS color composite image. [Adapted from Paudel et al. 2016]Most compact ellipticals are found in dense environments, particularly around massive galaxies. This has led astronomers to believe that compact ellipticals might form via the tidal stripping of a once-large galaxy in interactions with another, massive galaxy. In this model, once the original galaxys outer layers are stripped away, the compact inner bulge component would be left behind as a compact elliptical galaxy. Recent discoveries of a few isolated compact ellipticals, however, have strained this model.Now a new galaxy has been found to confuse our classification schemes: the first-ever compact elliptical to also display signs of an active galactic nucleus. Led by Sanjaya Paudel (Korea Astronomy and Space Science Institute), a team of scientists discovered SDSS J085431.18+173730.5 serendipitously in Sloan Digital Sky Survey data. The team used SDSS images and spectroscopy in combination with data from the Canada-France-Hawaii Telescope to learn more about this unique galaxy.Puzzling CharacteristicsSDSS J085431.18+173730.5 presents an interesting conundrum. Ancient compact ellipticals are supposed to be devoid of gas, with no fuel left to trigger nuclear activity. Yet SDSS J085431.18+173730.5 clearly shows the emission lines that indicate active accretion onto

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

    International Nuclear Information System (INIS)

    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 H I/M *, 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.

  16. 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.

  17. 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 atmosphere white dwarf stars (DAs) and helium atmosphere white 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 ultra-compact helium binary (AM...

  18. Radio Observations of Super Star Clusters in Dwarf Starburst Galaxies

    CERN Document Server

    Stevens, I R; Norris, R P; Stevens, Ian R.; Forbes, Duncan A.; Norris, Ray P.

    2002-01-01

    We present new radio continuum observations of two dwarf starburst galaxies, NGC3125 and NGC5408, with observations at 4.80GHz (6cm) and 8.64GHz (3cm), taken with the Australia Telescope Compact Array (ATCA). Both galaxies show a complex radio morphology with several emission regions, mostly coincident with massive young star clusters. The radio spectral indices of these regions are negative (with alpha ~ -0.5 - -0.7), indicating that the radio emission is dominated by synchrotron emission associated with supernova activity from the starburst. One emission region in NGC5408 has a flatter index (alpha ~ -0.1) indicative of optically thin free-free emission, which could indicate it is a younger cluster. Consequently, in these galaxies we do not see regions with the characteristic positive spectral index indicative of optically obscured star-formation regions, as seen in other dwarf starbursts such as Hen 2-10.

  19. About Exobiology: The Case for Dwarf K Stars

    Science.gov (United States)

    Cuntz, M.; Guinan, E. F.

    2016-08-01

    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.

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

    Science.gov (United States)

    Kepler, S. O.; Pelisoli, I.; Koester, D.; Ourique, G.; Romero, A. D.; Reindl, N.; Kleinman, S. J.; Eisenstein, D. J.; Valois, A. D. M.; Amaral, L. A.

    2016-02-01

    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 atmosphere white dwarf stars (DAs) and helium atmosphere white 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 CVn), one oxygen line-dominated white dwarf, 15 hot DO/PG1159s, 12 new cataclysmic variables, 36 magnetic white dwarf stars, 54 DQs, 115 helium-dominated white dwarfs, 148 white dwarf + main-sequence star binaries, 236 metal-polluted white dwarfs, 300 continuum spectra DCs, 230 hot subdwarfs, 2936 new hydrogen-dominated white dwarf stars, and 2675 cool hydrogen-dominated subdwarf stars. We calculate the mass distribution of all 5883 DAs with S/N ≥ 15 in DR12, including the ones in DR7 and DR10, with an average S/N = 26, corrected to the 3D convection scale, and also the distribution after correcting for the observed volume, using 1/Vmax.

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

    CERN Document Server

    Kepler, S O; 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 atmosphere white dwarf stars (DAs) and helium atmosphere white 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 ultra-compact helium binary (AM CVn), one oxygen line dominated white dwarf, 15 hot DO/PG1159s, 12 new cataclysmic variables, 36 magnetic white dwarf stars, 54 DQs, 115 helium dominated white dwarfs, 148 white dwarf+main sequence star binaries, 236 metal polluted white dwarfs, 300 continuum spectra DCs, 230 hot subdwarfs, 2936 new hydrogen dominated white dwarf stars, and 2675 cool hydrogen dominated subdwarf stars. We calculate the mass distribution of all 5883 DAs with S/N>15 in DR12, including the ones in DR7 and DR10, with an average S/N=26, correc...

  2. Super Star Clusters in the Blue Dwarf Galaxy UM 462

    OpenAIRE

    Vanzi, Leonardo

    2003-01-01

    I present optical observations of the Blue Compact Dwarf Galaxy UM 462. The images of this galaxy show several bright compact sources. A careful study of these sources has revealed their nature of young Super Star Clusters. The ages determined from the analysis of the stellar continuum and $H\\alpha$ are between few and few tens Myr. The total star formation taking place into the clusters is about 0.05 $\\mathrm{M_{\\odot}/yr}$. The clusters seem to be located at the edges of two large round-lik...

  3. Star formation around supermassive black holes.

    Science.gov (United States)

    Bonnell, I A; Rice, W K M

    2008-08-22

    The presence of young massive stars orbiting on eccentric rings within a few tenths of a parsec of the supermassive black hole in the galactic center is challenging for theories of star formation. The high tidal shear from the black hole should tear apart the molecular clouds that form stars elsewhere in the Galaxy, and transport of stars to the galactic center also appears unlikely during their lifetimes. We conducted numerical simulations of the infall of a giant molecular cloud that interacts with the black hole. The transfer of energy during closest approach allows part of the cloud to become bound to the black hole, forming an eccentric disk that quickly fragments to form stars. Compressional heating due to the black hole raises the temperature of the gas up to several hundred to several thousand kelvin, ensuring that the fragmentation produces relatively high stellar masses. These stars retain the eccentricity of the disk and, for a sufficiently massive initial cloud, produce an extremely top-heavy distribution of stellar masses. This potentially repetitive process may explain the presence of multiple eccentric rings of young stars in the presence of a supermassive black hole. PMID:18719276

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

    CERN Document Server

    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.

  5. Star Formation at Low Metallicity in Local Dwarf Irregular Galaxies

    Science.gov (United States)

    Elmegreen, Bruce; Hunter, Deidre Ann; Rubio, Monica; Brinks, Elias; Cortés, Juan R.; Cigan, Phil

    2016-01-01

    The radial profiles of star formation rates and surface mass densities for gas and stars have been compiled for 20 local dwarf irregular galaxies and converted into disk scale heights and Toomre Q values. The scale heights are relatively large compared to the galaxy sizes (~0.6 times the local radii) and generally increase with radius in a flare. The gaseous Q values are high, ~4, at most radii and even higher for the stars. Star formation proceeds even with these high Q values in a normal exponential disk as viewed in the far ultraviolet. Such normal star formation suggests that Q is not relevant to star formation in dIrrs. The star formation rate per unit area always equals approximately the gas surface density divided by the midplane free fall time with an efficiency factor of about 1% that decreases systematically with radius in approximate proportion to the gas surface density. We view this efficiency variation as a result of a changing molecular fraction in a disk where atomic gas dominates both stars and molecules. In a related study, CO observations with ALMA of star-forming regions at the low metallicities of these dwarfs, which averages 13% solar, shows, in the case of the WLM galaxy, tiny CO clouds inside much larger molecular and atomic hydrogen envelopes. The CO cloud mass fraction within the molecular region is only one percent or so. Nevertheless, the CO clouds have properties that are similar to solar neighborhood clouds: they satisfy the size-linewidth relation observed in the LMC, SMC, and other local dwarfs where CO has been observed, and the same virial mass versus luminosity relation. This uniforming of CO cloud properties seems to be the result of a confining pressure from the weight of the overlying molecular and atomic shielding layers. Star formation at low metallicity therefore appears to be a three dimensional process independent of 2D instabilities involving Q, in highly atomic gas with relatively small CO cores, activated at a rate

  6. Black Hole - Neutron Star Binary Mergers

    Data.gov (United States)

    National Aeronautics and Space Administration — Gravitational radiation waveforms for black hole-neutron star coalescence calculations. The physical input is Newtonian physics, an ideal gas equation of state with...

  7. Atmospheric activity in red dwarf stars

    International Nuclear Information System (INIS)

    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

  8. In what sense a neutron star-black hole binary is the holy grail for testing gravity?

    CERN Document Server

    Bagchi, Manjari

    2014-01-01

    Pulsars in binary systems have been very successful to test the validity of general relativity in the strong field regime. So far, such binaries include neutron star-white dwarf (NS-WD) and neutron star-neutron star (NS-NS) systems. It is commonly believed that a neutron star-black hole (NS-BH) binary will be much superior for this purpose. But in what sense is this true? Does it apply to all possible deviations?

  9. In what sense a neutron star-black hole binary is the holy grail for testing gravity?

    Energy Technology Data Exchange (ETDEWEB)

    Bagchi, Manjari [International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560012 (India); Torres, Diego F., E-mail: manjari.bagchi@icts.res.in, E-mail: dtorres@ieec.uab.es [ICREA and Institute of Space Sciences, Barcelona 2a Planta E-08193 (Spain)

    2014-08-01

    Pulsars in binary systems have been very successful to test the validity of general relativity in the strong field regime [1-4]. So far, such binaries include neutron star-white dwarf (NS-WD) and neutron star-neutron star (NS-NS) systems. It is commonly believed that a neutron star-black hole (NS-BH) binary will be much superior for this purpose. But in what sense is this true? Does it apply to all possible deviations?.

  10. In what sense a neutron star-black hole binary is the holy grail for testing gravity?

    International Nuclear Information System (INIS)

    Pulsars in binary systems have been very successful to test the validity of general relativity in the strong field regime [1-4]. So far, such binaries include neutron star-white dwarf (NS-WD) and neutron star-neutron star (NS-NS) systems. It is commonly believed that a neutron star-black hole (NS-BH) binary will be much superior for this purpose. But in what sense is this true? Does it apply to all possible deviations?

  11. New white dwarf stars in the Sloan Digital Sky Survey Data Release 10

    OpenAIRE

    Kepler, S.O.; Pelisoli, Ingrid; Koester, Detlev; Ourique, Gustavo; Kleinman, Scot J.; Romero, Alejandra Daniela; Nitta, Atsuko; Eisenstein, Daniel J.; Costa, José Eduardo da Silveira; Külebi, Baybars; Jordan, Stefan; Dufour, Patrick; Giommi, Paolo; Rebassa-Mansergas, Alberto

    2014-01-01

    We report the discovery of 9 089 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmos...

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

    Science.gov (United States)

    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.

  13. Carbon Star Survey of Local Group Dwarf Galaxies. II. Pegasus, DDO 210 and Tucana

    CERN Document Server

    Battinelli, P

    2000-01-01

    We present the latest results of our ongoing four filter photometric survey of C stars in Local Group dwarf irregular galaxies. Observations of the two low luminosity dwarf irregular galaxies, Pegasus and DDO 210, revealed respectively 40 and 3 C stars, assuming that the reddening of Pegasus is negligible. No C stars were identified in Tucana. Our observations permit the estimation of the CMD contamination by foreground M dwarf thus yielding reliable C/M ratios. Our R, I photometry of the C stars cannot be used to solve the extinction controversy toward Pegasus. The three C stars in DDO 210 are quite bright when compared to C star populations in dwarf galaxies. A larger fainter population in that galaxy seems however improbable. The statistics of C stars, currently on hand for dwarf galaxies, show a well-defined trend with the absolute magnitude of galaxies.

  14. Are nuclear star clusters the precursors of massive black holes?

    CERN Document Server

    Neumayer, Nadine

    2012-01-01

    We present new upper limits for black hole masses in extremely late type spiral galaxies. We confirm that this class of galaxies has black holes with masses less than 10^6 Msolar, if any. We also derive new upper limits for nuclear star cluster (NC) masses in massive galaxies with previously determined black hole masses. We use the newly derived upper limits and a literature compilation to study the low mass end of the global-to-nucleus relations. We find the following (1) The M_BH-sigma relation cannot flatten at low masses, but may steepen. (2) The M_BH-M_bulge relation may well flatten in contrast. (3) The M_BH-Sersic n relation is able to account for the large scatter in black hole masses in low-mass disk galaxies. Outliers in the M_BH-Sersic n relation seem to be dwarf elliptical galaxies. When plotting M_BH versus M_NC we find three different regimes: (a) nuclear cluster dominated nuclei, (b) a transition region, and (c) black hole-dominated nuclei. This is consistent with the picture, in which black ho...

  15. Gravitational waves from the collision of tidally disrupted stars with massive black holes

    International Nuclear Information System (INIS)

    We use simulations of hydrodynamics coupled with full general relativity to investigate the gravitational waves produced by a star colliding with a massive black hole when the star's tidal disruption radius lies far outside of the black hole horizon. We consider both main-sequence and white-dwarf compaction stars, and nonspinning black holes, as well as those with near-extremal spin. We study the regime in between where the star can be accurately modeled by a point particle, and where tidal effects completely suppress the gravitational wave signal. We find that nonnegligible gravitational waves can be produced even when the star is strongly affected by tidal forces, as well as when it collides with large angular momentum. We discuss the implications that these results have for the potential observation of gravitational waves from these sources with future detectors.

  16. Accretion-induced variability links young stellar objects, white dwarfs, and black holes.

    Science.gov (United States)

    Scaringi, Simone; Maccarone, Thomas J; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R; Aranzana, Ester; Dhillon, Vikram S; Barros, Susana C C

    2015-10-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies. PMID:26601307

  17. Accretion-induced variability links young stellar objects, white dwarfs, and black holes

    CERN Document Server

    Scaringi, S; Koerding, E; Knigge, C; Vaughan, S; Marsh, T R; Aranzana, E; Dhillon, V; Barros, S C C

    2015-01-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies.

  18. Distribution of star-forming complexes in dwarf irregular galaxies

    CERN Document Server

    Parodi, B R

    2003-01-01

    We study the distribution of bright star-forming complexes in a homogeneous sample of 72 late-type (``irregular'') dwarf galaxies located within the 10 Mpc volume. Star-forming complexes are identified as bright lumps in B-band galaxy images and isolated by means of the unsharp-masking method. For the sample as a whole the radial number distribution of bright lumps largely traces the underlying exponential-disk light profiles, but peaks at a 10 percent smaller scale length. Moreover, the presence of a tail of star forming regions out to at least six optical scale lengths provides evidence against a systematic star formation truncation within that galaxy extension. Considering these findings, we apply a scale length-independent concentration index, taking into account the implied non-uniform random spread of star formation regions throughout the disk. The number profiles frequently manifest a second, minor peak at about two scale lengths. Relying on a two-dimensional stochastic self-propagating star formation ...

  19. Black holes and stars in Horndeski theory

    CERN Document Server

    Babichev, Eugeny; Lehébel, Antoine

    2016-01-01

    We review black hole and star solutions for Horndeski theory. For non-shift symmetric theories, black holes involve a Kaluza-Klein reduction of higher dimensional Lovelock solutions. On the other hand, for shift symmetric theories of Horndeski and beyond Horndeski, black holes involve two classes of solutions: those that include, at the level of the action, a linear coupling to the Gauss-Bonnet term and those that involve time dependence in the galileon field. We analyze the latter class in detail for a specific subclass of Horndeski theory, discussing the general solution of a static and spherically symmetric spacetime. We then discuss stability issues, slowly rotating solutions as well as black holes coupled to matter. The latter case involves a conformally coupled scalar field as well as an electromagnetic field and the (primary) hair black holes thus obtained. We review and discuss the recent results on neutron stars in Horndeski theories.

  20. Black holes and stars in Horndeski theory

    Science.gov (United States)

    Babichev, Eugeny; Charmousis, Christos; Lehébel, Antoine

    2016-08-01

    We review black hole and star solutions for Horndeski theory. For non-shift symmetric theories, black holes involve a Kaluza–Klein reduction of higher dimensional Lovelock solutions. On the other hand, for shift symmetric theories of Horndeski and beyond Horndeski, black holes involve two classes of solutions: those that include, at the level of the action, a linear coupling to the Gauss–Bonnet term and those that involve time dependence in the galileon field. We analyze the latter class in detail for a specific subclass of Horndeski theory, discussing the general solution of a static and spherically symmetric spacetime. We then discuss stability issues, slowly rotating solutions as well as black holes coupled to matter. The latter case involves a conformally coupled scalar field as well as an electromagnetic field and the (primary) hair black holes thus obtained. We review and discuss the recent results on neutron stars in Horndeski theories.

  1. 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 (<0.7 arcsec). With these data we examine the spatially resolved kinematics and star formation history of this unusual object. We find no evidence of a rise in the central velocity dispersion of the UCD, suggesting that this UCD lacks a massive central black hole like those found in some other compact stellar systems, a conclusion confirmed by detailed dynamical modelling. Finally, we are able to use our extremely high signal-to-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.

  2. The Death of a Star

    Science.gov (United States)

    Thorne, Kip S.

    1971-01-01

    Theories associated with the gravitational collapse of a star into black holes" are described. Suggests that the collapse and compression might go through the stages from white dwarf star to neutron core to black hole." (TS)

  3. White Dwarfs Near Black Holes: A New Paradigm for Type I Supernovae

    OpenAIRE

    Wilson, J.R.; Mathews, G. J.

    2003-01-01

    We present calculations indicating the possibility of a new class of Type I supernovae. In this new paradigm relativistic terms enhance the self gravity of a carbon-oxygen white dwarf as it passes or orbits near a black hole. This relativistic compression can cause the central density to exceed the threshold for pycnonuclear reactions so that athermonuclear runaway ensues. We consider three possible environments: 1) white dwarfs orbiting a low-mass black hole; 2) white dwarfs encountering a m...

  4. Tidal disruption of white dwarfs by intermediate mass black holes

    Directory of Open Access Journals (Sweden)

    Bode T.

    2012-12-01

    Full Text Available Modeling ultra-close encounters between a white dwarf and a spinning, intermediate mass black hole requires a full general relativistic treatment of gravity. This paper summarizes results from such a study. Our results show that the disruption process and prompt accretion of the debris strongly depend on the magnitude and orientation of the black hole spin. On the other hand, the late-time accretion onto the black hole follows the same decay, Ṁ ∝  t−5/3, estimated from Newtonian gravity disruption studies. The spectrum of the fallback material peaks in the soft X-rays and sustains Eddington luminosity for 1–3 yrs after the disruption. The orientation of the black hole spin has also a profound effect on how the outflowing debris obscures the central region. The disruption produces a burst of gravitational radiation with characteristic frequencies of ∼3.2 Hz and strain amplitudes of ∼10−18 for galactic intermediate mass black holes.

  5. Search for carbon stars and DZ white dwarfs in SDSS spectra survey through machine learning

    Science.gov (United States)

    Si, JianMin; Luo, ALi; Li, YinBi; Zhang, JianNan; Wei, Peng; Wu, YiHong; Wu, FuChao; Zhao, YongHeng

    2014-01-01

    Carbon stars and DZ white dwarfs are two types of rare objects in the Galaxy. In this paper, we have applied the label propagation algorithm to search for these two types of stars from Data Release Eight (DR8) of the Sloan Digital Sky Survey (SDSS), which is verified to be efficient by calculating precision and recall. From nearly two million spectra including stars, galaxies and QSOs, we have found 260 new carbon stars in which 96 stars have been identified as dwarfs and 7 identified as giants, and 11 composition spectrum systems (each of them consists of a white dwarf and a carbon star). Similarly, using the label propagation method, we have obtained 29 new DZ white dwarfs from SDSS DR8. Compared with PCA reconstructed spectra, the 29 findings are typical DZ white dwarfs. We have also investigated their proper motions by comparing them with proper motion distribution of 9,374 white dwarfs, and found that they satisfy the current observed white dwarfs by SDSS generally have large proper motions. In addition, we have estimated their effective temperatures by fitting the polynomial relationship between effective temperature and g-r color of known DZ white dwarfs, and found 12 of the 29 new DZ white dwarfs are cool, in which nine are between 6,000 K and 6,600 K, and three are below 6,000 K.

  6. Calibrating UV Star Formation Rates for Dwarf Galaxies from STARBIRDS

    Science.gov (United States)

    McQuinn, Kristen B. W.; Skillman, Evan D.; Dolphin, Andrew E.; Mitchell, Noah P.

    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. Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA), and the Canadian Astronomy Data Centre (CADC/NRC/CSA).

  7. Double-mode RR Lyrae stars in the Draco dwarf galaxy and in other stellar systems

    International Nuclear Information System (INIS)

    Eleven double-mode (dm) RR Lyrae stars, with periods midway between the periods for the c-type and ab-type RR Lyrae stars, have been identified in the Draco dwarf galaxy by reanalyzing the photometry of Baade and Swope (1961) for 35 stars. The stars are V11, 72, 75, 83, 112, 138, 143, 156, 165, 169 and 190. (Auth.)

  8. Stability of relativistic stars and black holes

    International Nuclear Information System (INIS)

    Relativistic stability theory is reviewed, with an emphasis on work done over the last decade. Instabilities of uniformly rotating stars are of two types: instability to axisymmetric perturbations - to collapse, which can be diagnosed by a turning point method; and instability to non-axisymmetric perturbations, which requires a more detailed knowledge of the normal modes of viscous stars. Black holes are almost certainly stable, and recent work by Whiting appears to prove that there are no unstable normal modes. (author)

  9. CEPHEID VARIABLE STARS IN THE PEGASUS DWARF IRREGULAR GALAXY: CONSTRAINTS ON THE STAR FORMATION HISTORY

    International Nuclear Information System (INIS)

    Observations of the resolved stars obtained over a period of 11 years in the Local Group dwarf irregular galaxy Pegasus have been used to search for Cepheid variable stars. Images were obtained in 55 epochs in the V band and in 24 epochs in the I band. We have identified 26 Cepheids and have obtained their light curves and periods. On the basis of their position in the period-luminosity (PL) diagram, we have classified them as 18 fundamental modes and eight first overtone Cepheids. Two PL relations for Cepheids have been used to derive the distance, resulting in 1.07 ± 0.05 Mpc. We present the VARFINDER code which finds the variable stars and their predicted periods in a given synthetic color-magnitude diagram computed with IAC-star and we propose the use of the Cepheid population as a constraint of the star formation history of Pegasus.

  10. About Exobiology: The Case for Dwarf K Stars

    CERN Document Server

    Cuntz, M

    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 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 t...

  11. First stars VII. Lithium in extremely metal poor dwarfs

    CERN Document Server

    Bonifacio, P; Sivarani, T; Cayrel, R; Spite, M; Spite, F; Plez, B; Andersen, J; Barbuy, B; Beers, T C; Depagne, E; Hill, V; François, P; Nordström, B; Primas, F

    2006-01-01

    Aims. This study aims to determine the level and constancy of the Spite plateau as definitively as possible from homogeneous high-quality VLT-UVES spectra of 19 of the most metal-poor dwarf stars known. Methods. Our high-resolution (R ~ 43000), high S/N spectra are analysed with OSMARCS 1D LTE model atmospheres and turbospectrum synthetic spectra to determine effective temperatures, surface gravities, and metallicities as well as Li abundances for our stars. Results. Eliminating a cool subgiant and a spectroscopic binary, we find 8 stars to have -3.5 < [Fe/H] < -3.0 and 9 stars with -3.0 < [Fe/H] < -2.5. Our best value for the mean level of the plateau is A(Li) =2.10 +- 0.09. The scatter around the mean is entirely explained by our estimate of the observational error and does not allow for any intrinsic scatter in the Li abundances. In addition, we conclude that a systematic error of the order of 200 K in any of the current temperature scales remains possible. The iron excitation equilibria in our...

  12. Black holes and stars in Horndeski theory

    OpenAIRE

    Babichev, Eugeny; Charmousis, Christos; Lehébel, Antoine

    2016-01-01

    We review black hole and star solutions for Horndeski theory. For non-shift symmetric theories, black holes involve a Kaluza-Klein reduction of higher dimensional Lovelock solutions. On the other hand, for shift symmetric theories of Horndeski and beyond Horndeski, black holes involve two classes of solutions: those that include, at the level of the action, a linear coupling to the Gauss-Bonnet term and those that involve time dependence in the galileon field. We analyze the latter class in d...

  13. ULXs: Neutron Stars vs Black Holes

    CERN Document Server

    King, Andrew

    2016-01-01

    We consider ultraluminous X-ray sources (ULXs) where the accretor is a neutron star rather than a black hole. We show that the recently-discovered example (M82 X-2) fits naturally into the simple picture of ULXs as beamed X-ray sources fed at super-Eddington rates, provided that its magnetic field is weaker ($\\simeq 10^{11}{\\rm G}$) than a new-born X-ray pulsar, as expected if there has been mass gain. Continuing accretion is likely to weaken the field to the point that pulsing stops, and make the system indistinguishable from a ULX containing a black hole. Accordingly we suggest that a significant fraction of all ULXs may actually contain neutron star accretors rather than black holes, reflecting the neutron-star fraction among their X-ray binary progenitors. We emphasize that neutron-star ULXs are likely to have {\\it higher} apparent luminosities than black hole ULXs for a given mass transfer rate, as their tighter beaming outweighs their lower Eddington luminosities. This further increases the likely propo...

  14. 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.

  15. PULSATING VARIABLE STARS IN THE COMA BERENICES DWARF SPHEROIDAL GALAXY

    International Nuclear Information System (INIS)

    We present B, V, I time-series photometry of the Coma Berenices dwarf spheroidal galaxy, a faint Milky Way (MW) satellite, recently discovered by the Sloan Digital Sky Survey. We have obtained V, B - V and V, V - I color-magnitude diagrams that reach V ∼ 23.0-23.2 mag showing the galaxy turnoff at V ∼ 21.7 mag, and have performed the first study of the variable star population of this new MW companion. Two RR Lyrae stars (a fundamental-mode, RRab, and a first overtone, RRc, pulsator) and a short period variable with period P = 0.12468 days were identified in the galaxy. The RRab star has a rather long period of P ab = 0.66971 days and is about 0.2 mag brighter than the RRc variable and other nonvariable stars on the galaxy horizontal branch (HB). In the period-amplitude diagram, the RRab variable falls closer to the loci of Oosterhoff type-II systems and evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3. The average apparent magnitude of the galaxy HB, (V HB) = 18.64 ± 0.04 mag, leads to a distance modulus for the Coma dSph μ0 = 18.13 ± 0.08 mag, corresponding to a distance d = 42+2-1 kpc, by adopting a reddening E(B - V) = 0.045 ± 0.015 mag and a metallicity [Fe/H] =-2.53 ± 0.05 dex.

  16. Ruthenium and hafnium abundances in giant and dwarf barium stars

    CERN Document Server

    Allen, D M

    2007-01-01

    We present abundances for Ru and Hf, compare them to abundances of other heavy elements, and discuss the problems found in determining Ru and Hf abundances with laboratory gf-values in the spectra of barium stars. We determined Ru and Hf abundances in a sample of giant and dwarf barium stars, by the spectral synthesis of two RuI (4080.574A and 4757.856A) and two HfII (4080.437A and 4093.155A) transitions. The stellar spectra were observed with FEROS/ESO, and the stellar atmospheric parameters lie in the range 4300 < Teff/K < 6500, -1.2 < [Fe/H] <= 0 and 1.4 <= log g < 4.6. The HfII 4080A and the RuI 4758A observed transitions result in a unreasonably high solar abundance, given certain known uncertainties, when fitted with laboratory gf-values. For these two transitions we determined empirical gf-values by fitting the observed line profiles of the spectra of the Sun and Arcturus. For the sample stars, this procedure resulted in a good agreement of Ru and Hf abundances given by the two availa...

  17. 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.

  18. Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars

    OpenAIRE

    Jeffery, C. Simon; Karakas, Amanda I.; Saio, Hideyuki

    2011-01-01

    The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB) stars are discussed in terms of the merger of a carbon-oxygen white dwarf with a helium white dwarf. The model is expressed as a linear mixture of the individual layers of both constituent white dwarfs, taking account of the specific evolution of each star. In developing this recipe from previous versions, particular attention has been given to the inter-shell abundances of the asymptotic giant branch star which evolv...

  19. Carbon-to-Oxygen Ratios in M dwarfs and Solar-type stars

    CERN Document Server

    Nakajima, Tadashi

    2016-01-01

    Initial C/O ratios of circumstellar disks have been obtained in solar-type stars by differential photospheric abundance analysis with respect to the Sun. We present C/O ratios of M dwarfs obtained by absolute abundance analysis derived from CO and H2O spectra in the K-band. We compare the distributions of C/O ratios in M dwarfs and solar-type stars using the Kolmogorov-Smirnov test. The C/O distribution of M dwarfs is consistent with low frequency of high C/O ratios in solar-type stars.

  20. A Second Stellar Color Locus: a Bridge from White Dwarfs to M stars

    CERN Document Server

    Smolcic, V; Knapp, G R; Lupton, R H; Pavlovski, K; Ilijic, S; Schlegel, D J; Smith, J A; McGehee, P M; Silvestri, N M; Hawley, S L; Rockosi, C M; Gunn, J E; Strauss, M A; Fan, X; Eisenstein, D J; Harris, H

    2004-01-01

    We report the discovery of a locus of stars in the SDSS g-r vs. u-g color-color diagram that connects the colors of white dwarfs and M dwarfs. While its contrast with respect to the main stellar locus is only ~1:2300, this previously unrecognized feature includes 863 stars from the SDSS Data Release 1. The position and shape of the feature are in good agreement with predictions of a simple binary star model that consists of a white dwarf and an M dwarf, with the components' luminosity ratio controlling the position along this binary system locus. SDSS DR1 spectra for 47 of these objects strongly support this model. The absolute magnitude--color distribution inferred for the white dwarf component is in good agreement with the models of Bergeron et al. (1995).

  1. Stellar evolution on the borderline of white dwarf and neutron star formation

    NARCIS (Netherlands)

    Poelarends, A.J.T.

    2007-01-01

    This thesis is about the evolution of stars, specifically about the final fate of stars at the borderline between the formation of white dwarfs and neutron stars. It is well known that the mass and the metallicity are the two determining factors in stellar evolution, and for a given initial chemical

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

    CERN Document Server

    Pasetto, S; Fujita, Y; Chiosi, C; Grebel, E K

    2014-01-01

    In this paper we develop a simple analytical criterion to investigate the role of the environment on the onset of star formation. We will consider the main external agents that influence the star formation (i.e. ram pressure, tidal interaction, Rayleigh-Taylor and Kelvin-Helmholtz instabilities) in a spherical galaxy moving through an external environment. The theoretical framework developed here 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. We develop an analytic formalism to solve the fluid dynamics equations in a non-inertial reference frame mapped with spherical coordinates. 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. The solution presented here is quite general, allowing us to investi...

  3. The imprint of reionization on the star formation histories of dwarf galaxies

    Science.gov (United States)

    Benítez-Llambay, A.; Navarro, J. F.; Abadi, M. G.; Gottlöber, S.; Yepes, G.; Hoffman, Y.; Steinmetz, M.

    2015-07-01

    We use a compilation of star formation histories (SFHs) and cosmological simulations to explore the impact of cosmic reionization on nearby isolated dwarf galaxies. Nearby dwarfs show a wide diversity of SFHs; from ancient systems that completed their star formation (SF) ˜10 Gyr ago to young dwarfs that formed the majority of their stars in the past ˜5 Gyr to `two-component' systems characterized by the overlap of old and young stars. As an ensemble, SF in nearby dwarfs dips to lower-than-average rates at intermediate times (4 < t/Gyr < 8), a feature caused in the simulation by cosmic reionization. Reionization heats the gas and drives it out of low-mass haloes, affecting especially systems with virial temperatures of ˜2 × 104 K at zreion. SF begins before zreion in systems above this threshold; its associated feedback compounds the effects of reionization, emptying the haloes of gas and leaving behind old stellar systems. In haloes below the threshold at zreion, reionization leads to a delay in the onset of SF that lasts until the halo grows massive enough to allow gas to cool and form stars, leading to a system with a prominent young stellar component. `Two-component' systems may be traced to late accretion events that allow young stars to form in systems slightly above the threshold at zreion. The dearth of intermediate-age stars in nearby dwarfs might be the clearest signature of the imprint of cosmic reionization on the SFHs of dwarf galaxies.

  4. Superradiant amplification by stars and black holes

    CERN Document Server

    Rosa, João Luís de Figueiredo

    2016-01-01

    In this thesis we study the phenomenon of superradiance and its implications to the stability of black-holes (BH) and perfect-fluid stars. Superradiance is a radiation enhancement process that involves rotating dissipative systems. In BH spacetimes, superradiance is due to dissipation at the event horizon, with interesting associated phenomena, namely floating orbits and BH-bombs. BH superradiance is a very interdisciplinary topic, and its study allows us to obtain important results in the area of particle physics. The scattering of a scalar field by a rotating BH leads to the formation of quasi-boundstates. In rotational systems, these states can give rise to superradiant instabilities. These results were recently used to impose constraints to the mass of fundamental particles and darkmatter candidates. In this work, it is shown that, when dissipation is properly included, similar results are achievable in self-gravitating systems other than black-holes, such as perfect fluid stars. It is also demonstrated t...

  5. Electromagnetic jets from stars and black holes

    Science.gov (United States)

    Gralla, Samuel E.; Lupsasca, Alexandru; Rodriguez, Maria J.

    2016-02-01

    We present analytic force-free solutions modeling rotating stars and black holes immersed in the magnetic field of a thin disk that terminates at an inner radius. The solutions are exact in flat spacetime and approximate in Kerr spacetime. The compact object produces a conical jet whose properties carry information about its nature. For example, the jet from a star is surrounded by a current sheet, while that of a black hole is smooth. We compute an effective resistance in each case and compare to the canonical values used in circuit models of energy extraction. These solutions illustrate all of the basic features of the Blandford-Znajek process for energy extraction and jet formation in a clean setting.

  6. Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxies

    Science.gov (United States)

    Hunt, Leslie K.; Madden, Suzanne C.; Schneider, Raffaella

    2008-12-01

    'Shea and Michael L. Norman; 16. Damped Lyα systems as probes of chemical evolution over cosmological timescales Miroslava Dessauges-Zavadsky; 17. Connecting high-redshift galaxy populations through observations of local damped Lyman alpha dwarf galaxies Regina E. Schulte-Ladbeck; 18. Chemical enrichment and feedback in low metallicity environments: constraints on galaxy formation Francesca Matteucci; 19. Effects of reionization on dwarf galaxy formation Massimo Ricotti; 20. The importance of following the evolution of the dust in galaxies on their SEDs A. Schurer, F. Calura, L. Silva, A. Pipino, G. L. Granato, F. Matteucci and R. Maiolino; 21. About the chemical evolution of dSphs (and the peculiar globular cluster ωCen) Andrea Marcolini and Annibale D'Ercole; 22. Young star clusters in the small Magellanic cloud: impact of local and global conditions on star formation Elena Sabbi, Linda J. Smith, Lynn R. Carlson, Antonella Nota, Monca Tosi, Michele Cignoni, Jay S. Gallagher III, Marco Sirianni and Margaret Meixner; 23. Modeling the ISM properties of metal-poor galaxies and gamma-ray burst hosts Emily M. Levesque, Lisa J. Kewley, Kirsten Larson and Leonie Snijders; 24. Dwarf galaxies and the magnetisation of the IGM Uli Klein; Session III. Explosive Events in Low-Metallicity Environments: 25. Supernovae and their evolution in a low metallicity ISM Roger A. Chevalier; 26. First stars - type Ib supernovae connection Ken'ichi Nomoto, Masaomi Tanaka, Yasuomi Kamiya, Nozomu Tominaga and Keiichi Maeda; 27. Supernova nucleosynthesis in the early universe Nozomu Tominaga, Hideyuki Umeda, Keiichi Maeda, Ken'ichi Nomoto and Nobuyuki Iwamoto; 28. Powerful explosions at Z = 0? Sylvia Ekström, Georges Meynet, Raphael Hirschi and André Maeder; 29. Wind anisotropy and stellar evolution Cyril Georgy, Georges Meynet and André Maeder; 30. Low-mass and metal-poor gamma-ray burst

  7. Boson stars, neutron stars and black holes in five dimensions

    CERN Document Server

    Brihaye, Y

    2016-01-01

    Different types of gravitating compact objects occuring in d=5 space-time are considered: boson stars, hairy black holes and perfect fluid solutions. All these solutions of the Einstein equations coupled to matter have well established counterparts in d=4; in particular neutron stars can be modell{\\S}ed more or less realistically by a perfect fluid. A special emphasis is set on the possibility -and/or the necessity- for these solutions to have an intrinsic angular momentum or spin. The influence of a cosmological constant on their pattern is also studied. Several physical properties are presented from which common features to boson and neutron stars clearly emerge. We finally point out qualitative differences of the gravitational interaction supporting these classical lumps between four and five dimensions.

  8. Can Red Dwarf stars support Earth-like vegetation?

    Science.gov (United States)

    Gale, Joseph; Wandel, Amri

    2016-07-01

    The Kepler mission has shown that Earthlike planets are common. Of particular interest in our search for extra-solar-system, life-clement conditions, are planets orbiting Red Dwarf (RD) stars, the most numerous stellar type in the Milky Way galaxy. Early considerations indicated that conditions on RD planets would be inimical to life, as their Habitable Zones would be so close as to make planets tidally locked to their star. This was expected to engender tempestuous climates and to expose life forms to flares of ionizing electro-magnetic radiation and charged particles. Moreover, the less photon energy of the radiation of the relatively cool RDs would be too low in the 300-700nm waveband required for Oxygenic Photosynthesis (OP). Recent calculations show that these negative factors are less severe than originally estimated. Many authors have suggested that OP may evolve on RP planets to utilize infrared photons in the 700-1000nm waveband. However, projecting from OP and the vegetation in analogous regions on Earth, we argue that the evolutionary pressure to do so would be small. On RD planets there will be regions receiving continuous illumination, of moderate intensity, containing a significant component of photosynthetic 400-700nm radiation. On Earth, OP has been an essential factor in producing the Biosphere environment that enabled the appearance and evolution of complex life. We conclude that the conditions for OP could exist on RD planets and consequently the evolution of vegetation and complex life is possible (albeit not necessary). Furthermore, the huge number of RDs and their long lifetimes, make advanced vegetation, OP and consequently complex life on RD planets probable, and statistically more likely than on planets of solar type stars.

  9. Bursting with Stars and Black Holes

    Science.gov (United States)

    2007-01-01

    A growing black hole, called a quasar, can be seen at the center of a faraway galaxy in this artist's concept. Astronomers using NASA's Spitzer and Chandra space telescopes discovered swarms of similar quasars hiding in dusty galaxies in the distant universe. The quasar is the orange object at the center of the large, irregular-shaped galaxy. It consists of a dusty, doughnut-shaped cloud of gas and dust that feeds a central supermassive black hole. As the black hole feeds, the gas and dust heat up and spray out X-rays, as illustrated by the white rays. Beyond the quasar, stars can be seen forming in clumps throughout the galaxy. Other similar galaxies hosting quasars are visible in the background. The newfound quasars belong to a long-lost population that had been theorized to be buried inside dusty, distant galaxies, but were never actually seen. While some quasars are easy to detect because they are oriented in such a way that their X-rays point toward Earth, others are oriented with their surrounding doughnut-clouds blocking the X-rays from our point of view. In addition, dust and gas in the galaxy itself can block the X-rays. Astronomers had observed the most energetic of this dusty, or obscured, bunch before, but the 'masses,' or more typical members of the population, remained missing. Using data from Spitzer and Chandra, the scientists uncovered many of these lost quasars in the bellies of massive galaxies between 9 and 11 billion light-years away. Because the galaxies were also busy making stars, the scientists now believe most massive galaxies spent their adolescence building up their stars and black holes simultaneously. The Spitzer observations were made as part of the Great Observatories Origins Deep Survey program, which aims to image the faintest distant galaxies using a variety of wavelengths.

  10. How the first stars shaped the faintest gas-dominated dwarf galaxies

    CERN Document Server

    Verbeke, Robbert; De Rijcke, Sven

    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 exists 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 o...

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

    International Nuclear Information System (INIS)

    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

  12. Hypervelocity binary stars: smoking gun of massive binary black holes

    CERN Document Server

    Lu, Youjun; Lin, D N C

    2007-01-01

    The hypervelocity stars recently found in the Galactic halo are expelled from the Galactic center through interactions between binary stars and the central massive black hole or between single stars and a hypothetical massive binary black hole. In this paper, we demonstrate that binary stars can be ejected out of the Galactic center with velocities up to 10^3 km/s, while preserving their integrity, through interactions with a massive binary black hole. Binary stars are unlikely to attain such high velocities via scattering by a single massive black hole or through any other mechanisms. Based on the above theoretical prediction, we propose a search for binary systems among the hypervelocity stars. Discovery of hypervelocity binary stars, even one, is a definitive evidence of the existence of a massive binary black hole in the Galactic center.

  13. Prompt emission from tidal disruptions of white dwarfs by intermediate mass black holes

    Directory of Open Access Journals (Sweden)

    Laguna P.

    2012-12-01

    Full Text Available We present a qualitative picture of prompt emission from tidal disruptions of white dwarfs (WD by intermediate mass black holes (IMBH. The smaller size of an IMBH compared to a supermassive black hole and a smaller tidal radius of a WD disruption lead to a very fast event with high peak luminosity. Magnetic field is generated in situ following the tidal disruption, which leads to effective accretion. Since large-scale magnetic field is also produced, geometrically thick super-Eddington inflow leads to a relativistic jet. The dense jet possesses a photosphere, which emits quasi-thermal radiation in soft X-rays. The source can be classified as a long low-luminosity gamma-ray burst (ll-GRB. Tidal compression of a WD causes nuclear ignition, which is observable as an accompanying supernova. We suggest that GRB060218 and SN2006aj is such a pair of ll-GRB and supernova. We argue that in a flux-limited sample the disruptions of WDs by IMBHs are more frequent then the disruptions of other stars by IMBHs.

  14. The Ecology of Black Holes in Star Clusters

    OpenAIRE

    Zwart, Simon Portegies

    2004-01-01

    In this lecture we investigate the formation and evolution of black holes in star clusters. The star clusters under consideration are generally rich, containing more than 10^4 stars, and with a density exceeding 10^4 stars/pc^3. Among these are young dense clusters (YoDeCs), globular cluster and the nuclei of galaxies. We will also address the the possible evolutionary link between stellar mass black holes, via intermediate mass black holes to supermassive black holes, mainly focus on the eco...

  15. On the mass segregation of stars and brown dwarfs in Taurus

    CERN Document Server

    Parker, Richard J; Goodwin, Simon P; Moraux, Estelle; Allison, Richard J; Guieu, Sylvain; Guedel, Manuel

    2010-01-01

    We use the new minimum spanning tree (MST) method to look for mass segregation in the Taurus association. The method computes the ratio of MST lengths of any chosen subset of objects, including the most massive stars and brown dwarfs, to the MST lengths of random sets of stars and brown dwarfs in the cluster. This mass segregation ratio (Lambda_MSR) enables a quantitative measure of the spatial distribution of high-mass and low-mass stars, and brown dwarfs to be made in Taurus. We find that the most massive stars in Taurus are inversely mass segregated, with Lambda_MSR = 0.70 +/- 0.10 (Lambda_MSR = 1 corresponds to no mass segregation), which differs from the strong mass segregation signatures found in more dense and massive clusters such as Orion. The brown dwarfs in Taurus are not mass segregated, although we find evidence that some low-mass stars are, with an Lambda_MSR = 1.25 +/- 0.15. Finally, we compare our results to previous measures of the spatial distribution of stars and brown dwarfs in Taurus, and...

  16. PROTOPLANETARY DISK MASSES FROM STARS TO BROWN DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Mohanty, Subhanjoy; Mortlock, Daniel [Imperial College London, 1010 Blackett Lab, Prince Consort Rd., London SW7 2AZ (United Kingdom); Greaves, Jane [SUPA, Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS (United Kingdom); Pascucci, Ilaria; Apai, Daniel [Department of Planetary Sciences and Lunar and Planetary Laboratory, University of Arizona, Tucson AZ 85721 (United States); Scholz, Aleks [School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2 (Ireland); Thompson, Mark [Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB (United Kingdom); Lodato, Giuseppe [Dipartimento di Fisica, Universita Degli Studi di Milano, Via Celoria 16, I-20133 Milano (Italy); Looper, Dagny, E-mail: s.mohanty@imperial.ac.uk [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822 (United States)

    2013-08-20

    We present SCUBA-2 850 {mu}m 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 3{sigma} limits correspond to a dust mass of 1.2 M{sub Circled-Plus} in Taurus and a mere 0.2 M{sub Circled-Plus} in the TWA (3-10 Multiplication-Sign deeper than previous work). We combine our data with other submillimeter/millimeter (sub-mm/mm) surveys of Taurus, {rho} 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 {approx}100 AU for intermediate-mass stars, solar types, and VLMS, and {approx}20 AU for BDs. (2) While the upper envelope of apparent disk masses increases with M{sub *} 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 {rho} Oph intermediate-mass and solar-type stars evince an opacity index of {beta} {approx} 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 {mu}m 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

  17. Supernova SN 2011fe from an exploding carbon–oxygen white dwarf star

    OpenAIRE

    Nugent, Peter E.; S. R. Kulkarni; Kasliwal, Mansi M.; Ofek, Eran O.

    2011-01-01

    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 ...

  18. Local Group Dwarf Galaxies and the Star Formation Law at High Redshift

    CERN Document Server

    Gnedin, N Yu

    2000-01-01

    I show how the existing observational data on Local Group dwarf galaxies can be used to estimate the average star formation law during the first 3 Gyr of the history of the universe. I find that the observational data are consistent with the orthodox Schmidt law with a star formation efficiency of about 4 percent if the star formation is continuous (during the first 3 Gyr). The efficiency is proportionally higher if most of the gas in the dwarfs was consumed (and never replenished) in a short time interval well before the universe turned 3 Gyr.

  19. 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.

  20. Can boson stars supplant black holes?

    International Nuclear Information System (INIS)

    The time-like geodesics of spherically symmetric boson stars (BS) are compared to those of the Schwarzschild black hole (BH). It is shown that the compactness of the BS is the quantity that determines how similar time-like geodesics are to those of a BH with the same mass. It is also found that the self-interaction of the scalarfield the BS is made of, determines how compact a stable BS can be. The combination of these two results indicates that BSs could supplant BHs better when they are stable, have strong self-interaction and high central density. If boson stars will be considered as serious toy models for astrophysical BH candidates it will be important to choose correctly the free parameters of the scalarfield; here the basic guidelines are pointed out for the case of spherical symmetry

  1. ULXs: Neutron Stars vs Black Holes

    OpenAIRE

    King, Andrew; Lasota, Jean-Pierre

    2016-01-01

    We consider ultraluminous X-ray sources (ULXs) where the accretor is a neutron star rather than a black hole. We show that the recently-discovered example (M82 X-2) fits naturally into the simple picture of ULXs as beamed X-ray sources fed at super-Eddington rates, provided that its magnetic field is weaker ($\\simeq 10^{11}{\\rm G}$) than a new-born X-ray pulsar, as expected if there has been mass gain. Continuing accretion is likely to weaken the field to the point that pulsing stops, and mak...

  2. 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.

  3. The Rotation of Young Low-Mass Stars and Brown Dwarfs

    CERN Document Server

    Herbst, W; Mundt, R; Scholz, A

    2006-01-01

    We review the current state of our knowledge concerning the rotation and angular momentum evolution of young stellar objects and brown dwarfs from a primarily observational view point. Periods are typically accurate to 1% and available for about 1700 stars and 30 brown dwarfs in young clusters. Discussion of angular momentum evolution also requires knowledge of stellar radii, which are poorly known for pre-main sequence stars. It is clear that rotation rates at a given age depend strongly on mass; higher mass stars (0.4-1.2 M$_\\odot$) have longer periods than lower mass stars and brown dwarfs. On the other hand, specific angular momentum is approximately independent of mass for low mass pre-main sequence stars and young brown dwarfs. A spread of about a factor of 30 is seen at any given mass and age. The evolution of rotation of solar-like stars during the first 100 Myr is discussed. A broad, bimodal distribution exists at the earliest observable phases ($\\sim$1 Myr) for stars more massive than 0.4 M$_\\odot$....

  4. Homogeneous Photometry VI: Variable Stars in the Leo I Dwarf Spheroidal Galaxy?

    CERN Document Server

    Stetson, Peter B; Bono, Giuseppe; Bernard, Edouard J; Monelli, Matteo; Iannicola, Giacinto; Gallart, Carme; Ferraro, Ivan

    2014-01-01

    We have characterized the pulsation properties of 164 candidate RR Lyrae variables (RRLs) and 55 candidate Anomalous and/or short-period Cepheids in Leo I dwarf spheroidal galaxy. On the basis of its RRLs Leo I is confirmed to be an Oosterhoff-intermediate type galaxy, like several other dwarfs. We show that in their pulsation properties, the RRLs representing the oldest stellar population in the galaxy are not significantly different from those of five other nearby, isolated dwarf spheroidal galaxies. A similar result is obtained when comparing them to RR Lyrae stars in recently discovered ultra-faint dwarf galaxies. We are able to compare the period distributions and period-amplitude relations for a statistically significant sample of ab type RR Lyrae stars in dwarf galaxies (~1300stars) with those in the Galactic halo field (~14,000stars) and globular clusters (~1000stars). Field RRLs show a significant change in their period distribution when moving from the inner (dG14kpc) halo regions. This suggests tha...

  5. 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.

  6. How Dry is the Brown Dwarf Desert?: Quantifying the Relative Number of Planets, Brown Dwarfs and Stellar Companions around Nearby Sun-like Stars

    CERN Document Server

    Grether, D; Grether, Daniel; Lineweaver, Charles H.

    2004-01-01

    Sun-like stars have stellar, brown dwarf and planetary companions. To help constrain their formation and migration scenarios, we analyse the close companions (orbital period 2 M_Solar respectively. However, we find no evidence that companion mass scales with host mass in general. Approximately 16% of Sun-like stars have close (P < 5 years) companions more massive than Jupiter: 11% are stellar, 1% are brown dwarf and 4% are giant planets. The companion mass function in the brown dwarf and stellar mass range, has a different shape than the initial mass function of individual stars and free-floating brown dwarfs. This suggests either a different spectrum of gravitational fragmentation in the formation environment or post-formation migratory processes disinclined to leave brown dwarfs in close orbits.

  7. The Future Evolution of White Dwarf Stars Through Baryon Decay and Time Varying Gravitational Constant

    OpenAIRE

    Ketchum, Jacob A.; Adams, Fred C.

    2008-01-01

    Motivated by the possibility that the fundamental ``constants'' of nature could vary with time, this paper considers the long term evolution of white dwarf stars under the combined action of proton decay and variations in the gravitational constant. White dwarfs are thus used as a theoretical laboratory to study the effects of possible time variations, especially their implications for the future history of the universe. More specifically, we consider the gravitational constant $G$ to vary ac...

  8. White dwarf mergers and the origin of R coronae borealis stars

    OpenAIRE

    Lorén Aguilar, Pablo; Longland, Richard Leigh; José Pont, Jordi; García-Berro Montilla, Enrique; Althaus, Leandro Gabriel; Isern-fontanet, J.

    2012-01-01

    We present a nucleosynthesis study of the merger of a 0.4 solar masses helium white dwarf with a 0.8 solar masses carbon-oxygen white dwarf, coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.

  9. The History of Black Star Picture Agency: "Life's" European Connection.

    Science.gov (United States)

    Smith, C. Zoe

    Historians of photography have failed to explore the origins of the Black Star Picture Agency and how it introduced experienced photojournalists to Henry Luce, a publisher attempting to break new ground in American journalism with the introduction of a picture magazine, "Life," in 1936. Black Star's founders, Ernest Mayer, Kurt Kornfeld, and Kurt…

  10. Nucleosynthesis during the merger of white dwarfs and the origin of R Coronae Borealis stars

    OpenAIRE

    Longland, Richard Leigh; Lorén Aguilar, Pablo; José Pont, Jordi; García-Berro Montilla, Enrique; Althaus, Leandro Gabriel; Isern Vilaboy, Jordi

    2011-01-01

    Many hydrogen deficient stars are characterised by surface abundance patterns that are hard to reconcile with conventional stellar evolution. Instead, it has been suggested that they may represent the result of a merger episode between a helium and a carbon-oxygen white dwarf. In this Letter, we present a nucleosynthesis study of the merger of a 0.4 M_sol helium white dwarf with a 0.8 M_sol carbon-oxygen white dwarf, by coupling the thermodynamic history of Smoothed Particle Hydrodynamics par...

  11. Milky Way Red Dwarfs in the BoRG Survey; Galactic Scale-height and the Distribution of Dwarf Stars in WFC3 Imaging

    Science.gov (United States)

    Holwerda, B. W.; Trenti, M.; Clarkson, W.; Sahu, K.; Bradley, L.; Stiavelli, M.; Pirzkal, N.; De Marchi, G.; Andersen, M.; Bouwens, R.; Ryan, R.

    2014-06-01

    We present a tally of Milky Way late-type dwarf stars in 68 Wide Field Camera 3 (WFC3) pure-parallel fields (227 arcmin2) 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. The relative ubiquity

  12. 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)

    We present a tally of Milky Way late-type dwarf stars in 68 Wide Field Camera 3 (WFC3) pure-parallel fields (227 arcmin2) 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. The

  13. The Future Evolution of White Dwarf Stars Through Baryon Decay and Time Varying Gravitational Constant

    CERN Document Server

    Ketchum, Jacob A

    2008-01-01

    Motivated by the possibility that the fundamental ``constants'' of nature could vary with time, this paper considers the long term evolution of white dwarf stars under the combined action of proton decay and variations in the gravitational constant. White dwarfs are thus used as a theoretical laboratory to study the effects of possible time variations, especially their implications for the future history of the universe. More specifically, we consider the gravitational constant $G$ to vary according to the parametric relation $G = G_0 (1 + t/t_\\ast)^{-p}$, where the time scale $t_\\ast$ is the same order as the proton lifetime. We then study the long term fate and evolution of white dwarf stars. This treatment begins when proton decay dominates the stellar luminosity, and ends when the star becomes optically thin to its internal radiation.

  14. Discrete Classification with Principal Component Analysis Discrimination of Giant and Dwarf Spectra in K-stars

    CERN Document Server

    Ibata, R A; Ibata, Rodrigo; Irwin, Michael

    1997-01-01

    We demonstrate the use of a variant of Principal Component Analysis (PCA) for discrimination problems in astronomy. This variant of PCA is shown to provide the best linear discrimination between data classes. As a test case, we present the problem of discrimination between K giant and K dwarf stars from intermediate resolution spectra near the Mg `b' feature. The discrimination procedure is trained on a set of 24 standard K giants and 24 standard K dwarfs, and then used to perform giant - dwarf classification on a sample of approximately 1500 field K stars of unknown luminosity class which were initially classified visually. For the highest S/N spectra, the automated classification agrees very well (at the 90 - 95% level) with the visual classification. Most importantly, however, the automated method is found to classify stars in a repeatable fashion, and, according to numerical experiments, is very robust to signal to noise (S/N) degradation.

  15. Head-on collisions of binary white dwarf-neutron stars: Simulations in full general relativity

    International Nuclear Information System (INIS)

    We simulate head-on collisions from rest at large separation of binary white dwarf-neutron stars (WDNSs) in full general relativity. Our study serves as a prelude to our analysis of the circular binary WDNS problem. We focus on compact binaries whose total mass exceeds the maximum mass that a cold-degenerate star can support, and our goal is to determine the fate of such systems. A fully general relativistic hydrodynamic computation of a realistic WDNS head-on collision is prohibitive due to the large range of dynamical time scales and length scales involved. For this reason, we construct an equation of state (EOS) which captures the main physical features of neutron stars (NSs) while, at the same time, scales down the size of white dwarfs (WDs). We call these scaled-down WD models 'pseudo-WDs (pWDs)'. Using pWDs, we can study these systems via a sequence of simulations where the size of the pWD gradually increases toward the realistic case. We perform two sets of simulations; One set studies the effects of the NS mass on the final outcome, when the pWD is kept fixed. The other set studies the effect of the pWD compaction on the final outcome, when the pWD mass and the NS are kept fixed. All simulations show that after the collision, 14%-18% of the initial total rest mass escapes to infinity. All remnant masses still exceed the maximum rest mass that our cold EOS can support (1.92M·), but no case leads to prompt collapse to a black hole. This outcome arises because the final configurations are hot. All cases settle into spherical, quasiequilibrium configurations consisting of a cold NS core surrounded by a hot mantle, resembling Thorne-Zytkow objects. Extrapolating our results to realistic WD compactions, we predict that the likely outcome of a head-on collision of a realistic, massive WDNS system will be the formation of a quasiequilibrium Thorne-Zytkow-like object.

  16. Chemistry of Stars in the Sculptor Dwarf Galaxy from VLT-FLAMES

    Science.gov (United States)

    Venn, Kim A.; Hill, V.

    The chemical composition of 91 stars in the Sculptor dwarf spheroidal galaxy is presented as determined from spectra taken with the FLAMES multiobject spectrograph in the Medusa mode. The analysis methods are outlined. The [α/Fe] ratios are shown for Mg, Ca, and Ti, and compared with those of Galactic stars. Heavy element abundance ratios (Y, Ba, and Eu) are also presented. Since the Sculptor dwarf galaxy has had a significantly different star formation history and chemical evolution than the Galaxy, then comparison of Sculptor's metal-poor (old) stars to similar metallicity stars in the Galaxy can be used to discuss galaxy formation scenarios, as well as test some of our fundamental assumptions in stellar nucleosynthesis.

  17. The Different Environmental Dependencies of Star Formation for Giant and Dwarf Galaxies

    Science.gov (United States)

    Haines, C. P.; La Barbera, F.; Mercurio, A.; Merluzzi, P.; Busarello, G.

    2006-08-01

    We examine the origins of the bimodality observed in the global properties of galaxies around a stellar mass of 3×1010 Msolar by comparing the environmental dependencies of star formation for the giant and dwarf galaxy populations. The Sloan Digital Sky Survey DR4 spectroscopic data set is used to produce a sample of galaxies in the vicinity of the supercluster centered on the cluster A2199 at z=0.03 that is >~90% complete to M*r+3.3. From this we measure global trends with environment for both giant (Mr7 Gyr) or passive (EW [Hα] ~80% in the cluster cores to ~40% in field regions beyond 3-4Rvir, as found in previous studies. In contrast, we find that the dwarf galaxy population shows a sharp transition at ~1Rvir, from being predominantly old/passive within the cluster, to outside where virtually all galaxies are forming stars and old/passive galaxies are only found as satellites to more massive galaxies. These results imply fundamental differences in the evolution of giant and dwarf galaxies: whereas the star formation histories of giant galaxies are determined primarily by their merger history, star formation in dwarf galaxies is much more resilient to the effects of major mergers. Instead, dwarf galaxies become passive only once they become satellites within a more massive halo either by losing their halo gas reservoir to the host halo or through other environment-related processes such as galaxy harassment and ram pressure stripping.

  18. Black holes, pregalactic stars, and the dark matter problem

    International Nuclear Information System (INIS)

    We review the different ways in which black holes might form and discuss their various astrophysical and cosmological consequences. We then consider the various constraints on the form of the dark matter and conclude that black holes could have a significant cosmological density only if they are of primordial origin or remnants of a population of pregalactic stars. This leads us to discuss the other cosmological effects of primordial black holes and pregalactic stars. 239 refs., 7 figs., 5 tabs

  19. Black holes, pregalactic stars, and the dark matter problem

    Energy Technology Data Exchange (ETDEWEB)

    Carr, B.J.

    1985-06-01

    We review the different ways in which black holes might form and discuss their various astrophysical and cosmological consequences. We then consider the various constraints on the form of the dark matter and conclude that black holes could have a significant cosmological density only if they are of primordial origin or remnants of a population of pregalactic stars. This leads us to discuss the other cosmological effects of primordial black holes and pregalactic stars. 239 refs., 7 figs., 5 tabs.

  20. Black holes, pregalactic stars, and the dark matter problem

    International Nuclear Information System (INIS)

    The authors review the different ways in which black holes might form and discuss their various astrophysical and cosmological consequences. They then consider the various constraints on the form of the dark matter and conclude that black holes could have a significant cosmological density only if they are of primordial origin or remnants of a population of pregalactic stars. This leads them to discuss their cosmological effects of primordial black holes and pregalactic stars

  1. Quantified HI Morphology VII: star-formation and tidal influence on local dwarf HI morphology

    OpenAIRE

    Holwerda, B. W.; Pirzkal, N.; Blok, W. J. G. de; Blyth, S-L

    2013-01-01

    Scale-invariant morphology parameters applied to atomic hydrogen maps (HI) of galaxies can be used to quantify the effects of tidal interaction or star-formation on the ISM. Here we apply these parameters, Concentration, Asymmetry, Smoothness, Gini, M20, and the GM parameter, to two public surveys of nearby dwarf galaxies, the VLA-ANGST and LITTLE-THINGS survey, to explore whether tidal interaction or the ongoing or past star-formation is a dominant force shaping the HI disk of these dwarfs. ...

  2. Dark influences II. Gas and star formation in minor mergers of dwarf galaxies with dark satellites

    Science.gov (United States)

    Starkenburg, T. K.; Helmi, A.; Sales, L. V.

    2016-03-01

    Context. It has been proposed that mergers induce starbursts and lead to important morphological changes in galaxies. Most studies so far have focused on large galaxies, but dwarfs might also experience such events, since the halo mass function is scale-free in the concordance cosmological model. Notably, because of their low mass, most of their interactions will be with dark satellites. Aims: In this paper we follow the evolution of gas-rich disky dwarf galaxies as they experience a minor merger with a dark satellite. We aim to characterize the effects of such an interaction on the dwarf's star formation, morphology, and kinematical properties. Methods: We performed a suite of carefully set-up hydrodynamical simulations of dwarf galaxies that include dark matter, gas, and stars merging with a satellite consisting solely of dark matter. For the host system we vary the gas fraction, disk size and thickness, halo mass, and concentration, while we explore different masses, concentrations, and orbits for the satellite. Results: We find that the interactions cause strong starbursts of both short and long duration in the dwarfs. Their star formation rates increase by factors of a few to 10 or more. They are strongest for systems with extended gas disks and high gas fractions merging with a high-concentration satellite on a planar, radial orbit. In contrast to analogous simulations of Milky Way-mass galaxies, many of the systems experience strong morphological changes and become spheroidal even in the presence of significant amounts of gas. Conclusions: The simulated systems compare remarkably well with the observational properties of a large selection of irregular dwarf galaxies and blue compact dwarfs. This implies that mergers with dark satellites might well be happening but not be fully evident, and may thus play a role in the diversity of the dwarf galaxy population.

  3. Titan under a red dwarf star and as a rogue planet: requirements for liquid methane

    Science.gov (United States)

    Gilliam, Ashley E.; McKay, Christopher P.

    2011-07-01

    Titan has a surface temperature of 94 K and a surface pressure of 1.4 atmospheres. These conditions make it possible for liquid methane solutions to be present on the surface. Here, we consider how Titan could have liquid methane while orbiting around an M4 red dwarf star, and a special case of Titan orbiting the red dwarf star Gliese 581. Because light from a red dwarf star has a higher fraction of infrared than the Sun, more of the starlight will reach the surface of Titan because its atmospheric haze is more transparent to infrared wavelengths. If Titan was placed at a distance from a red dwarf star such that it received the same average flux as it receives from the Sun, we calculate the increased infrared fraction, which will warm surface temperatures by an additional ˜10 K. Compared to the Sun, red dwarf stars have less blackbody ultraviolet light but can have more Lyman α and particle radiation associated with flares. Thus depending on the details, the haze production may be much higher or much lower than for the current Titan. With the haze reduced by a factor of 100, Titan would have a surface temperature of 94 K at a distance of 0.23 AU from an M4 star and at a distance of 1.66 AU, for Gliese 581. If the haze is increased by a factor of 100 the distances become 0.08 and 0.6 AU for the M4-star and Gliese 581, respectively. As a rogue planet, with no incident stellar flux, Titan would need 1.6 W/m 2 of geothermal heat to maintain its current surface temperature, or an atmospheric opacity of 20× its present amount with 0.1 W/m 2 of geothermal heat. Thus Titan-like worlds beyond our solar system may provide environment supporting surface liquid methane.

  4. The Imprint of Reionization on the Star Formation Histories of Dwarf Galaxies

    CERN Document Server

    Benitez-Llambay, Alejandro; Abadi, Mario G; Gottloeber, Stefan; Yepes, Gustavo; Hoffman, Yehuda; Steinmetz, Matthias

    2014-01-01

    We explore the impact of cosmic reionization on nearby isolated dwarf galaxies using a compilation of SFHs estimated from deep HST data and a cosmological hydrodynamical simulation of the Local Group. The nearby dwarfs show a wide diversity of star formation histories; from ancient systems that have largely completed their star formation $\\sim 10$ Gyr ago to young dwarfs that have formed the majority of their stars in the past $\\sim 5$ Gyr to two-component systems characterized by the overlap of comparable numbers of old and young stars. Taken as an ensemble, star formation in nearby dwarfs dips to lower-than-average rates at intermediate times ($4

  5. 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. PMID:27135924

  6. 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

    2011-12-15

    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. PMID:22170680

  7. Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars II. Sulfur and Phosphorus

    CERN Document Server

    Visscher, C

    2005-01-01

    We use thermochemical equilibrium and kinetic calculations to model sulfur and phosphorus chemistry in the atmospheres of giant planets, brown dwarfs, low-mass stars, and extrasolar giant planets (EGPs). The chemical behavior of individual S- and P-bearing gases and condensates is determined as a function of pressure, temperature, and metallicity. Our results are independent of any particular model atmosphere and the behavior of different gases can be used to constrain atmospheric structure and metallicity. Hydrogen sulfide is the dominant sulfur gas in substellar atmospheres and approximately represents the atmospheric sulfur inventory. Depending on the prevailing S and C chemistry, the abundance of minor sulfur gases may constrain atmospheric temperatures or metallicity. Disequilibrium abundances of PH3 are expected in the observable atmospheres of substellar objects, and PH3 is representative of the total P abundance in giant planets and T dwarfs. A number of other phosphorus gases become relatively abunda...

  8. Star Formation History and Chemical Evolution of the Sextans Dwarf Spheroidal Galaxy

    OpenAIRE

    Lee, Myung Gyoon; Yuk, In-Soo; Park, Hong Soo; Harris, Jason; Zaritsky, Dennis

    2009-01-01

    We present the star formation history and chemical evolution of the Sextans dSph dwarf galaxy as a function of galactocentric distance. We derive these from the $VI$ photometry of stars in the $42' \\times 28'$ field using the SMART model developed by Yuk & Lee (2007, ApJ, 668, 876) and adopting a closed-box model for chemical evolution. For the adopted age of Sextans 15 Gyr, we find that $>$84% of the stars formed prior to 11 Gyr ago, significant star formation extends from 15 to 11 Gyr ago (...

  9. Detection of the white dwarf and the secondary star in the new SU UMa dwarf nova HS 2219+1824

    Science.gov (United States)

    Rodríguez-Gil, P.; Gänsicke, B. T.; Hagen, H.-J.; Marsh, T. R.; Harlaftis, E. T.; Kitsionas, S.; Engels, D.

    2005-02-01

    We report the discovery of a new, non-eclipsing SU UMa-type dwarf nova, HS 2219+1824. Photometry obtained in quiescence (V≈17.5) reveals a double-humped light curve from which we derive an orbital period of ≃86.2 min. Additional photometry obtained during a superoutburst reaching V≃12.0 clearly shows superhumps with a period of ≃89.05 min. The optical spectrum contains double-peaked Balmer and He I emission lines from the accretion disc as well as broad absorption troughs of Hβ, Hγ, and Hδ from the white dwarf primary star. Modelling of the optical spectrum implies a white dwarf temperature of 13 000 K⪉Teff⪉17 000 K, a distance of 180 pc⪉ d⪉230 pc, and suggests that the spectral type of the donor star is later than M 5. Phase-resolved spectroscopy obtained during quiescence reveals a narrow Hα emission line component which has a radial velocity amplitude and phase consistent with an origin on the secondary star, possibly on the irradiated hemisphere facing the white dwarf. This constitutes the first detection of line emission from the secondary star in a quiescent SU UMa star. Based in part on observations obtained at the German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie, Heidelberg, jointly with the Spanish National Commission for Astronomy; on observations made with the IAC80 and OGS telescopes, operated on the island of Tenerife by the Instituto de Astrofísica de Canarias (IAC) and the European Space Agency (ESA), respectively, in the Spanish Observatorio del Teide of the IAC; on observations made at the 1.2 m telescope, located at Kryoneri Korinthias, and owned by the National Observatory of Athens, Greece; and on observations made with the William Herschel Telescope, which is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the IAC.

  10. 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

  11. How the First Stars Shaped the Faintest Gas-dominated Dwarf Galaxies

    Science.gov (United States)

    Verbeke, R.; Vandenbroucke, B.; De Rijcke, S.

    2015-12-01

    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.

  12. A VLT/FORS2 spectroscopic survey of individual stars in a transforming dwarf galaxy

    CERN Document Server

    Battaglia, G; Rejkuba, M

    2016-01-01

    Understanding the properties of dwarf galaxies is important not only to put them in their proper cosmological context, but also to understand the formation and evolution of the most common type of galaxies. Dwarf galaxies are divided into two main classes, dwarf irregulars (dIrrs) and dwarf spheroidals (dSphs), which differ from each other mainly because the former are gas-rich objects currently forming stars, while the latter are gas-deficient with no on-going star formation. Transition types (dT) are thought to represent dIs in the process of losing their gas, and can therefore shed light into the possible process of dwarf irregulars (dIrrs) becoming gas-deficient, passively evolving galaxies. Here we present preliminary results from our wide-area VLT/FORS2 MXU spectroscopic survey of the Phoenix dT, from which we obtained line-of-sight velocities and metallicities from the nIR Ca II triplet lines for a large sample of individual Red Giant Branch stars.

  13. Episodic Model For Star Formation History and Chemical Abundances in Giant and Dwarf Galaxies

    CERN Document Server

    Debsarma, Suma; Das, Sukanta; Pfenniger, Daniel

    2016-01-01

    In search for a synthetic understanding, a scenario for the evolution of the star formation rate and the chemical abundances in galaxies is proposed, combining gas infall from galactic halos, outflow of gas by supernova explosions, and an oscillatory star formation process. The oscillatory star formation model is a consequence of the modelling of the fractional masses changes of the hot, warm and cold components of the interstellar medium. The observed periods of oscillation vary in the range $(0.1-3.0)\\times10^{7}$\\,yr depending on various parameters existing from giant to dwarf galaxies. The evolution of metallicity varies in giant and dwarf galaxies and depends on the outflow process. Observed abundances in dwarf galaxies can be reproduced under fast outflow together with slow evaporation of cold gases into hot gas whereas slow outflow and fast evaporation is preferred for giant galaxies. The variation of metallicities in dwarf galaxies supports the fact that low rate of SNII production in dwarf galaxies i...

  14. Lifestyles of the Stars.

    Science.gov (United States)

    National Aeronautics and Space Administration, Cocoa Beach, FL. John F. Kennedy Space Center.

    Some general information on stars is provided in this National Aeronautics and Space Administration pamphlet. Topic areas briefly discussed are: (1) the birth of a star; (2) main sequence stars; (3) red giants; (4) white dwarfs; (5) neutron stars; (6) supernovae; (7) pulsars; and (8) black holes. (JN)

  15. Suppression of star formation in dwarf galaxies by grain photoelectric feedback

    CERN Document Server

    Forbes, John C; Goldbaum, Nathan J; Dekel, Avishai

    2016-01-01

    Photoelectric heating has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies found some indication that photoelectric heating could suppress star formation. However, simulations that include photoelectric heating have typically found that it has little effect on the rate of star formation in either spiral galaxies or dwarfs suggesting that supernovae and not photoelectric heating are responsible for setting the star formation law in galaxies. This result is in tension with recent work indicating that a star formation law that depends on galaxy metallicity, as expected for photoelectric heating but not for supernovae, reproduces the present-day galaxy population better than a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, where the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space- and time-dependent photoelectric heating, and we resolve...

  16. CHARACTERIZING THE BROWN DWARF FORMATION CHANNELS FROM THE INITIAL MASS FUNCTION AND BINARY-STAR DYNAMICS

    International Nuclear Information System (INIS)

    The stellar initial mass function (IMF) is a key property of stellar populations. There is growing evidence that the classical star-formation mechanism by the direct cloud fragmentation process has difficulties reproducing the observed abundance and binary properties of brown dwarfs and very-low-mass stars. In particular, recent analytical derivations of the stellar IMF exhibit a deficit of brown dwarfs compared to observational data. Here we derive the residual mass function of brown dwarfs as an empirical measure of the brown dwarf deficiency in recent star-formation models with respect to observations and show that it is compatible with the substellar part of the Thies-Kroupa IMF and the mass function obtained by numerical simulations. We conclude that the existing models may be further improved by including a substellar correction term that accounts for additional formation channels like disk or filament fragmentation. The term ''peripheral fragmentation'' is introduced here for such additional formation channels. In addition, we present an updated analytical model of stellar and substellar binarity. The resulting binary fraction and the dynamically evolved companion mass-ratio distribution are in good agreement with observational data on stellar and very-low-mass binaries in the Galactic field, in clusters, and in dynamically unprocessed groups of stars if all stars form as binaries with stellar companions. Cautionary notes are given on the proper analysis of mass functions and the companion mass-ratio distribution and the interpretation of the results. The existence of accretion disks around young brown dwarfs does not imply that these form just like stars in direct fragmentation

  17. CHARACTERIZING THE BROWN DWARF FORMATION CHANNELS FROM THE INITIAL MASS FUNCTION AND BINARY-STAR DYNAMICS

    Energy Technology Data Exchange (ETDEWEB)

    Thies, Ingo; Pflamm-Altenburg, Jan; Kroupa, Pavel; Marks, Michael [Helmholtz-Institut für Strahlen- und Kernphysik (HISKP), Universität Bonn, Nussallee 14-16, D-53115 Bonn (Germany)

    2015-02-10

    The stellar initial mass function (IMF) is a key property of stellar populations. There is growing evidence that the classical star-formation mechanism by the direct cloud fragmentation process has difficulties reproducing the observed abundance and binary properties of brown dwarfs and very-low-mass stars. In particular, recent analytical derivations of the stellar IMF exhibit a deficit of brown dwarfs compared to observational data. Here we derive the residual mass function of brown dwarfs as an empirical measure of the brown dwarf deficiency in recent star-formation models with respect to observations and show that it is compatible with the substellar part of the Thies-Kroupa IMF and the mass function obtained by numerical simulations. We conclude that the existing models may be further improved by including a substellar correction term that accounts for additional formation channels like disk or filament fragmentation. The term ''peripheral fragmentation'' is introduced here for such additional formation channels. In addition, we present an updated analytical model of stellar and substellar binarity. The resulting binary fraction and the dynamically evolved companion mass-ratio distribution are in good agreement with observational data on stellar and very-low-mass binaries in the Galactic field, in clusters, and in dynamically unprocessed groups of stars if all stars form as binaries with stellar companions. Cautionary notes are given on the proper analysis of mass functions and the companion mass-ratio distribution and the interpretation of the results. The existence of accretion disks around young brown dwarfs does not imply that these form just like stars in direct fragmentation.

  18. Sensitivity of Biosignatures on Earth-like Planets orbiting in the Habitable Zone of Cool M-Dwarf Stars to varying Stellar UV Radiation and Surface Biomass Emissions

    CERN Document Server

    Grenfell, John Lee; von Paris, Philip; Godolt, Mareike; Rauer, Heike

    2015-01-01

    We find that variations in the UV emissions of cool M-dwarf stars have a potentially large impact upon atmospheric biosignatures in simulations of Earth-like exoplanets i.e. planets with Earths development, and biomass and a molecular nitrogen-oxygen dominated atmosphere. Starting with an assumed black-body stellar emission for an M7 class dwarf star, the stellar UV irradiation was increased stepwise and the resulting climate-photochemical response of the planetary atmosphere was calculated. Results suggest a Goldilocks effect with respect to the spectral detection of ozone. At weak UV levels, the ozone column was weak (due to weaker production from the Chapman mechanism) hence its spectral detection was challenging. At strong UV levels, ozone formation is stronger but its associated stratospheric heating leads to a weakening in temperature gradients between the stratosphere and troposphere, which results in weakened spectral bands. Also, increased UV levels can lead to enhanced abundances of hydrogen oxides ...

  19. REVERSAL OF FORTUNE: INCREASED STAR FORMATION EFFICIENCIES IN THE EARLY HISTORIES OF DWARF GALAXIES?

    International Nuclear Information System (INIS)

    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

  20. Temperate Earth-sized planets transiting a nearby ultracool dwarf star

    CERN Document Server

    Gillon, Michael; Lederer, Susan M; Delrez, Laetitia; de Wit, Julien; Burdanov, Artem; Van Grootel, Valerie; Burgasser, Adam J; Triaud, Amaury H M J; Opitom, Cyrielle; Demory, Brice-Olivier; Sahu, Devendra K; Gagliuffi, Daniella Bardalez; Magain, Pierre; Queloz, Didier

    2016-01-01

    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 disk, 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 poss...

  1. The white dwarf companion of the B a 2 star zeta Cap

    Science.gov (United States)

    Boehm-Vitense, E.

    1981-01-01

    The Ba II star zeta Cap has a white dwarf companion. Its T (sub eff) is determined to be 22000 K, its mass is approximately one solar mass. The importance of this finding for the explanation of abundance peculiarities is discussed.

  2. They are Small Worlds After All: Revised Properties of Kepler M Dwarf Stars and their Planets

    CERN Document Server

    Gaidos, E; Kraus, A L; Ireland, M

    2015-01-01

    We classified the reddest (r-J> 2.2) stars observed by the NASA Kepler mission into main sequence dwarf or evolved giant stars and determined the properties of 4216 M dwarfs based on a comparison of available photometry with that of nearby calibrator stars, as well as available proper motions and spectra. We then revised the properties of candidate transiting planets using the stellar parameters, high-resolution imaging and aperture masking to identify companion stars, and refitting of the light curves to identify the component most likely to host the planet. We inferred the intrinsic distribution of M dwarf planets using the method of iterative Monte Carlo simulation. We compared several models of planet orbital geometry and clustering and found that one where planets are exponentially distributed and almost precisely coplanar best describes the distribution of multi-planet systems. We determined that Kepler M dwarfs host an average of 1.9+/-0.3 planets with radii of 1-4Re and orbital periods of 1.5-180d. Th...

  3. Investigating coronal saturation and super-saturation in fast-rotating M-dwarf stars

    CERN Document Server

    Jeffries, R D; Briggs, K R; Evans, P A; Pye, J P

    2010-01-01

    At fast rotation rates the coronal activity of G- and K-type stars has been observed to "saturate" and then decline again at even faster rotation rates -- a phenomenon dubbed "super-saturation". In this paper we investigate coronal activity in fast-rotating M-dwarfs using deep XMM-Newton observations of 97 low-mass stars of known rotation period in the young open cluster NGC 2547, and combine these with published X-ray surveys of low-mass field and cluster stars of known rotation period. Like G- and K-dwarfs, we find that M-dwarfs exhibit increasing coronal activity with decreasing Rossby number N_R, the ratio of period to convective turnover time, and that activity saturates at L_x/L_bol ~ 10^-3 for log N_R < -0.8. However, super-saturation is not convincingly displayed by M-dwarfs, despite the presence of many objects in our sample with log N_R < -1.8, where super-saturation is observed to occur in higher mass stars. Instead, it appears that a short rotation period is the primary predictor of super-sa...

  4. Red Dwarf Stars: Ages, Rotation, Magnetic Dynamo Activity and the Habitability of Hosted Planets

    CERN Document Server

    Engle, Scott G

    2011-01-01

    We report on our continued efforts to understand and delineate the magnetic dynamo-induced behavior/variability of red dwarf (K5 V - M6 V) stars over their long lifetimes. These properties include: rotation, light variations (from star spots), coronal-chromospheric XUV activity and flares. This study is being carried out as part of the NSF-sponsored Living with a Red Dwarf program. The Living with a Red Dwarf program's database of dM stars with photometrically determined rotation rates (from starspot modulations) continues to expand, as does the inventory of archival XUV observations. When all data sets are combined with ages from cluster/population memberships and kinematics, the determination of Age-Rotation-Activity relationships is possible. Such relationships have broad impacts not only on the studies of magnetic dynamo theory & angular momentum loss of low-mass stars with deep convective zones, but also on the suitability of planets hosted by red dwarfs to support life. With intrinsically low lumino...

  5. Tidal disruption rate of stars by spinning supermassive black holes

    CERN Document Server

    Kesden, Michael

    2011-01-01

    A supermassive black hole can disrupt a star when its tidal field exceeds the star's self-gravity, and can directly capture stars that cross its event horizon. For black holes with mass M > 10^7 solar masses, tidal disruption of main-sequence stars occurs close enough to the event horizon that a Newtonian treatment of the tidal field is no longer valid. The fraction of stars that are directly captured is also no longer negligible. We calculate generically oriented stellar orbits in the Kerr metric, and evaluate the relativistic tidal tensor at pericenter for those stars not directly captured by the black hole. We combine this relativistic analysis with previous calculations of how these orbits are populated to determine tidal-disruption rates for spinning black holes. We find, consistent with previous results, that black-hole spin increases the upper limit on the mass of a black hole capable of tidally disrupting solar-like stars to ~7 x 10^8 solar masses. More quantitatively, we find that direct stellar capt...

  6. Linear theory radial and nonradial pulsations of DA dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Starrfield, S.; Cox, A.N.; Hodson, S.; Pesnell, W.D.

    1982-07-28

    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. (GHT)

  7. Relating Follicly-Challenged Compact Stars to Bald Black Holes

    CERN Document Server

    Yagi, Kent

    2015-01-01

    Compact stars satisfy certain no-hair relations through which their multipole moments are given by their mass, spin and quadrupole moment. These relations are approximately independent of their equation of state, relating pressure to density. Such relations are similar to the black hole no-hair theorems, but these possess event horizons inside which information that led to their formation can hide. Compact stars do not possess horizons, so whether their no-hair relations are related to the black hole ones is unclear. We here show numerically that the compact star no-hair relations approach the black hole ones as the compactness approaches that of a black hole. We moreover show that compact stars become progressively oblate in this limit, even if prolate at low compactness due to strong anisotropies.

  8. The first carbon-enhanced metal-poor star found in the Sculptor dwarf spheroidal

    CERN Document Server

    Skuladottir, Asa; Salvadori, Stefania; Hill, Vanessa; Pettini, Max; Shetrone, Matthew D; Starkenburg, Else

    2014-01-01

    The origin of carbon-enhanced metal-poor (CEMP) stars and their possible connection with the chemical elements produced by the first stellar generation is still highly debated. In contrast to the Galactic halo, not many CEMP stars have been found in the dwarf spheroidal galaxies around the Milky Way. Here we present detailed abundances from ESO VLT/UVES high-resolution spectroscopy for ET0097, the first CEMP star found in the Sculptor dwarf spheroidal. This star has $\\text{[Fe/H]}=-2.03\\pm0.10$, $\\text{[C/Fe]}=0.51\\pm0.10$ and $\\text{[N/Fe]}=1.18\\pm0.20$. The traditional definition of CEMP stars is $\\text{[C/Fe]}\\geq0.70$, but taking into account that this luminous red giant branch star has undergone mixing, it was intrinsically less nitrogen enhanced and more carbon-rich when it was formed, and so it falls under the definition of CEMP stars, as proposed by Aoki et al. (2007) to account for this effect. By making corrections for this mixing, we conclude that the star had $\\text{[C/Fe]}\\approx0.8$ during its e...

  9. A search for mass segregation of stars and brown dwarfs in \\rho\\ Ophiuchi

    CERN Document Server

    Parker, Richard J; de Oliveira, Catarina Alves

    2012-01-01

    We apply two different algorithms to search for mass segregation to a recent observational census of the rho Ophiuchi star forming region. Firstly, we apply the Lambda_MSR method, which compares the minimum spanning tree (MST) of a chosen subset of stars to MSTs of random subsets of stars in the cluster, and determine the mass segregation ratio, Lambda_MSR. Secondly, we apply the m-Sigma method, which calculates the local stellar surface density around each star and determines the statistical significance of the average surface density for a chosen mass bin, compared to the average surface density in the whole cluster. Using both methods, we find no indication of mass segregation (normal or inverse) in the spatial distribution of stars and brown dwarfs in rho Ophiuchi. Although rho Ophiuchi suffers from high visual extinction, we show that a significant mass segregation signature would be detectable, albeit slightly diluted, despite dust obscuration of centrally located massive stars.

  10. Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs

    CERN Document Server

    Werner, K; Kruk, J W

    2007-01-01

    We report the first discovery of argon in hot evolved stars and white dwarfs. We have identified the ArVII 1063.55A line in some of the hottest known (Teff=95000-110000 K) central stars of planetary nebulae and (pre-) white dwarfs of various spectral type. We determine the argon abundance and compare it to theoretical predictions from stellar evolution theory as well as from diffusion calculations. We analyze high-resolution spectra taken with the Far Ultraviolet Spectroscopic Explorer. We use non-LTE line-blanketed model atmospheres and perform line-formation calculations to compute synthetic argon line profiles. We find a solar argon abundance in the H-rich central star NGC1360 and in the H-deficient PG1159 star PG1424+535. This confirms stellar evolution modeling that predicts that the argon abundance remains almost unaffected by nucleosynthesis. For the DAO-type central star NGC7293 and the hot DA white dwarfs PG0948+534 and REJ1738+669 we find argon abundances that are up to three orders of magnitude sma...

  11. Young star clusters in the outer disks of LITTLE THINGS dwarf irregular galaxies

    CERN Document Server

    Hunter, Deidre A; Gehret, Elizabeth

    2016-01-01

    We examine FUV images of the LITTLE THINGS sample of nearby dwarf irregular (dIrr) and Blue Compact Dwarf (BCD) galaxies to identify distinct young regions in their far outer disks. We use these data, obtained with the Galaxy Evolution Explorer satellite, to determine the furthest radius at which in situ star formation can currently be identified. The FUV knots are found at distances from the center of the galaxies of 1 to 8 disk scale lengths and have ages of <20 Myrs and masses of 20 to 1E5 Msolar. The presence of young clusters and OB associations in the outer disks of dwarf galaxies shows that dIrrs do have star formation taking place there in spite of the extreme nature of the environment. Most regions are found where the HI surface density is ~1 Msolar per pc2, although both the HI and dispersed old stars go out much further. This limiting density suggests a cutoff in the ability to form distinct OB associations and perhaps even stars. We compare the star formation rates in the FUV regions to the ave...

  12. 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

    2016-05-01

    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.

  13. The Star Formation History of the Pegasus Dwarf Irregular Galaxy

    OpenAIRE

    Aparicio, A.; Gallart, C.; Bertelli, G

    1997-01-01

    The star formation history (SFH) of the Pegasus dIr, a likely Local Group member at 0.95 Mpc from the Milky Way, is investigated. We characterize the SFH by two basic functions: the star formation rate, psi(t), and the chemical enrichment law, Z(t). It has been derived by comparing the color-magnitude diagram of the resolved stars in Pegasus, with a total of 189 model diagrams produced with different psi(t) and Z(t) laws. Star formation began in Pegasus about 15 Gyr ago and was larger, on ave...

  14. Investigating coronal saturation and super-saturation in fast-rotating M-dwarf stars

    OpenAIRE

    Jeffries, R. D.; Jackson, R J; Briggs, K. R.; Evans, P A; Pye, J. P.

    2010-01-01

    At fast rotation rates the coronal activity of G- and K-type stars has been observed to "saturate" and then decline again at even faster rotation rates -- a phenomenon dubbed "super-saturation". In this paper we investigate coronal activity in fast-rotating M-dwarfs using deep XMM-Newton observations of 97 low-mass stars of known rotation period in the young open cluster NGC 2547, and combine these with published X-ray surveys of low-mass field and cluster stars of known rotation period. Like...

  15. The Potential of Planets Orbiting Red Dwarf Stars to Support Oxygenic Photosynthesis and Complex Life

    OpenAIRE

    Gale, Joseph; Wandel, Amri

    2015-01-01

    We review the latest findings on extra-solar planets and their potential to support Earth-like life. Focusing on planets orbiting Red Dwarf (RD) stars, the most abundant stellar type, we show that including RDs as potential host stars could increase the probability of finding biotic planets by a factor of up to a thousand, and reduce the estimate of the distance to our nearest biotic neighbor by up to 10. We argue that binary and multiple star systems need to be taken into account when discus...

  16. The Optical Gravitational Lensing Experiment. Variable stars in the Sculptor dwarf speroidal galaxy

    OpenAIRE

    Kaluzny, J.; Kubiak, M.; Szymanski, M.; Udalski, A; Krzeminski, W.; M Mateo

    1994-01-01

    The central area of the Sculptor dwarf galaxy was surveyed for variable stars as a side-program of the OGLE project. Light curves in the V band were obtained for 226 RR Lyr stars and for 3 anomalous cepheids. One previously unknown anomalous cepheid was identified. We discovered also two variables located at the tip of the red giant branch of Sculptor. Out of 226 RR Lyr variables 135 were classified as RRab, 88 as RRc and 2 as RRd. Distribution of periods for RRab stars shows a sharp cut-off ...

  17. The Star Formation & Chemical Evolution History of the Fornax Dwarf Spheroidal Galaxy

    CERN Document Server

    de Boer, T J L; Hill, V; Saha, A; Olszewski, E W; Mateo, M; Starkenburg, E; Battaglia, G; Walker, M G

    2012-01-01

    We present deep photometry in the B,V and I filters from CTIO/MOSAIC for about 270.000 stars in the Fornax dwarf Spheroidal galaxy, out to a radius of r_ell\\sim0.8 degrees. By combining the accurately calibrated photometry with the spectroscopic metallicity distributions of individual Red Giant Branch stars we obtain the detailed star formation and chemical evolution history of Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations, but also includes ancient (10-14 Gyr), and young (-1.5 dex, with a clear trend in age.

  18. Stabilization of CO2 Atmospheres on Exoplanets around M Dwarf Stars

    CERN Document Server

    Gao, Peter; Robinson, Tyler D; Li, Cheng; Yung, Yuk L

    2015-01-01

    We investigate the chemical stability of CO2-dominated atmospheres of M dwarf terrestrial exoplanets using a 1-dimensional photochemical model. On planets orbiting Sun-like stars, the photolysis of CO2 by Far-UV (FUV) radiation is balanced by the reaction between CO and OH, the rate of which depends on H2O abundance. By comparison, planets orbiting M dwarf stars experience higher FUV radiation compared to planets orbiting Sun-like stars, and they are also likely to have low H2O abundance due to M dwarfs having a prolonged, high-luminosity pre-main sequence (Luger & Barnes 2015). We show that, for H2O-depleted planets around M dwarfs, a CO2-dominated atmosphere is stable to conversion to CO and O2 by relying on a catalytic cycle involving H2O2 photolysis. However, this cycle breaks down for planets with atmospheric hydrogen mixing ratios below ~1 ppm, resulting in ~40% of the atmospheric CO2 being converted to CO and O2 on a time scale of 1 Myr. The increased abundance of O2 also results in high O3 concent...

  19. Quantified HI Morphology VII: star-formation and tidal influence on local dwarf HI morphology

    CERN Document Server

    Holwerda, B W; de Blok, W J G; Blyth, S-L

    2013-01-01

    Scale-invariant morphology parameters applied to atomic hydrogen maps (HI) of galaxies can be used to quantify the effects of tidal interaction or star-formation on the ISM. Here we apply these parameters, Concentration, Asymmetry, Smoothness, Gini, M20, and the GM parameter, to two public surveys of nearby dwarf galaxies, the VLA-ANGST and LITTLE-THINGS survey, to explore whether tidal interaction or the ongoing or past star-formation is a dominant force shaping the HI disk of these dwarfs. Previously, HI morphological criteria were identified for ongoing spiral-spiral interactions. When we apply these to the Irregular dwarf population, they either select almost all or none of the population. We find that only the Asymmetry-based criteria can be used to identify very isolated dwarfs (i.e., these have a low tidal indication). Otherwise, there is little or no relation between the level of tidal interaction and the HI morphology. We compare the HI morphology to three star-formation rates based on either Halpha,...

  20. Star Formation in Dwarf Galaxies of the Nearby Centaurus A Group

    CERN Document Server

    Cote, Stephanie; Skillman, Evan D; Miller, Bryan W

    2009-01-01

    We present Halpha narrow-band imaging of 17 dwarf irregular galaxies (dIs) in the nearby Centaurus A Group. Although all large galaxies of the group have a current or recent enhanced star formation episode, the dIs have normal star formation rates and do not contain a larger fraction of dwarf starbursts than other nearby groups. Relative distances between dIs and larger galaxies of the group can be computed in 3D since most of them have now fairly accurately known distances. We find that the dI star formation rates do not depend on local environment, and in particular they do not show any correlation with the distance of the dI to the nearest large galaxy of the group. There is a clear morphology-density relation in the Centaurus A Group, similarly to the Sculptor and Local Groups, in the sense that dEs/dSphs tend to be at small distances from the more massive galaxies of the group, while dIs are on average at larger distances. We find four transition dwarfs in the Group, dwarfs that show characteristics of b...

  1. THE NEAR-ULTRAVIOLET LUMINOSITY FUNCTION OF YOUNG, EARLY M-TYPE DWARF STARS

    International Nuclear Information System (INIS)

    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

  2. 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.

  3. WASP-30b: a 61 Mjup brown dwarf transiting a V=12, F8 star

    CERN Document Server

    Anderson, D R; Hellier, C; Lendl, M; Maxted, P F L; Pollacco, D; Queloz, D; Smalley, B; Smith, A M S; Todd, I; Triaud, A H M J; West, R G; Barros, S C C; Enoch, B; Gillon, M; Lister, T A; Pepe, F; Ségransan, D; Street, R A; Udry, S

    2010-01-01

    We report the discovery of a 61-Jupiter-mass brown dwarf, which transits its F8V, rotationally-synchronised host star, WASP-30, every 4.16 days. From a range of age indicators, we estimate the system age to be 1-2 Gyr. We derive a radius (0.89 \\pm 0.02 RJup) for the companion that is consistent with that predicted (0.914 RJup) by a model of a 1-Gyr-old, non-irradiated brown dwarf with a dusty atmosphere.

  4. Bursts of star formation in computer simulations of dwarf galaxies

    Energy Technology Data Exchange (ETDEWEB)

    Comins, N.F.

    1984-09-01

    A three-dimensional Stochastic Self-Propagating Star Formation (SSPSF) model of compact galacies is presented. Two phases of gas, active and inactive, are present, and permanent depletion of gas in the form of long lived, low mass stars and remnants occurs. Similarly, global infall of gas from a galactic halo or through galactic cannibalism is permitted. We base our parameters on the observed properties of the compact blue galaxy I Zw 36. Our results are that bursts of star formation occur much more frequently in these runs than continuous nonbursting star formation, suggesting that the blue compact galaxies are probably undergoing bursts rather than continuous, nonbursting low-level star formation activity.

  5. Eccentricity boost of stars around shrinking massive black hole binaries

    Science.gov (United States)

    Iwasa, Mao; Seto, Naoki

    2016-06-01

    Based on a simple geometrical approach, we analyze the evolution of the Kozai-Lidov mechanism for stars around shrinking massive black hole binaries on circular orbits. We find that, due to a peculiar bifurcation pattern induced by the Newtonian potential of stellar clusters, the orbit of stars could become highly eccentric. This transition occurs abruptly for stars with small initial eccentricities. The approach presented in this paper may be useful for studying the Kozai-Lidov mechanism in various astrophysical contexts.

  6. Visual Distortions Near a Neutron Star and Black Hole

    OpenAIRE

    Nemiroff, Robert J.

    1993-01-01

    The visual distortion effects visible to an observer traveling around and descending to the surface of an extremely compact star are described. Specifically, trips to a ``normal" neutron star, a black hole, and an ultracompact neutron star with extremely high surface gravity, are described. Concepts such as multiple imaging, red- and blue-shifting, conservation of surface brightness, the photon sphere, and the existence of multiple Einstein rings are discussed in terms of what the viewer woul...

  7. The Milky Way's nuclear star cluster and massive black hole

    CERN Document Server

    Schödel, Rainer

    2015-01-01

    Because of its nearness to Earth, the centre of the Milky Way is the only galaxy nucleus in which we can study the characteristics, distribution, kinematics, and dynamics of the stars on milli-parsec scales. We have accurate and precise measurements of the Galactic centre's central black hole, Sagittarius A*, and can study its interaction with the surrounding nuclear star cluster in detail. This contribution aims at providing a concise overview of our current knowledge about the Milky Way's central black hole and nuclear star cluster, at highlighting the observational challenges and limitations, and at discussing some of the current key areas of investigation.

  8. The Milky Way's nuclear star cluster and massive black hole

    Science.gov (United States)

    Schödel, Rainer

    2016-02-01

    Because of its nearness to Earth, the centre of the Milky Way is the only galaxy nucleus in which we can study the characteristics, distribution, kinematics, and dynamics of the stars on milli-parsec scales. We have accurate and precise measurements of the Galactic centre's central black hole, Sagittarius A*, and can study its interaction with the surrounding nuclear star cluster in detail. This contribution aims at providing a concise overview of our current knowledge about the Milky Way's central black hole and nuclear star cluster, at highlighting the observational challenges and limitations, and at discussing some of the current key areas of investigation.

  9. About Exobiology: The Case for Dwarf K Stars

    OpenAIRE

    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 an...

  10. Effect of Black Holes in Local Dwarf Spheroidal Galaxies on Gamma-Ray Constraints on Dark Matter Annihilation

    OpenAIRE

    Gonzalez-Morales, Alma X.; Profumo, Stefano; Queiroz, Farinaldo S.(Department of Physics, Santa Cruz Institute for Particle Physics, University of California, 95064, Santa Cruz, CA, USA)

    2014-01-01

    Recent discoveries of optical signatures of black holes in dwarf galaxies indicates that low-mass galaxies can indeed host intermediate massive black holes. This motivates the assessment of the resulting effect on the host dark matter density profile, and the consequences for the constraints on the plane of the dark matter annihilation cross section versus mass, stemming from the non-observation of gamma rays from local dwarf spheroidals with the Fermi Large Area Telescope. We compute the den...

  11. Kepler-445, Kepler-446 and the Occurrence of Compact Multiples Orbiting Mid-M Dwarf Stars

    CERN Document Server

    Muirhead, Philip S; Vanderburg, Andrew; Morton, Timothy D; Kraus, Adam; Ireland, Michael; Swift, Jonathan J; Feiden, Gregory A; Gaidos, Eric; Gazak, J Zachary

    2015-01-01

    We confirm and characterize the exoplanetary systems Kepler-445 and Kepler-446: two mid-M dwarf stars, each with multiple, small, short-period transiting planets. Kepler-445 is a metal-rich ([Fe/H]=+0.25 $\\pm$ 0.10) M4 dwarf with three transiting planets, and Kepler-446 is a metal-poor ([Fe/H]=-0.30 $\\pm$ 0.10) M4 dwarf also with three transiting planets. Kepler-445c is similar to GJ 1214b: both in planetary radius and the properties of the host star. The Kepler-446 system is similar to the Kepler-42 system: both are metal-poor with large galactic space velocities and three short-period, likely-rocky transiting planets that were initially assigned erroneously large planet-to-star radius ratios. We independently determined stellar parameters from spectroscopy and searched for and fitted the transit light curves for the planets, imposing a strict prior on stellar density in order to remove correlations between the fitted impact parameter and planet-to-star radius ratio for short-duration transits. Combining Kep...

  12. Suppression of star formation in dwarf galaxies by photoelectric grain heating feedback.

    Science.gov (United States)

    Forbes, John C; Krumholz, Mark R; Goldbaum, Nathan J; Dekel, Avishai

    2016-07-28

    Photoelectric heating--heating of dust grains by far-ultraviolet photons--has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity--as is expected with photoelectric heating,but not with supernovae--reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time,suppressing the rate by more than an order of magnitude relative to simulations with only supernovae. PMID:27350244

  13. They are small worlds after all: revised properties of Kepler M dwarf stars and their planets

    Science.gov (United States)

    Gaidos, E.; Mann, A. W.; Kraus, A. L.; Ireland, M.

    2016-04-01

    We classified the reddest (r - J > 2.2) stars observed by the NASA Kepler mission into main-sequence dwarf or evolved giant stars and determined the properties of 4216 M dwarfs based on a comparison of available photometry with that of nearby calibrator stars, as well as available proper motions and spectra. We revised the properties of candidate transiting planets using the stellar parameters, high-resolution imaging to identify companion stars, and, in the case of binaries, fitting light curves to identify the likely planet host. In 49 of 54 systems, we validated the primary as the host star. We inferred the intrinsic distribution of M dwarf planets using the method of iterative Monte Carlo simulation. We compared several models of planet orbital geometry and clustering and found that one where planets are exponentially distributed and almost precisely coplanar best describes the distribution of multiplanet systems. We determined that Kepler M dwarfs host an average of 2.2 ± 0.3 planets with radii of 1-4 R⊕ and orbital periods of 1.5-180 d. The radius distribution peaks at ˜1.2 R⊕ and is essentially zero at 4 R⊕, although we identify three giant planet candidates other than the previously confirmed Kepler-45b. There is suggestive but not significant evidence that the radius distribution varies with orbital period. The distribution with logarithmic orbital period is flat except for a decline for orbits less than a few days. 12 candidate planets, including two Jupiter-size objects, experience an irradiance below the threshold level for a runaway greenhouse on an Earth-like planet and are thus in a `habitable zone'.

  14. Suppression of star formation in dwarf galaxies by photoelectric grain heating feedback

    Science.gov (United States)

    Forbes, John C.; Krumholz, Mark R.; Goldbaum, Nathan J.; Dekel, Avishai

    2016-07-01

    Photoelectric heating—heating of dust grains by far-ultraviolet photons—has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity—as is expected with photoelectric heating, but not with supernovae—reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space- and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time, suppressing the rate by more than an order of magnitude relative to simulations with only supernovae.

  15. 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.

  16. Probing Black Holes and Relativistic Stars with Gravitational Waves

    CERN Document Server

    Thorne, K S

    1997-01-01

    In the coming decade, gravitational waves will convert the study of general relativistic aspects of black holes and stars from a largely theoretical enterprise to a highly interactive, observational/theoretical one. For example, gravitational-wave observations should enable us to observationally map the spacetime geometries around quiescient black holes, study quantitatively the highly nonlinear vibrations of curved spacetime in black-hole collisions, probe the structures of neutron stars and their equation of state, search for exotic types of general relativistic objects such as boson stars, soliton stars, and naked singularities, and probe aspects of general relativity that have never yet been seen such as the gravitational fields of gravitons and the influence of gravitational-wave tails on radiation reaction.

  17. Young Star Clusters in the Outer Disks of LITTLE THINGS Dwarf Irregular Galaxies

    Science.gov (United States)

    Hunter, Deidre A.; Elmegreen, Bruce G.; Gehret, Elizabeth

    2016-06-01

    We examine FUV images of the LITTLE THINGS sample of nearby dwarf irregular (dIrr) and Blue Compact Dwarf galaxies to identify distinct young regions in their far outer disks. We use these data, obtained with the Galaxy Evolution Explorer satellite, to determine the furthest radius at which in situ star formation can currently be identified. The FUV knots are found at distances from the center of the galaxies of 1–8 disk scale lengths and have ages of ≤slant 20 Myr and masses of 20 M{}ȯ to 1 × 105M{}ȯ . The presence of young clusters and OB associations in the outer disks of dwarf galaxies shows that dIrrs do have star formation taking place there in spite of the extreme nature of the environment. Most regions are found where the H i surface density is ∼1 M{}ȯ pc‑2, though both the H i and dispersed old stars go out much further. This limiting density suggests a cutoff in the ability to form distinct OB associations and perhaps even stars. We compare the star formation rates in the FUV regions to the average rates expected at their radii and beyond from the observed gas, using the conventional correlation for gas-rich regions. The localized rates are typically 10% of the expected average rates for the outer disks. Either star formation in dIrrs at surface densities \\lt 1 {M}ȯ pc‑2 occurs without forming distinct associations, or the Kennicutt–Schmidt relation over-predicts the rate beyond this point. In the latter case, the stellar disks in the far-outer parts of dIrrs result from scattering of stars from the inner disk.

  18. Star formation and molecular hydrogen in dwarf galaxies: a non-equilibrium view

    Science.gov (United States)

    Hu, Chia-Yu; Naab, Thorsten; Walch, Stefanie; Glover, Simon C. O.; Clark, Paul C.

    2016-06-01

    We study the connection of star formation to atomic (H I) and molecular hydrogen (H2) in isolated, low-metallicity dwarf galaxies with high-resolution (mgas = 4 M⊙, Nngb = 100) smoothed particle hydrodynamics simulations. The model includes self-gravity, non-equilibrium cooling, shielding from a uniform and constant interstellar radiation field, the chemistry of H2 formation, H2-independent star formation, supernova feedback and metal enrichment. We find that the H2 mass fraction is sensitive to the adopted dust-to-gas ratio and the strength of the interstellar radiation field, while the star formation rate is not. Star formation is regulated by stellar feedback, keeping the gas out of thermal equilibrium for densities n cold (T ≤ 100 K) gas, but this dense and cold gas - the reservoir for star formation - is dominated by H I, not H2. In addition, a significant fraction of H2 resides in a diffuse, warm phase, which is not star-forming. The interstellar medium is dominated by warm gas (100 K cold gas is always confined to a thin layer in the mid-plane. The cold gas fraction is regulated by feedback at small radii and by the assumed radiation field at large radii. The decreasing cold gas fractions result in a rapid increase in depletion time (up to 100 Gyr) for total gas surface densities Σ _{H I+H_2} ≲ 10 M⊙ pc-2, in agreement with observations of dwarf galaxies in the Kennicutt-Schmidt plane.

  19. Two new pulsating low-mass pre-white dwarfs or SX Phenix stars?*

    CERN Document Server

    Corti, M A; Córsico, A H; Kepler, S O; Althaus, L G; Koester, D; Arias, J P Sánchez

    2016-01-01

    Context. The discovery of pulsations in low-mass stars opens an opportunity for probing their interiors and to determine their evolution, by employing the tools of asteroseismology. Aims. We aim to analyze high-speed photometry of SDSSJ145847.02$+$070754.46 and SDSSJ173001.94$+$070600.25 and discover brightness variabilities. In order to locate these stars in the $T_{\\rm eff} - \\log g$ 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 obtained by us for these stars with the 2.15m telescope at CASLEO, Argentina. We analyzed their light curves and we apply the Discrete Fourier Transform to determine the pulsation frequencies. Finally, we compare both stars in the $T_{\\rm eff} - \\log g$ diagram, with known two pre-white dwarfs, seven pulsating pre-ELM white dwarf stars, $\\delta$ Scuti and SX Phe stars. Results. We report the discovery of pulsations in SDSSJ145847.02$+$070754.46 and SDSSJ173001.9...

  20. Investigation of the Puzzling Abundance Pattern in the Stars of the Fornax Dwarf Spheroidal Galaxy

    CERN Document Server

    Li, Hongjie; Zhang, Bo

    2013-01-01

    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 the 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 monotonously 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 massive stars to $\\alpha$ e...

  1. Extrasolar planets and brown dwarfs around AF-type stars. IX. The HARPS southern sample

    CERN Document Server

    Borgniet, Simon; Meunier, Nadège; Galland, Franck

    2016-01-01

    Massive, Main-Sequence AF-type stars have so far remained unexplored in past radial velocity surveys, due to their small number of spectral lines and their high rotational velocities that prevent the classic RV computation method. Our aim was to search for giant planets around AF MS stars, to get first statistical information on their occurrence rate and to compare the results with evolved stars and lower-mass MS stars. We used the HARPS spectrograph located on the 3.6m telescope at ESO La Silla Observatory to observe 108 AF MS stars with B-V in the -0.04 to 0.58 range and masses in the range 1.1-3.6 Msun. We used our SAFIR software specifically developed to compute the radial velocities of these early-type stars. We report the new detection of a mpsini = 4.51 Mjup companion with a ~826-day period to the F6V dwarf HD111998. We present new data on the 2-planet system around the F6IV-V dwarf HD60532. We also report the detection of 14 binaries with long-term RV trends. 70% of our targets show detection limits b...

  2. Scl-1013644: a CEMP-s star in the Sculptor Dwarf Spheroidal Galaxy

    CERN Document Server

    Salgado, C; Yong, D; Norris, J E

    2016-01-01

    Recent studies of the Milky Way and its satellites have paid special attention to the importance of carbon-enhanced metal-poor (CEMP) stars due to their involvement in Galactic formation history and their possible connection with the chemical elements originating in the first stellar generation. In an ongoing study of red giants in the Sculptor dwarf galaxy we have discovered a star with extremely strong CN and CH molecular bands. This star, Scl-1013644, has previously been identified by Geisler et al. (2005) as a star with an enrichment in the heavy elements. Spectrum synthesis has been used to derive the carbon, nitrogen and barium abundances for Scl-1013644. Our findings are [C/Fe] = +0.8, [N/Fe] = -0.3 and [Ba/Fe] = +2.1 with the latter result consistent with the value found by Geisler et al. (2005). These results reveal Scl-1013644 as a CEMP-s star, the third such star discovered in this dwarf galaxy.

  3. Improved synthetic spectra of helium-core white dwarf stars

    CERN Document Server

    Rohrmann, R D; Althaus, L G; Benvenuto, O G

    2002-01-01

    We examine the emergent fluxes from helium-core white dwarfs following their evolution from the end of pre-white dwarf stages down to advanced cooling stages. For this purpose, we include a detailed treatment of the physics of the atmosphere, particularly an improved representation of the state of the gas by taking into account non-ideal effects according to the so-called occupation probability formalism. The present calculations also incorporate hydrogen line opacity from Lyman, Balmer and Paschen series, pseudo-continuum absorptions and new updated induced-dipole absorption from H$_2$-H$_2$, H$_2$-He and H-He pairs. We find that the non-ideal effects and line absorption alter the appearance of the stellar spectrum and have a significant influence upon the photometric colours in the UBVRI-JHKL system. This occurs specially for hot models $T_{\\rm eff}\\ga 8000$ due to line and pseudo-continuum opacities, and for cool models $T_{\\rm eff}\\la 4000$ where the perturbation of atoms and molecules by neighbour partic...

  4. Star formation history and evolution of gas-rich dwarf galaxies in the Centaurus A group

    CERN Document Server

    Grossi, M; Pritzl, B J; Knezek, P M; Gallagher, J S; Minchin, R F; Freeman, K C

    2006-01-01

    We analyse the properties of three unusual dwarf galaxies in the Centaurus A group discovered with the HIPASS survey. From their optical morphology they appear to be low surface brightness dwarf spheroidals, yet they are gas-rich (M_{HI}/L_{B} > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. Therefore these systems appear different from any dwarfs of the Local Group. They should be favoured hosts for starburst, whereas we find a faint star formation region in only one object. We have obtained 21-cm data and Hubble Space Telescope photometry in V and I bands, and have constructed Colour Magnitude Diagrams (CMDs) to investigate their stellar populations and to set a constraint on their age. From the comparison of the observed and model CMDs we infer that all three galaxies are at least older than 2 Gyr (possibly even as old as 10 Gyr) and remain gas-rich because their star formation rates (SFRs) have been very low (< 10^{-3} M_{sun}/yr) throughout. In such systems, sta...

  5. The seismic properties of low-mass He-core white dwarf stars

    CERN Document Server

    Córsico, A H; Althaus, L G; Hermes, J J

    2012-01-01

    We present here a detailed pulsational study applied to low-mass He-core white dwarfs, based on full evolutionary models representative of these objects. The background stellar models on which our pulsational analysis was carried out were derived by taking into account the complete evolutionary history of the progenitor stars, with special emphasis on the diffusion processes acting during the white dwarf cooling phase. We computed nonradial $g$-modes to assess the dependence of the pulsational properties of these objects with stellar parameters such as the stellar mass and the effective temperature, and also with element diffusion processes. We also performed a g- and p-mode pulsational stability analysis on our models and found well-defined blue edges of the instability domain, where these stars should start to exhibit pulsations. We found substantial differences in the seismic properties of white dwarfs with $M_* \\gtrsim 0.20 M_{\\odot}$ and the extremely low-mass (ELM) white dwarfs ($M_* \\lesssim 0.20 M_{\\o...

  6. Initial data for black hole-neutron star binaries, with rotating stars

    CERN Document Server

    Tacik, Nick; Pfeiffer, Harald P; Muhlberger, Curran; Kidder, Lawrence E; Scheel, Mark A; Szilagyi, Bela

    2016-01-01

    The coalescence of a neutron star with a black hole is a primary science target of ground-based gravitational wave detectors. Constraining or measuring the neutron star spin directly from gravitational wave observations requires knowledge of the dependence of the emission properties of these systems on the neutron star spin. This paper lays foundations for this task, by developing a numerical method to construct initial data for black hole--neutron star binaries with arbitrary spin on the neutron star. We demonstrate the robustness of the code by constructing initial-data sets in large regions of the parameter space. In addition to varying the neutron star spin-magnitude and spin-direction, we also explore neutron star compactness, mass-ratio, black hole spin, and black hole spin-direction. Specifically, we are able to construct initial data sets with neutron stars spinning near centrifugal break-up, and with black hole spins as large as $S_{\\rm BH}/M_{\\rm BH}^2=0.99$.

  7. Weak Galactic halo--dwarf spheroidal connection from RR Lyrae stars

    CERN Document Server

    Fiorentino, Giuliana; Monelli, Matteo; Stetson, Peter B; Tolstoy, Eline; Gallart, Carme; Salaris, Maurizio; Martinez, Clara; Bernard, Edouard J

    2014-01-01

    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 six dwarf spheroidals and eleven ultra faint dwarf galaxies (1,300) show a Gaussian period distribution well peaked around a mean period of =0.610+-0.001 days (sigma=0.03). The Halo RRL (15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dwarf spheroidals apart from Sagittarius are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P 0.75mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered eighteen globulars covering a broad range in metallicity (-2.3< [Fe/H]< -1.1) and hosting more than 35 R...

  8. GALEX Ultraviolet Imaging of Dwarf Galaxies and Star Formation

    CERN Document Server

    Hunter, Deidre A; Ludka, Bonnie C

    2009-01-01

    We present ultraviolet integrated and azimuthally-averaged surface photometric properties of a sample of 44 dIm, BCD, and Sm galaxies measured from archival NUV and FUV images obtained with GALEX. We compare the UV to Halpha and V-band properties and convert FUV, Halpha, and V-band luminosities into star formation rates (SFRs). We also model the star formation history from colors and compare the integrated SFRs and SFR profiles with radius for these methods. In most galaxies, the UV photometry extends beyond Halpha in radius, providing a better measure of the star formation activity in the outer disks. The Halpha appears to be lacking in the outer disk because of faintness in low density gas. The FUV and V-band profiles are continuous with radius, although they sometimes have a kink from a double exponential disk. There is no obvious difference in star formation properties between the inner and outer disks. No disk edges have been observed, even to stellar surface densities as low as 0.1 Msun/pc2 and star for...

  9. Metallicity Distribution Functions of Dwarf Galaxies: A Probe of Star Formation History and Baryonic Physics

    Science.gov (United States)

    Escala, Ivanna; Kirby, Evan N.; Wetzel, Andrew R.; Hopkins, Philip F.

    2016-06-01

    We examine the metallicity distribution functions (MDFs) of simulated, isolated dwarf galaxies (M_{star} = 4 × 10^{4} - 3 × 10^{8} M_{⊙}) from the Feedback in Realistic Environments (FIRE) project to quantify the impact of star formation history (SFH) and baryonic physics. These high-resolution cosmological simulations include realistic treatments of stellar evolution and complex gas dynamics and do not require the usual approximations (e.g., instantaneous recycling and instantaneous mixing) of analytic chemical evolution models. The evolution of the MDF with redshift informs which processes drive the dominant contributions to the distribution at z = 0, thus enabling a reconstruction of the SFH and gas loss/accretion history. We then compare the theoretical MDFs to the observed MDFs of Local Group dwarf galaxies to infer plausible SFHs for each matched galaxy.

  10. Simulating the photometric study of pulsating white dwarf stars in the physics laboratory

    CERN Document Server

    Chote, Paul

    2015-01-01

    We have designed a realistic simulation of astronomical observing using a relatively low-cost commercial CCD camera and a microcontroller-based circuit that drives LEDs inside a light-tight box with time-varying intensities. As part of a laboratory experiment, students can acquire sequences of images using the camera, and then perform data analysis using a language such as MATLAB or Python to: (a) extract the intensity of the imaged LEDs, (b) perform basic calibrations on the time-series data, and (c) convert their data into the frequency domain where they can then identify the frequency structure. The primary focus is on studying light curves produced by the pulsating white dwarf stars. The exercise provides an introduction to CCD observing, a framework for teaching concepts in numerical data analysis and Fourier techniques, and connections with the physics of white dwarf stars.

  11. Brane-world stars and (microscopic) black holes

    Energy Technology Data Exchange (ETDEWEB)

    Casadio, R., E-mail: casadio@bo.infn.it [Dipartimento di Fisica, Universita di Bologna, via Irnerio 46, 40126 Bologna (Italy); Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, via Irnerio 46, 40126 Bologna (Italy); Ovalle, J., E-mail: jovalle@usb.ve [Departamento de Fisica, Universidad Simon Bolivar, Apartado 89000, Caracas 1080A (Venezuela, Bolivarian Republic of)

    2012-08-29

    We study stars in the brane-world by employing the principle of minimal geometric deformation and find that brane-world black hole metrics with a tidal charge can be consistently recovered in a suitable limit. This procedure allows us to determine the tidal charge as a function of the ADM mass of the black hole (and brane tension). A minimum mass for semiclassical microscopic black holes can then be derived, with a relevant impact for the description of black hole events at the LHC.

  12. Brane-world stars and (microscopic) black holes

    International Nuclear Information System (INIS)

    We study stars in the brane-world by employing the principle of minimal geometric deformation and find that brane-world black hole metrics with a tidal charge can be consistently recovered in a suitable limit. This procedure allows us to determine the tidal charge as a function of the ADM mass of the black hole (and brane tension). A minimum mass for semiclassical microscopic black holes can then be derived, with a relevant impact for the description of black hole events at the LHC.

  13. INVESTIGATION OF THE PUZZLING ABUNDANCE PATTERN IN THE STARS OF THE FORNAX DWARF SPHEROIDAL GALAXY

    International Nuclear Information System (INIS)

    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 α 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 α 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 α elements did not halt for [Fe/H] ∼< –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 [α/Fe] versus [Fe/H], [iron group/Fe] versus [Fe/H], and [neutron-capture/Fe] versus [Fe/H] for the dSph stars can be explained

  14. Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars

    OpenAIRE

    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), whic...

  15. Self-consistent photometric and spectroscopic Star Formation Histories in Dwarf Galaxies

    Science.gov (United States)

    García-Benito, R.; Pérez, E.; Pérez-Montero, E.; González Delgado, R.; Vílchez, J. M.

    2016-06-01

    This project aims to unify the spectroscopic and stellar photometric views by performing a comprehensive study of a sample of the nearest Blue Compact Dwarf Galaxies (BCDs). We plan to derive Star Formation Histories (SFH) both by means of Color-Magnitude Diagrams (CMDs) from extant Hubble Space Telescope (HST) optical imaging and with spectral fitting methods techniques using MUSE, allowing us to obtain state-of-the-art 2D stellar properties and abundances of the gas in BCDs.

  16. Continuum and line emission of flares on red dwarf stars

    CERN Document Server

    Morchenko, Egor; Livshits, Moisey

    2015-01-01

    The emission spectrum has been calculated of a homogeneous pure hydrogen layer, which parameters are typical for a flare on a red dwarf. The ionization and excitation states were determined by the solution of steady-state equations taking into account the continuum and all discrete hydrogen levels. We consider the following elementary processes: electron-impact transitions, spontaneous and induced radiative transitions, and ionization by the bremsstrahlung and recombination radiation of the layer itself. The Biberman--Holstein approximation was used to calculate the scattering of line radiation. Asymptotic formulae for the escape probability are obtained for a symmetric line profile taking into account the Stark and Doppler effects. The approximation for the core of the H$-\\alpha$ line by a gaussian curve has been substantiated. The spectral intensity of the continuous spectrum, the intensity of the lines of the Balmer series and the magnitude of the Balmer jump have been calculated. The conditions have been ...

  17. Star Clusters and Super Massive Black Holes: High Velocity Stars Production

    CERN Document Server

    Fragione, Giacomo

    2016-01-01

    One possible origin of high velocity stars in the Galaxy is that they are the product of the interaction of binary systems and supermassive black holes. We investigate a new production channel of high velocity stars as due to the close interaction between a star cluster and supermassive black holes in galactic centres. The high velocity acquired by some stars of the cluster comes from combined effect of extraction of their gravitational binding energy and from the slingshot due to the interaction with the black holes. Stars could reach a velocity sufficient to travel in the halo and even overcome the galactic potential well, while some of them are just stripped from the cluster and start orbiting around the galactic centre.

  18. Constraints on MACHO Dark Matter from the Star Cluster in the Dwarf Galaxy Eridanus II

    OpenAIRE

    Brandt, Timothy D.

    2016-01-01

    I show that a recently discovered star cluster near the center of the ultra-faint dwarf galaxy Eridanus II provides strong constraints on massive compact halo objects (MACHOs) of >~5 M_sun as the main component of dark matter. MACHO dark matter will dynamically heat the cluster, driving it to larger sizes and higher velocity dispersions until it dissolves into its host galaxy. The star cluster has a luminosity of just ~2000 L_sun and is relatively puffy, with a half-light radius of 13 pc, mak...

  19. ST Chamaeleontis and BP Coronae Australis: Two Southern Dwarf Novae Confirmed as Z Cam Stars

    CERN Document Server

    Simonsen, Mike; Hambsch, Franz-Josef; Stubbings, Rod

    2014-01-01

    Z Camelopardalis (Z Cam) stars are a subset of dwarf novae distinguished by the occurrence of standstills, periods of relative constant brightness one to one and a half magnitudes fainter than maximum brightness. As part of an ongoing observing campaign, the Z CamPaign, the authors focused attention on several Z Cam suspects in the southern hemisphere. Two stars, BP Coronae Australis and ST Chamaeleontis were found to exhibit standstill behavior in 2013, thus confirming them as Z Cam type systems. This adds two more bona fide members to the 19 confirmed Z Cams, bringing the total to 21.

  20. The Evolution of Dwarf Galaxies with Star Formation in Outward Propagating Super Shell

    OpenAIRE

    Mori, Masao; Yoshii, Yuzuru; Tsujimoto, Takuji; Nomoto, Ken'ichi

    1997-01-01

    We simulate the dynamical and chemical evolution of a dwarf galaxy embedded in a dark matter halo, using a three-dimensional N-body/SPH simulation code combined with stellar population synthesis. The initial condition is adopted in accord with a $10^{10}M_\\odot$ virialized sphere in a 1-sigma CDM perturbation which contains 10% baryonic mass. A supersonic spherical outflow is driven by the first star burst near the center of the galaxy and produces an expanding super shell in which stars are ...

  1. Do R Coronae Borealis Stars Form from Double White Dwarf Mergers?

    CERN Document Server

    Staff, Jan E; Herwig, Falk; Even, Wesley; Fryer, Chris L; Motl, Patrick M; Geballe, Tom; Pignatari, Marco; Clayton, Geoffrey C; Tohline, Joel E

    2012-01-01

    A leading formation scenario for R Coronae Borealis (RCB) stars invokes the merger of degenerate He and CO white dwarfs (WD) in a binary. The observed ratio of 16O/18O for RCB stars is in the range of 0.3-20 much smaller than the solar value of ~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 white dwarf. We present the results of five 3-dimensional hydrodynamic simulations of the merger of a double white dwarf system where the total mass is 0.9 Mdot and the initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations with $q\\lesssim0.7$ a feature around the merged stars where the temperatures and densities are suitable for forming 18O. However, more 16O is being dredged-up from the C- and O-rich accretor during the merger than the amount of 18O that is produced. Therefore, on a dynamical time scale over which our hydrodynamics simulation runs, a 16O/18O ratio of ~2000 in the "best" case is found. If the conditions fou...

  2. Black Holes and Pulsars in the Introductory Physics Course

    Science.gov (United States)

    Orear, Jay; Salpeter, E. E.

    1973-01-01

    Discusses the phenomenon of formation of white dwarfs, neutron stars, and black holes from dying stars for the purpose of providing college teachers with materials usable in the introductory physics course. (CC)

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

    International Nuclear Information System (INIS)

    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-[α/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] ∼ –3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo.

  4. Model for common growth of supermassive black holes, bulges and globular star clusters: ripping off Jeans clusters

    CERN Document Server

    Nieuwenhuizen, Theo M

    2011-01-01

    It is assumed that a galaxy starts as a dark halo of a few million Jeans clusters (JCs), each of which consists of nearly a trillion micro brown dwarfs, MACHOs of earth mass. JCs in the galaxy center heat up their MACHOs by tidal forces, which makes them expand, so that coagulation and star formation occurs. Being continuously fed by matter from bypassing JCs, the star(s) may transform into a super massive black hole. It has a fast $t^3$ growth during the first mega years, and a slow $t^{1/3}$ growth at giga years. JCs disrupted by a close encounter can provide matter for the bulge. Those that survive can be so agitated that they form stars and become globular star clusters. Thus black holes mostly arise together with galactic bulges in their own environment and are about as old as the oldest globular clusters. The age 13.2 Gyr of the star HE 1523-0901 (Frebel et al. 2007) puts forward that the Galactic halo was fully assembled at that moment. In case of merging super massive black holes the JCs passing near ...

  5. Radio wavelength observations of magnetic fields on active dwarf M, RS CVn and magnetic stars

    Science.gov (United States)

    Lang, Kenneth R.

    1986-01-01

    The dwarf M stars, YZ Canis Minoris and AD Leonis, exhibit narrow-band, slowly varying (hours) microwave emission that cannot be explained by conventional thermal radiation mechanisms. The dwarf M stars, AD Leonis and Wolf 424, emit rapid spikes whose high brightness temperatures similarly require a nonthermal radiation process. They are attributed to coherent mechanisms such as an electron-cyclotron maser or coherent plasma radiation. If the electron-cyclotron maser emits at the second or third harmonic gyrofrequency, the coronal magnetic field strength equals 250 G or 167 G, and constraints on the plasma frequency imply an electron density of 6 x 10 to the 9th/cu cm. Radio spikes from AD Leonis and Wolf 424 have rise times less than or equal to 5 ms, indicating a linear size of less than or equal to 1.5 x 10 to the 8th cm, or less than 0.005 of the stellar radius. Although Ap magnetic stars have strong dipole magnetic fields, they exhibit no detectable gyroresonant radiation, suggesting that these stars do not have hot, dense coronae. The binary RS CVn star UX Arietis exhibits variable emission at 6 cm wavelength on time scales ranging from 30 s to more than one hour.

  6. A Study of the Star-forming Dwarf Galaxy NGC 855 with Spitzer

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    We present a study of the dwarf elliptical galaxy NGC 855 using the narrow-band Ha and Spitzer data. Both the Ha and Spitzer IRAC images confirm star-forming activity in the center of NGC 855. We obtained a star formation rate (SFR) of 0.022 and 0.025 M☉yr-1, respectively, from the Spitzer IRAC 8.0 μm and MIPS 24 μm emission data. The HI observa tion suggests that the star-forming activity might be triggered by a minor merger. We also find that there is a distinct IR emission region in 5.8 and 8.0μm bands, located at about 10 "away from the nucleus of NGC 855. Given the strong 8.0μm but faint Hα emission, we expect that it is a heavily obscured star-forming region, which needs to be confirmed by further optical spectroscopic observations.

  7. Variable Stars in the Field of the Hydra II Ultra-Faint Dwarf Galaxy

    Science.gov (United States)

    Vivas, Anna Katherina; Olsen, Knut A.; Blum, Robert D.; Nidever, David L.; Walker, Alistair R.; Martin, Nicolas; Besla, Gurtina; Gallart, Carme; Van Der Marel, Roeland P.; Majewski, Steven R.; Munoz, Ricardo; Kaleida, Catherine C.; Saha, Abhijit; Conn, Blair; Jin, Shoko

    2016-06-01

    We searched for variable stars in Hydra II, one of the recently discovered ultra-faint dwarf satellites of the Milky Way, using gri time-series obtained with the Dark Energy Camera (DECam) at Cerro Tololo Inter-American Observatory, Chile. We discovered one RR Lyrae star in the galaxy which was used to derive a distance of 154±8 kpc to this system and to re-calculate its absolute magnitude and half-light radius.A comparison with other RR Lyrae stars in ultra-faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid) which are likely not related with Hydra II.

  8. Inclusion of horizontal branch stars in the derivation of star formation histories of dwarf galaxies: The Carina dSph

    Science.gov (United States)

    Savino, Alessandro; Salaris, Maurizio; Tolstoy, Eline

    2015-11-01

    We present a detailed analysis of the horizontal branch of the Carina dwarf spheroidal galaxy by means of synthetic modelling techniques, taking consistently into account the star formation history and metallicity evolution as determined from main sequence and red giant branch spectroscopic observations. We found that a range of integrated red giant branch mass loss values of 0.1-0.14 M⊙ increasing with metallicity is able to reproduce the colour extension of the old horizontal branch. Nonetheless, leaving the mass loss as the only free parameter is not enough to match the detailed morphology of Carina horizontal branch. We then investigated the role played by the star formation history on the discrepancies between synthetic and observed horizontal branches. We derived a "toy" bursty star formation history that reproduces well the observed horizontal branch star counts, and also matches qualitatively the red giant and the turn-off regions. This bursty star formation history is made of a subset of age and [M/H] components of the star formation history based on turn off and red giants only, and entails four separate bursts of star formation of different strengths, centred at 2, 5, 8.6, and 11.5 Gyr, respectively, with mean [M/H] decreasing from ~-1.7 to ~-2.2 when the age of the burst increases, and with a Gaussian spread of σ 0.1 dex around these mean values. The comparison between the metallicity distribution function of our bursty star formation history and the one measured from the infrared CaT feature using a CaT-[Fe/H] calibration shows a qualitative agreement, once the range of [Ca/Fe] abundances measured in a sample of Carina stars have been taken into account, that causes a bias of the derived [Fe/H] distribution toward values that are too low. In conclusion, we show how the information contained within the horizontal branch of Carina (and dwarf galaxies in general) can be extracted and interpreted to refine the star formation history derived exclusively

  9. Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-like Stars

    Science.gov (United States)

    Kay, C.; Opher, M.; Kornbleuth, M.

    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.

  10. 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.

  11. Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, Galactic chemical evolution, and exoplanets

    CERN Document Server

    Ramirez, I; Lambert, D L; Prieto, C Allende

    2012-01-01

    We derive atmospheric parameters and lithium abundances for 671 stars and include our measurements in a literature compilation of 1381 dwarf and subgiant stars. First, a "lithium desert" in the effective temperature (Teff) versus lithium abundance (A_Li) plane is observed such that no stars with Teff~6075 K and A_Li~1.8 are found. We speculate that most of the stars on the low A_Li side of the desert have experienced a short-lived period of severe surface lithium destruction as main-sequence or subgiant stars. Next, we search for differences in the lithium content of thin-disk and thick-disk stars, but we find that internal processes have erased from the stellar photospheres their possibly different histories of lithium enrichment. Nevertheless, we note that the maximum lithium abundance of thick-disk stars is nearly constant from [Fe/H]=-1.0 to -0.1, at a value that is similar to that measured in very metal-poor halo stars (A_Li~2.2). Finally, differences in the lithium abundance distribution of known planet...

  12. Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy

    CERN Document Server

    Matsuura, M; Bernard-Salas, J; Menzies, J W; Sloan, G C; Whitelock, P A; Wood, P R; Cioni, M -R L; Feast, M W; Lagadec, E; van Loon, J Th; Groenewegen, M A T; Harris, G J

    2007-01-01

    We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal (dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space Telescope. The stars were selected from a near-infrared survey of Fornax and include the three reddest stars, with presumably the highest mass-loss rates, in that galaxy. Such carbon stars probably belong to the intermediate-age population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary aim of this paper is to investigate mass-loss rate, as a function of luminosity and metallicity, by comparing AGB stars in several galaxies with different metallicities. The spectra of three stars are fitted with a radiative transfer model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We find no evidence that these rates depend on metallicity, although we do suggest that the gas-to-dust ratio could be higher than at solar metallicity, in the range 240 to 800. The C2H...

  13. Selection and Semi-Dwarf Allele Mutants Segregation Pattern as the Result of Gamma Ray Irradiation of West Sumatera Black Rice

    Directory of Open Access Journals (Sweden)

    Benny Warman

    2015-01-01

    Full Text Available Black rice is a local rice originating from East Pasaman which has a low yield potential, long panicles, too high of longevity and plant height and soeasily fall. To reduce  plant height has been irradiated rice seedsas much as 200 grams of black rice with a dose of 200 Gy of gamma rays. This study aimed to obtain information about the frequency of mutant semi-dwarf/dwarf and segregation patterns in mutant alleles black rice as the genetic improvement of West Sumatra local rice through mutation induction.  From the selection of the M2 generation gained 31 candidates dwarf/semi dwarf mutant frequency of 0.062%. also obtained information that the segregation of alleles that make up the character ofthe semi dwarf/dwarf rodon rice mutant semi dwarf/dwarf trunked together with the distribution pattern of segregation according to Mendel's Law. On the character of the semi-dwarf/dwarf stem is influenced by one or two groups of alleles, where one of the alleles controlling the properties of semi-dwarf/dwarf stem, while the other controls the high stemproperties. Alleles from each group are in a locus that its chromosomes separate.It is evident that the semi-dwarf/dwarf on the mutant is occured due to one or more alleles is dominant on the high stem properties controlling alleles group toward the recessive.

  14. Binary stars as probes of dark substructures in dwarf galaxies

    CERN Document Server

    Penarrubia, Jorge; Walker, Matthew G; Gilmore, Gerry; Evans, N Wyn; Mackay, Craig D

    2010-01-01

    We use analytical and N-body methods to examine the survival of wide stellar binaries against repeated encounters with dark substructures orbiting in the dark matter haloes of dwarf spheroidal galaxies (dSphs). Our models adopt cosmologically-motivated conditions wherein dSphs are dark-matter dominated systems that form hierarchically and orbit about a host galaxy. Our analytical estimates show that wide binaries are disrupted at a rate that is proportional to the local density of dark substructures averaged over the life-time of the binary population. The fact that external tides can efficiently strip dark substructures from the outskirts of dSphs implies that the present number and distribution of binaries is strongly coupled with the mass evolution of individual galaxies. Yet we show that for the range of dynamical masses and Galactocentric distances spanned by Milky Way dSphs, a truncation in the separation function at a_max <~ 0.1 pc is expected in all these galaxies. An exception may be the Sagittari...

  15. Angular momentum and the formation of stars and black holes

    International Nuclear Information System (INIS)

    The formation of compact objects such as stars and black holes is strongly constrained by the requirement that nearly all of the initial angular momentum of the diffuse material from which they form must be removed or redistributed during the formation process. The mechanisms that may be involved and their implications are discussed for (1) low-mass stars, most of which probably form in binary or multiple systems; (2) massive stars, which typically form in clusters and (3) supermassive black holes that form in galactic nuclei. It is suggested that in all cases, gravitational interactions with other stars or mass concentrations in a forming system play an important role in redistributing angular momentum and thereby enabling the formation of a compact object. If this is true, the formation of stars and black holes must be a more complex, dynamic and chaotic process than in standard models. The gravitational interactions that redistribute angular momentum tend to couple the mass of a forming object to the mass of the system, and this may have important implications for mass ratios in binaries, the upper stellar IMF in clusters, and the masses of supermassive black holes in galaxies.

  16. Inclusion of Horizontal Branch stars in the derivation of star formation histories of dwarf galaxies: the Carina dSph

    CERN Document Server

    Savino, Alessandro; Tolstoy, Eline

    2015-01-01

    We present a detailed analysis of the Horizontal Branch of the Carina Dwarf Spheroidal Galaxy by means of synthetic modelling techniques, taking consistently into account the star formation history and metallicity evolution as determined from main sequence and red giant branch spectroscopic observations. We found that a range of integrated red giant branch mass loss values of 0.1-0.14 M, increasing with metallicity, is able to reproduce the colour extension of the old Horizontal Branch. However, leaving the mass loss as the only free parameter is not enough to match the detailed morphology of Carina Horizontal Branch. We explored the role played by the star formation history on the discrepancies between synthetic and observed Horizontal Branches. We derived a toy bursty star formation history that reproduces the horizontal branch star counts, and also matches qualitatively the red giant and the turn off regions. This star formation history is made of a subset of age and [M/H] components of the star formation ...

  17. EL CVn-type binaries - Discovery of 17 helium white dwarf precursors in bright eclipsing binary star systems

    CERN Document Server

    Maxted, P F L; Heber, U; Geier, S; Wheatley, P J; Marsh, T R; Breedt, E; Sebastian, D; Faillace, G; Owen, C; Pulley, D; Smith, D; Kolb, U; Haswell, C A; Southworth, J; Anderson, D R; Smalley, B; Cameron, A Collier; Hebb, L; Simpson, E K; West, R G; Bochinski, J; Busuttil, R; Hadigal, S

    2013-01-01

    The star 1SWASP J024743.37-251549.2 was recently discovered to be a binary star in which an A-type dwarf star eclipses the remnant of a disrupted red giant star (WASP0247-25B). The remnant is in a rarely-observed state evolving to higher effective temperatures at nearly constant luminosity prior to becoming a very low-mass white dwarf composed almost entirely of helium, i.e., it is a pre-He-WD. We have used the WASP photometric database to find 17 eclipsing binary stars with orbital periods P=0.7 to 2.2 days with similar lightcurves to 1SWASP J024743.37-251549.2. The only star in this group previously identified as a variable star is the brightest one, EL CVn, which we adopt as the prototype for this class of eclipsing binary star. The characteristic lightcurves of EL CVn-type stars show a total eclipse by an A-type dwarf star of a smaller, hotter star and a secondary eclipse of comparable depth to the primary eclipse. We have used new spectroscopic observations for 6 of these systems to confirm that the comp...

  18. Insights into Pre-Enrichment of Star Clusters and Self-Enrichment of Dwarf Galaxies from their Intrinsic Metallicity Dispersions

    CERN Document Server

    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 $\\bar{Z}$ and intrinsic spread in metallicity $\\sigma(Z)^{2}$. A plot of $\\sigma(Z)^{2}$ versus $\\bar{Z}$ 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 (e.g., Oey 2000), where the star cluster and dwarf galaxy behaviour in the $\\sigma(Z)^{2}-\\bar{Z}$ diagram is reproduced in terms of the number of enrichment events, covering fraction, and intrinsic size of the...

  19. Variable Stars in the Field of the Hydra II Ultra-faint Dwarf Galaxy

    Science.gov (United States)

    Vivas, A. Katherina; Olsen, Knut; Blum, Robert; Nidever, David L.; Walker, Alistair R.; Martin, Nicolas F.; Besla, Gurtina; Gallart, Carme; van der Marel, Roeland P.; Majewski, Steven R.; Kaleida, Catherine C.; Muñoz, Ricardo R.; Saha, Abhijit; Conn, Blair C.; Jin, Shoko

    2016-05-01

    We report the discovery of one RR Lyrae star in the ultra-faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at Cerro Tololo Inter-American Observatory, Chile. The association of the RR Lyrae star discovered here with Hydra II is clear because is located at 42\\prime\\prime from the center of the dwarf, well within its half-light radius of 102\\prime\\prime . The RR Lyrae star has a mean magnitude of i=21.30+/- 0.04 which is too faint to be a field halo star. This magnitude translates to a heliocentric distance of 151 ± 8 kpc for Hydra II; this value is ∼ 13% larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of {76}-10+12 pc and a brighter absolute magnitude of {M}V=-5.1+/- 0.3, which keeps this object within the realm of the dwarf galaxies. A comparison with other RR Lyrae stars in ultra-faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid) which are likely not related with Hydra II.

  20. Axion cyclotron emissivity of magnetized white dwarfs and neutron stars

    OpenAIRE

    Kachelriess, M.; Wilke, C.; Wunner, G.

    1997-01-01

    The energy loss rate of a magnetized electron gas emitting axions a due to the process $e^- \\to e^- +a$ is derived for arbitrary magnetic field strength B. Requiring that for a strongly magnetized neutron star the axion luminosity is smaller than the neutrino luminosity we obtain the bound $g_{ae}\\lsim 10^{-10}$ for the axion electron coupling constant. This limit is considerably weaker than the bound derived earlier by Borisov and Grishina using the same method. Applying a similar argument t...

  1. First axion bounds from a pulsating helium-rich white dwarf star

    CERN Document Server

    Battich, Tiara; Althaus, Leandro Gabriel; Bertolami, Marcelo Miguel Miller

    2016-01-01

    The Peccei-Quinn mechanism proposed to solve the CP problem of Quantum Chromodynamics has as consequence the existence of axions, hypothetical weakly interacting particles whose mass is constrained to be on the sub-eV range. If these particles exist and interact with electrons, they would be emitted from the dense interior of white dwarfs, becoming an important energy sink for the star. Due to their well known physics, white dwarfs are good laboratories to study the properties of fundamental particles such as the axions. We study the general effect of axion emission on the evolution of helium-rich white dwarfs and on their pulsational properties. To this aim, we calculate evolutionary helium-rich white dwarf models with axion emission, and asses the pulsational properties of this models. Our results indicate that the rates of change of pulsation periods are significantly affected by the existence of axions. We are able for the first time to independently constrain the mass of the axion from the study of pulsa...

  2. VLT/FLAMES spectroscopy of Red Giant Branch stars in the Carina dwarf spheroidal galaxy

    CERN Document Server

    Lemasle, B; Tolstoy, E; Venn, K A; Shetrone, M D; Irwin, M J; de Boer, T J L; Starkenburg, E; Salvadori, S

    2011-01-01

    The ages of individual Red Giant Branch stars (RGB) can range from 1 Gyr old to the age of the Universe, and it is believed that the abundances of most chemical elements in their photospheres remain unchanged with time (those that are not affected by the 1st dredge-up). This means that they trace the ISM in the galaxy at the time the star formed, and hence the chemical enrichment history of the galaxy. CMD analysis has shown the Carina dwarf spheroidal (dSph) to have had an unusually episodic star formation history (SFH) which is expected to be reflected in the abundances of different chemical elements. We use the VLT-FLAMES spectrograph in HR mode (R~20000) to measure the abundances of several chemical elements in a sample of 35 RGB stars in Carina. We also combine these abundances with photometry to derive age estimates for these stars. This allows us to determine which of two distinct star formation (SF) episodes the stars in our sample belong to, and thus to define the relationship between SF and chemical...

  3. Subaru/HDS Abundances in Three Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy

    CERN Document Server

    Sadakane, K; Ikuta, C; Aoki, W; Jablonka, P; Tajitsu, A

    2004-01-01

    With the HDS (High Dispersion Spectrograph) on the Subaru telescope, we obtained high resolution optical region spectra of three red giant stars (cos 4, cos 82, and cos 347) in the Ursa Minor dwarf spheriodal galaxy. Chemical abundances in these stars have been analysed for 26 elements including alpha-, iron-peak, and neutron capture elements. All three stars show low abundances of alpha-elements (Mg, Si, and Ca) and two stars (cos 82 and cos 347) show high abundance of Mn compared to Galactic halo stars of similar metallicity. One star (cos 4) has been confirmed to be very metal deficient ([Fe/H]=-2.7) and found to show anomalously low abundances of Mn, Cu, and Ba. In another star cos 82 ([Fe/H]=-1.5), we have found large excess of heavy neutron-capture elements with the general abundance pattern similar to the scaled solar system r-process abundance curve. These observational results are rather puzzling: low abundances of alpha-elements and high abundance of Mn seem to sugggest a significant contribution of...

  4. Jets from Tidal Disruptions of Stars by Black Holes

    OpenAIRE

    Krolik, Julian H.; Piran, Tsvi

    2012-01-01

    Tidal disruption of main sequence stars by black holes has generally been thought to lead to a signal dominated by UV emission. If, however, the black hole spins rapidly and the poloidal magnetic field intensity on the black hole horizon is comparable to the inner accretion disk pressure, a powerful jet may form whose luminosity can easily exceed the thermal UV luminosity. When the jet beam points at Earth, its non-thermal luminosity can dominate the emitted spectrum. The thermal and non-ther...

  5. PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS

    International Nuclear Information System (INIS)

    We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (≳1 M 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 ☉) 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 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 Jup; L0−1+2; 120 ± 20 AU), GJ 3629 B (64−23+30 M Jup; M7.5 ± 0.5; 6.5 ± 0.5 AU), 1RXS J034231.8+121622 B (35 ± 8 M Jup; L0 ± 1; 19.8 ± 0.9 AU), and 2MASS J15594729+4403595 B (43 ± 9 M 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 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 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 Jup) companions to single M dwarfs between 10-100 AU is 2.8−1.5+2.4%. Altogether we find that

  6. Planets around Low-mass Stars (PALMS). IV. The Outer Architecture of M Dwarf Planetary Systems

    Science.gov (United States)

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Tamura, Motohide

    2015-01-01

    We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (gsim1 M Jup) around 122 newly identified nearby (lsim40 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 ⊙) 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 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 Jup; L0+2-1; 120 ± 20 AU), GJ 3629 B (64+30-23 M Jup; M7.5 ± 0.5; 6.5 ± 0.5 AU), 1RXS J034231.8+121622 B (35 ± 8 M Jup; L0 ± 1; 19.8 ± 0.9 AU), and 2MASS J15594729+4403595 B (43 ± 9 M 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 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 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 Jup) companions to single M dwarfs between 10-100 AU is 2.8+2.4-1.5%. Altogether we find that giant planets, especially massive ones, are rare

  7. 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

  8. Satellite dwarf galaxies in a hierarchical universe: the prevalence of dwarf-dwarf major mergers

    International Nuclear Information System (INIS)

    Mergers are a common phenomenon in hierarchical structure formation, especially for massive galaxies and clusters, but their importance for dwarf galaxies in the Local Group remains poorly understood. We investigate the frequency of major mergers between dwarf galaxies in the Local Group using the ELVIS suite of cosmological zoom-in dissipationless simulations of Milky Way- and M31-like host halos. We find that ∼10% of satellite dwarf galaxies with M star > 106 M ☉ that are within the host virial radius experienced a major merger of stellar mass ratio closer than 0.1 since z = 1, with a lower fraction for lower mass dwarf galaxies. Recent merger remnants are biased toward larger radial distance and more recent virial infall times, because most recent mergers occurred shortly before crossing within the virial radius of the host halo. Satellite-satellite mergers also occur within the host halo after virial infall, catalyzed by the large fraction of dwarf galaxies that fell in as part of a group. The merger fraction doubles for dwarf galaxies outside of the host virial radius, so the most distant dwarf galaxies in the Local Group are the most likely to have experienced a recent major merger. We discuss the implications of these results on observable dwarf merger remnants, their star formation histories, the gas content of mergers, and massive black holes in dwarf galaxies.

  9. A Survey of Local Group Galaxies Currently Forming Stars: \\\\II. UBVRI Photometry of Stars in Seven Dwarfs

    CERN Document Server

    Massey, P; Hodge, P W; Jacoby, G H; McNeill, R T; Smith, R C; Strong, S B; Massey, Philip; Hodge, Paul W.; Jacoby, George H.; Neill, Reagin T. Mc; Strong, Shay B.

    2007-01-01

    We have obtained UBVRI images with the Kitt Peak and Cerro Tololo 4-m telescopes and Mosaic cameras of seven dwarfs in (or near) the Local Group, all of which have known evidence of recent star formation: IC10, NGC 6822, WLM, Sextans B, Sextans A, Pegasus,and Phoenix. We construct color-magnitude diagrams (CMDs) of these systems, as well as neighboring regions that can be used to evaluate the degree of foreground contamination by stars in the Milky Way. Inter-comparison of these CMDs with those of M31, M33, the LMC, and the SMC permits us to determine improved reddening values for a typical OB star found within these galaxies. All of the CMDs reveal a strong or modest number of blue supergiants. All but Pegasus and Phoenix also show the clear presence of red supergiants in the CMD, although IC10 appears to be deficient in these objects given its large WR population. The bright stars of intermediate color in the CMD are badly contaminated by foreground stars (30-100%), and considerable spectroscopy is needed b...

  10. Stationarity of Extremum Entropy Stars and Black Holes

    Science.gov (United States)

    Schiffrin, Joshua

    2015-04-01

    For axisymmetric perfect-fluid stars in general relativity, we show that extrema of total entropy at fixed mass, angular momentum, and particle number are stationary. For axisymmetric vacuum black holes, we show that extrema of apparent-horizon area at fixed mass and angular momentum are stationary.

  11. Radio continuum observations of the candidate supermassive black hole in the dwarf elliptical VCC128

    CERN Document Server

    Buyle, Pieter; Debattista, Victor P; Ferreras, Ignacio; Pasquali, Anna; Seth, Anil; Morelli, Lorenzo

    2008-01-01

    The presence of black holes (BHs) at the centers of dwarf elliptical galaxies (dEs) has been argued both theoretically and observationally. Using archival HST/WFPC2 data, we found the Virgo cluster dwarf elliptical galaxy VCC128 to harbor a binary nucleus, a feature that is usually interpreted as the observable signature of a stellar disk orbiting a central massive black hole. Debattista et al. 2006 estimated its mass M sim 6 10^6 - 5 10^7 Msun. One of the most robust means of verifying the existence of a BH is radio continuum and/or X-ray emission, however because of the deficiency of gas in dEs, radio continuum emission forms the best option here. We have tried to detect the X-band radio emission coming from the putative black hole in VCC128 when it accretes gas from the surrounding ISM. While we made a positive 4 sigma detection of a point source 4.63'' south-west of the binary nucleus, no statistically significant evidence for emission associated with the nuclei themselves was detected. This implies eithe...

  12. 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.

  13. Star formation and molecular hydrogen in dwarf galaxies: a non-equilibrium view

    CERN Document Server

    Hu, Chia-Yu; Walch, Stefanie; Glover, Simon C O; Clark, Paul C

    2015-01-01

    We study the connection of star formation to atomic (HI) and molecular hydrogen (H$_2$) in isolated, low metallicity dwarf galaxies with high-resolution ($m_{\\rm gas}$ = 4 M$_\\odot$, $N_{\\rm ngb}$ = 100) SPH simulations. The model includes self-gravity, non-equilibrium cooling, shielding from an interstellar radiation field, the chemistry of H$_2$ formation, H$_2$-independent star formation, supernova feedback and metal enrichment. We find that the H$_2$ mass fraction is sensitive to the adopted dust-to-gas ratio and the strength of the interstellar radiation field, while the star formation rate is not. Star formation is regulated by stellar feedback, keeping the gas out of thermal equilibrium for densities $n <$ 1 cm$^{-3}$. Because of the long chemical timescales, the H$_2$ mass remains out of chemical equilibrium throughout the simulation. Star formation is well-correlated with cold ( T $\\leqslant$ 100 K ) gas, but this dense and cold gas - the reservoir for star formation - is dominated by HI, not H$_2...

  14. Stellar populations and Star Formation History of the Metal-Poor Dwarf Galaxy DDO 68

    CERN Document Server

    Sacchi, E; Cignoni, M; Aloisi, A; Sohn, T; Tosi, M; van der Marel, R P; Grocholski, A J; James, B

    2016-01-01

    We present the star formation history of the extremely metal-poor dwarf galaxy DDO~68, based on our $V-$ and $I-$ band photometry with the Advanced Camera for Surveys on board of the Hubble Space Telescope. With a metallicity of only $12+\\log(O/H)=7.15$ and an isolated location in the periphery of the nearby Lynx-Cancer void, DDO~68 is one of the most metal poor galaxies known. It has been argued in the past that DDO~68 is a young system that started forming stars only $\\sim 0.15$~Gyr ago. Our data provide a deep and uncontaminated optical color-magnitude diagram that now allows us to disprove this hypothesis, since we find a population of at least $\\sim 1$~Gyr old stars. The star formation activity has been fairly continuous over all the look-back time. The current rate is quite low, and the highest activity occurred between 10 and 100 Myr ago. The average star formation rate over the whole Hubble time is \\mbox{$\\simeq 0.01$ M$_{\\odot}$ yr$^{-1}$}, providing a total mass of formed stars of \\mbox{$\\simeq 1.3 ...

  15. Multi-fibre optical spectroscopy of low-mass stars and brown dwarfs in Upper Sco

    CERN Document Server

    Lodieu, N; Hambly, N C

    2011-01-01

    We have obtained multi-fibre intermediate-resolution optical spectroscopy of 94 photometric and proper motion selected low-mass star and brown dwarf candidates in Upper Sco with AAT/AAOmega. We have estimated the spectral types and measured the equivalent widths of youth and gravity diagnostic features to confirm the spectroscopic membership of about 95% of the candidates extracted from 6.5 square degrees in Upper Sco. We also detect lithium in the spectra with the highest signal-to-noise, consolidating our conclusions about their youth. Furthermore, we derive an estimate of our selections using spectroscopic data obtained for a large number of stars falling into the instrument's field-of-view. We have estimated the effective temperatures and masses for each new spectroscopic member using the latest evolutionary models available for low-mass stars and brown dwarfs. Combining the current optical spectroscopy presented here with near-infrared spectroscopy obtained for the faintest photometric candidates, we con...

  16. Chemical gradients in the Milky Way from the RAVE data. I. Dwarf stars

    CERN Document Server

    Boeche, C; Piffl, T; Just, A; Steinmetz, M; Sharma, S; Kordopatis, G; Gilmore, G; Chiappini, C; Williams, M; Grebel, E K; Bland-Hawthorn, J; Gibson, B K; Munari, U; Siviero, A; Bienaymé, O; Navarro, J F; Parker, Q A; Reid, W; Seabroke, G M; Watson, F G; Wyse, R F G; Zwitter, T

    2013-01-01

    Aim: We aim at measuring the chemical gradients of the elements Mg, Al, Si, and Fe along the Galactic radius to provide new constraints on the chemical evolution models of the Galaxy and Galaxy models such as the Besancon model. Methods: We analysed three different samples selected from three independent datasets: a sample of 19,962 dwarf stars selected from the RAVE database, a sample of 10,616 dwarf stars selected from the Geneva-Copenhagen Survey (GCS) dataset, and a mock sample (equivalent to the RAVE sample) created by using the GALAXIA code, which is based on the Besancon model. We measured the chemical gradients as functions of the guiding radius (Rg) at different distances from the Galactic plane reached by the stars along their orbit (Zmax). Results: The chemical gradients of the RAVE and GCS samples are negative and show consistent trends, although they are not equal: at Zmax<0.4 kpc and 4.5

  17. Constraints on MACHO Dark Matter from the Star Cluster in the Dwarf Galaxy Eridanus II

    CERN Document Server

    Brandt, Timothy D

    2016-01-01

    I show that a recently discovered star cluster near the center of the ultra-faint dwarf galaxy Eridanus II provides strong constraints on massive compact halo objects (MACHOs) of >~5 M_sun as the main component of dark matter. MACHO dark matter will dynamically heat the cluster, driving it to larger sizes and higher velocity dispersions until it dissolves into its host galaxy. The star cluster has a luminosity of just ~2000 L_sun and is relatively puffy, with a half-light radius of 13 pc, making it much more fragile than other known clusters in dwarf galaxies. For a wide range of plausible dark matter halo properties, Eri II's star cluster combines with existing constraints from microlensing, wide binaries, and disk kinematics to rule out dark matter composed entirely of MACHOs from ~10$^{-7}$ M_sun up to arbitrarily high masses. The cluster in Eri II closes the ~20--100 M_sun window of allowed MACHO dark matter and provides much stronger constraints than wide Galactic binaries for MACHOs of up to thousands o...

  18. 3D molecular line formation in dwarf carbon-enhanced metal-poor stars

    CERN Document Server

    Behara, N T; Bonifacio, P; Sbordone, L; Hernandez, J I Gonzalez; Caffau, E

    2009-01-01

    We present a detailed analysis of the carbon and nitrogen abundances of two dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229 and SDSS J0912+0216. We also report the oxygen abundance of SDSS J1349-0229. These stars are metal-poor, with [Fe/H] < -2.5, and were selected from our ongoing survey of extremely metal-poor dwarf candidates from the Sloan Digital SkySurvey (SDSS). The carbon, nitrogen and oxygen abundances rely on molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by `classical' 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. Using CO5BOLD 3D hydrodynamical model atmospheres and the Linfor3D line formation code, molecular lines of CH, NH, OH and C2 were computed, and 3D carbon, nitrogen and oxygen abundances were determined. The resulting carbon abundances were compared to abundances derived using atom...

  19. The Recent Star Formation Histories of M81 Group Dwarf Irregular Galaxies

    CERN Document Server

    Weisz, Daniel R; Cannon, John M; Dolphin, Andrew E; Kennicutt, Robert C; Lee, Janice; Walter, Fabian

    2008-01-01

    We present observations and analysis of nine dwarf irregular galaxies (dIs) in the M81 Group taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The nine galaxy sample (the Garland, M81 Dwarf A, DDO 53, Ho IX, Ho I, DDO 165, NGC 2366, Ho II, and IC 2574) spans 6 magnitudes in luminosity, a factor of 1000 in current star formation rate, and 0.5 dex in metallicity. Here we use color-magnitude diagrams of resolved stellar populations to study the star formation histories (SFHs) of these galaxies. We divide the sample into faint and bright galaxies, with a dividing line of M_${B}$ = -15, and then analyze the similarities and differences in the SFHs, birthrate parameters, fraction of stars formed per time interval, and spatial distribution of stellar components. Comparing these parameters as a function of luminosity, we find only minor differences in SF characteristics. We extend our comparison to select dIs in the Local Group (LG), and find only minor differences in SF parameters. The fr...

  20. Carbon and nitrogen abundances of individual stars in the Sculptor dwarf spheroidal galaxy

    CERN Document Server

    Lardo, C; Pancino, E; Romano, D; de Boer, T J L; Starkenburg, E; Tolstoy, E; Irwin, M J; Jablonka, P; Tosi, M

    2015-01-01

    We present [C/Fe] and [N/Fe] abundance ratios and CH({\\lambda}4300) and S({\\lambda}3883) index measurements for 94 red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving power R= 1150 at 4020 {\\AA}. This is the first time that [N/Fe] abundances are derived for a large number of stars in a dwarf spheroidal. We found a trend for the [C/Fe] abundance to decrease with increasing luminosity on the RGB across the whole metallicity range, a phenomenon observed in both field and globular cluster giants, which can be interpreted in the framework of evolutionary mixing of partially processed CNO material. Both our measurements of [C/Fe] and [N/Fe] are in good agreement with the theoretical predictions for stars at similar luminosity and metallicity. We detected a dispersion in the carbon abundance at a given [Fe/H], which cannot be ascribed to measurement uncertainties alone. We interpret this observational evidence as the result of the contribution of differ...

  1. Stark Broadening of Carbon and Oxygen Lines in Hot DQ White Dwarf Stars: Recent Results and Applications

    OpenAIRE

    Dufour, P.; Nessib, N. Ben; Sahal-Bréchot, S.; Dimitrijević, M. S.

    2011-01-01

    White dwarf stars are traditionally found to have surface compositions made primarily of hydrogen or helium. However, a new family has recently been uncovered, the so-called Hot DQ white dwarfs, which have surface compositions dominated by carbon and oxygen with little or no trace of hydrogen and helium (Dufour et al. 2007, 2008, 2010). Deriving precise atmospheric parameters for these objects (such as the effective temperature and the surface gravity) requires detailed modeling of spectral l...

  2. Black hole formation from axion stars

    CERN Document Server

    Helfer, Thomas; Clough, Katy; Fairbairn, Malcolm; Lim, Eugene A; Becerril, Ricardo

    2016-01-01

    The classical equations of motion for an axion with potential $V(\\phi)=m_a^2f_a^2 [1-\\cos (\\phi/f_a)]$ possess quasi-stable, localized, oscillating solutions, which we refer to as "axion stars". We study, for the first time, collapse of axion stars numerically using the full non-linear Einstein equations of general relativity and the full non-perturbative cosine potential. We map regions on an "axion star stability diagram", parameterized by the initial ADM mass, $M_{\\rm ADM}$, and axion decay constant, $f_a$. We identify three regions of the parameter space: i) long-lived oscillating axion star solutions, with a base frequency, $m_a$, modulated by self-interactions, ii) collapse to a BH and iii) complete dispersal due to gravitational cooling and interactions. We locate the boundaries of these three regions and an approximate "triple point" $(M_{\\rm TP},f_{\\rm TP})\\sim (2.4 M_{pl}^2/m_a,0.3 M_{pl})$. For $f_a$ below the triple point BH formation proceeds during winding (in the complex $U(1)$ picture) of the ...

  3. Quantifying the Coexistence of Massive Black Holes and Dense Nuclear Star Clusters

    CERN Document Server

    Graham, Alister W

    2009-01-01

    In large spheroidal stellar systems, such as elliptical galaxies, one invariably finds a 10^6-10^9 M_Sun supermassive black hole at their centre. In contrast, within dwarf elliptical galaxies one predominantly observes a 10^5-10^7 M_Sun nuclear star cluster. To date, few galaxies have been found with both type of nuclei coexisting and even less have had the masses determined for both central components. Here we identify one dozen galaxies housing nuclear star clusters and supermassive black holes whose masses have been measured. This doubles the known number of such hermaphrodite nuclei - which are expected to be fruitful sources of gravitational radiation. Over the host spheroid (stellar) mass range from 10^8 to 10^11 M_Sun, we find that a galaxy's nucleus-to-spheroid (baryon) mass ratio is not a constant value but decreases from a few percent to ~0.3 percent such that log[(M_BH+M_NC)/M_sph] = -(0.39+/-0.07)log[M_sph/10^10 M_Sun] -(2.18+/-0.07). Once dry merging has commenced by M_sph ~ 10^11 M_Sun and the n...

  4. Implications of primordial black holes on the first stars and the origin of the super--massive black holes

    OpenAIRE

    Bambi, Cosimo; Spolyar, Douglas; Dolgov, Alexander D.; Freese, Katherine; Volonteri, Marta

    2008-01-01

    If the cosmological dark matter has a component made of small primordial black holes, they may have a significant impact on the physics of the first stars and on the subsequent formation of massive black holes. Primordial black holes would be adiabatically contracted into these stars and then would sink to the stellar center by dynamical friction, creating a larger black hole which may quickly swallow the whole star. If these primordial black holes are heavier than $\\sim 10^{22} {\\rm g}$, the...

  5. Variable Stars in the Newly Discovered Milky Way Dwarf Spheroidal Satellite Canes Venatici I

    CERN Document Server

    Kuehn, Charles; Ripepi, Vincenzo; Clementini, Gisella; Dall'Ora, Massimo; Di Fabrizio, Luca; Rodgers, Christopher T; Greco, Claudia; Marconi, Marcella; Musella, Ilaria; Smith, Horace A; Catelan, Márcio; Beers, Timothy C; Pritzl, Barton J

    2007-01-01

    We have identified 23 RR Lyrae stars and 3 possible Anomalous Cepheids among 84 candidate variables in the recently discovered Canes Venatici I dwarf spheroidal galaxy. The mean period of 18 RRab type stars is = 0.60 +/-0.01 days. This period, and the location of these stars in the period-amplitude diagram, suggest that Canes Venatici I is likely an Oosterhoff-intermediate system. The average apparent magnitude of the RR Lyrae stars = 22.17 +/-0.02 is used to obtain a precision distance estimate of 210 +7/-5 kpc, for an adopted reddening E(B-V)=0.03 mag. We present a B,V color-magnitude diagram of Canes Venatici I that reaches V about 5 mag, and shows that the galaxy has a mainly old stellar population with a metal abundance near [Fe/H] = -2.0 dex. The width of the red giant branch and the location of the candidate Anomalous Cepheids on the color-magnitude diagram may indicate that the galaxy hosts a complex stellar population with stars from about 13 Gyr to as young as about 0.6 Gyr.

  6. THE STELLAR POPULATION AND STAR FORMATION PROPERTIES OF BLUE COMPACT DWARF GALAXIES

    International Nuclear Information System (INIS)

    We study stellar populations, star formation histories (SFHs), and star formation properties for a sample of blue compact dwarf galaxies (BCDs) selected by cross-correlating the Gil de Paz et al. sample with the Sloan Digital Sky Survey Data Release 6. The sample includes 31 BCDs, which span a large range of galactic parameters. Using a stellar population synthesis method, we derive stellar populations and reconstruct SFHs for these BCDs. Our studies confirm that BCDs are not young systems experiencing their first star formation, but old systems undergoing a starburst activity. The stellar mass-weighted ages can be up to 10 Gyr, while the luminosity-weighted ages might be up to approximately three orders of magnitude younger (∼10 Myr) for most galaxies. Based on multiwavelength data, we also study the integrated star formation properties. The star formation rate (SFR) for our sample galaxies spans nearly three orders of magnitude, from a few 10-3 to ∼1 Msun yr-1, with a median value of ∼0.1 Msun yr-1. We find that about 90% of BCDs in our sample have their birthrate parameter (the ratio of the current SFR to the averaged past SFR) b>2-3. We further discuss correlations of the current SFR with the integrated galactic stellar mass and explore the connection between SFR and metallicity.

  7. The Stellar Population and Star Formation Properties of Blue Compact Dwarf Galaxies

    CERN Document Server

    Zhao, Yinghe; Gao, Yu

    2010-01-01

    We study the stellar populations, star formation histories and star formation properties for a sample of blue compact dwarf galaxies (BCDs) selected by cross-correlating the Gil de Paz et al. (2003) sample with the Sloan Digital Sky Survey Data Release 6 (SDSS DR6). The sample includes 31 BCDs, which span a large range in galactic parameters. Using a stellar population synthesis method, we derive the stellar populations and reconstruct the star formation histories for these BCDs. Our studies confirm that BCDs are not young systems experiencing their first star formation but old systems undergoing a starburst activity. The stellar mass-weighted ages can be as old as 10 Gyr while the luminosity-weighted ages might be up to $\\sim 3$ orders of magnitude younger ($\\sim 10$ Myr) for most galaxies. Based on multi-wavelength data, we also study the integrated star formation properties. The SFR for our sample galaxies spans nearly 3 orders of magnitude, from a few $10^{-3}$ to $\\sim1\\,M_\\odot$ yr$^{-1}$, with the medi...

  8. The Star Formation Histories of Local Group Dwarf Galaxies II. Searching For Signatures of Reionization

    CERN Document Server

    Weisz, Daniel R; Skillman, Evan D; Holtzman, Jon; Gilbert, Karoline M; Dalcanton, Julianne J; Williams, Benjamin F

    2014-01-01

    We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (10$^4$ $<$ M$_{\\star}$ $<$ 10$^9$ M$_{\\odot}$). 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, 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 & 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 environment...

  9. Variable stars in the field of the Hydra II ultra-faint dwarf galaxy

    CERN Document Server

    Vivas, A Katherina; Olsen, Knut; Blum, Robert; Walker, Alistair; Martin, Nicolas; Besla, Gurtina; Gallart, Carme; van der Marel, Roeland; Majewski, Steven; Kaleida, Catherine; Muñoz, Ricardo; Conn, Blair; Jin, Shoko

    2015-01-01

    We report the discovery of one RR Lyrae star in the ultra--faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at Cerro Tololo Interamerican Observatory, Chile. The RR Lyrae star has a mean magnitude of $i = 21.30\\pm 0.04$ which translates to a heliocentric distance of $151\\pm 8$ kpc for Hydra II; this value is $\\sim 13\\%$ larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of $76^{+12}_{-10}$ pc and a brighter absolute magnitude of $M_V = -5.1 \\pm 0.3$, which keeps this object within the realm of the dwarf galaxies. The pulsational properties of the RR Lyrae star ($P=0.645$ d, $\\Delta g = 0.68$ mag) suggest Hydra II may be a member of the intermediate Oosterhoff or Oosterhoff II group. A comparison with other RR Lyrae stars in ultra--faint systems indicates similar pulsational properties among them...

  10. The Fermi Gamma-Ray Space Telescope, Exploding Stars, Neutron Stars, and Black Holes

    Science.gov (United States)

    Thompson, David J.

    2010-01-01

    Since August, 2008, the Fermi Gamma-ray Space Telescope has been scanning the sky, producing a full-sky image every three hours. These cosmic gamma-rays come from extreme astrophysical phenomena, many related to exploding stars (supernovae) or what these explosions leave behind: supernova remnants, neutron stars, and black holes. This talk uses sample Fermi results, plus simple demonstrations, to illustrate the exotic properties of these endpoints of stellar evolution.

  11. Planets Around Low-Mass Stars (PALMS). IV. The Outer Architecture of M Dwarf Planetary Systems

    CERN Document Server

    Bowler, Brendan P; Shkolnik, Evgenya L; Tamura, Motohide

    2014-01-01

    We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (>1 MJup) 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). 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 corresponds to limiting planet masses of 0.5-10 MJup at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 $\\pm$ 6 MJup; L0$^{+2}_{-1}$; 120 $\\pm$ 20 AU), GJ 3629 B (64$^{+30}_{-23}$ MJup; M7.5 $\\pm$ 0.5; 6.5 $\\pm$ 0.5 AU), 1RXS J034231.8+121622 B (35 $\\pm$ 8 MJup; L0 $\\pm$ 1; 19.8 $\\pm$ 0.9 AU), and 2MASS J15594729+4403595 B (43 $\\pm$ 9 MJup; M8.0 $\\pm$ 0.5; 190 $\\pm$ 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...

  12. Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars

    CERN Document Server

    Kay, C; 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, the location of the minimum in the background magnetic field. Solar-type CMEs behave similarly, but have much smaller deflections and do not get trapped at the Astrospheric Current Sheet. The probability of planetary impact decreases with increasing inclination of the planetary orbi...

  13. Constraining the nature of dark matter with the star formation history of the faintest Local Group dwarf galaxy satellites

    OpenAIRE

    Chau, Alice; Mayer, Lucio; Governato, Fabio

    2016-01-01

    $\\Lambda$-Warm Dark Matter (WDM) has been proposed as alternative scenario to $\\Lambda$ cold dark matter (CDM), motivated by discrepancies at the scale of dwarf galaxies, with less small-scale power and realized by collisionless particles with energies in the range $1-3$ keV. We present a new approach to constrain the viability of such WDM models using star formation histories of the dwarf spheroidal galaxies (dSphs) in the Local Group. We compare their high time-resolution star formation his...

  14. The Nature of Starbursts: I. The Star Formation Histories of Eighteen Nearby Starburst Dwarf Galaxies

    CERN Document Server

    McQuinn, Kristen B W; Cannon, John M; Dalcanton, Julianne; Dolphin, Andrew; Hidalgo-Rodríguez, Sebastian; Holtzman, Jon; Stark, David; Weisz, Daniel; Williams, Benjamin

    2010-01-01

    We use archival HST observations of resolved stellar populations to derive the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In this first paper we present the observations, color-magnitude diagrams, and the SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only currently bursting galaxies but also "fossil" bursts increasing the sample size of starburst galaxies in the nearby (D100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid "self-quenching" of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but...

  15. The Solar Neighborhood XXVIII: The Multiplicity Fraction of Nearby Stars from 5 to 70 AU and the Brown Dwarf Desert Around M Dwarfs

    CERN Document Server

    Dieterich, Sergio B; Golimowski, David A; Krist, John E; Tanner, Angelle M

    2012-01-01

    We report on our analysis of HST/NICMOS snapshot high resolution images of 255 stars in 201 systems within ~10 parsecs of the Sun. Photometry was obtained through filters F110W, F180M, F207M, and F222M using NICMOS Camera 2. These filters were selected to permit clear identification of cool brown dwarfs through methane contrast imaging. With a plate scale of 76 mas/pixel, NICMOS can easily resolve binaries with sub-arcsecond separations in the 19".5x19".5 field of view. We previously reported five companions to nearby M and L dwarfs from this search. No new companions were discovered during the second phase of data analysis presented here, confirming that stellar/substellar binaries are rare. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of 0.0{-0.0}^{+3.5}% for L companions to M dwarfs in...

  16. Bright Transients from Black Hole - Neutron Star Mergers

    CERN Document Server

    D'Orazio, Daniel J; Murray, Norman W; Price, Larry

    2016-01-01

    Direct detection of black hole-neutron star (BHNS) pairs is anticipated with the advent of aLIGO. Electromagnetic counterparts may be crucial for a confident gravitational-wave detection as well as for extraction of astronomical information. Yet BHNS star pairs are notoriously dark and so inaccessible to telescopes. Contrary to this expectation, a bright electromagnetic transient can occur in the final moments before merger as long as the neutron star is highly magnetized. The orbital motion of the neutron star magnet creates a Faraday flux and corresponding power available for luminosity. A spectrum of curvature radiation ramps up until the rapid injection of energy ignites a fireball, which would appear as an energetic blackbody peaking in the X-ray to gamma-rays for neutron star field strengths ranging from $10^{12}$G to $10^{16}$G respectively and a $10M_{\\odot}$ black hole. The fireball event may last from a few milliseconds to a few seconds depending on the NS magnetic field strength, and may be observa...

  17. The Different Environmental Dependencies of Star-formation for Giant and Dwarf Galaxies

    CERN Document Server

    Haines, C P; Mercurio, A; Merluzzi, P; Busarello, G

    2006-01-01

    We examine the origins of the bimodality observed in the global properties of galaxies around a stellar mass of 3x10^10 M_sun by comparing the environmental dependencies of star-formation for the giant and dwarf galaxy populations. The Sloan Digital Sky Survey DR4 spectroscopic dataset is used to produce a sample of galaxies in the vicinity of the supercluster centered on the cluster A2199 at z=0.03 that is ~90% complete to a magnitude limit of M*+3.3. From these we measure global trends with environment for both giant (M_r7 Gyr) or passive (EW[H_alpha]<4 A) falls gradually from ~80% in the cluster cores to ~40% in field regions beyond 3-4 R_virial, as found in previous studies. In contrast, we find that the dwarf galaxy population shows a sharp transition at ~1 R_virial, from being predominantly old/passive within the cluster, to outside where virtually all galaxies are forming stars and old/passive galaxies are only found as satellites to more massive galaxies. These results imply fundamental differences...

  18. An M dwarf Companion to an F-type Star in a young main-sequence binary

    CERN Document Server

    Eigmüller, Ph; Csizmadia, Sz; Lehmann, H; Erikson, A; Fridlund, M; Hartmann, M; Hatzes, A; Pasternacki, Th; Rauer, H; Tkachenko, A; Voss, H

    2016-01-01

    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. Spectro- scopic 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) Msun and a radius of (1.474 +- 0.040) Rsun. The companion is an M dwarf with a mass of (0.188 +- 0.014) Msun and a radius of (0.234 +- 0.009) Rsun. The orbital period is (1.35121 +- 0:00001)d. 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 Myrs. The mass-radius re...

  19. Living with a Red Dwarf: Rotation and X-ray and Ultraviolet Properties of the Halo Population Kapteyn's Star

    CERN Document Server

    Guinan, Edward F; Durbin, Allyn

    2016-01-01

    As part of Villanova's Living with a Red Dwarf program, we have obtained UV, X-ray and optical data of the Population II red dwarf -- Kapteyn's Star. Kapteyn's Star is noteworthy for its large proper motions and high RV of ~+245 km s^-1. As the nearest Pop II red dwarf, it serves as an old age anchor for calibrating Activity/Irradiance-Rotation-Age relations, and an important test bed for stellar dynamos and the resulting X-ray -- UV emissions of slowly rotating, near-fully convective red dwarf stars. Adding to the notoriety, Kapteyn's Star has recently been reported to host two super-Earth candidates, one of which (Kapteyn b) is orbiting within the habitable zone (Anglada-Escude et al. 2014a, 2015). However, Robertson et al. (2015) questioned the planet's existence since its orbital period may be an artifact of activity, related to the star's rotation period. Because of its large Doppler-shift, measures of the important, chromospheric H I Lyman-alpha 1215.67A emission line can be reliably made, because it is...

  20. MAPPING THE SHORES OF THE BROWN DWARF DESERT. II. MULTIPLE STAR FORMATION IN TAURUS-AURIGA

    International Nuclear Information System (INIS)

    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 Msun, 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 Msun) is nearly log-flat over separations of 3-5000 AU, but lower-mass stars (0.25-0.7 Msun) 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 = MB /MA ) 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.

  1. Gravitational waves from perturbed black holes and relativistic stars

    International Nuclear Information System (INIS)

    These lectures aim at providing an introduction to the properties of gravitational waves and in particular to those gravitational waves that are expected as a consequence of perturbations of black holes and neutron stars. Imprinted in the gravitational radiation emitted by these objects is, in fact, a wealth of physical information. In the case of black holes, a detailed knowledge of the gravitational radiation emitted as a response to perturbations will reveal us important details about their mass and spin, but also about the fundamental properties of the event horizon. In the case of neutron stars, on the other hand, this information can provide a detailed map of their internal structure and tell us about the equation of state of matter at very high density, thus filling-in a gap in energies and densities that cannot be investigated by experiments in terrestrial laboratories. (author)

  2. Quasi-Normal Modes of Stars and Black Holes

    Directory of Open Access Journals (Sweden)

    Kokkotas Kostas

    1999-01-01

    Full Text Available Perturbations of stars and black holes have been one of the main topics of relativistic astrophysics for the last few decades. They are of particular importance today, because of their relevance to gravitational wave astronomy. In this review we present the theory of quasi-normal modes of compact objects from both the mathematical and astrophysical points of view. The discussion includes perturbations of black holes (Schwarzschild, Reissner-Nordström, Kerr and Kerr-Newman and relativistic stars (non-rotating and slowly-rotating. The properties of the various families of quasi-normal modes are described, and numerical techniques for calculating quasi-normal modes reviewed. The successes, as well as the limits, of perturbation theory are presented, and its role in the emerging era of numerical relativity and supercomputers is discussed.

  3. Compact Binaries in Star Clusters I - Black Hole Binaries Inside Globular Clusters

    OpenAIRE

    Downing, J. M. B.; Benacquista, M. J.; Giersz, M.; Spurzem, R.

    2009-01-01

    We study the compact binary population in star clusters, focusing on binaries containing black holes, using a self-consistent Monte Carlo treatment of dynamics and full stellar evolution. We find that the black holes experience strong mass segregation and become centrally concentrated. In the core the black holes interact strongly with each other and black hole-black hole binaries are formed very efficiently. The strong interactions, however, also destroy or eject the black hole-black hole bi...

  4. Boson Stars Alternatives to primordial black holes?

    CERN Document Server

    Mielke, E W; Mielke, Eckehard W.; Schunck, Franz E.

    2000-01-01

    The present surge for the astrophysical relevance of boson stars stems from the speculative possibility that these compact objects could provide a considerable fraction of the non-baryonic part of dark matter within the halo of galaxies. For a very light `universal' axion of effective string models, their total gravitational mass will be in the most likely range of \\sim 0.5 M_ødot of MACHOs. According to this framework, gravitational microlensing is indirectly ``weighing" the axion mass, resulting in \\sim 10^{-10} eV/c^2. This conclusion is not changing much, if we use a dilaton type self-interaction for the bosons. Moreover, we review their formation, rotation and stability as likely candidates of astrophysical importance.

  5. The star formation histories of local group dwarf galaxies. II. Searching for signatures of reionization

    International Nuclear Information System (INIS)

    We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (104 < M* < 109 M☉). 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).

  6. STAR FORMATION IN ULTRA-FAINT DWARFS: CONTINUOUS OR SINGLE-AGE BURSTS?

    International Nuclear Information System (INIS)

    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 (Mvir∼107 M⊙), rather than being stripped remnants of much larger systems

  7. 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.

  8. 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).

  9. 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}

  10. The Potential of Planets Orbiting Red Dwarf Stars to Support Oxygenic Photosynthesis and Complex Life

    CERN Document Server

    Gale, Joseph

    2015-01-01

    We review the latest findings on extra-solar planets and their potential to support Earth-like life. Focusing on planets orbiting Red Dwarf (RD) stars, the most abundant stellar type, we show that including RDs as potential host stars could increase the probability of finding biotic planets by a factor of up to a thousand, and reduce the estimate of the distance to our nearest biotic neighbor by up to 10. We argue that binary and multiple star systems need to be taken into account when discussing exoplanet habitability. Early considerations indicated that conditions on RD planets would be inimical to life, as their Habitable Zones (where liquid water could exist) would be so close as to make planets tidally locked to their star. This was thought to cause an erratic climate and expose life forms to flares of ionizing radiation. Recent calculations show that these negative factors are less severe than originally thought. It has been argued that the lesser photon energy of the radiation of the relatively cool RD...

  11. Neutrinos in mergers of neutron stars with black holes

    Science.gov (United States)

    Deaton, Michael Brett

    Mergers of a neutron star and a black hole are interesting because of the dual complexity of the black hole's strong gravity and the neutron star's nuclear-density fluid. Mergers can yield short-lived nuclear accretion disks, emitting copious neutrinos. This radiation may change the thermodynamic state of the disk itself, may drive an ultrarelativistic jet of electrons and positrons, may oscillate in its flavor content, may irradiate surrounding matter, playing a role in nucleosynthesis, and may be detected directly. In this thesis I present a model of such a merger, its remnant accretion disk, and its neutrino emission. In particular, we evolve a neutron star---black hole merger through ˜100 ms, solving the full general relativistic hydrodynamics equations, from inspiral through merger and accretion epochs. We treat the neutrinos approximately, using a leakage framework, which accounts for local energy losses and composition drift in the fluid due to escaping neutrinos. We use geodesic ray tracing on a late time slice of the model to calculate the full spatial-, angular-, and energy-dependence of the neutrino distribution function around the accretion disk. This distribution then serves in a computation of the energy available to form a jet via neutrino-antineutrino annihilation in the disk funnel. In this scenario, we find that enough energy is deposited to drive a jet of short-gamma-ray-burst-energy by neutrino processes alone.

  12. OUTSIDE-IN SHRINKING OF THE STAR-FORMING DISK OF DWARF IRREGULAR GALAXIES

    International Nuclear Information System (INIS)

    We have studied multi-band surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies. Our data include Galaxy Evolution Explorer (GALEX) FUV/NUV, UBV, and Hα and Spitzer 3.6 μm images. These galaxies constitute the majority of the LITTLE THINGS survey (Local Irregulars That Trace Luminosity Extremes—The H I Nearby Galaxy Survey). By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation histories, we derived the stellar mass surface density distributions and the star formation rate averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr, and a Hubble time. We find that, for ∼80% (27 galaxies) of our sample galaxies, radial profiles (at least in the outer part) at shorter wavelengths, corresponding to younger stellar populations, have shorter disk scale lengths than those at longer wavelengths, corresponding to older stellar populations. This indicates that the star-forming disk has been shrinking. In addition, the radial distributions of the stellar mass surface density are well described as piece-wise exponential profiles, and ∼80% of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of the star formation rate in the outer parts compared to that in the inner disks gives a natural explanation for the down-bending stellar mass surface density profiles. Within the inner disks, our sample galaxies on average have constant ratios of recent star formation rate to stellar mass with radius. Nevertheless, ∼35% (12 galaxies, among which 7 have baryonic mass ∼8 M☉ ) of the sample exhibit negative slopes across the observed disk, which is in contrast with the so-called inside-out disk growth scenario suggested for luminous spiral galaxies. The tendency of star formation to become concentrated toward the inner disks in low-mass dwarf irregular galaxies is interpreted as a result of their susceptibility

  13. Jets from Tidal Disruptions of Stars by Black Holes

    CERN Document Server

    Krolik, Julian H

    2011-01-01

    Tidal disruption of main sequence stars by black holes has generally been thought to lead to a signal dominated by UV emission. If, however, the black hole spins rapidly and the poloidal magnetic field intensity on the black hole horizon is comparable to the inner accretion disk pressure, a powerful jet may form whose luminosity can easily exceed the thermal UV luminosity. When the jet beam points at Earth, its non-thermal luminosity can dominate the emitted spectrum. The thermal and non-thermal components decay differently with time. In particular, the thermal emission should remain roughly constant for a significant time after the period of maximum accretion, beginning to diminish only after a delay, whereas after the peak accretion rate, the non-thermal jet emission decays, but then reaches a plateau. When the newly-found flare source Swift J2058 is analyzed in terms of this model, it is found to be consistent with an event in which a main sequence solar-type star is disrupted by a black hole of mass at le...

  14. Neutron, quark, and proto-neutron stars at the onset of formation of black-holes: the memory effect

    Science.gov (United States)

    Claret, A.

    2014-02-01

    Context. In previous papers it was shown that the function Γ(M,EOS) ≡ [αβ]GR/Λ0.8(R) is invariant (≈0.40) for pre main-sequence stars (PMS), white dwarfs (WD), and for neutron stars (NS) computed with equations of state using relativistic mean-field nucleon interactions. The form-factors αGR and βGR are related to the relativistic gravitational potential energy and the moment of inertia and are a key to handling the Jacobi virial equation, which is a powerful tool for investigating the stellar internal structure and evolution. We also found that Γ(M,EOS) is invariant for gaseous planets. Moreover, a macroscopic criterion of stability for NS was derived. Aims: To test if the invariance of Γ(M,EOS) also holds for an equation of state (EOS) in the non-relativistic framework, we compute NS models by adopting four different EOS prescriptions. We also computed models for hybrid and pure quark stars to extend the range of validity of the Γ(M,EOS) memory effect. To complete the three known final scenarios for stellar evolution, we follow the core-collapse supernova until the onset of formation of a black hole. Methods: Calculations from the PMS up to the WD stages were performed using the MESA code. Neutron, hybrid, and pure quark star models were computed using a modified version of the NSCool/TOV subroutines. The core-collapse supernova simulation was carried out using the code AGILE-IDSA. The relativistic moment of inertia and gravitational potential energy were computed through a fourth-order Runge-Kutta method. Results: We confirm that the function Γ(M,EOS) is invariant for PMS, WD, NS, hybrid, and pure quark stars and is independent of the mass and of the EOS (relativistic and non-relativistic frameworks). We show that our macroscopic criterion of stability is also valid for all mentioned compact stars. In a core-collapse supernova simulation, the PMS value of Γ(M,EOS) is recovered at the onset of formation of a black hole. Therefore, we conclude that

  15. The properties of brown dwarfs and low-mass hydrogen-burning stars formed by disc fragmentation

    CERN Document Server

    Stamatellos, Dimitris

    2008-01-01

    We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive extended discs around Sun-like stars. Such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs fragment within a few thousand years, and produce mainlybrown dwarf (BDs) stars, but also planetary mass (PM) stars and very low-mass hydrogen-burning (HB) stars. Most of the the PM stars and BDs are ejected by mutual interactions. We analyse the statistical properties of these stars, and compare them with observations. After a few hundred thousand years the Sun-like primary is typically left with a close low-mass HB companion, and two much wider companions: a low-mass HB star and a BD star, or a BD-BD binary. There is a BD desert extending out to at least ~100 AU; this is because BDs tend to be formed further out than low-mass HB stars, and then they tend to be scattered...

  16. The quiescent chromospheres and transition regions of active dwarf stars: what are we learning from recent observations and models

    International Nuclear Information System (INIS)

    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.)

  17. Sensitivity of biosignatures on Earth-like planets orbiting in the habitable zone of cool M-dwarf Stars to varying stellar UV radiation and surface biomass emissions

    Science.gov (United States)

    Grenfell, J. L.; Gebauer, S.; v. Paris, P.; Godolt, M.; Rauer, H.

    2014-08-01

    We find that variations in the UV emissions of cool M-dwarf stars have a potentially large impact upon atmospheric biosignatures in simulations of Earth-like exoplanets i.e. planets with Earth's development, and biomass and a molecular nitrogen-oxygen dominated atmosphere. Starting with an assumed black-body stellar emission for an M7 class dwarf star, the stellar UV irradiation was increased stepwise and the resulting climate-photochemical response of the planetary atmosphere was calculated. Results suggest a "Goldilocks" effect with respect to the spectral detection of ozone. At weak UV levels, the ozone column was weak (due to weaker production from the Chapman mechanism) hence its spectral detection was challenging. At strong UV levels, ozone formation is stronger but its associated stratospheric heating leads to a weakening in temperature gradients between the stratosphere and troposphere, which results in weakened spectral bands. Also, increased UV levels can lead to enhanced abundances of hydrogen oxides which oppose the ozone formation effect. At intermediate UV (i.e. with ×10 the stellar UV radiative flux of black body Planck curves corresponding to spectral class M7) the conditions are "just right" for spectral detection. Results suggest that the planetary ozone profile is sensitive to the UV output of the star from ~200-350 nm.We also investigated the effect of increasing the top-of-atmosphere incoming Lyman-α radiation but this had only a minimal effect on the biosignatures since it was efficiently absorbed in the uppermost planetary atmospheric layer, mainly by abundant methane. Earlier studies have suggested that the planetary methane is an important stratospheric heater which critically affects the vertical temperature gradient, hence the strength of spectral emission bands. We therefore varied methane and nitrous oxide biomass emissions, finding e.g. that a lowering in methane emissions by ×100 compared with the Earth can influence temperature

  18. Implications of primordial black holes on the first stars and the origin of the super--massive black holes

    CERN Document Server

    Bambi, Cosimo; Dolgov, Alexander D; Freese, Katherine; Volonteri, Marta

    2008-01-01

    If the cosmological dark matter has a component made of small primordial black holes, they may have a significant impact on the physics of the first stars and on the subsequent formation of massive black holes. Primordial black holes would be adiabatically contracted into these stars and then would sink to the stellar center by dynamical friction, creating a larger black hole which may quickly swallow the whole star. The first stars would thus live only for a very short time and would not contribute much to reionization of the universe. They would instead become $10 - 10^3 M_\\odot$ black holes which (depending on subsequent accretion) could serve as seeds for the super--massive black holes seen at high redshifts as well as those inside galaxies today.

  19. The star formation and chemical evolution history of the Fornax dwarf spheroidal galaxy

    Science.gov (United States)

    de Boer, T. J. L.; Tolstoy, E.; Hill, V.; Saha, A.; Olszewski, E. W.; Mateo, M.; Starkenburg, E.; Battaglia, G.; Walker, M. G.

    2012-08-01

    We present deep photometry in the B, V and I filters from CTIO/MOSAIC for about 270 000 stars in the Fornax dwarf spheroidal galaxy, out to a radius of rell ≈ 0.8 degrees. By combining the accurately calibrated photometry with the spectroscopic metallicity distributions of individual red giant branch stars we obtain the detailed star formation and chemical evolution history of Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations, but also includes ancient (10-14 Gyr), and young (≤1 Gyr) stars. We show that Fornax displays a radial age gradient, with younger, more metal-rich populations dominating the central region. This confirms results from previous works. Within an elliptical radius of 0.8 degrees, or 1.9 kpc from the centre, a total mass in stars of 4.3 × 107 M⊙ was formed, from the earliest times until 250 Myr ago. Using the detailed star formation history, age estimates are determined for individual stars on the upper RGB, for which spectroscopic abundances are available, giving an age-metallicity relation of the Fornax dSph from individual stars. This shows that the average metallicity of Fornax went up rapidly from [Fe/H] ≤ -2.5 dex to [Fe/H] = -1.5 dex between 8-12 Gyr ago, after which a more gradual enrichment resulted in a narrow, well-defined sequence which reaches [Fe/H] ≈ -0.8 dex, ≈3 Gyr ago. These ages also allow us to measure the build-up of chemical elements as a function of time, and thus determine detailed timescales for the evolution of individual chemical elements. A rapid decrease in [Mg/Fe] is seen for the stars with [Fe/H] ≥ -1.5 dex, with a clear trend in age. Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/544/A73

  20. MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. II. CATALOG OF STARS IN MILKY WAY DWARF SATELLITE GALAXIES

    International Nuclear Information System (INIS)

    We present a catalog of Fe, Mg, Si, Ca, and Ti abundances for 2961 stars in eight dwarf satellite galaxies of the Milky Way (MW): Sculptor, Fornax, Leo I, Sextans, Leo II, Canes Venatici I, Ursa Minor, and Draco. For the purposes of validating our measurements, we also observed 445 red giants in MW globular clusters and 21 field red giants in the MW halo. The measurements are based on Keck/DEIMOS medium-resolution spectroscopy (MRS) combined with spectral synthesis. We estimate uncertainties in [Fe/H] by quantifying the dispersion of [Fe/H] measurements in a sample of stars in monometallic globular clusters (GCs). We estimate uncertainties in Mg, Si, Ca, and Ti abundances by comparing to high-resolution spectroscopic abundances of the same stars. For this purpose, a sample of 132 stars with published high-resolution spectroscopy in GCs, the MW halo field, and dwarf galaxies has been observed with MRS. The standard deviations of the differences in [Fe/H] and ([α/Fe]) (the average of [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe]) between the two samples is 0.15 and 0.16, respectively. This catalog represents the largest sample of multi-element abundances in dwarf galaxies to date. The next papers in this series draw conclusions on the chemical evolution, gas dynamics, and star formation histories from the catalog presented here. The wide range of dwarf galaxy luminosity reveals the dependence of dwarf galaxy chemical evolution on galaxy stellar mass.

  1. 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.

  2. A HERSCHEL SURVEY OF COLD DUST IN DISKS AROUND BROWN DWARFS AND LOW-MASS STARS

    International Nuclear Information System (INIS)

    We report the complete photometric results from our Herschel study which is the first comprehensive program to search for far-infrared emission from cold dust around young brown dwarfs (BDs). We surveyed 50 fields containing 51 known or suspected BDs and very low mass stars that have evidence of circumstellar disks based on Spitzer photometry and/or spectroscopy. The objects with known spectral types range from M3 to M9.5. Four of the candidates were subsequently identified as extragalactic objects. Of the remaining 47 we have successfully detected 36 at 70 μm and 14 at 160 μm with signal-to-noise ratio (S/N) greater than 3, as well as several additional possible detections with low S/N. The objects exhibit a range of [24]-[70] μm colors suggesting a range in mass and/or structure of the outer disk. We present modeling of the spectral energy distributions of the sample and discuss trends visible in the data. Using two Monte Carlo radiative transfer codes we investigate disk masses and geometry. We find a very wide range in modeled total disk masses from less than 10–6 M☉ up to 10–3 M☉ with a median disk mass of the order of 3 × 10–5 M☉, suggesting that the median ratio of disk mass to central object mass may be lower than for T Tauri stars. The disk scale heights and flaring angles, however, cover a range consistent with those seen around T Tauri stars. The host clouds in which the young BDs and low-mass stars are located span a range in estimated age from ∼1-3 Myr to ∼10 Myr and represent a variety of star-forming environments. No obvious dependence on cloud location or age is seen in the disk properties, though the statistical significance of this conclusion is not strong.

  3. Sowing the seeds of massive black holes in small galaxies: Young clusters as the building blocks of ultracompact dwarf galaxies

    International Nuclear Information System (INIS)

    Interacting galaxies often have complexes of hundreds of young stellar clusters of individual masses ∼104-106 M ☉ in regions that are a few hundred parsecs across. These cluster complexes interact dynamically, and their coalescence is a candidate for the origin of some ultracompact dwarf galaxies. Individual clusters with short relaxation times are candidates for the production of intermediate-mass black holes of a few hundred solar masses, via runaway stellar collisions prior to the first supernovae in a cluster. It is therefore possible that a cluster complex hosts multiple intermediate-mass black holes that may be ejected from their individual clusters due to mergers or binary processes, but bound to the complex as a whole. Here we explore the dynamical interaction between initially free-flying massive black holes and clusters in an evolving cluster complex. We find that, after hitting some clusters, it is plausible that the massive black hole will be captured in an ultracompact dwarf forming near the center of the complex. In the process, the hole typically triggers electromagnetic flares via stellar disruptions, and is also likely to be a prominent source of gravitational radiation for the advanced ground-based detectors LIGO and VIRGO. We also discuss other implications of this scenario, notably that the central black hole could be considerably larger than expected in other formation scenarios for ultracompact dwarfs.

  4. Sowing the seeds of massive black holes in small galaxies: Young clusters as the building blocks of ultracompact dwarf galaxies

    Energy Technology Data Exchange (ETDEWEB)

    Amaro-Seoane, Pau [Max Planck Institut für Gravitationsphysik (Albert-Einstein-Institut), D-14476 Potsdam (Germany); Konstantinidis, Symeon [Astronomisches Rechen-Institut, Zentrum für Astronomie, Universität Heidelberg, Mönchhofstraße 12-14, Heidelberg D-69120 (Germany); Freitag, Marc Dewi [Institute of Astronomy, Madingley Road, CB3 0HA Cambridge (United Kingdom); Coleman Miller, M. [Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742-2421 (United States); Rasio, Frederic A., E-mail: Pau.Amaro-Seoane@aei.mpg.de, E-mail: simos@ari.uni-heidelberg.de, E-mail: marc.freitag@gmail.com, E-mail: miller@astro.umd.edu, E-mail: rasio@northwestern.edu [Department of Physics and Astronomy, and Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, Evanston, IL 60208 (United States)

    2014-02-20

    Interacting galaxies often have complexes of hundreds of young stellar clusters of individual masses ∼10{sup 4}-10{sup 6} M {sub ☉} in regions that are a few hundred parsecs across. These cluster complexes interact dynamically, and their coalescence is a candidate for the origin of some ultracompact dwarf galaxies. Individual clusters with short relaxation times are candidates for the production of intermediate-mass black holes of a few hundred solar masses, via runaway stellar collisions prior to the first supernovae in a cluster. It is therefore possible that a cluster complex hosts multiple intermediate-mass black holes that may be ejected from their individual clusters due to mergers or binary processes, but bound to the complex as a whole. Here we explore the dynamical interaction between initially free-flying massive black holes and clusters in an evolving cluster complex. We find that, after hitting some clusters, it is plausible that the massive black hole will be captured in an ultracompact dwarf forming near the center of the complex. In the process, the hole typically triggers electromagnetic flares via stellar disruptions, and is also likely to be a prominent source of gravitational radiation for the advanced ground-based detectors LIGO and VIRGO. We also discuss other implications of this scenario, notably that the central black hole could be considerably larger than expected in other formation scenarios for ultracompact dwarfs.

  5. Flares from Disruptions of White Dwarfs by Intermediate Mass Black Holes

    Science.gov (United States)

    Shcherbakov, Roman; Haas, R.; Bode, T.; Laguna, P.

    2011-05-01

    Tidal disruptions of white dwarfs (WDs) by intermediate mass black holes (IMBHs) may produce detectable electromagnetic/gravitational wave signature. They may provide the evidence for existence of IMBHs. We simulate the disruptions in full numerical general relativity (GR) with MayaKranc + Whisky code combination for several BH spin values. We model X-ray and UV time-dependent spectra from the simulations. Direct emission from hot fallback material is substantial, when the BH spin is aligned with WD orbital angular momentum. However, the fallback material can be completely shielded by large Compton optical depth in a misaligned case. This emphasizes the importance of realistic GR simulations with misaligned spin. Properties of flares from shock breakout region depend less on spin orientation and value. We discuss the prospects of positive identifications of candidates from GALEX and other surveys.

  6. Not Alone: Tracing the Origins of Very Low Mass Stars and Brown Dwarfs Through Multiplicity Studies

    CERN Document Server

    Burgasser, A J; Siegler, N; Close, L; Allen, P; Lowrance, P J; Gizis, J; Burgasser, Adam J.; Siegler, Nick; Close, Laird; Allen, Peter; Lowrance, Patrick; Gizis, John

    2006-01-01

    The properties of multiple stellar systems have long provided important empirical constraints for star formation theories, enabling (along with several other lines of evidence) a concrete, qualitative picture of the birth and early evolution of normal stars. At very low masses (VLM; M = 0.8) occurring infrequently (perhaps 10-30%). Both the frequency and maximum separation of stellar and brown dwarf binaries steadily decrease for lower system masses, suggesting that VLM binary formation and/or evolution may be a mass-dependent process. There is evidence for a fairly rapid decline in the number of loosely-bound systems below ~0.3 M_sun, corresponding to a factor of 10-20 increase in the minimum binding energy of VLM binaries as compared to more massive stellar binaries. This wide-separation ``desert'' is present among both field (~1-5 Gyr) and older (> 100 Myr) cluster systems, while the youngest (<~10 Myr) VLM binaries, particularly those in nearby, low-density star forming regions, appear to have somewhat...

  7. The SONYC survey: Towards a complete census of brown dwarfs in star forming regions

    CERN Document Server

    Muzic, K; Geers, V C; Jayawardhana, R; Tamura, M; Dawson, P; Ray, T P

    2013-01-01

    SONYC, short for "Substellar Objects in Nearby Young Clusters", is a survey program to provide a census of the substellar population in nearby star forming regions. We have conducted deep optical and near-infrared photometry in five young regions (NGC1333, rho Ophiuchi, Chamaeleon-I, Upper Sco, and Lupus-3), combined with proper motions, and followed by extensive spectroscopic campaigns with Subaru and VLT, in which we have obtained more than 700 spectra of candidate low-mass objects. We have identified and characterized more than 60 new substellar objects, among them a handful of objects with masses close to, or below the Deuterium burning limit. Through SONYC and surveys by other groups, the substellar IMF is now well characterized down to ~ 5 - 10 MJup, and we find that the ratio of the number of stars with respect to brown dwarfs lies between 2 and 6. A comprehensive survey of NGC 1333 reveals that, down to ~5MJup, free-floating objects with planetary masses are 20-50 times less numerous than stars, i.e. ...

  8. Carbon Shell or Core Ignitions in White Dwarfs Accreting from Helium Stars

    CERN Document Server

    Brooks, Jared; Schwab, Josiah; Paxton, Bill

    2016-01-01

    White dwarfs accreting from helium stars can stably burn at the accreted rate and avoid the challenge of mass loss associated with unstable Helium burning that is a concern for many Type Ia supernovae scenarios. We study binaries with helium stars of mass $1.25 M_\\odot\\le M_{\\rm{He}} \\le 1.8 M_\\odot$, which have lost their hydrogen rich envelopes in an earlier common envelope event and now orbit with periods ($P_{\\rm orb}$) of several hours with non-rotating $0.84$ and $1.0 M_\\odot$ C/O WDs. The helium stars fill their Roche lobes (RLs) after exhaustion of central helium and donate helium on their thermal timescales (${\\sim}10^5$yr). As shown by others, these mass transfer rates coincide with the steady helium burning range for WDs, and grow the WD core up to near the Chandrasekhar mass ($M_{\\rm Ch}$) and a core carbon ignition. We show here, however, that many of these scenarios lead to an ignition of hot carbon ashes near the outer edge of the WD and an inward going carbon flame that does not cause an explo...

  9. GIANO Y-band spectroscopy of dwarf stars: Phosphorus, Sulphur, and Strontium abundances

    CERN Document Server

    Caffau, E; Korotin, S; Origlia, L; Oliva, E; Sanna, N; Ludwig, H -G; Bonifacio, P

    2015-01-01

    In recent years a number of poorly studied chemical elements, such as phosphorus, sulphur, and strontium, have received special attention as important tracers of the Galactic chemical evolution. By exploiting the capabilities of the infrared echelle spectrograph GIANO mounted at the Telescopio Nazionale Galileo, we acquired high resolution spectra of four Galactic dwarf stars spanning the metallicity range between about one-third and twice the solar value. We performed a detailed feasibility study about the effectiveness of the P, S, and Sr line diagnostics in the Y band between 1.03 and 1.10 microm. Accurate chemical abundances have been derived using one-dimensional model atmospheres computed in local thermodynamic equilibrium (LTE). We computed the line formation assuming LTE for P, while we performed non-LTE analysis to derive S and Sr abundances. We were able to derive phosphorus abundance for three stars and an upper limit for one star, while we obtained the abundance of sulphur and strontium for all of...

  10. HATS-7b: A Hot Super Neptune Transiting a Quiet K Dwarf Star

    CERN Document Server

    Bakos, G Á; Bayliss, D; Hartman, J D; Zhou, G; Brahm, R; Mancini, L; deVal-Borro, M; Bhatti, W; Jordán, A; Rabus, M; Espinoza, N; Csubry, Z; Howard, A W; Fulton, B J; Buchhave, L A; Ciceri, S; Henning, T; Schmidt, B; Isaacson, H; Noyes, R W; Marcy, G W; Suc, V; Howe, A R; Burrows, A S; Lázár, J; Papp, I; Sári, P

    2015-01-01

    We report the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune with a mass of 0.120+/-0.012 M_Jup, a radius of 0.563+0.046-0.034 R_Jup, and an orbital period of 3.1853 days. The host star is a moderately bright (V = 13.340+/-0.010 mag, K_S = 10.976+/-0.026 mag) K dwarf star with a mass of 0.849+/-0.027 M_Sun, a radius of 0.815+0.049-0.035 R_Sun, and a metallicity of [Fe/H]= +0.250+/-0.080. The star is photometrically quiet to within the precision of the HATSouth measurements, has low RV jitter, and shows no evidence for chromospheric activity in its spectrum. HATS-7b is the second smallest radius planet discovered by a wide-field ground-based transit survey, and one of only a handful of Neptune-size planets with mass and radius determined to 10% precision. Theoretical modeling of HATS-7b yields a hydrogen-helium fraction of 18+/-4% (rock-iron core and H2-He envelope), or 9+/-4% (ice core and H2-He envelope), i.e.it has a composition broadly similar to that of Uranus and Neptune, and ve...

  11. ON THE SURVIVAL OF BROWN DWARFS AND PLANETS ENGULFED BY THEIR GIANT HOST STAR

    Energy Technology Data Exchange (ETDEWEB)

    Passy, Jean-Claude; Mac Low, Mordecai-Mark [Department of Astrophysics, American Museum of Natural History, New York, NY (United States); De Marco, Orsola [Department of Physics and Astronomy, Macquarie University, Sydney, NSW (Australia)

    2012-11-10

    The recent discovery of two Earth-mass planets in close orbits around an evolved star has raised questions as to whether substellar companions can survive encounters with their host stars. We consider whether these companions could have been stripped of significant amounts of mass during the phase when they orbited through the dense inner envelopes of the giant. We apply the criterion derived by Murray et al. for disruption of gravitationally bound objects by ram pressure to determine whether mass loss may have played a role in the histories of these and other recently discovered low-mass companions to evolved stars. We find that the brown dwarf and Jovian-mass objects circling WD 0137-349, SDSS J08205+0008, and HIP 13044 are most unlikely to have lost significant mass during the common envelope phase. However, the Earth-mass planets found around KIC 05807616 could well be the remnants of one or two Jovian-mass planets that lost extensive mass during the common envelope phase.

  12. Homogeneous abundance analysis of dwarf, subgiant and giant FGK stars with and without giant planets

    CERN Document Server

    da Silva, R; Rocha-Pinto, H J

    2015-01-01

    We have analyzed high-resolution and high signal-to-noise ratio optical spectra of nearby FGK stars with and without detected giant planets in order to homogeneously measure their photospheric parameters, mass, age, and the abundances of volatile (C, N, and O) and refractory (Na, Mg, Si, Ca, Ti, V, Mn, Fe, Ni, Cu, and Ba) elements. Our sample contains 309 stars from the solar neighborhood (up to the distance of 100 pc), out of which 140 are dwarfs, 29 are subgiants, and 140 are giants. The photospheric parameters are derived from the equivalent widths of Fe I and Fe II lines. Masses and ages come from the interpolation in evolutionary tracks and isochrones on the HR diagram. The abundance determination is based on the equivalent widths of selected atomic lines of the refractory elements and on the spectral synthesis of C_2, CN, C I, O I, and Na I features. We apply a set of statistical methods to analyze the abundances derived for the three subsamples. Our results show that: i) giant stars systematically exhi...

  13. Stars and brown dwarfs in the sigma Orionis cluster. II. A proper motion study

    CERN Document Server

    Caballero, Jose A

    2010-01-01

    AIMS. I seek to fully know the stellar and substellar populations in the young sigma Orionis open cluster, which is a benchmark for star-forming studies. Because of the very low proper motion of the cluster, late-type dwarfs with appreciable proper motion in the foreground of sigma Orionis can be easily discarded for expensive spectroscopic follow-up. METHODS. I use the Aladin sky atlas, USNO-B1, public astrometric catalogues, and photographic plate digitisations to identify stars with proper motions inconsistent with cluster membership in a circular area of radius 30 arcmin centred on the early-type multiple system sigma Ori. Mostly due to the long time baseline, of more than half a century, the errors in the measured proper motions are lower than 2 mas/a. RESULTS. Of the 42 stars selected for astrometric follow-up, 37 of them are proper-motion cluster interlopers. Some USNO-B1 measurements were affected by partially resolved (visual) multiplicity and target faintness. Because of their late spectral types an...

  14. Boron Depletion in F and G Dwarf Stars and the Beryllium-Boron Correlation

    CERN Document Server

    Boesgaard, A M; Steinhauer, A; Boesgaard, Ann Merchant; Deliyannis, Constantine P.; Steinhauer, Aaron

    2004-01-01

    Boron provides a special probe below the stellar surface as it survives to greater depths than do Li and Be. To search for B depletions we have observed B in 13 F and G dwarfs with large Be depletions; for comparison we also obtained spectra of five stars which are undepleted in Li and Be. We have used HST with STIS to obtain spectra of the B I resonance line at 2497 \\AA. The spectral resolution is 30,000 or 114,000 and the median signal-to-noise ratio is 70 per pixel. New Be and Li spectra have been obtained at Keck I with HIRES of four of the five standard stars at $\\sim$48,000 resolution. Abundances have been determined by the spectrum synthesis method with MOOG. A comparison between the standard stars and those with severe Be depletions shows a distinct difference in the B abundances between the two groups of 0.22 dex. We have discovered a correlation between the Be and B abundances. The slope between A(Be) and A(B)$_{NLTE}$ is 0.22 $\\pm$0.05 (where A(element) = log N(element)/N(H) + 12.00) which, as expe...

  15. New PARSEC evolutionary tracks of massive stars at low metallicity: testing canonical stellar evolution in nearby star forming dwarf galaxies

    CERN Document Server

    Tang, Jing; Rosenfield, Philip; Slemer, Alessandra; Marigo, Paola; Girardi, Léo; Bianchi, Luciana

    2014-01-01

    We extend the {\\sl\\,PARSEC} library of stellar evolutionary tracks by computing new models of massive stars, from 14\\Msun to 350\\Msun. The input physics is the same used in the {\\sl\\,PARSEC}~V1.1 version, but for the mass-loss rate which is included by considering the most recent updates in literature. We focus on low metallicity, $Z$=0.001 and $Z$=0.004, for which the metal poor dwarf irregular star forming galaxies, Sextans A, WLM and NCG6822, provide simple but powerful workbenches. The models reproduce fairly well the observed CMDs but the stellar colour distributions indicate that the predicted blue loop is not hot enough in models with canonical extent of overshooting. In the framework of a mild extended mixing during central hydrogen burning, the only way to reconcile the discrepancy is to enhance the overshooting at the base of the convective envelope (EO) during the first dredge-UP. The mixing scales required to reproduce the observed loops, EO=2\\HP or EO=4\\HP, are definitely larger than those derive...

  16. Importance of Tides for Periastron Precession in Eccentric Neutron Star - White Dwarf Binaries

    CERN Document Server

    Sravan, Niharika; Kalogera, Vassiliki; Althaus, Leandro G

    2014-01-01

    Although not nearly as numerous as binaries with two white dwarfs, eccentric neutron star-white dwarf (NS-WD) binaries are important gravitational wave sources for the next generation of space-based detectors sensitive to low frequency waves. Here we investigate periastron precession in these sources as a result of general relativistic, tidal, and rotational effects; such precession is expected to be detectable for at least some of the detected binaries of this type. Currently, two eccentric NS-WD binaries are known in the galactic field, PSR J1141-6545 and PSR B2303+46, both of which have orbits too wide to be relevant in their current state to gravitational-wave observations. However, population synthesis studies predict the existence of a significant Galactic population of such systems. We find that the contribution from tides should not be neglected when analyzing periastron precession signatures in gravitational-wave signals: not accounting for tides can produce errors as high as a factor of 80 in the WD...

  17. Evolution and colours of helium-core white dwarf stars the case of low metallicity progenitors

    CERN Document Server

    Serenelli, A M; Rohrmann, R D; Benvenuto, O G

    2002-01-01

    The present work is designed to explore the evolution of helium-core white dwarf (HeWD) stars for the case of metallicities much lower than the solar one (Z=0.001 and Z=0.0002). Evolution is followed in a self-consistent way with the predictions of detalied and new non-grey atmospheres, time-dependent element diffusion and the history of the white dwarf progenitor. Reliable initial models for low mass HeWDs are obtained by applying mass loss rates to a 1msun stellar model. The loss of angular momentum caused by gravitational wave emission and magnetic stellar wind braking are considered. Model atmospheres, based on a detailed treatment of the microphysics entering the WD atmosphere enable us to provide accurate colours and magnitudes at both early and advanced evolutionary stages. We find that most of our evolutionary sequences experience several episodes of hydrogen thermonuclear flashes. In particular, the lower the metallicity, the larger the minimum stellar mass for the occurrence fo flashes induced by CN...

  18. Probing Galactic Structure with the Spatial Correlation Function of SEGUE G-dwarf Stars

    CERN Document Server

    Mao, Qingqing; Holley-Bockelmann, Kelly; Schlesinger, Katharine J; Johnson, Jennifer A; Rockosi, Constance M; Beers, Timothy C; Schneider, Donald P; Pan, Kaike; Bizyaev, Dmitry; Malanushenko, Elena

    2015-01-01

    We measure the two-point correlation function of G-dwarf stars within 1-3 kpc of the Sun in multiple lines-of-sight using the Schlesinger et al. G-dwarf sample from the SDSS SEGUE survey. The shapes of the correlation functions along individual SEGUE lines-of-sight depend sensitively on both the stellar-density gradients and the survey geometry. We fit smooth disk galaxy models to our SEGUE clustering measurements, and obtain strong constraints on the thin- and thick-disk components of the Milky Way. Specifically, we constrain the values of the thin- and thick-disk scale heights with 3% and 2% precision, respectively, and the values of the thin- and thick-disk scale lengths with 20% and 8% precision, respectively. Moreover, we find that a two-disk model is unable to fully explain our clustering measurements, which exhibit an excess of clustering at small scales (< 50 pc). This suggests the presence of small-scale substructure in the disk system of the Milky Way.

  19. Searching for star-forming dwarf galaxies in the Antlia cluster

    CERN Document Server

    Vaduvescu, O; Bassino, L P; Castelli, A V Smith; Calderon, J P

    2014-01-01

    The formation and evolution of dwarf galaxies in clusters need to be understood, and this requires large aperture telescopes. In this sense, we selected the Antlia cluster to continue our previous work in the Virgo, Fornax, and Hydra clusters and in the Local Volume (LV). Because of the scarce available literature data, we selected a small sample of five blue compact dwarf (BCD) candidates in Antlia for observation. Using the Gemini South and GMOS camera, we acquired the Halpha imaging needed to detect star-forming regions in this sample. With the long-slit spectroscopic data of the brightest seven knots detected in three BCD candidates, we derived their basic chemical properties. Using archival VISTA VHS survey images, we derived K_S magnitudes and surface brightness profile fits for the whole sample to assess basic physical properties. FS90-98, FS90-106, and FS90-147 are confirmed as BCDs and cluster members, based on their morphology, K_S surface photometry, oxygen abundance, and velocity redshift. FS90-15...

  20. A brown dwarf companion to the intermediate-mass star HR6037

    CERN Document Server

    Huelamo, N; Ivanov, V D; Chauvin, G; Carraro, G; Sterzik, M F; Melo, C H F; Bonnefoy, M; Hartung, M; Haubois, X; Foellmi, C

    2010-01-01

    In the course of an imaging survey we have detected a visual companion to the intermediate-mass star HR 6037. In this letter, we present two epoch observations of the binary with NACO/VLT, and near-IR spectroscopy of the secondary with ISAAC/VLT. The NACO observations allow us to confirm HR 6037B as a co-moving companion. Its J and H band ISAAC spectra suggest the object has an spectral type of M9+-1, with a surface gravity intermediate between that of 10 Myr dwarfs and field dwarfs with identical spectral type. The comparison of its Ks-band photometry with evolutionary tracks allows us to derive a mass, effective temperature, and surface gravity of 62+-20 MJup, Teff = 2330+-200 K, and log g = 5.1+-0.2, respectively. The small mass ratio of the binary, -0.03, and its long orbital period, -5000 yr, makes HR 6037 a rare and uncommon binary system.

  1. The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations

    CERN Document Server

    Weisz, Daniel R; Skillman, Evan D; Holtzman, Jon; Gilbert, Karoline M; Dalcanton, Julianne J; Williams, Benjamin F

    2014-01-01

    We present uniformly measured star formation histories (SFHs) of 40 Local Group dwarf galaxies based on color-magnitude diagram (CMD) analysis from archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can be recovered from CMDs that do not reach the oldest main sequence turn-off (MSTO), but emphasize that the oldest MSTO is critical for precisely constraining the earliest epochs of star formation. We find that: (1) the average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an exponentially declining SFH with $\\tau$ $\\sim$ 5 Gyr; (2) lower luminosity dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the average SFHs of dwarf irregulars (dIrrs), transition dwarfs (dTrans), and dwarf ellipticals (dEs) can be approximated by the combination of an exponentially declining SFH ($\\tau$ $\\sim$ 3-4 Gyr) for lookback ages $>$ 10-12 Gyr ago and a constant SFH thereafter; (4) the observed fraction of stellar mass formed prior to z=2 ranges considerably (80\\%...

  2. Dwarf nova-type cataclysmic variable stars are significant radio emitters

    CERN Document Server

    Coppejans, Deanne L; Miller-Jones, James C A; Rupen, Michael P; Sivakoff, Gregory R; Knigge, Christian; Groot, Paul J; Woudt, Patrick A; Waagen, Elizabeth O; Templeton, Matthew

    2016-01-01

    We present 8--12\\,GHz radio light curves of five dwarf nova (DN) type Cataclysmic Variable stars (CVs) in outburst (RX And, U Gem and Z Cam), or superoutburst (SU UMa and YZ Cnc), increasing the number of radio-detected DN by a factor of two. The observed radio emission was variable on time-scales of minutes to days, and we argue that it is likely to be synchrotron emission. This sample shows no correlation between the radio luminosity and optical luminosity, orbital period, CV class, or outburst type; however higher-cadence observations are necessary to test this, as the measured luminosity is dependent on the timing of the observations in these variable objects. The observations show that the previously detected radio emission from SS Cyg is not unique in type, luminosity (in the plateau phase of the outburst), or variability time-scales. Our results prove that DN, as a class, are radio emitters in outburst.

  3. The Galactic Formation Rate of Eccentric Neutron Star-White Dwarf Binaries

    CERN Document Server

    Kalogera, V; Lorimer, D R; Ihm, M; Belczynski, K

    2004-01-01

    In this paper we consider the population of eccentric binaries with a neutron star and a white dwarf that has been revealed in our galaxy in recent years through binary pulsar observations. We apply our statistical analysis method (Kim, Kalogera, & Lorimer 2003)and calculate the Galactic formation rate of these binaries empirically. We then compare our results with rate predictions based on binary population synthesis from various research groups and for various ranges of model input parameters. For our reference moel, we find the Galactic formation rate of these eccentric systems to be ~7 per Myr, about an order of magnitude smaller than results from binary evolution estimations. However, the empirical estimates are calculated with no correction for pulsar beaming, and therefore they should be taken as lower limits. Despite uncertainties that exceed an order of magnitude, there is significant overlap of the various rate calculations. This consistency lends confidence that our current understanding of the...

  4. A spectroscopic measure of the star-formation rate density in dwarf galaxies at z~1

    CERN Document Server

    Davies, G T; Glazebrook, Karl; Bower, Richard; Baldry, I K; Balogh, Michael; Hau, G K T; Li, I H; McCarthy, P; Savaglio, S

    2009-01-01

    We use a K-selected (22.5 < K_AB < 24.0) sample of dwarf galaxies (8.4 < log(M*/Msun) < 10) at 0.89star-formation rate density (SFRD), as inferred from their [OII] flux. By comparing with [OII]-based studies of higher stellar mass galaxies, we robustly measure a turnover in the [OII] luminosity density at a stellar mass of M~10^10 Msun. By comparison with the [OII]-based SFRD measured from the Sloan Digital Sky Survey we confirm that, while the SFRD of the lowest-mass galaxies changes very little with time, the SFRD of more massive galaxies evolves strongly, such that they dominate the SFRD at z = 1.

  5. WASP-30b: A 61 MJup BROWN DWARF TRANSITING A V = 12, F8 STAR

    International Nuclear Information System (INIS)

    We report the discovery of a 61-Jupiter-mass brown dwarf (BD), which transits its F8V host star, WASP-30, every 4.16 days. From a range of age indicators we estimate the system age to be 1-2 Gyr. We derive a radius (0.89 ± 0.02 RJup) for the companion that is consistent with that predicted (0.914 RJup) by a model of a 1 Gyr old, non-irradiated BD with a dusty atmosphere. The location of WASP-30b in the minimum of the mass-radius relation is consistent with the quantitative prediction of Chabrier and Baraffe, thus confirming the theory.

  6. Effect of Generalized Uncertainty Principle on Main-Sequence Stars and White Dwarfs

    Directory of Open Access Journals (Sweden)

    Mohamed Moussa

    2015-01-01

    Full Text Available This paper addresses the effect of generalized uncertainty principle, emerged from different approaches of quantum gravity within Planck scale, on thermodynamic properties of photon, nonrelativistic ideal gases, and degenerate fermions. A modification in pressure, particle number, and energy density are calculated. Astrophysical objects such as main-sequence stars and white dwarfs are examined and discussed as an application. A modification in Lane-Emden equation due to a change in a polytropic relation caused by the presence of quantum gravity is investigated. The applicable range of quantum gravity parameters is estimated. The bounds in the perturbed parameters are relatively large but they may be considered reasonable values in the astrophysical regime.

  7. Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. IV. Two bulge populations

    CERN Document Server

    Bensby, T; Meléndez, J; Gould, A; Feltzing, S; Asplund, M; Johnson, J A; Lucatello, S; Yee, J C; Ramírez, I; Cohen, J G; Thompson, I; Gal-Yam, A; Sumi, T; Bond, I A

    2011-01-01

    [ABRIDGED] Based on high-resolution (R~42000 to 48000) and high signal-to-noise (S/N~50 to 150) spectra obtained with UVES/VLT, we present detailed elemental abundances (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, and Ba) and stellar ages for 26 microlensed dwarf and subgiant stars in the Galactic bulge. The analysis is based on equivalent width measurements and standard 1-D LTE MARCS model stellar atmospheres. We also present NLTE Li abundances based on line synthesis of the 7Li line at 670.8 nm. We show that the bulge metallicity distribution (MDF) is double-peaked; one peak at [Fe/H]= -0.6 and one at [Fe/H]=+0.3, and with a dearth of stars around solar metallicity. This is in contrast to the MDF derived from red giants in Baade's window, which peaks at this exact value. A simple significance test shows that it is extremely unlikely to have such a gap in the microlensed dwarf star MDF if the dwarf stars are drawn from the giant star MDF. To resolve this issue we discuss several possibilities, but we can n...

  8. Is beryllium ultra-depletion in solar-type stars linked to the presence of a white dwarf companion?

    CERN Document Server

    Desidera, S; Lugaro, M

    2015-01-01

    Abundance studies of solar-type stars revealed a small fraction of objects with extreme depletion of beryllium. We investigate the possible link between the beryllium depletion and the presence of companions. The classical methods (radial velocity, astrometry, imaging) used to search for binary companions were exploited. We also performed a chemical analysis to identify binaries by the alteration in abundances that is produced by the accretion of material lost by a former evolved companion. We found that all the four previously investigated stars that were found to be ultra--depleted in Be are binaries. In two cases the companion is a white dwarf, and in the other two cases the companion might be a white dwarf or a main-sequence star. One new barium star was identified. We speculate that the interaction with the white dwarf progenitor caused an alteration in the abundance pattern of the star, which resulted in severe beryllium depletion. Possible mechanisms such as thermohaline mixing, episodic accretion, and...

  9. Identification of a DO white dwarf and a PG1159 star in the ESO SN Ia progenitor survey (SPY)

    OpenAIRE

    Werner, K; Rauch, T.; Napiwotzki, R.; Christlieb, N.; Reimers, D.; Karl, C. A.

    2004-01-01

    We present high-resolution VLT spectra of a new helium-rich DO white dwarf and a new PG1159 star, which we identified in the ESO SPY survey. We performed NLTE model atmosphere analyses and found that the PG1159 star is a low-gravity, extremely hot (Teff=160,000K, logg=6) star, having a carbon-helium dominated atmosphere with considerable amounts of oxygen and neon (He=38%, C=54%, O=6%, Ne=2% by mass). It is located within the planetary nebula nuclei instability strip, hence, future searches f...

  10. Southern rice black-streaked dwarf virus: A new proposed Fijivirus species in the family Reoviridae

    Institute of Scientific and Technical Information of China (English)

    ZHOU GuoHui; WEN JingJung; CAI DeJiang; LI Peng; XU DongLin; ZHANG ShuGuang

    2008-01-01

    For the past several years, a novel dwarf disease has been observed on rice (Oryza sativa) in some regions of Guangdong Province and Hainan Province, southern China. Infected plants showed stunting,dark leaf and small enations on stem and leaf back. Typical Fijivirus viroplasma containing crystalline arrayed spherical virons approximately 70-75 nm in diameter and tubular structures were detected in ultrathin sections by an electron microscope in parenchyma phloem cells of the infected plants. The virus was transmitted to rice seedlings by white-backed planthoppers, Sogatella furcifera (Hemiptera:Delphacidae), collected in the diseased fields. Analysis of dsRNA extracts from infected plants revealed ten linear segments, which were similar to the electrophoretic profile of Rice black-streaked dwarf virus (RBSDV). RT-PCR with a single primer which matched to a linker sequence ligated to both 3' ends of the viral genomic dsRNAs resulted in amplification of genome segments 9 (S9) and 10 (S10) cDNA products. The complete nucleotide sequences of S9 and S10 were obtained from clones of the RT-PCR amplicon exhibited characteristic properties of Fijivirus including low GC content (34.5% and 35.6%), genus conserved 5' and 3' termini sequences and similar genome organization. Blast searches indicated that the sequences of S9 and S10 shared 68.8%-74.9% and 67.1 %-77.4% nucleotide identities with those of viruses in the Fijivirus group 2, respectively. These values were similar to those among other viruses in the Fijivirus group 2 and considerably lower than those among RBSDV isolates. Phylogenetic trees based on S9 and S10 nucleotide sequences and their putative amino acid sequences showed that this virus represented a separate branch among other Fijiviruses. The virus was also detected by a nested RT-PCR assay in corn (Zea mays), barnyard grass (Echinochloa crusgalli), Juncellus serotinus and flaccidgrass (Pennisetum flaccidum) in and/or adjacent to the infected rice fields

  11. AN UNDERSTANDING OF THE SHOULDER OF GIANTS: JOVIAN PLANETS AROUND LATE K DWARF STARS AND THE TREND WITH STELLAR MASS

    Energy Technology Data Exchange (ETDEWEB)

    Gaidos, Eric [Department of Geology and Geophysics, University of Hawai' i at Manoa, Honolulu, HI 96822 (United States); Fischer, Debra A. [Department of Astronomy, Yale University, New Haven, CT 06520 (United States); Mann, Andrew W.; Howard, Andrew W., E-mail: gaidos@hawaii.edu [Institute for Astronomy, University of Hawai' i at Manoa, Honolulu, HI 96822 (United States)

    2013-07-01

    Analyses of exoplanet statistics suggest a trend of giant planet occurrence with host star mass, a clue to how planets like Jupiter form. One missing piece of the puzzle is the occurrence around late K dwarf stars (masses of 0.5-0.75 M{sub Sun} and effective temperatures of 3900-4800 K). We analyzed four years of Doppler radial velocity (RVs) data for 110 late K dwarfs, one of which hosts two previously reported giant planets. We estimate that 4.0% {+-} 2.3% of these stars have Saturn-mass or larger planets with orbital periods <245 days, depending on the planet mass distribution and RV variability of stars without giant planets. We also estimate that 0.7% {+-} 0.5% of similar stars observed by Kepler have giant planets. This Kepler rate is significantly (99% confidence) lower than that derived from our Doppler survey, but the difference vanishes if only the single Doppler system (HIP 57274) with completely resolved orbits is considered. The difference could also be explained by the exclusion of close binaries (without giant planets) from the Doppler but not Kepler surveys, the effect of long-period companions and stellar noise on the Doppler data, or an intrinsic difference between the two populations. Our estimates for late K dwarfs bridge those for solar-type stars and M dwarfs, and support a positive trend with stellar mass. Small sample size precludes statements about finer structure, e.g., a ''shoulder'' in the distribution of giant planets with stellar mass. Future surveys such as the Next Generation Transit Survey and the Transiting Exoplanet Satellite Survey will ameliorate this deficiency.

  12. The abundance of biotic exoplanets and life on planets of Red Dwarf stars

    Science.gov (United States)

    Wandel, Amri; Gale, Joseph

    2016-07-01

    The Kepler mission has shown that Earthlike planets orbiting within the Habitable Zones of their host stars are common. We derive an expression for the abundance of life bearing (biotic) extra-solar-system planets (exoplanets) in terms of the (yet unknown) probability for the evolution of biotic life. This "biotic probability" may be estimated by future missions and observations, e.g. spectral analyses of the atmospheres of exoplanets, looking for biomarkers. We show that a biotic probability in the range 0.001-1 implies that a biotic planet may be expected within ~10-100 light years from Earth. Of particular interest in the search for exolife are planets orbiting Red Dwarf (RD) stars, the most frequent stellar type. Previous researches suggested that conditions on planets near RDs would be inimical to life, e.g. the Habitable Zone of RDs is small, so their habitable planets would be close enough to be tidally locked. Recent calculations show that this and other properties of RDs, presumed hostile for the evolution of life, are less severe than originally estimated. We conclude that RD planets could be hospitable for the evolution of life as we know it, not less so than planets of solar-type stars. This result, together with the large number of RDs and their Kepler planet-statistics, makes finding life on RD planets ~10-1000 times more likely than on planets of solar-type stars. Our nearest biotic RD-planet is likely to be 2-10 times closer than the nearest solar-type one.

  13. Turning Point Instabilities for Relativistic Stars and Black Holes

    CERN Document Server

    Schiffrin, Joshua S

    2014-01-01

    In the light of recent results relating dynamic and thermodynamic stability of relativistic stars and black holes, we re-examine the relationship between "turning points"---i.e., extrema of thermodynamic variables along a one-parameter family of solutions---and instabilities. We give a proof of Sorkin's general result---showing the existence of a thermodynamic instability on one side of a turning point---that does not rely on heuristic arguments involving infinite dimensional manifold structure. We use the turning point results to prove the existence of a dynamic instability of black rings in 5 spacetime dimensions in the region where $c_J > 0$, in agreement with a result of Figueras, Murata, and Reall.

  14. Circularization of Tidally Disrupted Stars around Spinning Supermassive Black Holes

    CERN Document Server

    Hayasaki, Kimitake; Loeb, Abraham

    2015-01-01

    We study the circularization of tidally disrupted stars on bound orbits around spinning supermassive black holes by performing three-dimensional smoothed particle hydrodynamic simulations with Post-Newtonian corrections. Our simulations reveal that debris circularization depends sensitively on the efficiency of radiative cooling. There are two stages in debris circularization if radiative cooling is inefficient: first, the stellar debris streams self-intersect due to relativistic apsidal precession; shocks at the intersection points thermalize orbital energy and the debris forms a geometrically thick, ring-like structure around the black hole. The ring rapidly spreads via viscous diffusion, leading to the formation of a geometrically thick accretion disk. In contrast, if radiative cooling is efficient, the stellar debris circularizes due to self-intersection shocks and forms a geometrically thin ring-like structure. In this case, the dissipated energy can be emitted during debris circularization as a precurso...

  15. Self-force and motion of stars around black holes

    CERN Document Server

    Spallicci, A

    2009-01-01

    Through detection by low gravitational wave space interferometers, the capture of stars by supermassive black holes will constitute a giant step forward in the understanding of gravitation in strong field. The impact of the perturbations on the motion of the star is computed via the tail, the back-scattered part of the perturbations, or via a radiative Green function. In the former approach, the self-force acts upon the background geodesic, while in the latter, the geodesic is conceived in the total (background plus perturbations) field. Regularisations (mode-sum and Riemann-Hurwitz $\\zeta$ function) intervene to cancel divergencies coming from the infinitesimal size of the particle. The non-adiabatic trajectories require the most sophisticated techniques for studying the evolution of the motion, like the self-consistent approach.

  16. Characteristic initial data for a star orbiting a black hole

    International Nuclear Information System (INIS)

    We take further steps in the development of the characteristic approach to enable handling the physical problem of a compact self-gravitating object, such as a neutron star, in close orbit around a black hole. We examine different options for setting the initial data for this problem and, in order to shed light on their physical relevance, we carry out short time evolution of this data. To this end we express the matter part of the characteristic gravity code so that the hydrodynamics are in conservation form. The resulting gravity plus matter relativity code provides a starting point for more refined future efforts at longer term evolution. In the present work we find that, independently of the details of the initial gravitational data, the system quickly flushes out spurious gravitational radiation and relaxes to a quasiequilibrium state with an approximate helical symmetry corresponding to the circular orbit of the star

  17. Growth of black holes in the interior of rotating neutron stars

    DEFF Research Database (Denmark)

    Kouvaris, C.; Tinyakov, P.

    2014-01-01

    Mini-black holes made of dark matter that can potentially form in the interior of neutron stars always have been thought to grow by accreting the matter of the core of the star via a spherical Bondi accretion. However, neutron stars have sometimes significant angular velocities that can...... in principle stall the spherical accretion and potentially change the conclusions derived about the time it takes for black holes to destroy a star. We study the effect of the star rotation on the growth of such black holes and the evolution of the black hole spin. Assuming no mechanisms of angular momentum...... evacuation, we find that even moderate rotation rates can in fact destroy spherical accretion at the early stages of the black hole growth. However, we demonstrate that the viscosity of nuclear matter can alleviate the effect of rotation, making it possible for the black hole to maintain spherical accretion...

  18. Nonspinning black hole-neutron star mergers: a model for the amplitude of gravitational waveforms

    CERN Document Server

    Pannarale, Francesco; Kyutoku, Koutarou; Shibata, Masaru

    2013-01-01

    Black hole-neutron star binary mergers display a much richer phenomenology than black hole-black hole mergers, even in the relatively simple case - considered in this paper - in which both the black hole and the neutron star are nonspinning. When the neutron star is tidally disrupted, the gravitational wave emission is radically different from the black hole-black hole case and it can be broadly classified in two groups, depending on the spatial extent of the disrupted material. We present a phenomenological model for the gravitational waveform amplitude in the frequency domain that encompasses the three possible outcomes of the merger: no tidal disruption, "mild", and "strong" tidal disruption. The model is calibrated to general relativistic numerical simulations using piecewise polytropic neutron star equations of state. It should prove useful to extract information on the nuclear equation of state from future gravitational-wave observations, and also to obtain more accurate estimates of black hole-neutron ...

  19. A NEW CHEMICAL EVOLUTION MODEL FOR DWARF SPHEROIDAL GALAXIES BASED ON OBSERVED LONG STAR FORMATION HISTORIES

    International Nuclear Information System (INIS)

    We present a new chemical evolution model for dwarf spheroidal galaxies (dSphs) in the local universe. Our main aim is to explain both their observed star formation histories and metallicity distribution functions simultaneously. Applying our new model for the four local dSphs, that is, Fornax, Sculptor, Leo II, and Sextans, we find that our new model reproduces the observed chemical properties of the dSphs consistently. Our results show that the dSphs have evolved with both a low star formation efficiency and a large gas outflow efficiency compared with the Milky Way, as suggested by previous works. Comparing the observed [α/Fe]-[Fe/H] relation of the dSphs with the model predictions, we find that our model favors a longer onset time of Type Ia supernovae (i.e., 0.5 Gyr) than that suggested in previous studies (i.e., 0.1 Gyr). We discuss the origin of this discrepancy in detail

  20. Dust-to-gas ratio and star formation history of blue compact dwarf galaxies

    CERN Document Server

    Hirashita, H; Kamaya, H

    2002-01-01

    This paper investigates the origin of the observed large variety in dust-to-gas ratio among blue compact dwarf galaxies (BCDs). By applying our chemical evolution model, we find that the dust destruction can largely suppress the dust-to-gas ratio when the metallicity of a BCD reaches $12+\\log{\\rm (O/H)}\\sim 8$, i.e., a typical metallicity level of BCDs. We also show that dust-to-gas ratio is largely varied owing to the change of dust destruction efficiency that has two effects: (i) a significant contribution of Type Ia supernovae to total supernova rate; (ii) variation of gas mass contained in a star-forming region. While mass loss from BCDs was previously thought to be the major cause for the variance of dust-to-gas ratio, we suggest that the other two effects are also important. We finally discuss the intermittent star formation history, which naturally explains the large dispersion of dust-to-gas ratio among BCDs.

  1. Molecular gas and star formation in the Tidal Dwarf Galaxy VCC 2062

    CERN Document Server

    Lisenfeld, U; Duc, P A; Boquien, M; Brinks, E; Bournaud, F; Lelli, F; Charmandaris, V

    2016-01-01

    The physical mechanisms driving star formation (SF) in galaxies are still not fully understood. Tidal dwarf galaxies (TDGs), made of gas ejected during galaxy interactions, seem to be devoid of dark matter and have a near-solar metallicity. The latter makes it possible to study molecular gas and its link to SF using standard tracers (CO, dust) in a peculiar environment. We present a detailed study of a nearby TDG in the Virgo Cluster, VCC 2062, using new high-resolution CO(1--0) data from the Plateau de Bure, deep optical imaging from the Next Generation Virgo Cluster Survey (NGVS), and complementary multiwavelength data. Until now, there was some doubt whether VCC 2062 was a true TDG, but the new deep optical images from the NGVS reveal a stellar bridge between VCC 2062 and its parent galaxy, NGC 4694, which is clear proof of its tidal origin. Several high-resolution tracers (\\halpha, UV, 8~\\mi, and 24~\\mi) of the star formation rate (SFR) are compared to the molecular gas distribution as traced by the CO(1-...

  2. Dwarf spheroidal satellites of M31: I. Variable stars and stellar populations in Andromeda XIX

    CERN Document Server

    Cusano, Felice; Garofalo, Alessia; Cignoni, Michele; Federici, Luciana; Marconi, Marcella; Musella, Ilaria; Ripepi, Vincenzo; Boutsia, Konstantina; Fumana, Marco; Gallozzi, Stefano; Testa, Vincenzo

    2013-01-01

    We present B,V time-series photometry of Andromeda XIX (And XIX), the most extended (half-light radius of 6.2') of Andromeda's dwarf spheroidal companions, that we observed with the Large Binocular Cameras at the Large Binocular Telescope. We surveyed a 23'x 23' area centered on And XIX and present the deepest color magnitude diagram (CMD) ever obtained for this galaxy, reaching, at V~26.3 mag, about one magnitude below the horizontal branch (HB). The CMD shows a prominent and slightly widened red giant branch, along with a predominantly red HB, which, however, extends to the blue to significantly populate the classical instability strip. We have identified 39 pulsating variable stars, of which 31 are of RR Lyrae type and 8 are Anomalous Cepheids (ACs). Twelve of the RR Lyrae variables and 3 of the ACs are located within And XIX's half light radius. The average period of the fundamental mode RR Lyrae stars ( = 0.62 d, \\sigma= 0.03 d) and the period-amplitude diagram qualify And XIX as an Oosterhoff-Intermedia...

  3. Illuminating massive black holes with white dwarfs: orbital dynamics and high-energy transients from tidal interactions

    International Nuclear Information System (INIS)

    White dwarfs (WDs) can be tidally disrupted only by massive black holes (MBHs) with masses less than ∼105 M ☉. These tidal interactions feed material to the MBH well above its Eddington limit, with the potential to launch a relativistic jet. The corresponding beamed emission is a promising indication of an otherwise quiescent MBH of relatively low mass. We show that the mass transfer history, and thus the light curve, is quite different when the disruptive orbit is parabolic, eccentric, or circular. The mass lost each orbit exponentiates in the eccentric-orbit case, leading to the destruction of the WD after several tens of orbits. We examine the stellar dynamics of clusters surrounding MBHs to show that single-passage WD disruptions are substantially more common than repeating encounters. The 1049 erg s–1 peak luminosity of these events makes them visible to cosmological distances. They may be detectible at rates of as many as tens per year by instruments like Swift. In fact, WD-disruption transients significantly outshine their main-sequence star counterparts and are the tidal interaction most likely to be detected arising from MBHs with masses less than 105 M ☉. The detection or nondetection of such WD-disruption transients by Swift is, therefore, a powerful tool to constrain the lower end of the MBH mass function. The emerging ultralong gamma-ray burst class of events all have peak luminosities and durations reminiscent of WD disruptions, offering a hint that WD-disruption transients may already be present in existing data sets.

  4. Nuclear planetology: understanding habitable planets as Galactic bulge stellar remnants (black dwarfs) in a Hertzsprung-Russell (HR) diagram

    Science.gov (United States)

    Roller, Goetz

    2016-04-01

    The Hertzsprung-Russell (HR) diagram is one of the most important diagrams in astronomy. In a HR diagram, the luminosity of stars and/or stellar remnants (white dwarf stars, WD's), relative to the luminosity of the sun, is plotted versus their surface temperatures (Teff). The Earth shows a striking similarity in size (radius ≈ 6.370 km) and Teff of its outer core surface (Teff ≈ 3800 K at the core-mantle-boundary) with old WD's (radius ≈ 6.300 km) like WD0346+246 (Teff ≈ 3820 K after ≈ 12.7 Ga [1]), which plot in the HR diagram close to the low-mass extension of the stellar population or main sequence. In the light of nuclear planetology [2], Earth-like planets are regarded as old, down-cooled and differentiated black dwarfs (Fe-C BLD's) after massive decompression, the most important nuclear reactions involved being 56Fe(γ,α)52Cr (etc.), possibly responsible for extreme terrestrial glaciations events ("snowball" Earth), together with (γ,n), (γ,p) and fusion reactions like 12C(α,γ)16O. The latter reaction might have caused oxidation of the planet from inside out. Nuclear planetology is a new research field, tightly constrained by a coupled 187Re-232Th-238U systematics. By means of nuclear/quantum physics and taking the theory of relativity into account, it aims at understanding the thermal and chemical evolution of Fe-C BLD's after gravitational contraction (e.g. Mercury) or Fermi-pressure controlled collapse (e.g. Earth) events after massive decompression, leading possibly to an r-process event, towards the end of their cooling period [2]. So far and based upon 187Re-232Th-238U nuclear geochronometry, the Fe-C BLD hypothesis can successfully explain the global terrestrial MORB 232Th/238U signature [3]. Thus, it may help to elucidate the DM (depleted mantle), EMI (enriched mantle 1), EMII (enriched mantle 2) or HIMU (high U/Pb) reservoirs, and the 187Os/188Os isotopic dichotomy in Archean magmatic rocks and sediments [4]. Here I present a conceptual

  5. The pulsating DA white dwarf star EC 14012-1446: results from four epochs of time-resolved photometry

    CERN Document Server

    Handler, G; Provencal, J L; Sanchawala, K; Wood, M A; Silver, I; Chen, W -P

    2008-01-01

    The pulsating DA white dwarfs are the coolest degenerate stars that undergo self-driven oscillations. Understanding their interior structure will help to understand the previous evolution of the star. To this end, we report the analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA white dwarf star EC 14012-1446 acquired during four observing epochs in three different years, including a coordinated three-site campaign. A total of 19 independent frequencies in the star's light variations together with 148 combination signals up to fifth order could be detected. We are unable to obtain the period spacing of the normal modes and therefore a mass estimate of the star, but we infer a fairly short rotation period of 0.61 +/- 0.03 d, assuming the rotationally split modes are l=1. The pulsation modes of the star undergo amplitude and frequency variations, in the sense that modes with higher radial overtone show more pronounced variability and that amplitude changes are always accompanied by f...

  6. Collapse of differentially rotating supermassive stars: Post black hole formation

    International Nuclear Information System (INIS)

    We investigate the collapse of differentially rotating supermassive stars (SMSs) by means of 3+1 hydrodynamic simulations in general relativity. We particularly focus on the onset of collapse to understand the final outcome of collapsing SMSs. We find that the estimated ratio of the mass between the black hole and the surrounding disk from the equilibrium star is roughly the same as the results from numerical simulation. This suggests that the picture of axisymmetric collapse is adequate, in the absence of nonaxisymmetric instabilities, to illustrate the final state of the collapse. We also find that quasiperiodic gravitational waves continue to be emitted after the quasinormal mode frequency has decayed. We furthermore have found that when the newly formed black hole is almost extreme Kerr, the amplitude of the quasiperiodic oscillation is enhanced during the late stages of the evolution. Geometrical features, shock waves, and instabilities of the fluid are suggested as a cause of this amplification behavior. This alternative scenario for the collapse of differentially rotating SMSs might be observable by the Laser Interferometer Space Antenna.

  7. The star formation histories of local group dwarf galaxies. I. Hubble space telescope/wide field planetary camera 2 observations

    International Nuclear Information System (INIS)

    We present uniformly measured star formation histories (SFHs) of 40 Local Group (LG) dwarf galaxies based on color-magnitude diagram (CMD) analysis from archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can be recovered from CMDs that do not reach the oldest main sequence turn-off (MSTO), but emphasize that the oldest MSTO is critical for precisely constraining the earliest epochs of star formation. We find that: (1) the average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an exponentially declining SFH with τ ∼ 5 Gyr; (2) lower luminosity dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the average SFHs of dwarf irregulars (dIrrs), transition dwarfs, and dwarf ellipticals can be approximated by the combination of an exponentially declining SFH (τ ∼ 3-4 Gyr) for lookback ages >10-12 Gyr ago and a constant SFH thereafter; (4) the observed fraction of stellar mass formed prior to z = 2 ranges considerably (80% for galaxies with M < 105 M☉ to 30% for galaxies with M > 107 M☉) and is largely explained by environment; (5) the distinction between 'ultra-faint' and 'classical' dSphs is arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass prior to z = 2 than the Sloan Digital Sky Survey galaxies from Leitner and the SFHs from the abundance matching models of Behroozi et al. This may indicate higher than expected star formation efficiencies at early times in low mass galaxies. Finally, we provide all the SFHs in tabulated electronic format for use by the community.

  8. The star formation histories of local group dwarf galaxies. I. Hubble space telescope/wide field planetary camera 2 observations

    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 present uniformly measured star formation histories (SFHs) of 40 Local Group (LG) dwarf galaxies based on color-magnitude diagram (CMD) analysis from archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can be recovered from CMDs that do not reach the oldest main sequence turn-off (MSTO), but emphasize that the oldest MSTO is critical for precisely constraining the earliest epochs of star formation. We find that: (1) the average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an exponentially declining SFH with τ ∼ 5 Gyr; (2) lower luminosity dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the average SFHs of dwarf irregulars (dIrrs), transition dwarfs, and dwarf ellipticals can be approximated by the combination of an exponentially declining SFH (τ ∼ 3-4 Gyr) for lookback ages >10-12 Gyr ago and a constant SFH thereafter; (4) the observed fraction of stellar mass formed prior to z = 2 ranges considerably (80% for galaxies with M < 10{sup 5} M{sub ☉} to 30% for galaxies with M > 10{sup 7} M{sub ☉}) and is largely explained by environment; (5) the distinction between 'ultra-faint' and 'classical' dSphs is arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass prior to z = 2 than the Sloan Digital Sky Survey galaxies from Leitner and the SFHs from the abundance matching models of Behroozi et al. This may indicate higher than expected star formation efficiencies at early times in low mass galaxies. Finally, we provide all the SFHs in tabulated electronic format for use by the community.

  9. SEARCH FOR BLUE COMPACT DWARF GALAXIES DURING QUIESCENCE. II. METALLICITIES OF GAS AND STARS, AGES, AND STAR FORMATION RATES

    International Nuclear Information System (INIS)

    We examine the metallicity and age of a large set of Sloan Digital Sky Survey/Data Release 6 galaxies that may be blue compact dwarf (BCD) galaxies during quiescence (QBCDs). The individual spectra are first classified and then averaged to reduce noise. The metallicity inferred from emission lines (tracing ionized gas) exceeds by ∼0.35 dex the metallicity inferred from absorption lines (tracing stars). Such a small difference is significant according to our error budget estimate. The same procedure was applied to a reference sample of BCDs, and in this case the two metallicities agree, being also consistent with the stellar metallicity in QBCDs. Chemical evolution models indicate that the gas metallicity of QBCDs is too high to be representative of the galaxy as a whole, but it can represent a small fraction of the galactic gas, self-enriched by previous starbursts. The luminosity-weighted stellar age of QBCDs spans the whole range between 1 and 10 Gyr, whereas it is always smaller than 1 Gyr for BCDs. Our stellar ages and metallicities rely on a single stellar population spectrum fitting procedure, which we have specifically developed for this work using the stellar library MILES.

  10. CYG X-3: A GALACTIC DOUBLE BLACK HOLE OR BLACK-HOLE-NEUTRON-STAR PROGENITOR

    Energy Technology Data Exchange (ETDEWEB)

    Belczynski, Krzysztof; Bulik, Tomasz [Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw (Poland); Mandel, Ilya [School of Physics and Astronomy, University of Birmingham, Edgbaston B15 2TT (United Kingdom); Sathyaprakash, B. S. [School of Physics and Astronomy, Cardiff University, 5, The Parade, Cardiff CF24 3YB (United Kingdom); Zdziarski, Andrzej A.; Mikolajewska, Joanna [Centrum Astronomiczne im. M. Kopernika, Bartycka 18, PL-00-716 Warszawa (Poland)

    2013-02-10

    There are no known stellar-origin double black hole (BH-BH) or black-hole-neutron-star (BH-NS) systems. We argue that Cyg X-3 is a very likely BH-BH or BH-NS progenitor. This Galactic X-ray binary consists of a compact object, wind-fed by a Wolf-Rayet (W-R) type companion. Based on a comprehensive analysis of observational data, it was recently argued that Cyg X-3 harbors a 2-4.5 M {sub Sun} black hole (BH) and a 7.5-14.2 M {sub Sun} W-R companion. We find that the fate of such a binary leads to the prompt ({approx}< 1 Myr) formation of a close BH-BH system for the high end of the allowed W-R mass (M {sub W-R} {approx}> 13 M {sub Sun }). For the low- to mid-mass range of the W-R star (M {sub W-R} {approx} 7-10 M {sub Sun }) Cyg X-3 is most likely (probability 70%) disrupted when W-R ends up as a supernova. However, with smaller probability, it may form a wide (15%) or a close (15%) BH-NS system. The advanced LIGO/VIRGO detection rate for mergers of BH-BH systems from the Cyg X-3 formation channel is {approx}10 yr{sup -1}, while it drops down to {approx}0.1 yr{sup -1} for BH-NS systems. If Cyg X-3 in fact hosts a low-mass black hole and massive W-R star, it lends additional support for the existence of BH-BH/BH-NS systems.

  11. Electromagnetic extraction of energy from black hole-neutron star binaries

    CERN Document Server

    McWilliams, Sean T

    2011-01-01

    The coalescence of black hole-neutron star binaries is expected to be a principal source of gravitational waves for the next generation of detectors, Advanced LIGO and Advanced Virgo. Ideally, these and other gravitational wave sources would have a distinct electromagnetic counterpart, as significantly more information could be gained through two separate channels. In addition, since these detectors will probe distances with non-negligible redshift, a coincident observation of an electromagnetic counterpart to a gravitational wave signal would facilitate a novel measurement of dark energy [1]. For black hole masses not much larger than the neutron star mass, the tidal disruption and subsequent accretion of the neutron star by the black hole provides one avenue for generating an electromagnetic counterpart [2]. However, in this work, we demonstrate that, for all black hole-neutron star binaries observable by Advanced LIGO/Virgo, the interaction of the black hole with the magnetic field of the neutron star will...

  12. The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    CERN Document Server

    Deason, Alis J; 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_sun) 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_sun. Halos with more quiescent accretion histories tend to have lower mass progenitors (10^8-10^9 M_sun), and lower overall accreted stellar masses. Ultra-faint mass (M_star 10^8 M_sun 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 surv...

  13. Binary frequency of planet-host stars at wide separations: A new brown dwarf companion to a planet-host star

    CERN Document Server

    Lodieu, N; Bejar, V J S; Gauza, B; Ruiz, M T; Rebolo, R; Pinfield, D J; Martin, E L

    2014-01-01

    The aim of the project is to improve our knowledge on the multiplicity of planet-host stars at wide physical separations. We cross-matched approximately 6200 square degree area of the Southern sky imaged by the Visible Infrared Survey Telescope for Astronomy (VISTA) Hemisphere Survey (VHS) with the Two Micron All Sky Survey (2MASS) to look for wide common proper motion companions to known planet-host stars. We complemented our astrometric search with photometric criteria. We confirmed spectroscopically the co-moving nature of seven sources out of 16 companion candidates and discarded eight, while the remaining one stays as a candidate. Among these new wide companions to planet-host stars, we discovered a T4.5 dwarf companion at 6.3 arcmin (~9000 au) from HIP70849, a K7V star which hosts a 9 Jupiter mass planet with an eccentric orbit. We also report two new stellar M dwarf companions to one G and one metal-rich K star. We infer stellar and substellar binary frequencies for our complete sample of 37 targets of...

  14. Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Bo\\"otes I

    CERN Document Server

    Ishigaki, Miho N; Arimoto, Nobuo; Okamoto, Sakurako

    2014-01-01

    Ultra-faint dwarf galaxies recently discovered around the Milky Way (MW) contain extremely metal-poor stars, and might represent the building blocks of low-metallicity components of the MW. Among them, the Bo\\"otes I dwarf spheroidal galaxy is of particular interest because of its exclusively old stellar population. We determine chemical compositions of six red giant stars in Bo\\"otes I, based on the high-resolution spectra obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of 12 elements, including C, Na, alpha, Fe-peak, and neutron capture elements, were determined for the sample stars. The abundance results were compared to those in field MW halo stars previously obtained using an abundance analysis technique similar to the present study. We confirm the low metallicity of Boo-094 ([Fe/H]=-3.4). Except for this star, the abundance ratios ([X/Fe]) of elements lighter than zinc are generally homogeneous with small scatter around the mean values in the metallicities span...

  15. An Understanding of the Shoulder of Giants: Jovian Planets around Late K Dwarf Stars and the Trend with Stellar Mass

    CERN Document Server

    Gaidos, Eric; Mann, Andrew W; Howard, Andrew W

    2013-01-01

    Analyses of exoplanet statistics suggest a trend of giant planet occurrence with host star mass, a clue to how planets like Jupiter form. One missing piece of the puzzle is the occurrence around late K dwarf stars (masses of 0.5-0.75Msun and effective temperatures of 3900-4800K). We analyzed four years of Doppler radial velocities data of 110 late K dwarfs, one of which hosts two previously reported giant planets. We estimate that 4.0+/-2.3% of these stars have Saturn-mass or larger planets with orbital periods <245d, depending on the planet mass distribution and RV variability of stars without giant planets. We also estimate that 0.7+/-0.5% of similar stars observed by Kepler have giant planets. This Kepler rate is significantly (99% confidence) lower than that derived from our Doppler survey, but the difference vanishes if only the single Doppler system (HIP 57274) with completely resolved orbits is considered. The difference could also be explained by the exclusion of close binaries (without giant plane...

  16. Tidal stripping of stars near supermassive black holes

    Directory of Open Access Journals (Sweden)

    Blandford R.

    2012-12-01

    Full Text Available In a binary system composed of a supermassive black hole and a star orbiting the hole in an equatorial, circular orbit, the stellar orbit will shrink due to the action of gravitational radiation, until the star fills its Roche lobe outside the Innermost Stable Circular Orbit (ISCO of the hole or plunges into the hole. In the former case, gas will flow through the inner Lagrange point (L1 to the hole. If this tidal stripping process happens on a time scale faster than the thermal time scale but slower than the dynamical time scale, the entropy as a function of the interior mass is conserved. The star will evolve adiabatically, and, in most cases, will recede from the hole while filling its Roche lobe. We calculate how the stellar equilibrium properties change, which determines how the stellar orbital period and mass-transfer rate change through the “Roche evolution” for various types of stars in the relativistic regime. We envisage that the mass stream eventually hits the accretion disc, where it forms a hot spot orbiting the hole and may ultimately modulate the luminosity with the stellar orbital frequency. The ultimate goal is to probe the mass and spin of the hole and provide a test of general relativity in the strong-field regime from the resultant quasi-periodic signals. The observability of such a modulation is discussed along with a possible interpretation of an intermittent 1 hour period in the X-ray emission of RE J1034+ 396.

  17. Long-term stellar activity variations of stars from the HARPS M-dwarf sample: Comparison between activity indices

    CERN Document Server

    da Silva, J Gomes; Bonfils, X

    2010-01-01

    We used four known chromospheric activity indicators to measure long-term activity variations in a sample of 23 M-dwarf stars from the HARPS planet search program. We compared the indices using weighted Pearson correlation coefficients and found that in general (i) the correlation between $S_{CaII}$ and \\ion{Na}{i} is very strong and does not depend on the activity level of the stars, (ii) the correlation between our $S_{CaII}$ and H$\\alpha$ seems to depend on the activity level of the stars, and (iii) there is no strong correlation between $S_{CaII}$ and \\ion{He}{i} for these type of stars.

  18. Double White Dwarfs as Probes of Single and Binary Star Evolution

    Science.gov (United States)

    Andrews, Jeffrey John

    2016-01-01

    As the endpoints of stars less massive than roughly eight solar masses, the population of Galactic white dwarfs (WD) contain information about complex stellar evolution processes. Associated pairs of WDs add an extra degree of leverage; both WDs must have formed and evolved together. The work presented in this dissertation uses various populations of double WDs (DWD) to constrain evolution of both single and binary stars. One example is the set of low-mass WDs with unseen WD companions, which are formed through a dynamically-unstable mass loss process called the common envelope. To work toward a quantitative understanding of the common envelope, we develop and apply a Bayesian statistical technique to identify the masses of the unseen WD companions. We provide results which can be compared to evolutionary models and hence a deeper understanding of how binary stars evolve through a common envelope. The statistical technique we develop can be applied to any population of single-line spectroscopic binaries. Binaries widely separated enough that they avoid any significant interaction independently evolve into separate WDs that can be identified in photometric and astrometric surveys. We discuss techniques for finding these objects, known as wide DWDs. We present a catalog of 142 candidate wide DWDs, combining both previously detected systems and systems we identify in our searches in the Sloan Digital Sky Survey. Having been born at the same time, the masses and cooling ages of the WDs in wide DWDs, obtained with our spectroscopic follow-up campaign can be used to constrain the initial-final mass relation, which relates a main sequence star to the mass of the WD into which it will evolve. We develop a novel Bayesian technique to interpret our data and present our resulting constraints on this relation which are particularly strong for initial masses between two and four solar masses. During this process, we identified one wide DWD, HS 2220+2146, that was peculiar since

  19. Hairpin RNA Targeting Multiple Viral Genes Confers Strong Resistance to Rice Black-Streaked Dwarf Virus

    Science.gov (United States)

    Wang, Fangquan; Li, Wenqi; Zhu, Jinyan; Fan, Fangjun; Wang, Jun; Zhong, Weigong; Wang, Ming-Bo; Liu, Qing; Zhu, Qian-Hao; Zhou, Tong; Lan, Ying; Zhou, Yijun; Yang, Jie

    2016-01-01

    Rice black-streaked dwarf virus (RBSDV) belongs to the genus Fijivirus in the family of Reoviridae and causes severe yield loss in rice-producing areas in Asia. RNA silencing, as a natural defence mechanism against plant viruses, has been successfully exploited for engineering virus resistance in plants, including rice. In this study, we generated transgenic rice lines harbouring a hairpin RNA (hpRNA) construct targeting four RBSDV genes, S1, S2, S6 and S10, encoding the RNA-dependent RNA polymerase, the putative core protein, the RNA silencing suppressor and the outer capsid protein, respectively. Both field nursery and artificial inoculation assays of three generations of the transgenic lines showed that they had strong resistance to RBSDV infection. The RBSDV resistance in the segregating transgenic populations correlated perfectly with the presence of the hpRNA transgene. Furthermore, the hpRNA transgene was expressed in the highly resistant transgenic lines, giving rise to abundant levels of 21–24 nt small interfering RNA (siRNA). By small RNA deep sequencing, the RBSDV-resistant transgenic lines detected siRNAs from all four viral gene sequences in the hpRNA transgene, indicating that the whole chimeric fusion sequence can be efficiently processed by Dicer into siRNAs. Taken together, our results suggest that long hpRNA targeting multiple viral genes can be used to generate stable and durable virus resistance in rice, as well as other plant species. PMID:27187354

  20. Hairpin RNA Targeting Multiple Viral Genes Confers Strong Resistance to Rice Black-Streaked Dwarf Virus

    Directory of Open Access Journals (Sweden)

    Fangquan Wang

    2016-05-01

    Full Text Available Rice black-streaked dwarf virus (RBSDV belongs to the genus Fijivirus in the family of Reoviridae and causes severe yield loss in rice-producing areas in Asia. RNA silencing, as a natural defence mechanism against plant viruses, has been successfully exploited for engineering virus resistance in plants, including rice. In this study, we generated transgenic rice lines harbouring a hairpin RNA (hpRNA construct targeting four RBSDV genes, S1, S2, S6 and S10, encoding the RNA-dependent RNA polymerase, the putative core protein, the RNA silencing suppressor and the outer capsid protein, respectively. Both field nursery and artificial inoculation assays of three generations of the transgenic lines showed that they had strong resistance to RBSDV infection. The RBSDV resistance in the segregating transgenic populations correlated perfectly with the presence of the hpRNA transgene. Furthermore, the hpRNA transgene was expressed in the highly resistant transgenic lines, giving rise to abundant levels of 21–24 nt small interfering RNA (siRNA. By small RNA deep sequencing, the RBSDV-resistant transgenic lines detected siRNAs from all four viral gene sequences in the hpRNA transgene, indicating that the whole chimeric fusion sequence can be efficiently processed by Dicer into siRNAs. Taken together, our results suggest that long hpRNA targeting multiple viral genes can be used to generate stable and durable virus resistance in rice, as well as other plant species.

  1. THE NATURE OF STARBURSTS. I. THE STAR FORMATION HISTORIES OF EIGHTEEN NEARBY STARBURST DWARF GALAXIES

    International Nuclear Information System (INIS)

    We use archival Hubble Space Telescope observations of resolved stellar populations to derive the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In this first paper, we present the observations, color-magnitude diagrams (CMDs), and the SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only the currently bursting galaxies but also 'fossil' bursts increasing the sample size of starburst galaxies in the nearby (D 100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid 'self-quenching' of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but one galaxy confirming the short-lived nature of starbursts based on fuel limitations. Additionally, we find that the strength of the Hα emission usually correlates with the CMD-based SFR during the last 4-10 Myr. However, in four cases, the Hα emission is significantly less than what is expected for models of starbursts; the discrepancy is due to the SFR changing on timescales of a few Myr. The inherently short timescale of the Hα emission limits identifying galaxies as starbursts based on the current characteristics which may or may not be representative of the recent SFH of a galaxy.

  2. Spectroscopic Study of Extended Star Clusters in Dwarf Galaxy NGC 6822

    Science.gov (United States)

    Hwang, Narae; Park, Hong Soo; Lee, Myung Gyoon; Lim, Sungsoon; Hodge, Paul W.; Kim, Sang Chul; Miller, Bryan; Weisz, Daniel

    2014-03-01

    We present a spectroscopic study of the four extended star clusters (ESCs) in NGC 6822 based on the data obtained with the Gemini Multi-Object Spectrograph on the Gemini-South 8.1 m telescope. The radial velocities derived from the spectra range from -61.2 ± 20.4 km s-1 (for C1) to -115.34 ± 57.9 km s-1 (for C4) and, unlike the intermediate-age carbon stars, they do not display any sign of systematic rotation around NGC 6822. The ages and metallicities derived using the Lick indices show that the ESCs are old (>=8 Gyr) and metal poor ([Fe/H] = 20'(2.6 kpc). The kinematics, old ages, and low metallicities of ESCs suggest that ESCs may have accreted into the halo of NGC 6822. Based on the velocity distribution of ESCs, we have determined the total mass and the mass-to-light ratio of NGC 6822: M_{N6822} = 7.5^{+4.5}_{-0.1} \\times 10^{9}\\ M_{\\odot } and (M/L)_{N6822} = 75^{+45}_{-1} (M/L)_{\\odot }. It shows that NGC 6822 is one of the most dark matter dominated dwarf galaxies in the Local Group. Based on observations obtained at the Gemini Observatory, 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 Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

  3. Coalescence of Black Hole-Neutron Star Binaries

    Directory of Open Access Journals (Sweden)

    Masaru Shibata

    2011-08-01

    Full Text Available We review the current status of general relativistic studies for the coalescence of black hole-neutron star (BH-NS binaries. First, procedures for a solution of BH-NS binaries in quasi-equilibrium circular orbits and the numerical results, such as quasi-equilibrium sequence and mass-shedding limit, of the high-precision computation, are summarized. Then, the current status of numerical-relativity simulations for the merger of BH-NS binaries is described. We summarize our understanding for the merger and/or tidal disruption processes, the criterion for tidal disruption, the properties of the remnant formed after the tidal disruption, gravitational waveform, and gravitational-wave spectrum.

  4. Warm Dust around Cool Stars: Field M Dwarfs with WISE 12 or 22 Micron Excess Emission

    CERN Document Server

    Theissen, Christopher A

    2014-01-01

    Using the SDSS DR7 spectroscopic catalog, we searched the WISE AllWISE catalog to investigate the occurrence of warm dust, as inferred from IR excesses, around field M dwarfs (dMs). We developed SDSS/WISE color selection criteria to identify 175 dMs (from 70,841) that show IR flux greater than typical dM photosphere levels at 12 and/or 22 $\\mu$m, including seven new stars within the Orion OB1 footprint. We characterize the dust populations inferred from each IR excess, and investigate the possibility that these excesses could arise from ultracool binary companions by modeling combined SEDs. Our observed IR fluxes are greater than levels expected from ultracool companions ($>3\\sigma$). We also estimate that the probability the observed IR excesses are due to chance alignments with extragalactic sources is $<$ 0.1%. Using SDSS spectra we measure surface gravity dependent features (K, Na, and CaH 3), and find $<$ 15% of our sample indicate low surface gravities. Examining tracers of youth (H$\\alpha$, UV fl...

  5. A Neutron Star-White Dwarf Binary Model for Repeating Fast Radio Burst 121102

    CERN Document Server

    Gu, Wei-Min; Liu, Tong; Ma, Renyi; Wang, Junfeng

    2016-01-01

    We propose a compact binary model for the fast radio burst (FRB) repeaters, where the system consists of a magnetic white dwarf (WD) and a neutron star (NS) with strong bipolar magnetic fields. When the WD fills its Roche lobe, mass transfer will occur from the WD to the NS through the inner Lagrange point. The accreted magnetized materials may trigger magnetic reconnection when they approach the NS surface, and therefore the electrons can be accelerated to an ultra-relativistic speed. In this scenario, the curvature radiation of the electrons moving along the NS magnetic field lines can account for the characteristic frequency and the timescale of an FRB. Owing to the conservation of angular momentum, the WD may be kicked away after a burst, and the next burst may appear when the system becomes semi-detached again through the gravitational radiation. By comparing our analyses with the observations, we show that such an intermittent Roche lobe overflow mechanism can be responsible for the observed repeating b...

  6. New Low-Mass Stars and Brown Dwarfs with Disks in Lupus

    CERN Document Server

    Allen, P R; Myers, P C; Megeath, S T; Allen, L E; Hartmann, L; Fazio, G G

    2007-01-01

    Using the Infrared Array Camera and the Multiband Imaging Photometer aboard the {\\it Spitzer Space Telescope}, we have obtained images of the Lupus 3 star-forming cloud at 3.6, 4.5, 5.8, 8.0, and 24 \\micron. We present photometry in these bands for the 41 previously known members that are within our images. In addition, we have identified 19 possible new members of the cloud based on red 3.6-8.0 \\micron colors that are indicative of circumstellar disks. We have performed optical spectroscopy on 6 of these candidates, all of which are confirmed as young low-mass members of Lupus 3. The spectral types of these new members range from M4.75 to M8, corresponding to masses of 0.2-0.03 $M_\\odot$ for ages of $\\sim1$ Myr according to theoretical evolutionary models. We also present optical spectroscopy of a candidate disk-bearing object in the vicinity of the Lupus 1 cloud, 2M 1541-3345, which Jayawardhana & Ivanov recently classified as a young brown dwarf ($M\\sim0.03$ $M_\\odot$) with a spectral type of M8. In co...

  7. Choirs, HI Galaxy Groups: Catalogue and Detection of Star-forming Dwarf Group Members

    CERN Document Server

    Sweet, Sarah M; Drinkwater, Michael J; Kilborn, Virginia; Dénes, Helga; Bekki, Kenji; Hanish, Dan; Ferguson, Henry; Knezek, Patricia; Bland-Hawthorn, Joss; Dopita, Michael; Doyle-Pegg, Marianne T; Elson, Ed; Freeman, Ken; Heckman, Tim; Kennicutt, Robert; Kim, Ji Hoon; Koribalski, Bärbel; Meyer, Martin; Putman, Mary; Ryan-Weber, Emma; Smith, Chris; Staveley-Smith, Lister; Wong, O Ivy; Webster, Rachel; Werk, Jessica; Zwaan, Martin

    2013-01-01

    H{\\alpha} observations centred on galaxies selected from the HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001) typically show one and sometimes two star-forming galaxies within the approximately 15-arcminute beam of the Parkes 64-m HI detections. In our Survey of Ionization in Neutral Gas Galaxies (SINGG, Meurer et al. 2006) we found fifteen cases of HIPASS sources containing four or more emission line galaxies (ELGs). We name these fields Choir groups. In the most extreme case we found a field with at least nine ELGs. In this paper we present a catalogue of Choir group members in the context of the wider SINGG sample. The dwarf galaxies in the Choir groups would not be individually detectable in HIPASS at the observed distances if they were isolated, but are detected in SINGG narrow-band imaging due to their membership of groups with sufficiently large total HI mass. The ELGs in these groups are similar to the wider SINGG sample in terms of size, H{\\alpha} equivalent width, and surface brightness. Eight ...

  8. BANYAN. VIII. New Low-Mass Stars and Brown Dwarfs with Candidate Circumstellar Disks

    CERN Document Server

    Boucher, Anne; Gagné, Jonathan; Malo, Lison; Faherty, Jacqueline K; Doyon, René; Chen, Christine H

    2016-01-01

    We present the results of a search for new circumstellar disks around low-mass stars and brown dwarfs with spectral types >K5 that are confirmed or candidate members of nearby young moving groups. Our search input sample was drawn from the BANYAN surveys of Malo et al. and Gagn\\'e et al. Two-Micron All-Sky Survey and Wide-field Infrared Survey Explorer data were used to detect near- to mid-infrared excesses that would reveal the presence of circumstellar disks. A total of 13 targets with convincing excesses were identified: four are new and nine were already known in the literature. The new candidates are 2MASS J05010082$-$4337102 (M4.5), J08561384$-$1342242 (M8$\\,\\gamma$), J12474428$-$3816464 (M9$\\,\\gamma$) and J02265658$-$5327032 (L0$\\,\\delta$), and are candidate members of the TW Hya ($\\sim10\\pm 3\\,$Myr), Columba ($\\sim 42^{+6}_{-4}\\,$Myr) and Tucana-Horologium ($\\sim 45\\pm 4\\,$Myr) associations, with masses of $120$ and $13-18\\,M_{\\mathrm{Jup}}$. The M8$-$L0 objects in Columba and Tucana-Horologium are po...

  9. A Search for Companions to Brown Dwarfs in the Taurus and Chamaeleon Star Forming Regions

    CERN Document Server

    Todorov, K O; Konopacky, Q M; McLeod, K K; Apai, D; Ghez, A M; Pascucci, I; Robberto, M

    2014-01-01

    We present the results of a search for companions to young brown dwarfs in the Taurus and Chamaeleon I star forming regions (1/2-3 Myr). We have used WFPC2 on board HST to obtain F791W and F850LP images of 47 members of these regions that have spectral types of M6-L0 (0.01-0.1 Msun). An additional late-type member of Taurus, FU Tau (M7.25+M9.25), was also observed with adaptive optics at Keck Observatory. We have applied PSF subtraction to the primaries and have searched the resulting images for objects that have colors and magnitudes that are indicative of young low-mass objects. Through this process, we have identified promising candidate companions to 2MASS J04414489+2301513 (rho=0.105"/15 AU), 2MASS J04221332+1934392 (rho=0.05"/7 AU), and ISO 217 (rho=0.03"/5 AU). We reported the discovery of the first candidate in a previous study, showing that it has a similar proper motion as the primary through a comparison of astrometry measured with WFPC2 and Gemini adaptive optics. We have collected an additional e...

  10. Self-consistent evolution of accreting low-mass stars and brown dwarfs

    CERN Document Server

    Baraffe, I; Vorobyov, E I; Chabrier, G

    2016-01-01

    We present self-consistent calculations coupling numerical hydrodynamics simulations of collapsing pre-stellar cores and stellar evolution models of accreting objects. We analyse the main impact of consistent accretion history on the evolution and lithium depletion of young low-mass stars and brown dwarfs. These consistent models confirm the generation of a luminosity spread in Herzsprung-Russell diagrams at ages $\\sim$ 1-10 Myr. They also confirm that early accretion can produce objects with abnormal Li depletion, as found in a previous study that was based on arbitrary accretion rates. The results strengthen that objects with anomalously high level of Li depletion in young clusters should be extremely rare. We also find that early phases of burst accretion can produce coeval models of similar mass with a range of different Li surface abundances, and in particular with Li-excess compared to the predictions of non-accreting counterparts. This result is due to a subtle competition between the effect of burst a...

  11. Gamma Ray Burst progenitors - a case for helium star mergers

    OpenAIRE

    Belczynski, Krzysztof; Bulik, Tomasz; Rudak, Bronislaw

    2000-01-01

    Recently much work in Gamma-Ray Burst (GRB) studies was devoted to revealing the nature of outburst mechanism and to looking for GRB progenitors. Several types of progenitors were proposed for GRBs. Most promising objects are collapsars, compact object binaries, Helium star mergers and recently discussed supernovae. In this paper we consider four proposed binary star progenitors of GRBs: double neutron star (NS-NS), black hole neutron star (BH-NS), black hole white dwarf (BH-WD) mergers and H...

  12. Atmospheres of Brown Dwarfs

    CERN Document Server

    Helling, Christiane

    2014-01-01

    Brown Dwarfs are the coolest class of stellar objects known to date. Our present perception is that Brown Dwarfs follow the principles of star formation, and that Brown Dwarfs share many characteristics with planets. Being the darkest and lowest mass stars known makes Brown Dwarfs also the coolest stars known. This has profound implication for their spectral fingerprints. Brown Dwarfs cover a range of effective temperatures which cause brown dwarfs atmospheres to be a sequence that gradually changes from a M-dwarf-like spectrum into a planet-like spectrum. This further implies that below an effective temperature of < 2800K, clouds form already in atmospheres of objects marking the boundary between M-Dwarfs and brown dwarfs. Recent developments have sparked the interest in plasma processes in such very cool atmospheres: sporadic and quiescent radio emission has been observed in combination with decaying Xray-activity indicators across the fully convective boundary.

  13. From the first stars to the first black holes

    Science.gov (United States)

    Valiante, Rosa; Schneider, Raffaella; Volonteri, Marta; Omukai, Kazuyuki

    2016-04-01

    The growth of the first supermassive black holes (SMBHs) at z > 6 is still a major challenge for theoretical models. If it starts from black hole (BH) remnants of Population III stars (light seeds with mass ˜100 M⊙), it requires super-Eddington accretion. An alternative route is to start from heavy seeds formed by the direct collapse of gas on to an ˜105 M⊙ BH. Here we investigate the relative role of light and heavy seeds as BH progenitors of the first SMBHs. We use the cosmological, data constrained semi-analytic model GAMETE/QSODUST to simulate several independent merger histories of z > 6 quasars. Using physically motivated prescriptions to form light and heavy seeds in the progenitor galaxies, we find that the formation of a few heavy seeds (between 3 and 30 in our reference model) enables the Eddington-limited growth of SMBHs at z > 6. This conclusion depends sensitively on the interplay between chemical, radiative and mechanical feedback effects, which easily erase the conditions that allow the suppression of gas cooling in the low-metallicity gas (Z Jcr). We find that heavy seeds cannot form if dust cooling triggers gas fragmentation above a critical dust-to-gas mass ratio (D ≥ D_cr). In addition, the relative importance of light and heavy seeds depends on the adopted mass range for light seeds, as this dramatically affects the history of cold gas along the merger tree, by both SN- and AGN-driven winds.

  14. From the first stars to the first black holes

    CERN Document Server

    Valiante, Rosa; Volonteri, Marta; Omukai, Kazuyuki

    2016-01-01

    The growth of the first super massive black holes (SMBHs) at z > 6 is still a major challenge for theoretical models. If it starts from black hole (BH) remnants of Population III stars (light seeds with mass ~ 100 Msun) it requires super-Eddington accretion. An alternative route is to start from heavy seeds formed by the direct collapse of gas onto a ~ 10^5 Msun BH. Here we investigate the relative role of light and heavy seeds as BH progenitors of the first SMBHs. We use the cosmological, data constrained semi-analytic model GAMETE/QSOdust to simulate several independent merger histories of z > 6 quasars. Using physically motivated prescriptions to form light and heavy seeds in the progenitor galaxies, we find that the formation of a few heavy seeds (between 3 and 30 in our reference model) enables the Eddington-limited growth of SMBHs at z > 6. This conclusion depends sensitively on the interplay between chemical, radiative and mechanical feedback effects, which easily erase the conditions that allow the su...

  15. Low-frequency quasi-periodic oscillations in black hole and neutron star LMXBs

    Science.gov (United States)

    Ingram, Adam

    2016-07-01

    Low-frequency quasi-periodic oscillations (QPOs) are routinely seen in the X-ray flux of accreting black holes and neutron stars. Since the QPO frequency correlates with the low frequency power spectral break in the same manner for both object classes, it is reasonable to believe that these oscillations have the same physical origin in neutron stars as they do in black holes. However, recent successes in modelling black hole low frequency QPOs as Lense-Thirring precession contrast sharply with failures of the same model in neutron stars. This could be attributable to the significant extra complexity, both in the physics and in the observed power spectra, of accreting neutron stars when compared with black holes. Alternatively, the QPO mechanism really is the same for the two object classes, but in that case, why does the Lense-Thirring model work so well for black holes? I will review the current state of this field.

  16. Cleaning spectroscopic samples of stars in nearby dwarf galaxies: The use of the nIR MgI line to weed out Milky Way contaminants

    CERN Document Server

    Battaglia, Giuseppina

    2012-01-01

    Dwarf galaxies provide insights on the processes of star formation and chemical enrichment at the low end of the galaxy mass function, as well as on the clustering of dark matter on small scales. In studies of Local Group dwarf galaxies, spectroscopic samples of individual stars are used to derive the internal kinematics and abundance properties of these galaxies. It is therefore important to clean these samples from Milky Way stars, not related to the dwarf galaxy, since they can contaminate the analysis of the properties of these objects. Here we introduce a new diagnostic for separating Milky Way contaminant stars -- that mainly constitute of dwarf stars -- and red giant branch stars targeted in dwarf galaxies. As discriminator we use the trends in the equivalent width of the nIR MgI line at 8806.8 \\AA\\ as a function of the equivalent width of CaII triplet lines. This method is particularly useful for works dealing with multi-object intermediate resolution spectroscopy focusing in the region of the nIR CaI...

  17. THE CHEMICAL SIGNATURE OF A RELIC STAR CLUSTER IN THE SEXTANS DWARF SPHEROIDAL GALAXY—IMPLICATIONS FOR NEAR-FIELD COSMOLOGY

    International Nuclear Information System (INIS)

    We present tentative evidence for the existence of a dissolved star cluster at [Fe/H] = –2.7 in the Sextans dwarf spheroidal galaxy. We use the technique of chemical tagging to identify stars that are highly clustered in a multi-dimensional chemical abundance space (C-space). In a sample of six stars, three, possibly four, stars are identified as potential cluster stars. The initial stellar mass of the parent cluster is estimated from two independent observations to be M*,init=1.9+1.5-0.9(1.6+1.2-0.8)×105 Msun, assuming a Salpeter (Kroupa) initial mass function. If corroborated by follow-up spectroscopy, this star cluster is the most metal-poor system identified to date. Chemical signatures of remnant clusters in dwarf galaxies like Sextans provide us with a very powerful probe to the high-redshift universe. From available observational data, we argue that the average star cluster mass in the majority of the newly discovered ultra-faint dwarf galaxies was notably lower than it is in the Galaxy today and possibly lower than in the more luminous, classical dwarf spheroidal galaxies. Furthermore, the mean cumulative metallicity function of the dwarf spheroidals falls below that of the ultra-faints, which increases with increasing metallicity as predicted from our stochastic chemical evolution model. These two findings, together with a possible difference in the ([Mg/Fe]) ratio suggest that the ultra-faint dwarf galaxy population, or a significant fraction thereof, and the dwarf spheroidal population were formed in different environments and would thus be distinct in origin.

  18. THE CHEMICAL SIGNATURE OF A RELIC STAR CLUSTER IN THE SEXTANS DWARF SPHEROIDAL GALAXY-IMPLICATIONS FOR NEAR-FIELD COSMOLOGY

    Energy Technology Data Exchange (ETDEWEB)

    Karlsson, Torgny [Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala (Sweden); Bland-Hawthorn, Joss [Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006 (Australia); Freeman, Ken C. [Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston ACT 2611 (Australia); Silk, Joe, E-mail: torgny.karlsson@physics.uu.se [Physics Department, University of Oxford, Oxford OX1 3RH (United Kingdom)

    2012-11-10

    We present tentative evidence for the existence of a dissolved star cluster at [Fe/H] = -2.7 in the Sextans dwarf spheroidal galaxy. We use the technique of chemical tagging to identify stars that are highly clustered in a multi-dimensional chemical abundance space (C-space). In a sample of six stars, three, possibly four, stars are identified as potential cluster stars. The initial stellar mass of the parent cluster is estimated from two independent observations to be M{sub *,init}=1.9{sup +1.5}{sub -0.9}(1.6{sup +1.2}{sub -0.8}) Multiplication-Sign 10{sup 5} M{sub sun}, assuming a Salpeter (Kroupa) initial mass function. If corroborated by follow-up spectroscopy, this star cluster is the most metal-poor system identified to date. Chemical signatures of remnant clusters in dwarf galaxies like Sextans provide us with a very powerful probe to the high-redshift universe. From available observational data, we argue that the average star cluster mass in the majority of the newly discovered ultra-faint dwarf galaxies was notably lower than it is in the Galaxy today and possibly lower than in the more luminous, classical dwarf spheroidal galaxies. Furthermore, the mean cumulative metallicity function of the dwarf spheroidals falls below that of the ultra-faints, which increases with increasing metallicity as predicted from our stochastic chemical evolution model. These two findings, together with a possible difference in the ([Mg/Fe]) ratio suggest that the ultra-faint dwarf galaxy population, or a significant fraction thereof, and the dwarf spheroidal population were formed in different environments and would thus be distinct in origin.

  19. Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf

    CERN Document Server

    Anglada-Escudé, Guillem; Tuomi, Mikko; Zechmeister, Mathias; Jenkins, James S; Ofir, Aviv; Dreizler, Stefan; Gerlach, Enrico; Marvin, Chris J; Reiners, Ansgar; Jeffers, Sandra V; Butler, R Paul; Vogt, Steven S; Amado, Pedro J; Rodríguez-López, Cristina; Berdiñas, Zaira M; Morin, Julian; Crane, Jeff D; Shectman, Stephen A; Thompson, Ian B; Díaz, Mateo; Rivera, Eugenio; Sarmiento, Luis F; Jones, Hugh R A

    2014-01-01

    Exoplanets of a few Earth masses can be now detected around nearby low-mass stars using Doppler spectroscopy. In this paper, we investigate the radial velocity variations of Kapteyn's star, which is both a sub-dwarf M-star and the nearest halo object to the Sun. The observations comprise archival and new HARPS, HIRES and PFS Doppler measurements. Two Doppler signals are detected at periods of 48 and 120 days using likelihood periodograms and a Bayesian analysis of the data. Using the same techniques, the activity indicies and archival ASAS-3 photometry show evidence for low-level activity periodicities of the order of several hundred days. However, there are no significant correlations with the radial velocity variations on the same time-scales. The inclusion of planetary Keplerian signals in the model results in levels of correlated and excess white noise that are remarkably low compared to younger G, K and M dwarfs. We conclude that Kapteyn's star is most probably orbited by two super-Earth mass planets, on...

  20. Rapid Growth of Black Holes in Massive Star-Forming Galaxies

    OpenAIRE

    Alexander, D. M.; Smail, I.; Bauer, F. E.; Chapman, S. C.; Blain, A. W.; Brandt, W. N.; Ivison, R. J.

    2005-01-01

    The tight relationship between the masses of black holes and galaxy spheroids in nearby galaxies implies a causal connection between the growth of these two components. Optically luminous quasars host the most prodigious accreting black holes in the Universe and can account for >30% of the total cosmological black-hole growth. As typical quasars are not, however, undergoing intense star formation and already host massive black holes [>10^(8) M(Sun)], there must have been an earlier pre-quasar...

  1. SPLAT: Using Spectral Indices to Identify and Characterize Ultracool Stars, Brown Dwarfs and Exoplanets in Deep Surveys and as Companions to Nearby Stars

    Science.gov (United States)

    Aganze, Christian; Burgasser, Adam J.; Martin, Eduardo; Konopacky, Quinn; Masters, Daniel C.

    2016-06-01

    The majority of ultracool dwarf stars and brown dwarfs currently known were identified in wide-field red optical and infrared surveys, enabling measures of the local, typically isolated, population in a relatively shallow (based on templates and tools developed as part of the SpeX Prism Library Analysis Toolkit. In particular, we define and characterize specifically-tuned sets spectral indices that optimize selection of cool dwarfs and distinguish rare populations (subdwarfs, young planetary-mass objects) based on low-resolution, limited-wavelength-coverage spectral data; and present a template-matching classification method for these instruments. We apply these techniques to HST/WFC3 parallel fields data in the WISPS and HST-3D programs, where our spectral index set allows high completeness and low contamination for searches of late M, L and T dwarfs to distances out to ~3 kpc.The material presented here is based on work supported by the National Aeronautics and Space Administration under Grant No. NNX15AI75G.

  2. 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.

  3. Hubble Space Telescope Observations of Binary Very-Low-Mass Stars and Brown Dwarfs

    OpenAIRE

    Gizis, J. E.; Reid, I N; Knapp, G. R.; Liebert, J.; Kirkpatrick, J. D.; Koerner, D. W.; Burgasser, A. J.

    2003-01-01

    We present analysis of Hubble Space Telescope images of 82 nearby field late-M and L dwarfs. We resolve 13 of these systems into double M/L dwarf systems and identify an additional possible binary. Combined with previous observations of 20 L dwarfs, we derive an observed binary fraction for ultracool dwarfs of 17+4-3%, where the statistics included systems with separations in the range 1.6-16 A.U. We argue that accounting for biases and incompleteness leads to an estimated binary fraction 15+...

  4. Trumpeting M dwarfs with CONCH-SHELL: a catalogue of nearby cool host-stars for habitable exoplanets and life

    Science.gov (United States)

    Gaidos, E.; Mann, A. W.; Lépine, S.; Buccino, A.; James, D.; Ansdell, M.; Petrucci, R.; Mauas, P.; Hilton, E. J.

    2014-09-01

    We present an all-sky catalogue of 2970 nearby (d ≲ 50 pc), bright (J stars, 86 per cent of which have been confirmed by spectroscopy. This catalogue will be useful for searches for Earth-size and possibly Earth-like planets by future space-based transit missions and ground-based infrared Doppler radial velocity surveys. Stars were selected from the SUPERBLINK proper motion catalogue according to absolute magnitudes, spectra, or a combination of reduced proper motions and photometric colours. From our spectra, we determined gravity-sensitive indices, and identified and removed 0.2 per cent of these as interloping hotter or evolved stars. 13 per cent of the stars exhibit Hα emission, an indication of stellar magnetic activity and possible youth. The mean metallicity is [Fe/H] = -0.07 with a standard deviation of 0.22 dex, similar to nearby solar-type stars. We determined stellar effective temperatures by least-squares fitting of spectra to model predictions calibrated by fits to stars with established bolometric temperatures, and estimated radii, luminosities, and masses using empirical relations. Six per cent of stars with images from integral field spectra are resolved doubles. We inferred the planet population around M dwarfs using Kepler data and applied this to our catalogue to predict detections by future exoplanet surveys.

  5. First Metallicty Distribution From CaT Spectroscopy of RGB Stars in the Dwarf Irregular Galaxy WLM

    Science.gov (United States)

    Leaman, Ryan; Cole, A.; Venn, K.; Tolstoy, E.; Irwin, M.; Szeifert, T.

    2007-07-01

    A metallicity distribution for the central bar region of the dwarf irregular galaxy WLM is presented from VLT FORS2 spectra of 46 red giant stars, as well as radial velocities for the member stars in this field. The [Fe/H] values were derived using the near infrared Ca II triplet lines as a tracer of metallicity (see Grocholski et al. 2006, Rutledge et al. 1997) and is conformed to a metallicity scale with the aid of four calibrating globular clusters. Although limited by small number statistics in this preliminary release, the ability to study the metallicitiy with respect to velocity and physical location of the member stars is invaluable in helping to characterize the formation and enrichment history of these kind of stellar populations - as has been found from CaT analysis of RGB stars in the Sculptor and Fornax galaxies. (Tolstoy et al. 2004, Battaglia et al. 2006) Specifically, the metallicty distribution for the WLM stellar population(s) can be tied to the recent HST star formation history study (Dolphin, 2000) which places estimates on the frequency and duration of star formation episodes in WLM. The isolated nature of WLM allows a unique opportunity to analyze the enrichment and star formation history of a low luminosity stellar population, which presumably has had a less complicated evolution due to minimal local group interactions. Research for this study was funded in part by NSERC Discovery Grant Program #327292-06.

  6. Constraining the nature of dark matter with the star formation history of the faintest Local Group dwarf galaxy satellites

    CERN Document Server

    Chau, Alice; Governato, Fabio

    2016-01-01

    $\\Lambda$-Warm Dark Matter (WDM) has been proposed as alternative scenario to $\\Lambda$ cold dark matter (CDM), motivated by discrepancies at the scale of dwarf galaxies, with less small-scale power and realized by collisionless particles with energies in the range $1-3$ keV. We present a new approach to constrain the viability of such WDM models using star formation histories of the dwarf spheroidal galaxies (dSphs) in the Local Group. We compare their high time-resolution star formation histories (SFHs) obtained with HST-based color magnitude diagrams with the range of possible collapse redshifts of their dark matter halos expected in CDM and in different WDM scenarios. The collapse redshift is inferred after determining a plausible infall mass of the subhalo. This is based on the current mass of individual dwarf inferred from stellar kinematics combined with results of cosmological simulations providing information on the subhalo evolution. Since WDM subhalos close to the filtering mass scale form signific...

  7. Membership, binarity and accretion among very low-mass stars and brown dwarfs of the σ Orionis cluster

    Science.gov (United States)

    Kenyon, M. J.; Jeffries, R. D.; Naylor, Tim; Oliveira, J. M.; Maxted, P. F. L.

    2005-01-01

    Intermediate-resolution (R~ 7000) spectroscopy is presented for 76 photometrically selected very low-mass (0.04 consistent with the cluster mean. Photometric selection alone therefore appears to be very effective in identifying cluster members in this mass range. Only six objects appear to be certain non-members; however, a substantial subset of 13 candidates have ambiguous or contradictory indications of membership and lack Li absorption. Together with an observed spread in the equivalent width of the Li absorption feature in the cooler stars of our sample, this indicates that there may be deficiencies in our understanding of the formation of this line in cool, low-gravity objects. Four candidate binary cluster members are identified. Consideration of sampling and radial velocity measurement precision leads us to conclude that either the fraction of very low-mass stars and brown dwarfs in small separation (a < 1 au) binary systems is larger than in field M-dwarfs, or the distribution of separations is much less skewed towards large separations. This conclusion hinges critically on the correct identification of the small number of binary candidates, although it remains significant even when only the candidate members displaying Li absorption are considered. Broadened Hα emission, indicative of circum(sub)stellar accretion discs is found in five or six of the candidate cluster members, three of which probably have substellar masses. The fraction of accretors (10 +/- 5 per cent) is similar to that found in stars of higher mass in the σ Ori cluster using Hα emission as a diagnostic, but much lower than found for very low-mass stars and brown dwarfs of younger clusters. The time-scale for accretion rates to drop to <~10-11 Msolar yr-1 is hence less than the age of the σ Ori cluster (3-7 Myr) for most low-mass objects.

  8. Detection of binary and multiple systems among rapidly rotating K and M dwarf stars from Kepler data

    CERN Document Server

    Oláh, Katalin; Joss, Matthew

    2016-01-01

    From an examination of ~18,000 Kepler light curves of K- and M-stars we find some 500 which exhibit rotational periods of less than 2 days. Among such stars, approximately 50 show two or more incommensurate periodicities. We discuss the tools that allow us to differentiate between rotational modulation and other types of light variations, e.g., due to pulsations or binary modulations. We find that these multiple periodicities are independent of each other and likely belong to different, but physically bound, stars. This scenario was checked directly by UKIRT and adaptive optics imaging, time-resolved Fourier transforms, and pixel-level analysis of the data. Our result is potentially important for discovering young multiple stellar systems among rapidly rotating K- and M-dwarfs.

  9. Fundamental M-dwarf parameters from high-resolution spectra using PHOENIX ACES models: I. Parameter accuracy and benchmark stars

    CERN Document Server

    Passegger, Vera Maria; Reiners, Ansgar

    2016-01-01

    M-dwarf stars are the most numerous stars in the Universe; they span a wide range in mass and are in the focus of ongoing and planned exoplanet surveys. To investigate and understand their physical nature, detailed spectral information and accurate stellar models are needed. We use a new synthetic atmosphere model generation and compare model spectra to observations. To test the model accuracy, we compared the models to four benchmark stars with atmospheric parameters for which independent information from interferometric radius measurements is available. We used $\\chi^2$ -based methods to determine parameters from high-resolution spectroscopic observations. Our synthetic spectra are based on the new PHOENIX grid that uses the ACES description for the equation of state. This is a model generation expected to be especially suitable for the low-temperature atmospheres. We identified suitable spectral tracers of atmospheric parameters and determined the uncertainties in $T_{\\rm eff}$, $\\log{g}$, and [Fe/H] resul...

  10. The angular sizes of dwarf stars and subgiants - Non-linear surface brightness relations in BVRcIc from interferometry

    CERN Document Server

    Kervella, Pierre

    2008-01-01

    Context: The prediction of stellar angular diameters from broadband photometry plays an important role for different applications. In particular, long-baseline interferometry, gravitational microlensing, extrasolar planet transits, and many other observing techniques require accurate predictions of the angular size of stars. These predictions are based on the surface brightness-colour (SBC) relations. Aims: Our goal is to calibrate general-purpose SBC relations using visible colours, the most commonly available data for most stars. Methods: We compiled the existing long-baseline interferometric observations of nearby dwarf and subgiant stars and the corresponding broadband photometry in the Johnson B V and Cousins Rc Ic bands. We then adjusted polynomial SBC models to these data. Results: Due to the presence of spectral features that depend on the effective temperature, the SBC relations are usually not linear for visible colours. We present polynomial fits that can be employed with BVRcIc based colours to pr...

  11. Magnetospheric structure and atmospheric Joule heating of habitable planets orbiting M-dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Cohen, O.; Drake, J. J.; Garraffo, C.; Poppenhaeger, K. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Glocer, A. [NASA/GSFC, Code 673, Greenbelt, MD 20771 (United States); Bell, J. M. [Center for Planetary Atmospheres and Flight Sciences, National Institute of Aerospace, Hampton, VA 23666 (United States); Ridley, A. J.; Gombosi, T. I. [Center for Space Environment Modeling, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109 (United States)

    2014-07-20

    We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvénic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvénic sectors, while no bow shock forms in the sub-Alfvénic sectors. The planets reside most of the time in the sub-Alfvénic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the intense stellar wind. For the steady-state solution, the heating is about 0.1%-3% of the total incoming stellar irradiation, and it is enhanced by 50% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport. Here we assume constant ionospheric Pedersen conductance similar to that of the Earth. The conductance could be greater due to the intense EUV radiation leading to smaller heating rates. We plan to quantify the ionospheric conductance in future study.

  12. Identification of dusty massive stars in star-forming dwarf irregular galaxies in the Local Group with mid-IR photometry

    CERN Document Server

    Britavskiy, N E; Mehner, A; Boyer, M L; McQuinn, K B W

    2015-01-01

    Increasing the statistics of spectroscopically confirmed evolved massive stars in the Local Group enables the investigation of the mass loss phenomena that occur in these stars in the late stages of their evolution. We aim to complete the census of luminous mid-IR sources in star-forming dwarf irregular (dIrr) galaxies of the Local Group. To achieve this we employed mid-IR photometric selection criteria to identify evolved massive stars, such as red supergiants (RSGs) and luminous blue variables (LBVs), by using the fact that these types of stars have infrared excess due to dust. The method is based on 3.6 $\\mu$m and 4.5 $\\mu$m photometry from archival ${\\it Spitzer}$ Space Telescope images of nearby galaxies. We applied our criteria to 4 dIrr galaxies: Pegasus, Phoenix, Sextans A, and WLM, selecting 79 point sources, which we observed with the VLT/FORS2 spectrograph in multi-object spectroscopy mode. We identified 13 RSGs, of which 6 are new discoveries, also 2 new emission line stars, and 1 candidate yellow...

  13. Pushing the limits, episode 2: K2 observations of extragalactic RR Lyrae stars in the dwarf galaxy Leo IV

    CERN Document Server

    Molnár, L; Plachy, E; Ripepi, V; Moretti, M I; Szabó, R; Kiss, L L

    2015-01-01

    We present the first observations of extragalactic pulsating stars in the K2 ecliptic survey of the Kepler space telescope. Variability of all three RR Lyrae stars in the dwarf spheroidal galaxy Leo IV were successfully detected, at a brightness of Kp~21.5 mag, from data collected during Campaign 1. We identified one modulated star and another likely Blazhko candidate with periods of 29.8+-0.9 d and more than 80 d, respectively. EPIC 210282473 represents the first star beyond the Magellanic Clouds for which the Blazhko period and cycle-to-cycle variations in the modulation were unambiguously measured.The photometric [Fe/H] indices of the stars agree with earlier results that Leo IV is a very metal-poor galaxy. Two out of three stars blend with brighter background galaxies in the K2 frames. We demonstrate that image subtraction can be reliably used to extract photometry from faint confused sources that will be crucial not only for the K2 mission but for future space photometric missions as well.

  14. Molecular gas and star formation in the tidal dwarf galaxy VCC 2062

    Science.gov (United States)

    Lisenfeld, U.; Braine, J.; Duc, P. A.; Boquien, M.; Brinks, E.; Bournaud, F.; Lelli, F.; Charmandaris, V.

    2016-05-01

    The physical mechanisms driving star formation (SF) in galaxies are still not fully understood. Tidal dwarf galaxies (TDGs), made of gas ejected during galaxy interactions, seem to be devoid of dark matter and have a near-solar metallicity. The latter makes it possible to study molecular gas and its link to SF using standard tracers (CO, dust) in a peculiar environment. We present a detailed study of a nearby TDG in the Virgo Cluster, VCC 2062, using new high-resolution CO(1-0) data from the Plateau de Bure, deep optical imaging from the Next Generation Virgo Cluster Survey (NGVS), and complementary multiwavelength data. Until now, there was some doubt whether VCC 2062 was a true TDG, but the new deep optical images from the NGVS reveal a stellar bridge between VCC 2062 and its parent galaxy, NGC 4694, which is clear proof of its tidal origin. Several high-resolution tracers (Hα, UV, 8 μm, and 24 μm) of the star formation rate (SFR) are compared to the molecular gas distribution as traced by the CO(1-0). Coupled with the SFR tracers, the NGVS data are used with the CIGALE code to model the stellar populations throughout VCC 2062, yielding a declining SFR in the recent past, consistent with the low Hα/UV ratio, and a high burst strength. HI emission covers VCC 2062, whereas the CO is concentrated near the HI maxima. The CO peaks correspond to two very distinct regions: one with moderate SF to the NE and one with only slightly weaker CO emission but with nearly no SF. Even where SF is clearly present, the SFR is below the value expected from the surface density of the molecular and the total gas as compared to spiral galaxies and other TDGs. After discussing different possible explanations, we conclude that the low surface brightness is a crucial parameter to understand the low SFR. The reduced data cubes are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/590/A92

  15. Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

    Science.gov (United States)

    Grossi, M.; Corbelli, E.; Bizzocchi, L.; Giovanardi, C.; Bomans, D.; Coelho, B.; De Looze, I.; Gonçalves, T. S.; Hunt, L. K.; Leonardo, E.; Madden, S.; Menéndez-Delmestre, K.; Pappalardo, C.; Riguccini, L.

    2016-05-01

    We present 12CO(1-0) and 12CO(2-1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log (O / H) ~ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 μm emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H2 molecular masses, namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses ≲ 109 M⊙, contrary to the atomic hydrogen fraction, MHI/M∗, which increases inversely with M∗. The flattening of the MH2/M∗ ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both Hi-deficient and Hi-normal dwarfs. The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity, confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium. Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals, but gas depletion timescales, τdep, are not constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs, but the molecular gas appears to be less affected at the current stage of evolution within the cluster. However, the correlation between Hi deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity. Based on observations carried out with the IRAM 30-m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany

  16. Multi-Element Abundance Measurements from Medium-Resolution Spectra. II. Catalog of Stars in Milky Way Dwarf Satellite Galaxies

    CERN Document Server

    Kirby, Evan N; Simon, Joshua D; Geha, Marla C; Rockosi, Constance M; Sneden, Christopher; Cohen, Judith G; Sohn, Sangmo Tony; Majewski, Steven R; Siegel, Michael

    2010-01-01

    We present a catalog of Fe, Mg, Si, Ca, and Ti abundances for 2961 red giant stars that are likely members of eight dwarf satellite galaxies of the Milky Way (MW): Sculptor, Fornax, Leo I, Sextans, Leo II, Canes Venatici I, Ursa Minor, and Draco. For the purposes of validating our measurements, we also observed 445 red giants in MW globular clusters and 21 field red giants in the MW halo. The measurements are based on Keck/DEIMOS medium-resolution spectroscopy combined with spectral synthesis. We estimate uncertainties in [Fe/H] by quantifying the dispersion of [Fe/H] measurements in a sample of stars in monometallic globular clusters. We estimate uncertainties in Mg, Si, Ca, and Ti abundances by comparing our medium-resolution spectroscopic measurements to high-resolution spectroscopic abundances of the same stars. For this purpose, our DEIMOS sample included 132 red giants with published high-resolution spectroscopy in globular clusters, the MW halo field, and dwarf galaxies. The standard deviations of the ...

  17. Herschel/PACS view of disks around low-mass stars and brown dwarfs in the TW Hya association

    CERN Document Server

    Liu, Yao; Gong, Munan; Allers, Katelyn N; Brown, Joanna M; Kraus, Adam L; Liu, Michael C; Shkolnik, Evgenya L; van Dishoeck, Ewine F

    2014-01-01

    We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at $70\\,\\mu{\\rm{m}}$ and $100\\,\\mu{\\rm{m}}$ and three targets at $160\\,\\mu{\\rm{m}}$. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct SEDs with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modelling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both...

  18. THE ACS NEARBY GALAXY SURVEY TREASURY. X. QUANTIFYING THE STAR CLUSTER FORMATION EFFICIENCY OF NEARBY DWARF GALAXIES

    International Nuclear Information System (INIS)

    We study the relationship between the field star formation and cluster formation properties in a large sample of nearby dwarf galaxies. We use optical data from the Hubble Space Telescope and from ground-based telescopes to derive the ages and masses of the young (tage ∼< 100 Myr) cluster sample. Our data provide the first constraints on two proposed relationships between the star formation rate (SFR) of galaxies and the properties of their cluster systems in the low SFR regime. The data show broad agreement with these relationships, but significant galaxy-to-galaxy scatter exists. In part, this scatter can be accounted for by simulating the small number of clusters detected from stochastically sampling the cluster mass function. However, this stochasticity does not fully account for the observed scatter in our data, suggesting that there may be true variations in the fraction of stars formed in clusters in dwarf galaxies. Comparison of the cluster formation and the brightest cluster in our sample galaxies also provide constraints on cluster destruction models.

  19. Response of Atmospheric Biomarkers to NOx-induced Photochemistry Generated by Stellar Cosmic Rays for Earth-like Planets in the Habitable Zone of M-Dwarf Stars

    OpenAIRE

    Grenfell, John Lee; Grriessmeier, Jean-Mathias; von Paris, Philip; Patzer, Beate; Lammer, Helmut; Stracke, Barbara; Gebauer, Stefanie; Schreier, Franz; Rauer, Heike

    2015-01-01

    Understanding whether M-dwarf stars may host habitable planets with Earth-like atmospheres and biospheres is a major goal in exoplanet research. If such planets exist, the question remains as to whether they could be identified via spectral signatures of biomarkers. Such planets may be exposed to extreme intensities of cosmic rays that could perturb their atmospheric photochemistry. Here, we consider stellar activity of M-dwarfs ranging from quiet up to strong flaring conditions and investiga...

  20. Sequence Analysis of Segments 8 and 10 of Rice black-streaked dwarf virus from Maize Plants

    OpenAIRE

    Bong Choon Lee; Sang-Yun Cho; Young-Nam Yoon; In Jeong Kang; Do Yeon Kwak; Dong Bum Shin; Hang-Won Kang

    2012-01-01

    Rice black-streaked dwarf virus (RBSDV) was reported to occur on maize plants in Gochang-gun ofJeonllabuk-do region in 2011. The symptoms typically include stunted and deformed leaves. Virus infectedplants usually produce poor or no head. RT-PCR analysis of genomic dsRNA extracted from the plantconfirmed the infection. Specific primers for full length genome of segments 8 and 10 were used for RNAamplification. Full-length genomes of S8 and S10 were cloned and sequenced. Sequence analysis reve...

  1. SPIN-PRECESSION: BREAKING THE BLACK HOLE-NEUTRON STAR DEGENERACY

    Energy Technology Data Exchange (ETDEWEB)

    Chatziioannou, Katerina; Cornish, Neil; Klein, Antoine; Yunes, Nicolás [Department of Physics, Montana State University, Bozeman, MT 59717 (United States)

    2015-01-01

    Mergers of compact stellar remnants are prime targets for the LIGO/Virgo gravitational wave detectors. The gravitational wave signals from these merger events can be used to study the mass and spin distribution of stellar remnants, and provide information about black hole horizons and the material properties of neutron stars. However, it has been suggested that degeneracies in the way that the star's mass and spin are imprinted in the waveforms may make it impossible to distinguish between black holes and neutron stars. Here we show that the precession of the orbital plane due to spin-orbit coupling breaks the mass-spin degeneracy, and allows us to distinguish between standard neutron stars and alternative possibilities, such as black holes or exotic neutron stars with large masses and spins.

  2. The Star Formation Histories of Local Group Dwarf Galaxies III. Characterizing Quenching in Low-Mass Galaxies

    OpenAIRE

    Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; Holtzman, Jon; Gilbert, Karoline M.; Dalcanton, Julianne J.; Williams, Benjamin F.

    2015-01-01

    We explore the quenching of low-mass galaxies (10^4 < Mstar < 10^8 Msun) as a function of lookback time using the star formation histories (SFHs) of 38 Local Group dwarf galaxies. The SFHs were derived from analyzing color-magnitude diagrams of resolved stellar populations in archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) Lower mass galaxies quench earlier than higher mass galaxies; (2) Inside of virial radius there is no correlation between a satellite's ...

  3. ELECTROMAGNETIC EXTRACTION OF ENERGY FROM BLACK-HOLE-NEUTRON-STAR BINARIES

    Energy Technology Data Exchange (ETDEWEB)

    McWilliams, Sean T.; Levin, Janna, E-mail: stmcwill@princeton.edu [Institute for Strings, Cosmology and Astroparticle Physics (ISCAP), Columbia University, New York, NY 10027 (United States)

    2011-12-01

    The coalescence of black-hole-neutron-star binaries is expected to be a principal source of gravitational waves for the next generation of detectors, Advanced LIGO and Advanced Virgo. For black hole masses not much larger than the neutron star mass, the tidal disruption of the neutron star by the black hole provides one avenue for generating an electromagnetic counterpart. However, in this work, we demonstrate that, for all black-hole-neutron-star binaries observable by Advanced LIGO/Virgo, the interaction of the black hole with the magnetic field of the neutron star will generate copious luminosity, comparable to supernovae and active galactic nuclei. This novel effect may have already been observed as a new class of very short gamma-ray bursts by the Swift Gamma-Ray Burst Telescope. These events may be observable to cosmological distances, so that any black-hole-neutron-star coalescence detectable with gravitational waves by Advanced LIGO/Virgo could also be detectable electromagnetically.

  4. Relativistic black hole-neutron star binaries in quasiequilibrium: effects of the black hole excision boundary condition

    OpenAIRE

    Taniguchi, Keisuke; Baumgarte, Thomas W.; Faber, Joshua A.; Shapiro, Stuart L.

    2007-01-01

    We construct new models of black hole-neutron star binaries in quasiequilibrium circular orbits by solving Einstein's constraint equations in the conformal thin-sandwich decomposition together with the relativistic equations of hydrostationary equilibrium. We adopt maximal slicing, assume spatial conformal flatness, and impose equilibrium boundary conditions on an excision surface (i.e., the apparent horizon) to model the black hole. In our previous treatment we adopted a "leading-order" appr...

  5. Black hole variability and the star formation-AGN connection: Do all star-forming galaxies host an AGN?

    OpenAIRE

    Hickox, R. C.; Mullaney, J. R.; Alexander, D. M.; Chen, C.-T.J.; Civano, F. M.; Goulding, A. D.; Hainline, K. N.

    2013-01-01

    We investigate the effect of active galactic nucleus (AGN) variability on the observed connection between star formation and black hole accretion in extragalactic surveys. Recent studies have reported relatively weak correlations between observed AGN luminosities and the properties of AGN hosts, which has been interpreted to imply that there is no direct connection between AGN activity and star formation. However, AGNs may be expected to vary significantly on a wide range of timescales (from ...

  6. HATS-7b: A Hot Super Neptune Transiting a Quiet K Dwarf Star

    Science.gov (United States)

    Bakos, G. Á.; Penev, K.; Bayliss, D.; Hartman, J. D.; Zhou, G.; Brahm, R.; Mancini, L.; de Val-Borro, M.; Bhatti, W.; Jordán, A.; Rabus, M.; Espinoza, N.; Csubry, Z.; Howard, A. W.; Fulton, B. J.; Buchhave, L. A.; Ciceri, S.; Henning, T.; Schmidt, B.; Isaacson, H.; Noyes, R. W.; Marcy, G. W.; Suc, V.; Howe, A. R.; Burrows, A. S.; Lázár, J.; Papp, I.; Sári, P.

    2015-11-01

    We report the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune with a mass of 0.120 ± 0.012 {M}{{J}}, a radius of {0.563}-0.034+0.046 {R}{{J}}, and an orbital period of 3.1853 days. The host star is a moderately bright (V=13.340\\+/- 0.010 mag, {K}S=10.976\\+/- 0.026 mag) K dwarf star with a mass of 0.849 ± 0.027 {M}⊙ , a radius of {0.815}-0.035+0.049 {R}⊙ , and a metallicity of [{Fe}/{{H}}] =+0.250\\+/- 0.080. The star is photometrically quiet to within the precision of the HATSouth measurements, has low RV jitter, and shows no evidence for chromospheric activity in its spectrum. HATS-7b is the second smallest radius planet discovered by a wide-field ground-based transit survey, and one of only a handful of Neptune-size planets with mass and radius determined to 10% precision. Theoretical modeling of HATS-7b yields a hydrogen-helium fraction of 18 ± 4% (rock-iron core and H2-He envelope), or 9 ± 4% (ice core and H2-He envelope), i.e., it has a composition broadly similar to that of Uranus and Neptune, and very different from that of Saturn, which has 75% of its mass in H2-He. Based on a sample of transiting exoplanets with accurately (<20%) determined parameters, we establish approximate power-law relations for the envelopes of the mass-density distribution of exoplanets. HATS-7b, which, together with the recently discovered HATS-8b, is one of the first two transiting super-Neptunes discovered in the Southern sky, is a prime target for additional follow-up observations with Southern hemisphere facilities to characterize the atmospheres of Super-Neptunes (which we define as objects with mass greater than that of Neptune, and smaller than halfway between that of Neptune and Saturn, i.e., 0.054 {M}{{J}}\\lt {M}{{p}}\\lt 0.18 {M}{{J}}). The HATSouth network is operated by a collaboration consisting of Princeton University (PU), the Max Planck Institute für Astronomie (MPIA), the Australian National University (ANU), and the Pontificia

  7. Suppression of star formation in early-type galaxies by feedback from supermassive black holes.

    Science.gov (United States)

    Schawinski, Kevin; Khochfar, Sadegh; Kaviraj, Sugata; Yi, Sukyoung K; Boselli, Alessandro; Barlow, Tom; Conrow, Tim; Forster, Karl; Friedman, Peter G; Martin, D Chris; Morrissey, Patrick; Neff, Susan; Schiminovich, David; Seibert, Mark; Small, Todd; Wyder, Ted K; Bianchi, Luciana; Donas, Jose; Heckman, Tim; Lee, Young-Wook; Madore, Barry; Milliard, Bruno; Rich, R Michael; Szalay, Alex

    2006-08-24

    Detailed high-resolution observations of the innermost regions of nearby galaxies have revealed the presence of supermassive black holes. These black holes may interact with their host galaxies by means of 'feedback' in the form of energy and material jets; this feedback affects the evolution of the host and gives rise to observed relations between the black hole and the host. Here we report observations of the ultraviolet emissions of massive early-type galaxies. We derive an empirical relation for a critical black-hole mass (as a function of velocity dispersion) above which the outflows from these black holes suppress star formation in their hosts by heating and expelling all available cold gas. Supermassive black holes are negligible in mass compared to their hosts but nevertheless seem to play a critical role in the star formation history of galaxies. PMID:16929291

  8. Rapid growth of black holes in massive star-forming galaxies.

    Science.gov (United States)

    Alexander, D M; Smail, I; Bauer, F E; Chapman, S C; Blain, A W; Brandt, W N; Ivison, R J

    2005-04-01

    The tight relationship between the masses of black holes and galaxy spheroids in nearby galaxies implies a causal connection between the growth of these two components. Optically luminous quasars host the most prodigious accreting black holes in the Universe, and can account for greater than or approximately equal to 30 per cent of the total cosmological black-hole growth. As typical quasars are not, however, undergoing intense star formation and already host massive black holes (> 10(8)M(o), where M(o) is the solar mass), there must have been an earlier pre-quasar phase when these black holes grew (mass range approximately (10(6)-10(8))M(o)). The likely signature of this earlier stage is simultaneous black-hole growth and star formation in distant (redshift z > 1; >8 billion light years away) luminous galaxies. Here we report ultra-deep X-ray observations of distant star-forming galaxies that are bright at submillimetre wavelengths. We find that the black holes in these galaxies are growing almost continuously throughout periods of intense star formation. This activity appears to be more tightly associated with these galaxies than any other coeval galaxy populations. We show that the black-hole growth from these galaxies is consistent with that expected for the pre-quasar phase. PMID:15815623

  9. A Spitzer Study of Asymptotic Giant Branch Stars. III. Dust Production and Gas Return in Local Group Dwarf Irregular Galaxies

    CERN Document Server

    Boyer, Martha L; van Loon, Jacco Th; Gehrz, Robert D; Woodward, Charles E

    2009-01-01

    We present the third and final part of a census of Asymptotic Giant Branch (AGB) stars in Local Group dwarf irregular galaxies. Papers I and II presented the results for WLM and IC 1613. Included here are Phoenix, LGS 3, DDO 210, Leo A, Pegasus dIrr, and Sextans A. Spitzer photometry at 3.6, 4.5, 5.8, and 8 are presented, along with a more thorough treatment of background galaxy contamination than was presented in papers I and II. We find that at least a small population of completely optically obscured AGB stars exists in each galaxy, regardless of the galaxy's metallicity, but that higher-metallicity galaxies tend to harbor more stars with slight IR excesses. The optical incompleteness increases for the redder AGB stars, in line with the expectation that some AGB stars are not detected in the optical due to large amounts of extinction associated with in situ dust production. Overall, there is an underrepresentation of 30% - 40% in the optical AGB within the 1 sigma errors for all of the galaxies in our samp...

  10. Interior Solutions for Non-singular Gravity and the Dark Star alternative to Black Holes

    OpenAIRE

    Cornish, Neil

    1994-01-01

    The general equations describing hydrostatic equilibrium are developed for Non-singular Gravity. A new type of astrophysical structure, a Super Dense Object (SDO) or "Dark Star", is shown to exist beyond Neutron star field strengths. These structures are intrinsically stable against gravitational collapse and represent the non-singular alternative to General Relativity's Black Holes.

  11. A CORRELATION BETWEEN STAR FORMATION RATE AND AVERAGE BLACK HOLE ACCRETION IN STAR-FORMING GALAXIES

    International Nuclear Information System (INIS)

    We present a measurement of the average supermassive black hole accretion rate (BHAR) as a function of the star formation rate (SFR) for galaxies in the redshift range 0.25 2 Boötes multi-wavelength survey field. The SFR is estimated using 250 μm observations from the Herschel Space Observatory, for which the contribution from the active galactic nucleus (AGN) is minimal. In this sample, 121 AGNs are directly identified using X-ray or mid-IR selection criteria. We combined these detected AGNs and an X-ray stacking analysis for undetected sources to study the average BHAR for all of the star-forming galaxies in our sample. We find an almost linear relation between the average BHAR (in M☉ yr–1) and the SFR (in M☉ yr–1) for galaxies across a wide SFR range 0.85 < log SFR < 2.56: log BHAR = (– 3.72 ± 0.52) + (1.05 ± 0.33)log SFR. This global correlation between SFR and average BHAR is consistent with a simple picture in which SFR and AGN activity are tightly linked over galaxy evolution timescales

  12. Diagnostics of models and observations in the contexts of exoplanets, brown dwarfs, and very low-mass stars.

    Science.gov (United States)

    Kopytova, Taisiya

    2016-01-01

    When studying isolated brown dwarfs and directly imaged exoplanets with insignificant orbital motion,we have to rely on theoretical models to determine basic parameters such as mass, age, effective temperature, and surface gravity.While stellar and atmospheric models are rapidly evolving, we need a powerful tool to test and calibrate them.In my thesis, I focussed on comparing interior and atmospheric models with observational data, in the effort of taking into account various systematic effects that can significantly influence the data analysis.As a first step, about 460 candidate member os the Hyades were screened for companions using diffraction limited imaging observation (both our own data and archival data). As a result I could establish the single star sequence for the Hyades comprising about 250 stars (Kopytova et al. 2015, accepted to A&A). Open clusters contain many coeval objects of the same chemical composition and age, and spanning a range of masses. We compare the obtained sequence with a set of theoretical isochrones identifying systematic offsets and revealing probable issues in the models.However, there are many cases when it is impossible to test models before comparing them with observations.As a second step, we apply atmospheric models for constraining parameters of WISE 0855-07, the coolest known Y dwarf(Kopytova et al. 2014, ApJ 797, 3). We demonstrate the limits of constraining effective temperature and the presence/absence of water clouds.As a third step, we introduce a novel method to take into account the above-mentioned systematics. We construct a "systematics vector" that allows us to reveal problematic wavelength ranges when fitting atmospheric models to observed near-infrared spectraof brown dwarfs and exoplanets (Kopytova et al., in prep.). This approach plays a crucial role when retrieving abundances for these objects, in particularly, a C/O ratio. The latter parameter is an important key to formation scenarios of brown dwarf and

  13. Thick to Thin: The Evolutionary Connection Between PG 1159 Stars and the Thin Helium-Enveloped Pulsating White Dwarf GD 358

    OpenAIRE

    Dehner, Benjamin T.; Kawaler, Steven D.

    1995-01-01

    Seismological observations with the Whole Earth Telescope (WET) allow the determination of the subsurface compositional structure of white dwarf stars. The hot DO PG 1159 has a helium surface layer with a mass of 0.001 Msun, while the cooler DB white dwarf GD 358 has a much thinner surface helium layer of 10^-6 Msun. These results imply that either there is no evolutionary relation between these two stars, or that there is an unknown mass loss mechanism. To investigate possible evolutionary l...

  14. 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.

  15. Relations between broad-band linear polarization and Ca II H and K emission in late-type dwarf stars

    Science.gov (United States)

    Huovelin, Juhani; Saar, Steven H.; Tuominen, Ilkka

    1988-01-01

    Broadband UBV linear polarization data acquired for a sample of late-type dwarfs are compared with contemporaneous measurements of Ca II H and K line core emission. A weighted average of the largest values of the polarization degree is shown to be the best parameter for chromospheric activity diagnosis. The average maximum polarization in the UV is found to increase from late-F to late-G stars. It is noted that polarization in the U band is considerably more sensitive to activity variations than that in the B or V bands. The results indicate that stellar magnetic fields and the resulting saturation in the Zeeman-sensitive absorption lines are the most probably source of linear polarization in late-type main-sequence stars.

  16. No time for dead time: timing analysis of bright black hole binaries with NuSTAR

    DEFF Research Database (Denmark)

    Bachetti, Matteo; Harrison, Fiona A.; Cook, Rick;

    2015-01-01

    Timing of high-count-rate sources with the NuSTAR Small Explorer Mission requires specialized analysis techniques. NuSTAR was primarily designed for spectroscopic observations of sources with relatively low count rates rather than for timing analysis of bright objects. The instrumental dead time ...... techniques. We apply this technique to NuSTAR observations of the black hole binaries GX 339-4, Cyg X-1, and GRS 1915+105....

  17. GRB060218 AS A TIDAL DISRUPTION OF A WHITE DWARF BY AN INTERMEDIATE-MASS BLACK HOLE

    Energy Technology Data Exchange (ETDEWEB)

    Shcherbakov, Roman V.; Reynolds, Christopher S. [Department of Astronomy, University of Maryland, College Park, MD 20742 (United States); Pe' er, Asaf [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Haas, Roland [Theoretical AstroPhysics Including Relativity, California Institute of Technology, Pasadena, CA 91125 (United States); Bode, Tanja; Laguna, Pablo [Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 (United States)

    2013-06-01

    The highly unusual pair of a gamma-ray burst (GRB) GRB060218 and an associated supernova, SN2006aj, has puzzled theorists for years. A supernova shock breakout and a jet from a newborn stellar mass compact object have been proposed to explain this pair's multiwavelength signature. Alternatively, we propose that the source is naturally explained by another channel: the tidal disruption of a white dwarf (WD) by an intermediate-mass black hole (IMBH). This tidal disruption is accompanied by a tidal pinching, which leads to the ignition of a WD and a supernova. Some debris falls back onto the IMBH, forms a disk, which quickly amplifies the magnetic field, and launches a jet. We successfully fit soft X-ray spectra with the Comptonized blackbody emission from a jet photosphere. The optical/UV emission is consistent with self-absorbed synchrotron emission from the expanding jet front. The temporal dependence of the accretion rate M-dot (t) in a tidal disruption provides a good fit to the soft X-ray light curve. The IMBH mass is found to be about 10{sup 4} M{sub Sun} in three independent estimates: (1) fitting the tidal disruption M-dot (t) to the soft X-ray light curve, (2) computing the jet base radius in a jet photospheric emission model, and (3) inferring the mass of the central black hole based on the host dwarf galaxy's stellar mass. The position of the supernova is consistent with the center of the host galaxy, while the low supernova ejecta mass is consistent with that of a WD. The high expected rate of tidal disruptions in dwarf galaxies is consistent with one source observed by the Swift satellite over several years at a distance of 150 Mpc measured for GRB060218. Encounters with WDs provide much fuel for the growth of IMBHs.

  18. Hydro-without-hydro framework for simulations of black hole-neutron star binaries

    International Nuclear Information System (INIS)

    We introduce a computational framework which avoids solving explicitly hydrodynamic equations and is suitable for studying the pre-merger evolution of black hole-neutron star binary systems. The essence of the method consists of constructing a neutron star model with a black hole companion and freezing the internal degrees of freedom of the neutron star during the course of the evolution of the spacetime geometry. We present the main ingredients of the framework, from the formulation of the problem to the appropriate computational techniques to study these binary systems. In addition, we present numerical results of the construction of initial data sets and evolutions that demonstrate the feasibility of this approach

  19. The Relationship Between Black Hole Growth and Star Formation in Seyfert Galaxies

    OpenAIRE

    Diamond-Stanic, Aleksandar M.; Rieke, George H.

    2011-01-01

    We present estimates of black hole accretion rates and nuclear, extended, and total star-formation rates for a complete sample of Seyfert galaxies. Using data from the Spitzer Space Telescope, we measure the active galactic nucleus (AGN) luminosity using the [O IV] 25.89 micron emission line and the star-forming luminosity using the 11.3 micron aromatic feature and extended 24 micron continuum emission. We find that black hole growth is strongly correlated with nuclear (r1 kpc) star formation...

  20. Photometric Study on Stellar Magnetic Activity: I. Flare Variability of Red Dwarf Stars in the Open Cluster M37

    CERN Document Server

    Chang, S -W; Hartman, J D

    2015-01-01

    Based on one-month long MMT time-series observations of the open cluster M37, we monitored light variations of nearly 2500 red dwarfs and successfully identified 420 flare events from 312 cluster M dwarf stars. For each flare light curve, we derived observational and physical parameters, such as flare shape, peak amplitude, duration, energy, and peak luminosity. We show that cool stars produce serendipitous flares energetic enough to be observed in the $r$-band, and their temporal and peak characteristics are almost the same as those in traditional $U$-band observations. We also found many large-amplitude flares with inferred $\\Delta u > 6$ mag in the cluster sample which had been rarely reported in previous ground-based observations. Following the ergodic hypothesis, we investigate in detail statistical properties of flare parameters over a range of energy ($E_{r}$ $\\simeq$ $10^{31}-10^{34}$ erg). As expected, there are no statistical differences in the distributions of flare timescales, energies, and freque...

  1. Limits on a Gravitational Field Dependence of the Proton--Electron Mass Ratio from H$_2$ in White Dwarf Stars

    CERN Document Server

    Bagdonaite, Julija; Preval, Simon P; Barstow, Martin A; Barrow, John D; Murphy, Michael T; Ubachs, Wim

    2014-01-01

    Spectra of molecular hydrogen (H$_2$) are employed to search for a possible proton-to-electron mass ratio ($\\mu$) dependence on gravity. The Lyman transitions of H$_2$, observed with the Hubble Space Telescope towards white dwarf stars that underwent a gravitational collapse, are compared to accurate laboratory spectra taking into account the high temperature conditions ($T \\sim 13\\,000$ K) of their photospheres. We derive sensitivity coefficients $K_i$ which define how the individual H$_2$ transitions shift due to $\\mu$-dependence. The spectrum of white dwarf star GD133 yields a $\\Delta\\mu/\\mu$ constraint of $(-2.7\\pm4.7_{\\rm stat}\\pm 0.2_{\\rm sys})\\times10^{-5}$ for a local environment of a gravitational potential $\\phi\\sim10^4\\ \\phi_\\textrm{Earth}$, while that of G29$-$38 yields $\\Delta\\mu/\\mu=(-5.8\\pm3.8_{\\rm stat}\\pm 0.3_{\\rm sys})\\times10^{-5}$ for a potential of $2 \\times 10^4$ $\\phi_\\textrm{Earth}$.

  2. A SPITZER STUDY OF ASYMPTOTIC GIANT BRANCH STARS. III. DUST PRODUCTION AND GAS RETURN IN LOCAL GROUP DWARF IRREGULAR GALAXIES

    International Nuclear Information System (INIS)

    We present the third and final part of a census of asymptotic giant branch (AGB) stars in Local Group dwarf irregular (dIrr) galaxies. Papers I and II presented the results for WLM and IC 1613. Included here are Phoenix, LGS 3, DDO 210, Leo A, Pegasus dIrr, and Sextans A. Spitzer photometry at 3.6, 4.5, 5.8, and 8 μm are presented, along with a more thorough treatment of background galaxy contamination than was presented in Papers I and II. We find that at least a small population of completely optically obscured AGB stars exists in each galaxy, regardless of the galaxy's metallicity, but that higher metallicity galaxies tend to harbor more stars with slight infrared excesses. The optical incompleteness increases for the redder AGB stars, in line with the expectation that some AGB stars are not detected in the optical due to large amounts of extinction associated with in situ dust production. Overall, there is an underrepresentation of 30%-40% in the optical AGB within the 1σ errors for all of the galaxies in our sample. This undetected population is large enough to affect star formation histories derived from optical color-magnitude diagrams. As measured from the [3.6] - [4.5] color excesses, we find average stellar mass-loss rates (MLRs) ranging from 3.1 x 10-7to6.6 x 10-6 M sun yr-1, and integrated galaxy MLRs ranging from 4.4 x 10-5to1.4 x 10-3 M sun yr-1. The integrated MLR is sufficient to sustain the current star formation rate in only LGS 3 and DDO 210, requiring either significant nondusty mass loss or gas accretion in Phoenix, Leo A, Pegasus dIrr, Sextans A, WLM, and IC 1613 if they are to maintain their status as gas-rich galaxies.

  3. The Properties of Hypervelocity Stars and S-stars Originating from an Eccentric Disk around a Supermassive Black Hole

    Science.gov (United States)

    Šubr, Ladislav; Haas, Jaroslav

    2016-09-01

    Hypervelocity stars (HVSs), which are observed in the Galactic halo, are believed to be accelerated to large velocities by a process of tidal disruption of binary stars passing close to the supermassive black hole (SMBH) which resides in the center of the Galaxy. It is, however, still unclear where these relatively young stars were born and what dynamical process pushed them to nearly radial orbits around the SMBH. In this paper we investigate the possibility that the young binaries originated from a thin eccentric disk, similar to the one currently observed in the Galactic center. By means of direct N-body simulations, we follow the dynamical evolution of an initially thin and eccentric disk of stars with a 100% binary fraction orbiting around the SMBH. Such a configuration leads to Kozai–Lidov oscillations of orbital elements, bringing a considerable number of binaries to the close vicinity of the black hole. Subsequent tidal disruption of these binaries accelerates one of their components to velocities well above the escape velocity from the SMBH, while the second component becomes tightly bound to the SMBH. We describe the main kinematic properties of the escaping and tightly bound stars within our model, and compare them qualitatively to the properties of the observed HVSs and S-stars, respectively. The most prominent feature is strong anisotropy in the directions of the escaping stars, which is observed for Galactic HVSs but has not yet been explained.

  4. Optical Thermonuclear Transients From Tidal Compression of White Dwarfs as Tracers of the Low End of the Massive Black Hole Mass Function

    CERN Document Server

    MacLeod, Morgan; Ramirez-Ruiz, Enrico; Kasen, Daniel; Rosswog, Stephan

    2015-01-01

    In this paper, we model the observable signatures of tidal disruptions of white dwarf (WD) stars by massive black holes (MBHs) of moderate mass, $\\approx 10^3 - 10^5 M_\\odot$. When the WD passes deep enough within the MBH's tidal field, these signatures include thermonuclear transients from burning during maximum compression. We combine a hydrodynamic simulation that includes nuclear burning of the disruption of a $0.6 M_\\odot$ C/O WD with a Monte Carlo radiative transfer calculation to synthesize the properties of a representative transient. The transient's emission emerges in the optical, with lightcurves and spectra reminiscent of type I SNe. The properties are strongly viewing-angle dependent, and key spectral signatures are $\\approx 10,000$ km s$^{-1}$ Doppler shifts due to the orbital motion of the unbound ejecta. Disruptions of He WDs likely produce large quantities of intermediate-mass elements, offering a possible production mechanism for Ca-rich transients. Accompanying multiwavelength transients ar...

  5. Aligned spin neutron star-black hole mergers: a gravitational waveform amplitude model

    CERN Document Server

    Pannarale, Francesco; Kyutoku, Koutarou; Lackey, Benjamin D; Shibata, Masaru

    2015-01-01

    The gravitational radiation emitted during the merger of a black hole with a neutron star is rather similar to the radiation from the merger of two black holes when the neutron star is not tidally disrupted. When tidal disruption occurs, gravitational waveforms can be broadly classified in two groups, depending on the spatial extent of the disrupted material. Extending previous work by some of us, here we present a phenomenological model for the gravitational waveform amplitude in the frequency domain encompassing the three possible outcomes of the merger: no tidal disruption, "mild" and "strong" tidal disruption. The model is calibrated to 134 general-relativistic numerical simulations of binaries where the black hole spin is either aligned or antialigned with the orbital angular momentum. All simulations were produced using the SACRA code and piecewise polytropic neutron star equations of state. The present model can be used to determine when black-hole binary waveforms are sufficient for gravitational-wave...

  6. Bright transients from strongly-magnetized neutron star-black hole mergers

    Science.gov (United States)

    D'Orazio, Daniel J.; Levin, Janna; Murray, Norman W.; Price, Larry

    2016-07-01

    Direct detection of black hole-neutron star pairs is anticipated with the advent of aLIGO. Electromagnetic counterparts may be crucial for a confident gravitational-wave detection as well as for extraction of astronomical information. Yet black hole-neutron star pairs are notoriously dark and so inaccessible to telescopes. Contrary to this expectation, a bright electromagnetic transient can occur in the final moments before merger as long as the neutron star is highly magnetized. The orbital motion of the neutron star magnet creates a Faraday flux and corresponding power available for luminosity. A spectrum of curvature radiation ramps up until the rapid injection of energy ignites a fireball, which would appear as an energetic blackbody peaking in the x ray to γ rays for neutron star field strengths ranging from 1012 to 1016 G respectively and a 10 M⊙ black hole. The fireball event may last from a few milliseconds to a few seconds depending on the neutron star magnetic-field strength, and may be observable with the Fermi Gamma-Ray Burst Monitor with a rate up to a few per year for neutron star field strengths ≳1014 G . We also discuss a possible decaying post-merger event which could accompany this signal. As an electromagnetic counterpart to these otherwise dark pairs, the black-hole battery should be of great value to the development of multi-messenger astronomy in the era of aLIGO.

  7. Hydrodynamical simulations of the tidal stripping of binary stars by massive black holes

    Science.gov (United States)

    Mainetti, Deborah; Lupi, Alessandro; Campana, Sergio; Colpi, Monica

    2016-04-01

    In a galactic nucleus, a star on a low angular momentum orbit around the central massive black hole can be fully or partially disrupted by the black hole tidal field, lighting up the compact object via gas accretion. This phenomenon can repeat if the star, not fully disrupted, is on a closed orbit. Because of the multiplicity of stars in binary systems, also binary stars may experience in pairs such a fate, immediately after being tidally separated. The consumption of both the binary components by the black hole is expected to power a double-peaked flare. In this paper, we perform for the first time, with GADGET2, a suite of smoothed particle hydrodynamics simulations of binary stars around a galactic central black hole in the Newtonian regime. We show that accretion luminosity light curves from double tidal disruptions reveal a more prominent knee, rather than a double peak, when decreasing the impact parameter of the encounter and when elevating the difference between the mass of the star which leaves the system after binary separation and the mass of the companion. The detection of a knee can anticipate the onset of periodic accretion luminosity flares if one of the stars, only partially disrupted, remains bound to the black hole after binary separation. Thus knees could be precursors of periodic flares, which can then be predicted, followed up and better modelled. Analytical estimates in the black hole mass range 105-108 M⊙ show that the knee signature is enhanced in the case of black holes of mass 106-107 M⊙.

  8. The Asymptotic Giant Branch and the Tip of the Red Giant Branch as Probes of Star Formation History: The Nearby Dwarf Irregular Galaxy KKH 98

    OpenAIRE

    Melbourne, J.; Williams, B.; Dalcanton, J.; Ammons, S. M.; Max, C.; Koo, D.C.; Girardi, Leo; Dolphin, A.

    2010-01-01

    We investigate the utility of the asymptotic giant branch (AGB) and the red giant branch (RGB) as probes of the star formation history (SFH) of the nearby (D=2.5 Mpc) dwarf irregular galaxy, KKH 98. Near-infrared (IR) Keck Laser Guide Star Adaptive Optics (AO) images resolve 592 IR bright stars reaching over 1 magnitude below the Tip of the Red Giant Branch. Significantly deeper optical (F475W and F814W) Hubble Space Telescope images of the same field contain over 2500 stars, reaching to the ...

  9. Mergers of magnetized neutron stars with spinning black holes: disruption, accretion, and fallback.

    Science.gov (United States)

    Chawla, Sarvnipun; Anderson, Matthew; Besselman, Michael; Lehner, Luis; Liebling, Steven L; Motl, Patrick M; Neilsen, David

    2010-09-10

    We investigate the merger of a neutron star in orbit about a spinning black hole in full general relativity with a mass ratio of 5:1, allowing the star to have an initial magnetization of 10(12)  G. We present the resulting gravitational waveform and analyze the fallback accretion as the star is disrupted. We see no significant dynamical effects in the simulations or changes in the gravitational waveform resulting from the initial magnetization. We find that only a negligible amount of matter becomes unbound; 99% of the neutron star material has a fallback time of 10 seconds or shorter to reach the region of the central engine and that 99.99% of the star will interact with the central disk and black hole within 3 hours. PMID:20867561

  10. Mergers of Magnetized Neutron Stars with Spinning Black Holes: Disruption, Accretion and Fallback

    CERN Document Server

    Chawla, Sarvnipun; Besselman, Michael; Lehner, Luis; Liebling, Steven L; Motl, Patrick M; Neilsen, David

    2010-01-01

    We investigate the merger of a neutron star (of compaction ratio $0.1$) in orbit about a spinning black hole in full general relativity with a mass ratio of $5:1$, allowing for the star to have an initial magnetization of $10^{12} {\\rm Gauss}$. We present the resulting gravitational waveform and analyze the fallback accretion as the star is disrupted. The evolutions suggest no significant effects from the initial magnetization. We find that only a negligible amount of matter becomes unbound; $99\\%$ of the neutron star material has a fallback time of 10 seconds or shorter to reach the region of the central engine and that $99.99\\%$ of the star will interact with the central disk and black hole within 3 hours.

  11. M dwarf stars-the by-product of X-ray selected AGN candidates

    Institute of Scientific and Technical Information of China (English)

    Yu Bai; Yan-Chun Sun; Xiang-Tao He; Yang Chen; Jiang-Hua Wu; Qing-Kang Li; Richard F.Green; Wolfgang Voges

    2012-01-01

    X-ray loud M dwarfs are a major source of by-products (contamination)in the X-ray band of the multiwavelength quasar survey.As a by-product,the low dispersion spectra of 22 M dwarfs are obtained in which the spectra of 16 sources are taken for the first time.The spectral types and distances of the sample arc given based on spectral indices CaH2,CaH3,and TiOS.The parameter (ι)TiO/CaH is calculated to separate the different metallicity classes among dwarfs,subdwarfs and extreme subdwarfs.We also discuss the distributions in the diagrams of log( Lx/ Lbol),the ratio between X-ray and bolometric luminosity versus spectral type and infrared colors.

  12. The Tidal Disruption of Giant Stars and Their Contribution to the Flaring Supermassive Black Hole Population

    CERN Document Server

    MacLeod, Morgan; Ramirez-Ruiz, Enrico

    2012-01-01

    Sun-like stars are thought to be regularly disrupted by supermassive black holes (SMBHs) within galactic nuclei. Yet, as stars evolve off the main sequence their vulnerability to tidal disruption increases drastically as they develop a bifurcated structure consisting of a dense core and a tenuous envelope. Here we present the first hydrodynamic simulations of the tidal disruption of giant stars and show that the core has a substantial influence on the star's ability to survive the encounter. Stars with more massive cores retain large fractions of their envelope mass, even in deep encounters. Accretion flares resulting from the disruption of giant stars should last for tens to hundreds of years. Their characteristic signature in transient searches would not be the $t^{-5/3}$ decay typically associated with tidal disruption events, but a correlated rise over many orders of magnitude in brightness on months to years timescales. We calculate the relative disruption rates of stars of varying evolutionary stages in...

  13. Fundamental M-dwarf parameters from high-resolution spectra using PHOENIX ACES models. I. Parameter accuracy and benchmark stars

    Science.gov (United States)

    Passegger, V. M.; Wende-von Berg, S.; Reiners, A.

    2016-03-01

    M-dwarf stars are the most numerous stars in the Universe; they span a wide range in mass and are in the focus of ongoing and planned exoplanet surveys. To investigate and understand their physical nature, detailed spectral information and accurate stellar models are needed. We use a new synthetic atmosphere model generation and compare model spectra to observations. To test the model accuracy, we compared the models to four benchmark stars with atmospheric parameters for which independent information from interferometric radius measurements is available. We used χ2-based methods to determine parameters from high-resolution spectroscopic observations. Our synthetic spectra are based on the new PHOENIX grid that uses the ACES description for the equation of state. This is a model generation expected to be especially suitable for the low-temperature atmospheres. We identified suitable spectral tracers of atmospheric parameters and determined the uncertainties in Teff, log g, and [Fe/H] resulting from degeneracies between parameters and from shortcomings of the model atmospheres. The inherent uncertainties we find are σTeff = 35 K, σlog g = 0.14, and σ[Fe/H] = 0.11. The new model spectra achieve a reliable match to our observed data; our results for Teff and log g are consistent with literature values to within 1σ. However, metallicities reported from earlier photometric and spectroscopic calibrations in some cases disagree with our results by more than 3σ. A possible explanation are systematic errors in earlier metallicity determinations that were based on insufficient descriptions of the cool atmospheres. At this point, however, we cannot definitely identify the reason for this discrepancy, but our analysis indicates that there is a large uncertainty in the accuracy of M-dwarf parameter estimates. Based on observations carried out with UVES at ESO VLT.

  14. UNLEASHING POSITIVE FEEDBACK: LINKING THE RATES OF STAR FORMATION, SUPERMASSIVE BLACK HOLE ACCRETION, AND OUTFLOWS IN DISTANT GALAXIES

    International Nuclear Information System (INIS)

    Pressure-regulated star formation is a simple variant on the usual supernova-regulated star formation efficiency that controls the global star formation rate as a function of cold gas content in star-forming galaxies, and accounts for the Schmidt-Kennicutt law in both nearby and distant galaxies. Inclusion of active galactic nucleus (AGN) induced pressure, by jets and/or winds that flow back onto a gas-rich disk, can lead, under some circumstances, to significantly enhanced star formation rates, especially at high redshift and most likely followed by the more widely accepted phase of star formation quenching. Simple expressions are derived that relate supermassive black hole growth, star formation, and outflow rates. The ratios of black hole to spheroid mass and of both black hole accretion and outflow rates to star formation rate are predicted as a function of time. I suggest various tests of the AGN-triggered star formation hypothesis

  15. Hydrodynamical simulations of the tidal stripping of binary stars by massive black holes

    CERN Document Server

    Mainetti, Deborah; Campana, Sergio; Colpi, Monica

    2016-01-01

    In a galactic nucleus, a star on a low angular momentum orbit around the central massive black hole can be fully or partially disrupted by the black hole tidal field, lighting up the compact object via gas accretion. This phenomenon can repeat if the star, not fully disrupted, is on a closed orbit. Because of the multiplicity of stars in binary systems, also binary stars may experience in pairs such a fate, immediately after being tidally separated. The consumption of both the binary components by the black hole is expected to power a double peaked flare (Mandel & Levin 2015). In this paper we perform for the first time, with GADGET2, a suite of SPH simulations of binary stars around a galactic central black hole in the Newtonian regime. We show that accretion luminosity light curves from double tidal disruptions reveal a more prominent knee, rather than a double peak, when decreasing the impact parameter of the encounter and when elevating the difference between the mass of the star which leaves the syst...

  16. The Gas Phase Mass Metallicity Relation for Dwarf Galaxies: Dependence on Star Formation Rate and H I Gas Mass

    Science.gov (United States)

    Jimmy; Tran, Kim-Vy; Saintonge, Amélie; Accurso, Gioacchino; Brough, Sarah; Oliva-Altamirano, Paola

    2015-10-01

    Using a sample of dwarf galaxies observed using the VIMOS IFU on the Very Large Telescope, we investigate the mass–metallicity relation (MZR) as a function of star formation rate (FMRSFR) as well as HI-gas mass (FMRHI). We combine our IFU data with a subsample of galaxies from the ALFALFA HI survey crossmatched to the Sloan Digital Sky Survey (SDSS) to study the FMRSFR and FMRHI across the stellar mass range 106.6–108.8 M⊙, with metallicities as low as 12 + log(O/H) = 7.67. We find the 1σ mean scatter in the MZR to be 0.05 dex. The 1σ mean scatter in the FMRSFR (0.02 dex) is significantly lower than that of the MZR. The FMRSFR is not consistent between the IFU observed galaxies and the ALFALFA/SDSS galaxies for SFRs lower than 10‑2.4 M⊙ yr‑1, however, this could be the result of limitations of our measurements in that regime. The lowest mean scatter (0.01 dex) is found in the FMRHI. We also find that the FMRHI is consistent between the IFU observed dwarf galaxies and the ALFALFA/SDSS crossmatched sample. We introduce the fundamental metallicity luminosity counterpart to the FMR, again characterized in terms of SFR (FMLSFR) and HI-gas mass (FMLHI). We find that the FMLHI relation is consistent between the IFU observed dwarf galaxy sample and the larger ALFALFA/SDSS sample. However, the 1σ scatter for the FMLHI relation is not improved over the FMRHI scenario. This leads us to conclude that the FMRHI is the best candidate for a physically motivated fundamental metallicity relation. Based on VLT service mode observations (Programs 081.B-0649 and 083.B-0662) gathered at the European Southern Observatory, Chile.

  17. Crystal Chemistry of Three-component White Dwarfs and Neutron Star Crusts: Phase Stability, Phase Stratification, and Physical Properties

    Science.gov (United States)

    Engstrom, T. A.; Yoder, N. C.; Crespi, V. H.

    2016-02-01

    A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are “bred” by a genetic algorithm and optimized at constant pressure under the assumption of linear response (Thomas-Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the T = 0 bulk phase diagrams, five of which are complicated multinary structures not previously predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low-density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase-layering diagram and mass-radius-composition dependence, both of which are reported for He-C-O and C-O-Ne white dwarfs. Finite thickness interfacial phases (“interphases”) show up at the boundaries between single-component body-centered cubic (bcc) crystalline regions, some of which have lower lattice symmetry than cubic. A second application—quasi-static settling of heavy nuclei in white dwarfs—builds on our equilibrium phase-layering method. Tests of this nonequilibrium method reveal extra phases that play the role of transient host phases for the settling species.

  18. A bag of tricks: Proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

    CERN Document Server

    Fabrizio, M; Brocato, E; Bellini, A; Libralato, M; Testa, V; Cantiello, M; Musella, I; Clementini, G; Carini, R; Marconi, M; Piotto, G; Ripepi, V; Buonanno, R; Sani, E; Speziali, R

    2014-01-01

    Hercules is the prototype of the ultra-faint dwarf (UFD) galaxies. To date, there are still no firm constraints on its total luminosity, due to the difficulty of disentangling Hercules bona-fide stars from the severe Galactic field contamination. In order to better constrain Hercules properties we aim at removing foreground and background contaminants in the galaxy field using the proper motions of the Milky Way stars and the colour-colour diagram. We have obtained images of Hercules in the rSloan, BBessel and Uspec bands with the Large Binocular Telescope (LBT) and LBC-BIN mode capabilities. The rSloan new data-set combined with data from the LBT archive span a time baseline of about 5 yr, allowing us to measure for the first time proper motions of stars in the Hercules direction. The Uspec data along with existing LBT photometry allowed us to use colour-colour diagram to further remove the field contamination. Thanks to a highly-accurate procedure to derive the rSloan-filter geometric distortion solution fo...

  19. Jet-induced star formation by accreting black holes: impact on stellar, galaxy, and cosmic evolution

    Science.gov (United States)

    Mirabel, Igor Felix

    2016-07-01

    Evidence that relativistic jets trigger star formation along their axis has been found associated to low redshift and high redshift accreting supermassive black holes. However, the physical processes by which jet-cloud interaction may trigger star formation has so far not been elucidated. To gain insight into this potentially important star formation mechanism during reionization, when microquasars were form prolifically before AGN, our international team is carrying out a muliwavelength study of a microquasar jet-induced star formation region in the Milky Way using data from space missions (Chandra, Integral, ISO, Herschel) and from the ground (at cm and mm wavelengths with the VLA and IRAM, and IR with Gemini and VLT). I will show that this relative nearby star forming region is an ideal laboratory to test models of jet-induced star formation elsewhere in the universe.

  20. Southern rice black-streaked dwarf virus: a white-backed planthopper transmitted fijivirus threadening rice production in Asia

    Directory of Open Access Journals (Sweden)

    GuohuiZhou

    2013-09-01

    Full Text Available Southern rice black-streaked dwarf virus (SRBSDV, a nonenveloped icosahedral virus with a genome of 10 double-stranded RNA segments, is a novel species in the genus Fijivirus (family Reoviridae first recognized in 2008. Rice plants infected with this virus exhibit symptoms similar to those caused by Rice black-streaked dwarf virus. Since 2009, the virus has rapidly spread and caused serious rice losses in East and Southeast Asia. Significant progress has been made in recent years in understanding this disease, especially about the functions of the viral genes, rice–virus–insect interactions, and epidemiology and control measures. The virus can be efficiently transmitted by the white-backed planthopper (WBPH, Sogatella furcifera in a persistent circulative propagative manner but cannot be transmitted by the brown planthopper (Nilaparvata lugens and small brown planthopper (Laodelphax striatellus. Rice, maize, Chinese sorghum (Coix lacryma-jobi and other grass weeds can be infected via WBPH. However, only rice plays a major role in the virus infection cycle because of the vector's preference. In Southeast Asia, WBPH is a long-distance migratory rice pest. The disease cycle can be described as follows: SRBSDV and its WBPH vector overwinter in warm tropical or sub-tropical areas; viruliferous WBPH adults carry the virus from south to north via long-distance migration in early spring, transmit the virus to rice seedlings in the newly colonized areas, and lay eggs on the infected seedlings; the next generation of WBPHs propagate on infected seedlings, become viruliferous, disperse, and cause new disease outbreaks. Several molecular and serological methods have been developed to detect SRBSDV in plant tissues and individual insects. Control measures based on protection from WBPH, including seedbed coverage, chemical seed treatments, and chemical spraying of seedlings, have proven effective in China.

  1. DARK MATTER CORES IN THE FORNAX AND SCULPTOR DWARF GALAXIES: JOINING HALO ASSEMBLY AND DETAILED STAR FORMATION HISTORIES

    International Nuclear Information System (INIS)

    We combine the detailed star formation histories of the Fornax and Sculptor dwarf spheroidals with the mass assembly history of their dark matter (DM) halo progenitors to estimate if the energy deposited by Type II supernovae (SNe II) is sufficient to create a substantial DM core. Assuming the efficiency of energy injection of the SNe II into DM particles is εgc = 0.05, we find that a single early episode, z ≳ z infall, that combines the energy of all SNe II due to explode over 0.5 Gyr is sufficient to create a core of several hundred parsecs in both Sculptor and Fornax. Therefore, our results suggest that it is energetically plausible to form cores in cold dark matter (CDM) halos via early episodic gas outflows triggered by SNe II. Furthermore, based on CDM merger rates and phase-space density considerations, we argue that the probability of a subsequent complete regeneration of the cusp is small for a substantial fraction of dwarf-size halos

  2. The Star Formation Histories of Local Group Dwarf Galaxies III. Characterizing Quenching in Low-Mass Galaxies

    CERN Document Server

    Weisz, Daniel R; Skillman, Evan D; Holtzman, Jon; Gilbert, Karoline M; Dalcanton, Julianne J; Williams, Benjamin F

    2015-01-01

    We explore the quenching of low-mass galaxies (10^4 < Mstar < 10^8 Msun) as a function of lookback time using the star formation histories (SFHs) of 38 Local Group dwarf galaxies. The SFHs were derived from analyzing color-magnitude diagrams of resolved stellar populations in archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) Lower mass galaxies quench earlier than higher mass galaxies; (2) Inside of virial radius there is no correlation between a satellite's current proximity to a massive host and its quenching epoch; (3) There are hints of systematic differences in quenching times of M31 and Milky Way (MW) satellites, although the sample sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive conclusions. Combined with literature results, we qualitatively consider the redshift evolution (z=0-1) of the quenched galaxy fraction over ~7 dex in stellar mass (10^4 < Mstar < 10^11.5 Msun). The quenched fraction of all galaxies generally increases to...

  3. Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

    CERN Document Server

    Grossi, M; Bizzocchi, L; Giovanardi, C; Bomans, D; Coelho, B; De Looze, I; Gonçalves, T S; Hunt, L K; Leonardo, E; Madden, S; Menéndez-Delmestre, K; Pappalardo, C; Riguccini, L

    2016-01-01

    We present $^{12}$CO(1-0) and $^{12}$CO(2-1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 $\\mu$m emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H2 molecular masses, namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses <~ 10$^9$ M$_{\\odot}$, contrary to the atomic hydrogen fraction, M$_{HI}$/M$_*$, which increases inversely with M$_*$. The flattening of the M$_{H_2}$/M$_*$ ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both HI-deficient and HI-normal dwa...

  4. MagAO Imaging of Long-period Objects (MILO). II. A Puzzling White Dwarf around the Sun-like Star HD 11112

    CERN Document Server

    Rodigas, Timothy J; Simon, Amelie; Arriagada, Pamela; Faherty, Jackie; Anglada-Escude, Guillem; Mamajek, Eric E; Weinberger, Alycia; Butler, R Paul; Males, Jared R; Morzinski, Katie; Close, Laird M; Hinz, Philip M; Bailey, Jeremy; Carter, Brad; Jenkins, James S; Jones, Hugh; O'Toole, Simon; Tinney, C G; Wittenmyer, Rob; Debes, John

    2016-01-01

    HD 11112 is an old, Sun-like star that has a long-term radial velocity (RV) trend indicative of a massive companion on a wide orbit. Here we present direct images of the source responsible for the trend using the Magellan Adaptive Optics system. We detect the object (HD 11112B) at a separation of 2\\fasec 2 (100 AU) at multiple wavelengths spanning 0.6-4 \\microns ~and show that it is most likely a gravitationally-bound cool white dwarf. Modeling its spectral energy distribution (SED) suggests that its mass is 0.9-1.1 \\msun, which corresponds to very high-eccentricity, near edge-on orbits from Markov chain Monte Carlo analysis of the RV and imaging data together. The total age of the white dwarf is $>2\\sigma$ discrepant with that of the primary star under most assumptions. The problem can be resolved if the white dwarf progenitor was initially a double white dwarf binary that then merged into the observed high-mass white dwarf. HD 11112B is a unique and intriguing benchmark object that can be used to calibrate ...

  5. THE RELATIONSHIP BETWEEN BLACK HOLE GROWTH AND STAR FORMATION IN SEYFERT GALAXIES

    International Nuclear Information System (INIS)

    We present estimates of black hole accretion rates (BHARs) and nuclear, extended, and total star formation rates for a complete sample of Seyfert galaxies. Using data from the Spitzer Space Telescope, we measure the active galactic nucleus (AGN) luminosity using the [O IV] λ25.89 μm emission line and the star-forming luminosity using the 11.3 μm aromatic feature and extended 24 μm continuum emission. We find that black hole growth is strongly correlated with nuclear (r 1 kpc) star formation in the host galaxy. In particular, the nuclear star formation rate (SFR) traced by the 11.3 μm aromatic feature follows a relationship with the BHAR of the form SFR∝ M-dotBH0.8, with an observed scatter of 0.5 dex. This SFR-BHAR relationship persists when additional star formation in physically matched r = 1 kpc apertures is included, taking the form SFR∝ M-dotBH0.6. However, the relationship becomes almost indiscernible when total SFRs are considered. This suggests a physical connection between the gas on sub-kiloparsec and sub-parsec scales in local Seyfert galaxies that is not related to external processes in the host galaxy. It also suggests that the observed scaling between star formation and black hole growth for samples of AGNs will depend on whether the star formation is dominated by a nuclear or an extended component. We estimate the integrated black hole and bulge growth that occurs in these galaxies and find that an AGN duty cycle of 5%-10% would maintain the ratio between black hole and bulge masses seen in the local universe.

  6. Tidal evolution of CoRoT massive planets and brown dwarfs and of their host stars

    CERN Document Server

    Ferraz-Mello, Sylvio

    2016-01-01

    Aims: Revisit and improvement of the main results obtained in the study of the tidal evolution of several massive CoRoT planets and brown dwarfs and of the rotation of their host stars. Methods: Simulations of the past and future evolution of the orbital and rotational elements of the systems under the joint action of the tidal torques and the braking due to the stellar wind. Results: Presentation of several paradigms and significant examples of tidal evolution in extrasolar planetary systems. It is shown that the high quality of the photometric and spectrographic observations of the CoRoT objects allow for a precise study of their past and future evolution and to estimate the tidal parameters ruling the dissipation in the systems.

  7. Crystal chemistry of three-component white dwarfs and neutron star crusts: phase stability, phase stratification, and physical properties

    CERN Document Server

    Engstrom, T A; Crespi, V H

    2015-01-01

    A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are "bred" by a genetic algorithm, and optimized at constant pressure under the assumption of linear response (Thomas-Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the $T=0$ bulk phase diagrams, five of which are complicated multinary structures not before predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravit...

  8. Discovery of a companion at the brown dwarf limit to the solar-type star Gliese 29

    Science.gov (United States)

    Chini, R.; Fuhrmann, K.; Pozo Nuñez, F.; Ramolla, M.; Kaderhandt, L.; Niedworok, N.; Hodapp, K.-W.

    2016-07-01

    Gliese 29 is a 7 to 8 Gyr old, southern Population I turnoff star with a large proper motion of 1 arcsec/yr. Using recent direct imaging observations with the 0.8 m Infrared Imaging System (IRIS) of the Universitätssternwarte Bochum near Cerro Armazones in Chile, we demonstrate that the faint source 2MASS J00402651-5927168 at a projected angular separation ρ=6.35 arcsec is a common-proper-motion companion to Gl 29. Provided this source is not part of a further subsystem, the IRIS J- and K_s-band photometry either implies a spectral type of about L2, based on its absolute magnitude, or an approximate mass M_B ≃ 0.077 M⊙, suggesting that it may even be a brown dwarf. Assuming a face-on circular orbit this faint companion orbits Gl 29 in 1880 years.

  9. Detailed abundances in extremely metal poor dwarf stars extracted from SDSS

    CERN Document Server

    Sbordone, Luca; Caffau, Elisabetta; Ludwig, Hans-Gunther

    2012-01-01

    We report on the result of an ongoing campaign to determine chemical abundances in extremely metal poor (EMP) turn-off (TO) stars selected from the Sloan Digital Sky Survey (SDSS) low resolution spectra. This contribution focuses principally on the largest part of the sample (18 stars out of 29), observed with UVES@VLT and analyzed by means of the automatic abundance analysis code MyGIsFOS to derive atmosphere parameters and detailed compositions. The most significant findings include i) the detection of a C-rich, strongly Mg-enhanced star ([Mg/Fe]=1.45); ii) a group of Mn-rich stars ([Mn/Fe]>-0.4); iii) a group of Ni-rich stars ([Ni/Fe]>0.2). Li is measured in twelve stars, while for three upper limits are derived.

  10. The SW Sex-type star 2MASS J01074282+4845188: an unusual bright accretion disk with non-steady emission and a hot white dwarf

    CERN Document Server

    Khruzina, T; Kjurkchieva, D; 10.1051/0004-6361/201220385

    2013-01-01

    We present new photometric and spectral observations of the newly discovered nova-like eclipsing star 2MASS J01074282+4845188. To obtain a light curve solution we used model of a nova-like star whose emission sources are a white dwarf surrounded by an accretion disk, a secondary star filling its Roche lobe, a hot spot and a hot line. 2MASS J01074282+4845188 shows the deepest permanent eclipse among the known nova-like stars. It is reproduced by covering the very bright accretion disk by the secondary component. The luminosity of the disk is much bigger than that of the rest light sources. The determined high temperature of the disk is typical for that observed during the outbursts of CVs. The primary of 2MASS J01074282+4845188 is one of the hottest white dwarfs in CVs. The temperature of 5090 K of its secondary is also quite high and more appropriate for a long-period SW Sex star. It might be explained by the intense heating from the hot white dwarf and the hot accretion disk of the target. The high mass accr...

  11. Aligned spin neutron star-black hole mergers: A gravitational waveform amplitude model

    Science.gov (United States)

    Pannarale, Francesco; Berti, Emanuele; Kyutoku, Koutarou; Lackey, Benjamin D.; Shibata, Masaru

    2015-10-01

    The gravitational radiation emitted during the merger of a black hole with a neutron star is rather similar to the radiation from the merger of two black holes when the neutron star is not tidally disrupted. When tidal disruption occurs, gravitational waveforms can be broadly classified in two groups, depending on the spatial extent of the disrupted material. Extending previous work by some of us, here we present a phenomenological model for the gravitational waveform amplitude in the frequency domain encompassing the three possible outcomes of the merger: no tidal disruption, and "mild" and "strong" tidal disruption. The model is calibrated to 134 general-relativistic numerical simulations of binaries where the black hole spin is either aligned or antialigned with the orbital angular momentum. All simulations were produced using the SACRA code and piecewise polytropic neutron star equations of state. The present model can be used to determine when black-hole binary waveforms are sufficient for gravitational-wave detection, to extract information on the equation of state from future gravitational-wave observations, to obtain more accurate estimates of black hole-neutron star merger event rates, and to determine the conditions under which these systems are plausible candidates as central engines of gamma-ray bursts and macronovae/kilonovae.

  12. Soft X-ray Spectroscopy of the Hot DA White Dwarf LB1919 and the PG1159 Star PG1520+525

    CERN Document Server

    Werner, K; Rauch, T; Schuh, S; Gautschy, A

    2006-01-01

    We have performed soft X-ray spectroscopy of two hot white dwarfs with the Chandra observatory using the Low Energy Transmission Grating. The first target is the hot DA white dwarf LB1919 (Teff=69000 K). This star is representative of a small group of hot DAs whose metallicities lie well below predictions from radiative levitation theory. The Chandra spectrum shows a rich absorption line spectrum which may allow to find the origin of the low-metallicity nature of these DAs. The second target is PG1520+525, a very hot non-pulsating PG1159 star. We find that it is hotter (Teff=150 000 K) than the pulsating prototype PG1159-035 (Teff=140 000) and conclude that both stars confine the blue edge of the GW Vir instability strip.

  13. Formation of massive black holes through runaway collisions in dense young star clusters.

    Science.gov (United States)

    Zwart, Simon F Portegies; Baumgardt, Holger; Hut, Piet; Makino, Junichiro; McMillan, Stephen L W

    2004-04-15

    A luminous X-ray source is associated with MGG 11--a cluster of young stars approximately 200 pc from the centre of the starburst galaxy M 82 (refs 1, 2). The properties of this source are best explained by invoking a black hole with a mass of at least 350 solar masses (350 M(o)), which is intermediate between stellar-mass and supermassive black holes. A nearby but somewhat more massive cluster (MGG 9) shows no evidence of such an intermediate-mass black hole, raising the issue of just what physical characteristics of the clusters can account for this difference. Here we report numerical simulations of the evolution and motion of stars within the clusters, where stars are allowed to merge with each other. We find that for MGG 11 dynamical friction leads to the massive stars sinking rapidly to the centre of the cluster, where they participate in a runaway collision. This produces a star of 800-3,000 M(o) which ultimately collapses to a black hole of intermediate mass. No such runaway occurs in the cluster MGG 9, because the larger cluster radius leads to a mass segregation timescale a factor of five longer than for MGG 11. PMID:15085124

  14. Gravitational waves from nonspinning black hole-neutron star binaries: dependence on equations of state

    CERN Document Server

    Kyutoku, Koutarou; Taniguchi, Keisuke

    2010-01-01

    We report results of a numerical-relativity simulation for the merger of a black hole-neutron star binary with a variety of equations of state (EOSs) modeled by piecewise polytropes. We focus in particular on the dependence of the gravitational waveform at the merger stage on the EOSs. The initial conditions are computed in the moving-puncture framework, assuming that the black hole is nonspinning and the neutron star has an irrotational velocity field. For a small mass ratio of the binaries (e.g., MBH/MNS = 2 where MBH and MNS are the masses of the black hole and neutron star, respectively), the neutron star is tidally disrupted before it is swallowed by the black hole irrespective of the EOS. Especially for less-compact neutron stars, the tidal disruption occurs at a more distant orbit. The tidal disruption is reflected in a cutoff frequency of the gravitational-wave spectrum, above which the spectrum amplitude exponentially decreases. A clear relation is found between the cutoff frequency of the gravitatio...

  15. The masses and spins of neutron stars and stellar-mass black holes

    International Nuclear Information System (INIS)

    Stellar-mass black holes and neutron stars represent extremes in gravity, density, and magnetic fields. They therefore serve as key objects in the study of multiple frontiers of physics. In addition, their origin (mainly in core-collapse supernovae) and evolution (via accretion or, for neutron stars, magnetic spindown and reconfiguration) touch upon multiple open issues in astrophysics. In this review, we discuss current mass and spin measurements and their reliability for neutron stars and stellar-mass black holes, as well as the overall importance of spins and masses for compact object astrophysics. Current masses are obtained primarily through electromagnetic observations of binaries, although future microlensing observations promise to enhance our understanding substantially. The spins of neutron stars are straightforward to measure for pulsars, but the birth spins of neutron stars are more difficult to determine. In contrast, even the current spins of stellar-mass black holes are challenging to measure. As we discuss, major inroads have been made in black hole spin estimates via analysis of iron lines and continuum emission, with reasonable agreement when both types of estimate are possible for individual objects, and future X-ray polarization measurements may provide additional independent information. We conclude by exploring the exciting prospects for mass and spin measurements from future gravitational wave detections, which are expected to revolutionize our understanding of strong gravity and compact objects

  16. High velocity stars from close interaction of a globular cluster and a super massive black hole

    CERN Document Server

    Capuzzo-Dolcetta, R

    2015-01-01

    Observations show the presence, in the halo of our Galaxy, of stars moving at velocities so high to require an acceleration mechanism involving the presence of a massive central black hole. Thus, in the frame of a galaxy hosting a supermassive black hole ($10^8$ $M_{\\odot}$) we investigated a mechanism for the production of high velocity stars, which was suggested by the results of N-body simulations of the close interaction between a massive, orbitally decayed, globular cluster and the super massive black hole. The high velocity acquired by some stars of the cluster comes from the transfer of gravitational binding energy into kinetic energy of the escaping star originally orbiting around the cluster. After the close interaction with the massive black hole, stars could reach a velocity sufficient to travel in the halo and even overcome the galactic gravitational well, while some of them are just stripped from the globular cluster and start orbiting on precessing loops around the galactic centre.

  17. Local behaviour of evaporating stars and black holes around the total evaporation event

    OpenAIRE

    Fayos Vallés, Francisco; Torres Herrera, Ramon

    2010-01-01

    Abstract Many models in which the object under study loses all its mass have appeared in the literature. One can find from evaporating stars to incipient black holes, all the way to evaporating black holes. In this article we try a semiclassical study of these evaporating models centered on the evaporating event itself. We analyze their common properties, behaviours and possibilities. Specifically, we pay special attention to the evaporating models as a means of avoiding singularities duri...

  18. Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30

    CERN Document Server

    Mondal, Aditya S; Pahari, Mayukh; Misra, Ranjeev; Kembhavi, Ajit K; Raychaudhuri, Biplab

    2016-01-01

    Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner accretion disk. The X-ray reflection features can be weakened due to strong magnetic fields or very low iron abundances such as is possible in X-ray binaries with low mass, first generation stars as companions. Here we investigate the reality of the broad iron line detected earlier from the neutron star low mass X-ray binary 4U~1820--30 with a degenerate helium dwarf companion. We perform a comprehensive, systematic broadband spectral study of the atoll source using \\suzaku{} and simultaneous \

  19. Insight into the structure and physics of M dwarf stars through determination of the rotation, metallicities, and radii of the nearby population

    Science.gov (United States)

    Newton, Elisabeth R.

    2016-01-01

    Despite the prevalence of M dwarfs, their fundamental properties--their sizes, compositions, and ages--are not well-constrained. Empirical determination of these properties is important for gaining insight into their stellar structure, magnetic field generation, and angular momentum evolution. Knowledge of the stellar parameters is also key to characterizing planetary systems. I used observations to empirically constrain the properties of nearby, mid-to-late M dwarfs targeted by the MEarth transiting planet survey. I obtained low-resolution (R=2000) NIR spectra of 450 M dwarfs using SpeX on IRTF. I measured their absolute radial velocities with an accuracy of 4 km/s by exploiting telluric lines to establish an absolute wavelength calibration, and developed techniques to estimate M dwarf metallicities from K-band spectral line equivalent widths (EWs) or 2MASS colors to 0.15 dex. Using stars with interferometric radii, I showed that H-band EWs can be used to infer K and M dwarf temperatures to 69K, and radii to 0.027Rsun. I applied these relations to planet-hosting stars from Kepler, showing that the typical planet is 15% larger than is inferred if adopting other stellar parameters. Using photometry from the MEarth-North Observatory, I measured rotation periods from 0.1 to 150 days for 350 M dwarfs. There is a prevalence of stable spot patterns, and no correlation between period and amplitude for fully-convective stars. Using galactic kinematics as a proxy for age, I demonstrated a smooth age-rotation relation. I found that rapid rotators (Pcommunity, and acknowledge the opportunity to conduct these observations.

  20. Local luminous infrared galaxies. III. Co-evolution of black hole growth and star formation activity?

    OpenAIRE

    Alonso-Herrero, A.; Pereira-Santaella, M.; Rieke, George H.; Diamond-Stanic, Aleksandar M.; Wang, Yiping; Hernán-Caballero, Antonio; Rigopoulou, Dimitra

    2013-01-01

    Local luminous infrared (IR) galaxies (LIRGs) have both high star formation rates (SFR) and a high AGN (Seyfert and AGN/starburst composite) incidence. Therefore, they are ideal candidates to explore the co-evolution of black hole (BH) growth and star formation (SF) activity, not necessarily associated with major mergers. Here, we use Spitzer/IRS spectroscopy of a complete volume-limited sample of local LIRGs (distances of

  1. Chandra and MMT observations of low-mass black hole active galactic nuclei accreting at low rates in dwarf galaxies

    International Nuclear Information System (INIS)

    We report on Chandra X-ray observations of four candidate low-mass black hole (M bh ≲ 106 M ☉) active galactic nuclei (AGNs) that have the estimated Eddington ratios among the lowest (∼10–2) found for this class. The aims are to validate the nature of their AGNs and to confirm the low Eddington ratios that are derived from the broad Hα line, and to explore this poorly studied regime in the AGN parameter space. Among them, two objects with the lowest significance of the broad lines are also observed with the Multi-Mirror Telescope, and the high-quality optical spectra taken confirm them as Seyfert 1 AGNs and as having small black hole masses. X-ray emission is detected from the nuclei of two of the galaxies, which is variable on timescales of ∼103 s, whereas no significant (or only marginal at best) detection is found for the remaining two. The X-ray luminosities are on the order of 1041 erg s–1 or even lower, on the order of 1040 erg s–1 for non-detections, which are among the lowest regimes ever probed for Seyfert galaxies. The low X-ray luminosities, compared to their black hole masses derived from Hα, confirm their low accretion rates assuming typical bolometric corrections. Our results hint at the existence of a possibly large population of under-luminous low-mass black holes in the local universe. An off-nucleus ultra-luminous X-ray source in one of the dwarf galaxies is detected serendipitously, with a luminosity (6-9)× 1039 erg s–1 in 2-10 keV.

  2. Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars.

    Science.gov (United States)

    Hu, Yongyun; Yang, Jun

    2014-01-14

    The distinctive feature of tidally locked exoplanets is the very uneven heating by stellar radiation between the dayside and nightside. Previous work has focused on the role of atmospheric heat transport in preventing atmospheric collapse on the nightside for terrestrial exoplanets in the habitable zone around M dwarfs. In the present paper, we carry out simulations with a fully coupled atmosphere-ocean general circulation model to investigate the role of ocean heat transport in climate states of tidally locked habitable exoplanets around M dwarfs. Our simulation results demonstrate that ocean heat transport substantially extends the area of open water along the equator, showing a lobster-like spatial pattern of open water, instead of an "eyeball." For sufficiently high-level greenhouse gases or strong stellar radiation, ocean heat transport can even lead to complete deglaciation of the nightside. Our simulations also suggest that ocean heat transport likely narrows the width of M dwarfs' habitable zone. This study provides a demonstration of the importance of exooceanography in determining climate states and habitability of exoplanets. PMID:24379386

  3. Upper limits on the rates of binary neutron star and neutron-star--black-hole mergers from Advanced LIGO's first observing run

    CERN Document Server

    Abbott, B P; Abbott, T D; Abernathy, M R; Acernese, F; Ackley, K; Adams, C; Adams, T; Addesso, P; Adhikari, R X; Adya, V B; Affeldt, C; Agathos, M; Agatsuma, K; Aggarwal, N; Aguiar, O D; Aiello, L; Ain, A; Ajith, P; Allen, B; Allocca, A; Altin, P A; Anderson, S B; Anderson, W G; Arai, K; Araya, M C; Arceneaux, C C; Areeda, J S; Arnaud, N; Arun, K G; Ascenzi, S; Ashton, G; Ast, M; Aston, S M; Astone, P; Aufmuth, P; Aulbert, C; Babak, S; Bacon, P; Bader, M K M; Baker, P T; Baldaccini, F; Ballardin, G; Ballmer, S W; Barayoga, J C; Barclay, S E; Barish, B C; Barker, D; Barone, F; Barr, B; Barsotti, L; Barsuglia, M; Barta, D; Bartlett, J; Bartos, I; Bassiri, R; Basti, A; Batch, J C; Baune, C; Bavigadda, V; Bazzan, M; Bejger, M; Bell, A S; Berger, B K; Bergmann, G; Berry, C P L; Bersanetti, D; Bertolini, A; Betzwieser, J; Bhagwat, S; Bhandare, R; Bilenko, I A; Billingsley, G; Birch, J; Birney, R; Biscans, S; Bisht, A; Bitossi, M; Biwer, C; Bizouard, M A; Blackburn, J K; Blair, C D; Blair, D G; Blair, R M; Bloemen, S; Bock, O; Boer, M; Bogaert, G; Bogan, C; Bohe, A; Bond, C; Bondu, F; Bonnand, R; Boom, B A; Bork, R; Boschi, V; Bose, S; Bouffanais, Y; Bozzi, A; Bradaschia, C; Brady, P R; Braginsky, V B; Branchesi, M; Brau, J E; Briant, T; Brillet, A; Brinkmann, M; Brisson, V; Brockill, P; Broida, J E; Brooks, A F; Brown, D A; Brown, D D; Brown, N M; Brunett, S; Buchanan, C C; Buikema, A; Bulik, T; Bulten, H J; Buonanno, A; Buskulic, D; Buy, C; Byer, R L; Cabero, M; Cadonati, L; Cagnoli, G; Cahillane, C; Bustillo, J Calder'on; Callister, T; Calloni, E; Camp, J B; Cannon, K C; Cao, J; Capano, C D; Capocasa, E; Carbognani, F; Caride, S; Diaz, J Casanueva; Casentini, C; Caudill, S; Cavagli`a, M; Cavalier, F; Cavalieri, R; Cella, G; Cepeda, C B; Baiardi, L Cerboni; Cerretani, G; Cesarini, E; Chamberlin, S J; Chan, M; Chao, S; Charlton, P; Chassande-Mottin, E; Cheeseboro, B D; Chen, H Y; Chen, Y; Cheng, C; Chincarini, A; Chiummo, A; Cho, H S; Cho, M; Chow, J H; Christensen, N; Chu, Q; Chua, S; Chung, S; Ciani, G; Clara, F; Clark, J A; Cleva, F; Coccia, E; Cohadon, P -F; Colla, A; Collette, C G; Cominsky, L; Constancio, M; Conte, A; Conti, L; Cook, D; Corbitt, T R; Cornish, N; Corsi, A; Cortese, S; Costa, C A; Coughlin, M W; Coughlin, S B; Coulon, J -P; Countryman, S T; Couvares, P; Cowan, E E; Coward, D M; Cowart, M J; Coyne, D C; Coyne, R; Craig, K; Creighton, J D E; Cripe, J; Crowder, S G; Cumming, A; Cunningham, L; Cuoco, E; Canton, T Dal; Danilishin, S L; D'Antonio, S; Danzmann, K; Darman, N S; Dasgupta, A; Costa, C F Da Silva; Dattilo, V; Dave, I; Davier, M; Davies, G S; Daw, E J; Day, R; De, S; DeBra, D; Debreczeni, G; Degallaix, J; De Laurentis, M; Del'eglise, S; Del Pozzo, W; Denker, T; Dent, T; Dergachev, V; De Rosa, R; DeRosa, R T; DeSalvo, R; Devine, R C; Dhurandhar, S; D'iaz, M C; Di Fiore, L; Di Giovanni, M; Di Girolamo, T; Di Lieto, A; Di Pace, S; Di Palma, I; Di Virgilio, A; Dolique, V; Donovan, F; Dooley, K L; Doravari, S; Douglas, R; Downes, T P; Drago, M; Drever, R W P; Driggers, J C; Ducrot, M; Dwyer, S E; Edo, T B; Edwards, M C; Effler, A; Eggenstein, H -B; Ehrens, P; Eichholz, J; Eikenberry, S S; Engels, W; Essick, R C; Etzel, T; Evans, M; Evans, T M; Everett, R; Factourovich, M; Fafone, V; Fair, H; Fairhurst, S; Fan, X; Fang, Q; Farinon, S; Farr, B; Farr, W M; Favata, M; Fays, M; Fehrmann, H; Fejer, M M; Fenyvesi, E; Ferrante, I; Ferreira, E C; Ferrini, F; Fidecaro, F; Fiori, I; Fiorucci, D; Fisher, R P; Flaminio, R; Fletcher, M; Fournier, J -D; Frasca, S; Frasconi, F; Frei, Z; Freise, A; Frey, R; Frey, V; Fritschel, P; Frolov, V V; Fulda, P; Fyffe, M; Gabbard, H A G; Gair, J R; Gammaitoni, L; Gaonkar, S G; Garufi, F; Gaur, G; Gehrels, N; Gemme, G; Geng, P; Genin, E; Gennai, A; George, J; Gergely, L; Germain, V; Ghosh, Abhirup; Ghosh, Archisman; Ghosh, S; Giaime, J A; Giardina, K D; Giazotto, A; Gill, K; Glaefke, A; Goetz, E; Goetz, R; Gondan, L; Gonz'alez, G; Castro, J M Gonzalez; Gopakumar, A; Gordon, N A; Gorodetsky, M L; Gossan, S E; Gosselin, M; Gouaty, R; Grado, A; Graef, C; Graff, P B; Granata, M; Grant, A; Gras, S; Gray, C; Greco, G; Green, A C; Groot, P; Grote, H; Grunewald, S; Guidi, G M; Guo, X; Gupta, A; Gupta, M K; Gushwa, K E; Gustafson, E K; Gustafson, R; Hacker, J J; Hall, B R; Hall, E D; Hammond, G; Haney, M; Hanke, M M; Hanks, J; Hanna, C; Hannam, M D; Hanson, J; Hardwick, T; Harms, J; Harry, G M; Harry, I W; Hart, M J; Hartman, M T; Haster, C -J; Haughian, K; Heidmann, A; Heintze, M C; Heitmann, H; Hello, P; Hemming, G; Hendry, M; Heng, I S; Hennig, J; Henry, J; Heptonstall, A W; Heurs, M; Hild, S; Hoak, D; Hofman, D; Holt, K; Holz, D E; Hopkins, P; Hough, J; Houston, E A; Howell, E J; Hu, Y M; Huang, S; Huerta, E A; Huet, D; Hughey, B; Husa, S; Huttner, S H; Huynh-Dinh, T; Indik, N; Ingram, D R; Inta, R; Isa, H N; Isac, J -M; Isi, M; Isogai, T; Iyer, B R; Izumi, K; Jacqmin, T; Jang, H; Jani, K; Jaranowski, P; Jawahar, S; Jian, L; Jim'enez-Forteza, F; Johnson, W W; Jones, D I; Jones, R; Jonker, R J G; Ju, L; K, Haris; Kalaghatgi, C V; Kalogera, V; Kandhasamy, S; Kang, G; Kanner, J B; Kapadia, S J; Karki, S; Karvinen, K S; Kasprzack, M; Katsavounidis, E; Katzman, W; Kaufer, S; Kaur, T; Kawabe, K; K'ef'elian, F; Kehl, M S; Keitel, D; Kelley, D B; Kells, W; Kennedy, R; Key, J S; Khalili, F Y; Khan, I; Khan, S; Khan, Z; Khazanov, E A; Kijbunchoo, N; Kim, Chi-Woong; Kim, Chunglee; Kim, J; Kim, K; Kim, N; Kim, W; Kim, Y -M; Kimbrell, S J; King, E J; King, P J; Kissel, J S; Klein, B; Kleybolte, L; Klimenko, S; Koehlenbeck, S M; Koley, S; Kondrashov, V; Kontos, A; Korobko, M; Korth, W Z; Kowalska, I; Kozak, D B; Kringel, V; Krishnan, B; Kr'olak, A; Krueger, C; Kuehn, G; Kumar, P; Kumar, R; Kuo, L; Kutynia, A; Lackey, B D; Landry, M; Lange, J; Lantz, B; Lasky, P D; Laxen, M; Lazzarini, A; Lazzaro, C; Leaci, P; Leavey, S; Lebigot, E O; Lee, C H; Lee, H K; Lee, H M; Lee, K; Lenon, A; Leonardi, M; Leong, J R; Leroy, N; Letendre, N; Levin, Y; Lewis, J B; Li, T G F; Libson, A; Littenberg, T B; Lockerbie, N A; Lombardi, A L; London, L T; Lord, J E; Lorenzini, M; Loriette, V; Lormand, M; Losurdo, G; Lough, J D; L"uck, H; Lundgren, A P; Lynch, R; Ma, Y; Machenschalk, B; MacInnis, M; Macleod, D M; Magana-Sandoval, F; Zertuche, L Magana; Magee, R M; Majorana, E; Maksimovic, I; Malvezzi, V; Man, N; Mandic, V; Mangano, V; Mansell, G L; Manske, M; Mantovani, M; Marchesoni, F; Marion, F; M'arka, S; M'arka, Z; Markosyan, A S; Maros, E; Martelli, F; Martellini, L; Martin, I W; Martynov, D V; Marx, J N; Mason, K; Masserot, A; Massinger, T J; Masso-Reid, M; Mastrogiovanni, S; Matichard, F; Matone, L; Mavalvala, N; Mazumder, N; McCarthy, R; McClelland, D E; McCormick, S; McGuire, S C; McIntyre, G; McIver, J; McManus, D J; McRae, T; McWilliams, S T; Meacher, D; Meadors, G D; Meidam, J; Melatos, A; Mendell, G; Mercer, R A; Merilh, E L; Merzougui, M; Meshkov, S; Messenger, C; Messick, C; Metzdorff, R; Meyers, P M; Mezzani, F; Miao, H; Michel, C; Middleton, H; Mikhailov, E E; Milano, L; Miller, A L; Miller, A; Miller, B B; Miller, J; Millhouse, M; Minenkov, Y; Ming, J; Mirshekari, S; Mishra, C; Mitra, S; Mitrofanov, V P; Mitselmakher, G; Mittleman, R; Moggi, A; Mohan, M; Mohapatra, S R P; Montani, M; Moore, B C; Moore, C J; Moraru, D; Moreno, G; Morriss, S R; Mossavi, K; Mours, B; Mow-Lowry, C M; Mueller, G; Muir, A W; Mukherjee, Arunava; Mukherjee, D; Mukherjee, S; Mukund, N; Mullavey, A; Munch, J; Murphy, D J; Murray, P G; Mytidis, A; Nardecchia, I; Naticchioni, L; Nayak, R K; Nedkova, K; Nelemans, G; Nelson, T J N; Neri, M; Neunzert, A; Newton, G; Nguyen, T T; Nielsen, A B; Nissanke, S; Nitz, A; Nocera, F; Nolting, D; Normandin, M E N; Nuttall, L K; Oberling, J; Ochsner, E; O'Dell, J; Oelker, E; Ogin, G H; Oh, J J; Oh, S H; Ohme, F; Oliver, M; Oppermann, P; Oram, Richard J; O'Reilly, B; O'Shaughnessy, R; Ottaway, D J; Overmier, H; Owen, B J; Pai, A; Pai, S A; Palamos, J R; Palashov, O; Palomba, C; Pal-Singh, A; Pan, H; Pankow, C; Pannarale, F; Pant, B C; Paoletti, F; Paoli, A; Papa, M A; Paris, H R; Parker, W; Pascucci, D; Pasqualetti, A; Passaquieti, R; Passuello, D; Patricelli, B; Patrick, Z; Pearlstone, B L; Pedraza, M; Pedurand, R; Pekowsky, L; Pele, A; Penn, S; Perreca, A; Perri, L M; Phelps, M; Piccinni, O J; Pichot, M; Piergiovanni, F; Pierro, V; Pillant, G; Pinard, L; Pinto, I M; Pitkin, M; Poe, M; Poggiani, R; Popolizio, P; Post, A; Powell, J; Prasad, J; Pratt, J; Predoi, V; Prestegard, T; Price, L R; Prijatelj, M; Principe, M; Privitera, S; Prix, R; Prodi, G A; Prokhorov, L; Puncken, O; Punturo, M; Puppo, P; P"urrer, M; Qi, H; Qin, J; Qiu, S; Quetschke, V; Quintero, E A; Quitzow-James, R; Raab, F J; Rabeling, D S; Radkins, H; Raffai, P; Raja, S; Rajan, C; Rakhmanov, M; Rapagnani, P; Raymond, V; Razzano, M; Re, V; Read, J; Reed, C M; Regimbau, T; Rei, L; Reid, S; Reitze, D H; Rew, H; Reyes, S D; Ricci, F; Riles, K; Rizzo, M; Robertson, N A; Robie, R; Robinet, F; Rocchi, A; Rolland, L; Rollins, J G; Roma, V J; Romano, R; Romanov, G; Romie, J H; Rosi'nska, D; Rowan, S; R"udiger, A; Ruggi, P; Ryan, K; Sachdev, S; Sadecki, T; Sadeghian, L; Sakellariadou, M; Salconi, L; Saleem, M; Salemi, F; Samajdar, A; Sammut, L; Sanchez, E J; Sandberg, V; Sandeen, B; Sanders, J R; Sassolas, B; Sathyaprakash, B S; Saulson, P R; Sauter, O E S; Savage, R L; Sawadsky, A; Schale, P; Schilling, R; Schmidt, J; Schmidt, P; Schnabel, R; Schofield, R M S; Sch"onbeck, A; Schreiber, E; Schuette, D; Schutz, B F; Scott, J; Scott, S M; Sellers, D; Sengupta, A S; Sentenac, D; Sequino, V; Sergeev, A; Setyawati, Y; Shaddock, D A; Shaffer, T; Shahriar, M S; Shaltev, M; Shapiro, B; Shawhan, P; Sheperd, A; Shoemaker, D H; Shoemaker, D M; Siellez, K; Siemens, X; Sieniawska, M; Sigg, D; Silva, A D; Singer, A; Singer, L P; Singh, A; Singh, R; Singhal, A; Sintes, A M; Slagmolen, B J J; Smith, J R; Smith, N D; Smith, R J E; Son, E J; Sorazu, B; Sorrentino, F; Souradeep, T; Srivastava, A K; Staley, A; Steinke, M; Steinlechner, J; Steinlechner, S; Steinmeyer, D; Stephens, B C; Stone, R; Strain, K A; Straniero, N; Stratta, G; Strauss, N A; Strigin, S; Sturani, R; Stuver, A L; Summerscales, T Z; Sun, L; Sunil, S; Sutton, P J; Swinkels, B L; Szczepa'nczyk, M J; Tacca, M; Talukder, D; Tanner, D B; T'apai, M; Tarabrin, S P; Taracchini, A; Taylor, R; Theeg, T; Thirugnanasambandam, M P; Thomas, E G; Thomas, M; Thomas, P; Thorne, K A; Thrane, E; Tiwari, S; Tiwari, V; Tokmakov, K V; Toland, K; Tomlinson, C; Tonelli, M; Tornasi, Z; Torres, C V; Torrie, C I; T"oyr"a, D; Travasso, F; Traylor, G; Trifir`o, D; Tringali, M C; Trozzo, L; Tse, M; Turconi, M; Tuyenbayev, D; Ugolini, D; Unnikrishnan, C S; Urban, A L; Usman, S A; Vahlbruch, H; Vajente, G; Valdes, G; van Bakel, N; van Beuzekom, M; Brand, J F J van den; Broeck, C Van Den; Vander-Hyde, D C; van der Schaaf, L; van Heijningen, J V; van Veggel, A A; Vardaro, M; Vass, S; Vas'uth, M; Vaulin, R; Vecchio, A; Vedovato, G; Veitch, J; Veitch, P J; Venkateswara, K; Verkindt, D; Vetrano, F; Vicer'e, A; Vinciguerra, S; Vine, D J; Vinet, J -Y; Vitale, S; Vo, T; Vocca, H; Vorvick, C; Voss, D V; Vousden, W D; Vyatchanin, S P; Wade, A R; Wade, L E; Wade, M; Walker, M; Wallace, L; Walsh, S; Wang, G; Wang, H; Wang, M; Wang, X; Wang, Y; Ward, R L; Warner, J; Was, M; Weaver, B; Wei, L -W; Weinert, M; Weinstein, A J; Weiss, R; Wen, L; Wessels, P; Westphal, T; Wette, K; Whelan, J T; Whiting, B F; Williams, R D; Williamson, A R; Willis, J L; Willke, B; Wimmer, M H; Winkler, W; Wipf, C C; Wittel, H; Woan, G; Woehler, J; Worden, J; Wright, J L; Wu, D S; Wu, G; Yablon, J; Yam, W; Yamamoto, H; Yancey, C C; Yu, H; Yvert, M; zny, A Zadro; Zangrando, L; Zanolin, M; Zendri, J -P; Zevin, M; Zhang, L; Zhang, M; Zhang, Y; Zhao, C; Zhou, M; Zhou, Z; Zhu, X J; Zucker, M E; Zuraw, S E; Zweizig, J

    2016-01-01

    We report here the non-detection of gravitational waves from the merger of binary neutron star systems and neutron-star--black-hole systems during the first observing run of Advanced LIGO. In particular we searched for gravitational wave signals from binary neutron star systems with component masses $\\in [1,3] M_{\\odot}$ and component dimensionless spins $< 0.05$. We also searched for neutron-star--black-hole systems with the same neutron star parameters, black hole mass $\\in [2,99] M_{\\odot}$ and no restriction on the black hole spin magnitude. We assess the sensitivity of the two LIGO detectors to these systems, and find that they could have detected the merger of binary neutron star systems with component mass distributions of $1.35\\pm0.13 M_{\\odot}$ at a volume-weighted average distance of $\\sim$ 70Mpc, and for neutron-star--black-hole systems with neutron star masses of $1.4M_\\odot$ and black hole masses of at least $5M_\\odot$, a volume-weighted average distance of at least $\\sim$ 110Mpc. From this we...

  4. The r-Process in Metal Poor Stars and Black Hole Formation

    International Nuclear Information System (INIS)

    Nucleosynthesis of heavy nuclei in metal-poor stars is generally ascribed to the r-process, as the abundance pattern in many such stars agrees with the inferred Solar r-process abundances. Nonetheless, a significant number of these stars do not share this r-process template. they suggest that many such stars have begun an r-process, but it was prevented from running to completion in more massive stars by collapse to black holes, creating a 'truncated r-process,' or 'tr-process'. The observed fraction of tr-process stars is found to be consistent with expectations from the initial mass function (IMF), and they suggest that an apparent sharp truncation observed at around mass 160 could result from a combination of collapses to black holes and the difficulty of observing the higher mass rare earths. They test the tr-process hypothesis with calculations that are terminated before all r-process trajectories have been ejected. These produce qualitative agreement with observation when both black hole collapse and observational realities are taken into account.

  5. THE r-PROCESS IN METAL-POOR STARS AND BLACK HOLE FORMATION

    International Nuclear Information System (INIS)

    Nucleosynthesis of heavy nuclei in metal-poor stars is generally thought to occur via the r-process because the r-process is a primary process that would have operated early in the Galaxy's history. This idea is strongly supported by the fact that the abundance pattern in many metal-poor stars matches well the inferred solar r-process abundance pattern in the mass range between the second and third r-process abundance peaks. Nevertheless, a significant number of metal-poor stars do not share this standard r-process template. In this Letter, we suggest that the nuclides observed in many of these stars are produced by the r-process, but that it is prevented from running to completion in more massive stars by collapse to black holes before the r-process is completed, creating a 'truncated r-process', or 'tr-process'. We find that the observed fraction of tr-process stars is qualitatively what one would expect from the initial mass function and that an apparent sharp truncation observed at around mass 160 could result from a combination of collapses to black holes and the difficulty of observing the higher mass rare-earth elements. We test the tr-process hypothesis with r-process calculations that are terminated before all r-process trajectories have been ejected. We find qualitative agreement between observation and theory when black hole collapse and observational realities are taken into account.

  6. Super massive black hole in galactic nuclei with tidal disruption of stars

    International Nuclear Information System (INIS)

    Tidal disruption of stars by super massive central black holes from dense star clusters is modeled by high-accuracy direct N-body simulation. The time evolution of the stellar tidal disruption rate, the effect of tidal disruption on the stellar density profile, and, for the first time, the detailed origin of tidally disrupted stars are carefully examined and compared with classic papers in the field. Up to 128k particles are used in simulation to model the star cluster around a super massive black hole, and we use the particle number and the tidal radius of the black hole as free parameters for a scaling analysis. The transition from full to empty loss-cone is analyzed in our data, and the tidal disruption rate scales with the particle number, N, in the expected way for both cases. For the first time in numerical simulations (under certain conditions) we can support the concept of a critical radius of Frank and Rees, which claims that most stars are tidally accreted on highly eccentric orbits originating from regions far outside the tidal radius. Due to the consumption of stars moving on radial orbits, a velocity anisotropy is found inside the cluster. Finally we estimate the real galactic center based on our simulation results and the scaling analysis.

  7. The r-Process in Metal Poor Stars and Black Hole Formation

    Energy Technology Data Exchange (ETDEWEB)

    Boyd, R N; Famiano, M A; Meyer, B S; Motizuki, Y; Kajino, T; Roederer, I U

    2011-11-30

    Nucleosynthesis of heavy nuclei in metal-poor stars is generally ascribed to the r-process, as the abundance pattern in many such stars agrees with the inferred Solar r-process abundances. Nonetheless, a significant number of these stars do not share this r-process template. they suggest that many such stars have begun an r-process, but it was prevented from running to completion in more massive stars by collapse to black holes, creating a 'truncated r-process,' or 'tr-process'. The observed fraction of tr-process stars is found to be consistent with expectations from the initial mass function (IMF), and they suggest that an apparent sharp truncation observed at around mass 160 could result from a combination of collapses to black holes and the difficulty of observing the higher mass rare earths. They test the tr-process hypothesis with calculations that are terminated before all r-process trajectories have been ejected. These produce qualitative agreement with observation when both black hole collapse and observational realities are taken into account.

  8. I-Love-Q Relations: From Compact Stars to Black Holes

    CERN Document Server

    Yagi, Kent

    2016-01-01

    The relations between most observables associated with a compact star, such as the mass and radius of a neutron star or a quark star, typically depend strongly on their unknown internal structure. The I-Love-Q relations (between the moment of inertia, the tidal deformability and the quadrupole moment) are however approximately insensitive to this structure. These relations become exact for stationary black holes in General Relativity as shown by the no-hair theorems. In this paper, we take the first steps toward studying how the approximate I-Love-Q relations become exact in the limit as compact stars become black holes. To do so, we consider a toy model, i.e. incompressible stars with anisotropic pressure, which allows us to model an equilibrium sequence of stars with their compactness approaching the black hole limit arbitrarily closely. We extract the I-Love-Q trio by numerically constructing such a sequence in the slow-rotation and small-tide approximations. We find that the I-Love-Q relations approach th...

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

    OpenAIRE

    Hidalgo, Sebastian L.; Monelli, Matteo; Aparicio, Antonio; Gallart, Carme; Skillman, Evan D.; Cassisi, Santi; Bernard, Edouard J.; Mayer, Lucio; Stetson, Peter; Cole, Andrew; Dolphin, Andrew

    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 differenc...

  10. Soft X-ray Spectroscopy of the Hot DA White Dwarf LB1919 and the PG1159 Star PG1520+525

    OpenAIRE

    Werner, K.; Drake, J. J.; Rauch, T.; Schuh, S.; Gautschy, A.

    2006-01-01

    We have performed soft X-ray spectroscopy of two hot white dwarfs with the Chandra observatory using the Low Energy Transmission Grating. The first target is the hot DA white dwarf LB1919 (Teff=69000 K). This star is representative of a small group of hot DAs whose metallicities lie well below predictions from radiative levitation theory. The Chandra spectrum shows a rich absorption line spectrum which may allow to find the origin of the low-metallicity nature of these DAs. The second target ...

  11. Can mixed star-plus-wormhole systems mimic black holes?

    CERN Document Server

    Dzhunushaliev, Vladimir; Kleihaus, Burkhard; Kunz, Jutta

    2016-01-01

    We consider mixed strongly gravitating configurations consisting of a wormhole threaded by two types of ordinary matter. For such systems, the possibility of obtaining static spherically symmetric solutions describing compact massive central objects enclosed by high-redshift surfaces (black-hole-like configurations) is studied. Using the standard thin accretion disk model, we exhibit potentially observable differences allowing to distinguish the mixed systems from ordinary black holes with the same masses.

  12. New PARSEC evolutionary tracks of massive stars at low metallicity: testing canonical stellar evolution in nearby star-forming dwarf galaxies

    Science.gov (United States)

    Tang, Jing; Bressan, Alessandro; Rosenfield, Philip; Slemer, Alessandra; Marigo, Paola; Girardi, Léo; Bianchi, Luciana

    2014-12-01

    We extend the PARSEC library of stellar evolutionary tracks by computing new models of massive stars, from 14 to 350 M⊙. The input physics is the same used in the PARSEC V1.1 version, but for the mass-loss rate from considering the most recent updates in the literature. We focus on low metallicity, Z = 0.001 and Z = 0.004, for which the metal-poor dwarf irregular star-forming galaxies, Sextans A, the Wolf-Lundmark-Melotte galaxy and NGC 6822, provide simple but powerful workbenches. The models reproduce fairly well the observed colour-magnitude diagrams (CMDs) but the stellar colour distributions indicate that the predicted blue loop is not hot enough in models with a canonical extent of overshooting. In the framework of a mild extended mixing during central hydrogen burning, the only way to reconcile the discrepancy is to enhance the overshooting at the base of the convective envelope (EO) during the first dredge-up. The mixing scales required to reproduce the observed loops, EO = 2HP or EO = 4HP, are definitely larger than those derived from, e.g. the observed location of the red-giant-branch bump in low mass stars. This effect, if confirmed, would imply a strong dependence of the mixing scale below the formal Schwarzschild border, on the stellar mass or luminosity. Reproducing the features of the observed CMDs with standard values of envelope overshooting would require a metallicity significantly lower than the values measured in these galaxies. Other quantities, such as the star formation rate and the initial mass function, are only slightly sensitive to this effect. Future investigations will consider other metallicities and different mixing schemes.

  13. Chemical evolution with bursts of star formation - Element ratios in dwarf galaxies

    International Nuclear Information System (INIS)

    It is demonstrated that galaxies in which star formation proceeds in a small number of bursts evolve their chemical elements in ratios which are very different from those in galaxies with continuing star formation. Systematic changes of element ratios with overall chemical abundance are determined to a large extent by the onset and duration of the star formation bursts and can be very different from those seen in the solar neighborhood. In particular, it is shown that an underabundance of oxygen relative to iron, such as is observed in the Large Magellanic Cloud, occurs naturally when star formation proceeds in a small number of widely separated bursts, as is inferred from the age distribution of LMC field stars and clusters. There is no need to invoke either variations in the stellar initial mass function or metal-enhanced winds. 25 refs

  14. Local behaviour of evaporating stars and black holes around the total evaporation event

    International Nuclear Information System (INIS)

    Many models in which the object under study loses all its mass have appeared in the literature. This can be found from evaporating stars to incipient black holes, all the way to evaporating black holes. In this paper we try a semiclassical study of these evaporating models centred on the evaporating event itself. We analyse their common properties, behaviours and possibilities. Specifically, we pay special attention to the evaporating models as a means of avoiding singularities during the collapse. In the case of any pre-existing non-spacelike curvature singularity, we show that these models tend to evaporate it. Finally, we introduce a new class of evaporating black holes.

  15. Mergers of Magnetized Neutron Stars with Spinning Black Holes: Disruption, Accretion and Fallback

    OpenAIRE

    Chawla, Sarvnipun; Anderson, Matthew; Besselman, Michael; Lehner, Luis; Liebling, Steven L.; Motl, Patrick M; Neilsen, David

    2010-01-01

    We investigate the merger of a neutron star (of compaction ratio $0.1$) in orbit about a spinning black hole in full general relativity with a mass ratio of $5:1$, allowing for the star to have an initial magnetization of $10^{12} {\\rm Gauss}$. We present the resulting gravitational waveform and analyze the fallback accretion as the star is disrupted. The evolutions suggest no significant effects from the initial magnetization. We find that only a negligible amount of matter becomes unbound; ...

  16. Spinning boson stars and Kerr black holes with scalar hair: the effect of self-interactions

    CERN Document Server

    Herdeiro, Carlos A R; Rúnarsson, Helgi F

    2016-01-01

    Self-interacting boson stars have been shown to alleviate the astrophysically low maximal mass of their non-self-interacting counterparts. We report some physical features of spinning self-interacting boson stars, namely their compactness, the occurence of ergo-regions and the scalar field profiles, for a sample of values of the coupling parameter. The results agree with the general picture that these boson stars are comparatively less compact than the non-self-interacting ones. We also briefly discuss the effect of scalar self-interactions on the properties of Kerr black holes with scalar hair.

  17. Analysis for Mar Vel Black and acetylene soot low reflectivity surfaces for star tracker sunshade applications

    Science.gov (United States)

    Yung, E.

    1974-01-01

    Mar Vel Black is a revolutionary new extremely low reflectivity anodized coating developed by Martin Marietta of Denver. It is of great interest in optics in general, and in star trackers specifically because it can reduce extraneous light reflections. A sample of Mar Vel Black was evaluated. Mar Vel Black looks much like a super black surface with many small peaks and very steep sides so that any light incident upon the surface will tend to reflect many times before exiting that surface. Even a high reflectivity surface would thus appear to have a very low reflectivity under such conditions. Conversely, acetylene soot does not have the magnified surface appearance of a super black surface. Its performance is, however, predictable from the surface structure, considering the known configuration of virtually pure carbon.

  18. The properties of hypervelocity stars and S-stars originating from an eccentric disc around a supermassive black hole

    CERN Document Server

    Subr, Ladislav

    2016-01-01

    Hypervelocity stars (HVSs) that are observed in the Galactic halo, are believed to be accelerated to large velocities by a process of tidal disruption of binary stars passing close to a supermassive black hole (SMBH) which resides in the center of the Galaxy. It is, however, still unclear, where these relatively young stars were born and which dynamical process pushed them to nearly radial orbits around the SMBH. In this paper we investigate the possibility that the young binaries originated from a thin eccentric disc, similar to the one observed in the Galactic center nowadays. By means of direct N-body simulations, we follow the dynamical evolution of an initially thin and eccentric disc of stars with a 100% binary fraction orbiting around the SMBH. Such a configuration leads to Kozai-Lidov oscillations of orbital elements, bringing considerable amount of binaries to close vicinity of the black hole. Subsequent tidal disruption of these binaries accelerates one of their component to velocities well above th...

  19. Dwarf galaxies with ionizing radiation feedback. II. Spatially resolved star formation relation

    International Nuclear Information System (INIS)

    We investigate the spatially resolved star formation relation using a galactic disk formed in a comprehensive high-resolution (3.8 pc) simulation. Our new implementation of stellar feedback includes ionizing radiation as well as supernova explosions, and we handle ionizing radiation by solving the radiative transfer equation rather than by a subgrid model. Photoheating by stellar radiation stabilizes gas against Jeans fragmentation, reducing the star formation rate (SFR). Because we have self-consistently calculated the location of ionized gas, we are able to make simulated, spatially resolved observations of star formation tracers, such as Hα emission. We can also observe how stellar feedback manifests itself in the correlation between ionized and molecular gas. Applying our techniques to the disk in a galactic halo of 2.3 × 1011 M ☉, we find that the correlation between SFR density (estimated from mock Hα emission) and H2 density shows large scatter, especially at high resolutions of ≲75 pc that are comparable to the size of giant molecular clouds (GMCs). This is because an aperture of GMC size captures only particular stages of GMC evolution and because Hα traces hot gas around star-forming regions and is displaced from the H2 peaks themselves. By examining the evolving environment around star clusters, we speculate that the breakdown of the traditional star formation laws of the Kennicutt-Schmidt type at small scales is further aided by a combination of stars drifting from their birthplaces and molecular clouds being dispersed via stellar feedback.

  20. Backreaction of Hawking radiation on a gravitationally collapsing star I: Black holes?

    Directory of Open Access Journals (Sweden)

    Laura Mersini-Houghton

    2014-11-01

    Full Text Available Particle creation leading to Hawking radiation is produced by the changing gravitational field of the collapsing star. The two main initial conditions in the far past placed on the quantum field from which particles arise, are the Hartle–Hawking vacuum and the Unruh vacuum. The former leads to a time-symmetric thermal bath of radiation, while the latter to a flux of radiation coming out of the collapsing star. The energy of Hawking radiation in the interior of the collapsing star is negative and equal in magnitude to its value at future infinity. This work investigates the backreaction of Hawking radiation on the interior of a gravitationally collapsing star, in a Hartle–Hawking initial vacuum. It shows that due to the negative energy Hawking radiation in the interior, the collapse of the star stops at a finite radius, before the singularity and the event horizon of a black hole have a chance to form. That is, the star bounces instead of collapsing to a black hole. A trapped surface near the last stage of the star's collapse to its minimum size may still exist temporarily. Its formation depends on the details of collapse. Results for the case of Hawking flux of radiation with the Unruh initial state, will be given in a companion paper II.

  1. Backreaction of Hawking radiation on a gravitationally collapsing star I: Black holes?

    Energy Technology Data Exchange (ETDEWEB)

    Mersini-Houghton, Laura [DAMTP, University of Cambridge, Wilberforce Rd., Cambridge, CB3 0WA, England (United Kingdom); Department of Physics and Astronomy, UNC, Chapel Hill, NC 27599 (United States)

    2014-11-10

    Particle creation leading to Hawking radiation is produced by the changing gravitational field of the collapsing star. The two main initial conditions in the far past placed on the quantum field from which particles arise, are the Hartle–Hawking vacuum and the Unruh vacuum. The former leads to a time-symmetric thermal bath of radiation, while the latter to a flux of radiation coming out of the collapsing star. The energy of Hawking radiation in the interior of the collapsing star is negative and equal in magnitude to its value at future infinity. This work investigates the backreaction of Hawking radiation on the interior of a gravitationally collapsing star, in a Hartle–Hawking initial vacuum. It shows that due to the negative energy Hawking radiation in the interior, the collapse of the star stops at a finite radius, before the singularity and the event horizon of a black hole have a chance to form. That is, the star bounces instead of collapsing to a black hole. A trapped surface near the last stage of the star's collapse to its minimum size may still exist temporarily. Its formation depends on the details of collapse. Results for the case of Hawking flux of radiation with the Unruh initial state, will be given in a companion paper II.

  2. Supermassive Black Holes in Galactic Nuclei with Tidal Disruption of Stars: Paper II - Axisymmetric Nuclei

    CERN Document Server

    Zhong, Shiyan; Spurzem, Rainer

    2015-01-01

    Tidal Disruption of stars by supermassive central black holes from dense rotating star clusters is modelled by high-accuracy direct N-body simulation. As in a previous paper on spherical star clusters we study the time evolution of the stellar tidal disruption rate and the origin of tidally disrupted stars, now according to several classes of orbits which only occur in axisymmetric systems (short axis tube and saucer). Compared with that in spherical systems, we found a higher TD rate in axisymmetric systems. The enhancement can be explained by an enlarged loss-cone in phase space which is raised from the fact that total angular momentum $\\bf J$ is not conserved. As in the case of spherical systems, the distribution of the last apocenter distance of tidally accreted stars peaks at the classical critical radius. However, the angular distribution of the origin of the accreted stars reveals interesting features. Inside the influence radius of the supermassive black hole the angular distribution of disrupted star...

  3. Constraining Asymmetric Dark Matter through observations of compact stars

    DEFF Research Database (Denmark)

    Kouvaris, Christoforos; Tinyakov, Peter

    2011-01-01

    We put constraints on asymmetric dark matter candidates with spin-dependent interactions based on the simple existence of white dwarfs and neutron stars in globular clusters. For a wide range of the parameters (WIMP mass and WIMP-nucleon cross section), WIMPs can be trapped in progenitors in large...... numbers and once the original star collapses to a white dwarf or a neutron star, these WIMPs might self-gravitate and eventually collapse forming a mini-black hole that eventually destroys the star. We impose constraints competitive to direct dark matter search experiments, for WIMPs with masses down...

  4. Modeling the Cloudy Atmospheres of Cool Stars, Brown Dwarfs and Hot Exoplanets

    DEFF Research Database (Denmark)

    Juncher, Diana

    -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...... properties of its host star, it is crucial that the stellar models linking the observations of a star to its properties are as precise as possible. The primary goal of this project is therefore to merge the model atmosphere code MARCS with the dust model code DRIFT, thus facilitating the computation of self...

  5. Quantitative spectral analysis of the sdB star HD 188112: a helium-core white dwarf progenitor

    CERN Document Server

    Latour, M; Irrgang, A; Schaffenroth, V; Geier, S; Hillebrandt, W; Roepke, F K; Taubenberger, S; Kromer, M; Fink, M

    2016-01-01

    HD 188112 is a bright (V = 10.2 mag) hot subdwarf B (sdB) star with a mass too low to sustain core helium burning and is therefore considered as a pre-extremely low mass (ELM) white dwarf (WD). ELM WDs (M $\\le$ 0.3 Msun) are He-core objects produced by the evolution of compact binary systems. We present in this paper a detailed abundance analysis of HD 188112 based on high-resolution Hubble Space Telescope (HST) near and far-ultraviolet spectroscopy. We also constrain the mass of the star's companion. We use hybrid non-LTE model atmospheres to fit the observed spectral lines and derive the abundances of more than a dozen elements as well as the rotational broadening of metallic lines. We confirm the previous binary system parameters by combining radial velocities measured in our UV spectra with the already published ones. The system has a period of 0.6065858 days and a WD companion with M $\\geq$ 0.70 Msun. By assuming a tidally locked rotation, combined with the projected rotational velocity (v sin i = 7.9 $\\...

  6. The Interaction of Venus-like, M-dwarf Planets with the Stellar Wind of Their Host Star

    CERN Document Server

    Cohen, O; Drake, J J; Glocer, A; Garraffo, C; Bell, J M; Gombosi, T I

    2015-01-01

    We study the interaction between the atmospheres of Venus-like, non-magnetized exoplanets orbiting an M-dwarf star, and the stellar wind using a multi-species Magnetohydrodynaic (MHD) model. We focus our investigation on the effect of enhanced stellar wind and enhanced EUV flux as the planetary distance from the star decreases. Our simulations reveal different topologies of the planetary space environment for sub- and super-Alfvenic stellar wind conditions, which could lead to dynamic energy deposition in to the atmosphere during the transition along the planetary orbit. We find that the stellar wind penetration for non-magnetized planets is very deep, up to a few hundreds of kilometers. We estimate a lower limit for the atmospheric mass-loss rate and find that it is insignificant over the lifetime of the planet. However, we predict that when accounting for atmospheric ion acceleration, a significant amount of the planetary atmosphere could be eroded over the course of a billion years.

  7. Planets Around Low-Mass Stars (PALMS). III. A Young Dusty L Dwarf Companion at the Deuterium-Burning Limit

    CERN Document Server

    Bowler, Brendan P; Shkolnik, Evgenya L; Dupuy, Trent J

    2013-01-01

    We report the discovery of an L-type companion to the young M3.5V star 2MASS J01225093-2439505 at a projected separation of 1.45" (~52 AU) as part of our adaptive optics imaging search for extrasolar giant planets around young low-mass stars. 2MASS 0122-2439 B has very red near-infrared colors similar to the HR 8799 planets and the reddest known young/dusty L dwarfs in the field. Moderate-resolution (R~3800) 1.5-2.4 $\\mu$m spectroscopy reveals a near-infrared spectral type of L4-L6 and an angular H-band shape, confirming its cool temperature and young age. The kinematics of 2MASS 0122-2439 AB are marginally consistent with members of the ~120 Myr AB Dor young moving group based on the photometric distance to the primary (36 +/- 4 pc) and our radial velocity measurement of 2MASS 0122-2439 A from Keck/HIRES. We adopt the AB Dor group age for the system, but the high energy emission, lack of Li I $\\lambda$6707 absorption, and spectral shape of 2MASS 0122-2439 B suggest a range of ~10-120 Myr is possible. The age...

  8. Ultra-Close Encounters of Stars With Massive Black Holes: Tidal Disruption Events With Prompt Hyperaccretion

    CERN Document Server

    Evans, Christopher; Eracleous, Michael

    2015-01-01

    A bright flare from a galactic nucleus followed at late times by a $t^{-5/3}$ decay in luminosity is often considered to be the signature of a tidal disruption of a star by a massive black hole. The flare and afterglow are produced when the stream of stellar debris released by the disruption returns to the vicinity of the black hole, self-intersects, and eventually forms an accretion disk or torus. In the canonical scenario of a solar-type star disrupted by a $10^{6}\\; M_\\odot$ black hole, the time between the disruption of the star and the formation of the accretion torus could be years. We present fully general relativistic simulations of a new class of tidal disruption events involving ultra-close encounters of solar-type stars with intermediate mass black holes. In these encounters, a thick disk forms promptly after disruption, on timescales of hours. After a brief initial flare, the accretion rate remains steady and highly super-Eddington for a few days at $\\sim 10^2\\,M_\\odot\\,{\\rm yr}^{-1}$.

  9. Big Black Hole, Little Neutron Star: Magnetic Dipole Fields in the Rindler Spacetime

    CERN Document Server

    D'Orazio, Daniel J

    2013-01-01

    As a black hole and neutron star approach during inspiral, the field lines of a magnetized neutron star eventually thread the black hole event horizon and a short-lived electromagnetic circuit is established. The black hole acts as a battery that provides power to the circuit, thereby lighting up the pair just before merger. Although originally suggested as a promising electromagnetic counterpart to gravitational-wave detection, the luminous signals are promising more generally as potentially detectable phenomena, such as short gamma-ray bursts. To aid in the theoretical understanding, we present a analytic solutions for the electromagnetic fields of a magnetic dipole in the presence of an event horizon. In the limit that the neutron star is very close to a Schwarzschild horizon, the Rindler limit, we can solve Maxwell's equations exactly for a magnetic dipole on an arbitrary worldline. We present these solutions here and investigate a proxy for a small segment of the neutron star orbit around a big black hol...

  10. Ultra-close Encounters of Stars with Massive Black Holes: Tidal Disruption Events with Prompt Hyperaccretion

    Science.gov (United States)

    Evans, Christopher; Laguna, Pablo; Eracleous, Michael

    2015-06-01

    A bright flare from a galactic nucleus followed at late times by a {{t}-5/3} decay in luminosity is often considered the signature of the complete tidal disruption of a star by a massive black hole. The flare and power-law decay are produced when the stream of bound debris returns to the black hole, self-intersects, and eventually forms an accretion disk or torus. In the canonical scenario of a solar-type star disrupted by a {{10}6} {{M}⊙ } black hole, the time between the disruption of the star and the formation of the accretion torus could be years. We present fully general relativistic simulations of a new class of tidal disruption events involving ultra-close encounters of solar-type stars with intermediate mass black holes. In these encounters, a thick disk forms promptly after disruption, on timescales of hours. After a brief initial flare, the accretion rate remains steady and highly super-Eddington for a few days at ˜ {{10}2} {{M}⊙ } y{{r}-1}.

  11. Optical Thermonuclear Transients from Tidal Compression of White Dwarfs as Tracers of the Low End of the Massive Black Hole Mass Function

    Science.gov (United States)

    MacLeod, Morgan; Guillochon, James; Ramirez-Ruiz, Enrico; Kasen, Daniel; Rosswog, Stephan

    2016-03-01

    In this paper, we model the observable signatures of tidal disruptions of white dwarf (WD) stars using massive black holes (MBHs) of moderate mass, ≈103-105 M⊙. When the WD passes deep enough within the MBH’s tidal field, these signatures include thermonuclear transients from burning during maximum compression. We combine a hydrodynamic simulation that includes nuclear burning of the disruption of a 0.6 M⊙ C/O WD with a Monte Carlo radiative transfer calculation to synthesize the properties of a representative transient. The transient’s emission emerges in the optical, with light curves and spectra reminiscent of Type I supernovae. The properties are strongly viewing angle dependent, and key spectral signatures are ≈10,000 km s-1 doppler shifts, due to the orbital motion of the unbound ejecta. Disruptions of He WDs likely produce large quantities of intermediate-mass elements, offering a possible production mechanism for Ca-rich transients. Accompanying multi-wavelength transients are fueled by accretion and arise from the nascent accretion disk and relativistic jet. If MBHs of moderate mass exist with number densities similar to those of supermassive BHs, both high-energy wide-field monitors and upcoming optical surveys should detect tens to hundreds of WD tidal disruptions per year. The current best strategy for their detection may therefore be deep optical follow-up of high-energy transients of unusually long duration. The detection rate or the nondetection of these transients by current and upcoming surveys can thus be used to place meaningful constraints on the extrapolation of the MBH mass function to moderate masses.

  12. Mass-radius relations for white dwarf stars of different internal compositions

    OpenAIRE

    Panei, J. A.; Althaus, L.G.; Benvenuto, O. G.

    1999-01-01

    The purpose of this work is to present accurate and detailed mass-radius relations for white dwarf (WD) models with helium, carbon, oxygen, silicon and iron cores, by using a fully updated stellar evolutionary code. We considered masses from 0.15 to 0.5 Mo for the case of helium core, from 0.45 to 1.2 Mo for carbon, oxygen and silicon cores and from 0.45 to 1.0 Mo for the case of an iron core. In view of recent measurements made by Hipparcos that strongly suggest the existence of WDs with an ...

  13. VCC 144 - a star-bursting dwarf galaxy in the Virgo Cluster

    OpenAIRE

    Brosch, N.; Almoznino, E.; L. Hoffman

    1997-01-01

    We describe results of a multi-spectral study of a blue compact dwarf galaxy in Virgo. The object was observed with broad-band and H$\\alpha$ imaging, UV observations, and radio synthesis. Our data were combined with published optical observations, with HI single-beam observation and with FIR data, and were compared to results of evolutionary synthesis programs. The radio observations revealed a compact source of HI coincident with the optical galaxy, embedded in a diffuse, HI cloud which has ...

  14. Gyrochronology of Low-mass Stars - Age-Rotation-Activity Relations for Young M Dwarfs

    Science.gov (United States)

    Kidder, Benjamin; Shkolnik, E.; Skiff, B.

    2014-01-01

    New rotation periods for 34 young intention of strengthening age-rotation-activity relations and assessing the possible use of gyrochronology in young, low-mass stars. We compared ages and rotation periods of our target stars to cluster members spanning 1-600 Myr. Rotation periods at every age exhibit a large scatter, with values typically ranging from 0.2 to 15 days. This suggests that gyrochronology for individual field stars will not be possible without a better understanding of the underlying mechanisms that govern angular momentum evolution. Yet, on average, the data still support the predicted trends for spin-up during contraction and spin-down on the main sequence, with the turnover occurring at around 150 Myr for early Ms. This suggests that rotation period distributions can be helpful in evaluating the ages of coeval groups of stars. Many thanks to the National Science Foundation for their support through the Research Experience for Undergraduates Grant AST- 1004107.

  15. BANYAN. V. A SYSTEMATIC ALL-SKY SURVEY FOR NEW VERY LATE-TYPE LOW-MASS STARS AND BROWN DWARFS IN NEARBY YOUNG MOVING GROUPS

    Energy Technology Data Exchange (ETDEWEB)

    Gagné, Jonathan; Lafrenière, David; Doyon, René; Malo, Lison; Artigau, Étienne [Département de Physique, Université de Montréal, C.P. 6128 Succ. Centre-ville, Montréal, QC H3C 3J7 (Canada)

    2015-01-10

    We present the BANYAN All-Sky Survey (BASS) catalog, consisting of 228 new late-type (M4-L6) candidate members of nearby young moving groups (YMGs) with an expected false-positive rate of ∼13%. This sample includes 79 new candidate young brown dwarfs and 22 planetary-mass objects. These candidates were identified through the first systematic all-sky survey for late-type low-mass stars and brown dwarfs in YMGs. We cross-matched the Two Micron All Sky Survey and AllWISE catalogs outside of the galactic plane to build a sample of 98,970 potential ≥M5 dwarfs in the solar neighborhood and calculated their proper motions with typical precisions of 5-15 mas yr{sup –1}. We selected highly probable candidate members of several YMGs from this sample using the Bayesian Analysis for Nearby Young AssociatioNs II tool (BANYAN II). We used the most probable statistical distances inferred from BANYAN II to estimate the spectral type and mass of these candidate YMG members. We used this unique sample to show tentative signs of mass segregation in the AB Doradus moving group and the Tucana-Horologium and Columba associations. The BASS sample has already been successful in identifying several new young brown dwarfs in earlier publications, and will be of great interest in studying the initial mass function of YMGs and for the search of exoplanets by direct imaging; the input sample of potential close-by ≥M5 dwarfs will be useful to study the kinematics of low-mass stars and brown dwarfs and search for new proper motion pairs.

  16. Giants reveal what dwarfs conceal: Li abundance in lower red giant branch stars as diagnostic of the primordial Li

    Science.gov (United States)

    Mucciarelli, A.; Salaris, M.; Bonifacio, P.

    2012-01-01

    The discrepancy between cosmological Li abundance inferred from Population II dwarf stars and that derived from big bang nucleosynthesis calculations is still far from being satisfactorily solved. We investigated, as an alternative route, the use of Li abundances in Population II lower red giant branch stars as empirical diagnostic of the cosmological Li. Both theory and observations suggest that the surface Li abundance in metal-poor red giants after the completion of the first dredge-up and before the red giant branch bump is significantly less sensitive to the efficiency of atomic diffusion, compared with dwarf stars. The surface Li abundances in these objects - after the dilution caused by the first dredge-up - are predicted to be sensitive to the total Li content left in the star, i.e. they are affected only by the total amount of Li eventually burned during the previous main-sequence phase. Standard stellar models computed under different physical assumptions show that the inclusion of the atomic diffusion has an impact of about 0.07 dex in the determination of the primordial Li abundance - much smaller than the case of metal-poor main-sequence turnoff stars - and it is basically unaffected by reasonable variations of other parameters (overshooting, age, initial He abundance and mixing length). We have determined from spectroscopy the surface Li content of 17 halo lower red giant branch stars, in the metallicity range between [Fe/H] ˜- 3.4 and ˜- 1.4 dex, evolving before the extramixing episode that sets in at the red giant branch bump. The initial Li (customarily taken as estimate of the cosmological Li abundance A(Li)0) has then been inferred by accounting for the difference between initial and post-dredge-up Li abundances in the appropriate stellar models. It depends mainly on the Teff scale adopted in the spectroscopic analysis, and is only weakly sensitive to the efficiency of atomic diffusion in the models, so long as one neglects Li destruction

  17. The shortest-known-period star orbiting our Galaxy's supermassive black hole.

    Science.gov (United States)

    Meyer, L; Ghez, A M; Schödel, R; Yelda, S; Boehle, A; Lu, J R; Do, T; Morris, M R; Becklin, E E; Matthews, K

    2012-10-01

    Stars with short orbital periods at the center of our Galaxy offer a powerful probe of a supermassive black hole. Over the past 17 years, the W. M. Keck Observatory has been used to image the galactic center at the highest angular resolution possible today. By adding to this data set and advancing methodologies, we have detected S0-102, a star orbiting our Galaxy's supermassive black hole with a period of just 11.5 years. S0-102 doubles the number of known stars with full phase coverage and periods of less than 20 years. It thereby provides the opportunity, with future measurements, to resolve degeneracies in the parameters describing the central gravitational potential and to test Einstein's theory of general relativity in an unexplored regime. PMID:23042888

  18. Dual transcriptome analysis reveals insights into the response to Rice black-streaked dwarf virus in maize

    Science.gov (United States)

    Zhou, Yu; Xu, Zhennan; Duan, Canxing; Chen, Yanping; Meng, Qingchang; Wu, Jirong; Hao, Zhuanfang; Wang, Zhenhua; Li, Mingshun; Yong, Hongjun; Zhang, Degui; Zhang, Shihuang; Weng, Jianfeng; Li, Xinhai

    2016-01-01

    Maize rough dwarf disease (MRDD) is a viral infection that results in heavy yield losses in maize worldwide, particularly in the summer maize-growing regions of China. MRDD is caused by the Rice black-streaked dwarf virus (RBSDV). In the present study, analyses of microRNAs (miRNAs), the degradome, and transcriptome sequences were used to elucidate the RBSDV-responsive pathway(s) in maize. Genomic analysis indicated that the expression of three non-conserved and 28 conserved miRNAs, representing 17 known miRNA families and 14 novel miRNAs, were significantly altered in response to RBSDV when maize was inoculated at the V3 (third leaf) stage. A total of 99 target transcripts from 48 genes of 10 known miRNAs were found to be responsive to RBSDV infection. The annotations of these target genes include a SQUAMOSA promoter binding (SPB) protein, a P450 reductase, an oxidoreductase, and a ubiquitin-related gene, among others. Characterization of the entire transcriptome suggested that a total of 28 and 1085 differentially expressed genes (DEGs) were detected at 1.5 and 3.0 d, respectively, after artificial inoculation with RBSDV. The expression patterns of cell wall- and chloroplast-related genes, and disease resistance- and stress-related genes changed significantly in response to RBSDV infection. The negatively regulated genes GRMZM2G069316 and GRMZM2G031169, which are the target genes for miR169i-p5 and miR8155, were identified as a nucleolin and a NAD(P)-binding Rossmann-fold superfamily protein in maize, respectively. The gene ontology term GO:0003824, including GRMZM2G031169 and other 51 DEGs, was designated as responsive to RBSDV. PMID:27493226

  19. Dual transcriptome analysis reveals insights into the response to Rice black-streaked dwarf virus in maize.

    Science.gov (United States)

    Zhou, Yu; Xu, Zhennan; Duan, Canxing; Chen, Yanping; Meng, Qingchang; Wu, Jirong; Hao, Zhuanfang; Wang, Zhenhua; Li, Mingshun; Yong, Hongjun; Zhang, Degui; Zhang, Shihuang; Weng, Jianfeng; Li, Xinhai

    2016-08-01

    Maize rough dwarf disease (MRDD) is a viral infection that results in heavy yield losses in maize worldwide, particularly in the summer maize-growing regions of China. MRDD is caused by the Rice black-streaked dwarf virus (RBSDV). In the present study, analyses of microRNAs (miRNAs), the degradome, and transcriptome sequences were used to elucidate the RBSDV-responsive pathway(s) in maize. Genomic analysis indicated that the expression of three non-conserved and 28 conserved miRNAs, representing 17 known miRNA families and 14 novel miRNAs, were significantly altered in response to RBSDV when maize was inoculated at the V3 (third leaf) stage. A total of 99 target transcripts from 48 genes of 10 known miRNAs were found to be responsive to RBSDV infection. The annotations of these target genes include a SQUAMOSA promoter binding (SPB) protein, a P450 reductase, an oxidoreductase, and a ubiquitin-related gene, among others. Characterization of the entire transcriptome suggested that a total of 28 and 1085 differentially expressed genes (DEGs) were detected at 1.5 and 3.0 d, respectively, after artificial inoculation with RBSDV. The expression patterns of cell wall- and chloroplast-related genes, and disease resistance- and stress-related genes changed significantly in response to RBSDV infection. The negatively regulated genes GRMZM2G069316 and GRMZM2G031169, which are the target genes for miR169i-p5 and miR8155, were identified as a nucleolin and a NAD(P)-binding Rossmann-fold superfamily protein in maize, respectively. The gene ontology term GO:0003824, including GRMZM2G031169 and other 51 DEGs, was designated as responsive to RBSDV. PMID:27493226

  20. White dwarf pulsations

    International Nuclear Information System (INIS)

    The DA white dwarfs are those which show only the Stark-broadened lines of hydrogen in their spectra. They comprise about 80% of the total white dwarf population. A subset of the DA dwarfs, the ZZ Ceti stars, form a highly homogeneous class of nonradially pulsating variable stars. In this paper we shall review the observations from which both the physical properties of the stars and the characteristics of the pulsations have been derived. Data obtained since the last review of these variables (Robinson 1979) is stressed, as these data are forcing a somewhat revised understanding of the ZZ Ceti stars and their relationship to investigations of white dwarfs and to pulsating variable stars, in general. (orig.)