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Sample records for low-mass white dwarfs

  1. Most Double Degenerate Low Mass White Dwarf Binaries Merge

    CERN Document Server

    Brown, Warren R; Kenyon, Scott J; Gianninas, A

    2016-01-01

    We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM) <0.3 Msun white dwarfs in the Galaxy. Such white dwarfs are detectable for timescales of 0.1 Gyr -- 1 Gyr in the ELM Survey; the binaries they reside in have gravitational wave merger times of 0.001 Gyr -- 100 Gyr. To explain the observed distribution requires that most ELM white dwarf binary progenitors detach from the common envelope phase with <1 hr orbital periods. We calculate the local space density of ELM white dwarf binaries and estimate a merger rate of 3e-3/yr over the entire disk of the Milky Way; the merger rate in the halo is 10 times smaller. The ELM white dwarf binary merger rate exceeds by a factor of 40 the formation rate of stable mass transfer AM CVn binaries, marginally exceeds the rate of underluminous supernovae, and is identical to the formation rate of R CrB stars. On this basis, we conclude that ELM white dwarf binaries can be the progenitors of all observed AM CVn and possibly underlum...

  2. The ELM Survey. VII. Orbital Properties of Low Mass White Dwarf Binaries

    CERN Document Server

    Brown, Warren R; Kilic, Mukremin; Kenyon, Scott J; Prieto, Carlos Allende

    2016-01-01

    We present the discovery of 15 extremely low mass (5 < log{g} < 7) white dwarf candidates, 9 of which are in ultra-compact double-degenerate binaries. Our targeted ELM Survey sample now includes 76 binaries. The sample has a lognormal distribution of orbital periods with a median period of 5.4 hr. The velocity amplitudes imply that the binary companions have a normal distribution of mass with 0.76 Msun mean and 0.25 Msun dispersion. Thus extremely low mass white dwarfs are found in binaries with a typical mass ratio of 1:4. Statistically speaking, 95% of the white dwarf binaries have a total mass below the Chandrasekhar mass and thus are not Type Ia supernova progenitors. Yet half of the observed binaries will merge in less than 6 Gyr due to gravitational wave radiation; probable outcomes include single massive white dwarfs and stable mass transfer AM CVn binaries.

  3. Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. I. Adiabatic properties

    Science.gov (United States)

    Córsico, A. H.; Althaus, L. G.

    2014-09-01

    Context. Many low-mass white dwarfs with masses M∗/M⊙ ≲ 0.45, including the so-called extremely low-mass white dwarfs (M∗/M⊙ ≲ 0.20 - 0.25), have recently been discovered in the field of our Galaxy through dedicated photometric surveys. The subsequent discovery of pulsations in some of them has opened the unprecedented opportunity of probing the internal structure of these ancient stars. Aims: We present a detailed adiabatic pulsational study of these stars based on full evolutionary sequences derived from binary star evolution computations. The main aim of this study is to provide a detailed theoretical basis of reference for interpreting present and future observations of variable low-mass white dwarfs. Methods: Our pulsational analysis is based on a new set of He-core white-dwarf models with masses ranging from 0.1554 to 0.4352 M⊙ derived by computing the non-conservative evolution of a binary system consisting of an initially 1 M⊙ ZAMS star and a 1.4 M⊙ neutron star. We computed adiabatic radial (ℓ = 0) and non-radial (ℓ = 1,2) p and g modes to assess the dependence of the pulsational properties of these objects on stellar parameters such as the stellar mass and the effective temperature, as well as the effects of element diffusion. Results: We found that for white dwarf models with masses below ~ 0.18 M⊙, g modes mainly probe the core regions and p modes the envelope, therefore pulsations offer the opportunity of constraining both the core and envelope chemical structure of these stars via asteroseismology. For models with M∗ ≳ 0.18 M⊙, on the other hand, g modes are very sensitive to the He/H compositional gradient and therefore can be used as a diagnostic tool for constraining the H envelope thickness. Because both types of objects have not only very distinct evolutionary histories (according to whether the progenitor stars have experienced CNO-flashes or not), but also have strongly different pulsation properties, we propose to

  4. Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: I. Adiabatic properties

    CERN Document Server

    Córsico, A H

    2014-01-01

    The discovery of pulsations in some low-mass white dwarfs, including the so-called extremely low-mass white dwarfs, has opened the unprecedented opportunity of probing the internal structure of these ancient stars. We present a detailed adiabatic pulsational study of these stars based on a new set of He-core white-dwarf models with masses ranging from $0.1554$ to $0.4352 M_{\\odot}$ derived by computing the non-conservative evolution of a binary system consisting of an initially $1 M_{\\odot}$ ZAMS star and a $1.4 M_{\\odot}$ neutron star. We computed adiabatic radial ($\\ell= 0$) and non-radial ($\\ell= 1, 2$) $p$ and $g$ modes to assess the dependence of the pulsational properties of these objects on stellar parameters such as the stellar mass and the effective temperature, as well as the effects of element diffusion. We found that for white dwarf models with masses below $\\sim 0.18 M_{\\odot}$, $g$ modes mainly probe the core regions and $p$ modes the envelope, therefore pulsations offer the opportunity of const...

  5. Recent advances in the theoretical modeling of pulsating low-mass He-core white dwarfs

    CERN Document Server

    Córsico, A H; Calcaferro, L M; Serenelli, A M; Kepler, S O; Jeffery, C S

    2016-01-01

    Many extremely low-mass (ELM) white-dwarf (WD) stars are currently being found in the field of the Milky Way. Some of these stars exhibit long-period nonradial $g$-mode pulsations, and constitute the class of ELMV pulsating WDs. In addition, several low-mass pre-WDs, which could be precursors of ELM WDs, have been observed to show short-period photometric variations likely due to nonradial $p$ modes and radial modes. They could constitute a new class of pulsating low-mass pre-WD stars, the pre-ELMV stars. Here, we present the recent results of a thorough theoretical study of the nonadiabatic pulsation properties of low-mass He-core WDs and pre-WDs on the basis of fully evolutionary models representative of these stars.

  6. Pruning The ELM Survey: Characterizing Candidate Low-mass White Dwarfs through Photometric Variability

    Science.gov (United States)

    Bell, Keaton J.; Gianninas, A.; Hermes, J. J.; Winget, D. E.; Kilic, Mukremin; Montgomery, M. H.; Castanheira, B. G.; Vanderbosch, Z.; Winget, K. I.; Brown, Warren R.

    2017-02-01

    We assess the photometric variability of nine stars with spectroscopic Teff and log g values from the ELM Survey that locates them near the empirical extremely low-mass (ELM) white dwarf instability strip. We discover three new pulsating stars: SDSS J135512.34+195645.4, SDSS J173521.69+213440.6, and SDSS J213907.42+222708.9. However, these are among the few ELM Survey objects that do not show radial velocity (RV) variations that confirm the binary nature expected of helium-core white dwarfs. The dominant 4.31 hr pulsation in SDSS J135512.34+195645.4 far exceeds the theoretical cut-off for surface reflection in a white dwarf, and this target is likely a high-amplitude δ Scuti pulsator with an overestimated surface gravity. We estimate the probability to be less than 0.0008 that the lack of measured RV variations in four of eight other pulsating candidate ELM white dwarfs could be due to low orbital inclination. Two other targets exhibit variability as photometric binaries. Partial coverage of the 19.342 hr orbit of WD J030818.19+514011.5 reveals deep eclipses that imply a primary radius >0.4 R⊙—too large to be consistent with an ELM white dwarf. The only object for which our time series photometry adds support to ELM white dwarf classification is SDSS J105435.78‑212155.9, which has consistent signatures of Doppler beaming and ellipsoidal variations. We conclude that the ELM Survey contains multiple false positives from another stellar population at Teff ≲ 9000 K, possibly related to the sdA stars recently reported from SDSS spectra.

  7. Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: II. Nonadiabatic analysis

    CERN Document Server

    Córsico, Alejandro H

    2016-01-01

    Low-mass ($M_{\\star}/M_{\\sun} \\lesssim 0.45$) white dwarfs, including the so called extremely low-mass white dwarfs (ELM, $M_{\\star}/M_{\\sun } \\lesssim 0.18-0.20$), are being currently discovered in the field of our Galaxy through dedicated photometric surveys. The fact that some of them pulsate opens the unparalleled chance for sounding their interiors. We present a detailed nonadiabatic pulsational analysis of such stars based on a new set of He-core white-dwarf models with masses ranging from $0.1554$ to $0.4352 M_{\\sun}$ derived by computing the non-conservative evolution of a binary system consisting of an initially $1 M_{\\sun}$ ZAMS star and a $1.4 M_{\\sun}$ neutron star. We have computed nonadiabatic radial modes and nonradial g and p modes to assess the dependence of the pulsational stability properties of these objects with stellar parameters such as the stellar mass, the effective temperature, and the convective efficiency. We found that a dense spectrum of unstable radial modes and nonradial g and ...

  8. Discovery of a Bright, Extremely Low-Mass White Dwarf in a Close Double Degenerate System

    CERN Document Server

    Vennes, S; Kawka, A; Nemeth, P; Skinner, J N; Pigulski, A; Steslicki, M; Kolaczkowski, Z; Srodka, P; 10.1088/2041-8205/737/1/L16

    2011-01-01

    We report the discovery of a bright (V ~ 13.7), extremely low-mass white dwarf in a close double degenerate system. We originally selected GALEX J171708.5+675712 for spectroscopic follow-up among a group of white dwarf candidates in an ultraviolet-optical reduced proper-motion diagram. The new white dwarf has a mass of 0.18 M_solar and is the primary component of a close double degenerate system (P=0.246137 d, K_1 = 288 km/s) comprising a fainter white dwarf secondary with M_2 ~ 0.9 M_solar. Light curves phased with the orbital ephemeris show evidence of relativistic beaming and weaker ellipsoidal variations. The light curves also reveal secondary eclipses (depth ~ 8 mmag) while the primary eclipses appear partially compensated by the secondary gravitational deflection and are below detection limits. Photospheric abundance measurements show a nearly solar composition of Si, Ca, and Fe (0.1-1 solar), while the normal kinematics suggest a relatively recent formation history. Close binary evolutionary scenarios ...

  9. 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.

  10. Asteroseismic test of rotational mixing in low-mass white dwarfs

    CERN Document Server

    Istrate, A G; Gianninas, A; Grassitelli, L; Marchant, P; Tauris, T M; Langer, N

    2016-01-01

    We exploit the recent discovery of pulsations in mixed-atmosphere (He/H), extremely low-mass white dwarf precursors (ELM proto-WDs) to test the proposition that rotational mixing is a fundamental process in the formation and evolution of low-mass helium core white dwarfs. Rotational mixing has been shown to be a mechanism able to compete efficiently against gravitational settling, thus accounting naturally for the presence of He, as well as traces of metals such as Mg and Ca, typically found in the atmospheres of ELM proto-WDs. Here we investigate whether rotational mixing can maintain a sufficient amount of He in the deeper driving region of the star, such that it can fuel, through HeII-HeIII ionization, the observed pulsations in this type of stars. Using state-of-the-art evolutionary models computed with MESA, we show that rotational mixing can indeed explain qualitatively the very existence and general properties of the known pulsating, mixed-atmosphere ELM proto-WDs. Moreover, such objects are very likel...

  11. Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: III. The pre-ELM white dwarf instability strip

    CERN Document Server

    Córsico, A H; Serenelli, A M; Kepler, S O; Jeffery, C S; Corti, M A

    2016-01-01

    Two low-mass pre-white dwarfs, which could be precursors of ELM white dwarfs, have been observed to show multiperiodic photometric variations. They could constitute a new class of pulsating low-mass pre-white dwarf stars. We present a detailed nonadiabatic pulsation study of such stars, employing full evolutionary sequences of low-mass He-core pre-white dwarf models. We have considered models in which element diffusion is accounted for and also models in which it is neglected. We confirm and explore in detail a new instability strip in the domain of low gravities and low effective temperatures of the $T_{\\rm eff}-\\log g$ diagram, where low-mass pre-white dwarfs are currently found. The destabilized modes are radial and nonradial $p$ and $g$ modes excited by the $\\kappa-\\gamma$ mechanism acting mainly at the zone of the second partial ionization of He, with non-negligible contributions from the region of the first partial ionization of He and the partial ionization of H. The computations with element diffusion...

  12. 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...

  13. SHORT-PERIOD g-MODE PULSATIONS IN LOW-MASS WHITE DWARFS TRIGGERED BY H-SHELL BURNING

    Energy Technology Data Exchange (ETDEWEB)

    Córsico, A. H.; Althaus, L. G., E-mail: acorsico@fcaglp.unlp.edu.ar [Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata (Argentina)

    2014-09-20

    The detection of pulsations in white dwarfs with low mass offers the possibility of probing their internal structures through asteroseismology and placing constraints on the binary evolutionary processes involved in their formation. In this Letter, we assess the impact of stable H burning on the pulsational stability properties of low-mass He-core white dwarf models resulting from binary star evolutionary calculations. We found that besides a dense spectrum of unstable radial modes and nonradial g and p modes driven by the κ mechanism due to the partial ionization of H in the stellar envelope, some unstable g modes with short pulsation periods are also powered by H burning via the ε mechanism of mode driving. This is the first time that ε destabilized modes are found in models representative of cool white dwarf stars. The short periods recently detected in the pulsating low-mass white dwarf SDSS J111215.82+111745.0 could constitute the first evidence of the existence of stable H burning in these stars, in particular in the so-called extremely low-mass white dwarfs.

  14. A Radio Search For Pulsar Companions To SDSS Low-Mass White Dwarfs

    CERN Document Server

    Agueros, Marcel A; Silvestri, Nicole M; Kleinman, S J; Anderson, Scott F; Liebert, James W

    2009-01-01

    We have conducted a search for pulsar companions to 15 low-mass white dwarfs (LMWDs; M < 0.4 M_Sun) at 820 MHz with the NRAO Green Bank Telescope (GBT). These LMWDs were spectroscopically identified in the Sloan Digital Sky Survey (SDSS), and do not show the photometric excess or spectroscopic signature associated with a companion in their discovery data. However, LMWDs are believed to evolve in binary systems and to have either a more massive WD or a neutron star as a companion. Indeed, evolutionary models of low-mass X-ray binaries, the precursors of millisecond pulsars (MSPs), produce significant numbers of LMWDs (e.g., Benvenuto & De Vito 2005), suggesting that the SDSS LMWDs may have neutron star companions. No convincing pulsar signal is detected in our data. This is consistent with the findings of van Leeuwen et al. (2007), who conducted a GBT search for radio pulsations at 340 MHz from unseen companions to eight SDSS WDs (five are still considered LMWDs; the three others are now classified as "...

  15. Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. IV. The secular rate of period change

    Science.gov (United States)

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

    2017-04-01

    Context. An increasing number of low-mass (M⋆/M⊙ ≲ 0.45) and extremely low-mass (ELM, M⋆/M⊙ ≲ 0.18-0.20) white-dwarf stars are being discovered in the field of the Milky Way. Some of these stars exhibit long-period g-mode pulsations, and are called ELMV variable stars. Also, some low-mass pre-white dwarf stars show short-period p-mode (and likely radial-mode) photometric variations, and are designated as pre-ELMV variable stars. The existence of these new classes of pulsating white dwarfs and pre-white dwarfs opens the prospect of exploring the binary formation channels of these low-mass white dwarfs through asteroseismology. Aims: We aim to present a theoretical assessment of the expected temporal rates of change of periods (\\dot{Π}) for such stars, based on fully evolutionary low-mass He-core white dwarf and pre-white dwarf models. Methods: Our analysis is based on a large set of adiabatic periods of radial and nonradial pulsation modes computed on a suite of low-mass He-core white dwarf and pre-white dwarf models with masses ranging from 0.1554 to 0.4352 M⊙, which were derived by computing the non-conservative evolution of a binary system consisting of an initially 1 M⊙ ZAMS star and a 1.4 M⊙ neutron star companion. Results: We computed the secular rates of period change of radial (ℓ = 0) and nonradial (ℓ = 1,2) g and p modes for stellar models representative of ELMV and pre-ELMV stars, as well as for stellar objects that are evolving just before the occurrence of CNO flashes at the early cooling branches. We find that the theoretically expected magnitude of \\dot{Π} of g modes for pre-ELMVs is by far larger than for ELMVs. In turn, \\dot{Π} of g modes for models evolving before the occurrence of CNO flashes are larger than the maximum values of the rates of period change predicted for pre-ELMV stars. Regarding p and radial modes, we find that the larger absolute values of \\dot{Π} correspond to pre-ELMV models. Conclusions: We

  16. Discovery of Three Pulsating, Mixed-atmosphere, Extremely Low-mass White Dwarf Precursors

    Science.gov (United States)

    Gianninas, A.; Curd, Brandon; Fontaine, G.; Brown, Warren R.; Kilic, Mukremin

    2016-05-01

    We report the discovery of pulsations in three mixed-atmosphere, extremely low-mass white dwarf (ELM WD, M ≤slant 0.3 M ⊙) precursors. Following the recent discoveries of pulsations in both ELM and pre-ELM WDs, we targeted pre-ELM WDs with mixed H/He atmospheres with high-speed photometry. We find significant optical variability in all three observed targets with periods in the range 320-590 s, consistent in timescale with theoretical predictions of p-mode pulsations in mixed-atmosphere ≈0.18 M ⊙ He-core pre-ELM WDs. This represents the first empirical evidence that pulsations in pre-ELM WDs can only occur if a significant amount of He is present in the atmosphere. Future, more extensive, timeseries photometry of the brightest of the three new pulsators offers an excellent opportunity to constrain the thickness of the surface H layer, which regulates the cooling timescales for ELM WDs. 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 National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil).

  17. Discovery of Three Pulsating, Mixed Atmosphere, Extremely Low-Mass White Dwarf Precursors

    CERN Document Server

    Gianninas, A; Fontaine, G; Browm, Warren R; Kilic, Mukremin

    2016-01-01

    We report the discovery of pulsations in three mixed atmosphere, extremely low-mass white dwarf (ELM WD, M $<$ 0.3 M$_{\\odot}$) precursors. Following the recent discoveries of pulsations in both ELM and pre-ELM WDs, we targeted pre-ELM WDs with mixed H/He atmospheres with high-speed photometry. We find significant optical variability in all three observed targets with periods in the range 320--590 s, consistent in time-scale with theoretical predictions of p-mode pulsations in mixed-atmosphere $\\approx$ 0.18 M$_{\\odot}$ He-core pre-ELM WDs. This represents the first empirical evidence that pulsations in pre-ELM WDs can only occur if a significant amount of He is present in the atmosphere. Future, more extensive, time-series photometry of the brightest of the three new pulsators offers an excellent opportunity to constrain the thickness of the surface H layer which regulates the cooling timescales for ELM WDs.

  18. A New Code for Nonradial Stellar Pulsations and its Application to Low - Mass, Helium White Dwarfs

    CERN Document Server

    Corsico, A H

    2002-01-01

    We present a finite difference code intended for computing linear, adiabatic, nonradial pulsations of spherical stars. This code is based on a general Newton - Raphson technique in order to handle the relaxation of the eigenvalue (square of the eigenfrequency) of the modes and their corresponding eigenfunctions. This code has been tested computing the pulsation spectra of polytropic spheres finding a good agreement with previous work. Then, we have coupled this code to our evolutionary code and applied it to the computation of the pulsation spectrum of a low mass, pure - helium white dwarf of 0.3 M_{sun} for a wide range of effective temperatures. In making this calculation we have taken an evolutionary time step short enough such that eigenmodes corresponding to a given model are used as initial approximation to those of the next one. Specifically, we have computed periods, period spacing, eigenfunctions, weight functions, kinetic energies and variational periods for a wide range of modes. To our notice this...

  19. The formation of low-mass helium white dwarfs orbiting pulsars: Evolution of low-mass X-ray binaries below the bifurcation period

    CERN Document Server

    Istrate, Alina; Langer, Norbert

    2014-01-01

    Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs) which have been spun up to high rotation rates via accretion of matter from a companion star in a low-mass X-ray binary (LMXB). However, many details of this recycling scenario remain to be understood. Here we investigate binary evolution in close LMXBs to study the formation of radio MSPs with low-mass helium white dwarf companions (He WDs) in tight binaries with orbital periods P_orb = 2-9 hr. In particular, we examine: i) if such observed systems can be reproduced from theoretical modelling using standard prescriptions of orbital angular momentum losses (i.e. with respect to the nature and the strength of magnetic braking), ii) if our computations of the Roche-lobe detachments can match the observed orbital periods, and iii) if the correlation between WD mass and orbital period (M_WD, P_orb) is valid for systems with P_orb < 2 days. Numerical calculations with a detailed stellar evolution code were used to trace the mass-tra...

  20. Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing

    CERN Document Server

    Istrate, Alina; Tauris, Thomas M; Langer, Norbert; Stancliffe, Richard J; Grassitelli, Luca

    2016-01-01

    A large number of extremely low-mass helium white dwarfs (ELM WDs) have been discovered in recent years. The majority of them are found in close binary systems suggesting they are formed either through a common-envelope phase or via stable mass transfer in a low-mass X-ray binary (LMXB) or a cataclysmic variable (CV) system. Here, we investigate the formation of these objects through the LMXB channel with emphasis on the proto-WD evolution in environments with different metallicities. We study, for the first time, the combined effects of rotational mixing and element diffusion (e.g. gravitational settling, thermal and chemical diffusion) on the evolution of proto-WDs and on the cooling properties of the resulting WDs. We present state-of-the-art binary stellar evolution models computed with MESA for metallicities between Z=0.0002 and Z=0.02, producing WDs with masses between 0.16-0.45 M$_{\\odot}$. Our results confirm that element diffusion plays a significant role in the evolution of proto-WDs that experience...

  1. 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

  2. DISCOVERY OF PULSATIONS, INCLUDING POSSIBLE PRESSURE MODES, IN TWO NEW EXTREMELY LOW MASS, He-CORE WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Bell, Keaton J.; Harrold, Samuel T. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Brown, Warren R.; Kenyon, Scott J. [Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 (United States); Gianninas, A.; Kilic, Mukremin, E-mail: jjhermes@astro.as.utexas.edu [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK 73019 (United States)

    2013-03-10

    We report the discovery of the second and third pulsating extremely low mass (ELM) white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 M{sub Sun} and effective temperatures below 10, 000 K, establishing these putatively He-core WDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Ceti stars). The short-period pulsations evidenced in the light curve of J1112 may also represent the first observation of acoustic (p-mode) pulsations in any WD, which provide an exciting opportunity to probe this WD in a complimentary way compared to the long-period g-modes that are also present. J1112 is a T{sub eff} =9590 {+-} 140 K and log g =6.36 {+-} 0.06 WD. The star displays sinusoidal variability at five distinct periodicities between 1792 and 2855 s. In this star, we also see short-period variability, strongest at 134.3 s, well short of the expected g-modes for such a low-mass WD. The other new pulsating WD, J1518, is a T{sub eff} =9900 {+-} 140 K and log g =6.80 {+-} 0.05 WD. The light curve of J1518 is highly non-sinusoidal, with at least seven significant periods between 1335 and 3848 s. Consistent with the expectation that ELM WDs must be formed in binaries, these two new pulsating He-core WDs, in addition to the prototype SDSS J184037.78+642312.3, have close companions. However, the observed variability is inconsistent with tidally induced pulsations and is so far best explained by the same hydrogen partial-ionization driving mechanism at work in classic C/O-core ZZ Ceti stars.

  3. SOPHIE velocimetry of Kepler transit candidates. I. Detection of the low-mass white dwarf KOI 74b

    Science.gov (United States)

    Ehrenreich, D.; Lagrange, A.-M.; Bouchy, F.; Perrier, C.; Hébrard, G.; Boisse, I.; Bonfils, X.; Arnold, L.; Delfosse, X.; Desort, M.; Díaz, R. F.; Eggenberger, A.; Forveille, T.; Lovis, C.; Moutou, C.; Pepe, F.; Pont, F.; Santos, N. C.; Santerne, A.; Ségransan, D.; Udry, S.; Vidal-Madjar, A.

    2011-01-01

    The Kepler mission has detected transits and occultations of a hot compact object around an early-type star, the Kepler Object of Interest KOI 74. The mass of this transiting object was photometrically assessed in a previous study using the presence of the relativistic beaming effect (so-called “Doppler boosting”) in the light curve. Our aim was to provide a spectroscopic validation of this pioneering approach. We measured the radial velocity variations of the A1V star KOI 74 with the SOPHIE spectrograph at the 1.93-m telescope of the Observatoire de Haute-Provence (France). Radial velocity measurements of this star are challenging because of the high level of stellar pulsations and the few available spectral lines. Using a technique dedicated to early-type main-sequence stars, we measured radial velocity variations compatible with a companion of mass 0.252 ± 0.025 {M_⊙}, in good agreement with the value derived from the Kepler light curve. This work strengthens the scenario suggesting that KOI 74 is a blue straggler orbited by a stellar core despoiled of its envelope, the low-mass white dwarf KOI 74b. Based on observations collected with the SOPHIE spectrograph on the 1.93-m telescope at Observatoire de Haute-Provence (CNRS), France, by the SOPHIE consortium (program 10A.PNP.CONS).

  4. New evolutionary sequences for extremely low mass white dwarfs: Homogeneous mass and age determinations, and asteroseismic prospects

    CERN Document Server

    Althaus, Leandro G; Córsico, Alejandro H

    2013-01-01

    We provide a fine and homogeneous grid of evolutionary sequences for He-core white dwarfs with masses 0.15-0.45 Msun, including the mass range for ELM white dwarfs (<0.20Msun). The grid is appropriate for mass and age determination, and to study their pulsational properties. White dwarf sequences have been computed by performing full evolutionary calculations that consider the main energy sources and processes of chemical abundance changes during white dwarf evolution. Initial models for the evolving white dwarfs have been obtained by computing the non-conservative evolution of a binary system consisting of a Msun ZAMS star and a 1.4 Msun neutron star for various initial orbital periods. To derive cooling ages and masses for He-core white dwarf we perform a least square fitting of the M(Teff, g) and Age(Teff, g) relations provided by our sequences by using a scheme that takes into account the time spent by models in different regions of the Teff-g plane. This is useful when multiple solutions for cooling a...

  5. Hydrogen-Accreting Carbon-Oxygen White Dwarfs of Low Mass Thermal and Chemical Behavior of Burning Shells

    CERN Document Server

    Piersanti, L; Iben, I; Tornambé, A

    2000-01-01

    Numerical experiments have been performed to investigate the thermal behavior of a cooled down white dwarf of initial mass $M_{\\rm WD} = 0.516 M_{\\sun}$ which accretes hydrogen-rich matter with Z = 0.02 at the rate $\\dot{M}=10^{-8}$ \\msun \\yrm1, typical for a recurrent hydrogen shell flash regime. The evolution of the main physical quantities of a model during a pulse cycle is examined in detail. From selected models in the mass range $M_{\\rm WD} = 0.52\\div 0.68$ \\msunend, we derive the borders in the $M_{\\rm WD}$ - $\\dot{M}$ plane of the steady state accretion regime when hydrogen is burned at a constant rate as rapidly as it is accreted. The physical properties during a hydrogen shell flash in white dwarfs accreting hydrogen-rich matter with metallicities Z = 0.001 and Z = 0.0001 are also studied. For a fixed accretion rate, a decrease in the metallicity of the accreted matter leads to an increase in the thickness of the hydrogen-rich layer at outburst and a decrease in the hydrogen-burning shell efficiency...

  6. Orbital properties of an unusually low-mass sdB star in a close binary system with a white dwarf

    CERN Document Server

    Silvotti, R; Bloemen, S; Telting, J H; Heber, U; Oreiro, R; Reed, M D; Farris, L E; O'Toole, S J; Lanteri, L; Degroote, P; Hu, H; Baran, A S; Hermes, J J; Althaus, L G; Marsh, T R; Charpinet, S; Li, J; Morris, R L; Sanderfer, D T

    2012-01-01

    We have used 605 days of photometric data from the Kepler spacecraft to study KIC 6614501, a close binary system with an orbital period of 0.15749747(25) days (3.779939 hours), that consists of a low-mass subdwarf B (sdB) star and a white dwarf. As seen in many other similar systems, the gravitational field of the white dwarf produces an ellipsoidal deformation of the sdB which appears in the light curve as a modulation at two times the orbital frequency. The ellipsoidal deformation of the sdB implies that the system has a maximum inclination of \\sim40 degrees, with i \\approx 20\\degrees being the most likely. The orbital radial velocity of the sdB star is high enough to produce a Doppler beaming effect with an amplitude of 432 \\pm 5 ppm, clearly visible in the folded light curve. The photometric amplitude that we obtain, K1 = 85.8 km/s, is \\sim 12 per cent less than the spectroscopic RV amplitude of 97.2 \\pm 2.0 km/s. The discrepancy is due to the photometric contamination from a close object at about 5 arcse...

  7. Brown Dwarf Companions to White Dwarfs

    CERN Document Server

    Burleigh, M R; Dobbie, P D; Farihi, J; Napiwotzki, R; Maxted, P F L; Barstow, M A; Jameson, R F; Casewell, S L; Gänsicke, B T; Marsh, T R

    2011-01-01

    Brown dwarf companions to white dwarfs are rare, but recent infra-red surveys are slowly reveal- ing examples. We present new observations of the post-common envelope binary WD0137-349, which reveals the effects of irradiation on the ~ 0.05M* secondary, and new observations of GD 1400 which show that it too is a close, post-comon envelope system. We also present the lat- est results in a near-infrared photometric search for unresolved ultracool companions and to white dwarfs with UKIDSS. Twenty five DA white dwarfs were identified as having photometric excesses indicative of a low mass companion, with 8-10 of these having a predicted mass in the range asso- ciated with brown dwarfs. The results of this survey show that the unresolved (< 2") brown dwarf companion fraction to DA white dwarfs is 0.3 \\leq fWD+BD \\leq 1.3%.

  8. Heavy metals in a light white dwarf: Abundances of the metal-rich, extremely low-mass GALEX J1717+6757

    CERN Document Server

    Hermes, J J; Koester, D; Bours, M C P; Townsley, D M; Farihi, J; Marsh, T R; Littlefair, Stuart; Dhillon, V S; Gianninas, A; Breedt, E; Raddi, R

    2014-01-01

    Using the Hubble Space Telescope, we detail the first abundance analysis enabled by far-ultraviolet spectroscopy of a low-mass (~0.19 Msun) white dwarf (WD), GALEX J1717+6757, which is in a 5.9-hr binary with a fainter, more-massive companion. We see absorption from nine metals, including roughly solar abundances of Ca, Fe, Ti, and P. We detect a significantly sub-solar abundance of C, and put upper limits on N and O that are also markedly sub-solar. Updated diffusion calculations indicate that all metals should settle out of the atmosphere of this 14,900 K, log(g) = 5.67 WD in the absence of radiative forces in less than 20 yr, orders of magnitude faster than the cooling age of hundreds of Myr. We demonstrate that ongoing accretion of rocky material that is often the cause of atmospheric metals in isolated, more massive WDs is unlikely to explain the observed abundances in GALEX J1717+6757. Using new radiative levitation calculations, we determine that radiative forces can counteract diffusion and support ma...

  9. BEER Analysis of Kepler and CoRoT Light Curves. IV. Discovery of Four New Low-mass White-Dwarf Companions in the Kepler Data

    Science.gov (United States)

    Faigler, S.; Kull, I.; Mazeh, T.; Kiefer, F.; Latham, D. W.; Bloemen, S.

    2015-12-01

    We report the discovery of four short-period eclipsing systems in the Kepler light curves, consisting of an A-star primary and a low-mass white dwarf (WD) secondary (dA+WD)—KIC 4169521, KOI-3818, KIC 2851474, and KIC 9285587. The systems show BEaming, Ellipsoidal and Reflection (BEER) phase modulations together with primary and secondary eclipses. These add to the 6 Kepler and 18 WASP short-period eclipsing dA+WD binaries that were previously known. The light curves, together with follow-up spectroscopic observations, allow us to derive the masses, radii, and effective temperatures of the two components of the four systems. The orbital periods, of 1.17-3.82 days, and WD masses, of 0.19-0.22 M⊙, are similar to those of the previously known systems. The WD radii of KOI-3818, KIC 2851474, and KIC 9285587 are 0.026, 0.035, and 0.026 R⊙, respectively, the smallest WD radii derived so far for short-period eclipsing dA+WD binaries. These three binaries extend the previously known population to older systems with cooler and smaller WD secondaries. KOI-3818 displays evidence for a fast-rotating primary and a minute but significant eccentricity, ˜1.5 × 10-3. These features are probably the outcome of the mass-transfer process.

  10. BEER ANALYSIS OF KEPLER AND CoRoT LIGHT CURVES. IV. DISCOVERY OF FOUR NEW LOW-MASS WHITE DWARF COMPANIONS IN THE KEPLER DATA

    Energy Technology Data Exchange (ETDEWEB)

    Faigler, S.; Kull, I.; Mazeh, T.; Kiefer, F. [School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978 (Israel); Latham, D. W. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Bloemen, S. [Department of Astrophysics, IMAPP, Radboud University Nijmegen, P.O. BOX 9010, NL-6500 GL Nijmegen (Netherlands)

    2015-12-10

    We report the discovery of four short-period eclipsing systems in the Kepler light curves, consisting of an A-star primary and a low-mass white dwarf (WD) secondary (dA+WD)—KIC 4169521, KOI-3818, KIC 2851474, and KIC 9285587. The systems show BEaming, Ellipsoidal and Reflection (BEER) phase modulations together with primary and secondary eclipses. These add to the 6 Kepler and 18 WASP short-period eclipsing dA+WD binaries that were previously known. The light curves, together with follow-up spectroscopic observations, allow us to derive the masses, radii, and effective temperatures of the two components of the four systems. The orbital periods, of 1.17–3.82 days, and WD masses, of 0.19–0.22 M{sub ⊙}, are similar to those of the previously known systems. The WD radii of KOI-3818, KIC 2851474, and KIC 9285587 are 0.026, 0.035, and 0.026 R{sub ⊙}, respectively, the smallest WD radii derived so far for short-period eclipsing dA+WD binaries. These three binaries extend the previously known population to older systems with cooler and smaller WD secondaries. KOI-3818 displays evidence for a fast-rotating primary and a minute but significant eccentricity, ∼1.5 × 10{sup −3}. These features are probably the outcome of the mass-transfer process.

  11. BEER analysis of Kepler and CoRoT light curves: IV. Discovery of four new low-mass white-dwarf companions in the Kepler data

    CERN Document Server

    Faigler, Simchon; Mazeh, Tsevi; Kiefer, Flavien; Latham, David W; Bloemen, Steven

    2015-01-01

    We report the discovery of four short-period eclipsing systems in the Kepler light curves, consisting of an A-star primary and a low-mass white-dwarf (WD) secondary (dA+WD) - KIC 4169521, KOI-3818, KIC 2851474 and KIC 9285587. The systems show BEaming, Ellipsoidal and Reflection (BEER) phase modulations together with primary and secondary eclipses. These add to the 6 Kepler, and 18 WASP, previously known short-period eclipsing dA+WD binaries. The light curves together with follow-up spectroscopic observations allow us to derive the masses, radii and effective temperatures of the two components of the four systems. The orbital periods, of 1.17-3.82 d, and WD masses, of 0.19-0.22 Msun, are similar to those of the previously known systems. The WD radii of KOI-3818, KIC 2851474, and KIC 9285587 are 0.026, 0.035 and 0.026 Rsun, respectively, the smallest WD radii derived so far for short-period eclipsing dA+WD binaries. These three binaries extend the previously known population to older systems with cooler and sm...

  12. SDSS J074511.56+194926.5: Discovery of a metal-rich and tidally distorted extremely low mass white dwarf

    Energy Technology Data Exchange (ETDEWEB)

    Gianninas, A.; Barber, Sara D.; Kilic, Mukremin [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States); Hermes, J. J.; Harrold, Samuel T. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Brown, Warren R.; Kenyon, Scott J. [Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138 (United States); Dufour, P., E-mail: alexg@nhn.ou.edu [Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7 (Canada)

    2014-02-01

    We present the discovery of an unusual, tidally distorted extremely low mass white dwarf (WD) with nearly solar metallicity. Radial velocity measurements confirm that this is a compact binary with an orbital period of 2.6975 hr and a velocity semi-amplitude of K = 108.7 km s{sup –1}. Analysis of the hydrogen Balmer lines yields an effective temperature of T {sub eff} = 8380 K and a surface gravity of log g = 6.21 that in turn indicate a mass of M = 0.16 M {sub ☉} and a cooling age of 4.2 Gyr. In addition, a detailed analysis of the observed metal lines yields abundances of log (Mg/H) = –3.90, log (Ca/H) = –5.80, log (Ti/H) = –6.10, log (Cr/H) = –5.60, and log (Fe/H) = –4.50, similar to the sun. We see no evidence of a debris disk from which these metals would be accreted, though the possibility cannot entirely be ruled out. Other potential mechanisms to explain the presence of heavy elements are discussed. Finally, we expect this system to ultimately undergo unstable mass transfer and merge to form a ∼0.3-0.6 M {sub ☉} WD in a few Gyr.

  13. White Dwarf Mass Distribution

    CERN Document Server

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

    2016-01-01

    We present the mass distribution for all S/N > 15 pure DA white dwarfs detected in the Sloan Digital Sky Survey up to Data Release 12, fitted with Koester models for ML2/alpha=0.8, and with Teff > 10 000 K, and for DBs with S/N >10, fitted with ML2/alpha=1.25, for Teff > 16 000 K. These mass distributions are for log g > 6.5 stars, i.e., excluding the Extremely Low Mass white dwarfs. We also present the mass distributions corrected by volume with the 1/Vmax approach, for stars brighter than g=19. Both distributions have a maximum at M=0.624 Msun but very distinct shapes. From the estimated z-distances, we deduce a disk scale height of 300 pc. We also present 10 probable halo white dwarfs, from their galactic U, V, W velocities.

  14. The formation of low-mass stars and brown dwarfs

    CERN Document Server

    Stamatellos, Dimitris

    2013-01-01

    It is estimated that ~60% of all stars (including brown dwarfs) have masses below 0.2Msun. Currently, there is no consensus on how these objects form. I will briefly review the four main theories for the formation of low-mass objects: turbulent fragmentation, ejection of protostellar embryos, disc fragmentation, and photo-erosion of prestellar cores. I will focus on the disc fragmentation theory and discuss how it addresses critical observational constraints, i.e. the low-mass initial mass function, the brown dwarf desert, and the binary statistics of low-mass stars and brown dwarfs. I will examine whether observations may be used to distinguish between different formation mechanisms, and give a few examples of systems that strongly favour a specific formation scenario. Finally, I will argue that it is likely that all mechanisms may play a role in low-mass star and brown dwarf formation.

  15. Luminosity functions for very low mass stars and brown dwarfs

    Science.gov (United States)

    Laughlin, Gregory; Bodenheimer, Peter

    1993-01-01

    A theoretical investigation of the luminosity function for low-mass objects to constrain the stellar initial mass function at the low-mass end is reported. The ways in which luminosity functions for low-mass stars are affected by star formation histories, brown dwarf and premain-sequence cooling rates and main-sequence mass luminosity relations, and the IMF are examined. Cooling rates and the mass-luminosity relation are determined through a new series of evolutionary calculations for very low mass stars and brown dwarfs in the range 0.05-0.50 solar mass. Model luminosity functions are constructed for specific comparison with the results of four recent observational surveys. The likelihood that the stellar mass function in the solar neighborhood is increasing at masses near the bottom of the main sequence and perhaps at lower masses is confirmed. In the most optimistic case, brown dwarfs contribute half of the local missing disk mass. The actual contribution is likely to be considerably less.

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

    CERN Document Server

    Reid, I. Neill

    2005-01-01

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

  17. Formation of low-mass stars and brown dwarfs

    CERN Document Server

    Hennebelle, Patrick

    2012-01-01

    These lectures attempt to expose the most important ideas, which have been proposed to explain the formation of stars with particular emphasis on the formation of brown dwarfs and low-mass stars. We first describe the important physical processes which trigger the collapse of a self-gravitating piece of fluid and regulate the star formation rate in molecular clouds. Then we review the various theories which have been proposed along the years to explain the origin of the stellar initial mass function paying particular attention to four models, namely the competitive accretion and the theories based respectively on stopped accretion, MHD shocks and turbulent dispersion. As it is yet unsettled whether the brown dwarfs form as low-mass stars, we present the theory of brown dwarfs based on disk fragmentation stressing all the uncertainties due to the radiative feedback and magnetic field. Finally, we describe the results of large scale simulations performed to explain the collapse and fragmentation of molecular cl...

  18. Formation of Low-Mass Stars and Brown Dwarfs

    Science.gov (United States)

    Hennebelle, P.

    2012-11-01

    These lectures attempt to expose the most important ideas, which have been proposed to explain the formation of stars with particular emphasis on the formation of brown dwarfs and low-mass stars. We first describe the important physical processes which trigger the collapse of a self-gravitating piece of fluid and regulate the star formation rate in molecular clouds. Then we review the various theories which have been proposed along the years to explain the origin of the stellar initial mass function paying particular attention to four models, namely the competitive accretion and the theories based respectively on stopped accretion, MHD shocks and turbulent dispersion. As it is yet unsettled whether the brown dwarfs form as low-mass stars, we present the theory of brown dwarfs based on disk fragmentation stressing all the uncertainties due to the radiative feedback and magnetic field. Finally, we describe the results of large scale simulations performed to explain the collapse and fragmentation of molecular clouds.

  19. The Merger Rate of Extremely Low Mass White Dwarf Binaries: Links to the Formation of AM CVn Stars and Underluminous Supernovae

    CERN Document Server

    Brown, Warren R; Prieto, Carlos Allende; Kenyon, Scott J

    2010-01-01

    We study a complete, colour-selected sample of double-degenerate binary systems containing extremely low mass (ELM) 3% of AM CVn stars. More importantly, the ELM WD systems that may detonate merge at a rate comparable to the estimated rate of underluminous SNe, rare explosions estimated to produce only ~0.2 Msol worth of ejecta. At least 25% of our ELM WD sample belong to the old thick disk and halo components of the Milky Way. Thus, if merging ELM WD systems are the progenitors of underluminous SNe, transient surveys must find them in both elliptical and spiral galaxies.

  20. Formation of low-mass stars and brown dwarfs

    OpenAIRE

    Hennebelle, Patrick

    2012-01-01

    These lectures attempt to expose the most important ideas, which have been proposed to explain the formation of stars with particular emphasis on the formation of brown dwarfs and low-mass stars. We first describe the important physical processes which trigger the collapse of a self-gravitating piece of fluid and regulate the star formation rate in molecular clouds. Then we review the various theories which have been proposed along the years to explain the origin of the stellar initial mass f...

  1. Model Atmospheres From Very Low Mass Stars to Brown Dwarfs

    CERN Document Server

    Allard, F; Freytag, B

    2010-01-01

    Since the discovery of brown dwarfs in 1994, and the discovery of dust cloud formation in the latest Very Low Mass Stars (VLMs) and Brown Dwarfs (BDs) in 1996, the most important challenge in modeling their atmospheres as become the understanding of cloud formation and advective mixing. For this purpose, we have developed radiation hydrodynamic 2D model atmosphere simulations to study the formation of forsterite dust in presence of advection, condensation, and sedimentation across the M-L-T VLMs to BDs sequence (Teff = 2800 K to 900 K, Freytag et al. 2010). We discovered the formation of gravity waves as a driving mechanism for the formation of clouds in these atmospheres, and derived a rule for the velocity field versus atmospheric depth and Teff , which is relatively insensitive to gravity. This rule has been used in the construction of the new model atmosphere grid, BT-Settl, to determine the microturbulence velocity, the diffusion coefficient, and the advective mixing of molecules as a function of depth. ...

  2. Low-mass visual companions to nearby G-dwarfs

    CERN Document Server

    Tokovinin, Andrei

    2010-01-01

    Complete census of wide visual companions to nearby G-dwarf stars can be achieved by selecting candidates from the 2MASS Point-Source Catalog and checking their status by second-epoch imaging. Such data are obtained for 124 candidates with separations up to 20", 47 of which are shown to be new physical low-mass stellar companions. A list of visual binaries with G-dwarf primaries is produced by combining newly found companions with historical data. Maximum likelihood analysis leads to the companion frequency of 0.13+-0.015 per decade of separation. The mass ratio is distributed almost uniformly, with a power-law index between -0.4 and 0. The remaining uncertainty in the index is related to modeling of the companion detection threshold in 2MASS. These findings are confirmed by alternative analysis of wider companions in 2MASS, removing the contamination by background stars statistically. Extension of this work will lead to a complete detection of visual companions - a necessary step towards reaching unbiased mu...

  3. Axions and White Dwarfs

    CERN Document Server

    Isern, J; Garcia-Berro, E; Salaris, M; Torres, S

    2010-01-01

    White dwarfs are almost completely degenerate objects that cannot obtain energy from the thermonuclear sources and their evolution is just a gravothermal process of cooling. The simplicity of these objects, the fact that the physical inputs necessary to understand them are well identified, although not always well understood, and the impressive observational background about white dwarfs make them the most well studied Galactic population. These characteristics allow to use them as laboratories to test new ideas of physics. In this contribution we discuss the robustness of the method and its application to the axion case.

  4. White Dwarfs in Globular Clusters

    CERN Document Server

    Möhler, S

    2008-01-01

    We review empirical and theoretical findings concerning white dwarfs in Galactic globular clusters. Since their detection is a critical issue we describe in detail the various efforts to find white dwarfs in globular clusters. We then outline the advantages of using cluster white dwarfs to investigate the formation and evolution of white dwarfs and concentrate on evolutionary channels that appear to be unique to globular clusters. We also discuss the usefulness of globular cluster white dwarfs to provide independent information on the distances and ages of globular clusters, information that is very important far beyond the immediate field of white dwarf research. Finally, we mention possible future avenues concerning globular cluster white dwarfs, like the study of strange quark matter or plasma neutrinos.

  5. The white dwarf luminosity function

    Science.gov (United States)

    García-Berro, Enrique; Oswalt, Terry D.

    2016-06-01

    White dwarfs are the final remnants of low- and intermediate-mass stars. Their evolution is essentially a cooling process that lasts for ∼ 10 Gyr. Their observed properties provide information about the history of the Galaxy, its dark matter content and a host of other interesting astrophysical problems. Examples of these include an independent determination of the past history of the local star formation rate, identification of the objects responsible for the reported microlensing events, constraints on the rate of change of the gravitational constant, and upper limits to the mass of weakly interacting massive particles. To carry on these tasks the essential observational tools are the luminosity and mass functions of white dwarfs, whereas the theoretical tools are the evolutionary sequences of white dwarf progenitors, and the corresponding white dwarf cooling sequences. In particular, the observed white dwarf luminosity function is the key manifestation of the white dwarf cooling theory, although other relevant ingredients are needed to compare theory and observations. In this review we summarize the recent attempts to empirically determine the white dwarf luminosity function for the different Galactic populations. We also discuss the biases that may affect its interpretation. Finally, we elaborate on the theoretical ingredients needed to model the white dwarf luminosity function, paying special attention to the remaining uncertainties, and we comment on some applications of the white dwarf cooling theory. Astrophysical problems for which white dwarf stars may provide useful leverage in the near future are also discussed.

  6. Hystereses in dwarf nova outbursts and low-mass X-ray binaries

    Science.gov (United States)

    Hameury, J.-M.; Lasota, J.-P.; Knigge, C.; Körding, E. G.

    2017-04-01

    Context. The disc instability model (DIM) successfully explains why many accreting compact binary systems exhibit outbursts during which their luminosity increases by orders of magnitude. The DIM correctly predicts which systems should be transient and works regardless of whether the accretor is a black hole, a neutron star, or a white dwarf. However, it has been known for some time that the outbursts of X-ray binaries, which contain neutron-star or black-hole accretors, exhibit hysteresis in the X-ray hardness-intensity diagram (HID). More recently, it has been shown that the outbursts of accreting white dwarfs also show hysteresis, but in a diagram combining optical, EUV, and X-ray fluxes. Aims: We examine the nature of the hysteresis observed in cataclysmic variables and low-mass X-ray binaries. Methods: We used our disc evolution code for modelling dwarf nova outbursts, and constructed the hardness intensity diagram as predicted by the disc instability model. Results: We show explicitly that the standard DIM, modified only to account for disc truncation, can explain the hysteresis observed in accreting white dwarfs, but cannot explain that observed in X-ray binaries. Conclusions: The spectral evidence for the existence of different accretion regimes or components (disc, corona, jets, etc.) should only be based on wavebands that are specific to the innermost parts of the discs, i.e. EUV and X-rays; this task is difficult because of interstellar absorption. The existing data, however, indicate that a hysteresis is in the EUV - X-ray domain is present in SS Cyg.

  7. The white dwarf luminosity function

    CERN Document Server

    García-Berro, Enrique

    2016-01-01

    White dwarfs are the final remnants of low- and intermediate-mass stars. Their evolution is essentially a cooling process that lasts for $\\sim 10$ Gyr. Their observed properties provide information about the history of the Galaxy, its dark matter content and a host of other interesting astrophysical problems. Examples of these include an independent determination of the past history of the local star formation rate, identification of the objects responsible for the reported microlensing events, constraints on the rate of change of the gravitational constant, and upper limits to the mass of weakly interacting massive particles. To carry on these tasks the essential observational tools are the luminosity and mass functions of white dwarfs, whereas the theoretical tools are the evolutionary sequences of white dwarf progenitors, and the corresponding white dwarf cooling sequences. In particular, the observed white dwarf luminosity function is the key manifestation of the white dwarf cooling theory, although other...

  8. White dwarfs, red dwarfs and halo dark matter

    Energy Technology Data Exchange (ETDEWEB)

    GarcIa-Berro, E; Torres, S; Camacho, J [Departament de Fisica Aplicada, Escola Politecnica Superior de Castelldefels, Universitat Politecnica de Catalunya, Av. del Canal Olimpic, s/n, 08860 Castelldefels (Spain); Isern, J, E-mail: garcia@fa.upc.ed [Institut de Ciencies de l' Espai, CSIC, Campus UAB, Facultat de Ciencies, Torre C-5, 08193 Bellaterra (Spain)

    2009-06-01

    The nature of the microlensing events observed by the MACHO team towards the LMC still remains controversial. Low-mass substellar objects and stars with masses larger than approx 1M{sub o-dot} have been ruled out, while stars of approx 0.5 M{sub o-dot} are the most probable candidates. This means that the microlenses should be either red or white dwarfs. Consequently, we assess jointly the relative contributions of both types of stars to the mass budget of the Galactic halo. We use a Monte Carlo code that incorporates up-to-date evolutionary sequences of both red dwarfs and white dwarfs as well as detailed descriptions of both our Galaxy and the LMC and we compare the synthetic populations obtained with our simulator with the results obtained by the MACHO and EROS experiments. We find that the contribution of the red dwarf population is not enough to explain the number of events measured by the MACHO team. Even though, the optical depth obtained in our simulations almost doubles that obtained when taking into account the white dwarf population alone. Finally, we also find that the contribution to the halo dark matter of the entire population under study is smaller than 10%, at the 95% confidence level.

  9. Magnetized White Dwarfs

    CERN Document Server

    Terrero, D Alvear; Martínez, A Pérez

    2016-01-01

    The purpose of this thesis is to obtain more realistic equations of state to describe the matter forming magnetized white dwarfs, and use them to solve its structure equations. The equations of state are determined by considering the weak magnetic field approximation $Bwhite dwarfs. Also, we consider the energy and pressure correction due to the Coulomb interaction of the electron gas with the ions located in a crystal lattice. Moreover, spherically symmetric Tolman-Oppenheimer-Volkoff structure equations are solved independently for the perpendicular and parallel pressures, confirming the necessity of using axisymmetric structure equations, more adequate to describe the anisotropic system. Therefore, we study the solutions in cylindrical coordinates. In this case, the mass per longitude unit is obtained instead of the total mass of the whit...

  10. Axion cooling of white dwarfs

    CERN Document Server

    Isern, J; Garcia--Berro, E; Salaris, M; Torres, S

    2013-01-01

    The evolution of white dwarfs is a simple gravothermal process. This process can be tested in two ways, through the luminosity function of these stars and through the secular variation of the period of pulsation of those stars that are variable. Here we show how the mass of the axion can be constrained using the white dwarf luminosity function.

  11. Microlensing Discovery of a Population of Very Tight, Very Low-mass Binary Brown Dwarfs

    CERN Document Server

    Choi, J -Y; Udalski, A; Sumi, T; Gaudi, B S; Gould, A; Bennett, D P; Dominik, M; Beaulieu, J -P; Tsapras, Y; Bozza, V; Abe, F; Bond, I A; Botzler, C S; Chote, P; Freeman, M; Fukui, A; Furusawa, K; Itow, Y; Ling, C H; Masuda, K; Matsubara, Y; Miyake, N; Muraki, Y; Ohnishi, K; Rattenbury, N J; Saito, To; Sullivan, D J; Suzuki, K; Sweatman, W L; Suzuki, D; Takino, S; Tristram, P J; Wada, K; Yock, P C M; Szymański, M K; Kubiak, M; Pietrzyński, G; Soszyński, I; Skowron, J; Kozłowski, S; Poleski, R; Ulaczyk, K; Wyrzykowski, Ł; Pietrukowicz, P; Almeida, L A; DePoy, D L; Dong, Subo; Gorbikov, E; Jablonski, F; Henderson, C B; Hwang, K -H; Janczak, J; Jung, Y -K; Kaspi, S; Lee, C -U; Malamud, U; Maoz, D; McGregor, D; Munoz, J A; Park, B -G; Park, H; Pogge, R W; Shvartzvald, Y; Shin, I -G; Yee, J C; Alsubai, K A; Browne, P; Burgdorf, M J; Novati, S Calchi; Dodds, P; Fang, X -S; Finet, F; Glitrup, M; Grundahl, F; Gu, S -H; Hardis, S; Harpsøe, K; Hinse, T C; Hornstrup, A; Hundertmark, M; Jessen-Hansen, J; Jørgensen, U G; Kains, N; Kerins, E; Liebig, C; Lund, M N; Lundkvist, M; Maier, G; Mancini, L; Mathiasen, M; Penny, M T; Rahvar, S; Ricci, D; Scarpetta, G; Skottfelt, J; Snodgrass, C; Southworth, J; Surdej, J; Tregloan-Reed, J; Wambsganss, J; Wertz, O; Zimmer, F; Albrow, M D; Bachelet, E; Batista, V; Brillant, S; Cassan, A; Cole, A A; Coutures, C; Dieters, S; Prester, D Dominis; Donatowicz, J; Fouqué, P; Greenhill, J; Kubas, D; Marquette, J -B; Menzies, J W; Sahu, K C; Zub, M; Bramich, D M; Horne, K; Steele, I A; Street, R A

    2013-01-01

    Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs are poorly understood. The multiplicity properties and minimum mass of the brown-dwarf mass function provide critical empirical diagnostics of these mechanisms. We present the discovery via gravitational microlensing of two very low-mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 Msun and 0.034 Msun, and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field brown-dwarf binaries known. The discovery of a population of such binaries indicates that brown dwarf binaries can robustly form at least down to masses of ~0.02 Msun. Future microlensing surveys will measure a mass-selected sample of brown-dwarf binary systems, which can then be directly compared to similar samples of stellar binaries.

  12. 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 ...

  13. Throwing Icebergs at White Dwarfs

    Science.gov (United States)

    Kohler, Susanna

    2017-08-01

    Where do the metals come from that pollute the atmospheres of many white dwarfs? Close-in asteroids may not be the only culprits! A new study shows that distant planet-size and icy objects could share some of the blame.Pollution ProblemsArtists impression of rocky debris lying close around a white dwarf star. [NASA/ESA/STScI/G. Bacon]When a low- to intermediate-mass star reaches the end of its life, its outer layers are blown off, leaving behind its compact core. The strong gravity of this white dwarf causes elements heavier than hydrogen and helium to rapidly sink to its center in a process known as sedimentation, leaving an atmosphere that should be free of metallic elements.Therefore its perhaps surprising that roughly 2550% of all white dwarfs are observed to have atmospheric pollution by heavy elements. The short timescales for sedimentation suggest that these elements were added to the white dwarf recently but how did they get there?Bringing Ice InwardIn the generally accepted theory, pre-existing rocky bodies or an orbiting asteroid belt survive the stars evolution, later accreting onto the final white dwarf. But this scenario doesnt explain a few observations that suggest white dwarfs might be accreting larger planetary-size bodies and bodies with ices and volatile materials.Dynamical evolution of a Neptune-like planet (a) and a Kuiper belt analog object (b) in wide binary star systems. Both have large eccentricity excitations during the white dwarf phase. [Stephan et al. 2017]How might you get large or icy objects which would begin on very wide orbits close enough to a white dwarf to become disrupted and accrete? Led by Alexander Stephan, a team of scientists at UCLA now suggest that the key is for the white dwarf to be in a binary system.Influence of a CompanionIn the authors model, the white-dwarf progenitor is orbited by both a distant stellar companion (a common occurrence) and a number of large potential polluters, which could have masses between that

  14. White Dwarf - Red Dwarf Systems Resolved with the Hubble Space Telescope. II. Full Snapshot Survey Results

    CERN Document Server

    Farihi, J; Wachter, S

    2010-01-01

    {Abrigded} Results are presented for a Hubble Space Telescope Advanced Camera for Surveys high-resolution imaging campaign of 90 white dwarfs with known or suspected low mass stellar and substellar companions. Of the 72 targets which remain candidate and confirmed white dwarfs with near-infrared excess, 43 are spatially resolved into two or more components, and a total of 12 systems are potentially triples. There is a possible, slight deficit of earlier spectral types (bluer colors) among the spatially unresolved companions, exactly the opposite of expectations if significant mass is transferred to the companion during the common envelope phase. Using the best available distance estimates, the low mass companions to white dwarfs exhibit a bimodal distribution in projected separation. This result supports the hypothesis that during the giant phases of the white dwarf progenitor, any unevolved companions either migrate inward to short periods of hours to days, or outward to periods of hundreds to thousands of y...

  15. 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...

  16. Pleiades low-mass brown dwarfs: the cluster L dwarf sequence

    CERN Document Server

    Bihain, G; Béjar, V J S; Caballero, J A; Bailer-Jones, C A L; Mundt, R; Acosta-Pulido, J A; Torres, A M

    2006-01-01

    We present a search for low-mass brown dwarfs in the Pleiades open cluster. The identification of Pleiades members fainter and cooler than those currently known allows us to constrain evolutionary models for L dwarfs and to extend the study of the cluster mass function to lower masses. We conducted a 1.8 deg^2 near-infrared J-band survey at the 3.5m Calar Alto Telescope, with completeness J~19.0. The detected sources were correlated with those of previously available optical I-band images (completeness I~22). Using a J versus I-J colour-magnitude diagram, we identified 18 faint red L-type candidates, with magnitudes 17.43.2. If Pleiades members, their masses would span ~0.040-0.020 M_Sol. We performed follow-up HKs-band imaging to further confirm their cluster membership by photometry and proper motion. Out of 11 IJ candidates with proper motion measurements, we find six cluster members, two non-members and three whose membership is uncertain and depends on the intrinsic velocity dispersion of Pleiades brown ...

  17. The DODO Survey: Imaging Planets around White Dwarfs

    Science.gov (United States)

    Hogan, E.; Burleigh, M. R.; Clarke, F. J.

    2007-09-01

    The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to directly image very low mass (⪆2 MJup) common proper motion companions in wide orbits around nearby white dwarfs. These proceedings contribution presents detailed results for three interesting white dwarfs from this survey and briefly describes the results from 19 other northern hemisphere and equatorial white dwarfs. So far, these results are consistent with the conclusions of tet{t40_mz2004}, that no more than ˜3% of stars harbour 5 - 10 MJup planets in wide orbits.

  18. 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...

  19. Singing and dancing white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Mukadam, Anjum S; Szkody, Paula [Department of Astronomy, University of Washington, Seattle, WA 98195 (United States); Gaensicke, Boris T [Department of Physics, University of Warwick, Coventry, CV4 7AL (United Kingdom); Nitta, Atsuko, E-mail: anjum@astro.washington.ed [Gemini Observatory, Hilo, HI 96720 (United States)

    2009-06-01

    Accreting white dwarfs have recently been shown to exhibit non-radial pulsations similar to their non-interacting counterparts. This allows us to probe the interior of the accreting white dwarf using seismology, and may be the only way to determine masses for non-eclipsing cataclysmic variables. Improving our understanding of accreting white dwarfs will have implications for models of supernovae Type Ia. Pulsating white dwarfs in cataclysmic variables are also useful in establishing the effects of accretion on pulsations. A search for nonradial pulsations among suitable candidates has led to the discovery of twelve such systems known to date. With the goal of establishing an instability strip (or strips) for these pulsating accretors, we acquired HST ultra-violet time-series spectroscopy of six pulsating white dwarfs in cataclysmic variables in 2007 and 2008. This approach enables us to measure the effective temperature of the white dwarf using the co-added spectrum, and to simultaneously characterize the pulsations. We also intended to constrain the pulsation mode identification by comparing the ultra-violet amplitudes to those from near-simultaneous ground-based photometry. Our preliminary results indicate a broad instability strip in the temperature range of 10500-15400 K.

  20. Measuring the Initial Mass Function of Low Mass Stars and Brown Dwarfs

    CERN Document Server

    Jeffries, R D

    2012-01-01

    I review efforts to determine the form and any lower limit to the initial mass function in the Galactic disk, using observations of low-mass stars and brown dwarfs in the field, young clusters and star forming regions. I focus on the methodologies that have been used and the uncertainties that exist due to observational limitations and to systematic uncertainties in calibrations and theoretical models. I conclude that whilst it is possible that the low-mass IMFs deduced from the field and most young clusters are similar, there are too many problems to be sure; there are examples of low-mass cluster IMFs that appear to be very discrepant and the IMFs for brown dwarfs in the field and young clusters have yet to be reconciled convincingly.

  1. Direct evidence of hierarchical assembly at low masses from isolated dwarf galaxy groups

    Science.gov (United States)

    Stierwalt, S.; Liss, S. E.; Johnson, K. E.; Patton, D. R.; Privon, G. C.; Besla, G.; Kallivayalil, N.; Putman, M.

    2017-01-01

    The demographics of dwarf galaxy populations have long been in tension with predictions from the Λ cold dark matter (ΛCDM) paradigm 1-4 . If primordial density fluctuations were scale-free as predicted, dwarf galaxies should themselves host dark-matter subhaloes 5 , the most massive of which may have undergone star formation resulting in dwarf galaxy groups. Ensembles of dwarf galaxies are observed as sate­llites of more massive galaxies 6-9 , and there is observational 10 and theoretical 11 evidence to suggest that these satellites at redshift z = 0 were captured by the massive host halo as a group. However, the evolution of dwarf galaxies is highly susceptible to environment 12-14 , making these satellite groups imperfect probes of ΛCDM in the low-mass regime. Here we report one of the clearest examples yet of hierarchical structure formation at low masses: using deep multi-wavelength data, we identify seven isolated, spectroscopically confirmed groups of only dwarf galaxies. Each group hosts three to five known members, has a baryonic mass of ~4.4 × 109 to 2 × 1010 solar masses (M ⊙), and requires a mass-to-light ratio of <100 to be gravitationally bound. Such groups are predicted to be rare theoretically and found to be rare observationally at the current epoch, and thus provide a unique window into the possible formation mechanism of more massive, isolated galaxies.

  2. Spitzer Spectroscopy of Low-Mass Dwarfs - Clouds and Chemistry at the Bottom of the IMF

    Science.gov (United States)

    Roellig, Thomas L.

    2006-01-01

    Brown dwarfs and low-mass stars show evidence of complicated atmospheres, including a variety of molecular species and clouds. Infrared observations are one of the best probes of the physics of these objects, but up until recently these observations have been limited in studies from ground-based telescopes by atmospheric absorption and insufficient sensitivity. With the launch of the Spitzer Space Telescope with its Infrared Spectrograph (IRS) instrument we now have the capability to undertake a systematic study of the atmospheric structure and chemistry in these cool objects. The IRS Dim Suns team has compiled spectra from objects ranging from M1 dwarfs with effective temperatures 3,800K of down to T8 dwarfs with effective temperatures of 700. This talk will present these results and discuss their implications for our understanding of cool dwarf atmospheric physics and structure.

  3. Spitzer Spectroscopy of Low-Mass Dwarfs - Clouds and Chemistry at the Bottom of the IMF

    Science.gov (United States)

    Roellig, Thomas L.

    2006-01-01

    Brown dwarfs and low-mass stars show evidence of complicated atmospheres, including a variety of molecular species and clouds. Infrared observations are one of the best probes of the physics of these objects, but up until recently these observations have been limited in studies from ground-based telescopes by atmospheric absorption and insufficient sensitivity. With the launch of the Spitzer Space Telescope with its Infrared Spectrograph (IRS) instrument we now have the capability to undertake a systematic study of the atmospheric structure and chemistry in these cool objects. The IRS Dim Suns team has compiled spectra from objects ranging from M1 dwarfs with effective temperatures 3,800K of down to T8 dwarfs with effective temperatures of 700. This talk will present these results and discuss their implications for our understanding of cool dwarf atmospheric physics and structure.

  4. The evolution of iron white dwarf stars

    Directory of Open Access Journals (Sweden)

    J. A. Panei

    2001-01-01

    Full Text Available Recent measurements by Hipparcos provide strong observational evidence supporting the existence of white dwarf stars with iron-rich core composition. Here we examine the evolution of iron-rich white dwarfs, for which the cooling is substancially accelerated as compared with the standard carbon-oxigen white dwarfs.

  5. Branes constrictions with White Dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    García-Aspeitia, Miguel A., E-mail: aspeitia@fisica.uaz.edu.mx [Consejo Nacional de Ciencia y Tecnología, Av, Insurgentes Sur 1582, Colonia Crédito Constructor, Del. Benito Juárez, C.P. 03940, Mexico, D.F. (Mexico); Unidad Académica de Física, Universidad Autónoma de Zacatecas, Calzada Solidaridad esquina con Paseo a la Bufa S/N, C.P. 98060, Zacatecas (Mexico)

    2015-11-06

    We consider here a robust study of stellar dynamics for white dwarf stars with polytropic matter in the weak-field approximation using the Lane–Emden equation from the brane-world scenario. We also derive an analytical solution to the nonlocal energy density and show the behavior and sensitivity of these stars to the presence of extra dimensions. Similarly, we analyze stability and compactness, in order to show whether it is possible to agree with the conventional wisdom of white dwarfs dynamics. Our results predict an average value of the brane tension of <λ>≳84.818 MeV{sup 4}, with a standard deviation σ≃82.021 MeV{sup 4}, which comes from a sample of dwarf stars, being weaker than other astrophysical observations but remaining higher than cosmological results provided by nucleosynthesis among others.

  6. Branes constrictions with White Dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Garcia-Aspeitia, Miguel A. [Consejo Nacional de Ciencia y Tecnologia, Mexico (Mexico); Unidad Academica de Fisica, Universidad Autonoma de Zacatecas (Mexico)

    2015-11-15

    We consider here a robust study of stellar dynamics for white dwarf stars with polytropic matter in the weak-field approximation using the Lane-Emden equation from the brane-world scenario. We also derive an analytical solution to the nonlocal energy density and show the behavior and sensitivity of these stars to the presence of extra dimensions. Similarly, we analyze stability and compactness, in order to show whether it is possible to agree with the conventional wisdom of white dwarfs dynamics. Our results predict an average value of the brane tension of left angle λ right angle >or similar 84.818 MeV{sup 4}, with a standard deviation σ ≅ 82.021 MeV{sup 4}, which comes from a sample of dwarf stars, being weaker than other astrophysical observations but remaining higher than cosmological results provided by nucleosynthesis among others. (orig.)

  7. White dwarfs in cataclysmic variables

    Science.gov (United States)

    Gaensicke, Boris

    2016-07-01

    Cataclysmic variables (CVs) provide excellent laboratories to study the effect that the accretion of matter, energy and angular momentum has on the structure of white dwarfs, with important implications on the evolution of these compact binaries, the ignition of thermonuclear surface burning, and potentially their explosion as SNIa. I will provide an overview of our current understanding of CV white dwarfs, with a particular emphasis on the results of a recent large HST program. I will review our knowledge regarding the mass distribution of CV white dwarfs, as well as the secular mean accretion rates that can be inferred from their effective temperatures, and compare those statistics with predictions from CV population models. I will also discuss a sub-set of CVs which underwent thermal-time scale mass transfer, one of the channels that is often discussed as a pathway to SN Ia, and I will illustrate how the study of these "failed SNIa" can contribute to the discussion of SNIa progenitors. Finally, I will discuss the occurrence of non-radial pulsations in white dwarfs, both in CVs and their detached progenitors.

  8. A Pulsar and White Dwarf in an Unexpected Orbit

    Science.gov (United States)

    Kohler, Susanna

    2016-11-01

    Astronomers have discovered a binary system consisting of a low-mass white dwarf and a millisecond pulsar but its eccentric orbit defies all expectations of how such binaries form.Observed orbital periods and binary eccentricities for binary millisecond pulsars. PSR J2234+0511 is the furthest right of the green stars that mark the five known eccentric systems. [Antoniadis et al. 2016]Unusual EccentricityIt would take a low-mass (0.4 solar masses) white dwarf over 100 billion years to form from the evolution of a single star. Since this is longer than the age of the universe, we believe that these lightweights are instead products of binary-star evolution and indeed, we observe many of these stars to still be in binary systems.But the binary evolution that can create a low-mass white dwarf includes a period of mass transfer, in which efficient tidal dissipation damps the systems orbital eccentricity. Because of this, we would expect all systems containing low-mass white dwarfs to have circular orbits.In the past, our observations of low-mass white dwarfmillisecond pulsar binaries have all been consistent with this expectation. But a new detection has thrown a wrench in the works: the unambiguous identification of a low-mass white dwarf thats in an eccentric (e=0.13) orbit with the millisecond pulsar PSR J2234+0511. How could this system have formed?Eliminating Formation ModelsLed by John Antoniadis (Dunlap Institute at University of Toronto), a team of scientists has used newly obtained optical photometry (from the Sloan Digital Sky Survey) and spectroscopy (from the Very Large Telescope in Chile) of the white dwarf to confirm the identification of this system.Antoniadis and collaborators then use measurements of the bodies masses (0.28 and 1.4 solar masses for the white dwarf and pulsar, respectively) and velocities, and constraints on the white dwarfs temperature, radius and surface gravity, to address three proposed models for the formation of this system.The 3D

  9. The statistical challenge of constraining the low-mass IMF in Local Group dwarf galaxies

    Science.gov (United States)

    El-Badry, Kareem; Weisz, Daniel R.; Quataert, Eliot

    2017-06-01

    We use Monte Carlo simulations to explore the statistical challenges of constraining the characteristic mass (mc) and width (σ) of a lognormal sub-solar initial mass function (IMF) in Local Group dwarf galaxies using direct star counts. For a typical Milky Way (MW) satellite (MV = -8), jointly constraining mc and σ to a precision of ≲ 20 per cent requires that observations be complete to ≲ 0.2 M⊙, if the IMF is similar to the MW IMF. A similar statistical precision can be obtained if observations are only complete down to 0.4 M⊙, but this requires measurement of nearly 100× more stars, and thus, a significantly more massive satellite (MV ˜ -12). In the absence of sufficiently deep data to constrain the low-mass turnover, it is common practice to fit a single-sloped power law to the low-mass IMF, or to fit mc for a lognormal while holding σ fixed. We show that the former approximation leads to best-fitting power-law slopes that vary with the mass range observed and can largely explain existing claims of low-mass IMF variations in MW satellites, even if satellite galaxies have the same IMF as the MW. In addition, fixing σ during fitting leads to substantially underestimated uncertainties in the recovered value of mc (by a factor of ˜4 for typical observations). If the IMFs of nearby dwarf galaxies are lognormal and do vary, observations must reach down to ˜mc in order to robustly detect these variations. The high-sensitivity, near-infrared capabilities of the James Webb Space Telescope and Wide-Field Infrared Survey Telescope have the potential to dramatically improve constraints on the low-mass IMF. We present an efficient observational strategy for using these facilities to measure the IMFs of Local Group dwarf galaxies.

  10. Oscillations of red dwarfs in evolved low-mass binaries with neutron stars

    Science.gov (United States)

    Sarna, Marek J.; Lee, Umin; Muslimov, Alexander G.

    1994-01-01

    We investigate a novel aspect of a problem related to the properties of low-mass binaries (LMBs) with millisecond pulsars: the pulsations of the red dwarf (donor) companion of the neutron star (NS). The illumination of the donor star by the pulsar's high-energy nonthermal radiation and relativistic wind may substantially affect its structure. We present a quantitative analysis of the oscillation spectrum of a red dwarf which has evolved in an LMB and has undergone the stage of evaporation. We calculate the p- and g-modes for red dwarfs with masses in the interval (0.2-0.6) stellar mass. For comparison, similar calculations are presented for zero age main-sequence (ZAMS) stars of the same masses. For less massive donor stars (approximately 0.2 stellar mass) the oscillation spectrum becomes quantitatively different from that of their ZAMS counterparts. The differnce is due to the fact that a ZAMS star of 0.2 stellar mass is fully convective, while the donor star in an LMB is expected to be far from thermal equilibrium and not fully convective. As a result, in contrast to a low-mass ZAMS star, a red dwarf of the same mass in an LMB allows the existence of g-modes. We also consider tidally forced g-modes, and perform a linear analysis of these oscillations for different degrees of nonsynchronism between the orbital and spin rotation of the red dwarf component. We demonstrate the existence of a series of reasonances for the low-order g-modes which may occur in LMBs at a late stage of their evolution. We discuss the possibility that these oscillations may trigger Roche lobe overflow and sudden mass loss by the donor star. Further implications of this effect for gamma- and X-ray burst phenomena are outlined.

  11. Abundance ratios of red giants in low mass ultra faint dwarf spheroidal galaxies

    CERN Document Server

    François, P; Bonifacio, P; Bidin, C Moni; Geisler, D; Sbordone, L

    2015-01-01

    Low mass dwarf spheroidal galaxies are key objects for our understanding of the chemical evolution of the pristine Universe and the Local Group of galaxies. Abundance ratios in stars of these objects can be used to better understand their star formation and chemical evolution. We report on the analysis of a sample of 11 stars belonging to 5 different ultra faint dwarf spheroidal galaxies (UfDSph) based on X-Shooter spectra obtained at the VLT. Medium resolution spectra have been used to determine the detailed chemical composition of their atmosphere. We performed a standard 1D LTE analysis to compute the abundances. Considering all the stars as representative of the same population of low mass galaxies, we found that the [alpha/Fe] ratios vs [Fe/H] decreases as the metallicity of the star increases in a way similar to what is found for the population of stars belonging to dwarf spheroidal galaxies. The main difference is that the solar [alpha/Fe] is reached at a much lower metallicity for the UfDSph than the ...

  12. Magnetars and White Dwarf Pulsars

    CERN Document Server

    Lobato, Ronaldo V; Coelho, Jaziel G

    2016-01-01

    The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are a class of pulsars understood as neutron stars (NSs) with super strong surface magnetic fields, namely $B\\gtrsim10^{14}$ G, and for that reason are known as Magnetars. However, in the last years some SGRs/AXPs with low surface magnetic fields $B\\sim(10^{12}-10^{13})$ G have been detected, challenging the Magnetar description. Moreover, some fast and very magnetic white dwarfs (WDs) have also been observed, and at least one showed X-Ray energy emission as an ordinary pulsar. Following this fact, an alternative model based on white dwarfs pulsars has been proposed to explain this special class of pulsars. In this model, AXPs and SGRs as dense and magnetized white dwarfs can have surface magnetic field $B\\sim 10^{7}-10^{10}$ G and rotate very fast with frequencies $\\Omega\\sim 1$ rad/s, consistent with the observed rotation periods $P\\sim (2-12)$ s.

  13. MICROLENSING DISCOVERY OF A POPULATION OF VERY TIGHT, VERY LOW MASS BINARY BROWN DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Choi, J.-Y.; Han, C. [Department of Physics, Institute for Astrophysics, Chungbuk National University, Cheongju 371-763 (Korea, Republic of); Udalski, A. [Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa (Poland); Sumi, T. [Department of Earth and Space Science, Osaka University, Osaka 560-0043 (Japan); Gaudi, B. S.; Gould, A. [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Bennett, D. P. [Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556-5670 (United States); Dominik, M. [SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS (United Kingdom); Beaulieu, J.-P. [Institut dAstrophysique de Paris, UMR7095 CNRS-Universite Pierre and Marie Curie, 98 bis boulevard Arago, F-75014 Paris (France); Tsapras, Y. [Las Cumbres Observatory Global Telescope Network, 6740B Cortona Drive, Goleta, CA 93117 (United States); Bozza, V. [INFN, Sezione di Napoli, I-80126 Napoli (Italy); Abe, F.; Furusawa, K.; Itow, Y. [Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601 (Japan); Bond, I. A.; Ling, C. H. [Institute of Information and Mathematical Sciences, Massey University, Private Bag 102-904, North Shore Mail Centre, Auckland 0745 (New Zealand); Botzler, C. S.; Freeman, M. [Department of Physics, University of Auckland, Private Bag 92-019, Auckland 1001 (New Zealand); Chote, P. [School of Chemical and Physical Sciences, Victoria University, Wellington 6140 (New Zealand); Fukui, A. [Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Asakuchi, Okayama 719-0232 (Japan); Collaboration: MOA Collaboration; OGLE Collaboration; muFUN Collaboration; MiNDSTEp Consortium; PLANET Collaboration; RoboNet Collaboration; and others

    2013-05-10

    Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs (BDs) are poorly understood. The multiplicity properties and minimum mass of the BD mass function provide critical empirical diagnostics of these mechanisms. We present the discovery via gravitational microlensing of two very low mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 M{sub Sun} and 0.034 M{sub Sun }, and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field BD binaries known. The discovery of a population of such binaries indicates that BD binaries can robustly form at least down to masses of {approx}0.02 M{sub Sun }. Future microlensing surveys will measure a mass-selected sample of BD binary systems, which can then be directly compared to similar samples of stellar binaries.

  14. Evaporation and Accretion of Extrasolar Comets Following White Dwarf Kicks

    CERN Document Server

    Stone, Nicholas; Loeb, Abraham

    2014-01-01

    Several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass loss. If other stars possess extrasolar analogues to the Solar Oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. Although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation or tidal disruption. The dusty debris from these comets will manifest itself as a debris disk temporarily visible around newborn white dwarfs; examples of such disks may already have been seen in the Helix Nebula, and around several other young WDs. Future observations with the James Webb Space Telescope will distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. If interpreted as indeed being cometary in origin, the observation that >15% of young WDs possess such disks provides indirect evidence that low mass gas giants (thought necessary to produce an Oort cloud) are common in the out...

  15. Red Optical Planet Survey: A radial velocity search for low mass M dwarf planets

    Science.gov (United States)

    Barnes, J. R.; Jenkins, J. S.; Jones, H. R. A.; Rojo, P.; Arriagada, P.; Jordán, A.; Minniti, D.; Tuomi, M.; Jeffers, S. V.; Pinfield, D.

    2013-04-01

    We present radial velocity results from our Red Optical Planet Survey (ROPS), aimed at detecting low-mass planets orbiting mid-late M dwarfs. The ˜10 ms-1 precision achieved over 2 consecutive nights with the MIKE spectrograph at Magellan Clay is also found on week long timescales with UVES at VLT. Since we find that UVES is expected to attain photon limited precision of order 2 ms-1 using our novel deconvolution technique, we are limited only by the (≤10 ms-1) stability of atmospheric lines. Rocky planet frequencies of η⊕ = 0.3-0.7 lead us to expect high planet yields, enabling determination of η⊕ for the uncharted mid-late M dwarfs with modest surveys.

  16. Models of very-low-mass stars, brown dwarfs and exoplanets.

    Science.gov (United States)

    Allard, F; Homeier, D; Freytag, B

    2012-06-13

    Within the next few years, GAIA and several instruments aiming to image extrasolar planets will be ready. In parallel, low-mass planets are being sought around red dwarfs, which offer more favourable conditions, for both radial velocity detection and transit studies, than solar-type stars. In this paper, the authors of a model atmosphere code that has allowed the detection of water vapour in the atmosphere of hot Jupiters review recent advances in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow the photometric and spectroscopic properties of this transition to be reproduced for the first time. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets.

  17. Red Optical Planet Survey: A radial velocity search for low mass M dwarf planets

    Directory of Open Access Journals (Sweden)

    Minniti D.

    2013-04-01

    Full Text Available We present radial velocity results from our Red Optical Planet Survey (ROPS, aimed at detecting low-mass planets orbiting mid-late M dwarfs. The ∼10 ms−1 precision achieved over 2 consecutive nights with the MIKE spectrograph at Magellan Clay is also found on week long timescales with UVES at VLT. Since we find that UVES is expected to attain photon limited precision of order 2 ms−1 using our novel deconvolution technique, we are limited only by the (≤10 ms−1 stability of atmospheric lines. Rocky planet frequencies of η⊕ = 0.3−0.7 lead us to expect high planet yields, enabling determination of η⊕ for the uncharted mid-late M dwarfs with modest surveys.

  18. 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...

  19. White dwarf cooling sequences and cosmochronology

    CERN Document Server

    Isern, J; Garcia-Berro, E

    2012-01-01

    The evolution of white dwarfs is a simple gravothermal process. This means that their luminosity function, i.e. the number of white dwarfs per unit bolometric magnitude and unit volume as a function of bolometric magnitude, is a monotonically increasing function that decreases abruptly as a consequence of the finite age of the Galaxy. The precision and the accuracy of the white dwarf luminosity functions obtained with the recent large surveys together with the improved quality of the theoretical models of evolution of white dwarfs allow to feed the hope that in a near future it will be possible to reconstruct the history of the different Galactic populations.

  20. Dynamical Masses of Accreting White Dwarfs

    Science.gov (United States)

    Pala, A. F.; Gänsckie, B. T.

    2017-03-01

    The mass retention efficiency is a key question in both the theoretical and observational study of accreting white dwarfs in interacting binaries, with important implications for their potential as progenitors for type Ia supernovae (SNe Ia). Canonical wisdom is that classical nova eruptions erode the white dwarf mass, and consequently, cataclysmic variables (CVs) have been excluded from the SN Ia progenitor discussion. However the average mass of white dwarfs in CVs is substantially higher (≃ 0.83 M⊙) than that of single white dwarfs (≃ 0.64 M ⊙), in stark contrast to expectations based on current classical nova models. This finding is based on a sample of ≃ 30 CV white dwarfs with accurate mass measurements, most of them in eclipsing systems. Given the fundamental importance of the mass evolution of accreting white dwarfs, it is necessary to enlarge this sample and to diversify the methods used for measuring masses. We have begun a systematic study of 27 CVs to almost double the number of CV white dwarfs with an accurate mass measurement. Using VLT/X-shooter phase-resolved observations, we can measure the white dwarf masses to a few percent, and will be able to answer the question whether accreting CV white dwarfs grow in mass.

  1. 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...

  2. Massive double white dwarfs and the AM CVn birthrate

    Science.gov (United States)

    Kilic, Mukremin; Brown, Warren R.; Heinke, Craig O.; Gianninas, A.; Benni, P.; Agüeros, M. A.

    2016-08-01

    We present Chandra and Swift X-ray observations of four extremely low-mass (ELM) white dwarfs with massive companions. We place stringent limits on X-ray emission from all four systems, indicating that neutron star companions are extremely unlikely and that the companions are almost certainly white dwarfs. Given the observed orbital periods and radial velocity amplitudes, the total masses of these binaries are greater than 1.02-1.39 M⊙. The extreme mass ratios between the two components make it unlikely that these binary white dwarfs will merge and explode as Type Ia or underluminous supernovae. Instead, they will likely go through stable mass transfer through an accretion disc and turn into interacting AM CVn. Along with three previously known systems, we identify two of our targets, J0811 and J2132, as systems that will definitely undergo stable mass transfer. In addition, we use the binary white dwarf sample from the ELM Survey to constrain the inspiral rate of systems with extreme mass ratios. This rate, 1.7 × 10-4 yr-1, is consistent with the AM CVn space density estimated from the Sloan Digital Sky Survey. Hence, stable mass transfer double white dwarf progenitors can account for the entire AM CVn population in the Galaxy.

  3. Double White Dwarf Merger Rates

    Science.gov (United States)

    Toonen, Silvia; Nelemans, Gijs; Portegies Zwart, Simon

    2013-01-01

    Type Ia supernovae (SNe Ia) are very successfully used as standard candles on cosmological distance scales, but so far the nature of the progenitor(s) is unclear. A possible scenario for SNe Ia are merging carbon/oxygen white dwarfs with a combined mass exceeding the Chandrasekhar mass. We determine the theoretical rates and delay time distribution of these mergers for two different common envelope prescriptions and metallicities. The shape of the delay time distributions is rather insensitive to the assumptions. The normalization is a factor ~3-13 too low compared to observations.

  4. The Survey of HI in Extremely Low-mass Dwarfs (SHIELD)

    CERN Document Server

    Cannon, John M; Haynes, Martha P; Janowiecki, Steven; Parker, Angela; Salzer, John J; Adams, Elizabeth A K; Engstrom, Eric; Huang, Shan; McQuinn, Kristen B W; Ott, Juergen; Saintonge, Amelie; Skillman, Evan D; Allan, John; Erny, Grace; Fliss, Palmer; Smith, AnnaLeigh

    2011-01-01

    We present first results from the "Survey of HI in Extremely Low-mass Dwarfs" (SHIELD), a multi-configuration EVLA study of the neutral gas contents and dynamics of galaxies with HI masses in the 10^6-10^7 Solar mass range detected by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We describe the survey motivation and concept demonstration using VLA imaging of 6 low-mass galaxies detected in early ALFALFA data products. We then describe the primary scientific goals of SHIELD and present preliminary EVLA and WIYN 3.5m imaging of the 12 SHIELD galaxies. With only a few exceptions, the neutral gas distributions of these extremely low-mass galaxies are centrally concentrated. In only 1 system have we detected HI column densities higher than 10^21 cm^-2. Despite this, the stellar populations of all of these systems are dominated by blue stars. Further, we find ongoing star formation as traced by H alpha emission in 10 of the 11 galaxies with H alpha imaging obtained to date. Taken together these results suggest th...

  5. AR Sco: A White Dwarf Synchronar

    CERN Document Server

    Katz, J I

    2016-01-01

    The emission of the white dwarf-M dwarf binary AR Sco is driven by the rapid synchronization of its white dwarf, rather than by accretion. This requires a comparatively large magnetic field $\\sim 100$ gauss at the M dwarf and $\\sim 10^8$ gauss on the white dwarf, larger than the fields of most intermediate polars but within the range of fields of known magnetic white dwarfs. The spindown power is dissipated in the atmosphere of the M dwarf by magnetic reconnection, accelerating particles that produce the observed synchrotron radiation. The displacement of the optical maximum from conjunction may be explained either by dissipation in a bow wave as the white dwarf's magnetic field sweeps past the M dwarf or by a misaligned white dwarf's rotation axis and oblique magnetic moment. In the latter case the rotation axis precesses with a period of decades, predicting a drift in the orbital phase of maximum. Binaries whose emission is powered by synchronization may be termed synchronars, in analogy to magnetars.

  6. The effect of feedback and reionization on star formation in low-mass dwarf galaxy haloes

    CERN Document Server

    Simpson, Christine M; Johnston, Kathryn V; Smith, Britton D; Mac Low, Mordecai-Mark; Sharma, Sanjib; Tumlinson, Jason

    2012-01-01

    We simulate the evolution of a 10^9 Msun dark matter halo in a cosmological setting with an adaptive-mesh refinement code as an analogue to local low luminosity dwarf irregular and dwarf spheroidal galaxies. The primary goal of our study is to investigate the roles of reionization and supernova feedback in determining the star formation histories of low mass dwarf galaxies. We include a wide range of physical effects, including metal cooling, molecular hydrogen formation and cooling, photoionization and photodissociation from a metagalactic background, a simple prescription for self-shielding, star formation, and a simple model for supernova driven energetic feedback. We carry out simulations excluding each major effect in turn. We find that reionization is primarily responsible for expelling most of the gas in our simulations, but that supernova feedback is required to disperse the dense, cold gas in the core of the halo. Moreover, we show that the timing of reionization can produce an order of magnitude dif...

  7. White Dwarf Critical Tests for Modified Gravity

    CERN Document Server

    Jain, Rajeev Kumar; Nielsen, Niklas Grønlund

    2015-01-01

    Scalar-tensor theories of gravity can lead to modifications of the gravitational force inside astrophysical objects. We exhibit that compact stars such as white dwarfs provide a unique set-up to test such deviations from Newtonian gravitational physics inside the stars. We obtain stringent and independent constraints on the parameter $\\Upsilon$ characterizing the deviations from gravity using the mass-radius relation, the Chandrasekhar mass limit and the maximal rotational frequency of white dwarfs. We find that white dwarfs impose stronger constraints on $\\Upsilon$ than the red and brown dwarfs.

  8. Magnetic White Dwarfs with Heavy Elements

    CERN Document Server

    Hardy, François; Jordan, Stefan

    2016-01-01

    Using our newly developed model atmosphere code appropriate for magnetic white dwarfs with metal lines in the Paschen-Back regime, we study various magnetic white dwarfs and explore the effects of various parameters such as the field geometry and the convective efficiency

  9. A white dwarf with an oxygen atmosphere

    Science.gov (United States)

    Kepler, S. O.; Koester, Detlev; Ourique, Gustavo

    2016-04-01

    Stars born with masses below around 10 solar masses end their lives as white dwarf stars. Their atmospheres are dominated by the lightest elements because gravitational diffusion brings the lightest element to the surface. We report the discovery of a white dwarf with an atmosphere completely dominated by oxygen, SDSS J124043.01+671034.68. After oxygen, the next most abundant elements in its atmosphere are neon and magnesium, but these are lower by a factor of ≥25 by number. The fact that no hydrogen or helium are observed is surprising. Oxygen, neon, and magnesium are the products of carbon burning, which occurs in stars at the high-mass end of pre-white dwarf formation. This star, a possible oxygen-neon white dwarf, will provide a rare observational test of the evolutionary paths toward white dwarfs.

  10. AR Sco: A Precessing White Dwarf Synchronar?

    Science.gov (United States)

    Katz, J. I.

    2017-02-01

    The emission of the white dwarf–M dwarf binary AR Sco is driven by the rapid synchronization of its white dwarf, rather than by accretion. Synchronization requires a magnetic field ∼100 Gauss at the M dwarf and ∼ {10}8 Gauss at the white dwarf, larger than the fields of most intermediate polars but within the range of fields of known magnetic white dwarfs. The spindown power is dissipated in the atmosphere of the M dwarf, within the near zone of the rotating white dwarf’s field, by magnetic reconnection, accelerating particles that produce the observed synchrotron radiation. The displacement of the optical maximum from conjunction may be explained either by dissipation in a bow wave as the white dwarf’s magnetic field sweeps past the M dwarf or by a misaligned white dwarf rotation axis and oblique magnetic moment. In the latter case the rotation axis precesses with a period of decades, predicting a drift in the orbital phase of the optical maximum. Binaries whose emission is powered by synchronization may be termed synchronars, in analogy to magnetars.

  11. 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

  12. 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

  13. A Pulsation Search Among Young Brown Dwarfs and Very Low Mass Stars

    CERN Document Server

    Cody, Ann Marie

    2014-01-01

    In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very low mass stars (VLMSs; <0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of 1-4 hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters $\\sigma$ Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to obs...

  14. A pulsation search among young brown dwarfs and very-low-mass stars

    Energy Technology Data Exchange (ETDEWEB)

    Cody, Ann Marie; Hillenbrand, Lynne A., E-mail: amc@ipac.caltech.edu [California Institute of Technology, Department of Astrophysics, MC 249-17, Pasadena, CA 91125 (United States)

    2014-12-01

    In 2005, Palla and Baraffe proposed that brown dwarfs (BDs) and very-low-mass stars (VLMSs; < 0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of one to four hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters σ Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to observable amplitudes in the early pre-main sequence. In spite of the nondetection, we did uncover a rich set of variability behavior—both periodic and aperiodic—on day to week timescales. We present new compilations of variable sources from our sample, as well as three new candidate cluster members in Chamaeleon I.

  15. A Hybrid Scenario for the Formation of Brown Dwarfs and Very Low Mass Stars

    CERN Document Server

    Basu, Shantanu

    2012-01-01

    We present a calculation of protostellar disk formation and evolution in which gaseous clumps (essentially, the first Larson cores formed via disk fragmentation) are ejected from the disk during the early stage of evolution. This is a universal process related to the phenomenon of ejection in multiple systems of point masses. However, it occurs in our model entirely due to the interaction of compact, gravitationally-bound gaseous clumps and is free from the smoothing-length uncertainty that is characteristic of models using sink particles. Clumps that survive ejection span a mass range of 0.08--0.35 $M_\\odot$, and have ejection velocities $0.8 \\pm 0.35$ km s$^{-1}$, which are several times greater than the escape speed. We suggest that, upon contraction, these clumps can form substellar or low-mass stellar objects with notable disks, or even close-separation very-low-mass binaries. In this hybrid scenario, allowing for ejection of clumps rather than finished protostars/proto--brown-dwarfs, disk formation and ...

  16. Throwing Icebergs at White Dwarfs

    Science.gov (United States)

    Stephan, Alexander P.; Naoz, Smadar; Zuckerman, B.

    2017-08-01

    White dwarfs (WDs) have atmospheres that are expected to consist nearly entirely of hydrogen and helium, since heavier elements will sink out of sight on short timescales. However, observations have revealed atmospheric pollution by heavier elements in about a quarter to a half of all WDs. While most of the pollution can be accounted for with asteroidal or dwarf planetary material, recent observations indicate that larger planetary bodies, as well as icy and volatile material from Kuiper belt analog objects, are also viable sources of pollution. The commonly accepted pollution mechanisms, namely scattering interactions between planetary bodies orbiting the WDs, can hardly account for pollution by objects with large masses or long-period orbits. Here we report on a mechanism that naturally leads to the emergence of massive body and icy and volatile material pollution. This mechanism occurs in wide binary stellar systems, where the mass loss of the planets’ host stars during post main sequence stellar evolution can trigger the Eccentric Kozai-Lidov mechanism. This mechanism leads to large eccentricity excitations, which can bring massive and long-period objects close enough to the WDs to be accreted. We find that this mechanism readily explains and is consistent with observations.

  17. A census of very-low-mass stars and brown dwarfs in the sigma Orionis cluster

    CERN Document Server

    Lodieu, N; Rebolo, R; Martín, E L; Hambly, N C

    2009-01-01

    (ABRIDGED) We have analysed the near-infrared photometric data from the Fourth Data Release (DR4) of the UKIRT Infrared Deep Sky Suvey (UKIDSS) Galactic Clusters Survey (GCS) to derive the cluster luminosity and mass functions, evaluate the extent of the cluster, and study the distribution and variability of low-mass stars and brown dwarfs down to the deuterium-burning limit. We have recovered most of the previously published members and found a total of 287 candidate members within the central 30 arcmin in the 0.5-0.009 Msun mass range, including new objects not previously reported in the literature. This new catalogue represents a homogeneous dataset of brown dwarf member candidates over the central 30 arcmin of the cluster. The expected photometric contamination by field objects with similar magnitudes and colours to sigma Orionis members is ~15%. We present evidence of variability at the 99.5% confidence level over ~yearly timescales in 10 member candidates that exhibit signs of youth and the presence of ...

  18. Magnetars and white dwarf pulsars

    Science.gov (United States)

    Lobato, Ronaldo V.; Malheiro, Manuel; Coelho, Jaziel G.

    2016-07-01

    The anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are a class of pulsars understood as neutron stars (NSs) with super strong surface magnetic fields, namely B ≳ 1014G, and for that reason are known as magnetars. However, in the last years, some SGRs/AXPs with low surface magnetic fields B ˜ (1012-1013)G have been detected, challenging the magnetar description. Moreover, some fast and very magnetic white dwarfs (WDs) have also been observed, and at least one showed X-ray energy emission as an ordinary pulsar. Following this fact, an alternative model based on WDs pulsars has been proposed to explain this special class of pulsars. In this model, AXPs and SGRs as dense and magnetized WDs can have surface magnetic field B ˜ 107-1010 G and rotate very fast with frequencies Ω ˜ 1rad/s, consistent with the observed rotation periods P ˜ (2-12)s.

  19. Dyson Spheres around White Dwarfs

    CERN Document Server

    Semiz, İbrahim

    2015-01-01

    A Dyson Sphere is a hypothetical structure that an advanced civilization might build around a star to intercept all of the star's light for its energy needs. One usually thinks of it as a spherical shell about one astronomical unit (AU) in radius, and surrounding a more or less Sun-like star; and might be detectable as an infrared point source. We point out that Dyson Spheres could also be built around white dwarfs. This type would avoid the need for artificial gravity technology, in contrast to the AU-scale Dyson Spheres. In fact, we show that parameters can be found to build Dyson Spheres suitable --temperature- and gravity-wise-- for human habitation. This type would be much harder to detect.

  20. SDSS DR7 White Dwarf Catalog

    CERN Document Server

    Kleinman, S J; Koester, D; Pelisoli, Ingrid; Peçanha, Viviane; Nitta, A; Costa, J E S; Krzesinski, J; Dufour, P; Lachapelle, F -R; Bergeron, P; Yip, Ching-Wa; Harris, Hugh C; Eisenstein, Daniel J; Althaus, L; Córsico, A

    2012-01-01

    We present a new catalog of spectroscopically-confirmed white dwarf stars from the Sloan Digital Sky Survey Data Release 7 spectroscopic catalog. We find 20,407 white dwarf spectra, representing 19,712 stars, and provide atmospheric model fits to 14,120 DA and 1011 DB white dwarf spectra from 12,843 and 923 stars, respectively. These numbers represent a more than factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalog based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log(g) if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for...

  1. SDSS DR7 WHITE DWARF CATALOG

    Energy Technology Data Exchange (ETDEWEB)

    Kleinman, S. J.; Nitta, A. [Gemini Observatory, 670 North A' ohoku Place, Hilo, HI 96720 (United States); Kepler, S. O.; Pelisoli, Ingrid; Pecanha, Viviane; Costa, J. E. S. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS (Brazil); Koester, D. [Institut fuer Theoretische Physik und Astrophysik, Universitaet Kiel, D-24098 Kiel (Germany); Krzesinski, J. [Mt. Suhora Observatory, Pedagogical University of Cracow, ul. Podchorazych 2, 30-084 Cracow (Poland); Dufour, P.; Lachapelle, F.-R.; Bergeron, P. [Departement de Physique, Universite de Montreal, C. P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada); Yip, Ching-Wa [Department of Physics and Astronomy, The Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218 (United States); Harris, Hugh C. [United States Naval Observatory, Flagstaff Station, 10391 West Naval Observatory Road, Flagstaff, AZ 86001-8521 (United States); Eisenstein, Daniel J. [Harvard Smithsonian Center for Astrophysics, 60 Garden Street, MS 20, Cambridge, MA 02138 (United States); Althaus, L.; Corsico, A., E-mail: hch@nofs.navy.mil [Facultad de Ciencias Astronomicas y Geofisicas, Paseo del Bosque S/N, (1900) La Plata (Argentina)

    2013-01-15

    We present a new catalog of spectroscopically confirmed white dwarf stars from the Sloan Digital Sky Survey (SDSS) Data Release 7 spectroscopic catalog. We find 20,407 white dwarf spectra, representing 19,712 stars, and provide atmospheric model fits to 14,120 DA and 1011 DB white dwarf spectra from 12,843 and 923 stars, respectively. These numbers represent more than a factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalogs based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log g if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for the DAs.

  2. White dwarf constraints on a varying $G$

    CERN Document Server

    García-Berro, Enrique; Althaus, Leandro G; Córsico, Alejandro H; Lorén-Aguilar, Pablo; Romero, Alejandra D; Isern, Jordi

    2013-01-01

    A secular variation of $G$ modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code, an up-to-date pulsational code, and a detailed population synthesis code we demonstrate that the effects of a running $G$ are obvious both in the properties of individual white dwarfs, and in those of the white dwarf populations in clusters. Specifically, we show that the white dwarf evolutionary sequences depend on both the value of $\\dot G/G$, and on the value of $G$ when the white dwarf was born. We show as well that the pulsational properties of variable white dwarfs can be used to constrain $\\dot G/G$. Finally, we also show that the ensemble properties of of white dwarfs in clusters can also be used to set upper bounds to $\\dot G/G$. Precisely, the tightest bound --- $\\dot G/G \\sim -1.8 10^{-12}$ yr$^{-1}$ --- is obtained studying the population of the old, metal-rich, well populated, open cluster NGC 6791. Less stringent upper limits can be obtained compari...

  3. Empirical Determination of Convection in Pulsating White Dwarfs

    Science.gov (United States)

    Provencal, Judith L.; Hermes, J. J.; Montgomery, M.; Reed, Mike; Shipman, Harry; Fraga, Luciano

    2013-02-01

    We propose high speed photometric observations of WD J1518+0658 with SOAR and the KPNO 2m as important components of a coordinated international campaign designed to survey the properties of convection in white dwarf atmospheres. Convection remains the largest source of theoretical uncertainty in our understanding of stellar physics. Asteroseismology has proven a powerful tool to attack this problem. White dwarf pulsations appear as local surface temperature variations. The extreme temperature sensitivity of convection leads to local variations in the convection zone's depth. This in turn modulates the local energy flux, producing nonsinusoidal light curves. The observed nonlinearities provide a self-consistent observational test of convection in white dwarf atmospheres. WD J1518+0658 is a member of the newly discovered class of extremely low mass white dwarf pulsators (ELMVs). ELMVs offer the opportunity to extend our investigation to unexplored regions of lower effective temperatures and surface gravities, where conditions are closer to those found in main sequence stars. High precision light curves from SOAR, combined with frequency, amplitude, and phase information provided by the KPNO 2m and the entire WET run, will allow us to recover WD J1518+0658's convective thermal response timescale.

  4. Axions and the cooling of white dwarf stars

    CERN Document Server

    Isern, J; Torres, S; Catalan, S

    2008-01-01

    White dwarfs are the end-product of the lifes of intermediate- and low-mass stars and their evolution is described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. We show here that the shape of the bright branch of this function is only sensitive to the averaged cooling rate of white dwarfs and we propose to use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. Our results indicate that the inclusion of the emission of axions in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function. The best fit is obtained for m_a cos^2 \\beta ~ 5 meV, where m_a is the mass of the axion and cos^2 \\beta is a free parameter. We also show that values larger than 10 meV are clearly excluded. The existin...

  5. Massive Double White Dwarfs and the AM CVn Birthrate

    CERN Document Server

    Kilic, Mukremin; Heinke, Craig O; Gianninas, A; Benni, P; Agueros, M A

    2016-01-01

    We present Chandra and Swift X-ray observations of four extremely low-mass (ELM) white dwarfs with massive companions. We place stringent limits on X-ray emission from all four systems, indicating that neutron star companions are extremely unlikely and that the companions are almost certainly white dwarfs. Given the observed orbital periods and radial velocity amplitudes, the total masses of these binaries are greater than 1.02 to 1.39 Msun. The extreme mass ratios between the two components make it unlikely that these binary white dwarfs will merge and explode as Type Ia or underluminous supernovae. Instead, they will likely go through stable mass transfer through an accretion disk and turn into interacting AM CVn. Along with three previously known systems, we identify two of our targets, J0811 and J2132, as systems that will definitely undergo stable mass transfer. In addition, we use the binary white dwarf sample from the ELM Survey to constrain the inspiral rate of systems with extreme mass ratios. This r...

  6. White Dwarf Period Tables I. Pulsators with hydrogen-dominated atmospheres

    Science.gov (United States)

    Bognar, Zs.; Sodor, A.

    2016-09-01

    We aimed at collecting all known white dwarf pulsators with hydrogen-dominated atmospheres and list their main photometric and atmospheric parameters together with their pulsation periods and amplitudes observed at different epochs. For this purpose, we explored the pulsating white dwarf related literature with the systematic use of the SIMBAD and the NASA's Astrophysics Data System (ADS) databases. We summarized our results in four tables listing seven ZZ Ceti stars in detached white dwarf plus main-sequence binaries, seven extremely low-mass DA pulsators, three hot DAVs and 180 ZZ Ceti stars.

  7. White Dwarf Period Tables - I. Pulsators with hydrogen-dominated atmospheres

    CERN Document Server

    Bognár, Zs

    2016-01-01

    We aimed at collecting all known white dwarf pulsators with hydrogen-dominated atmospheres and list their main photometric and atmospheric parameters together with their pulsation periods and amplitudes observed at different epochs. For this purpose, we explored the pulsating white dwarf related literature with the systematic use of the SIMBAD and the NASA's Astrophysics Data System (ADS) databases. We summarized our results in four tables listing seven ZZ Ceti stars in detached white dwarf plus main-sequence binaries, seven extremely low-mass DA pulsators, three hot DAVs and 180 ZZ Ceti stars.

  8. Merging White Dwarfs and Thermonuclear Supernovae

    CERN Document Server

    van Kerkwijk, Marten H

    2012-01-01

    Thermonuclear supernovae result when interaction with a companion reignites nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway, a catastrophic gain in pressure, and the disintegration of the whole white dwarf. It is usually thought that fusion is reignited in near-pycnonuclear conditions when the white dwarf approaches the Chandrasekhar mass. I briefly describe two long-standing problems faced by this scenario, and our suggestion that these supernovae instead result from mergers of carbon-oxygen white dwarfs, including those that produce sub-Chandrasekhar mass remnants. I then turn to possible observational tests, in particular those that test the absence or presence of electron captures during the burning.

  9. Magnetic white dwarfs with debris disks

    CERN Document Server

    Külebi, Baybars; Lorén-Aguilar, Pablo; Isern, Jordi; García-Berro, Enrique

    2012-01-01

    It has long been accepted that a possible mechanism for explaining the existence of magnetic white dwarfs is the merger of a binary white dwarf system, as there are viable mechanisms for producing sustainable magnetism within the merger product. However, the lack of rapid rotators in the magnetic white dwarf population has been always considered a problematic issue of this scenario. In order to explain this discrepancy we build a model in which the interaction between the magnetosphere of the star and the disk induces angular momentum transfer. Our model predicts that the magnetospheric interaction of magnetic white dwarfs with their disks results in a significant spin down, and we show that the observed rotation period of REJ 0317-853, which is suggested to be a product of a double degenerate merger, can be reproduced.

  10. White Dwarfs, Neutron Stars and Black Holes

    Science.gov (United States)

    Szekeres, P.

    1977-01-01

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

  11. 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.

  12. Pulsating White Dwarf Stars and Precision Asteroseismology

    Science.gov (United States)

    Winget, D. E.; Kepler, S. O.

    2008-09-01

    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.

  13. Abundance analysis of DAZ white dwarfs

    CERN Document Server

    Kawka, Adela; Dinnbier, Frantisek; Cibulkova, Helena; Nemeth, Peter

    2010-01-01

    We present an abundance analysis of a sample of 33 hydrogen-rich (DA) white dwarfs. We have used archival high-resolution spectra to measure abundances of calcium, magnesium and iron in a set of 30 objects. In addition, we present preliminary calcium abundances in three new white dwarfs based on low-dispersion spectra. We investigate some abundance ratios (Mg/Ca, Fe/Ca) that may help uncover the composition of the accretion source.

  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. Comparison of theoretical white dwarf cooling timescales

    CERN Document Server

    Salaris, Maurizio; García-Berro, Enrique

    2013-01-01

    An accurate assessment of white dwarf cooling times is paramount to place white dwarf cosmochronology of Galactic populations on more solid grounds. This issue is particularly relevant in view of the enhanced observational capabilities provided by the next generation of Extremely Large Telescopes, that will offer more avenues to employ white dwarfs as probes of Galactic evolution and test-beds of fundamental physics. We estimate for the first time the consistency of results obtained from two independent and widely used evolutionary codes (BaSTI and LPCODE) for white dwarf models with fixed mass and chemical stratification, when the same input physics is employed in both codes. We considered 0.55Msun white dwarf models with both pure carbon and uniform carbon-oxygen (50/50 mass fractions) core. We have assessed for the first time the maximum possible accuracy in the current estimates of white dwarf cooling times, resulting only from the different implementations of the stellar evolution equations and homogeneo...

  16. ON THE EVOLUTION OF MAGNETIC WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Tremblay, P.-E. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Fontaine, G.; Brassard, P. [Département de Physique, Université de Montréal, C. P. 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7 (Canada); Freytag, B. [Department of Physics and Astronomy at Uppsala University, Regementsvägen 1, Box 516, SE-75120 Uppsala (Sweden); Steiner, O. [Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, D-79104 Freiburg (Germany); Ludwig, H.-G. [Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117 Heidelberg (Germany); Steffen, M. [Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, D-14482 Potsdam (Germany); Wedemeyer, S., E-mail: tremblay@stsci.edu [Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo (Norway)

    2015-10-10

    We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1–50 kG, which is much smaller than the typical 1–1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B ≳ 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (T{sub eff}) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with T{sub eff} ≲ 10,000 K cool significantly slower than non-magnetic degenerates.

  17. The contribution of red dwarfs and white dwarfs to the halo dark matter

    CERN Document Server

    Torres, S; Isern, J; García-Berro, E

    2008-01-01

    The nature of the several microlensing events observed by the MACHO team towards the LMC still remains controversial. Low-mass substellar objects and stars with masses larger than ~1 M_{sun} have been ruled out as major components of a MACHO galactic halo, while stars of half solar masses are the most probable candidates. In this paper we assess jointly the relative contributions of both red dwarfs and white dwarfs to the mass budget of the galactic halo. In doing so we use a Monte Carlo simulator which incorporates up-to-date evolutionary sequences of both red dwarfs and white dwarfs as well as detailed descriptions of our Galaxy and of the LMC. We explore the complete mass range between 0.08 and 1 M_{sun} as possible microlensing candidates and we compare the synthetic populations obtained with our simulator with the results obtained by the MACHO and EROS experiments. Our results indicate that, despite that the contribution of the red dwarf population increases by a factor of 2 the value of the optical dept...

  18. Building Magnetic Fields in White Dwarfs

    Science.gov (United States)

    Kohler, Susanna

    2017-03-01

    White dwarfs, the compact remnants left over at the end of low- and medium-mass stars lifetimes, are often found to have magnetic fields with strengths ranging from thousands to billions of times that of Earth. But how do these fields form?MultiplePossibilitiesAround 1020% of white dwarfs have been observed to have measurable magnetic fields with a wide range of strengths. There are several theories as to how these fields might be generated:The fields are fossil.The original weak magnetic fields of the progenitor stars were amplified as the stars cores evolved into white dwarfs.The fields are caused by binary interactions.White dwarfs that formed in the merger of a binary pair might have had a magnetic field amplified as a result of a dynamo that was generated during the merger.The fields were produced by some other internal physical mechanism during the cooling of the white dwarf itself.In a recent publication, a team of authors led by Jordi Isern (Institute of Space Sciences, CSIC, and Institute for Space Studies of Catalonia, Spain) explored this third possibility.Dynamos from CrystallizationThe inner and outer boundaries of the convective mantle of carbon/oxygen white dwarfs of two different masses (top vs. bottom panel) as a function of luminosity. As the white dwarf cools (toward the right), the mantle grows thinner due to the crystallization and settling of material. [Isern et al. 2017]As white dwarfs have no nuclear fusion at their centers, they simply radiate heat and gradually cool over time. The structure of the white dwarf undergoes an interesting change as it cools, however: though the object begins as a fluid composed primarily of an ionized mixture of carbon and oxygen (and a few minor species like nickel and iron), it gradually crystallizes as its temperature drops.The crystallized phase of the white dwarf is oxygen-rich which is denser than the liquid, so the crystallized material sinks to the center of the dwarf as it solidifies. As a result, the

  19. 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...

  20. Evidence for Blow-out in the Low-mass Dwarf Galaxy Holmberg I

    CERN Document Server

    Ott, J; Brinks, E; Van Dyk, S D; Dirsch, B; Klein, U; Ott, Juergen; Walter, Fabian; Brinks, Elias; Dyk, Schuyler D. Van; Dirsch, Boris; Klein, Ulrich

    2001-01-01

    We present radio and optical observations of Holmberg I (HoI), a member of the M81 group of galaxies (distance 3.6Mpc). HoI is a low-mass, low surface- brightness dwarf galaxy. High-resolution multi-array VLA HI observations reveal a supergiant shell (diameter: 1.7 kpc) which covers about half the optical extent of HoI and which comprises 75% of the total HI content (total HI mass: 1.1 10^8 M_o). We set a tentative upper limit to the dark matter content of < 3.1 10^8 M_o. The HI data are complemented by deep, optical UBV(RI)_c and Halpha observations obtained at the Calar Alto 2.2m telescope. We find M_HI/L_B = 1.1 M_o/L_B_o. The total visible (stars plus gas) mass of HoI adds up to 2.4 10^8 M_o. This leads to a total mass of < 5.5 10^8 M_o. The origin of HoI's peculiar HI morphology is discussed in terms of a supergiant shell created by strong stellar winds and supernova explosions (energy: equiv. 20-260 type II SN; age: 80+-20 Myr). The morphological center of HoI is offset by 0.75 kpc with respect to...

  1. 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...

  2. 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...

  3. 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...

  4. Building an Unusual White-Dwarf Duo

    Science.gov (United States)

    Kohler, Susanna

    2016-09-01

    A new study has examined how the puzzling wide binary system HS 2220+2146 which consists of two white dwarfs orbiting each other might have formed. This system may be an example of a new evolutionary pathway for wide white-dwarf binaries.Evolution of a BinaryMore than 100 stellar systems have been discovered consisting of two white dwarfs in a wide orbit around each other. How do these binaries form? In the traditional picture, the system begins as a binary consisting of two main-sequence stars. Due to the large separation between the stars, the stars evolve independently, each passing through the main-sequence and giant branches and ending their lives as white dwarfs.An illustration of a hierarchical triple star system, in which two stars orbit each other, and a third star orbits the pair. [NASA/JPL-Caltech]Because more massive stars evolve more quickly, the most massive of the two stars in a binary pair should be the first to evolve into a white dwarf. Consequently, when we observe a double-white-dwarf binary, its usually a safe bet that the more massive of the two white dwarfs will also be the older and cooler of the pair, since it should have formed first.But in the case of the double-white-dwarf binary HS 2220+2146, the opposite is true: the more massive of the two white dwarfs appears to be the younger and hotter of the pair. If it wasnt created in the traditional way, then how did this system form?Two From Three?Led by Jeff Andrews (Foundation for Research and Technology-Hellas, Greece and Columbia University), a team of scientists recently examined this system more carefully, analyzing its spectra to confirm our understanding of the white dwarfs temperatures and masses.Based on their observations, Andrews and collaborators determined that there are no hidden additional companions that could have caused the unusual evolution of this system. Instead, the team proposed that this unusual binary might be an example of an evolutionary channel that involves three

  5. Brown dwarfs as close companions to white dwarfs

    Science.gov (United States)

    Stringfellow, Guy S.; Bodenheimer, Peter; Black, David C.

    1990-01-01

    The influence of the radiation flux emitted by a white dwarf primary on the evolution of a closely orbiting brown dwarf (BD) companion is investigated. Full stellar evolutionary calculations are presented for both isolated and thermal bath cases, including effects of large variations in the atmospheric grain opacities. High grain opacities significantly increase the radii of the BDs, but the thermal bath does not. The major influence of the thermal bath is to increase substantially the surface temperature and luminosity of the BD at a given age. These results are compared with the observational properties of the possible BD companion of the white dwarf G29-38. Inclusion of both physical effects, high grain opacities and thermal bath, increases the mass range (0.034-0.063 solar masses) of viable models significantly, yet the final determination of whether the object is indeed a BD requires improvements in the observations of the system's properties.

  6. Latest Results from the DODO Survey: Imaging Planets around White Dwarfs

    Science.gov (United States)

    Hogan, Emma; Burleigh, Matt R.; Clarke, Fraser J.

    2011-03-01

    The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures lower (DODO survey has the ability to directly image planets in post-main sequence analogues of these systems. These proceedings present the latest results of our multi-epoch J band common proper motion survey of nearby white dwarfs.

  7. Properties and Star Formation Histories of Intermediate Redshift Dwarf Low-Mass Star-Forming Galaxies

    Science.gov (United States)

    Rodríguez-Muñoz, L.; Gallego, J.; Pacifici, C.; Tresse, L.; Charlot, S.; Gil de Paz, A.; Barro, G.; Villar, V.

    2017-03-01

    The epoch when low-mass star-forming galaxies (LMSFGs) form the bulk of their stellar mass is uncertain. While some models predict an early formation, others favor a delayed scenario until later ages of the Universe. We present improved constraints on the physical properties and star formation histories (SFHs) of a sample of intermediate redshift LMSFGs selected by their stellar mass or blue-compact-dwarf-like properties. Our work takes advantage of the deep UV-to-FIR photometric coverage available on the Extended-Chandra Deep Field South and our own dedicated deep VLT/VIMOS optical spectroscopy programs. On the one hand, we estimate the stellar mass (M_{*}), star formation rate (SFR), and SFH of each galaxy modeling its spectral energy distribution. We use a novel approach by Pacifici et al. 2012, that (1) consistently combines photometric (broad-band) and spectroscopic (emission line fluxes and equivalent widths) data, and (2) uses physically-motivated SFHs with non-uniform variations of the SFR as a function of time. On the other hand, we characterize the properties of their interstellar medium by analyzing the emission line features visible in the VIMOS spectroscopy. The final sample includes 91 spectroscopically confirmed LMSFGs (7.3 ≤ logM_{*}/M_{⊙} ≤ 9.5) at 0.3

  8. Hints for Small Disks around Very Low Mass Stars and Brown Dwarfs

    Science.gov (United States)

    Hendler, Nathanial P.; Mulders, Gijs D.; Pascucci, Ilaria; Greenwood, Aaron; Kamp, Inga; Henning, Thomas; Ménard, François; Dent, William R. F.; Evans, Neal J., II

    2017-06-01

    The properties of disks around brown dwarfs and very low mass stars (hereafter VLMOs) provide important boundary conditions on the process of planet formation and inform us about the numbers and masses of planets than can form in this regime. We use the Herschel Space Observatory PACS spectrometer to measure the continuum and [O i] 63 μm line emission toward 11 VLMOs with known disks in the Taurus and Chamaeleon I star-forming regions. We fit radiative transfer models to the spectral energy distributions of these sources. Additionally, we carry out a grid of radiative transfer models run in a regime that connects the luminosity of our sources with brighter T Tauri stars. We find that VLMO disks with sizes 1.3-78 au, smaller than typical T Tauri disks, fit well the spectral energy distributions assuming that disk geometry and dust properties are stellar mass independent. Reducing the disk size increases the disk temperature, and we show that VLMOs do not follow previously derived disk temperature-stellar luminosity relationships if the disk outer radius scales with stellar mass. Only 2 out of 11 sources are detected in [O i] despite a better sensitivity than was achieved for T Tauri stars, suggesting that VLMO disks are underluminous. Using thermochemical models, we show that smaller disks can lead to the unexpected [O i] 63 μm nondetections in our sample. The disk outer radius is an important factor in determining the gas and dust observables. Hence, spatially resolved observations with ALMA—to establish if and how disk radii scale with stellar mass—should be pursued further. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

  9. New cooling sequences for old white dwarfs

    CERN Document Server

    Renedo, Isabel; Bertolami, Marcelo M Miller; Romero, Alejandra D; Corsico, Alejandro H; Rohrmann, Rene D; Garcia-Berro, Enrique

    2010-01-01

    We present full evolutionary calculations appropriate for the study of hydrogen-rich DA white dwarfs. This is done by evolving white dwarf progenitors from the zero age main sequence, through the core hydrogen burning phase, the helium burning phase and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. Complete evolutionary sequences are computed for a wide range of stellar masses and for two different metallicities: Z=0.01, which is representative of the solar neighborhood, and Z=0.001, which is appropriate for the study of old stellar systems, like globular clusters. During the white dwarf cooling stage we compute self-consistently the phase in which nuclear reactions are still important, the diffusive evolution of the elements in the outer layers and, finally, we also take into account all the relevant energy sources in the deep interior of the white dwarf, like the release of latent heat and the release of gravitational energy due to carbon-oxygen phase separation upon crystall...

  10. The (Double) White Dwarf Binary SDSS 1257+5428

    CERN Document Server

    Kulkarni, S R

    2010-01-01

    SDSS 1257+5428 is a white dwarf in a close orbit with a companion that has been suggested to be a neutron star. If so, it hosts the closest known neutron star, and its existence implies a great abundance of similar systems and a rate of white-dwarf neutron-star mergers similar to that of the type Ia supernova rate. Here, we present high signal-to-noise spectra of SDSS 1257+5428, which confirm an independent finding that the system is in fact composed of two white dwarfs, one relatively cool and with low mass, and the other hotter and more massive. With this, the demographics and merger rate are no longer puzzling (various factors combine to lower the latter by more than two orders of magnitude). We show that the spectra are fit well with a combination of two hydrogen model atmospheres, as long as the lines of the higher-gravity component are broadened significantly relative to what is expected from just pressure broadening. Interpreting this additional broadening as due to rotation, the inferred spin period i...

  11. Hubble COS Spectroscopy of the Dwarf Nova CW Mon: The White Dwarf in Quiescence?

    Science.gov (United States)

    Hause, Connor; Sion, Edward M.; Godon, Patrick; Gänsicke, Boris T.; Szkody, Paula; de Martino, Domitilla; Pala, Anna

    2017-08-01

    We present a synthetic spectral analysis of the HST COS spectrum of the U Geminorum-type dwarf nova CW Mon, taken during quiescence as part of our COS survey of accreting white dwarfs (WDs) in Cataclysmic Variables. We use a synthetic photosphere and optically thick accretion disk spectra to model the COS spectrum as well as archival IUE spectra obtained decades ago, when the system was in an even deeper quiescent state. Assuming a reddening of E(B-V) = 0.06, an inclination of 60° (CW Mon has eclipses of the accretion disk), and a WD mass of 0.8 {M}⊙ , our results indicate the presence of a 22-27,000 K WD and a low mass accretion rate \\dot{M}≲ {10}-10 {M}⊙ {{yr}}-1, for a derived distance of ˜200 to ˜300 pc. Based on observations made with the NASA-Hubble Space Telescope.

  12. 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...

  13. Mystery of a Dimming White Dwarf

    Science.gov (United States)

    Kohler, Susanna

    2015-12-01

    In the wake of the recent media attention over an enigmatic, dimming star, another intriguing object has been discovered: J1529+2928, a white dwarf that periodically dims. This mystery, however, may have a simple solution with interesting consequences for future surveys of white dwarfs.Unexpected VariabilityJ1529+2928 is an isolated white dwarf that appears to have a mass of slightly more than the Sun. But rather than radiating steadily, J1529+2928 dims once every 38 minutes almost as though it were being eclipsed.The team that discovered these variations, led by Mukremin Kilic (University of Oklahoma), used telescopes at the Apache Point Observatory and the McDonald Observatory to obtain follow-up photometric data of J1529+2928 spread across 66 days. The team also took spectra of the white dwarf with the Gemini North telescope.Kilic and collaborators then began, one by one, to rule out possible causes of this objects variability.Eliminating OptionsThe period of the variability is too long for J1529+2928 to be a pulsating white dwarf with luminosity variation caused by gravity-wave pulsations.The variability cant be due to an eclipse by a stellar or brown-dwarf companion, because there isnt any variation in J1529+2928s radial velocity.Its not due to the orbit of a solid-body planetary object; such a transit would be too short to explain observations.It cant be due to the orbit of a disintegrated planet; this wouldnt explain the light curves observed in different filters plus the light curve doesnt change over the 66-day span.Spotty SurfaceTop and middle two panels: light curves from three different nights observing J1529+2928s periodic dimming. Bottom panel: The Fourier transform shows a peak at 37.7 cycles/day (and another, smaller peak at its first harmonic). [Kilic et al. 2015]So what explanation is left? The authors suggest that J1529+2928s variability is likely caused by a starspot on the white dwarfs surface that rotates into and out of our view. Estimates

  14. The formation of high-field magnetic white dwarfs from common envelopes

    CERN Document Server

    Nordhaus, J; Spiegel, D S; Metzger, B D; Blackman, E G

    2010-01-01

    The origin of highly-magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star is engulfed by a post-main-sequence giant, the hydrodynamic drag in the envelope of the giant leads to a reduction of the companion's orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute...

  15. Formation of high-field magnetic white dwarfs from common envelopes.

    Science.gov (United States)

    Nordhaus, Jason; Wellons, Sarah; Spiegel, David S; Metzger, Brian D; Blackman, Eric G

    2011-02-22

    The origin of highly magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star, is engulfed by a post-main-sequence giant, gravitational torques in the envelope of the giant lead to a reduction of the companion's orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute to the origin of magnetar fields.

  16. 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.

  17. Pulsating White Dwarfs in Globular Clusters

    Science.gov (United States)

    Kanaan, A.; Zabot, A.; Fraga, L.

    2012-09-01

    We present our current efforts to detect pulsating white dwarfs in globular clusters and analyze the future of this area when the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT) and the Thirty-Meter Telescope (TMT) all become operational. Today we are able to detect pulsating white dwarfs in M 4, NGC 6397 and NGC 6752. When ELT comes on line we should be able to improve the quality of data for the nearby clusters and push the limit to at least 3 magnitudes further, up to NGC 6626, increasing the number of observable clusters from 3 to 20.

  18. White Dwarf Pulsational Constraints on Stellar Evolution

    Science.gov (United States)

    Dunlap, Bart H.; Clemens, J. Christopher; O'Brien, Patrick C.; Hermes, J. J.; Fuchs, Joshua T.

    2017-01-01

    The complex processes that convert a protostellar cloud into a carbon/oxygen-core white dwarf star are distilled and modeled in state of the art stellar evolution codes. Many of these processes are well-constrained, but several are uncertain or must be parameterized in the models because a complete treatment would be computationally prohibitive—turbulent motions such as convective overshoot cannot, for example, be modeled in 1D. Various free parameters in the models must therefore be calibrated. We will discuss how white dwarf pulsations can inform such calibrations. The results of all prior evolution are cemented into the interiors of white dwarf stars and, so, hidden from view. However, during certain phases of their cooling, pulsations translate the star's evolutionary history into observable surface phenomena. Because the periods of a pulsating white dwarf star depend on an internal structure assembled as it evolved to its final state, white dwarf pulsation periods can be viewed as observable endpoints of stellar evolution. For example, the thickness of the helium layer in a white dwarf directly affects its pulsations; the observed periods are, therefore, a function of the number of thermal pulses during which the star converts helium into core material on the asymptotic giant branch. Because they are also a function of several other significant evolutionary processes, several pulsation modes are necessary to tease all of these apart. Unfortunately, white dwarf pulsators typically do not display enough oscillation modes to constrain stellar evolution. To avoid this limitation, we consider the pulsations of the entire collection of hot pulsating hydrogen-atmosphere white dwarf stars (DAVs). Though any one star may not have sufficient information to place interesting constraints on its evolutionary history, taken together, the stars show a pattern of modes that allows us to test evolutionary models. For an example set of published evolutionary models, we show a

  19. The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters

    Science.gov (United States)

    Luhman, K. L.; Rieke, G. H.; Young, Erick T.; Cotera, Angela S.; Chen, H.; Rieke, Marcia J.; Schneider, Glenn; Thompson, Rodger I.

    2000-09-01

    We have obtained images of the Trapezium Cluster (140''×140'' 0.3 pc×0.3 pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Combining these data with new ground-based K-band spectra (R=800) and existing spectral types and photometry, we have constructed an H-R diagram and used it and other arguments to infer masses and ages. To allow comparison with the results of our previous studies of IC 348 and ρ Oph, we first use the models of D'Antona & Mazzitelli. With these models, the distributions of ages of comparable samples of stars in the Trapezium, ρ Oph, and IC 348 indicate median ages of ~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass initial mass functions (IMFs) in these sites of clustered star formation are similar over a wide range of stellar densities (ρ Oph, n=0.2-1×103 pc-3 IC 348, n=1×103 pc-3 Trapezium, n=1-5×104 pc-3) and other environmental conditions (e.g., presence or absence of OB stars). With current data, we cannot rule out modest variations in the substellar mass functions among these clusters. We then make the best estimate of the true form of the IMF in the Trapezium by using the evolutionary models of Baraffe et al. and an empirically adjusted temperature scale and compare this mass function to recent results for the Pleiades and the field. All of these data are consistent with an IMF that is flat or rises slowly from the substellar regime to about 0.6 Msolar and then rolls over into a power law that continues from about 1 Msolar to higher masses with a slope similar to or somewhat larger than the Salpeter value of 1.35. For the Trapezium, this behavior holds from our completeness limit of ~0.02 Msolar and probably, after a modest completeness correction, even from 0.01-0.02 Msolar. These data include ~50 likely brown dwarfs. We test the predictions of theories of the IMF against (1) the shape of the IMF, which is not log-normal, in clusters and the field, (2) the

  20. TiNy Titans: The Role of Dwarf-Dwarf Interactions in the Evolution of Low Mass Galaxies

    CERN Document Server

    Stierwalt, S; Patton, D; Johnson, K; Kallivayalil, N; Putman, M; Privon, G; Ross, G

    2014-01-01

    We introduce TiNy Titans (TNT), the first systematic study of star formation and the subsequent processing of the interstellar medium in interacting dwarf galaxies. Here we present the first results from a multiwavelength observational program based on a sample of 104 dwarf galaxy pairs selected from a range of environments within the SDSS and caught in various stages of interaction. The TNT dwarf pairs span mass ratios of M1/M2 100 A, occur in 20% of TNT dwarf pairs, regardless of environment, compared to only 6-8% of matched unpaired dwarfs. Starbursts can be triggered throughout the merger (out to large pair separations) and not just approaching coalescence. Despite their enhanced star formation, most TNT dwarf pairs have similar gas fractions relative to unpaired dwarfs. Thus, there may be significant reservoirs of diffuse, non-starforming gas surrounding the dwarf pairs or the gas consumption timescales may be long in the starburst phase. The only TNT dwarf pairs with low gas fractions (fgas <0.4) and...

  1. VERY LOW MASS STELLAR AND SUBSTELLAR COMPANIONS TO SOLAR-LIKE STARS FROM MARVELS. IV. A CANDIDATE BROWN DWARF OR LOW-MASS STELLAR COMPANION TO HIP 67526

    Energy Technology Data Exchange (ETDEWEB)

    Jiang Peng; Ge Jian; De Lee, Nathan; Fleming, Scott W.; Lee, Brian L.; Ma Bo; Wang, Ji [Astronomy Department, University of Florida, 211 Bryant Space Science Center, P.O. Box 112055, Gainesville, FL 32611 (United States); Cargile, Phillip; Hebb, Leslie; Stassun, Keivan G. [Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States); Crepp, Justin R. [Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556 (United States); Porto de Mello, Gustavo F.; Ferreira, Leticia D. [Observatorio do Valongo, Universidade Federal do Rio de Janeiro, Ladeira do Pedro Antonio, 43, CEP: 20080-090, Rio de Janeiro, RJ (Brazil); Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I. [Instituto de Astrofisica de Canarias, C/Via Lactea S/N, E-38200 La Laguna (Spain); Gaudi, B. Scott [Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Ghezzi, Luan [Laboratorio Interinstitucional de e-Astronomia (LIneA), Rio de Janeiro, RJ 20921-400 (Brazil); Wisniewski, John P. [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks Street, Norman, OK 73019 (United States); Agol, Eric, E-mail: jpaty@mail.ustc.edu.cn [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195-1580 (United States); and others

    2013-09-15

    We report the discovery of a candidate brown dwarf (BD) or a very low mass stellar companion (MARVELS-5b) to the star HIP 67526 from the Multi-object Apache point observatory Radial Velocity Exoplanet Large-area Survey (MARVELS). The radial velocity curve for this object contains 31 epochs spread over 2.5 yr. Our Keplerian fit, using a Markov Chain Monte Carlo approach, reveals that the companion has an orbital period of 90.2695{sup +0.0188}{sub -0.0187} days, an eccentricity of 0.4375 {+-} 0.0040, and a semi-amplitude of 2948.14{sup +16.65}{sub -16.55} m s{sup -1}. Using additional high-resolution spectroscopy, we find the host star has an effective temperature T{sub eff} = 6004 {+-} 34 K, a surface gravity log g (cgs) =4.55 {+-} 0.17, and a metallicity [Fe/H] =+0.04 {+-} 0.06. The stellar mass and radius determined through the empirical relationship of Torres et al. yields 1.10 {+-} 0.09 M{sub Sun} and 0.92 {+-} 0.19 R{sub Sun }. The minimum mass of MARVELS-5b is 65.0 {+-} 2.9M{sub Jup}, indicating that it is likely to be either a BD or a very low mass star, thus occupying a relatively sparsely populated region of the mass function of companions to solar-type stars. The distance to this system is 101 {+-} 10 pc from the astrometric measurements of Hipparcos. No stellar tertiary is detected in the high-contrast images taken by either FastCam lucky imaging or Keck adaptive optics imaging, ruling out any star with mass greater than 0.2 M{sub Sun} at a separation larger than 40 AU.

  2. Very Low Mass Stellar and Substellar Companions to Solar-like Stars from MARVELS. IV. A Candidate Brown Dwarf or Low-mass Stellar Companion to HIP 67526

    Science.gov (United States)

    Jiang, Peng; Ge, Jian; Cargile, Phillip; Crepp, Justin R.; De Lee, Nathan; Porto de Mello, Gustavo F.; Esposito, Massimiliano; Ferreira, Letícia D.; Femenia, Bruno; Fleming, Scott W.; Gaudi, B. Scott; Ghezzi, Luan; González Hernández, Jonay I.; Hebb, Leslie; Lee, Brian L.; Ma, Bo; Stassun, Keivan G.; Wang, Ji; Wisniewski, John P.; Agol, Eric; Bizyaev, Dmitry; Brewington, Howard; Chang, Liang; Nicolaci da Costa, Luiz; Eastman, Jason D.; Ebelke, Garrett; Gary, Bruce; Kane, Stephen R.; Li, Rui; Liu, Jian; Mahadevan, Suvrath; Maia, Marcio A. G.; Malanushenko, Viktor; Malanushenko, Elena; Muna, Demitri; Nguyen, Duy Cuong; Ogando, Ricardo L. C.; Oravetz, Audrey; Oravetz, Daniel; Pan, Kaike; Pepper, Joshua; Paegert, Martin; Allende Prieto, Carlos; Rebolo, Rafael; Santiago, Basilio X.; Schneider, Donald P.; Shelden Bradley, Alaina C.; Sivarani, Thirupathi; Snedden, Stephanie; van Eyken, J. C.; Wan, Xiaoke; Weaver, Benjamin A.; Zhao, Bo

    2013-09-01

    We report the discovery of a candidate brown dwarf (BD) or a very low mass stellar companion (MARVELS-5b) to the star HIP 67526 from the Multi-object Apache point observatory Radial Velocity Exoplanet Large-area Survey (MARVELS). The radial velocity curve for this object contains 31 epochs spread over 2.5 yr. Our Keplerian fit, using a Markov Chain Monte Carlo approach, reveals that the companion has an orbital period of 90.2695^{+0.0188}_{-0.0187} days, an eccentricity of 0.4375 ± 0.0040, and a semi-amplitude of 2948.14^{+16.65}_{-16.55} m s-1. Using additional high-resolution spectroscopy, we find the host star has an effective temperature T eff = 6004 ± 34 K, a surface gravity log g (cgs) =4.55 ± 0.17, and a metallicity [Fe/H] =+0.04 ± 0.06. The stellar mass and radius determined through the empirical relationship of Torres et al. yields 1.10 ± 0.09 M ⊙ and 0.92 ± 0.19 R ⊙. The minimum mass of MARVELS-5b is 65.0 ± 2.9M Jup, indicating that it is likely to be either a BD or a very low mass star, thus occupying a relatively sparsely populated region of the mass function of companions to solar-type stars. The distance to this system is 101 ± 10 pc from the astrometric measurements of Hipparcos. No stellar tertiary is detected in the high-contrast images taken by either FastCam lucky imaging or Keck adaptive optics imaging, ruling out any star with mass greater than 0.2 M ⊙ at a separation larger than 40 AU.

  3. The Observational Signature of Old White Dwarfs

    CERN Document Server

    Hansen, B M S

    1998-01-01

    The recent discovery of microlensing of stars in the Large Magellanic Cloud has excited much interest in the nature of the lensing population. Detailed analyses indicate that the mass of these objects ranges from 0.3-0.8 solar masses, suggesting that they might be white dwarfs, the faint remnants of stellar evolution. The confirmation of such an hypothesis would yield profound insights into the early history of our galaxy and the early generations of stars in the universe. Previous attempts have been made to place theoretical constraints on this scenario, but were unduly pessimistic because they relied on inadequate evolutionary models. Here we present the first results from detailed evolutionary models appropriate for the study of white dwarfs of truly cosmological vintage. We find that the commonly held notion that old white dwarfs are red to hold only for helium atmosphere dwarfs and that hydrogen atmosphere dwarfs will be blue, with colours similar to those of the faint point sources found in the Hubble D...

  4. The Discovery of Binary White Dwarfs that will Merge within 500 Myr

    CERN Document Server

    Kilic, Mukremin; Prieto, Carlos Allende; Kenyon, S J

    2009-01-01

    We present radial velocity observations of four extremely low-mass (0.2 Msol) white dwarfs. All four stars show peak-to-peak radial velocity variations of 540 - 710 km/s with 1.0 - 5.9 hr periods. The optical photometry rules out main-sequence companions. In addition, no milli-second pulsar companions are detected in radio observations. Thus the invisible companions are most likely white dwarfs. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.44 Msol; thus those are carbon/oxygen core white dwarfs. However, the chance of a supernova Ia event is only 1% to 5%. These systems will most likely form single R Coronae Borealis stars, providing evidence for a white dwarf + white dwarf merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70% chance of having a low-mass white d...

  5. DA white dwarfs in the Kepler field

    Science.gov (United States)

    Doyle, T. F.; Howell, S. B.; Petit, V.; Lépine, S.

    2017-01-01

    We present 16 new, and confirm 7 previously identified, DA white dwarfs in the Kepler field through ground-based spectroscopy with the Hale 200″, Kitt Peak 4-m, and Bok 2.3-m telescopes. Using atmospheric models, we determine their effective temperatures and surface gravities to constrain their position with respect to the ZZ Ceti (DA pulsator) instability strip, and look for the presence or absence of pulsation with Kepler's unprecedented photometry. Our results are as follows. (i) From our measurements of temperature and surface gravity, 12 of the 23 DA white dwarfs from this work fall well outside of the instability strip. The Kepler photometry available for 11 of these WDs allows us to confirm that none are pulsating. One of these 11 happens to be a presumed binary, KIC 11604781, with a period of ˜5 d. (ii) The remaining 11 DA white dwarfs are instability strip candidates, potentially falling within the current, empirical instability strip, after accounting for uncertainties. These WDs will help constrain the strip's location further, as eight are near the blue edge and three are near the red edge of the instability strip. Four of these WDs do not have Kepler photometry, so ground-based photometry is needed to determine the pulsation nature of these white dwarfs. The remaining seven have Kepler photometry available, but do not show any periodicity on typical WD pulsation time-scales.

  6. Magnetic white dwarfs with debris discs

    CERN Document Server

    Külebi, Baybars; Lorén-Aguilar, Pablo; Isern, Jordi; García-Berro, Enrique

    2013-01-01

    It has long been accepted that a possible mechanism for explaining the existence of magnetic white dwarfs is the merger of a binary white dwarf system, as there are viable mechanisms for producing sustainable magnetic fields within the merger product. However, the lack of rapid rotators in the magnetic white dwarf population has been always considered a problematic issue of this scenario. Smoothed Particle Hydrodynamics simulations show that in mergers in which the two white dwarfs have different masses a disc around the central compact object is formed. If the central object is magnetized it can interact with the disc through its magnetosphere. The torque applied by the disc changes the spin of the star, whereas the transferred angular momentum from the star to the disc determines the properties of the disc. In this work we build a model for the disc evolution under the effect of magnetic accretion, and for the angular momentum evolution of the star, which can be compared with the observations. Our model pre...

  7. The 25 parsec local white dwarf population

    Science.gov (United States)

    Holberg, J. B.; Oswalt, T. D.; Sion, E. M.; McCook, G. P.

    2016-11-01

    We have extended our detailed survey of the local white dwarf population from 20 to 25 pc, effectively doubling the sample volume, which now includes 232 stars. In the process, new stars within 20 pc have been added, a more uniform set of distance estimates as well as improved spectral and binary classifications are available. The present 25 pc sample is estimated to be about 68 per cent complete (the corresponding 20 pc sample is now 86 per cent complete). The space density of white dwarfs is unchanged at 4.8 ± 0.5 × 10-3 pc-3. This new study includes a white dwarf mass distribution and luminosity function based on the 232 stars in the 25 pc sample. We find a significant excess of single stars over systems containing one or more companions (74 per cent versus 26 per cent). This suggests mechanisms that result in the loss of companions during binary system evolution. In addition, this updated sample exhibits a pronounced deficiency of nearby `Sirius-like' systems. 11 such systems were found within the 20 pc volume versus only one additional system found in the volume between 20 and 25 pc. An estimate of white dwarf birth rates during the last ˜8 Gyr is derived from individual remnant cooling ages. A discussion of likely ways new members of the local sample may be found is provided.

  8. On The Evolution of Magnetic White Dwarfs

    CERN Document Server

    Tremblay, P -E; Freytag, B; Steiner, O; Ludwig, H -G; Steffen, M; Wedemeyer, S; Brassard, P

    2015-01-01

    We present the first radiation magnetohydrodynamics simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-beta parameter, the thermal to magnetic pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have then employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B > 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (Teff) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection owing to the convective cou...

  9. DA White Dwarfs in the Kepler Field

    Science.gov (United States)

    Doyle, T. F.; Howell, S. B.; Petit, V.; Lépine, S.

    2016-10-01

    We present 16 new, and confirm 7 previously identified, DA white dwarfs in the Kepler field through ground-based spectroscopy with the Hale 200″, Kitt Peak 4-meter, and Bok 2.3-meter telescopes. Using atmospheric models we determine their effective temperatures and surface gravities to constrain their position with respect to the ZZ Ceti (DA pulsator) instability strip, and look for the presence or absence of pulsation with Kepler's unprecedented photometry. Our results are as follows: i) From our measurements of temperature and surface gravity, 12 of the 23 DA white dwarfs from this work fall well outside of the instability strip. The Kepler photometry available for 11 of these WDs allows us to confirm that none are pulsating. One of these eleven happens to be a presumed binary, KIC 11604781, with a period of ˜5 days. ii) The remaining 11 DA white dwarfs are instability strip candidates, potentially falling within the current, empirical instability strip, after accounting for uncertainties. These WDs will help constrain the strip's location further, as eight are near the blue edge and three are near the red edge of the instability strip. Four of these WDs do not have Kepler photometry, so ground-based photometry is needed to determine the pulsation nature of these white dwarfs. The remaining seven have Kepler photometry available, but do not show any periodicity on typical WD pulsation timescales.

  10. Very Low Mass Stellar and Substellar Companions to Solar-like Stars From MARVELS IV: A Candidate Brown Dwarf or Low-Mass Stellar Companion to HIP 67526

    CERN Document Server

    Jiang, Peng; Cargile, Phillip; Crepp, Justin R; De Lee, Nathan; de Mello, Gustavo F Porto; Esposito, Massimiliano; Ferreira, Letícia D; Femenia, Bruno; Fleming, Scott W; Gaudi, B Scott; Ghezzi, Luan; Hernández, Jonay I González; Hebb, Leslie; Lee, Brian L; Ma, Bo; Stassun, Keivan G; Wang, Ji; Wisniewski, John P; Agol, Eric; Bizyaev, Dmitry; Brewington, Howard; Chang, Liang; da Costa, Luiz Nicolaci; Eastman, Jason D; Ebelke, Garrett; Gary, Bruce; Kane, Stephen R; Li, Rui; Liu, Jian; Mahadevan, Suvrath; Maia, Marcio A G; Malanushenko, Viktor; Malanushenko, Elena; Muna, Demitri; Nguyen, Duy Cuong; Ogando, Ricardo L C; Oravetz, Audrey; Oravetz, Daniel; Pan, Kaike; Pepper, Joshua; Paegert, Martin; Prieto, Carlos Allende; Rebolo, Rafael; Santiago, Basilio X; Schneider, Donald P; Bradley, Alaina C Shelden; Sivarani, Thirupathi; Snedden, Stephanie; van Eyken, J C; Wan, Xiaoke; Weaver, Benjamin A; Zhao, Bo

    2013-01-01

    We report the discovery of a candidate brown dwarf or a very low mass stellar companion (MARVELS-5b) to the star HIP 67526 from the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS). The radial velocity curve for this object contains 31 epochs spread over 2.5 years. Our Keplerian fit using a Markov Chain Monte Carlo approach, reveals that the companion has an orbital period of $90.2695^{+0.0188}_{-0.0187}$ days, an eccentricity of $0.4375 \\pm 0.0040$ and a semi-amplitude of $2948.14^{+16.65}_{-16.55}$ m s$^{-1}$. Using additional high-resolution spectroscopy, we find the host star has an effective temperature $T_{\\rm{eff}}=6004 \\pm 34$ K, a surface gravity $\\log g$ [cgs] $=4.55 \\pm 0.17$ and a metallicity [Fe/H] $=+0.04 \\pm 0.06$. The stellar mass and radius determined through the empirical relationship of Torres et al. (2010), yields 1.10$\\pm$0.09 $M_{\\sun}$ and 0.92$\\pm$0.19 $R_{\\sun}$. The minimum mass of MARVELS-5b is $65.0 \\pm 2.9 M_{Jup}$, indicating that it is likely to be either a...

  11. Young brown dwarfs at high cadence: Warm Spitzer time series monitoring of very low mass Sigma Orionis cluster members

    CERN Document Server

    Cody, Ann Marie

    2011-01-01

    The continuous temporal coverage and high photometric precision afforded by space observatories has opened up new opportunities for the study of variability processes in young stellar cluster members. Of particular interest is the phenomenon of deuterium-burning pulsation in brown dwarfs and very-low-mass stars, whose existence on 1-4 hours timescales has been proposed but not yet borne out by observations. To investigate short-timescale variability in young, low-mass objects, we carried out high-precision, high-cadence time series monitoring with the Warm Spitzer mission on 14 low mass stars and brown dwarfs in the ~3 Myr Sigma Orionis cluster. The flux in many of our raw light curves is strongly correlated with sub-pixel position and can vary systematically as much as 10%. We present a new approach to disentangle true stellar variability from this "pixel-phase effect," which is more pronounced in Warm Spitzer observations as compared to the cryogenic mission. The light curves after correction reveal that mo...

  12. Fluid Atmospheres of Cool White Dwarfs

    Science.gov (United States)

    Kowalski, P.

    2004-05-01

    We investigate quantitatively for the first time the dense fluid effects in the surface layers of very cool white dwarf stars. In general, the gas is so tenuous in stellar atmospheres that non-ideal gas effects are negligible. One important exception are the atmospheres of cool white dwarfs, especially those rich in helium, where temperature varies from 1000K to 10000K, the densities reach values as large as 2 \\ g/cm3, and pressure is as high as 1 \\ Mbar. Under such conditions, the atmosphere is no longer an ideal gas, but must be treated as a dense fluid. New physical effects occur. Helium atoms become strongly correlated and refraction effects are present. Opacity sources, such as He- free-free absorption, require different treatment from diluted gases. The refractive index departs from unity and can be as large as 1.35. We present the first solution of the radiative transfer in refractive atmospheres of cool white dwarfs. The importance of total internal reflection is discussed. We find that through the constraint of the radiative equilibrium, the total internal reflection warms the white dwarf atmosphere in optically thin surface regions. Strong curvature of rays results in a much weakened limb darkening effect. This preliminary result suggests that dense fluid effects may have a significant impact on studies of very cool white dwarf stars. This research was supported by NSF grant AST97-31438, NASA grant NAG5-8906, and by the United States Department of Energy under contract W-7405-ENG-36.

  13. White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up

    CERN Document Server

    Althaus, Leandro G; Bertolami, Marcelo M Miller; Córsico, Alejandro H; García-Berro, Enrique

    2015-01-01

    We present new white dwarf evolutionary sequences for low-metallicity progenitors. White dwarf sequences have been derived from full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally-pulsing and the post-asymptotic giant branch phases. We show that for progenitor metallicities in the range 0.00003--0.001, and in the absence of carbon enrichment due to the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases, and the planetary nebulae stage. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by stable H burning. Our study...

  14. Microlensing discovery of a population of very tight, very low mass binary brown dwarfs

    DEFF Research Database (Denmark)

    Choi, J.-Y.; Han, C.; Udalski, A.

    2013-01-01

    the discovery via gravitational microlensing of two very low mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 M ☉ and 0.034 M ☉, and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field BD binaries known...

  15. The Field White Dwarf Mass Distribution

    CERN Document Server

    Tremblay, P -E; Kalirai, J S; Gaensicke, B T; Gentile-Fusillo, N; Raddi, R

    2016-01-01

    We revisit the properties and astrophysical implications of the field white dwarf mass distribution in preparation of Gaia applications. Our study is based on the two samples with the best established completeness and most precise atmospheric parameters, the volume-complete survey within 20 pc and the Sloan Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of the observed mass distributions with Monte Carlo simulations, but find that it is difficult to constrain independently the initial mass function (IMF), the initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the variation of the Galactic disk vertical scale height as a function of stellar age, and binary evolution. Each of these input ingredients has a moderate effect on the predicted mass distributions, and we must also take into account biases owing to unidentified faint objects (20 pc sample), as well as unknown masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS sample). Never...

  16. 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.

  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. Six White Dwarfs with Circumstellar Silicates

    CERN Document Server

    Jura, M; Zuckerman, B

    2008-01-01

    Spitzer Space Telescope spectra reveal 10 micron silicate emission from circumstellar dust orbiting six externally-polluted white dwarfs. Micron-size glasses with an olivine stoichiometry can account for the distinctively broad wings that extend to 12 microns; these particles likely are produced by tidal-disruption of asteroids. The absence of infrared PAH features is consistent with a scenario where extrasolar rocky planets are assembled from carbon-poor solids.

  19. The field white dwarf mass distribution

    Science.gov (United States)

    Tremblay, P.-E.; Cummings, J.; Kalirai, J. S.; Gänsicke, B. T.; Gentile-Fusillo, N.; Raddi, R.

    2016-09-01

    We revisit the properties and astrophysical implications of the field white dwarf mass distribution in preparation of Gaia applications. Our study is based on the two samples with the best established completeness and most precise atmospheric parameters, the volume-complete survey within 20 pc and the Sloan Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of the observed mass distributions with Monte Carlo simulations, but find that it is difficult to constrain independently the initial mass function (IMF), the initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the variation of the Galactic disc vertical scale height as a function of stellar age, and binary evolution. Each of these input ingredients has a moderate effect on the predicted mass distributions, and we must also take into account biases owing to unidentified faint objects (20 pc sample), as well as unknown masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS sample). Nevertheless, we find that fixed standard assumptions for the above parameters result in predicted mean masses that are in good qualitative agreement with the observed values. It suggests that derived masses for both studied samples are consistent with our current knowledge of stellar and Galactic evolution. Our simulations overpredict by 40-50 per cent the number of massive white dwarfs (M > 0.75 M⊙) for both surveys, although we can not exclude a Salpeter IMF when we account for all biases. Furthermore, we find no evidence of a population of double white dwarf mergers in the observed mass distributions.

  20. Asteroseismology of DAV White Dwarf Stars

    Energy Technology Data Exchange (ETDEWEB)

    Bradley, Paul A.

    1997-12-31

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

  1. Observations and Modelling of DQ White Dwarfs

    CERN Document Server

    Vornanen, Tommi; Berdyugin, Andrei

    2012-01-01

    We present spectropolarimetric observations and modelling of 12 DQ white dwarfs. Modelling is based on the method presented in Berdyugina et al. (2005). We use the model to fit the C_2 absorption bands to get atmospheric parameters in different configurations, including stellar spots and stratified atmospheres, searching for the best possible fit. We still have problem to solve before we can give temperature estimates based on the Swan bands alone.

  2. Open Science Project in White Dwarf Research

    Science.gov (United States)

    Vornanen, T.

    2013-01-01

    I will propose a new way of advancing white dwarf research. Open science is a method of doing research that lets everyone who has something to say about the subject take part in the problem solving process. Already now, the amount of information we gather from observations, theory and modeling is too vast for any one individual to comprehend and turn into knowledge. And the amount of information just keeps growing in the future. A platform that promotes sharing of thoughts and ideas allows us to pool our collective knowledge of white dwarfs and get a clear picture of our research field. It will also make it possible for researchers in fields closely related to ours (AGB stars, planetary nebulae etc.) to join the scientific discourse. In the first stage this project would allow us to summarize what we know and what we don't, and what we should search for next. Later, it could grow into a large collaboration that would have the impact to, for example, suggest instrument requirements for future telescopes to satisfy the needs of the white dwarf community, or propose large surveys. A simple implementation would be a wiki page for collecting knowledge combined with a forum for more extensive discussions. These would be simple and cheap to maintain. A large community effort on the whole would be needed for the project to succeed, but individual workload should stay at a low level.

  3. The Masses of Population II White Dwarfs

    CERN Document Server

    Kalirai, Jason S; Richer, Harvey B; Bergeron, P; Catelan, Marcio; Hansen, Brad M S; Rich, R Michael

    2009-01-01

    Globular star clusters are among the first stellar populations to have formed in the Milky Way, and thus only a small sliver of their initial spectrum of stellar types are still burning hydrogen on the main-sequence today. Almost all of the stars born with more mass than 0.8 M_sun have evolved to form the white dwarf cooling sequence of these systems, and the distribution and properties of these remnants uniquely holds clues related to the nature of the now evolved progenitor stars. With ultra-deep HST imaging observations, rich white dwarf populations of four nearby Milky Way globular clusters have recently been uncovered, and are found to extend an impressive 5 - 8 magnitudes in the faint-blue region of the H-R diagram. In this paper, we characterize the properties of these population II remnants by presenting the first direct mass measurements of individual white dwarfs near the tip of the cooling sequence in the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS multiobject spectro...

  4. Axions and the white dwarf luminosity function

    CERN Document Server

    Isern, J; García-Berro, E; Torres, S

    2008-01-01

    The evolution of white dwarfs can be described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. Since the shape of the bright branch of this function is only sensitive to the average cooling rate, we use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. We show here that the inclusion of the axion emissivity in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function, thus providing the first positive indication that axions could exist. Our results indicate that the best fit is obtained for m_a cos^2beta ~ 2-6 meV, where m_a is the mass of the axion and cos^2beta is a free parameter, and that values larger than 10 meV are clearly excluded.

  5. The white dwarf population of NGC 6397

    CERN Document Server

    Torres, S; Althaus, L G; Camisassa, M E

    2015-01-01

    NGC 6397 is one of the most interesting, well observed and theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low metallicity progenitors,to determine its age and the percentage of unresolved binaries, and to assess other important characteristics of the cluster, like the slope of the initial mass function, or the fraction of white dwarfs with hydrogen deficient atmospheres. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. We do this using an up-to-date Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. We find a good agreement between our theoretical models and the observed data. In particular, we find that this agreement is best for those cooling sequences th...

  6. Unlocking the secrets of white dwarf stars

    CERN Document Server

    Van Horn, Hugh M

    2015-01-01

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

  7. The 25 Parsec Local White Dwarf Population

    CERN Document Server

    Oswalt, J B Holberg T D; McCook, G P

    2016-01-01

    We have extended our detailed survey of the local white dwarf population from 20 pc to 25 pc, effectively doubling the sample volume, which now includes 232 stars. In the process new stars within 20 pc have been added, a more uniform set of distance estimates as well as improved spectral and binary classifications are available. The present 25 pc sample is estimated to be about 68% complete (the corresponding 20 pc sample is now 86\\% complete). The space density of white dwarfs is unchanged at 4.8 \\pm 0.5 x 10^{-3} pc^{-3}. This new study includes a white dwarf mass distribution and luminosity function based on the 232 stars in the 25 pc sample. We find a significant excess of single stars over systems containing one or more companions (74\\% vs 26\\%). This suggests mechanisms that result in the loss of companions during binary system evolution. In addition this updated sample exhibits a pronounced deficiency of nearby Sirius-Like systems. Eleven such systems were found within the 20 pc volume vs, only one add...

  8. Detonations in white dwarf dynamical interactions

    CERN Document Server

    Aznar-Siguán, Gabriela; Lorén-Aguilar, Pablo; José, Jordi; Isern, Jordi

    2013-01-01

    In old, dense stellar systems collisions of white dwarfs are a rather frequent phenomenon. Here we present the results of a comprehensive set of Smoothed Particle Hydrodynamics simulations of close encounters of white dwarfs aimed to explore the outcome of the interaction and the nature of the final remnants for different initial conditions. Depending on the initial conditions and the white dwarf masses, three different outcomes are possible. Specifically, the outcome of the interaction can be either a direct or a lateral collision or the interaction can result in the formation of an eccentric binary system. In those cases in which a collision occurs, the infalling material is compressed and heated such that the physical conditions for a detonation may be reached during the most violent phases of the merger. While we find that detonations occur in a significant number of our simulations, in some of them the temperature increase in the shocked region rapidly lifts degeneracy, leading to the quenching of the bu...

  9. DA White Dwarfs in the Kepler Field

    CERN Document Server

    Doyle, T F; Petit, V; Lepine, S

    2016-01-01

    We present 16 new, and confirm 7 previously identified, DA white dwarfs in the Kepler field through ground-based spectroscopy with the Hale 200", Kitt Peak 4-meter, and Bok 2.3-meter telescopes. Using atmospheric models we determine their effective temperatures and surface gravities to constrain their position with respect to the ZZ Ceti (DA pulsator) instability strip, and look for the presence or absence of pulsation with Kepler's unprecedented photometry. Our results are as follows: i) From our measurements of temperature and surface gravity, 12 of the 23 DA white dwarfs from this work fall well outside of the instability strip. The Kepler photometry available for 11 of these WDs allows us to confirm that none are pulsating. One of these eleven happens to be a presumed binary, KIC 11604781, with a period of ~5 days. ii) The remaining 11 DA white dwarfs are instability strip candidates, potentially falling within the current, empirical instability strip, after accounting for uncertainties. These WDs will he...

  10. On the White Dwarf Mass Problem of Cataclysmic Variables

    CERN Document Server

    Liu, Wei-Min

    2016-01-01

    Recent observations show that the white dwarfs (WDs) in cataclysmic Variables (CVs) have an average mass significantly higher than isolated WDs and WDs in post-common envelope binaries (PCEBs), which are thought to the progenitors of CVs. This suggests that either the WDs have grown in mass during the PCEB/CV evolution or the binaries with low-mass WDs are unable to evolve to be CVs. In this paper, we calculate the evolution of accreting WD binaries with updated hydrogen accumulation efficiency and angular momentum loss prescriptions. We show that thermal timescale mass transfer is not effective in changing the average WD mass distribution. The WD mass discrepancy is most likely related to unstable mass transfer in WD binaries in which an efficient mechanism of angular momentum loss is required.

  11. THE ELM SURVEY. V. MERGING MASSIVE WHITE DWARF BINARIES

    Energy Technology Data Exchange (ETDEWEB)

    Brown, Warren R.; Kenyon, Scott J. [Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 (United States); Kilic, Mukremin; Gianninas, A. [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK, 73019 (United States); Allende Prieto, Carlos, E-mail: wbrown@cfa.harvard.edu, E-mail: skenyon@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu, E-mail: callende@iac.es [Instituto de Astrofisica de Canarias, E-38205, La Laguna, Tenerife (Spain)

    2013-05-20

    We present the discovery of 17 low-mass white dwarfs (WDs) in short-period (P {<=} 1 day) binaries. Our sample includes four objects with remarkable log g {approx_equal} 5 surface gravities and orbital solutions that require them to be double degenerate binaries. All of the lowest surface gravity WDs have metal lines in their spectra implying long gravitational settling times or ongoing accretion. Notably, six of the WDs in our sample have binary merger times <10 Gyr. Four have {approx}>0.9 M{sub Sun} companions. If the companions are massive WDs, these four binaries will evolve into stable mass transfer AM CVn systems and possibly explode as underluminous supernovae. If the companions are neutron stars, then these may be millisecond pulsar binaries. These discoveries increase the number of detached, double degenerate binaries in the ELM Survey to 54; 31 of these binaries will merge within a Hubble time.

  12. Search for very low-mass brown dwarfs and free-floating planetary-mass objects in Taurus

    CERN Document Server

    Quanz, Sascha P; Henning, Thomas; Brandner, Wolfgang; Burrows, Adam; Hofstetter, Lorne W

    2009-01-01

    The number of low-mass brown dwarfs and even free floating planetary mass objects in young nearby star-forming regions and associations is continuously increasing, offering the possibility to study the low-mass end of the IMF in greater detail. In this paper, we present six new candidates for (very) low-mass objects in the Taurus star-forming region one of which was recently discovered in parallel by Luhman et al. (2009). The underlying data we use is part of a new database from a deep near-infrared survey at the Calar Alto observatory. The survey is more than four magnitudes deeper than the 2MASS survey and covers currently ~1.5 square degree. Complementary optical photometry from SDSS were available for roughly 1.0 square degree. After selection of the candidates using different color indices, additional photometry from Spitzer/IRAC was included in the analysis. In greater detail we focus on two very faint objects for which we obtained J-band spectra. Based on comparison with reference spectra we derive a s...

  13. A statistical analysis of seeds and other high-contrast exoplanet surveys: massive planets or low-mass brown dwarfs?

    Energy Technology Data Exchange (ETDEWEB)

    Brandt, Timothy D.; Spiegel, David S. [Institute for Advanced Study, Princeton, NJ (United States); McElwain, Michael W.; Grady, C. A. [Exoplanets and Stellar Astrophysics Laboratory, Goddard Space Flight Center, Greenbelt, MD (United States); Turner, Edwin L. [Department of Astrophysical Sciences, Princeton University, Princeton, NJ (United States); Mede, Kyle; Kuzuhara, Masayuki [University of Tokyo, Tokyo (Japan); Schlieder, Joshua E.; Brandner, W.; Feldt, M. [Max Planck Institute for Astronomy, Heidelberg (Germany); Wisniewski, John P. [HL Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK (United States); Abe, L. [Laboratoire Hippolyte Fizeau, Nice (France); Biller, B. [University of Edinburgh, Edinburgh, Scotland (United Kingdom); Carson, J. [College of Charleston, Charleston, SC (United States); Currie, T. [Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON (Canada); Egner, S.; Golota, T.; Guyon, O. [Subaru Telescope, Hilo, Hawai' i (United States); Goto, M. [Universitäts-Sternwarte München, Munich (Germany); Hashimoto, J. [National Astronomical Observatory of Japan, Tokyo (Japan); and others

    2014-10-20

    We conduct a statistical analysis of a combined sample of direct imaging data, totalling nearly 250 stars. The stars cover a wide range of ages and spectral types, and include five detections (κ And b, two ∼60 M {sub J} brown dwarf companions in the Pleiades, PZ Tel B, and CD–35 2722B). For some analyses we add a currently unpublished set of SEEDS observations, including the detections GJ 504b and GJ 758B. We conduct a uniform, Bayesian analysis of all stellar ages using both membership in a kinematic moving group and activity/rotation age indicators. We then present a new statistical method for computing the likelihood of a substellar distribution function. By performing most of the integrals analytically, we achieve an enormous speedup over brute-force Monte Carlo. We use this method to place upper limits on the maximum semimajor axis of the distribution function derived from radial-velocity planets, finding model-dependent values of ∼30-100 AU. Finally, we model the entire substellar sample, from massive brown dwarfs to a theoretically motivated cutoff at ∼5 M {sub J}, with a single power-law distribution. We find that p(M, a)∝M {sup –0.65} {sup ±} {sup 0.60} a {sup –0.85} {sup ±} {sup 0.39} (1σ errors) provides an adequate fit to our data, with 1.0%-3.1% (68% confidence) of stars hosting 5-70 M {sub J} companions between 10 and 100 AU. This suggests that many of the directly imaged exoplanets known, including most (if not all) of the low-mass companions in our sample, formed by fragmentation in a cloud or disk, and represent the low-mass tail of the brown dwarfs.

  14. SHIELD: EVLA HI Spectral Line Observations of Low-mass Dwarfs

    Science.gov (United States)

    Miazzo, Masao; Ruvolo, Elizabeth; Cannon, John M.; McNichols, Andrew; Teich, Yaron; Adams, Elizabeth A.; Giovanelli, Riccardo; Haynes, Martha P.; McQuinn, Kristen B.; Salzer, John Joseph; Skillman, Evan D.; Dolphin, Andrew E.; Elson, Edward C.; Haurberg, Nathalie C.; Huang, Shan; Janowiecki, Steven; Jozsa, Gyula; Leisman, Luke; Ott, Juergen; Papastergis, Emmanouil; Rhode, Katherine L.; Saintonge, Amelie; Van Sistine, Angela; Warren, Steven R.

    2017-01-01

    The “Survey of HI in Extremely Low-mass Dwarfs” (SHIELD) is a multiwavelength study of local volume low-mass galaxies. Using the now-complete Arecibo Legacy Fast ALFA (ALFALFA) source catalog, 82 systems are identified that meet distance, line width, and HI flux criteria for being gas-rich, low-mass galaxies. These systems harbor neutral gas reservoirs smaller than 3x10^7 M_sun, thus populating the faint end of the HI mass function with statistical confidence for the first time. Here we present new Karl G. Jansky Very Large Array D-configuration HI spectral line observations of 32 previously unobserved galaxies. These low angular resolution (~40" beam) images localize the HI gas; with a few exceptions, the HI gas is co-spatial with the optical centers of the galaxies. These images provide the first glimpse of the neutral interstellar medium in these systems.Support for this work was provided by NSF grant 1211683 to JMC at Macalester College.

  15. 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...

  16. The WFCAM transit survey and cool white dwarfs

    Directory of Open Access Journals (Sweden)

    Pinfield D.

    2013-04-01

    Full Text Available We present results from our search for cool white dwarfs in the WTS (WFCAM Transit Survey. Repeat observations starting in 2007 allowed to produce deep stacked images in J and measure proper motions. We combine this with deep optical imaging to select cool white dwarf candidates (Teff < 5000 K. About 27 cool white dwarf candidates with proper motions above 0.10 arcsec/yr were identified in one of the fields representing 1/8th of the survey area. Follow-up spectroscopy with the 10.2 m GTC telescope at La Palma confirmed the white dwarf status for all observed candidates. On-going work is being carried out to increase the sample of cool white dwarfs that will allow a more comprehensive study of the thick disk/halo white dwarf population.

  17. A Search for Asteroids, Moons, and Rings Orbiting White Dwarfs

    CERN Document Server

    Di Stefano, Rosanne; Kawaler, Steven D

    2009-01-01

    Do white dwarfs host asteroid systems? Although several lines of argument suggest that white dwarfs may be orbited by large populations of asteroids, transits would provide the most direct evidence. We demonstrate that the Kepler mission has the capability to detect transits of white dwarfs by asteroids. Because white-dwarf asteroid systems, if they exist, are likely to contain many asteroids orbiting in a spatially extended distribution, discoveries of asteroid transits can be made by monitoring only a small number of white dwarfs, compatible with Kepler's primary mission, which is to monitor stars with potentially habitable planets. Possible future missions that survey ten times as many stars with similar sensitivity and minute-cadence monitoring can establish the characteristics of asteroid systems around white dwarfs, such as the distribution of asteroid sizes and semimajor axes. Transits by planets would be more dramatic, but the probability that they will occur is lower. Ensembles of planetary moons and...

  18. Anderson and Stoner Published White Dwarf Mass Limits Before Chandrasekhar

    CERN Document Server

    Blackman, Eric G

    2011-01-01

    In their engaging recountals of Chandrasekhar's extraordinary career (Physics Today, vol 63, Issue 12, Dec 2010), neither Dyson nor Wali mention that Chandrasekhar was the third person not the first, to publish a white dwarf mass limit incorporating a relativistic treatment of degenerate electrons. As it has become a common misconception that Chandrasekhar was the first, a clarifying reminder on this historical point is warranted. In short, the white dwarf mass limit widely attributed to Chandrasekhar (1931) should be the specific white dwarf mass limit calculated for a polytrope. The insight that a relativistic treatment of degeneracy leads to the existence of a white dwarf mass limit first appeared in papers of W. Anderson (1929) and E.C. Stoner (1930) for a uniform density star. Accordingly, Chandrasekhar (1931) cites Stoner (1930) and points out that the polytrope white dwarf mass limit is less than Stoner's uniform density white dwarf mass limit by about 20%.

  19. Stripped red giants - Helium core white dwarf progenitors and their sdB siblings

    CERN Document Server

    Heber, Ulrich

    2016-01-01

    Some gaps in the mosaic of binary star evolution have recently been filled by the discoveries of helium-core white dwarf progenitors (often called extremely low mass (ELM) white dwarfs) as stripped cores of first-giant branch objects. Two varieties can be distinguished. One class is made up by SB1 binaries, companions being white dwarfs as well, another class, the so-called EL CVn stars, are composite spectrum binaries, with A-Type companions. Pulsating stars are found among both classes. A riddle is posed by the apparently single objects. There is a one-to-one correspondence of the phenomena found for these new classes of star to those observed for sdB stars. In fact, standard evolutionary scenarios explain the origin of sdB stars as red giants that have been stripped close to the tip of first red giant branch. A subgroup of subluminous B stars can also be identified as stripped helium-cores of red giants. They form an extension of the ELM sequence to higher temperatures. Hence low mass white dwarfs of heliu...

  20. Stripped Red Giants - Helium Core White Dwarf Progenitors and their sdB Siblings

    Science.gov (United States)

    Heber, U.

    2017-03-01

    Some gaps in the mosaic of binary star evolution have recently been filled by the discoveries of helium-core white dwarf progenitors (often called extremely low mass (ELM) white dwarfs) as stripped cores of first-giant branch objects. Two varieties can be distinguished. One class is made up by SB1 binaries, companions being white dwarfs as well. Another class, the so-called EL CVn stars, are composite spectrum binaries, with A-Type companions. Pulsating stars are found among both classes. A riddle is posed by the apparently single objects. There is a one-to-one correspondence of the phenomena found for these new classes of star to those observed for sdB stars. In fact, standard evolutionary scenarios explain the origin of sdB stars as red giants that have been stripped close to the tip of first red giant branch. A subgroup of subluminous B stars can also be identified as stripped helium-cores of red giants. They form an extension of the ELM sequence to higher temperatures. Hence low mass white dwarfs of helium cores and sdB stars in binaries are close relatives in terms of stellar evolution.

  1. The Fate of Exomoons in White Dwarf Planetary Systems

    CERN Document Server

    Payne, Matthew J; Gaensicke, Boris T; Holman, Matthew J

    2016-01-01

    Roughly 1000 white dwarfs are known to be polluted with planetary material, and the progenitors of this material are typically assumed to be asteroids. The dynamical architectures which perturb asteroids into white dwarfs are still unknown, but may be crucially dependent on moons liberated from parent planets during post-main-sequence gravitational scattering. Here, we trace the fate of these exomoons, and show that they more easily achieve deep radial incursions towards the white dwarf than do scattered planets. Consequently, moons are likely to play a significant role in white dwarf pollution, and in some cases may be the progenitors of the pollution itself.

  2. Emission line diagnostics for accretion and outflows in young very low-mass stars and brown dwarfs

    Directory of Open Access Journals (Sweden)

    Stelzer B.

    2014-01-01

    Full Text Available We discuss accretion and outflow properties of three very low-mass young stellar objects based on broad-band mid-resolution X-Shooter/VLT spectra. Our targets (FU Tau A, 2M1207-39, and Par-Lup3-4 have spectral types between M5 and M8, ages between 1Myr and ~ 10Myr, and are known to be accreting from previous studies. The final objective of our project is the determination of mass outflow to accretion rate for objects near or within the substellar regime as a probe for the T Tauri phase of brown dwarfs and the investigation of variability in the accretion and outflow processes.

  3. Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii

    Science.gov (United States)

    Buckley, D. A. H.; Meintjes, P. J.; Potter, S. B.; Marsh, T. R.; Gänsicke, B. T.

    2017-01-01

    The variable star AR Scorpii (AR Sco) was recently discovered to pulse in brightness every 1.97 min from ultraviolet wavelengths into the radio regime. The system is composed of a cool, low-mass star in a tight, 3.55-hour orbit with a more massive white dwarf. Here we report new optical observations of AR Sco that show strong linear polarization (up to 40%) that varies strongly and periodically on both the spin period of the white dwarf and the beat period between the spin and orbital period, as well as low-level (up to a few per cent) circular polarization. These observations support the notion that, similar to neutron-star pulsars, the pulsed luminosity of AR Sco is powered by the spin-down of the rapidly rotating white dwarf that is highly magnetized (up to 500 MG). The morphology of the modulated linear polarization is similar to that seen in the Crab pulsar, albeit with a more complex waveform owing to the presence of two periodic signals of similar frequency. Magnetic interactions between the two component stars, coupled with synchrotron radiation from the white dwarf, power the observed polarized and non-polarized emission. AR Sco is therefore the first example of a white dwarf pulsar.

  4. Search for Higgs shifts in white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Onofrio, Roberto [Dipartimento di Fisica e Astronomia " Galileo Galilei," Università di Padova, Via Marzolo 8, I-35131 Padova (Italy); Wegner, Gary A., E-mail: onofrior@gmail.com, E-mail: gary.a.wegner@dartmouth.edu [Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755 (United States)

    2014-08-20

    We report on a search for differential shifts between electronic and vibronic transitions in carbon-rich white dwarfs BPM 27606 and Procyon B. The absence of differential shifts within the spectral resolution and taking into account systematic effects such as space motion and pressure shifts allows us to set the first upper bound of astrophysical origin on the coupling between the Higgs field and the Kreschmann curvature invariant. Our analysis provides the basis for a more general methodology to derive bounds to the coupling of long-range scalar fields to curvature invariants in an astrophysical setting complementary to the ones available from high-energy physics or table-top experiments.

  5. The Low-Mass Stellar IMF at High Redshift Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy

    CERN Document Server

    Wyse, R F G; Feltzing, S; Houdashelt, M L; Wyse, Rosemary F.G.; Gilmore, Gerard; Feltzing, Sofia; Houdashelt, Mark

    1999-01-01

    Low-mass stars, those with main-sequence lifetimes that are of order the age of the Universe, provide unique constraints on the Initial Mass Function (IMF) when they formed. Star counts in systems with simple star-formation histories are particularly straightforward to interpret, and those in old systems allow one to determine the low-mass stellar IMF at large look-back times and thus at high redshift. We present the faint stellar luminosity function (based on optical HST data) in an external galaxy, the Ursa Minor dwarf Spheroidal (dSph). This relatively-nearby (distance 70kpc) companion galaxy to the Milky Way has a stellar population with narrow distributions of age and of metallicity, remarkably similar to that of a classical halo globular cluster such as M92 or M15, i.e. old and metal-poor. Contrasting with globular clusters, the internal velocity dispersion of the Ursa Minor dSph indicates the presence of significant amounts of dark matter. We find that the main sequence stellar luminosity function of t...

  6. 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...

  7. Multi-Object and Long-Slit Spectroscopy of Very Low Mass Brown Dwarfs in Orion Nebular Cluster

    CERN Document Server

    Suenaga, Takuya; Kuzuhara, Masayuki; Yanagisawa, Kenshi; Ishii, Miki; Lucas, Philip W

    2013-01-01

    We present the results of a H- and K-band multi-object and long-slit spectroscopic survey of substellar mass candidates in the outer regions of the Orion Nebula Cluster. The spectra were obtained using MOIRCS on the 8.2-m Subaru telescope and ISLE on the 1.88-m telescope of Okayama Astronomical Observatory. Eight out of twelve spectra show strong water absorptions and we confirm that their effective temperatures are M6) from a chi-square fit to synthetic spectra. We plot our sources on an HR diagram overlaid with theoretical isochrones of low-mass objects and identify three new young brown dwarf candidates. One of the three new candidates is a cool object near the brown dwarf and planetary mass boundary. Based on our observations and those of previous studies, we determine the stellar (0.08 < M/Msun < 1) to substellar (0.03 < M/Msun < 0.08) mass number ratio in the outer regions of the Orion nebular cluster to be 3.5 +/- 0.8. In combination with the number ratio reported for the central region (3...

  8. Magnetic activity in the HARPS M dwarf sample. The rotation-activity relationship for very low-mass stars through

    Science.gov (United States)

    Astudillo-Defru, N.; Delfosse, X.; Bonfils, X.; Forveille, T.; Lovis, C.; Rameau, J.

    2017-03-01

    Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the Ca ii H and K doublet. Starting with the historical Mount Wilson monitoring program, these two spectral lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period (Prot) and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca ii emission is traditionally quantified through the -index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M dwarfs - the most numerous stars of the Galaxy - were left out of these analyses and no calibration of their Ca ii H and K emission to an exists to date. Aims: We set out to characterize the magnetic activity of the low- and very-low-mass stars by providing a calibration of the -index that extends to the realm of M dwarfs, and by evaluating the relationship between and the rotation period. Methods: We calibrated the bolometric and photospheric factors for M dwarfs to properly transform the S-index (which compares the flux in the Ca ii H and K lines to a close spectral continuum) into the . We monitored magnetic activity through the Ca ii H and K emission lines in the HARPS M dwarf sample. Results: The index, like the fractional X-ray luminosity LX/Lbol, shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the index saturates to approximately 10-4. Stellar mass modulates the Ca ii activity, with showing a constant basal activity above 0.6 M⊙ and then decreasing with mass between 0.6 M⊙ and the fully-convective limit of 0.35 M⊙. Short-term variability of the

  9. Open Science Project in White Dwarf Research

    CERN Document Server

    Vornanen, Tommi

    2012-01-01

    I will propose a new way of advancing white dwarf research. Open science is a method of doing research that lets everyone who has something to say about the subject take part in the problem solving process. Already now, the amount of information we gather from observations, theory and modelling is too vast for any one individual to comprehend and turn into knowledge. And the amount of information just keeps growing in the future. A platform that promotes sharing of thoughts and ideas allows us to pool our collective knowledge of white dwarfs and get a clear picture of our research field. It will also make it possible for researchers in fields closely related to ours (AGB stars, planetary nebulae etc.) to join the scientific discourse. In the first stage this project would allow us to summarize what we know and what we don't, and what we should search for next. Later, it could grow into a large collaboration that would have the impact to, for example, suggest instrument requirements for future telescopes to sa...

  10. Studying white dwarf merger remnants with FLASH

    Science.gov (United States)

    Jenks, Malia

    2017-01-01

    There is still uncertainty as to the progenitor systems of type Ia supernova (SN Ia). Both single and double degenerate systems have been suggested as progenitors. In a double degenerate system a merger between the two white dwarfs, with total mass at or exceeding the Chandrasekhar mass, leads to the supernova. If the explosion occurs during the merging process it is a violent merger. If an explosion doesn't occur while the stars merge the system becomes a white dwarf of unstable mass. For mergers of this type with differing starting masses it has been shown that during the viscous evolution carbon burning starts far from the center and stably converts the star to oxygen and neon. In this case the star will eventually collapse to a neutron star and not produce an SN Ia. The case of similar mass mergers has been much less explored. Using the results of a smooth particle hydrodynamic merger we simulate the viscous evolution of models of different mass ratios with FLASH. These simulations test if a similar mass merger can lead to an SN Ia, and begin to probe where the transition from similar to dissimilar mass occurs.

  11. Accretion on to Magnetic White Dwarfs

    Directory of Open Access Journals (Sweden)

    Wickramasinghe Dayal

    2014-01-01

    The polars have no counterparts in neutron star systems and their study provides unique insights into the complex nature of the magnetospheric boundary. The observed properties of accretion shocks at the white dwarf surface such as the anomalous soft-X-ray excess and its time variability provide strong support for the hypothesis that under certain circumstances the field channelled funnel flow is “blobby”. This has been attributed to interchange instabilities such as the Magnetic Rayleigh-Taylor instability in the shocked gas at the stream-magnetosphere boundary where the stream fragments into discrete clumps of gas. As the clumps penetrate into the magnetosphere, they are shredded into smaller mass blobs via the Kelvin-Helmholtz instability that then couple on to field lines over an extended inner transition region in the orbital plane. The more massive blobs penetrate deep into the photosphere of the white dwarf releasing their energy as a reprocessed soft-X-ray black body component. Although similar instabilities are expected in the inner transition region in disced accretion albeit on a different scale there has been no direct observational evidence for blobby accretion in the generally lower field and disced IPs.

  12. Remnant evolution after a carbon-oxygen white dwarf merger

    NARCIS (Netherlands)

    Yoon, S.C.; Podsiadlowski, P.; Rosswog, S.

    2007-01-01

    We systematically explore the evolution of the merger of two carbon-oxygen (CO) white dwarfs. The dynamical evolution of a 0.9Msolar + 0.6Msolar CO white dwarf merger is followed by a 3D smoothed particle hydrodynamics (SPH) simulation. The calculation uses a state-of-the-art equation of state that

  13. A wide binary trigger for white dwarf pollution

    CERN Document Server

    Bonsor, Amy

    2015-01-01

    Metal pollution in white dwarf atmospheres is likely to be a signature of remnant planetary systems. Most explanations for this pollution predict a sharp decrease in the number of polluted systems with white dwarf cooling age. Observations do not confirm this trend, and metal pollution in old (1-5 Gyr) white dwarfs is difficult to explain. We propose an alternative, time-independent mechanism to produce the white dwarf pollution. The orbit of a wide binary companion can be perturbed by Galactic tides, approaching close to the primary star for the first time after billions of years of evolution on the white dwarf branch. We show that such a close approach perturbs a planetary system orbiting the white dwarf, scattering planetesimals onto star-grazing orbits, in a manner that could pollute the white dwarf's atmosphere. Our estimates find that this mechanism is likely to contribute to metal pollution, alongside other mechanisms, in up to a few percent of an observed sample of white dwarfs with wide binary compan...

  14. Pure hydrogen atmosphere for very cool white dwarfs

    CERN Document Server

    Saumon, D S

    1999-01-01

    Microlensing events observed in the line of sight toward the LMC indicate that a significant fraction of the mass of the dark halo of the Galaxy is probably composed of white dwarfs. In addition, white dwarf sequences have now be observed in the HR diagrams of several globular clusters. Because of the unavailability of white dwarf atmospheres for Teff < 4000K, cooling time scales for white dwarfs older than ~ 10 Gyr are very uncertain. Moreover, the identification of a MACHO white dwarf population by direct observation depends on a knowledge of the colors and bolometric corrections of very-cool white dwarfs. In this paper we present the first detailed model atmospheres and spectra of very cool hydrogen white dwarfs for Teff < 4000K. We include the latest description of the opacities of hydrogen and significantly, we introduce a non-ideal equation of state in the atmosphere calculation. We find that due to strong absorption from H_2 in the infrared, very old white dwarfs are brightest in the V, R, and I ...

  15. Evolutionary and pulsational properties of white dwarf stars

    CERN Document Server

    Althaus, Leandro G; Isern, Jordi; a-Berro, Enrique Garcí

    2010-01-01

    Abridged. White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. The study of white dwarfs has potential applications to different fields of astrophysics. In particular, they can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, like our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow to use these stars as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. They can be used to constrain fundamental properties of elementary particles such as axions and neutrinos, and to study problems related to the variation of fundamental constants. In this work, we review the essentials of the physics of white dwarf stars. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs ...

  16. Effect of accretion on the pre-main-sequence evolution of low-mass stars and brown dwarfs

    Science.gov (United States)

    Vorobyov, Eduard I.; Elbakyan, Vardan; Hosokawa, Takashi; Sakurai, Yuya; Guedel, Manuel; Yorke, Harold

    2017-09-01

    Aims: The pre-main-sequence evolution of low-mass stars and brown dwarfs is studied numerically starting from the formation of a protostellar or proto-brown dwarf seed and taking into account the mass accretion onto the central object during the initial several Myr of evolution. Methods: The stellar evolution was computed using the STELLAR evolution code with recent modifications. The mass accretion rates were taken from numerical hydrodynamics models by computing the circumstellar disk evolution starting from the gravitational collapse of prestellar cloud cores of various mass and angular momentum. The resulting stellar evolution tracks were compared with the isochrones and isomasses calculated using non-accreting models. Results: We find that mass accretion in the initial several Myr of protostellar evolution can have a strong effect on the subsequent evolution of young stars and brown dwarfs. The disagreement between accreting and non-accreting models in terms of the total stellar luminosity L∗, stellar radius R∗, and effective temperature Teff depends on the thermal efficiency of accretion, that is, on the fraction of accretion energy that is absorbed by the central object. The largest mismatch is found for the cold accretion case, in which essentially all accretion energy is radiated away. The relative deviations in L∗ and R∗ in this case can reach 50% for objects 1.0 Myr old, and they remain notable even for objects 10 Myr old. In the hot and hybrid accretion cases, in which a constant fraction of accretion energy is absorbed, the disagreement between accreting and non-accreting models becomes less pronounced, but still remains notable for objects 1.0 Myr old. These disagreements may lead to an incorrect age estimate for objects of (sub-)solar mass when using the isochrones that are based on non-accreting models, as has also been noted previously. We find that objects with strong luminosity bursts exhibit notable excursions in the L∗-Teff diagram

  17. A DARK SPOT ON A MASSIVE WHITE DWARF

    Energy Technology Data Exchange (ETDEWEB)

    Kilic, Mukremin; Gianninas, Alexandros; Curd, Brandon; Wisniewski, John P. [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States); Bell, Keaton J.; Winget, D. E.; Winget, K. I. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Brown, Warren R. [Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138 (United States); Hermes, J. J. [Department of Physics, University of Warwick, Coventry CV4 7AL (United Kingdom); Dufour, Patrick [Institut de recherche sur les exoplanétes (iREx), Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7 (Canada)

    2015-12-01

    We present the serendipitous discovery of eclipse-like events around the massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We selected J1529+2928 for time-series photometry based on its spectroscopic temperature and surface gravity, which place it near the ZZ Ceti instability strip. Instead of pulsations, we detect photometric dips from this white dwarf every 38 minutes. Follow-up optical spectroscopy observations with Gemini reveal no significant radial velocity variations, ruling out stellar and brown dwarf companions. A disintegrating planet around this white dwarf cannot explain the observed light curves in different filters. Given the short period, the source of the photometric dips must be a dark spot that comes into view every 38 minutes due to the rotation of the white dwarf. Our optical spectroscopy does not show any evidence of Zeeman splitting of the Balmer lines, limiting the magnetic field strength to B < 70 kG. Since up to 15% of white dwarfs display kG magnetic fields, such eclipse-like events should be common around white dwarfs. We discuss the potential implications of this discovery on transient surveys targeting white dwarfs, like the K2 mission and the Large Synoptic Survey Telescope.

  18. The Survey of HI in Extremely Low-mass Dwarfs: A Multi-Wavelength Perspective on Low-Mass Galaxy Evolution

    Science.gov (United States)

    Cannon, John M.; McNichols, Andrew; Teich, Yaron; Adams, Elizabeth A.; Giovanelli, Riccardo; Haynes, Martha P.; McQuinn, Kristen B.; Salzer, John Joseph; Skillman, Evan D.; Dolphin, Andrew E.; Elson, Edward C.; Haurberg, Nathalie C.; Huang, Shan; Janowiecki, Steven; Jozsa, Gyula; Leisman, Luke; Ott, Juergen; Papastergis, Emmanouil; Rhode, Katherine L.; Saintonge, Amelie; Van Sistine, Angela; Warren, Steven R.

    2017-01-01

    The “Survey of HI in Extremely Low-mass Dwarfs” (SHIELD) is a multiwavelength study of local volume low-mass galaxies drawn from the Arecibo Legacy Fast ALFA (ALFALFA) catalog. HST/Spitzer joint program GO-12658 revealed the stellar populations of the first 12 SHIELD galaxies (Cannon et al. 2011), allowing accurate distance measurements (McQuinn et al. 2014) and detailed studies of the patterns of recent star formation in each galaxy (McQuinn et al. 2015). These HST and Spitzer images are a critical interpretive benchmark for ground-based optical imaging and spectroscopy (Haurberg et al. 2015), as well as for sensitive VLA HI spectral line imaging of the SHIELD galaxies (McNichols et al. 2016; Teich et al. 2016). These results have furthered our understanding of the evolution of galaxies in a mass regime that was previously only sparsely populated. With the low-redshift ALFALFA catalog now complete, the scope of the SHIELD program has been expanded to include all 82 galaxies that meet distance, line width, and HI flux criteria for being gas-rich, low-mass galaxies. In HST program 13750, images of 18 more SHIELD galaxies have again set the physical scales for supporting HI spectral line imaging with both the VLA and the WSRT (Gordon et al. 2016). Taken as a whole, the ongoing SHIELD program is one of the most comprehensive multiwavelength studies of the physical properties of low-mass galaxies outside of the Local Group.

  19. THREE NEW ECLIPSING WHITE-DWARF-M-DWARF BINARIES DISCOVERED IN A SEARCH FOR TRANSITING PLANETS AROUND M-DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Law, Nicholas M. [Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George St., Toronto, ON M5S 3H4 (Canada); Kraus, Adam L. [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Street, Rachel; Fulton, Benjamin J.; Shporer, Avi; Lister, Tim [Las Cumbres Observatory Global Telescope Network, Inc., 6740 Cortona Dr. Suite 102, Santa Barbara, CA 93117 (United States); Hillenbrand, Lynne A.; Baranec, Christoph; Bui, Khanh; Davis, Jack T. C.; Dekany, Richard G.; Kulkarni, S. R.; Ofek, Eran O. [Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States); Bloom, Joshua S.; Cenko, S. Bradley; Filippenko, Alexei V. [Department of Astronomy, University of California, Berkeley, CA 94720-3411 (United States); Burse, Mahesh P.; Das, H. K. [Inter-University Centre for Astronomy and Astrophysics, Ganeshkhind, Pune-411007 (India); Kasliwal, Mansi M. [Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101 (United States); Nugent, Peter [Computational Cosmology Center, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 (United States); and others

    2012-10-01

    We present three new eclipsing white-dwarf/M-dwarf binary systems discovered during a search for transiting planets around M-dwarfs. Unlike most known eclipsing systems of this type, the optical and infrared emission is dominated by the M-dwarf components, and the systems have optical colors and discovery light curves consistent with being Jupiter-radius transiting planets around early M-dwarfs. We detail the PTF/M-dwarf transiting planet survey, part of the Palomar Transient Factory (PTF). We present a graphics processing unit (GPU)-based box-least-squares search for transits that runs approximately 8 Multiplication-Sign faster than similar algorithms implemented on general purpose systems. For the discovered systems, we decompose low-resolution spectra of the systems into white-dwarf and M-dwarf components, and use radial velocity measurements and cooling models to estimate masses and radii for the white dwarfs. The systems are compact, with periods between 0.35 and 0.45 days and semimajor axes of approximately 2 R{sub Sun} (0.01 AU). The M-dwarfs have masses of approximately 0.35 M{sub Sun }, and the white dwarfs have hydrogen-rich atmospheres with temperatures of around 8000 K and have masses of approximately 0.5 M{sub Sun }. We use the Robo-AO laser guide star adaptive optics system to tentatively identify one of the objects as a triple system. We also use high-cadence photometry to put an upper limit on the white-dwarf radius of 0.025 R{sub Sun} (95% confidence) in one of the systems. Accounting for our detection efficiency and geometric factors, we estimate that 0.08%{sub -0.05%}{sup +0.10%} (90% confidence) of M-dwarfs are in these short-period, post-common-envelope white-dwarf/M-dwarf binaries where the optical light is dominated by the M-dwarf. The lack of detections at shorter periods, despite near-100% detection efficiency for such systems, suggests that binaries including these relatively low-temperature white dwarfs are preferentially found at

  20. The Star Formation Histories of Local Group Dwarf Galaxies. III. Characterizing Quenching in Low-mass Galaxies

    Science.gov (United States)

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

    2015-05-01

    We explore the quenching of low-mass galaxies (104 ≲ {{M}\\star } ≲ 108 {{M}⊙ }) as a function of lookback time using the star formation histories (SFHs) of 38 Local Group dwarf galaxies. The SFHs were derived by 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 Rvirial there is no correlation between a satellite’s current proximity to a massive host and its quenching epoch; and (3) there are hints of systematic differences in the quenching times of M31 and Milky Way (MW) satellites, although the sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive conclusions. Combined with results from the literature, we qualitatively consider the redshift evolution (z = 0-1) of the quenched galaxy fraction over ˜7 dex in stellar mass (104 ≲ {{M}\\star } ≲ 1011.5 {{M}⊙ }). The quenched fraction of all galaxies generally increases toward the present, with both the lowest and highest-mass systems exhibiting the largest quenched fractions at all redshifts. In contrast, galaxies between {{M}\\star } ˜ 108-1010 {{M}⊙ } have the lowest quenched fractions. We suggest that such intermediate-mass galaxies are the least efficient at quenching. Finally, we compare our quenching times with predictions for infall times for low-mass galaxies associated with the MW. We find that some of the lowest-mass satellites (e.g., CVn II, Leo IV) may have been quenched before infall, while higher-mass satellites (e.g., Leo I, Fornax) typically quench ˜1-4 Gyr after infall. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA constract NAS 5-26555.

  1. Implications for the origin of early-type dwarf galaxies - the discovery of rotation in isolated, low-mass early-type galaxies

    Science.gov (United States)

    Janz, Joachim; Penny, Samantha J.; Graham, Alister W.; Forbes, Duncan A.; Davies, Roger L.

    2017-07-01

    We present the discovery of rotation in quenched, low-mass early-type galaxies that are isolated. This finding challenges the claim that (all) rotating dwarf early-type galaxies in clusters were once spiral galaxies that have since been harassed and transformed into early-type galaxies. Our search of the Sloan Digital Sky Survey data within the Local Volume (z half-light radius Re in the best cases, and beyond Re/2 for all. They reveal a variety of behaviours similar to those of a comparison sample of early-type dwarf galaxies in the Virgo cluster observed by Toloba et al. Both samples have similar frequencies of slow and fast rotators, as well as kinematically decoupled cores. This, and especially the finding of rotating quenched low-mass galaxies in isolation, reveals that the early-type dwarfs in galaxy clusters need not be harassed or tidally stirred spiral galaxies.

  2. Future Observations of White Dwarfs from Space

    Science.gov (United States)

    Barstow, M. A.; Casewell, S. L.

    2015-06-01

    We outline two possible future space missions suitable for the study of white dwarfs. These lie at the extreme ends of the spectrum of such opportunities in terms of cost and timescale. The SIRIUS extreme ultraviolet spectrograph will be proposed for the ESA/Chinese Academy of Sciences small, ≍ 50M Euro, mission. If selected it will fly in 2021. ATLAST is planned to be a very large UVOIR space observatory to provide a true replacement for the capabilities of HST. If it goes ahead, it will be a several billion-dollar project and will not fly before 2030. The paper provides further details on the technical capabilities of these space telescopes.

  3. Liberating exomoons in white dwarf planetary systems

    CERN Document Server

    Payne, Matthew J; Holman, Matthew J; Gaensicke, Boris T

    2016-01-01

    Previous studies indicate that more than a quarter of all white dwarf (WD) atmospheres are polluted by remnant planetary material, with some WDs being observed to accrete the mass of Pluto in 10^6 years. The short sinking timescale for the pollutants indicate that the material must be frequently replenished. Moons may contribute decisively to this pollution process if they are liberated from their parent planets during the post-main-sequence evolution of the planetary systems. Here, we demonstrate that gravitational scattering events among planets in WD systems easily triggers moon ejection. Repeated close encounters within tenths of a planetary Hill radii are highly destructive to even the most massive, close-in moons. Consequently, scattering increases both the frequency of perturbing agents in WD systems, as well as the available mass of polluting material in those systems, thereby enhancing opportunities for collision and fragmentation and providing more dynamical pathways for smaller bodies to reach the ...

  4. Heavy Metals Resisting Gravity in White Dwarfs?

    Science.gov (United States)

    Rauch, T.; Gamrath, S.; Quinet, P.; Hoyer, D.; Werner, K.; Kruk, J. W.

    2017-03-01

    Spectral lines of heavy metals, identified in high-resolution ultraviolet spectra of the DO-type white dwarf RX J0503.9–2854 (RE 0503–289), allow precise abundance determinations of these species by means of advanced non-local thermodynamic equilibrium stellar-atmosphere models – provided that reliable atomic data is available. Such analyses of Zn (atomic number Z = 30), Ga (31), Ge (32), As (33), Mo (42), Kr (36), Zr (40), Xe (54), and Ba (56) have recently shown that, without exception, their abundances are unexpectedly strongly supersolar (up to about 5 dex). This is much higher than predicted by recent asymptotic giant branch nucleosynthesis calculations. Thus, the interplay of gravitational settling and radiative levitation may play an important role for their photospheric prominence.

  5. Evolution of iron core white dwarfs

    CERN Document Server

    Panei, J A; Benvenuto, O G

    1999-01-01

    Recent measurements made by Hipparcos (Provencal et al. 1998) present observational evidence supporting the existence of some white dwarf (WD) stars with iron - rich, core composition. In this connection, the present paper is aimed at exploring the structure and evolution of iron - core WDs by means of a detailed and updated evolutionary code. In particular, we examine the evolution of the central conditions, neutrino luminosity, surface gravity, crystallization, internal luminosity profiles and ages. We find that the evolution of iron - rich WDs is markedly different from that of their carbon - oxygen counterparts. In particular, cooling is strongly accelerated as compared with the standard case. Thus, if iron WDs were very numerous, some of them would have had time enough to evolve at lower luminosities than that corresponding to the fall - off in the observed WD luminosity function.

  6. Isochrones and Luminosity Functions for Old White Dwarfs

    CERN Document Server

    Richer, H B; Limongi, M; Chieffi, A; Straniero, O; Fahlman, G G; Richer, Harvey B.; Hansen, Brad; Limongi, Marco; Chieffi, Alessandro; Straniero, Oscar; Fahlman, Gregory G.

    1999-01-01

    Using a new grid of models of cooling white dwarfs, we calculate isochrones and luminosity functions in the Johnson-Kron/Cousins and HST filter sets for systems containing old white dwarfs. These new models incorporate a non-grey atmosphere which is necessary to properly describe the effects of molecular opacity at the cool temperatures of old white dwarfs. The various functions calculated and extensively tabulated and plotted are meant to be as utilitarian as possible for observers so all results are listed in quantities that observers will obtain. The tables and plots developed should eventually prove critical in interpreting the results of HST's Advanced Camera observations of the oldest white dwarfs in nearby globular clusters, in understanding the results of searches for old white dwarfs in the Galactic halo, and in determining ages for star clusters of all ages using white dwarfs. As a practical application we demonstrate the use of these results by deriving the white dwarf cooling age of the old Galact...

  7. Binary white dwarfs in the halo of the Milky Way

    CERN Document Server

    van Oirschot, Pim; Toonen, Silvia; Pols, Onno; Brown, Anthony G A; Helmi, Amina; Zwart, Simon Portegies

    2014-01-01

    Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~ 3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference w...

  8. The evolution of white dwarfs with a varying gravitational constant

    CERN Document Server

    Althaus, L G; Torres, S; Loren-Aguilar, P; Isern, J; Garcia-Berro, E

    2011-01-01

    Within the theoretical framework of some modern unification theories the constants of nature are functions of cosmological time. White dwarfs offer the possibility of testing a possible variation of G and, thus, to place constraints to these theories. We present full white dwarf evolutionary calculations in the case that G decreases with time. White dwarf evolution is computed in a self-consistent way, including the most up-to-date physical inputs, non-gray model atmospheres and a detailed core chemical composition that results from the calculation of the full evolution of progenitor stars. We find that the mechanical structure and the energy balance of white dwarfs are strongly modified by the presence of a varying G. In particular, for certain values of the rate of change of G, the evolution of cool white dwarfs is markedly affected. The impact of a varying G is more notorious in the case of more massive white dwarfs. In view of the recent results reporting that a very accurate white dwarf cooling age can b...

  9. 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...

  10. Investigating the rotational evolution of very low-mass stars and brown dwarfs in young clusters using Monte Carlo simulations

    CERN Document Server

    Vasconcelos, M J

    2016-01-01

    Context. Very low-mass (VLM) stars and brown dwarfs (BDs) present a different rotational behaviour from their solar mass counter-parts. Aims. We investigate the rotational evolution of young VLM stars and BDs using Monte Carlo simulations under the hypothesis of disk locking and stellar angular momentum conservation. Methods. We built a set of objects with masses ranging from 0.01 Mo to 0.4 Mo and considered models with single- and double- peaked initial period distributions with and without disk locking. An object is considered to be diskless when its mass accretion rate is below a given threshold. Results. Models with initial single-peaked period distributions reproduce the observations well given that BDs rotate faster than VLM stars. We observe a correlation between rotational period and mass when we relax the disk locking hypothesis, but with a shallower slope compared to some observational results. The angular momentum evolution of diskless stars is flatter than it is for stars with a disk which occurs ...

  11. Brown dwarfs and very low mass stars in the Praesepe open cluster: a dynamically unevolved mass function?

    CERN Document Server

    Boudreault, S; Goldman, B; Henning, T; Caballero, J A

    2009-01-01

    [Abridged] In this paper, we present the results of a photometric survey to identify low mass and brown dwarf members of the old open cluster Praesepe (age of 590[+150][-120]Myr and distance of 190[+6.0][-5.8]pc) and use this to infer its mass function which we compare with that of other clusters. We have performed an optical (Ic-band) and near-infrared (J and Ks-band) photometric survey of Praesepe with a spatial coverage of 3.1deg^2. With 5sigma detection limits of Ic=23.4 and J=20.0, our survey is sensitive to objects with masses from about 0.6 to 0.05Msol. The mass function of Praesepe rises from 0.6Msol down to 0.1Msol and then turns-over at ~0.1Msol. The rise observed is in agreement with the mass function derived by previous studies, including a survey based on proper motion and photometry. Comparing our mass function with that for another open cluster with a similar age, the Hyades (age ~ 600Myr), we see a significant difference. Possible reasons are that dynamical evaporation has not influenced the H...

  12. Physical Properties of Young Brown Dwarfs and Very Low-Mass Stars Inferred from High-Resolution Model Spectra

    CERN Document Server

    Rice, Emily L; McLean, Ian S; Prato, L; Kirkpatrick, J Davy

    2009-01-01

    By comparing near-infrared spectra with atmosphere models, we infer the effective temperature, surface gravity, projected rotational velocity, and radial velocity for 21 very-low-mass stars and brown dwarfs. The unique sample consists of two sequences in spectral type from M6-M9, one of 5-10 Myr objects and one of >1 Gyr field objects. A third sequence is comprised of only ~M6 objects with ages ranging from 1 Gyr. Spectra were obtained in the J band at medium (R~2,000) and high (R~20,000) resolutions with NIRSPEC on the Keck II telescope. Synthetic spectra were generated from atmospheric structures calculated with the PHOENIX model atmosphere code. Using multi-dimensional least-squares fitting and Monte Carlo routines we determine the best-fit model parameters for each observed spectrum and note which spectral regions provide consistent results. We identify successes in the reproduction of observed features by atmospheric models, including pressure-broadened KI lines, and investigate deficiencies in the model...

  13. Induced Compression of White Dwarfs by Angular Momentum Loss

    CERN Document Server

    Boshkayev, Kuantay; Ruffini, Remo; Zhami, Bakytzhan

    2016-01-01

    We investigate isolated sub- and super-Chandrasekhar white dwarfs which lose angular momentum through magnetic dipole braking. We construct constant rest mass sequences by fulfilling all stability criteria of rotating configurations and show how the main structure of white dwarfs such as the central density, mean radius and angular velocity change with time. We explicitly demonstrate that all isolated white dwarfs regardless of their masses, by angular momentum loss, shrink and increase their central density. We also analyze the effects of the structure parameters on the evolution timescale both in the case of constant magnetic field and constant magnetic flux.

  14. On the Evolution of Hydrogen-Deficient White Dwarfs

    Science.gov (United States)

    Camisassa, M. E.; Althaus, L. G.; Rohrmann, R. D.; García–Berro, E.; Córsico, A. H.

    2017-03-01

    We present full evolutionary calculations for hydrogen-deficient white dwarfs. We take into account the evolutionary history of the progenitor stars, all relevant energy sources, element diffusion, and outer boundary conditions provided by new and detailed non-gray white dwarf model atmospheres for pure helium composition. Model atmospheres are based on the most up-to-date physical inputs. The calculations are extended down to an effective temperature of 2500 K. Our calculations provide a homogeneous set of evolutionary cooling tracks appropriate for mass and age determinations of old hydrogen-deficient white dwarfs.

  15. A radio-pulsing white dwarf binary star

    Science.gov (United States)

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

    2016-09-01

    White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco’s optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 107-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf’s spin, they mainly originate from the cool star. AR Sco’s broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf’s magnetosphere.

  16. Active states and structure transformations in accreting white dwarfs

    Science.gov (United States)

    Boneva, Daniela; Kaygorodov, Pavel

    2016-07-01

    Active states in white dwarfs are usually associated with light curve's effects that concern to the bursts, flickering or flare-up occurrences. It is common that a gas-dynamics source exists for each of these processes there. We consider the white dwarf binary stars with accretion disc around the primary. We suggest a flow transformation modeling of the mechanisms that are responsible for ability to cause some flow instability and bring the white dwarfs system to the outburst's development. The processes that cause the accretion rate to sufficiently increase are discussed. Then the transition from a quiescent to an active state is realized. We analyze a quasi-periodic variability in the luminosity of white dwarf binary stars systems. The results are supported with an observational data.

  17. 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.

  18. General Relativistic White Dwarfs and Their Astrophysical Implications

    CERN Document Server

    Boshkayev, Kuantay; Ruffini, Remo; Siutsou, Ivan

    2014-01-01

    We consider applications of general relativistic uniformly-rotating white dwarfs to several astrophysical phenomena related to the spin-up and the spin-down epochs and to delayed type Ia supernova explosions of super-Chandrasekhar white dwarfs, where we estimate the "spinning down" lifetime due to magnetic-dipole braking. In addition, we describe the physical properties of Soft Gamma Repeaters and Anomalous X-Ray Pulsars as massive rapidly-rotating highly-magnetized white dwarfs. Particularly we consider one of the so-called low-magnetic-field magnetars SGR 0418+5729 as a massive rapidly-rotating highly-magnetized white dwarf and give bounds for the mass, radius, moment of inertia, and magnetic field by requiring the general relativistic uniformly-rotating configurations to be stable.

  19. Magnetic White Dwarfs: Observations, Theory, and Future Prospects

    CERN Document Server

    García-Berro, Enrique; Kepler, S O

    2015-01-01

    Isolated magnetic white dwarfs have field strengths ranging from kilogauss to gigagauss, and constitute an interesting class of objects. The origin of the magnetic field is still the subject of a hot debate. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of the progenitor of white dwarfs, or on the contrary, are the result of binary interactions or, finally, other physical mechanisms that could produce such large magnetic fields during the evolution of the white dwarf itself, remains to be elucidated. In this work we review the current status and paradigms of magnetic fields in white dwarfs, from both the theoretical and observational points of view.

  20. Mass-radius relations of white dwarfs at finite temperatures

    OpenAIRE

    Boshkayev, Kuantay; Rueda, Jorge A.; Ruffini, Remo; Zhami, Bakytzhan; Kalymova, Zhanerke; Balgimbekov, Galymdin

    2016-01-01

    We construct mass-radius relations of white dwarfs taking into account the effects of rotation and finite temperatures. We compare and contrast the theoretical mass-radius relations with observational data.

  1. General relativistic white dwarfs and their astrophysical implications

    Energy Technology Data Exchange (ETDEWEB)

    Boshkayev, Kuantay [Al-Farabi Kazakh National University, Almaty (Kazakhstan); Rueda, Jorge A.; Ruffini, Remo [Sapienza University of Rome, Rome (Italy); Siutsou, Ivan [ICRANet, Square of Republic, Pescara (Italy)

    2014-09-15

    We consider applications of general relativistic uniformly-rotating white dwarfs to several astrophysical phenomena related to the spin-up and the spin-down epochs and to delayed type Ia supernova explosions of super-Chandrasekhar white dwarfs, where we estimate the 'spinning down' lifetime due to magnetic-dipole braking. In addition, we describe the physical properties of Soft Gamma Repeaters and Anomalous X-Ray Pulsars as massive rapidly-rotating highly-magnetized white dwarfs. Particularly we consider one of the so-called low-magnetic-field magnetars SGR 0418+5729 as a massive rapidly-rotating highly- magnetized white dwarf and give bounds for the mass, radius, moment of inertia, and magnetic field by requiring the general relativistic uniformly rotating configurations to be stable.

  2. Outbursts in Two New Cool Pulsating DA White Dwarfs

    Science.gov (United States)

    Bell, Keaton J.; Hermes, J. J.; Montgomery, M. H.; Gentile Fusillo, N. P.; Raddi, R.; Gänsicke, B. T.; Winget, D. E.; Dennihy, E.; Gianninas, A.; Tremblay, P.-E.; Chote, P.; Winget, K. I.

    2016-10-01

    The unprecedented extent of coverage provided by Kepler observations recently revealed outbursts in two hydrogen-atmosphere pulsating white dwarfs (DAVs) that cause hours-long increases in the overall mean flux of up to 14%. We have identified two new outbursting pulsating white dwarfs in K2, bringing the total number of known outbursting white dwarfs to four. EPIC 211629697, with {T}{eff} = 10,780 ± 140 K and {log} g = 7.94 ± 0.08, shows outbursts recurring on average every 5.0 days, increasing the overall flux by up to 15%. EPIC 229227292, with {T}{eff} = 11,190 ± 170 K and {log} g = 8.02 ± 0.05, has outbursts that recur roughly every 2.4 days with amplitudes up to 9%. We establish that only the coolest pulsating white dwarfs within a small temperature range near the cool, red edge of the DAV instability strip exhibit these outbursts.

  3. Analysis of White Dwarfs with Strange-Matter Cores

    CERN Document Server

    Mathews, G J; O'Gorman, B; Lan, N Q; Zech, W; Otsuki, K; Weber, F

    2006-01-01

    We summarize masses and radii for a number of white dwarfs as deduced from a combination of proper motion studies, Hipparcos parallax distances, effective temperatures, and binary or spectroscopic masses. A puzzling feature of these data is that some stars appear to have radii which are significantly smaller than that expected for a standard electron-degenerate white-dwarf equations of state. We construct a projection of white-dwarf radii for fixed effective mass and conclude that there is at least marginal evidence for bimodality in the radius distribution forwhite dwarfs. We argue that if such compact white dwarfs exist it is unlikely that they contain an iron core. We propose an alternative of strange-quark matter within the white-dwarf core. We also discuss the impact of the so-called color-flavor locked (CFL) state in strange-matter core associated with color superconductivity. We show that the data exhibit several features consistent with the expected mass-radius relation of strange dwarfs. We identify ...

  4. Follow-up Observations of SPY White Dwarf + M-Dwarf Binaries

    NARCIS (Netherlands)

    Maxted, P.F.L.; Napiwotzki, R.; Marsh, T.R.; Burleigh, M.R.; Dobbie, P.D.; Hogan, E.; Nelemans, G.A.

    2007-01-01

    We present the results of follow-up observations of white-dwarf + M-dwarf binaries identified using spectra obtained as part of the SPY survey. Spectra of the Halpha region were obtained with the SPIRAL spectrograph on the AAT telescope. Of the eleven stars observed, seven are binaries with periods

  5. QUIESCENT NUCLEAR BURNING IN LOW-METALLICITY WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Miller Bertolami, Marcelo M.; Althaus, Leandro G. [Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata (Argentina); García-Berro, Enrique [Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, E-08860 Castelldefels (Spain)

    2013-09-20

    We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich (DA) white dwarfs with very low metallicity progenitors (Z = 0.0001). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main-sequence masses, covering the most usual interval of white dwarf masses—from 0.53 M {sub ☉} to 0.83 M {sub ☉}. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from those predicted by standard white dwarf models. In particular, we find that for this metallicity and for white dwarf masses smaller than about 0.6 M {sub ☉}, nuclear reactions are the main contributor to the stellar luminosity for luminosities as low as log (L/L {sub ☉}) ≅ –3.2. This, in turn, should have a noticeable impact in the white dwarf luminosity function of low-metallicity stellar populations.

  6. Binary white dwarfs in the halo of the Milky Way

    Science.gov (United States)

    van Oirschot, Pim; Nelemans, Gijs; Toonen, Silvia; Pols, Onno; Brown, Anthony G. A.; Helmi, Amina; Portegies Zwart, Simon

    2014-09-01

    Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference with a bottom-heavy IMF is small. A burst of star formation 13 Gyr ago fits slightly better than a star formation burst 10 Gyr ago and also slightly better than continuous star formation 10-13 Gyr ago. Gaia will be the first instument to constrain the bright end of the field halo WDLF, where contributions from binary WDs are considerable. Many of these will have He cores, of which a handful have atypical surface gravities (log g 0 in our standard model for WD cooling. These so called pre-WDs, if observed, can help us to constrain white dwarf cooling models and might teach us something about the fraction of halo stars that reside in binaries. Appendices are available in electronic form at http://www.aanda.org

  7. The Montreal White Dwarf Database: a Tool for the Community

    CERN Document Server

    Dufour, P; Coutu, S; Fortin-Archambault, M; Thibeault, C; Bergeron, P; Fontaine, G

    2016-01-01

    We present the "Montreal White Dwarf Database" (MWDD), an accessible database with sortable/filterable table and interactive plots that will, when fully completed, allow the community to explore the physical properties of all white dwarfs ever analyzed by the Montreal group, as well as display data and analyses from the literature. We present its current capability and show how it will continuously be updated to instantly reflect improvements made on both the theoretical and observational fronts.

  8. Broad-Band Molecular Polarization in White Dwarfs

    Science.gov (United States)

    Berdyugina, S. V.; Berdyugin, A. V.; Piirola, V.; Shapiro, A.

    2007-09-01

    We present novel calculations of broad-band polarization due to the molecular Paschen--Back effect in a strong magnetic field. Based on that, we analyze new spectropolarimetric observations of the cool magnetic helium-rich white dwarf G 99-37 which shows strongly polarized molecular bands in its spectrum. Combining the polarimetric observations with our model calculations for the CH bands at 4300 Å, we deduce a magnetic field of 8 MG on this unique magnetic white dwarf.

  9. Presupernova evolution of accreting white dwarfs with rotation

    CERN Document Server

    Yoon, S C

    2004-01-01

    We discuss the effects of rotation on the evolution of accreting carbon-oxygen white dwarfs, with the emphasis on possible consequences in Type Ia supernova (SN Ia) progenitors. Starting with a slowly rotating white dwarf, we simulate the accretion of matter and angular momentum from a quasi-Keplerian accretion disk. The role of the various rotationally induced hydrodynamic instabilities for the transport of angular momentum inside the white dwarf is investigated. We find that the dynamical shear instability is the most important one in the highly degenerate core. Our results imply that accreting white dwarfs rotate differentially throughout,with a shear rate close to the threshold value for the onset of the dynamical shear instability. As the latter depends on the temperature of the white dwarf, the thermal evolution of the white dwarf core is found to be relevant for the angular momentum redistribution. As found previously, significant rotation is shown to lead to carbon ignition masses well above 1.4 Msun....

  10. A disintegrating minor planet transiting a white dwarf

    CERN Document Server

    Vanderburg, Andrew; Rappaport, Saul; Bieryla, Allyson; Irwin, Jonathan; Lewis, John Arban; Kipping, David; Brown, Warren R; Dufour, Patrick; Ciardi, David R; Angus, Ruth; Schaefer, Laura; Latham, David W; Charbonneau, David; Beichman, Charles; Eastman, Jason; McCrady, Nate; Wittenmyer, Robert A; Wright, Jason T

    2015-01-01

    White dwarfs are the end state of most stars, including the Sun, after they exhaust their nuclear fuel. Between 1/4 and 1/2 of white dwarfs have elements heavier than helium in their atmospheres, even though these elements should rapidly settle into the stellar interiors unless they are occasionally replenished. The abundance ratios of heavy elements in white dwarf atmospheres are similar to rocky bodies in the Solar system. This and the existence of warm dusty debris disks around about 4% of white dwarfs suggest that rocky debris from white dwarf progenitors' planetary systems occasionally pollute the stars' atmospheres. The total accreted mass can be comparable to that of large asteroids in the solar system. However, the process of disrupting planetary material has not yet been observed. Here, we report observations of a white dwarf being transited by at least one and likely multiple disintegrating planetesimals with periods ranging from 4.5 hours to 4.9 hours. The strongest transit signals occur every 4.5 ...

  11. A Search for Fine Wines: Discovering Close Red Dwarf-White Dwarf Binaries

    Science.gov (United States)

    Boyd, Mark; Finch, C. T.; Hambly, N. C.; Henry, T. J.; Jao, W.; Riedel, A. R.; Subasavage, J. P.; Winters, J. G.; RECONS

    2012-01-01

    Like fine wines, stars come in both red and white varieties. Here we present initial results of the Fine Wines Project that targets red dwarf-white dwarf pairs. The two scientific goals of Fine Wines are (1) to develop methods to estimate ages for red dwarfs based on the cooling ages of the white dwarfs, and (2) to identify suitable pairs for dynamical mass determinations of white dwarfs to probe their interior structures. Here we focus on the search for Fine Wines, including sample selection, elimination of false positives, and initial reconnaissance. The sample was extracted via color-color plots from a pool of more than 30,000 proper motion systems examined during the SuperCOSMOS-RECONS (SCR) and UCAC3 Proper Motion (UPM) surveys. The initial sample of 75 best candidates is being observed for BVRI photometry and 3500-9500 A spectroscopy to confirm whether or not the systems are red dwarf-white dwarf pairs. Early results indicate that roughly 50% of the candidates selected are indeed Fine Wine systems. This effort is supported by the NSF through grant AST 09-08402 and via observations made possible by the SMARTS Consortium.

  12. Activity and Kinematics of White Dwarf-M Dwarf Binaries from the SUPERBLINK Proper Motion Survey

    Science.gov (United States)

    Skinner, Julie N.; Morgan, Dylan P.; West, Andrew A.; Lépine, Sébastien; Thorstensen, John R.

    2017-09-01

    We present an activity and kinematic analysis of high proper motion white dwarf-M dwarf binaries (WD+dMs) found in the SUPERBLINK survey, 178 of which are new identifications. To identify WD+dMs, we developed a UV–optical–IR color criterion and conducted a spectroscopic survey to confirm each candidate binary. For the newly identified systems, we fit the two components using model white dwarf spectra and M dwarf template spectra to determine physical parameters. We use Hα chromospheric emission to examine the magnetic activity of the M dwarf in each system, and investigate how its activity is affected by the presence of a white dwarf companion. We find that the fraction of WD+dM binaries with active M dwarfs is significantly higher than their single M dwarf counterparts at early and mid-spectral types. We corroborate previous studies that find high activity fractions at both close and intermediate separations. At more distant separations, the binary fraction appears to approach the activity fraction for single M dwarfs. Using derived radial velocities and the proper motions, we calculate 3D space velocities for the WD+dMs in SUPERBLINK. For the entire SUPERBLINK WD+dMs, we find a large vertical velocity dispersion, indicating a dynamically hotter population compared to high proper motion samples of single M dwarfs. We compare the kinematics for systems with active M dwarfs and those with inactive M dwarfs, and find signatures of asymmetric drift in the inactive sample, indicating that they are drawn from an older population. Based on observations obtained at the MDM Observatory operated by Dartmouth College, Columbia University, The Ohio State University, and the University of Michigan.

  13. Seismology of Rapidly Rotating Accreting White Dwarfs

    CERN Document Server

    Townsley, Dean M; Bildsten, Lars

    2016-01-01

    A number of White Dwarfs (WDs) in cataclysmic binaries have shown brightness variations consistent with non-radial oscillations as observed in isolated WDs. A few objects have been well-characterized with photometric campaigns in the hopes of gleaning information about the mass, spin, and possibly internal structural characteristics. The novel aspect of this work is the possiblity to measure or constrain the interior structure and spin rate of WDs which have spent gigayears accreting material from their companion, undergoing thousands of nova outbursts in the process. In addition, variations in the surface temperature affect the site of mode driving, and provide unique and challenging tests for mode driving theories previously applied to isolated WD's. Having undergone long-term accretion, these WDs are expected to have been spun up. Spin periods in the range 60-100 seconds have been measured by other means for two objects, GW Lib and V455 And. Compared to typical mode frequencies, the spin frequency may be s...

  14. Spin and Magnetism of White Dwarfs

    CERN Document Server

    Kissin, Yevgeni

    2015-01-01

    The magnetism and rotation of white dwarf (WD) stars are investigated in relation to a hydromagnetic dynamo operating in the progenitor during shell burning phases. We find that the downward pumping of angular momentum in the convective envelope can, by itself, trigger dynamo action near the core-envelope boundary in an isolated intermediate-mass star. A solar-mass star must receive additional angular momentum following its rotational braking on the main sequence, either by a merger with a planet, or by tidal interaction in a stellar binary. Several arguments point to the outer core as the source for a magnetic field in the WD remnant: i) the outer third of a ~0.55$M_\\odot$ WD is processed during the shell burning phases of the progenitor; ii) escape of magnetic helicity through the envelope mediates the growth of (compensating) helicity in the core, as is needed to maintain a stable magnetic field in the remnant; and iii) intense radiation flux at the core boundary facilitates magnetic buoyancy within a rela...

  15. On general relativistic uniformly rotating white dwarfs

    CERN Document Server

    Boshkayev, Kuantay; Ruffini, Remo; Siutsou, Ivan

    2012-01-01

    Uniformly rotating white dwarfs (RWDs) are analyzed within the framework of general relativity. The Hartle's formalism is applied to construct self-consistently the internal and external solutions to the Einstein equations. The relativistic Feynman-Metropolis-Teller EoS that generalizes the Salpeter's one taking fully into account the finite size of nuclei, the Coulomb interactions as well as electroweak equilibrium in a self-consistent relativistic fashion is used to describe the WD matter. The mass, radius, angular momentum, eccentricity and quadrupole moment of RWDs are calculated as a function of the central density and rotation angular velocity. We construct the region of stability of RWDs taking into account the mass-shedding limit, inverse beta-decay instability, and the boundary established by the turning points of constant angular momentum sequences that separates stable from secularly unstable configurations. We found the minimum rotation periods 0.3, 0.5, 0.7 and 2.2 seconds and maximum masses 1.50...

  16. White Dwarf Cosmochronology in the Solar Neighborhood

    CERN Document Server

    Tremblay, P -E; Soderblom, D R; Cignoni, M; Cummings, J

    2014-01-01

    The study of the stellar formation history in the solar neighborhood is a powerful technique to recover information about the early stages and evolution of the Milky Way. We present a new method which consists of directly probing the formation history from the nearby stellar remnants. We rely on the volume complete sample of white dwarfs within 20 pc, where accurate cooling ages and masses have been determined. The well characterized initial-final mass relation is employed in order to recover the initial masses (1 < M/Msun < 8) and total ages for the local degenerate sample. We correct for moderate biases that are necessary to transform our results to a global stellar formation rate, which can be compared to similar studies based on the properties of main-sequence stars in the solar neighborhood. Our method provides precise formation rates for all ages except in very recent times, and the results suggest an enhanced formation rate for the solar neighborhood in the last 5 Gyr compared to the range 5 <...

  17. Thermonuclear detonations ensuing white dwarf mergers

    CERN Document Server

    Dan, Marius; Brüggen, Marcus; Ramirez-Ruiz, Enrico; Rosswog, Stephan

    2015-01-01

    The merger of two white dwarfs (WDs) has for many years not been considered as the favoured model for the progenitor system of type Ia supernovae (SNe Ia). But recent years have seen a change of opinion as a number of studies, both observational and theoretical, have concluded that they should contribute significantly to the observed type Ia supernova rate. In this paper, we study the ignition and propagation of detonation through post-merger remnants and we follow the resulting nucleosynthesis up to the point where a homologous expansion is reached. In our study we cover the entire range of WD masses and compositions. For the emergence of a detonation we study several setups, guided by both merger remnants from our own simulations and by results taken from the literature. We carefully compare the nucleosynthetic yields of successful explosions with SN Ia observations. Only three of our models are consistent with all the imposed constraints and potentially lead to a standard type Ia event. The first one, a $0...

  18. The binary white dwarf LHS 3236

    Energy Technology Data Exchange (ETDEWEB)

    Harris, Hugh C.; Dahn, Conard C.; Canzian, Blaise; Guetter, Harry H.; Levine, Stephen E.; Luginbuhl, Christian B.; Monet, Alice K. B.; Stone, Ronald C.; Subasavage, John P.; Tilleman, Trudy; Walker, Richard L. [US Naval Observatory, 10391 West Naval Observatory Road, Flagstaff, AZ 86001-8521 (United States); Dupuy, Trent J.; Liu, Michael C. [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Hartkopf, William I. [US Naval Observatory, 3450 Massachusetts Avenue, N.W., Washington, DC 20392-5420 (United States); Ireland, Michael J. [Department of Physics and Astronomy, Macquarie University, New South Wales, NSW 2109 (Australia); Leggett, S. K., E-mail: hch@nofs.navy.mil [Gemini Observatory, 670 N. Aohoku Place, Hilo, HI 96720 (United States)

    2013-12-10

    The white dwarf LHS 3236 (WD1639+153) is shown to be a double-degenerate binary, with each component having a high mass. Astrometry at the U.S. Naval Observatory gives a parallax and distance of 30.86 ± 0.25 pc and a tangential velocity of 98 km s{sup –1}, and reveals binary orbital motion. The orbital parameters are determined from astrometry of the photocenter over more than three orbits of the 4.0 yr period. High-resolution imaging at the Keck Observatory resolves the pair with a separation of 31 and 124 mas at two epochs. Optical and near-IR photometry give a set of possible binary components. Consistency of all data indicates that the binary is a pair of DA stars with temperatures near 8000 and 7400 K and with masses of 0.93 and 0.91 M {sub ☉}; also possible is a DA primary and a helium DC secondary with temperatures near 8800 and 6000 K and with masses of 0.98 and 0.69 M {sub ☉}. In either case, the cooling ages of the stars are ∼3 Gyr and the total ages are <4 Gyr. The combined mass of the binary (1.66-1.84 M {sub ☉}) is well above the Chandrasekhar limit; however, the timescale for coalescence is long.

  19. Merging white dwarfs and SN Ia

    CERN Document Server

    Yungelson, L R

    2016-01-01

    Using population synthesis, we study a double-degenerate (DD) scenario for SNe Ia, aiming to estimate the maximum possible contribution to the rate of SNe from this scenario and the dependence of the delay-time distribution (DTD) on it. We make an extreme assumption that all mergers of super-Chandrasekhar pairs of CO white dwarfs (WDs) and mergers of CO WDs more massive than 0.47 $M_\\odot$ with hybrid or helium WDs more massive than 0.37$M_\\odot$ produce SNe Ia. The models are parametrized by the product of the common envelope efficiency and the parameter of binding energy of stellar envelopes $\\alpha_{ce}\\lambda$, which we vary between 0.25 and 2. The best agreement with observations is obtained for $\\alpha_{ce}\\lambda$=2. A substantial contribution to the rate of SNe Ia is provided by the pairs with a hybrid WD. The estimated Galactic rate of SNe Ia is $6.5 10^{-3}$ per yr (for the mass of the bulge and thin disk equal to $7.2 10^{10} M_\\odot$), which is comparable to the observational estimate $(5.4\\pm0.12...

  20. White dwarf masses in cataclysmic variables

    CERN Document Server

    Wijnen, T P G; Schreiber, M R

    2015-01-01

    The white dwarf (WD) mass distribution of cataclysmic variables (CVs) has recently been found to dramatically disagree with the predictions of the standard CV formation model. The high mean WD mass among CVs is not imprinted in the currently observed sample of CV progenitors and cannot be attributed to selection effects. Two possibilities have been put forward: either the WD grows in mass during CV evolution, or in a significant fraction of cases, CV formation is preceded by a (short) phase of thermal time-scale mass transfer (TTMT) in which the WD gains a sufficient amount of mass. We investigate if either of these two scenarios can bring theoretical predictions and observations into agreement. We employed binary population synthesis models to simulate the present intrinsic CV population. We incorporated aspects specific to CV evolution such as an appropriate mass-radius relation of the donor star and a more detailed prescription for the critical mass ratio for dynamically unstable mass transfer. We also imp...

  1. A Dark Spot on a Massive White Dwarf

    CERN Document Server

    Kilic, Mukremin; Bell, Keaton J; Curd, Brandon; Brown, Warren R; Hermes, J J; Dufour, Patrick; Wisniewski, John P; Winget, D E; Winget, K I

    2015-01-01

    We present the serendipitous discovery of eclipse-like events around the massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We selected J1529+2928 for time-series photometry based on its spectroscopic temperature and surface gravity, which place it near the ZZ Ceti instability strip. Instead of pulsations, we detect photometric dips from this white dwarf every 38 minutes. Follow-up optical spectroscopy observations with Gemini reveal no significant radial velocity variations, ruling out stellar and brown dwarf companions. A disintegrating planet around this white dwarf cannot explain the observed light curves in different filters. Given the short period, the source of the photometric dips must be a dark spot that comes into view every 38 min due to the rotation of the white dwarf. Our optical spectroscopy does not show any evidence of Zeeman splitting of the Balmer lines, limiting the magnetic field strength to B<70 kG. Since up to 15% of white dwarfs display kG magnetic fields, such ecli...

  2. Hubble Space Telescope Studies of Exposed White Dwarfs in Dwarf Novae

    Science.gov (United States)

    Sion, Edward M.

    Coordinated AAVSO optical observations and Hubble Space Telescope (HST) far ultraviolet (UV) spectroscopic observations of cataclysmic variables, during dwarf nova quiescence when the underlying white dwarf is exposed in the far UV, have yielded a number of new insights into accretional heating, photospheric abundances of the accreted atmosphere, and rotational velocities of the underlying degenerates. Recent results of synthetic spectral analyses of HST spectra are highlighted. Their impact on our understanding of accretion physics and the effect of accretion on the white dwarf are discussed.

  3. Constraining White Dwarf Masses Via Apsidal Precession in Eccentric Double White Dwarf Binaries

    CERN Document Server

    Valsecchi, Francesca; Willems, Bart; Deloye, Christopher J; Kalogera, Vicky

    2011-01-01

    Galactic short period double white dwarfs (DWD) are guaranteed gravitational wave (GW) sources for the next generation of space-based interferometers sensitive to low-frequency GWs (10^{-4}- 1 Hz). Here we investigate the possibility of constraining the white dwarf (WD) properties through measurements of apsidal precession in eccentric binaries. We analyze the general relativistic (GR), tidal, and rotational contributions to apsidal precession by using detailed He WD models. We find that apsidal precession can lead to a detectable shift in the emitted GW signal, the effect being stronger (weaker) for binaries hosting hot (cool) WDs. We find that in hot (cool) DWDs tides dominate the precession at orbital frequencies above ~0.01 mHz (~1 mHz). Analyzing the apsidal precession of these sources only accounting for GR would potentially lead to an extreme overestimate of the component masses. Finally, we derive a relation that ties the radius and apsidal precession constant of cool WD components to their masses, th...

  4. Spectroscopy of the DA white dwarfs - Automatic atmospheric parameterization and mass distribution

    Science.gov (United States)

    Mcmahan, Robert K.

    1989-01-01

    A method for the automatic calculation of the atmospheric parameters (Teff and log g) of hydrogen-rich degenerate stars from low-resolution spectra is described, and then applied to the spectra of 53 DA white dwarfs. A value for the width of the DA mass distribution of sigma M/solar-M not greater than +0.10 is obtained using the proposed approach. The data indicate that the distribution is asymmetrically skewed to low masses; however, there is also evidence of a high-mass non-Gaussian tail.

  5. 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.

  6. 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.

  7. Merging white dwarfs and Type Ia supernovae

    Science.gov (United States)

    Yungelson, L. R.; Kuranov, A. G.

    2017-01-01

    Using population synthesis, we study a double-degenerate (DD) scenario for Type Ia supernovae (SNe Ia), aiming to estimate the maximum possible contribution to the rate of SNe from this scenario and the dependence of the delay-time distribution (DTD) on it. We make an extreme assumption that all mergers of super-Chandrasekhar pairs of CO white dwarfs (WDs) and mergers of CO WDs more massive than 0.47 M⊙ with hybrid or helium WDs more massive than 0.37 M⊙ produce SNe Ia. The models are parametrized by the product of the common envelope efficiency and the parameter of binding energy of stellar envelopes, αce λ, which we vary between 0.25 and 2. The best agreement with observations is obtained for αce λ = 2. A substantial contribution to the rate of SNe Ia is provided by the pairs with a hybrid WD. The estimated Galactic rate of SNe Ia is 6.5 × 10-3 yr-1 (for the mass of the bulge and thin disc equal to 7.2 × 1010 M⊙), which is comparable to the observational estimate (5.4 ± 0.12) × 10-3 yr-1. The model DTD for 1 ≤ t ≤ 8 Gyr satisfactorily fits the DTD for SNe Ia in the field galaxies (Maoz, Mannucci & Brandt). For this epoch, the model DTD is ∝t-1.64. At earlier and later epochs, our DTD has a deficit of events, as in other studies. Marginal agreement with the observational DTD is achieved even if only CO+CO WDs with M1 ≥ 0.8 M⊙ and M2 ≥ 0.6 M⊙ produce SNe Ia. A better agreement of observed and modelled DTD may be obtained if tidal effects are weaker than assumed and/or the metallicity of the population is much lower than solar.

  8. Dissecting accretion and outflows in accreting white dwarf binaries

    CERN Document Server

    de Martino, D; Balman, S; Bernardini, F; Bianchini, A; Bode, M; Bonnet-Bidaud, J -M; Falanga, M; Greiner, J; Groot, P; Hernanz, M; Israel, G; Jose, J; Motch, C; Mouchet, M; Norton, A J; Nucita, A; Orio, M; Osborne, J; Ramsay, G; Rodriguez-Gil, P; Scaringi, S; Schwope, A; Traulsen, I; Tamburini, F

    2015-01-01

    This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of accreting white dwarfs. For a summary, we refer to the paper.

  9. Disk Accretion of Tidally Disrupted Rocky Bodies onto White Dwarfs

    Science.gov (United States)

    Feng, W.; Desch, S.

    2017-03-01

    The prevailing model for the pollution of white dwarf photospheres invokes accretion from a disk of gas and solid particles, fed by tidal disruption of rocky bodies inside the Roche radius. Current models can successfully explain the accretion rates of metals onto white dwarfs, provided the gaseous disks viscously spread at rates consistent with a partially suppressed magnetorotational instability (Metzger et al. 2012); however, these models do not explore the extent of the magnetorotational instability in disks by calculating the degree of ionization. We present ionization fractions for thermal and non-thermal processes to assess the extent of the magnetorotational instability in white dwarf disks. We determine that the disk viscosity parameter α can be as high as 0.1 in white disks, implying that the magnetorotational instability must be carefully modeled.

  10. Latest Results from the DODO Survey: Imaging Planets around White Dwarfs

    CERN Document Server

    Hogan, E; Clarke, F J

    2011-01-01

    The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures lower (< 500 K) than the coolest brown dwarfs currently observed. The discovery of planets around white dwarfs would prove that such objects can survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors (with masses between 1.5 - 8 M*, i.e., early B to mid-F). An increasing number of planetary mass companions have been directly imaged in wide orbits around young main sequence stars. For example, the planets around HR 8799 and 1RXS J160929.1 - 210524 are in wide orbits of 24 - 68 AU and 330 AU, respectively. The DODO survey has the ability to directly image planets in post-main sequence analogues of these systems. These proceedings ...

  11. Searching for benchmark systems containing ultra-cool dwarfs and white dwarfs

    Directory of Open Access Journals (Sweden)

    Pinfield D.J.

    2013-04-01

    Full Text Available We have used the 2MASS all-sky survey and WISE to look for ultracool dwarfs that are part of multiple systems containing main sequence stars. We cross-matched L dwarf candidates from the surveys with Hipparcos and Gliese stars, finding two new systems. We consider the binary fraction for L dwarfs and main sequence stars, and further assess possible unresolved multiplicity within the full companion sample. This analysis shows that some of the L dwarfs in this sample might actually be unresolved binaries themselves. We have also identified a sample of common proper motion systems in which a main sequence star has a white dwarf as wide companion. These systems can help explore key issues in star evolution theory, as the initial-final mass relationship of white dwarfs, or the chromospheric activity-age relationship for stars still in the main sequence. Spectroscopy for 50 white dwarf candidates, selected from the SuperCOSMOS Science Archive, was obtained. We have also observed 6 of the main sequence star companions, and have estimated their effective temperatures, rotational and microturbulent velocities and metallicities.

  12. On the long term evolution of white dwarfs in cataclysmic variables and their recurrence times

    Science.gov (United States)

    Sion, E. M.; Starrfield, S. G.

    1985-01-01

    The relevance of the long term quasi-static evolution of accreting white dwarfs to the outbursts of Z Andromeda-like symbiotics; the masses and accretion rates of classical nova white dwarfs; and the observed properties of white dwarfs detected optically and with IUE in low M dot cataclysmic variables is discussed. A surface luminosity versus time plot for a massive, hot white dwarf bears a remarkable similarity to the outburst behavior of the hot blue source in Z Andromeda. The long term quasi-static models of hot accreting white dwarfs provide convenient constraints on the theoretically permissible parameters to give a dynamical (nova-like) outburst of classic white dwarfs.

  13. A Code for Stellar Binary Evolution and its Application to the Formation of Helium White Dwarfs

    CERN Document Server

    Benvenuto, O G

    2003-01-01

    We present a numerical code intended for calculating stellar evolution in close binary systems. In doing so, we consider that mass transfer episodes occur when the stellar size overflows the corresponding Roche lobe. In such situation we equate the radius of the star with the equivalent radius of the Roche lobe. This equation is handled implicitly together with those corresponding to the whole structure of the star. We describe in detail the necessary modifications to the standard Henyey technique for treating the mass loss rate implicitly together with thin outer layers integrations. We have applied this code to the calculation of the formation of low mass, helium white dwarfs in low mass close binary systems. We found that the global numerical convergence properties are fairly good. In particular, the onset and end of mass transfer episodes is computed automatically.

  14. The First Mid-Infrared Spectra of Cool White Dwarfs

    CERN Document Server

    Kilic, Mukremin; Mullally, Fergal; Reach, William T; von Hippel, Ted

    2007-01-01

    We present the first mid-infrared spectra of two cool white dwarfs obtained with the Spitzer Space Telescope. We also present 3.5-8 micron photometry for 19 cool white dwarfs with 5000K < Teff < 9000K. We perform a detailed model atmosphere analysis of these white dwarfs by fitting their UBVRIJHK and Spitzer photometry with state-of-the-art model atmospheres, and demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our mid-IR photometry and 7.5-14.5 micron spectrum of WD0018-267 are consistent with a Teff = 5720K, pure hydrogen white dwarf model atmosphere. On the other hand, LHS 1126 remains peculiar with significant mid-IR flux deficits in all IRAC bands and a featureless spectrum in the 5.2-7.5 micron range. Even though this deficit is attributed to collision induced absorption (CIA) due to molecular hydrogen, the shape of the deficit cannot be explained with current CIA opacity calculations. The infrared portion of th...

  15. Outer boundary conditions for evolving cool white dwarfs

    CERN Document Server

    Rohrmann, R D; García-Berro, E; Córsico, A H; Bertolami, M M Miller

    2012-01-01

    White dwarf evolution is essentially a gravothermal cooling process, which,for cool white dwarfs, sensitively depends on the treatment of the outer boundary conditions. We provide detailed outer boundary conditions appropriate for computing the evolution of cool white dwarfs employing detailed non-gray model atmospheres for pure H composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are computed taken into account non-ideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman alpha quasi-molecular opacity. Our results show that the use of detailed outer boundary conditions becomes relevant for effective temperatures lower than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively. Detailed model atmospheres predict ages that are up to approx 10% shorter at log L/L_sun=-4 when compared with...

  16. Quiescent nuclear burning in low-metallicity white dwarfs

    CERN Document Server

    Bertolami, Marcelo M Miller; Garcia-Berro, Enrique

    2013-01-01

    We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich DA white dwarfs with very low metallicity progenitors ($Z=0.0001$). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main sequence masses, covering the most usual interval of white dwarf masses --- from $0.53\\, M_{\\sun}$ to $0.83\\, M_{\\sun}$. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase ...

  17. Discovery of kilogauss magnetic fields in three DA white dwarfs

    CERN Document Server

    Aznar Cuadrado, R; Napiwotzki, R; Schmid, H M; Solanki, S K; Mathys, G

    2004-01-01

    We have detected longitudinal magnetic fields between 2 and 4 kG in three (WD 0446$-$790, WD 1105$-$048, WD 2359$-$434) out of a sample of 12 normal DA white dwarfs by using optical spectropolarimetry done with the VLT Antu 8 m telescope equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first positive detections of magnetic fields in white dwarfs below 30 kG. Although suspected, it was not clear whether a significant fraction of white dwarfs contain magnetic fields at this level. These fields may be explained as fossil relics from magnetic fields in the main-sequence progenitors considerably enhanced by magnetic flux conservation during the shrinkage of the core. A detection rate of 25 % (3/12) may indicate now for the first time that a substantial fraction of white dwarfs have a weak magnetic field. This result, if confirmed by future observations, would form a cornerstone for our understanding on the evolution of stellar magnetic fields. Keywords: stars: white dwarfs - stars: magnetic...

  18. White Dwarfs in HETDEX: Preparation for the Survey

    Science.gov (United States)

    Castanheira, B. G.; Winget, D. E.

    2015-06-01

    In the past decade, large scale surveys have discovered a large number of white dwarf stars. Many new aspects have been revealed, including the discovery of the DQVs, close-in non-contact binary systems, and debris disks around many stars. Unfortunately, the population statistics of the newly discovered white dwarf stars are poorly constrained, because of the various methods used to assign objects to fibers for spectroscopic observations in the SDSS survey. A white dwarf sample that is magnitude limited, with well-constrained selection criteria, is needed. The HET Dark Energy Experiment (HETDEX) will use the 9.2 m HET at McDonald Observatory and a set of more than 100 spectrographs to map the three-dimensional positions of one million galaxies, to probe dark energy. In this unique magnitude limited survey, all objects that fall into the fibers will be observed. We expect to observe spectroscopically about 10,00 white dwarf stars. In this paper, we will present the specifications and current status of HETDEX, which will start taking data in Fall 2014. We will also show our first results from observations of white dwarf stars using an identical spectrograph with the 2.7m HJS Telescope and discuss some of the approaches we have been working on in preparation for this exciting survey.

  19. A Multi-Survey Approach to White Dwarf Discovery

    CERN Document Server

    Sayres, Conor; Bergeron, P; Dufour, P; Davenport, James R A; AlSayyad, Yusra; Tofflemire, Benjamin M

    2012-01-01

    By selecting astrometric and photometric data from the Sloan Digital Sky Survey (SDSS), the L{\\'e}pine & Shara Proper Motion North Catalog (LSPM-North), the Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a succession of methods to isolate white dwarf candidates for follow-up spectroscopy. Our methods include: reduced proper motion diagram cuts, color cuts, and atmospheric model adherence. We present spectroscopy of 26 white dwarfs obtained from the CTIO 4m and APO 3.5m telescopes. Additionally, we confirm 28 white dwarfs with spectra available in the SDSS DR7 database but unpublished elsewhere, presenting a total of 54 WDs. We label one of these as a recovered WD while the remaining 53 are new discoveries. We determine physical parameters and estimate distances based on atmospheric model analyses. Three new white dwarfs are modeled to lie within 25 pc. Two additional white dwarfs are confirmed to be metal-polluted (DAZ). Follow-up time series photometry confirms another object to be ...

  20. Detonability of white dwarf plasma: turbulence models at low densities

    Science.gov (United States)

    Fenn, D.; Plewa, T.

    2017-06-01

    We study the conditions required to produce self-sustained detonations in turbulent, carbon-oxygen degenerate plasma at low densities. We perform a series of three-dimensional hydrodynamic simulations of turbulence driven with various degrees of compressibility. The average conditions in the simulations are representative of models of merging binary white dwarfs. We find that material with very short ignition times is abundant in case turbulence is driven compressively. This material forms contiguous structures that persist over many ignition times, and that we identify as prospective detonation kernels. Detailed analysis of prospective kernels reveals that these objects are centrally condensed and their shape is characterized by low curvature, supportive of self-sustained detonations. The key characteristic of the newly proposed detonation mechanism is thus high degree of compressibility of turbulent drive. The simulated detonation kernels have sizes notably smaller than the spatial resolution of any white dwarf merger simulation performed to date. The resolution required to resolve kernels is 0.1 km. Our results indicate a high probability of detonations in such well-resolved simulations of carbon-oxygen white dwarf mergers. These simulations will likely produce detonations in systems of lower total mass, thus broadening the population of white dwarf binaries capable of producing Type Ia supernovae. Consequently, we expect a downward revision of the lower limit of the total merger mass that is capable of producing a prompt detonation. We review application of the new detonation mechanism to various explosion scenarios of single, Chandrasekhar-mass white dwarfs.

  1. Signs of a faint disc population at polluted white dwarfs

    CERN Document Server

    Bergfors, Carolina; Dufour, Patrick; Rocchetto, Marco

    2014-01-01

    Observations of atmospheric metals and dust discs around white dwarfs provide important clues to the fate of terrestrial planetary systems around intermediate mass stars. We present Spitzer IRAC observations of 15 metal polluted white dwarfs to investigate the occurrence and physical properties of circumstellar dust created by the disruption of planetary bodies. We find subtle infrared excess emission consistent with warm dust around KUV 15519+1730 and HS 2132+0941, and weaker excess around the DZ white dwarf G245-58, which, if real, makes it the coolest white dwarf known to exhibit a 3.6 micron excess and the first DZ star with a bright disc. All together our data corroborate a picture where 1) discs at metal-enriched white dwarfs are commonplace and most escape detection in the infrared (possibly as narrow rings), 2) the discs are long lived, having lifetimes on the order of 10^6 yr or longer, and 3) the frequency of bright, infrared detectable discs decreases with age, on a timescale of roughly 500 Myr, su...

  2. Pulsations powered by hydrogen shell burning in white dwarfs

    CERN Document Server

    Camisassa, María E; Althaus, Leandro G; Shibahashi, Hiromoto

    2016-01-01

    In the absence of a third dredge-up episode during the asymptotic giant branch phase, white dwarf models evolved from low-metallicity progenitors have a thick hydrogen envelope, which makes hydrogen shell burning be the most important energy source. We investigate the pulsational stability of white dwarf models with thick envelopes to see whether nonradial $g$-mode pulsations are triggered by hydrogen burning, with the aim of placing constraints on hydrogen shell burning in cool white dwarfs and on a third dredge-up during the asymptotic giant branch evolution of their progenitor stars. We construct white-dwarf sequences from low-metallicity progenitors by means of full evolutionary calculations, and analyze their pulsation stability for the models in the range of effective temperatures $T_{\\rm eff} \\sim 15\\,000\\,-\\, 8\\,000$ K. We demonstrate that, for white dwarf models with masses $M_{\\star} \\lesssim 0.71\\,\\rm M_{\\sun}$ and effective temperatures $8\\,500 \\lesssim T_{\\rm eff} \\lesssim 11\\,600$ K that evolved...

  3. Gravitational settling of 22Ne and white dwarf evolution

    CERN Document Server

    García--Berro, E; Córsico, A H; Isern, J

    2007-01-01

    We study the effects of the sedimentation of the trace element 22Ne in the cooling of white dwarfs. In contrast with previous studies, which adopted a simplified treatment of the effects of 22Ne sedimentation, this is done self-consistently for the first time, using an up-to-date stellar evolutionary code in which the diffusion equation is coupled with the full set of equations of stellar evolution. Due the large neutron excess of 22Ne, this isotope rapidly sediments in the interior of the white dwarf. Although we explore a wide range of parameters, we find that using the most reasonable assumptions concerning the diffusion coefficient and the physical state of the white dwarf interior the delay introduced by the ensuing chemical differentation is minor for a typical 0.6 Msun white dwarf. For more massive white dwarfs, say M_Wd about 1.0 Msun, the delay turns out to be considerably larger. These results are in qualitatively good accord with those obtained in previous studies, but we find that the magnitude of...

  4. Properties of the Eclipsing Double-White Dwarf Binary NLTT 11748

    CERN Document Server

    Kaplan, David L; Walker, Arielle N; Bildsten, Lars; Bours, Madelon C P; Breedt, Elmé; Copperwheat, Chris M; Dhillon, Vik S; Howell, Steve B; Littlefair, Stuart P; Shporer, Avi; Steinfadt, Justin D R

    2013-01-01

    We present high-quality ULTRACAM photometry of the eclipsing detached double-white dwarf binary NLTT 11748. This system consists of a carbon/oxygen white dwarf and an extremely-low mass (1.5 yr, we constrain the masses and radii of both objects in the NLTT 11748 system to a statistical uncertainty of a few percent. However, we find that overall uncertainty in the thickness of the envelope of the secondary carbon/oxygen white dwarf leads to a larger (~13%) systematic uncertainty in the primary He WD's mass. Over the full range of possible envelope thicknesses we find that our primary mass (0.136-0.162 Msun) and surface gravity (log(g)=6.32-6.38; radii are 0.0423-0.0433 Rsun) constraints do not agree with previous spectroscopic determinations. We use precise eclipse timing to detect the Romer delay at 7 sigma significance, providing an additional weak constraint on the masses and limiting the eccentricity to e*cos(omega)= -4e-5 +/- 5e-5. Finally, we use multi-color data to constrain the secondary's effective te...

  5. A search for binarity using FUSE observations of DAO white dwarfs

    CERN Document Server

    Good, S A; Burleigh, M R; Dobbie, P D; Holberg, J B

    2005-01-01

    We report on a search for evidence of binarity in Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of DAO white dwarfs. Spectra recorded by FUSE are built up from a number of separate exposures. Observation of changes in the position of photospheric heavy element absorption lines between exposures, with respect to the stationary interstellar medium lines, would reveal radial velocity changes - evidence of the presence of a binary system. This technique is successful in picking out all the white dwarfs already known to be binaries, which comprise 5 out of the sample of 16, but significant radial velocity shifts were found for only one additional star, Ton 320. This object is also known to have an infrared excess. DAOs can be separated broadly into low or normal mass objects. Low mass white dwarfs can be formed as a result of binary evolution, but it has been suggested that the lower mass DAOs evolve as single stars from the extended horizontal branch, and we find no evidence of binarity for 8 out of ...

  6. Direct formation of millisecond pulsars from rotationally delayed accretion-induced collapse of massive white dwarfs

    CERN Document Server

    Freire, Paulo C C

    2013-01-01

    Millisecond pulsars (MSPs) are believed to be old neutron stars, formed via Type Ib/c core-collapse supernovae, which have subsequently been spun up to high rotation rates via accretion from a companion star in a highly circularised low-mass X-ray binary. The recent discoveries of Galactic field binary MSPs in eccentric orbits, and mass functions compatible with that expected for helium white dwarf companions, PSR J2234+06 and PSR J1946+3417, therefore challenge this picture. Here we present a hypothesis for producing this new class of systems, where the MSPs are formed directly from a rotationally-delayed accretion-induced collapse of a super-Chandrasekhar mass white dwarf. We compute the orbital properties of the MSPs formed in such events and demonstrate that our hypothesis can reproduce the observed eccentricities, masses and orbital periods of the white dwarfs, as well as forecasting the pulsar masses and velocities. Finally, we compare this hypothesis to a triple star scenario.

  7. Does an Average White Dwarf Have Enough Mass to Prevent an Accretion Disk Tilt?

    CERN Document Server

    Montgomery, M M

    2010-01-01

    In a recent publication, we introduce the lift force as a common source to accretion disk tilt that is likely relevant to accretion disk systems. Lift is generated by slightly different supersonic gas stream speeds flowing over and under the disk at the bright spot. In this conference proceeding, we focus on whether the average white dwarf has enough mass to prevent a disk tilt in non-magnetic Cataclysmic Variables (CVs) with accretion disks. Assuming a white dwarf mass of 0.6M$_{\\odot}$ and a disk mass of 10$^{-11}$M$_{\\odot}$, we vary the secondary mass to establish theoretical minimum mass transfer rates needed to induce and maintain a disk tilt of four degrees around the line of nodes. For mass ratios in the range \\( (0.13 \\le q=M_{2}M^{-1} \\le 0.45) \\), we confirm that the secondary mass does not contribute significantly to disk tilt. We also confirm that the average white dwarf does not have enough mass to prevent a disk tilt. We find that disk tilt may be likely in low mass transfer rate systems such a...

  8. White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy

    CERN Document Server

    Torres, S; Isern, J; Garcia-Berro, E

    2010-01-01

    The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass substellar objects and stars with masses larger than ~M_{sun} have been ruled out as major components of a Massive Astrophysical Compact Halo Object (MACHO) Galactic halo, while stars of half a solar mass seem to be viable candidates. Main sequence stars have been already discarded, and there are tight restrictions on the role played by white dwarfs with hydrogen-dominated atmospheres. In this paper we evaluate the contribution to the dark matter content of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. For this purpose we use a Monte Carlo simulator which incorporates up-to-date evolutionary sequences of white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres. We also take into account detailed descriptions of the thick disk and the halo of our Galaxy as well as of a reliable model of the LMC. We find that the contribution of whit...

  9. The Spectral Energy Distributions of White Dwarfs in 47 Tucanae: The Distance to the Cluster

    CERN Document Server

    Woodley, Kristin A; Kalirai, Jason; Richer, Harvey; Tremblay, Pier-Emmanuel; Anderson, Jay; Bergeron, Pier; Dotter, Aaron; Esteves, Lisa; Fahlman, Greg; Hansen, Brad; Heyl, Jeremy; Hurley, Jarrod; Rich, R Michael; Shara, Michael; Stetson, Peter

    2011-01-01

    We present a new distance determination to the Galactic globular cluster 47 Tucanae by fitting the spectral energy distributions of its white dwarfs to pure hydrogen atmosphere white dwarf models. Our photometric dataset is obtained from a 121 orbit Hubble Space Telescope program using the Wide Field Camera 3 UVIS/IR channels, capturing F390W, F606W, F110W, and F160W images. These images cover more than 60 square arcmins and extend over a radial range of 5-13.7 arcmin (6.5-17.9 pc) within the globular cluster. Using a likelihood analysis, we obtain a best fitting unreddened distance modulus of (m - M)o=13.36+/-0.02+/-0.06 corresponding to a distance of 4.70+/-0.04+/-0.13 kpc, where the first error is random and the second is systematic. We also search the white dwarf photometry for infrared excess in the F160W filter, indicative of debris disks or low mass companions, and find no convincing cases within our sample.

  10. The Habitable Zone Planet Finder: A Proposed High Resolution NIR Spectrograph for the Hobby Eberly Telescope to Discover Low Mass Exoplanets around M Dwarfs

    CERN Document Server

    Mahadevan, Suvrath; Wright, Jason; Endl, Michael; Redman, Stephen; Bender, Chad; Roy, Arpita; Zonak, Stephanie; Troupe, Nathaniel; Engel, Leland; Sigurdsson, Steinn; Wolszczan, Alex; Zhao, Bo

    2010-01-01

    The Habitable Zone Planet Finder (HZPF) is a proposed instrument for the 10m class Hobby Eberly telescope that will be capable of discovering low mass planets around M dwarfs. HZPF will be fiber-fed, provide a spectral resolution R~ 50,000 and cover the wavelength range 0.9-1.65{\\mu}m, the Y, J and H NIR bands where most of the flux is emitted by mid-late type M stars, and where most of the radial velocity information is concentrated. Enclosed in a chilled vacuum vessel with active temperature control, fiber scrambling and mechanical agitation, HZPF is designed to achieve a radial velocity precision < 3m/s, with a desire to obtain <1m/s for the brightest targets. This instrument will enable a study of the properties of low mass planets around M dwarfs; discover planets in the habitable zones around these stars, as well serve as an essential radial velocity confirmation tool for astrometric and transit detections around late M dwarfs. Radial velocity observation in the near-infrared (NIR) will also enabl...

  11. Asteroseismology of pulsating DA white dwarfs with fully evolutionary models

    Directory of Open Access Journals (Sweden)

    Althaus L.G.

    2013-03-01

    Full Text Available We present a new approach for asteroseismology of DA white dwarfs that consists in the employment of a large set of non-static, physically sound, fully evolutionary models representative of these stars. We already have applied this approach with success to pulsating PG1159 stars (GW Vir variables. Our white dwarf models, which cover a wide range of stellar masses, effective temperatures, and envelope thicknesses, are the result of fully evolutionary computations that take into account the complete history of the progenitor stars from the ZAMS. In particular, the models are characterized by self-consistent chemical structures from the centre to the surface, a crucial aspect of white dwarf asteroseismology. We apply this approach to an ensemble of 44 bright DAV (ZZ Ceti stars.

  12. On high proper motion white dwarfs from photographic surveys

    CERN Document Server

    Reylé, C; Creze, M; Reyle, Celine; Robin, Annie C.; Creze, Michel

    2001-01-01

    The interpretation of high proper motion white dwarfs detected by Oppenheimer et al (2001) was the start of a tough controversy. While the discoverers identify a large fraction of their findings as dark halo members, others interpret the same sample as essentially made of disc and/or thick disc stars. We use the comprehensive description of galactic stellar populations provided by the "Besancon" model to produce a realistic simulation of Oppenheimer et al. data, including all observational selections and calibration biases. The conclusion is unambiguous: Thick disc white dwarfs resulting from ordinary hypotheses on the local density and kinematics are sufficient to explain the observed objects, there is no need for halo white dwarfs. This conclusion is robust to reasonable changes in model ingredients. The main cause of the misinterpretation seems to be that the velocity distribution of a proper motion selected star sample is severely biased in favour of high velocities. This has been neglected in previous an...

  13. I -Love- Q relations for white dwarf stars

    Science.gov (United States)

    Boshkayev, K.; Quevedo, H.; Zhami, B.

    2017-02-01

    We investigate the equilibrium configurations of uniformly rotating white dwarfs, using Chandrasekhar and Salpeter equations of state in the framework of Newtonian physics. The Hartle formalism is applied to integrate the field equation together with the hydrostatic equilibrium condition. We consider the equations of structure up to the second order in the angular velocity, and compute all basic parameters of rotating white dwarfs to test the so-called moment of inertia, rotational Love number, and quadrupole moment (I-Love-Q) relations. We found that the I-Love-Q relations are also valid for white dwarfs regardless of the equation of state and nuclear composition. In addition, we show that the moment of inertia, quadrupole moment, and eccentricity (I-Q-e) relations are valid as well.

  14. On the maximum mass of magnetised white dwarfs

    CERN Document Server

    Chatterjee, D; Chamel, N; Novak, J; Oertel, M

    2016-01-01

    We develop a detailed and self-consistent numerical model for extremely-magnetised white dwarfs, which have been proposed as progenitors of overluminous Type Ia supernovae. This model can describe fully-consistent equilibria of magnetic stars in axial symmetry, with rotation, general-relativistic effects and realistic equations of state (including electron-ion interactions and taking into account Landau quantisation of electrons due to the magnetic field). We study the influence of each of these ingredients onto the white dwarf structure and, in particular, on their maximum mass. We perform an extensive stability analysis of such objects, with their highest surface magnetic fields reaching $\\sim 10^{13}~G$ (at which point the star adopts a torus-like shape). We confirm previous speculations that although very massive strongly magnetised white dwarfs could potentially exist, the onset of electron captures and pycnonuclear reactions may severely limit their stability. Finally, the emission of gravitational wave...

  15. New DA white dwarf evolutionary models and their pulsational properties

    CERN Document Server

    Corsico, A H; Benvenuto, O G; Serenelli, A M

    2001-01-01

    In this letter we investigate the pulsational properties of ZZ Ceti stars on the basis of new white dwarf evolutionary models calculated in a self-consistent way with the predictions of time dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to the white dwarf formation. Emphasis is placed on the trapping properties of such models. By means of adiabatic, non-radial pulsation calculations, we find, as a result of time dependent diffusion, a much weaker mode trapping effect, particularly for the high-period regime of the pulsation g-spectrum. This result is valid at least for models with massive hydrogen-rich envelopes. Thus, mode trapping would not be an effective mechanism to explain the fact that all the high periods expected from standard models of stratified white dwarfs are not observed in the ZZ Ceti stars.

  16. Detection of a white dwarf in a visual binary system

    Science.gov (United States)

    Boehm-Vitense, Erika

    1992-01-01

    The F6 giant HD 160365 was detected to have a white dwarf companion about 8 arcsec south of the star. The UV energy distribution observed with IUE shows that the white dwarf has an effective temperature of 23,000 +/- 2000 K. If log g = 8 the Lya profile indicates an effective temperature around 24,500 K. Using the theoretical models by Wesemael et al. (1980) one finds a visual magnitude of m(V) about 16.5. For T(eff) = 24,500 K one expects for a white dwarf a luminosity of log L/L(solar) about 1.3 and M(V) about 10.67. This gives a distance modulus for the system of m(V) - M(V) = 5.83 and an absolute magnitude M(V)= 0.3 for the giant.

  17. Two white dwarfs in ultrashort binaries with detached, eclipsing, likely sub-stellar companions detected by K2

    Science.gov (United States)

    Parsons, S. G.; Hermes, J. J.; Marsh, T. R.; Gänsicke, B. T.; Tremblay, P.-E.; Littlefair, S. P.; Sahman, D. I.; Ashley, R. P.; Green, M.; Rattanasoon, S.; Dhillon, V. S.; Burleigh, M. R.; Casewell, S. L.; Buckley, D. A. H.; Braker, I. P.; Irawati, P.; Dennihy, E.; Rodríguez-Gil, P.; Winget, D. E.; Winget, K. I.; Bell, Keaton J.; Kilic, Mukremin

    2017-10-01

    Using data from the extended Kepler mission in K2 Campaign 10, we identify two eclipsing binaries containing white dwarfs with cool companions that have extremely short orbital periods of only 71.2 min (SDSS J1205-0242, a.k.a. EPIC 201283111) and 72.5 min (SDSS J1231+0041, a.k.a. EPIC 248368963). Despite their short periods, both systems are detached with small, low-mass companions, in one case a brown dwarf and in the other case either a brown dwarf or a low-mass star. We present follow-up photometry and spectroscopy of both binaries, as well as phase-resolved spectroscopy of the brighter system, and use these data to place preliminary estimates on the physical and binary parameters. SDSS J1205-0242 is composed of a 0.39 ± 0.02 M⊙ helium-core white dwarf that is totally eclipsed by a 0.049 ± 0.006 M⊙ (51 ± 6MJ) brown-dwarf companion, while SDSS J1231+0041 is composed of a 0.56 ± 0.07 M⊙ white dwarf that is partially eclipsed by a companion of mass ≲0.095 M⊙. In the case of SDSS J1205-0242, we look at the combined constraints from common-envelope evolution and brown-dwarf models; the system is compatible with similar constraints from other post-common-envelope binaries, given the current parameter uncertainties, but has potential for future refinement.

  18. Globular cluster interstellar media: ionized and ejected by white dwarfs

    Science.gov (United States)

    McDonald, I.; Zijlstra, A. A.

    2015-01-01

    Ultraviolet radiation from white dwarfs can efficiently clear Galactic globular clusters (GCs) of their intracluster medium (ICM). This solves the problem of the missing ICM in clusters, which is otherwise expected to build up to easily observable quantities. To show this, we recreate the ionizing flux in 47 Tuc, following randomly generated stars through their asymptotic giant branch (AGB), post-AGB and white dwarf evolution. Each white dwarf can ionize all the material injected into the cluster by stellar winds for ˜3 Myr of its evolution: ˜40 such white dwarfs exist at any point. Every GC's ICM should be ionized. The neutral cloud in M15 should be caused by a temporary overdensity. A pressure-supported ICM will expand over the cluster's tidal radius, where it will be truncated, allowing Jeans escape. The modelled Jeans mass-loss rate approximates the total stellar mass-loss rate, allowing efficient clearing of ICM. Any cluster's ICM mass should equal the mass injected by its stars over the sound travel time between the cluster core and tidal radius. We predict ˜11.3 M⊙ of ICM within 47 Tuc, cleared over ˜4 Myr, compared to a dynamical time-scale of 4.3 Myr. We present a new mass hierarchy, discussing the transition between GCs dwarf galaxies.

  19. A Dark Spot on a Massive White Dwarf

    Science.gov (United States)

    Kilic, Mukremin; Gianninas, Alexandros; Bell, Keaton J.; Curd, Brandon; Brown, Warren R.; Hermes, J. J.; Dufour, Patrick; Wisniewski, John P.; Winget, D. E.; Winget, K. I.

    2015-12-01

    We present the serendipitous discovery of eclipse-like events around the massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We selected J1529+2928 for time-series photometry based on its spectroscopic temperature and surface gravity, which place it near the ZZ Ceti instability strip. Instead of pulsations, we detect photometric dips from this white dwarf every 38 minutes. Follow-up optical spectroscopy observations with Gemini reveal no significant radial velocity variations, ruling out stellar and brown dwarf companions. A disintegrating planet around this white dwarf cannot explain the observed light curves in different filters. Given the short period, the source of the photometric dips must be a dark spot that comes into view every 38 minutes due to the rotation of the white dwarf. Our optical spectroscopy does not show any evidence of Zeeman splitting of the Balmer lines, limiting the magnetic field strength to B Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

  20. Constraining the neutrino magnetic dipole moment from white dwarf pulsations

    Energy Technology Data Exchange (ETDEWEB)

    Córsico, A.H.; Althaus, L.G. [Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, (1900) La Plata (Argentina); Bertolami, M.M. Miller [Instituto de Astrofísica La Plata, CONICET-UNLP, Paseo del Bosque s/n, (1900) La Plata (Argentina); Kepler, S.O. [Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, Porto Alegre 91501-970, RS (Brazil); García-Berro, E., E-mail: acorsico@fcaglp.unlp.edu.ar, E-mail: althaus@fcaglp.unlp.edu.ar, E-mail: marcelo@MPA-Garching.MPG.DE, E-mail: kepler@if.ufrgs.br, E-mail: enrique.garcia-berro@upc.edu [Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860, Castelldefels (Spain)

    2014-08-01

    Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment (μ{sub ν}) using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. We employ state-of-the-art evolutionary and pulsational codes which allow us to perform a detailed asteroseismological period fit based on fully DB white dwarf evolutionary sequences. Plasmon neutrino emission is the dominant cooling mechanism for this class of hot pulsating white dwarfs, and so it is the main contributor to the rate of change of period with time (Pidot) for the DBV class. Thus, the inclusion of an anomalous neutrino emission through a non-vanishing magnetic dipole moment in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DBV stars. By comparing the theoretical Pidot value with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment with an upper limit of μ{sub ν} ∼< 10{sup -11} μ{sub B}. This bound is somewhat less restrictive than, but still compatible with, other limits inferred from the white dwarf luminosity function or from the color-magnitude diagram of the Globular cluster M5. Further improvements of the measurement of the rate of period change of the dominant pulsation mode of PG 1351+489 will be necessary to confirm our bound.

  1. PSR J1738+0333: The First Millisecond Pulsar + Pulsating White Dwarf Binary

    CERN Document Server

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

    2014-01-01

    We report the discovery of the first millisecond pulsar with a pulsating white dwarf companion. Following the recent discoveries of pulsations in extremely low-mass (ELM, <0.3 Msol) white dwarfs (WDs), we targeted ELM WD companions to two millisecond pulsars with high-speed Gemini photometry. We find significant optical variability in PSR J1738+0333 with periods between roughly 1790-3060 s, consistent in timescale with theoretical and empirical observations of pulsations in 0.17 Msol He-core ELM WDs. We additionally put stringent limits on a lack of variability in PSR J1909-3744, showing this ELM WD is not variable to <0.1 per cent amplitude. Thanks to the accurate distance and radius estimates from radio timing measurements, PSR J1738+0333 becomes a benchmark for low-mass, pulsating WDs. Future, more extensive time-series photometry of this system offers an unprecedented opportunity to constrain the physical parameters (including the cooling age) and interior structure of this ELM WD, and in turn, the ...

  2. The gravitational waveforms of white dwarf collisions in globular clusters

    Energy Technology Data Exchange (ETDEWEB)

    Loren-Aguilar, P; Garcia-Berro, E [Departament de Fisica Aplicada, Escola Politecnica Superior de Castelldefels, Universitat Politecnica de Catalunya, Av. del Canal OlImpic, s/n, 08860 Castelldefels (Spain); Lobo, J A; Isern, J, E-mail: loren@fa.upc.ed [Institut de Ciencies de l' Espai, CSIC, Campus UAB, Facultat de Ciencies, Torre C-5, 08193 Bellaterra (Spain)

    2009-06-01

    In the dense central regions of globular clusters close encounters of two white dwarfs are relatively frequent. The estimated frequency is one or more strong encounters per star in the lifetime of the cluster. Such encounters should be then potential sources of gravitational wave radiation. Thus, it is foreseeable that these collisions could be either individually detected by LISA or they could contribute significantly to the background noise of the detector. We compute the pattern of gravitational wave emission from these encounters for a sufficiently broad range of system parameters, namely the masses, the relative velocities and the distances of the two white dwarfs involved in the encounter.

  3. Helium at White Dwarf Photospheric Conditions: Preliminary Laboratory Results

    Science.gov (United States)

    Schaeuble, M.; Falcon, R. E.; Gomez, T. A.; Winget, D. E.; Montgomery, M. H.; Bailey, J. E.

    2017-03-01

    We present preliminary results of an experimental study exploring helium at photospheric conditions of white dwarf stars. These data were collected at Sandia National Laboratories' Z-machine, the largest x-ray source on earth. Our helium results could have many applications ranging from validating current DB white dwarf model atmospheres to providing accurate He pressure shifts at varying temperatures and densities. In a much broader context, these helium data can be used to guide theoretical developments in new continuum-lowering models for two-electron atoms. We also discuss future applications of our updated experimental design, which enables us to sample a greater range of densities, temperatures, and gas compositions.

  4. Astro-archaeology - The white dwarfs and hot subwarfs

    Science.gov (United States)

    Van Horn, Hugh M.

    1991-01-01

    By 'astroarcheology' is presently meant the effort to ascertain the Galaxy's past in light of what is found in its most ancient, white dwarf constituents. Attention is given to the controversial role of the hot subdwarfs and the theory of white dwarf spectral evolution, as well as to the concept of the 'Whole Earth Telescope', involving continuous photometric coverage of rapidly varying astronomical sources and thereby eliminating the otherwise troublesome diurnal gaps in data. Much higher resolution of the power spectra of these objects is attainable by these means than any current alternative.

  5. White dwarfs as physics laboratories: the case of axions

    CERN Document Server

    Isern, J; Catalan, S; Corsico, A; Garcia-Berro, E; Salaris, M; Torres, S

    2012-01-01

    White dwarfs are almost completely degenerate objects that cannot obtain energy from thermonuclear sources, so their evolution is just a gravothermal cooling process. Recent improvements in the accuracy and precision of the luminosity function and in pulsational data of variable white dwarfs suggest that they are cooling faster than expected from conventional theory. In this contribution we show that the inclusion of an additional cooling term due to axions able to interact with electrons with a coupling constant g_ae ~(2-7)x10^{-13} allows to fit better the observations.

  6. Bayesian Evidence for Two Populations of White Dwarfs: Preliminary Results

    Science.gov (United States)

    Valentim, R.; Romero, A. D.; Kepler, S. O.; Horvath, J. E.; Rangel, E. M.

    2017-03-01

    White dwarf (WD) populations are analyzed using Bayesian tools, which allows inferring possible evolutionary paths through the study of the mass values. We employed a sample of 2761 DA white dwarf stars from the SDSS, and obtained the central mass values and their corresponding standard deviations using a bimodal population as an ansatz. The results indicate a population with M1 = 0.60 M⊙ and σ1 = 0.06 M⊙, corresponding to a single stellar evolution, and a second population with M2 = 1.00 M⊙ and σ1 = 0.11 M⊙ possibly due to binary evolution resulting from mergers.

  7. Astro-archaeology - The white dwarfs and hot subwarfs

    Science.gov (United States)

    Van Horn, Hugh M.

    1991-01-01

    By 'astroarcheology' is presently meant the effort to ascertain the Galaxy's past in light of what is found in its most ancient, white dwarf constituents. Attention is given to the controversial role of the hot subdwarfs and the theory of white dwarf spectral evolution, as well as to the concept of the 'Whole Earth Telescope', involving continuous photometric coverage of rapidly varying astronomical sources and thereby eliminating the otherwise troublesome diurnal gaps in data. Much higher resolution of the power spectra of these objects is attainable by these means than any current alternative.

  8. Helium at white dwarf photospheric conditions: preliminary laboratory results

    CERN Document Server

    Schaeuble, Marc; Gomez, Thomas A; Winget, Don E; Montgomery, Michael H; Bailey, James E

    2016-01-01

    We present preliminary results of an experimental study exploring helium at photospheric conditions of white dwarf stars. These data were collected at Sandia National Laboratories' Z-machine, the largest x-ray source on earth. Our helium results could have many applications ranging from validating current DB white dwarf model atmospheres to providing accurate He pressure shifts at varying temperatures and densities. In a much broader context, these helium data can be used to guide theoretical developments in new continuum-lowering models for two-electron atoms. We also discuss future applications of our updated experimental design, which enables us to sample a greater range of densities, temperatures, and gas compositions.

  9. SpeX spectroscopy of unresolved very low mass binaries. II. Identification of 14 candidate binaries with late-M/early-L and T dwarf components

    Energy Technology Data Exchange (ETDEWEB)

    Bardalez Gagliuffi, Daniella C.; Burgasser, Adam J.; Nicholls, Christine P. [Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Drive, Mail Code 0424, La Jolla, CA 92093 (United States); Gelino, Christopher R. [NASA Exoplanet Science Institute, Mail Code 100-22, California Institute of Technology, 770 South Wilson Avenue, Pasadena, CA 91125 (United States); Looper, Dagny L. [School of Mathematics and Physics, The University of Queensland, Brisbane, QLD 4072 (Australia); Schmidt, Sarah J. [Department of Physics and Astronomy, Hunter College, City University of New York, 695 Park Avenue, New York, NY 10065 (United States); Cruz, Kelle [Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024 (United States); West, Andrew A. [Department of Physics and Astronomy, University of Delaware, 104 The Green, Newark, DE 19716 (United States); Gizis, John E. [Department of Physics and Astronomy, Western University, London, ON N6A 3K7 (Canada); Metchev, Stanimir, E-mail: daniella@physics.ucsd.edu [Infrared Processing and Analysis Center, Mail Code 100-22, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125 (United States)

    2014-10-20

    Multiplicity is a key statistic for understanding the formation of very low mass (VLM) stars and brown dwarfs. Currently, the separation distribution of VLM binaries remains poorly constrained at small separations (≤1 AU), leading to uncertainty in the overall binary fraction. We approach this problem by searching for late-M/early-L plus T dwarf spectral binaries whose combined light spectra exhibit distinct peculiarities, allowing for separation-independent identification. We define a set of spectral indices designed to identify these systems, and we use a spectral template fitting method to confirm and characterize spectral binary candidates from a library of 815 spectra from the SpeX Prism Spectral Libraries. We present 11 new binary candidates, confirm 3 previously reported candidates, and rule out 2 previously identified candidates, all with primary and secondary spectral types in the range M7-L7 and T1-T8, respectively. We find that subdwarfs and blue L dwarfs are the primary contaminants in our sample and propose a method for segregating these sources. If confirmed by follow-up observations, these systems may add to the growing list of tight separation binaries, whose orbital properties may yield further insight into brown dwarf formation scenarios.

  10. Formation of millisecond pulsars with CO white dwarf companions - II. Accretion, spin-up, true ages and comparison to MSPs with He white dwarf companions

    CERN Document Server

    Tauris, Thomas M; Kramer, Michael

    2012-01-01

    Millisecond pulsars (MSPs) are mainly characterised by their spin periods, B-fields and masses - quantities which are largely affected by previous interactions with a companion star in a binary system. In this paper, we investigate the formation mechanism of MSPs by considering the pulsar recycling process in both intermediate-mass X-ray binaries (IMXBs) and low-mass X-ray binaries (LMXBs). The IMXBs mainly lead to the formation of binary MSPs with a massive carbon-oxygen (CO) or an oxygen-neon-magnesium white dwarf (ONeMg WD) companion, whereas the LMXBs form recycled pulsars with a helium white dwarf (He WD) companion. We discuss the accretion physics leading to the spin-up line in the PPdot-diagram and demonstrate that such a line cannot be uniquely defined. We derive a simple expression for the amount of accreted mass needed for any given pulsar to achieve its equilibrium spin and apply this to explain the observed differences of the spin distributions of recycled pulsars with different types of companion...

  11. Further Defining Spectral Type "Y" and Exploring the Low-mass End of the Field Brown Dwarf Mass Function

    CERN Document Server

    Kirkpatrick, J Davy; Cushing, Michael C; Mace, Gregory N; Griffith, Roger L; Skrutskie, Michael F; Marsh, Kenneth A; Wright, Edward L; Eisenhardt, Peter R; McLean, Ian S; Mainzer, Amanda K; Burgasser, Adam J; Tinney, C G; Parker, Stephen; Salter, Graeme

    2012-01-01

    We present the discovery of another seven Y dwarfs from the Wide-field Infrared Survey Explorer (WISE). Using these objects, as well as the first six WISE Y dwarf discoveries from Cushing et al., we further explore the transition between spectral types T and Y. We find that the T/Y boundary roughly coincides with the spot where the J-H colors of brown dwarfs, as predicted by models, turn back to the red. Moreover, we use preliminary trigonometric parallax measurements to show that the T/Y boundary may also correspond to the point at which the absolute H (1.6 um) and W2 (4.6 um) magnitudes plummet. We use these discoveries and their preliminary distances to place them in the larger context of the Solar Neighborhood. We present a table that updates the entire stellar and substellar constituency within 8 parsecs of the Sun, and we show that the current census has hydrogen-burning stars outnumbering brown dwarfs by roughly a factor of six. This factor will decrease with time as more brown dwarfs are identified wi...

  12. SDSS 1355+0856: a detached white dwarf + M star binary in the period gap discovered by the SWARMS survey

    Science.gov (United States)

    Badenes, Carles; van Kerkwijk, Marten H.; Kilic, Mukremin; Bickerton, Steven J.; Mazeh, Tsevi; Mullally, Fergal; Tal-Or, Lev; Thompson, Susan E.

    2013-03-01

    SDSS J135523.92 + 085645.4 (SDSS 1355+0856) was identified as a hot white dwarf with a companion from time-resolved Sloan Digital Sky Survey spectroscopy as part of the ongoing Sloan White Dwarf Radial velocity data Mining Survey survey. Follow-up observations with the Astrophysical Research Consortium 3.5 m telescope and the Multiple Mirror Telescope revealed weak emission lines in the central cores of the Balmer absorption lines during some phases of the orbit, but no line emission during other phases. This can be explained if SDSS 1355+0856 is a detached white dwarf + M dwarf binary similar to GD 448, where one of the hemispheres of the low-mass companion is irradiated by the proximity of the hot white dwarf. Based on the available data, we derive an orbital period of 0.114 38 ± 0.000 06 d, a primary mass of 0.46 ± 0.01 M⊙, a secondary mass between 0.083 and 0.097 M⊙, and an orbital inclination larger than 57°. This makes SDSS 1355+0856 one of the shortest period post-common envelope white dwarf + M dwarf binaries, and the record holder for the lowest mass stellar companion, which has interesting implications for our understanding of common envelope evolution and the phenomenology of cataclysmic variables. The short cooling time of the WD (25 Myr) implies that the system emerged from the common envelope phase with an orbital period very similar to what we observe today, and was born in the period gap of cataclysmic variables.

  13. A Binary Scenario for the Formation of Strongly Magnetized White Dwarfs

    CERN Document Server

    Nordhaus, J

    2011-01-01

    Since their initial discovery, the origin of isolated white dwarfs (WDs) with magnetic fields in excess of $\\sim$1 MG has remained a mystery. Recently, the formation of these high-field magnetic WDs has been observationally linked to strong binary interactions incurred during post-main-sequence evolution. Planetary, brown dwarf or stellar companions located within a few AU of main-sequence stars may become engulfed during the primary's expansion off the main sequence. Sufficiently low-mass companions in-spiral inside a common envelope until they are tidally shredded near the natal white dwarf. Formation of an accretion disk from the disrupted companion provides a source of turbulence and shear which act to amplify magnetic fields and transport them to the WD surface. We show that these disk-generated fields explain the observed range of magnetic field strengths for isolated, high-field magnetic WDs. Additionally, we discuss a high-mass binary analogue which generates a strongly-magnetized WD core inside a pre...

  14. A Common Origin of Magnetism from Planets to White Dwarfs

    Science.gov (United States)

    Isern, Jordi; García-Berro, Enrique; Külebi, Baybars; Lorén-Aguilar, Pablo

    2017-02-01

    Isolated magnetic white dwarfs have field strengths ranging from kilogauss to gigagauss. However, the origin of the magnetic field has not been hitherto elucidated. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of their progenitor stars, or are the result of binary interactions, or, finally, they are produced by other internal physical mechanisms during the cooling of the white dwarf itself, remains a mystery. At sufficiently low temperatures, white dwarfs crystallize. Upon solidification, phase separation of its main constituents, 12C and 16O, and of the impurities left by previous evolution occurs. This process leads to the formation of a Rayleigh–Taylor unstable liquid mantle on top of a solid core. This convective region, as it occurs in solar system planets like the Earth and Jupiter, can produce a dynamo able to yield magnetic fields of strengths of up to 0.1 MG, thus providing a mechanism that could explain magnetism in single white dwarfs.

  15. High-Resolution EUV Spectroscopy of White Dwarfs

    Science.gov (United States)

    Kowalski, Michael P.; Wood, K. S.; Barstow, M. A.

    2014-01-01

    We compare results of high-resolution EUV spectroscopic measurements of the isolated white dwarf G191-B2B and the binary system Feige 24 obtained with the J-PEX (Joint Plasmadynamic Experiment), which was sponsored jointly by the U.S. Naval Research Laboratory and NASA. J-PEX delivers the world's highest resolution in EUV and does so at high effective area (e.g., more effective area in a sounding rocket than is available with Chandra at adjacent energies, but in a waveband Chandra cannot reach). The capability J-PEX represents is applicable to the astrophysics of hot plasmas in stellar coronae, white dwarfs and the ISM. G191-B2B and Feige 24 are quite distinct hot white dwarf systems having in common that they are bright in the portion of the EUV where He emission features and edges occur, hence they can be exploited to probe both the stellar atmosphere and the ISM, separating those components by model-fitting that sums over all relevant (He) spectral features in the band. There is evidence from these fits that atmospheric He is being detected but the result is more conservatively cast as a pair of upper limits. We discuss how longer duration satellite observations with the same instrumentation could increase exposure to detect atmospheric He in these and other nearby hot white dwarfs.

  16. The Chandrasekhar's Equation for Two-Dimensional Hypothetical White Dwarfs

    CERN Document Server

    De, Sanchari

    2014-01-01

    In this article we have extended the original work of Chandrasekhar on the structure of white dwarfs to the two-dimensional case. Although such two-dimensional stellar objects are hypothetical in nature, we strongly believe that the work presented in this article may be prescribed as Master of Science level class problem for the students in physics.

  17. Central stars of planetary nebulae: The white dwarf connection

    CERN Document Server

    Werner, K

    2011-01-01

    This paper is focused on the transition phase between central stars and white dwarfs, i.e. objects in the effective temperature range 100,000 - 200,000 K. We confine our review to hydrogen-deficient stars because the common H-rich objects are subject of the paper by Ziegler et al. in these proceedings. We address the claimed iron-deficiency in PG1159 stars and [WC] central stars. The discovery of new Ne VII and Ne VIII lines in PG1159 stars suggests that the identification of O VII and O VIII lines that are used for spectral classification of [WCE] stars is wrong. We then present evidence for two distinct post-AGB evolutionary sequences for H-deficient stars based on abundance analyses of the He-dominated O(He) stars and the hot DO white dwarf KPD0005+5106. Finally, we report on evidence for an H-deficient post-super AGB evolution sequence represented by the hottest known, carbon/oxygen-atmosphere white dwarf H1504+65 and the recently discovered carbon-atmosphere "hot DQ" white dwarfs.

  18. Mass-Radius Relation of Strongly Magnetized White Dwarfs

    Science.gov (United States)

    Bera, P.; Bhattacharya, D.

    2017-03-01

    We study the strongly magnetized white dwarf configurations in a self-consistent manner as a progenitor of the over-luminous type-Ia supernovae. We compute static equilibria of white dwarf stars containing a strong magnetic field and present the modification of the white dwarf mass-radius relation caused by the magnetic field. From a static equilibrium study, we find that a maximum white dwarf mass of about 1.9 M⊙ may be supported if the interior poloidal field is as strong as approximately 1010 T. On the other hand if the field is purely toroidal the maximum mass can be more than 5 M⊙. All these modifications are mainly from the presence of the Lorenz force. The effects of i) modification of the equation of state due to Landau quantization, ii) electrostatic interaction due to ions, iii) general relativistic calculation on the stellar structure and, iv) field geometry are also considered. These strongly magnetised configurations are sensitive to magnetic instabilities where the perturbations grow at the corresponding Alfven time scales.

  19. Global properties of the white dwarf pulsar AR Scorpii

    CERN Document Server

    Franzon, B

    2016-01-01

    In view of the new recent observation and measurement of the fast-rotating and highly-magnetized white dwarf AR Sco \\cite{Marsh:2016uhc}, we determine bounds for its radius, magnetic fields, moment of inertia and gravitational wave (GW) emission by using observations of the luminosity, as well the rotation frequency $\

  20. Direct imaging searches for planets around white dwarf stars

    Science.gov (United States)

    Burleigh, Matt; Hogan, Emma; Clarke, Fraser

    White dwarfs are excellent targets for direct imaging searches for extra-solar planets, since they are up to 10^4 times fainter than their main sequence progenitors, providing a huge gain in the contrast problem. In addition, the orbits of planetary companions that lie beyond the maximum extent of the Red Giant envelope are expected to widen considerably, improving resolution and further encouraging direct detection. We discuss current searches for planetary companions to white dwarfs, including our own “DODO” programme. At the time of writing, no planetary companion to a white dwarf has been detected. The most sensitive searches have been capable of detecting companions ≳5M_{Jup}, and their non-detection is consistent with the conclusions of McCarthy & Zuckerman (2004), that no more than 3% of stars harbour 5-10M_{Jup} planets at orbits between 75-300AU. Extremely Large Telescopes are required to enable deeper searches sensitive to lower mass planets, and to provide larger target samples including more distant and older white dwarfs. ELTs will also enable spectroscopic follow-up for any resolved planets, and follow-up of any planetary companions discovered astrometrically by GAIA and SIM.

  1. Binary white dwarfs in the halo of the Milky Way

    NARCIS (Netherlands)

    van Oirschot, Pim; Nelemans, Gijs; Toonen, Silvia; Pols, Onno; Brown, Anthony G. A.; Helmi, Amina; Portegies Zwart, Simon

    2014-01-01

    Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolut

  2. LISA Astronomy of Double White Dwarf Binary Systems

    NARCIS (Netherlands)

    Stroeer, A.; Vecchio, A.; Nelemans, G.A.

    2005-01-01

    The Laser Interferometer Space Antenna (LISA) will provide us with the largest observational sample of (interacting) double white dwarf binaries, whose evolution is driven by the radiation reaction and other effects, such as tides and mass transfer. We show that, depending on the actual physical par

  3. 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...

  4. Axions and the pulsation periods of variable white dwarfs revisited

    CERN Document Server

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

    2010-01-01

    Axions are the natural consequence of the introduction of the Peccei-Quinn symmetry to solve the strong CP problem. All the efforts to detect such elusive particles have failed up to now. Nevertheless, it has been recently shown that the luminosity function of white dwarfs is best fitted if axions with a mass of a few meV are included in the evolutionary calculations. Our aim is to show that variable white dwarfs can provide additional and independent evidence about the existence of axions. The evolution of a white dwarf is a slow cooling process that translates into a secular increase of the pulsation periods of some variable white dwarfs, the so-called DAV and DBV types. Since axions can freely escape from such stars, their existence would increase the cooling rate and, consequently, the rate of change of the periods as compared with the standard ones. The present values of the rate of change of the pulsation period of G117-B15A are compatible with the existence of axions with the masses suggested by the lu...

  5. The white dwarf population within 40 pc of the Sun

    CERN Document Server

    Torres, S

    2016-01-01

    The white dwarf luminosity function is an important tool to understand the properties of the Solar neighborhood, like its star formation history, and its age. Here we present a population synthesis study of the white dwarf population within 40~pc from the Sun, and compare the results of this study with the properties of the observed sample. We use a state-of-the-art population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable white dwarf cooling sequences, an accurate description of the Galactic neighborhood, and a realistic treatment of all the known observational biases and selection procedures. We find a good agreement between our theoretical models and the observed data. In particular, our simulations reproduce a previously unexplained feature of the bright branch of the white dwarf luminosity function, which we argue is due to a recent episode of star formation. We also derive the age of the Solar neighborhood employing the position of the observed cut-off of ...

  6. Mass-radius relation of strongly magnetized white dwarfs

    Science.gov (United States)

    Bera, Prasanta; Bhattacharya, Dipankar

    2016-07-01

    We study the strongly magnetized white dwarf configurations in a self-consistent manner as a progenitor of the over-luminous type-Ia supernovae. We compute static equilibria of white dwarf stars containing a strong magnetic field and present the modification of white dwarf mass-radius relation caused by the magnetic field. From a static equilibrium study, we find that a maximum white dwarf mass of about 1.9 M_{⊙} may be supported if the interior poloidal field is as strong as approximately 10^{10} T. On the other hand, if the field is purely toroidal the maximum mass can be more than 5 M_⊙. All these modifications are mainly from the presence of Lorenz force. The effects of i) modification of equation of state due to Landau quantization ii) electrostatic interaction due to ions, ii) general relativistic calculation on the stellar structure and, iii) field geometry are also considered. These strongly magnetised configurations are sensitive to magnetic instabilities where the perturbations grow at the corresponding Alfven time scales.

  7. Evolutionary Grids of Accreting White Dwarf Companions in Cataclysmic Variables

    Science.gov (United States)

    Benjamin, J.; Jensen, M.; Nadeau, S.; Nelson, L. A.

    2003-12-01

    We analyze the evolution of accreting white dwarfs in binary systems for a wide range of initial conditions. Specifically, evolutionary tracks are calculated for CO white dwarfs with masses in the range of 0.6 - 1.3 solar masses and accreting H-rich gas at rates of between 10-6 to 10-10 solar masses per year. Since the white dwarfs in these binaries could be very young or very old at the onset of mass transfer we simulated this possibility by investigating the evolution for a large range of internal temperatures. Thus most of the sequences generated were not thermally relaxed at the onset of mass transfer (and the thermonuclear flashes were not cyclic). We discuss the temporal dependence of the interior properties (envelope readjustment on a thermal timescale and compressional heating) on the initial conditions. Particular attention is paid to the white dwarfs accretors that remained small (relative to the Roche lobe radius) during the shell flash event. Finally, we use the results of these models to comment on the observed properties of Supersoft X-ray sources. This research was supported in part by funds from the Natural Sciences and Engineering Research Council (Canada).

  8. Revisiting the luminosity function of single halo white dwarfs

    CERN Document Server

    Cojocaru, R; Althaus, L G; Isern, J; García-Berro, E

    2015-01-01

    White dwarfs are the fossils left by the evolution of low-and intermediate-mass stars, and have very long evolutionary timescales. This allows us to use them to explore the properties of old populations, like the Galactic halo. We present a population synthesis study of the luminosity function of halo white dwarfs, aimed at investigating which information can be derived from the currently available observed data. We employ an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences for metal poor progenitors as well as an accurate modeling of the observational biases. We find that because the observed sample of halo white dwarfs is restricted to the brightest stars only the hot branch of the white dwarf luminosity function can be used for such purposes, and that its shape function is almost insensitive to the most relevant inputs, like the adopted cooling sequences, the initial mass function, the density profile of the stellar spher...

  9. Planetary Engulfment as a Trigger for White Dwarf Pollution

    Science.gov (United States)

    Petrovich, Cristobal; Muñoz, Diego J.

    2017-01-01

    The presence of a planetary system can shield a planetesimal disk from the secular gravitational perturbations due to distant outer massive objects (planets or stellar companions). As the host star evolves off the main sequence to become a white dwarf, these planets can be engulfed during the giant phase, triggering secular instabilities and leading to the tidal disruptions of small rocky bodies. These disrupted bodies can feed the white dwarfs with rocky material and possibly explain the high-metallicity material in their atmospheres. We illustrate how this mechanism can operate when the gravitational perturbations are due to the KL mechanism from a stellar binary companion, a process that is activated only after the planet has been removed/engulfed. We show that this mechanism can explain the observed accretion rates if: (1) the planetary engulfment happens rapidly compared to the secular timescale, which is generally the case for wide binaries (> 100 au) and planetary engulfment during the asymptotic giant branch; (2) the planetesimal disk has a total mass of ∼ {10}-4-{10}-2{M}\\oplus . We show that this new mechanism can provide a steady supply of material throughout the entire life of the white dwarfs for all cooling ages and can account for a large fraction (up to nearly half) of the observed polluted white dwarfs.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2010-01-18

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

  11. Chandra Grating Spectroscopy of Three Hot White Dwarfs

    Science.gov (United States)

    Adamczak, J.; Werner, K.; Rauch, T.; Schuh, S.; Drake, J. J.; Kruk, J. W.

    2013-01-01

    High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB1919) and the other is a non-DA of spectral type PG1159 (PG1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD246). Aims. The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods. The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results. No metals could be identified in LB1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG1520+525 constrains the effective temperature to T(sub eff) = 150 000 +/- 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GWVir class (PG1159-035) defines the location of the blue edge of the GWVir instability region

  12. M dwarfs in the Local Milky Way: The Field Low-Mass Stellar Luminosity and Mass Functions

    Energy Technology Data Exchange (ETDEWEB)

    Bochanski, Jr, John J. [Univ. of Washington, Seattle, WA (United States)

    2008-01-01

    Modern sky surveys, such as the Sloan Digital Sky Survey (SDSS) and the Two-Micron All Sky Survey, have revolutionized how Astronomy is done. With millions of photometric and spectroscopic observations, global observational properties can be studied with unprecedented statistical significance. Low-mass stars dominate the local Milky Way, with tens of millions observed by SDSS within a few kpc. Thus, they make ideal tracers of the Galactic potential, and the thin and thick disks. In this thesis dissertation, I present my efforts to characterize the local low-mass stellar population, using a collection of observations from the Sloan Digital Sky Survey (SDSS). First, low-mass stellar template spectra were constructed from the co-addition of thousands of SDSS spectroscopic observations. These template spectra were used to quantify the observable changes introduced by chromospheric activity and metallicity. Furthermore, the average ugriz colors were measured as a function of spectral type. Next, the local kinematic structure of the Milky Way was quantified, using a special set of SDSS spectroscopic observations. Combining proper motions and radial velocities (measured using the spectral templates), along with distances, the full UVW space motions of over 7000 low-mass stars along one line of sight were computed. These stars were also separated kinematically to investigate other observational differences between the thin and thick disks. Finally, this dissertation details a project designed to measure the luminosity and mass functions of low-mass stars. Using a new technique optimized for large surveys, the field luminosity function (LF) and local stellar density profile are measured simultaneously. The sample size used to estimate the LF is nearly three orders of magnitude larger than any previous study, offering a definitive measurement of this quantity. The observed LF is transformed into a mass function (MF) and compared to previous studies.

  13. Sirius B - A still mysterious white dwarf

    Science.gov (United States)

    Wesemael, F.; Fontaine, G.

    1982-02-01

    Observations and knowledge of Sirius B, the companion star to Sirius A are reviewed, noting the solar mass and terrestrial radius of the dwarf. The system is 2.65 pc distant, with separation between the stars oscillating from 8-32 AU. Spectral observations are best obtained when the stars are at maximum distance, and redshifts which affirm the theory of relativity have been detected. Copernicus satellite observations have also revealed the emissions of UV and X rays from the dwarf. It is noted that Grecian records over 2000 yr old cite Sirius as a red star, which implies that the dwarf was then a red star since Sirius A is a main sequence star, an implication which does not correspond with the known evolution of stars. Another mystery is cited, that of the tribal records of the Sudanese Dogon, which maintains an ancient legend of Sirius A having an invisible companion called Digitaria, composed of a substance called segala, which cannot be lifted by all the humans on earth combined.

  14. Nonlinear Time Series Analysis of White Dwarf Light Curves

    Science.gov (United States)

    Jevtic, N.; Zelechoski, S.; Feldman, H.; Peterson, C.; Schweitzer, J.

    2001-12-01

    We use nonlinear time series analysis methods to examine the light intensity curves of white dwarf PG1351+489 obtained by the Whole Earth Telescope (WET). Though these methods were originally introduced to study chaotic systems, when a clear signature of determinism is found for the process generating an observable and it couples the active degrees of freedom of the system, then the notion of phase space provides a framework for exploring the system dynamics of nonlinear systems in general. With a pronounced single frequency, its harmonics and other frequencies of lower amplitude on a broadband background, the PG1351 light curve lends itself to the use of time delay coordinates. Our phase space reconstruction yields a triangular, toroidal three-dimensional shape. This differs from earlier results of a circular toroidal representation. We find a morphological similarity to a magnetic dynamo model developed for fast rotators that yields a union of both results: the circular phase space structure for the ascending portion of the cycle, and the triangular structure for the declining portion. The rise and fall of the dynamo cycle yield both different phase space representations and different correlation dimensions. Since PG1351 is known to have no significant fields, these results may stimulate the observation of light curves of known magnetic white dwarfs for comparison. Using other data obtained by the WET, we compare the phase space reconstruction of DB white dwarf PG1351 with that of GD 358 which has a more complex power spectrum. We also compare these results with those for PG1159. There is some general similarity between the results of the phase space reconstruction for the DB white dwarfs. As expected, the difference between the results for the DB white dwarfs and PG1159 is great.

  15. Monte Carlo simulations of the luminosity function of hot white dwarfs

    CERN Document Server

    Torres, S; Krzesinski, J; Kleinman, S J

    2012-01-01

    We present a detailed Monte Carlo simulation of the population of the hot branch of the white dwarf luminosity function. We used the most up-to-date stellar evolutionary models and we implemented a full description of the observational selection biases. Our theoretical results are compared with the luminosity function of hot white dwarfs obtained from the Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. For non-DA white dwarfs we find an excellent agreement with the observational data, while for DA white dwarfs our simulations show some discrepancies with the observations for the brightest luminosity bins, those corresponding to L>= 10 L_sun.

  16. The DODO survey - II. A Gemini direct imaging search for substellar and planetary mass companions around nearby equatorial and Northern hemisphere white dwarfs

    Science.gov (United States)

    Hogan, E.; Burleigh, M. R.; Clarke, F. J.

    2009-07-01

    The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low-mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures much lower (T8.5, and so could belong to the proposed Y dwarf spectral sequence. The detection of a planet around a white dwarf would prove that such objects can survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors (with masses between 1.5 and 8Msolar, i.e. early B to mid-F). This paper presents the results of a multi epoch J band common proper motion survey of 23 nearby equatorial and Northern hemisphere white dwarfs. We rule out the presence of any common proper motion companions, with limiting masses determined from the completeness limit of each observation, to 18 white dwarfs. For the remaining five targets, the motion of the white dwarf is not sufficiently separated from the non-moving background objects in each field. These targets require additional observations to conclusively rule out the presence of any common proper motion companions. From our completeness limits, we tentatively suggest that ~ 500 K between projected physical separations of 60-200 au.

  17. Magnetic activity in the HARPS M-dwarf sample. The rotation-activity relationship for very low-mass stars through R'HK

    CERN Document Server

    Astudillo-Defru, Nicola; Bonfils, Xavier; Forveille, Thierry; Lovis, Christophe; Rameau, Julien

    2016-01-01

    Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres. Starting with the historical Mount Wilson monitoring program, CaH&K lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca H&Kemission is traditionally quantified through the R'HK-index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M-dwarfs were left out of these analyses and no calibration of their Ca ii H&K emission to an R'HK exists to date. We set out to characterize the magnetic activity of the low and very low-mass stars by providing a calibration of the R'HK-index that extends to the realm of M-dwarfs, and by evaluating the relation between R'HK and the rot...

  18. A Population Synthesis Study of the White Dwarf Cooling Sequence of the Galactic Bulge

    Science.gov (United States)

    Torres, S.; García-Berro, E. G.; Cojocaru, R. E.; Calamida, A.

    2017-03-01

    Recent Hubble Space Telescope observations have allowed to determine, for the first time, the white dwarf cooling sequence of the Galactic bulge. However, observations show systematically redder objects than those predicted by the theoretical cooling tracks of carbon-oxygen white dwarfs. Here we present a population synthesis study of the white dwarf cooling sequence of the galactic bulge including both single white dwarfs and binary systems. These calculations incorporate the most up-to-date cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, for both white dwarfs with carbon-oxygen and helium cores, and also take into account detailed prescriptions of the evolution of binary systems and of the observational biases. This allows us to model with a high degree of realism the white dwarf population of the Galactic bulge. Among other interesting results we estimate the fraction of binaries and double degenerate systems of the Galactic bulge.

  19. The relativistic pulsar-white dwarf binary PSR J1738+0333 I. Mass determination and evolutionary history

    CERN Document Server

    Antoniadis, J; Koester, D; Freire, P C C; Wex, N; Tauris, T M; Kramer, M; Bassa, C G

    2012-01-01

    PSR J1738+0333 is one of the four millisecond pulsars known to be orbited by a white dwarf companion bright enough for optical spectroscopy. Of these, it has the shortest orbital period, making it especially interesting for a range of astrophysical and gravity related questions. We present a spectroscopic and photometric study of the white dwarf companion and infer its radial velocity curve, effective temperature, surface gravity and luminosity. We find that the white dwarf has properties consistent with those of low-mass white dwarfs with thick hydrogen envelopes, and use the corresponding mass-radius relation to infer its mass; M_WD = 0.181 +/- +0.007/-0.005 solar masses. Combined with the mass ratio q=8.1 +/- 0.2 inferred from the radial velocities and the precise pulsar timing ephemeris, the neutron star mass is constrained to M_PSR = 1.47 +/- +0.07/-0.06 solar masses. Contrary to expectations, the latter is only slightly above the Chandrasekhar limit. We find that, even if the birth mass of the neutron s...

  20. 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

  1. THE DYNAMICAL EVOLUTION OF LOW-MASS HYDROGEN-BURNING STARS, BROWN DWARFS, AND PLANETARY-MASS OBJECTS FORMED THROUGH DISK FRAGMENTATION

    Energy Technology Data Exchange (ETDEWEB)

    Li, Yun; Kouwenhoven, M. B. N. [Department of Astronomy, School of Physics, Peking University, Yiheyuan Lu 5, Haidian Qu, Beijing 100871 (China); Stamatellos, D. [Jeremiah Horrocks Institute for Mathematics, Physics and Astronomy, University of Central Lancashire, Preston, PR1 2HE (United Kingdom); Goodwin, S. P., E-mail: yunli@pku.edu.cn [Department of Physics and Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH (United Kingdom)

    2015-06-01

    Theory and simulations suggest that it is possible to form low-mass hydrogen-burning stars, brown dwarfs (BDs), and planetary-mass objects (PMOs) via disk fragmentation. As disk fragmentation results in the formation of several bodies at comparable distances to the host star, their orbits are generally unstable. Here, we study the dynamical evolution of these objects. We set up the initial conditions based on the outcomes of the smoothed-particle hydrodynamics simulations of Stamatellos and Whitworth, and for comparison we also study the evolution of systems resulting from lower-mass fragmenting disks. We refer to these two sets of simulations as set 1 and set 2, respectively. At 10 Myr, approximately half of the host stars have one companion left, and approximately 22% (set 1) to 9.8% (set 2) of the host stars are single. Systems with multiple secondaries in relatively stable configurations are common (about 30% and 44%, respectively). The majority of the companions are ejected within 1 Myr with velocities mostly below 5 km s{sup −1}, with some runaway escapers with velocities over 30 km s{sup −1}. Roughly 6% (set 1) and 2% (set 2) of the companions pair up into very low-mass binary systems, resulting in respective binary fractions of 3.2% and 1.2%. The majority of these pairs escape as very low-mass binaries, while others remain bound to the host star in hierarchical configurations (often with retrograde inner orbits). Physical collisions with the host star (0.43 and 0.18 events per host star for set 1 and set 2, respectively) and between companions (0.08 and 0.04 events per host star for set 1 and set 2, respectively) are relatively common and their frequency increases with increasing disk mass. Our study predicts observable properties of very low-mass binaries, low-mass hierarchical systems, the BD desert, and free-floating BDs and PMOs in and near young stellar groupings, which can be used to distinguish between different formation scenarios of very low-mass

  2. Adaptive Optics imaging of VHS 1256-1257: A Low Mass Companion to a Brown Dwarf Binary System

    CERN Document Server

    Stone, Jordan M; Kratter, Kaitlin M; Dupuy, Trent J; Close, Laird M; Eisner, Josh A; Fortney, Jonathan J; Hinz, Philip M; Males, Jared R; Morley, Caroline V; Morzinski, Katie M; Ward-Duong, Kimberly

    2016-01-01

    Recently, Gauza et al. (2015) reported the discovery of a companion to the late M-dwarf, VHS J125601.92-125723.9 (VHS 1256-1257). The companion's absolute photometry suggests its mass and atmosphere are similar to the HR 8799 planets. However, as a wide companion to a late-type star, it is more accessible to spectroscopic characterization. We discovered that the primary of this system is an equal-magnitude binary. For an age $\\sim300$ Myr the A and B components each have a mass of $64.6^{+0.8}_{-2.0}~M_{\\mathrm{Jup}}$, and the b component has a mass of $11.2^{+9.7}_{-1.8}$, making VHS 1256-1257 only the third brown dwarf triple system. There exists some tension between the spectrophotometric distance of $17.2\\pm2.6$ pc and the parallax distance of $12.7\\pm1.0$ pc. At 12.7 pc VHS1256-1257 A and B would be the faintest known M7.5 objects, and are even faint outliers among M8 types. If the larger spectrophotmetric distance is more accurate than the parallax, then the mass of each component increases. In particul...

  3. Primeval very low-mass stars and brown dwarfs. I. Six new L subdwarfs, classification and atmospheric properties

    CERN Document Server

    Zhang, Z H; Galvez-Ortiz, M C; Burningham, B; Lodieu, N; Marocco, F; Burgasser, A J; Day-Jones, A C; Allard, F; Jones, H R A; Homeier, D; Gomes, J; Smart, R L

    2016-01-01

    We have conducted a search for L subdwarf candidates within the photometric catalogues of the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey. Six of our candidates are confirmed as L subdwarfs spectroscopically at optical and/or near infrared wavelengths. We also present new optical spectra of three previously known L subdwarfs (WISEA J001450.17-083823.4, 2MASS J00412179+3547133, ULAS J124425.75+102439.3). We examined the spectral types and metallicity subclasses classification of known L subdwarfs. We summarised the spectroscopic properties of L subdwarfs with different spectral types and subclasses. We classify these new L subdwarfs by comparing their spectra to known L subdwarfs and L dwarf standards. We estimate temperatures and metallicities of 22 late type M and L subdwarfs by comparing their spectra to BT-Settl models. We find that L subdwarfs have temperatures between 1500 K and 2700 K, which are higher than similarly-typed L dwarfs by around 100-400 K depending on different subclasses an...

  4. Polluting white dwarfs with perturbed exo-comets

    Science.gov (United States)

    Caiazzo, Ilaria; Heyl, Jeremy S.

    2017-08-01

    We present a model to account for the observed debris discs around young white dwarfs and the presence of metal lines in their spectra. Stellar evolution models predict that the mass-loss on the AGB will be pulsed; furthermore, observations indicate that the bulk of the mass-loss occurs on the AGB. In this case, if the progenitors of the white dwarfs had remnants of planetary formation like the Sun's Oort cloud or the Kuiper Belt and a planet lying within that cloud or nearby, we find that up to 2 per cent of the planetesimals will fall either into planet-crossing orbits or into chaotic regions after the mass-loss, depending on the location and mass of the planet (from Mars to Neptune). This yields a sufficient mass of comets that can be scattered towards the star, form a debris disc and pollute the atmosphere.

  5. Discovery of five new massive pulsating white dwarf stars

    Science.gov (United States)

    Castanheira, B. G.; Kepler, S. O.; Kleinman, S. J.; Nitta, A.; Fraga, L.

    2013-03-01

    Using the SOuthern Astrophysical Research telescope (SOAR) Optical Imager at the SOAR 4.1 m telescope, we report on the discovery of five new massive pulsating white dwarf stars. Our results represent an increase of about 20 per cent in the number of massive pulsators. We have detected both short and long periods, low and high amplitude pulsation modes, covering the whole range of the ZZ Ceti instability strip. In this paper, we present a first seismological study of the new massive pulsators based on the few frequencies detected. Our analysis indicates that these stars have masses higher than average, in agreement with the spectroscopic determinations. In addition, we study for the first time the ensemble properties of the pulsating white dwarf stars with masses above 0.8 M⊙. We found a bimodal distribution of the main pulsation period with the effective temperature for the massive DAVs, which indicates mode selection mechanisms.

  6. Constraining Gravitational Theories by Observing Magnetic White Dwarfs

    CERN Document Server

    Preuss, O; Haugan, Mark P; Solanki, S K; Preuss, Oliver; Jordan, Stefan; Haugan, Mark P.; Solanki, Sami K.

    2004-01-01

    Under the assumption of a specific nonminimal coupling of torsion to Under the assumption of a specific nonminimal coupling of torsion to electromagnetism, spacetime is birefringent in the presence of a gravitational field leading to depolarization of light emitted from extended astrophysical sources. We use polarimetric data of the magnetic white dwarf RE J0317-853 to set for the very first time constraints on the essential coupling constant for this effect, giving k^2 <22 m^2. electromagnetism, spacetime is birefringent in the presence of a gravitational field leading to depolarization of light emitted from extended astrophysical sources. We use polarimetric data of the magnetic white dwarf RE J0317-853 to set for the very first time constraints on the essential coupling constant for this effect, giving k^2 <22 m^2.

  7. Limb-Darkening Coefficients for Eclipsing White Dwarfs

    CERN Document Server

    Gianninas, A; Kilic, Mukremin; Bergeron, P

    2013-01-01

    We present extensive calculations of linear and non-linear limb-darkening coefficients as well as complete intensity profiles appropriate for modeling the light-curves of eclipsing white dwarfs. We compute limb-darkening coefficients in the Johnson-Kron-Cousins UBVRI photometric system as well as the Large Synoptic Survey Telescope (LSST) ugrizy system using the most up-to-date model atmospheres available. In all, we provide the coefficients for seven different limb-darkening laws. We describe the variations of these coefficients as a function of the atmospheric parameters, including the effects of convection at low effective temperatures. Finally, we discuss the importance of having readily available limb-darkening coefficients in the context of present and future photometric surveys like the LSST, Palomar Transient Factory (PTF), and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The LSST, for example, may find ~10^5 eclipsing white dwarfs. The limb-darkening calculations presented h...

  8. The white dwarf cooling sequence of 47 Tucanae

    CERN Document Server

    García-Berro, Enrique; Althaus, Leandro G; Bertolami, Marcelo M Miller

    2014-01-01

    47 Tucanae is one of the most interesting and well observed and theoretically studied globular clusters. This allows us to study the reliability of our understanding of white dwarf cooling sequences, to confront different methods to determine its age, and to assess other important characteristics, like its star formation history. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the distribution of effective temperatures, the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. We do so using an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. We find a good agreement between our theoretical models and the observed data. Thus, our study, rules out previous claims that there are still missing physics in the white dwarf cooling models at moderately high e...

  9. On the origin of high-field magnetic white dwarfs

    CERN Document Server

    García-Berro, E; Lorén-Aguilar, P; Aznar-Siguán, G; Camacho, J; Külebi, B; Isern, J; Althaus, L G; Córsico, A H

    2012-01-01

    High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here we show that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores is able to produce magnetic fields of the required strength that do not decay for long timescales. We also show, using an state-of-the-art Monte Carlo simulator, that the expected number of high-field magnetic white dwarfs produced in this way is consistent with that found in the solar neighborhood.

  10. A Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs

    CERN Document Server

    Dufour, P; Fontaine, G

    2005-01-01

    We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz, and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the first data release of the Sloan Digital Sky Survey. Our analysis confirms that effective temperatures derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly the mean mass of the trigonometric parallax sample, = 0.62 Mo, is significantly lower than that obtained from pure helium models, = 0.73 Mo, and more consistent with the spectroscopic mean mass of DB stars, = 0.59 Mo, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well defined sequence in a ...

  11. The metal content of hot DA white dwarf spectra

    CERN Document Server

    Dickinson, Nathan

    2012-01-01

    A study of high ion metal absorption features present in the spectra of hot DA white dwarfs is presented. An analysis of three DAs is performed, where previous studies came to conflicting conclusions as to the stars' nitrogen configurations. The nitrogen abundances were found to be in keeping with DAs of higher Teff, with a homogeneous distribution. A search for circumstellar gas discs was performed on eight stars, where circumstellar pollution may explain the differences between predicted and observed metal abundances. No positive detections were made. Already the subject of previous studies, the circumstellar absorption features seen at many hot DAs were again analysed, using a more advanced technique than those implemented in previous studies. This allowed, for the first time, column density measurements for all non-photospheric absorbing material. The derived column density measurements are consistent with those predicted to exist in white dwarf Stromgren Spheres, and the velocities of the absorbing mater...

  12. Cool DZ white dwarfs I: Identification and spectral analysis

    Science.gov (United States)

    Hollands, M. A.; Koester, D.; Alekseev, V.; Herbert, E. L.; Gänsicke, B. T.

    2017-01-01

    White dwarfs with metal lines in their spectra act as signposts for post-main sequence planetary systems. Searching the Sloan Digital Sky Survey (SDSS) data release 12, we have identified 231 cool (absorption, extending the DZ cooling sequence to both higher metal abundances, lower temperatures, and hence longer cooler ages. Of these 231 systems, 104 are previously unknown white dwarfs. Compared with previous work, our spectral fitting uses improved model atmospheres with updated line profiles and line-lists, which we use to derive effective temperatures and abundances for up to 8 elements. We also determine spectroscopic distances to our sample, identifying two halo-members with tangential space-velocities >300 km s-1. The implications of our results on remnant planetary systems are to be discussed in a separate paper.

  13. Photometry of two DQ white dwarfs - search for spots

    CERN Document Server

    Vornanen, Tommi; 10.1051/0004-6361/201219255

    2012-01-01

    Aims. The intensity profiles of the C_2 Swan bands in cool DQ white dwarfs cannot be adequately fitted with models that otherwise succesfully reproduce spectral features of the molecule CH in these stars. Initial modelling showed that a two-component atmosphere in the style of a spot might be able to solve the problem. Methods. We photometrically observed the two cool DQ white dwarfs GJ1117 and EGGR78 to search for variability caused by stellar spots. Results. We have not found any such variability, but we estimate the effects of hypothetical spots on lightcurves. We also estimate detection probabilities for spots in different configurations. Alternative explanations of the problem are needed and briefly discussed.

  14. Onset of Convectionon a Pre-Runaway White Dwarf

    CERN Document Server

    Dursi, L J; Alexakis, A; Truran, J W; Rosner, R; Zingale, M A; Fryxell, B; Ricker, P M; Timmes, F X; Olson, K

    2002-01-01

    Observed novae abundances and explosion energies estimated from observations indicate that there must be significant mixing of the heavier material of the white dwarf (C+O) into the lighter accreted material (H+He). Accordingly, nova models must incorporate a mechanism that will dredge up the heavier white dwarf material, and fluid motions from an early convection phase is one proposed mechanism. We present results from two-dimensional simulations of classical nova precursor models that demonstrate the beginning of a convective phase during the `simmering' of a Nova precursor. We use a new hydrostatic equilibrium hydrodynamics module recently developed for the adaptive-mesh code FLASH. The two-dimensional models are based on the one-dimensional models of Ami Glasner, and were evolved with FLASH from a pre-convective state to the onset of convection.

  15. Gravitational wave emission from the coalescence of white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Garcia-Berro, E [Departament de Fisica Aplicada, Universitat Politecnica de Catalunya, Escola Politecnica Superior de Castelldefels, Avda del Canal OlImpic s/n, 08860 Castelldefels (Spain); Institut d' Estudis Espacials de Catalunya, Ed. Nexus, c/Gran Capita 2, 08034 Barcelona (Spain); Loren-Aguilar, P [Institut d' Estudis Espacials de Catalunya, Ed. Nexus, c/Gran Capita 2, 08034 Barcelona (Spain); Institut de Ciencies de l' Espai (CSIC), Campus UAB, Facultat de Ciencies, Torre C-5, 08193 Bellaterra (Spain); Isern, J [Institut d' Estudis Espacials de Catalunya, Ed. Nexus, c/Gran Capita 2, 08034 Barcelona (Spain); Institut de Ciencies de l' Espai (CSIC), Campus UAB, Facultat de Ciencies, Torre C-5, 08193 Bellaterra (Spain); Pedemonte, A G [Departament de Fisica Aplicada, Universitat Politecnica de Catalunya, Escola Politecnica Superior de Castelldefels, Avda del Canal OlImpic s/n, 08860 Castelldefels (Spain); Guerrero, J [Institut d' Estudis Espacials de Catalunya, Ed. Nexus, c/Gran Capita 2, 08034 Barcelona (Spain); Institut de Ciencies de l' Espai (CSIC), Campus UAB, Facultat de Ciencies, Torre C-5, 08193 Bellaterra (Spain); Lobo, J A [Institut d' Estudis Espacials de Catalunya, Ed. Nexus, c/Gran Capita 2, 08034 Barcelona (Spain); Departament de Fisica Fonamental, Universitat de Barcelona, c/MartI i Franques 1, 08028 Barcelona (Spain)

    2005-05-21

    We have computed the gravitational wave emission arising from the coalescence of several close white dwarf binary systems. In order to do so, we have followed the evolution of such systems using a smoothed particle hydrodynamics code. Here we present some of the results obtained so far, paying special attention to the detectability of the emitted gravitational waves. Within this context, we show which could be the impact of individual merging episodes for LISA.

  16. Stellar archaeology with Gaia: the Galactic white dwarf population

    CERN Document Server

    Gaensicke, Boris; Barstow, Martin; Bono, Giuseppe; Burleigh, Matt; Casewell, Sarah; Dhillon, Vik; Farihi, Jay; Garcia-Berro, Enrique; Geier, Stephan; Gentile-Fusillo, Nicola; Hermes, JJ; Hollands, Mark; Istrate, Alina; Jordan, Stefan; Knigge, Christian; Manser, Christopher; Marsh, Tom; Nelemans, Gijs; Pala, Anna; Raddi, Roberto; Tauris, Thomas; Toloza, Odette; Veras, Dimitri; Werner, Klaus; Wilson, David

    2015-01-01

    Gaia will identify several 1e5 white dwarfs, most of which will be in the solar neighborhood at distances of a few hundred parsecs. Ground-based optical follow-up spectroscopy of this sample of stellar remnants is essential to unlock the enormous scientific potential it holds for our understanding of stellar evolution, and the Galactic formation history of both stars and planets.

  17. On the possible observational signatures of white dwarf dynamical interactions

    OpenAIRE

    Aznar-Siguán, G.; García-Berro, E.; Magnien, M.; Lorén-Aguilar, P.

    2014-01-01

    We compute the possible observational signatures of white dwarf dynamical interactions in dense stellar environments. Specifically, we compute the emission of gravitational waves, and we compare it with the sensitivity curves of planned space-borne gravitational wave detectors. We also compute the light curves for those interactions in which a detonation occurs, and one of the stars is destroyed, as well as the corresponding neutrino luminosities. We find that for the three possible outcomes ...

  18. Planet-Planet Scattering and White Dwarf Pollution

    Science.gov (United States)

    Joasil, Arielle; Payne, Matthew John; Veras, Dimitri

    2017-01-01

    About one-quarter to one-half of white dwarfs are observed to have polluted atmospheres. White dwarfs (WD) are expected to be chemically stratified, with heavy elements rapidly sinking. The frequent observation of heavy element pollution in WD atmospheres indicates that there must be a copious and frequent supply of rocky material from remnant planetary systems acting as a pollutant. Recently, the white dwarf WD 1145+017 has been observed to have been transited by a rocky body apparently in the process of disintegrating (Vanderburg et al. 2015).Post-main sequence expansion may render the planetary system unstable (Veras 2016). Planets orbiting the white dwarf may perturb and scatter one another. If this scattering happens, any moons can be scattered about the system. As such, one possible source of the material polluting WDs is destabilized exomoons (Payne et al. 2016a, 2016b). Moons offer a plausible source of pollution due to their large total mass (in the Solar system), and their generally rocky composition that matches that found in the atmospheric pollution of WDs. During a planet-planet scattering event, the probability that a moon will be ejected from its parent planet is a function of the velocity of the perturbing planet and the distance between the perturbed moon and the perturbing planet (as well as the initial orbit of the moon). We review the results of Payne et al. (2016a, 2016b) and present new results illustrating the probability of moon ejection as a function of these key parameters. We demonstrate the utility of these results for (a) the pollution and WDs, and for (b) general planet-planet scattering scenarios around main-sequence stars.

  19. On mode trapping in pulsating DA white dwarf stars

    CERN Document Server

    Benvenuto, O G; Althaus, L G; Serenelli, A M

    2002-01-01

    The present work is designed to explore the effects of the time-dependent element diffusion on the mode trapping properties of DA white dwarf models with various thickness of the hydrogen envelope. Our predictions are compared with the standard assumption of diffusive equilibrium in the trace element approximation. We find that element diffusion markedly weakens the presence of mode trapping originated in the outer layers of the models, even for the case of thin hydrogen envelopes.

  20. White dwarf stars as strange quark matter detectors

    Energy Technology Data Exchange (ETDEWEB)

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

    2005-11-01

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

  1. A population synthesis study of the luminosity function of hot white dwarfs

    CERN Document Server

    Torres, S; Krzesinski, J; Kleinman, S J

    2014-01-01

    We present a coherent and detailed Monte Carlo simulation of the population of hot white dwarfs. We assess the statistical significance of the hot end of the white dwarf luminosity function and the role played by the bolometric corrections of hydrogen-rich white dwarfs at high effective temperatures. We use the most up-to-date stellar evolutionary models and implement a full description of the observational selection biases to obtain realistic simulations of the observed white dwarf population. Our theoretical results are compared with the luminosity function of hot white dwarfs obtained from the Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. We find that the theoretical results are in excellent agreement with the observational data for the population of white dwarfs with hydrogen deficient atmospheres (non-DA white dwarfs). For the population of white dwarfs with hydrogen-rich atmospheres (white dwarfs of the DA class), our simulations show some discrepancies with the observations for ...

  2. 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.

  3. 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...

  4. Towards an Understanding of the Atmospheres of Cool White Dwarfs

    CERN Document Server

    Kowalski, Piotr M; Holberg, Jay; Leggett, Sandy

    2012-01-01

    Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars that existed in our Galaxy. Their atmospheres, when properly characterized, can provide valuable information on white dwarf evolution and ultimately star formation through the history of the Milky Way. Understanding the atmospheres of these stars requires joined observational effort and reliable atmosphere modeling. We discuss and analyze recent observations of the near-ultraviolet (UV) and near-infrared (IR) spectrum of several cool white dwarfs including DQ/DQp stars showing carbon in their spectra. We present fits to the entire spectral energy distribution (SED) of selected cool stars, showing that the current pure-hydrogen atmosphere models are quite reliable, especially in the near-UV spectral region. Recently, we also performed an analysis of the coolest known DQ/DQp stars investigating further the origin of the C2 Swan bands-like spectral features that characterize the DQp stars. We show that the carbon abundances derived for ...

  5. COS Spectroscopy of White Dwarf Companions to Blue Stragglers

    Science.gov (United States)

    Gosnell, Natalie M.; Geller, Aaron M.; Knigge, Christian; Mathieu, Robert D.; Sills, Alison; Leiner, Emily; Leigh, Nathan

    2017-01-01

    Complete membership studies of open stellar clusters reveal that 25% of the evolved stars follow alternative pathways in stellar evolution, meaning something in the history of these stars changed their composition or mass (or both). In order to draw a complete picture of stellar evolution we must include these canonically "strange" stars in our definition of standard stellar populations. The formation mechanism of blue straggler stars, traditionally defined to be brighter and bluer than the main sequence turnoff in a star cluster, has been an outstanding question for almost six decades. Recent Hubble Space Telescope (HST) far-ultraviolet (far-UV) observations directly reveal that the blue straggler stars in the old (7 Gyr) open cluster NGC 188 are predominantly formed through mass transfer. We will present HST far-UV COS spectroscopy of white dwarf companions to blue stragglers. These white dwarfs are the remnants of the mass transfer formation process. The effective temperatures and surface gravities of the white dwarfs delineate the timeline of blue straggler formation in this cluster. The existence of these binaries in a well-studied cluster environment provides an unprecedented opportunity to observationally constrain mass transfer models and inform our understanding of many other alternative pathway stellar products.

  6. The age-metallicity dependence for white dwarfs

    CERN Document Server

    Romero, A D; Kepler, S O

    2015-01-01

    We present a theoretical study on the metallicity dependence of the initial$-$to$-$final mass relation and its influence on white dwarf age determinations. We compute a grid of evolutionary sequences from the main sequence to $\\sim 3\\, 000$ K on the white dwarf cooling curve, passing through all intermediate stages. During the thermally-pulsing asymptotic giant branch no third dredge-up episodes are considered and thus the photospheric C/O ratio is below unity for sequences with metallicities larger than $Z=0.0001$. We consider initial metallicities from $Z=0.0001$ to $Z=0.04$, accounting for stellar populations in the galactic disk and halo, with initial masses below $\\sim 3M_{\\odot}$. We found a clear dependence of the shape of the initial$-$to$-$final mass relation with the progenitor metallicity, where metal rich progenitors result in less massive white dwarf remnants, due to an enhancement of the mass loss rates associated to high metallicity values. By comparing our theoretical computations with semi em...

  7. Constraining the neutrino magnetic dipole moment from white dwarf pulsations

    CERN Document Server

    Córsico, Alejandro H; Bertolami, Marcelo M Miller; Kepler, S O; García-Berro, Enrique

    2014-01-01

    Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. By comparing the theoretical rate of change of period expected for this star with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment. Our upper limit for the neutrino magnetic dipole moment is somewhat less restrictive than, but still compat...

  8. Regimes of Internal Rotation in Differentially Rotating White Dwarfs

    Science.gov (United States)

    Wheeler, J. Craig; Ghosh, Pranab

    2017-01-01

    Most viable models of Type Ia supernovae (SN Ia) require the thermonuclear explosion of a carbon/oxygen white dwarf that has evolved in a binary system. Rotation could be an important aspect of any model for SN Ia, whether single or double degenerate, with the white dwarf mass at, below, or above the Chandrasekhar limit. Differential rotation is specifically invoked in attempts to account for the apparent excess mass in the super--Chandrasekhar events. Some earlier work has suggested that only uniform rotation is consistent with the expected mechanisms of angular momentum transport in white dwarfs, while others have found pronounced differential rotation. We show that if the baroclinic instability is active in degenerate matter and the effects of magnetic fields are neglected, both nearly-uniform and strongly-differential rotation are possible. We classify rotation regimes in terms of the Richardson number, Ri. At small values of Ri > 1 produce a regime of nearly-uniform rotation for which the baroclinic viscosity is of intermediate value and scales as σ3. We discuss the gap in understanding of the behavior at intermediate values of Ri and how observations may constrain the rotation regimes attained by nature.

  9. Differentially Rotating White Dwarfs I: Regimes of Internal Rotation

    Science.gov (United States)

    Ghosh, Pranab; Wheeler, J. Craig

    2017-01-01

    Most viable models of Type Ia supernovae (SNe Ia) require the thermonuclear explosion of a carbon/oxygen white dwarf that has evolved in a binary system. Rotation could be an important aspect of any model for SNe Ia, whether single or double degenerate, with the white dwarf mass at, below, or above the Chandrasekhar limit. Differential rotation is specifically invoked in attempts to account for the apparent excess mass in the super-Chandrasekhar events. Some earlier work has suggested that only uniform rotation is consistent with the expected mechanisms of angular momentum transport in white dwarfs, while others have found pronounced differential rotation. We show that if the baroclinic instability is active in degenerate matter and the effects of magnetic fields are neglected, both nearly uniform rotation and strongly differential rotation are possible. We classify rotation regimes in terms of the Richardson number, Ri. At small values of Ri ≤slant 0.1, we find both the low-viscosity Zahn regime with a nonmonotonic angular velocity profile and a new differential rotation regime for which the viscosity is high and scales linearly with the shear, σ. Employment of Kelvin–Helmholtz viscosity alone yields differential rotation. Large values of Ri ≫ 1 produce a regime of nearly uniform rotation for which the baroclinic viscosity is of intermediate value and scales as {σ }3. We discuss the gap in understanding of the behavior at intermediate values of Ri and how observations may constrain the rotation regimes attained by nature.

  10. Progenitors of the Accretion-Induced Collapse of White Dwarfs

    CERN Document Server

    Kwiatkowski, Damian

    2015-01-01

    Recent calculations of accretion-induced collapse of an oxygen-neon-magnesium white dwarf into a neutron star [Piro & Thompson 2014] allow for a potentially detectable transient electromagnetic signal. Motivated by these results, I present theoretical rates and physical properties of binary stars that can produce accretion-induced collapse. The rates are presented for various types of host galaxies (e.g. old ellipticals versus spirals) and are differentiated by the donor star type (e.g. large giant star versus compact helium-rich donor). Results presented in this thesis may help to guide near-future electromagnetic transient search campaigns to find likely candidates for accretion-induced collapse events. My predictions are based on binary evolution calculations that include the most recent updates on mass accretion and secular mass growth of white dwarfs. I find that the most likely systems that undergo accretion-induced collapse consist of an ONeMg white dwarf with a Hertzsprung gap star or a red giant ...

  11. The distribution of metals in hot DA white dwarfs

    CERN Document Server

    Dickinson, N J; Hubeny, I

    2012-01-01

    The importance to stellar evolution of understanding the metal abundances in hot white dwarfs is well known. Previous work has found the hot DA white dwarfs REJ 1032+532, REJ 1614-085 and GD 659 to have highly abundant, stratified photospheric nitrogen, due to the narrow absorption line profiles of the FUV N V doublet and the lack of EUV continuum absorption. A preliminary analysis of the extremely narrow, deep line profiles of the photospheric metal absorption features of PG 0948+534 suggested a similar photospheric metal configuration. However, other studies have found REJ 1032+532, REJ 1614-085 and GD 659 can be well described by homogeneous models, with nitrogen abundances more in keeping with those of white dwarfs with higher effective temperatures. Here, a re-analysis of the nitrogen absorption features seen in REJ 1032+532, REJ 1614-085 and GD 659 is presented, with the aim of better understanding the structure of these stars, to test which models better represent the observed data and apply the result...

  12. Evidence for Terrestrial Planetary System Remnants at White Dwarfs

    CERN Document Server

    Farihi, J

    2010-01-01

    The last several years have brought about a dynamic shift in the view of exoplanetary systems in the post-main sequence, perhaps epitomized by the evidence for surviving rocky planetary bodies at white dwarfs. Coinciding with the launch of the Spitzer Space Telescope, both space- and ground-based data have supported a picture whereby asteroid analogs persist at a significant fraction of cool white dwarfs, and are prone to tidal disruption when passing close to the compact stellar remnant. The ensuing debris can produce a detectable infrared excess, and the material gradually falls onto the star, polluting the atmosphere with heavy elements that can be used to determine the bulk composition of the destroyed planetary body. Based on the observations to date, the parent bodies inferred at white dwarfs are best described as asteroids, and have a distinctly rocky composition similar to material found in the inner Solar System. Their minimum masses are typical of large asteroids, and can approach or exceed the mass...

  13. Calibration of Synthetic Photometry Using DA White Dwarfs

    Science.gov (United States)

    Holberg, J. B.; Bergeron, Pierre

    2006-09-01

    We have calibrated four major ground-based photometric systems with respect to the Hubble Space Telescope (HST) absolute flux scale, which is defined by Vega and four fundamental DA white dwarfs. These photometric systems include the Johnson-Kron-Cousins UBVRI, the Strömgren uvby filters, the Two Micron All Sky Survey JHKs, and the Sloan Digital Sky Survey ugriz filters. Synthetic magnitudes are calculated from model white dwarf spectra folded through the published filter response functions; these magnitudes in turn are absolutely calibrated with respect to the HST flux scale. Effective zero-magnitude fluxes and zero-point offsets of each system are determined. In order to verify the external observational consistency, as well as to demonstrate the applicability of these definitions, the synthetic magnitudes are compared with the respective observed magnitudes of larger sets of DA white dwarfs that have well-determined effective temperatures and surface gravities and span a wide range in both of these parameters.

  14. Differentially Rotating White Dwarfs I: Regimes of Internal Rotation

    CERN Document Server

    Ghosh, Pranab

    2016-01-01

    Most viable models of Type Ia supernovae (SN~Ia) require the thermonuclear explosion of a carbon/oxygen white dwarf that has evolved in a binary system. Rotation could be an important aspect of any model for SN~Ia, whether single or double degenerate, with the white dwarf mass at, below, or above the Chandrasekhar limit. {\\sl Differential rotation} is specifically invoked in attempts to account for the apparent excess mass in the super--Chandrasekhar events. Some earlier work has suggested that only uniform rotation is consistent with the expected mechanisms of angular momentum transport in white dwarfs, while others have found pronounced differential rotation. We show that if the baroclinic instability is active in degenerate matter and the effects of magnetic fields are neglected, both nearly-uniform and strongly-differential rotation are possible. We classify rotation regimes in terms of the Richardson number, Ri. At small values of Ri $\\leq$ 0.1, we find both the low-viscosity Zahn regime with a non-monot...

  15. The double-layered chemical structure in DB white dwarfs

    CERN Document Server

    Althaus, L G

    2004-01-01

    We study the structure and evolution of white dwarf stars with helium-rich atmospheres (DB) in a self-consistent way with the predictions of time-dependent element diffusion. Our treatment of diffusion includes gravitational settling and chemical and thermal diffusion. OPAL radiative opacities for arbitrary metallicity and carbon-and oxygen-rich compositions are employed. Emphasis is placed on the evolution of the diffusion-modeled double-layered chemical structure. This structure, which is characterized by a pure helium envelope atop an intermediate remnant shell rich in helium, carbon and oxygen, is expected for pulsating DB white dwarfs, assuming that they are descendants of hydrogen-deficient PG1159 post-AGB stars. We find that, depending on the stellar mass, if DB white dwarf progenitors are formed with a helium content smaller than \\approx 10^-3 M_*, a single-layered configuration is expected to emerge during the DB pulsation instability strip. We also explore the consequences of diffusively evolving ch...

  16. Mass-radius relations for white dwarf stars of different internal compositions

    CERN Document Server

    Panei, J A; Benvenuto, O G

    2000-01-01

    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 iron-dominated core, we focus our attention mainly on the finite-temperature, mass-radius relations for WD models with iron interiors. Furthermore, we explore the effects of gravitational, chemical and thermal diffusion on low-mass helium WD models with hydrogen and helium envelopes.

  17. Multi-Dimensional Double Detonation of Sub-Chandrasekhar Mass White Dwarfs

    CERN Document Server

    Moll, Rainer

    2013-01-01

    Using 2D and 3D simulation, we study the "robustness" of the double detonation scenario for Type Ia supernovae, in which a detonation in the helium shell of a carbon-oxygen white dwarf induces a secondary detonation in the underlying core. We find that a helium detonation cannot easily descend into the core unless it commences (artificially) well above the hottest layer calculated for the helium shell in current presupernova models. Compressional waves induced by the sliding helium detonation, however, robustly generate hot spots which trigger a detonation in the core. Our simulations show that this is true even for non-axisymmetric initial conditions. If the helium is ignited at multiple points, the internal waves can pass through one another or be reflected, but this added complexity does not defeat the generation of the hot spot. The ignition of very low-mass helium shells depends on whether a thermonuclear runaway can simultaneously commence in a sufficiently large region.

  18. A Strange Star Scenario for the Formation of Eccentric Millisecond Pulsar/Helium White Dwarf Binaries

    CERN Document Server

    Jiang, Long; Dey, Jishnu; Dey, Mira

    2015-01-01

    According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during the binary evolution, as observed in most of the binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are PSRs J2234$+$06, J1946$+$3417 and J1950$+$2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive white dwarfs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from neutron stars to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitati...

  19. No Neutron Star Companion To The Lowest Mass SDSS White Dwarf

    CERN Document Server

    Agueros, Marcel; Camilo, Fernando; Kilic, Mukremin; Anderson, Scott; Freire, Paulo; Kleinman, Scot; Liebert, James; Silvestri, Nicole

    2009-01-01

    SDSS J091709.55+463821.8 (hereafter J0917+4638) is the lowest surface gravity white dwarf (WD) currently known, with log g = 5.55 +/- 0.05 (M ~ 0.17 M_sun; Kilic et al. 2007a,b). Such low-mass white dwarfs (LMWDs) are believed to originate in binaries that evolve into WD/WD or WD/neutron star (NS) systems. An optical search for J0917+4638's companion showed that it must be a compact object with a mass >= 0.28 M_sun (Kilic 2007b). Here we report on Green Bank Telescope 820 MHz and XMM-Newton X-ray observations of J0917+4638 intended to uncover a potential NS companion to the LMWD. No convincing pulsar signal is detected in our radio data. Our X-ray observation also failed to detect X-ray emission from J0917+4638's companion, while we would have detected any of the millisecond radio pulsars in 47 Tuc. We conclude that the companion is almost certainly another WD.

  20. A faint type of supernova from a white dwarf with a helium-rich companion.

    Science.gov (United States)

    Perets, H B; Gal-Yam, A; Mazzali, P A; Arnett, D; Kagan, D; Filippenko, A V; Li, W; Arcavi, I; Cenko, S B; Fox, D B; Leonard, D C; Moon, D-S; Sand, D J; Soderberg, A M; Anderson, J P; James, P A; Foley, R J; Ganeshalingam, M; Ofek, E O; Bildsten, L; Nelemans, G; Shen, K J; Weinberg, N N; Metzger, B D; Piro, A L; Quataert, E; Kiewe, M; Poznanski, D

    2010-05-20

    Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The 'old' environment near the supernova location, and the very low derived ejected mass ( approximately 0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive (44)Ti.

  1. 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...

  2. The formation of a helium white dwarf in a close binary system with diffusion

    Science.gov (United States)

    Benvenuto, O. G.; De Vito, M. A.

    2004-07-01

    We study the evolution of a system composed of a 1.4-Msolar neutron star and a normal, solar composition star of 2 Msolar in orbit with a period of 1 d. Calculations were performed employing the binary HYDRO code presented by Benvenuto & De Vito that handle the mass transfer rate in a fully implicit way. We then included the main standard physical ingredients together with the diffusion processes and a proper outer boundary condition. We have assumed fully non-conservative mass transfer episodes. In order to study the interplay of mass loss episodes and diffusion we considered evolutionary sequences with and without diffusion in which all Roche lobe overflows (RLOFs) produce mass transfer. Another two sequences in which thermonuclearly driven RLOFs were not allowed to drive mass transfer have been computed with and without diffusion. As far as we are aware, this study represents the first binary evolution calculations in which diffusion is considered. The system produces a helium white dwarf of ~0.21 Msolar in an orbit with a period of ~4.3 d for the four cases. We find that mass transfer episodes induced by hydrogen thermonuclear flashes drive a tiny amount of mass transfer. As diffusion produces stronger flashes, the amount of hydrogen-rich matter transferred is slightly higher than in the models without diffusion. We find that diffusion is the main agent in determining the evolutionary time-scale of low-mass white dwarfs even in the presence of mass transfer episodes.

  3. A THERMAL INFRARED IMAGING STUDY OF VERY LOW MASS, WIDE-SEPARATION BROWN DWARF COMPANIONS TO UPPER SCORPIUS STARS: CONSTRAINING CIRCUMSTELLAR ENVIRONMENTS

    Energy Technology Data Exchange (ETDEWEB)

    Bailey, Vanessa; Hinz, Philip M.; Su, Kate Y. L.; Hoffmann, William F.; Rieke, George; Rodigas, Timothy; Skemer, Andrew; Vaitheeswaran, Vidhya [Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States); Currie, Thayne [Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada); Esposito, Simone; Pinna, Enrico; Puglisi, Alfio [Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze (Italy); Hill, John M. [Large Binocular Telescope Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States); Jones, Terry [School of Physics and Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455 (United States); Kim, Jihun [College of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, AZ 85721 (United States); Leisenring, Jarron; Meyer, Michael [Institut fuer Angewandte Physik, Eidgenoessische Technische Hochschule-Zuerich, CH-8093 (Switzerland); Murray-Clay, Ruth; Skrutskie, Michael F. [Harvard-Smithsonian Center for Astrophysics, Harvard University, 60 Garden St., Cambridge, MA 02138 (United States); Nelson, Matthew J., E-mail: vbailey@as.arizona.edu [Department of Astronomy, University of Virginia, Charlottesville, VA 22904 (United States); and others

    2013-04-10

    We present a 3-5 {mu}m LBT/MMT adaptive optics imaging study of three Upper Scorpius stars with brown dwarf (BD) companions with very low masses/mass ratios (M{sub BD} <25 M{sub Jup}; M{sub BD}/M{sub *} Almost-Equal-To 1%-2%) and wide separations (300-700 AU): GSC 06214, 1RXS 1609, and HIP 78530. We combine these new thermal IR data with existing 1-4 {mu}m and 24 {mu}m photometry to constrain the properties of the BDs and identify evidence for circumprimary/circumsecondary disks in these unusual systems. We confirm that GSC 06214B is surrounded by a disk, further showing that this disk produces a broadband IR excess due to small dust near the dust sublimation radius. An unresolved 24 {mu}m excess in the system may be explained by the contribution from this disk. 1RXS 1609B exhibits no 3-4 {mu}m excess, nor does its primary; however, the system as a whole has a modest 24 {mu}m excess, which may come from warm dust around the primary and/or BD. Neither object in the HIP 78530 system exhibits near- to mid-IR excesses. We additionally find that the 1-4 {mu}m colors of HIP 78530B match a spectral type of M3 {+-} 2, inconsistent with the M8 spectral type assigned based on its near-IR spectrum, indicating that it may be a low-mass star rather than a BD. We present new upper limits on additional low-mass companions in the system (<5 M{sub Jup} beyond 175 AU). Finally, we examine the utility of circumsecondary disks as probes of the formation histories of wide BD companions, finding that the presence of a disk may disfavor BD formation near the primary with subsequent outward scattering.

  4. Peculiar variations of white dwarf pulsation frequencies and maestro

    Science.gov (United States)

    Dalessio, James Ruland

    In Part I we report on variations of the normal mode frequencies of the pulsating DB white dwarfs EC 20058-5234 and KIC 8626021 and the pulsating DA white dwarf GD 66. The observations of EC 20058-5234 and KIC 8626021 were motivated by the possibility of measuring the plasmon neutrino production rate of a white dwarf, while the observations of GD 66 were part of a white dwarf pulsation timing based planet search. We announce the discovery of periodic and quasi-periodic variations of multiple normal mode frequencies that cannot be due to the presence of planetary companions. We note the possible signature of a planetary companion to EC 20058-5234 and show that GD 66 cannot have a planet in a several AU orbit down to half a Jupiter mass. We also announce the discovery of secular variations of the normal mode frequencies of all three stars that are inconsistent with cooling alone. Importantly, the rates of period change of several modes of KIC 8626021 are consistent with evolutionary cooling, but are not yet statistically significant. These modes offer the best possibility of measuring the neutrino production rate in a white dwarf. We also observe periodic and secular variations in the frequency of a combination mode that exactly matches the variations predicted by the parent modes, strong observational evidence that combination modes are created by the convection zone and are not normal modes. Periodic variations in the amplitudes of many of these modes is also noted. We hypothesize that these frequency variations are caused by complex variations of the magnetic field strength and geometry, analogous to behavior observed in the Sun. In Part II we describe the MAESTRO software framework and the MAESTRO REDUCE algorithm. MAESTRO is a collection of astronomy specific MatLab software developed by the Whole Earth Telescope. REDUCE is an an algorithm that can extract the brightness of stars on a set of CCD images with minimal configuration and human interaction. The key to

  5. Exoplanet recycling in massive white-dwarf debris discs

    Science.gov (United States)

    Van Lieshout, Rik

    2017-06-01

    When a star evolves into a white dwarf, the planetary system it hosts can become unstable. Planets in such systems may then be scattered onto star-grazing orbits, leading to their tidal disruption as they pass within the white dwarf’s Roche limit. We study the massive, compact debris discs that may arrise from this process using a combination of analytical estimates and numerical modelling. The discs are gravitationally unstable, resulting in an enhanced effective viscosity due to angular momentum transport associated with self-gravity wakes. For disc masses greater than ~1026 g (corresponding to progenitor objects comparable to the Galilean moons), viscous spreading dominates over Poynting-Robertson drag in the outer parts of the disc. In such massive discs, mass is transported both in- and outwards. When the outward-flowing material spreads beyond the Roche limit, it coagulates into new (minor) planets in a process analogous to the ongoing formation of Saturn’s innermost moonlets. This process recycles a substantial fraction of the original disc mass (tens of percents), with the bulk of the mass locked in a single large body orbitting in a 2:1 mean-motion resonance with the Roche limit. As such, the recycling of a tidally disrupted super-Earth could yield an Earth-mass planet on a 10--20 hr orbit. For white dwarfs with a temperature below 6000-7000 K (corresponding to a cooling age of >1--2 Gyr), this orbit is located in the white dwarf’s habitable zone. The recycling process also creates a string of smaller bodies just outside the Roche limit. These may account for the collection of minor planets postulated to orbit white dwarf WD 1145+017.

  6. CFBDSIR 2149-0403: young isolated planetary-mass object or high-metallicity low-mass brown dwarf?

    Science.gov (United States)

    Delorme, P.; Dupuy, T.; Gagné, J.; Reylé, C.; Forveille, T.; Liu, M. C.; Artigau, E.; Albert, L.; Delfosse, X.; Allard, F.; Homeier, D.; Malo, L.; Morley, C.; Naud, M. E.; Bonnefoy, M.

    2017-06-01

    Aims: We conducted a multi-wavelength, multi-instrument observational characterisation of the candidate free-floating planet CFBDSIR J214947.2-040308.9, a late T-dwarf with possible low-gravity features, in order to constrain its physical properties. Methods: We analysed nine hours of X-shooter spectroscopy with signal detectable from 0.8 to 2.3 μm, as well as additional photometry in the mid-infrared using the Spitzer Space Telescope. Combined with a VLT/HAWK-I astrometric parallax, this enabled a full characterisation of the absolute flux from the visible to 5 μm, encompassing more than 90% of the expected energy emitted by such a cool late T-type object. Our analysis of the spectrum also provided the radial velocity and therefore the determination of its full 3D kinematics. Results: While our new spectrum confirms the low gravity and/or high metallicity of CFBDSIR 2149, the parallax and kinematics safely rule out membership to any known young moving group, including AB Doradus. We use the equivalent width of the K i doublet at 1.25 μm as a promising tool to discriminate the effects of low-gravity from the effects of high-metallicity on the emission spectra of cool atmospheres. In the case of CFBDSIR 2149, the observed K i doublet clearly favours the low-gravity solution. Conclusions: CFBDSIR 2149 is therefore a peculiar late-T dwarf that is probably a young, planetary-mass object (2-13 MJup, X-shooter on VLT-UT2 at ESO-Paranal (run 091.D-0723). Based on observations obtained with HAWKI on VLT-UT4 (run 089.C-0952, 090.C-0483, 091.C-0543,092.C-0548,293.C-5019(A) and run 086.C-0655(A)). Based on observations obtained with ISAAC on VLT-UT3 at ESO-Paranal (run 290.C-5083). Based on observation obtained with WIRCam at CFHT (program 2012BF12). Based on Spitzer Space telescope DDT observation (program 10166).

  7. X-Shooter study of accretion in $\\rho$-Ophiucus: very low-mass stars and brown dwarfs

    CERN Document Server

    Manara, C F; Natta, A; Alcalá, J M

    2015-01-01

    We present new VLT/X-Shooter optical and NIR spectra of a sample of 17 candidate young low-mass stars and BDs in the rho-Ophiucus cluster. We derived SpT and Av for all the targets, and then we determined their physical parameters. All the objects but one have M*<0.6 Msun, and 8 have mass below or close to the hydrogen-burning limit. Using the intensity of various emission lines present in their spectra, we determined the Lacc and Macc for all the objects. When compared with previous works targeting the same sample, we find that, in general, these objects are not as strongly accreting as previously reported, and we suggest that the reason is our more accurate estimate of the photospheric parameters. We also compare our findings with recent works in other slightly older star-forming regions to investigate possible differences in the accretion properties, but we find that the accretion properties for our targets have the same dependence on the stellar and substellar parameters as in the other regions. This l...

  8. Chandra and MMT observations of low-mass black hole active galactic nuclei accreting at low rates in dwarf galaxies

    CERN Document Server

    Yuan, W; Dou, L; Dong, X -B; Fan, X; Wang, T -G

    2014-01-01

    We report on Chandra X-ray observations of four candidate low-mass black hole (<10^6Msun) 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 H_alpha 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 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 10^3s, whereas no significant (or only marginal at best) detection is found for the remaining two. The X-ray luminosities are on the order of 10^(41) ergs/s or even lower, on the order of 10^(40) ergs/s for non-detections, which are among the lowest regimes ever probed ...

  9. Deuterium Burning in Massive Giant Planets and Low-Mass Brown Dwarfs formed by Core-Nucleated Accretion

    CERN Document Server

    Bodenheimer, Peter; Lissauer, Jack J; Fortney, Jonathan J; Saumon, Didier

    2013-01-01

    Formation of bodies near the deuterium-burning limit is considered by detailed numerical simulations according to the core-nucleated giant planet accretion scenario. The objects, with heavy-element cores in the range 5-30 Mearth, are assumed to accrete gas up to final masses of 10-15 Jupiter masses (Mjup). After the formation process, which lasts 1-5 Myr and which ends with a 'cold-start', low-entropy configuration, the bodies evolve at constant mass up to an age of several Gyr. Deuterium burning via proton capture is included in the calculation, and we determined the mass, M50, above which more than 50% of the initial deuterium is burned. This often-quoted borderline between giant planets and brown dwarfs is found to depend only slightly on parameters, such as core mass, stellar mass, formation location, solid surface density in the protoplanetary disk, disk viscosity, and dust opacity. The values for M50 fall in the range 11.6-13.6 Mjup, in agreement with previous determinations that do not take the formati...

  10. Microlensing discovery of a tight, low-mass-ratio planetary-mass object around an old field brown dwarf

    Energy Technology Data Exchange (ETDEWEB)

    Han, C.; Jung, Y. K. [Department of Physics, Chungbuk National University, Cheongju 371-763 (Korea, Republic of); Udalski, A.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Soszyński, I.; Skowron, J.; Kozłowski, S.; Poleski, R.; Ulaczyk, K.; Wyrzykowski, Ł.; Pietrukowicz, P. [Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa (Poland); Sumi, T. [Department of Earth and Space Science, Osaka University, Osaka 560-0043 (Japan); Gaudi, B. S.; Gould, A. [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Bennett, D. P. [University of Notre Dame, Department of Physics, 225 Nieuwland Science Hall, Notre Dame, IN 46556-5670 (United States); Tsapras, Y. [Las Cumbres Observatory Global Telescope Network, 6740B Cortona Dr, Goleta, CA 93117 (United States); Abe, F. [Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601 (Japan); Bond, I. A. [Institute of Information and Mathematical Sciences, Massey University, Private Bag 102-904, North Shore Mail Centre, Auckland (New Zealand); Collaboration: OGLE Collaboration; MOA Collaboration; μFUN Collaboration; RoboNet Collaboration; and others

    2013-11-20

    Observations of accretion disks around young brown dwarfs (BDs) have led to the speculation that they may form planetary systems similar to normal stars. While there have been several detections of planetary-mass objects around BDs (2MASS 1207-3932 and 2MASS 0441-2301), these companions have relatively large mass ratios and projected separations, suggesting that they formed in a manner analogous to stellar binaries. We present the discovery of a planetary-mass object orbiting a field BD via gravitational microlensing, OGLE-2012-BLG-0358Lb. The system is a low secondary/primary mass ratio (0.080 ± 0.001), relatively tightly separated (∼0.87 AU) binary composed of a planetary-mass object with 1.9 ± 0.2 Jupiter masses orbiting a BD with a mass 0.022 M {sub ☉}. The relatively small mass ratio and separation suggest that the companion may have formed in a protoplanetary disk around the BD host in a manner analogous to planets.

  11. An Analysis of the SEEDS High-Contrast Exoplanet Survey: Massive Planets or Low-Mass Brown Dwarfs?

    CERN Document Server

    Brandt, Timothy D; Turner, Edwin L; Mede, Kyle; Spiegel, David S; Kuzuhara, Masayuki; Schlieder, Joshua E; Wisniewski, John P; Abe, L; Brandner, W; Carson, J; Currie, T; Egner, S; Feldt, M; Golota, T; Goto, M; Grady, C A; Guyon, O; Hashimoto, J; Hayano, Y; Hayashi, M; Hayashi, S; Henning, T; Hodapp, K W; Inutsuka, S; Ishii, M; Iye, M; Janson, M; Kandori, R; Knapp, G R; Kudo, T; Kusakabe, N; Kwon, J; Matsuo, T; Miyama, S; Morino, J -I; Moro-Martín, A; Nishimura, T; Pyo, T -S; Serabyn, E; Suto, H; Suzuki, R; Takami, M; Takato, N; Terada, H; Thalmann, C; Tomono, D; Watanabe, M; Yamada, T; Takami, H; Usuda, T; Tamura, M

    2014-01-01

    We conduct a statistical analysis of a combined sample of direct imaging data, totalling nearly 250 stars observed by HiCIAO on the Subaru Telescope, NIRI on Gemini North, and NICI on Gemini South. The stars cover a wide range of ages and spectral types, and include five detections (kap And b, two ~60 M_J brown dwarf companions in the Pleiades, PZ Tel B, and CD-35 2722 B). We conduct a uniform, Bayesian analysis of the ages of our entire sample, using both membership in a kinematic moving group and activity/rotation age indicators, to obtain posterior age distributions. We then present a new statistical method for computing the likelihood of a substellar distribution function. By performing most integrals analytically, we achieve an enormous speedup over brute-force Monte Carlo. We use this method to place upper limits on the maximum semimajor axis beyond which the distribution function for radial-velocity planets cannot extend, finding model-dependent values of ~30--100 AU. Finally, we treat our entire subst...

  12. Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey

    CERN Document Server

    Gates, E; Harris, H C; Subba-Rao, M; Anderson, S; Kleinman, S J; Liebert, J; Brewington, H; Brinkmann, J; Harvanek, M; Krzesínski, J; Lamb, D Q; Long, D; Neilsen, E H; Newman, P R; Nitta, A; Snedden, S A; Gates, Evalyn; Gyuk, Geza; Harris, Hugh C.; Subbarao, Mark; Anderson, Scott; Liebert, James; Brewington, Howard; Harvanek, Michael; Krzesinski, Jurek; Lamb, Don Q.; Long, Dan; Neilsen, Eric H.; Newman, Peter R.; Nitta, Atsuko; Snedden, Stephanie A.

    2004-01-01

    We report the discovery of five very cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced absorption (CIA) from molecular hydrogen and are similar in color to the three previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402. The fifth, an ultracool white dwarf candidate, shows milder CIA flux suppression and has a color and spectral shape similar to WD 0346+246. All five new white dwarfs are faint (g > 18.9) and have significant proper motions. One of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.

  13. GW Librae: A unique laboratory for pulsations in an accreting white dwarf

    CERN Document Server

    Toloza, O; Hermes, J J; Townsley, D M; Schreiber, M R; Szkody, P; Pala, A; Beuermann, K; Bildsten, L; Breedt, E; Cook, M; Godon, P; Henden, A A; Hubeny, I; Knigge, C; Long, K S; Marsh, T R; de Martino, D; Mukadam, A S; Myers, G; Nelson, P; Oksanen, A; Patterson, J; Sion, E M; Zorotovic, M

    2016-01-01

    Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of HST ultraviolet spectroscopy taken in 2002, 2010 and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in May~2013, we obtained new HST/COS ultraviolet observations that displayed unexpected behaviour: besides showing variability at ~275s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhi...

  14. Constraining white-dwarf kicks in globular clusters : IV. Retarding Core Collapse

    CERN Document Server

    Heyl, Jeremy S

    2009-01-01

    Observations of white dwarfs in the globular clusters NGC 6397 and Omega Centauri indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. The kicks may be large enough to dramatically change the radial distribution of young white dwarfs, giving them larger energies than other stars in the cluster. As these energetic white dwarfs travel through the cluster they can impart their excess energy on the other stars in the cluster. A Monte-Carlo simualtion of the white-dwarfs kicks combined with estimate of the phase-space diffusion of the white dwarfs reveals that as the white dwarfs equilibrate, they lose most of their energy in the central region of the cluster. They could possibly mimic the effect of binaries, puffing up the cluster and delaying core collapse.

  15. The evolutionary state of short period magnetic white dwarf binaries

    CERN Document Server

    Breedt, E; Girven, J; Drake, A J; Copperwheat, C M; Parsons, S G; Marsh, T R

    2012-01-01

    We present phase-resolved spectroscopy of two new short period low accretion rate magnetic binaries, SDSSJ125044.42+154957.3 (Porb = 86 min) and SDSSJ151415.65+074446.5 (Porb = 89 min). Both systems were previously identified as magnetic white dwarfs from the Zeeman splitting of the Balmer absorption lines in their optical spectra. Their spectral energy distributions exhibit a large near-infrared excess, which we interpret as a combination of cyclotron emission and possibly a late type companion star. No absorption features from the companion are seen in our optical spectra. We derive the orbital periods from a narrow, variable H_alpha emission line which we show to originate on the companion star. The high radial velocity amplitude measured in both systems suggests a high orbital inclination, but we find no evidence for eclipses in our data. The two new systems resemble the polar EF Eri in its prolonged low state and also SDSSJ121209.31+013627.7, a known magnetic white dwarf plus possible brown dwarf binary,...

  16. DEUTERIUM BURNING IN MASSIVE GIANT PLANETS AND LOW-MASS BROWN DWARFS FORMED BY CORE-NUCLEATED ACCRETION

    Energy Technology Data Exchange (ETDEWEB)

    Bodenheimer, Peter [UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); D' Angelo, Gennaro; Lissauer, Jack J. [Space Science and Astrobiology Division, NASA-Ames Research Center, Moffett Field, CA 94035 (United States); Fortney, Jonathan J. [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); Saumon, Didier, E-mail: peter@ucolick.org, E-mail: gennaro.dangelo@nasa.gov, E-mail: Jack.J.Lissauer@nasa.gov, E-mail: jfortney@ucolick.org, E-mail: dsaumon@lanl.gov [Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545 (United States)

    2013-06-20

    Using detailed numerical simulations, we study the formation of bodies near the deuterium-burning limit according to the core-nucleated giant planet accretion scenario. The objects, with heavy-element cores in the range 5-30 M{sub Circled-Plus }, are assumed to accrete gas up to final masses of 10-15 Jupiter masses (M{sub Jup}). After the formation process, which lasts 1-5 Myr and which ends with a ''cold-start'', low-entropy configuration, the bodies evolve at constant mass up to an age of several Gyr. Deuterium burning via proton capture is included in the calculation, and we determined the mass, M{sub 50}, above which more than 50% of the initial deuterium is burned. This often-quoted borderline between giant planets and brown dwarfs is found to depend only slightly on parameters, such as core mass, stellar mass, formation location, solid surface density in the protoplanetary disk, disk viscosity, and dust opacity. The values for M{sub 50} fall in the range 11.6-13.6 M{sub Jup}, in agreement with previous determinations that do not take the formation process into account. For a given opacity law during the formation process, objects with higher core masses form more quickly. The result is higher entropy in the envelope at the completion of accretion, yielding lower values of M{sub 50}. For masses above M{sub 50}, during the deuterium-burning phase, objects expand and increase in luminosity by one to three orders of magnitude. Evolutionary tracks in the luminosity versus time diagram are compared with the observed position of the companion to Beta Pictoris.

  17. The DODO Survey II: A Gemini Direct Imaging Search for Substellar and Planetary Mass Companions around Nearby Equatorial and Northern Hemisphere White Dwarfs

    CERN Document Server

    Hogan, E; Clarke, F J

    2009-01-01

    The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures much lower ( T8.5 and so could belong to the proposed Y dwarf spectral sequence. The detection of a planet around a white dwarf would prove that such objects can survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors (with masses between 1.5 - 8 solar masses, i.e., early B to mid F). This paper presents the results of a multi-epoch J band common proper motion survey of 23 nearby equatorial and northern hemisphere white dwarfs. We rule out the presence of any common proper motion companions, with limiting masses determined from the completeness limit of each observation, to 18 white dwarfs. For the remaining five targets, the motion...

  18. A Model of White Dwarf Pulsar AR Scorpii

    CERN Document Server

    Geng, Jin-Jun; Huang, Yong-Feng

    2016-01-01

    A 3.56-hour white dwarf (WD) - M dwarf (MD) close binary system, AR Scorpii, was recently reported to show pulsating emission in radio, IR, optical, and UV, with a 1.97-minute period, which suggests the existence of a WD with a rotation period of 1.95 minutes. We propose a model to explain the temporal and spectral characteristics of the system. The WD is a nearly perpendicular rotator, with both open field line beams sweeping the MD stellar wind periodically. A bow shock propagating into the stellar wind accelerates electrons in the wind. Synchrotron radiation of these shocked electrons can naturally account for the broad-band (from radio to X-rays) spectral energy distribution of the system.

  19. Hot C-rich white dwarfs: testing the DB-DQ transition through pulsations

    CERN Document Server

    Córsico, A H; Althaus, L G; García-Berro, E

    2009-01-01

    Hot DQ white dwarfs constitute a new class of white dwarf stars, uncovered recently within the framework of the SDSS project. Recently, three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. Here, we present a nonadiabatic pulsation analysis of the recently discovered carbon-rich hot DQ white dwarf stars. One of our main aims is to test the convective-mixing picture for the origin of hot DQs through the pulsational properties. Our study relies on the full evolutionary models of hot DQ white dwarfs recently developed by Althaus et al. (2009), that consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Specifically, we present a stability analysis of white dwarf models from stages before the blue edge of the DBV instability strip until the domain of the hot DQ white dwarfs, including the transition DB --> hot DQ white dwarf. We found that at evolutionary phases in which the models are characteriz...

  20. Cool Companions to White Dwarf Stars from the Two Micron All Sky Survey All Sky Data Release

    CERN Document Server

    Hoard, D W; Sturch, L K; Widhalm, A M; Weiler, K P; Pretorius, M L; Wellhouse, J W; Gibiansky, M; Sturch, Laura K.; Widhalm, Allison M.; Weiler, Kevin P.; Pretorius, Magaretha L.; Wellhouse, Joseph W.; Gibiansky, Maxsim

    2007-01-01

    We present the culmination of our near-infrared survey of the optically spectroscopically identified white dwarf stars from the McCook & Sion catalog, conducted using photometric data from the Two Micron All Sky Survey final All Sky Data Release. The color-selection technique, which identifies candidate binaries containing a white dwarf and a low mass stellar (or sub-stellar) companion via their distinctive locus in the near-infrared color-color diagram, is demonstrated to be simple to apply and to yield candidates with a high rate of subsequent confirmation. We recover 105 confirmed binaries, and identify 28 firm candidates (20 of which are new to this work) and 21 tentative candidates (17 of which are new to this work) from the 2MASS data. Only a small number of candidates from our survey have likely companion spectral types later than M5, none of which is an obvious L type (i.e., potential brown dwarf) companion. Only one previously known WD + brown dwarf binary is detected. This result is discussed in...

  1. Orbital periods and component masses of three double white dwarfs

    Science.gov (United States)

    Rebassa-Mansergas, A.; Parsons, S. G.; García-Berro, E.; Gänsicke, B. T.; Schreiber, M. R.; Rybicka, M.; Koester, D.

    2017-04-01

    The merger of close double white dwarfs (CDWDs) is one of the favourite evolutionary channels for producing Type Ia supernovae (SN Ia). Unfortunately, current theories of the evolution and formation of CDWDs are still poorly constrained and have several serious uncertainties that affect the predicted SN Ia rates. Moreover, current observational constraints on this evolutionary pathway for SN Ia mainly rely on only 17 double-lined and/or eclipsing CDWDs with measured orbital and stellar parameters for both white dwarfs. In this paper, we present the orbital periods and the individual masses of three new double-lined CDWDs, derived using a new method. This method employs mass ratios, the Hα core ratios and spectral model fitting to constrain the masses of the components of the pair. The three CDWDs are WD0028-474 (Porb = 9.350 ± 0.007 h, M1 = 0.60 ± 0.06 M⊙, M2 = 0.45 ± 0.04 M⊙), HE0410-1137 (Porb = 12.208 ± 0.008 h, M1 = 0.51 ± 0.04 M⊙, M2 = 0.39 ± 0.03 M⊙) and SDSSJ031813.25-010711.7 (Porb = 45.908 ± 0.006 h, among the longest period systems, M1 = 0.40 ± 0.05 M⊙, M2 = 0.49 ± 0.05 M⊙). While the three systems studied here will merge in time-scales longer than the Hubble time and are expected to become single massive ( ≳ 0.9 M⊙) white dwarfs rather than exploding as SN Ia, increasing the small sample of CDWDs with determined stellar parameters is crucial for a better overall understanding of their evolution.

  2. Three New Eclipsing White-dwarf - M-dwarf Binaries Discovered in a Search for Transiting Planets Around M-dwarfs

    CERN Document Server

    Law, Nicholas M; Street, Rachel; Fulton, Benjamin J; Hillenbrand, Lynne A; Shporer, Avi; Lister, Tim; Baranec, Christoph; Bloom, Joshua S; Bui, Khanh; Burse, Mahesh P; Cenko, S Bradley; Das, H K; Davis, Jack T C; Dekany, Richard G; Filippenko, Alexei V; Kasliwal, Mansi M; Kulkarni, S R; Nugent, Peter; Ofek, Eran O; Poznanski, Dovi; Quimby, Robert M; Ramaprakash, A N; Riddle, Reed; Silverman, Jeffrey M; Sivanandam, Suresh; Tendulkar, Shriharsh

    2011-01-01

    We present three new eclipsing white-dwarf / M-dwarf binary systems discovered during a search for transiting planets around M-dwarfs. Unlike most known eclipsing systems of this type, the optical and infrared emission is dominated by the M-dwarf components, and the systems have optical colors and discovery light curves consistent with being Jupiter-radius transiting planets around early M-dwarfs. We detail the PTF/M-dwarf transiting planet survey, part of the Palomar Transient Factory (PTF). We present a Graphics Processing Unit (GPU)-based box-least-squares search for transits that runs approximately 8X faster than similar algorithms implemented on general purpose systems. For the discovered systems, we decompose low-resolution spectra of the systems into white-dwarf and M-dwarf components, and use radial velocity measurements and cooling models to estimate masses and radii for the white dwarfs. The systems are compact, with periods between 0.35 and 0.45 days and semimajor axes of approximately 2 solar radi...

  3. The Past and Future of Detached Double White Dwarfs with Helium Donors

    CERN Document Server

    Macias, Phillip J; Ramirez-Ruiz, Enrico

    2015-01-01

    We present a method for modeling the evolution of detached double white dwarf (DWD) binaries hosting helium donors from the end of the common envelope (CE) phase to the onset of Roche Lobe overflow (RLOF). This is achieved by combining detailed stellar evolution calculations of extremely low mass (ELM) helium WDs possessing hydrogen envelopes with the the orbital shrinking of the binary driven by gravitational radiation. We show that the consideration of hydrogen fusion in these systems is crucial, as a significant fraction ($\\approx$50%) of future donors are expected to still be burning when mass transfer commences. We apply our method to two detached eclipsing DWD systems, SDSS J0651+2844 and NLTT-11748, in order to demonstrate the effect that carbon-nitrogen-oxygen (CNO) flashes have on constraining the evolutionary history of such systems. We find that when CNO flashes are absent on the low mass WD ($M_{2}$ < $0.18 M_{\\odot}$), such as in NLTT-11748, we are able to self consistently solve for the donor...

  4. The response of a helium white dwarf to an exploding type Ia supernova

    CERN Document Server

    Papish, Oded; García-Berro, Enrique; Aznar-Siguán, Gabriela

    2014-01-01

    We conduct numerical simulations of the interacting ejecta from an exploding CO white dwarf (WD) with the He~WD donor in the double-detonation scenario for Type Ia supernovae (SNe Ia), and find that the descendant supernova remnant (SNR) is highly asymmetrical, in contradiction with observations. When the donor He~WD has low mass, M_WD =0.2 Msun, it is at a distance of ~0.08 Rsun from the explosion, and helium is not ignited. The low mass He~WD casts an `ejecta shadow' behind it, that has imprint in the SN remnant (SNR) hundreds of years later. The outer parts of the shadowed side are fainter and its boundary with the ambient gas is somewhat flat. These features are not found in known SNRs. More massive He~WD donors, M_WD ~ 0.4 Msun, must be closer to the CO~WD to transfer mass. At a distance a < 0.045 Rsun helium is ignited and the He~WD explodes. This explosion leads to a highly asymmetrical SNR and to ejection of ~0.15 Msun of helium, both of which contradict observations of SNe Ia.

  5. An eccentric binary millisecond pulsar with a helium white dwarf companion in the Galactic Field

    CERN Document Server

    Antoniadis, John; Stovall, Kevin; Freire, Paulo C; Deneva, Julia S; Koester, Detlev; Jenet, Frederick; Martinez, Jose

    2016-01-01

    Low-mass white dwarfs (LMWDs) are believed to be exclusive products of binary evolution, as the Universe is not yet old enough to produce them from single stars. Because of the strong tidal forces operating during the binary interaction phase, the remnant host systems observed today are expected to have negligible eccentricities. Here, we report on the first unambiguous identification of a LMWD in an eccentric (e=0.13) orbit with a millisecond pulsar, which directly contradicts this picture. We use our spectra and radio-timing solution (derived elsewhere) to infer the WD temperature T_eff = 8600 +/- 190 K) and 3D systemic velocity (179.5 km\\s). We also place model-independent constraints on the WD radius (R_WD = 0.024+/- 0.004/0.002 R_sun) and surface gravity (log g = 7.11 +/- 0.08/0.16 dex). The WD and kinematic properties are consistent with the expectations for low-mass X-ray binary evolution and disfavour a three-body formation channel. In the case of the high eccentricity being the result of a spontaneou...

  6. The Discovery of the First White Dwarf: Sirius B

    Science.gov (United States)

    Holberg, J. B.

    2005-08-01

    The major events surrounding the discovery of the white dwarf Sirius B, from its prediction in 1844 by Bessel, to its discovery by Alvan Graham Clark in 1862, to the demonstration of its subluminous nature by Adams in 1915 are relatively well known. However, numerous influential individuals, including C. A. F. Peters, A. Auwers, G. P. Bond., U. J. J. Le Verrier, L. Foucault and W. Struve, all played little known but influential roles in the initial discovery of Sirius B and in the subsequent interpretation of its nature.

  7. White dwarfs in an ungravity-inspired model

    Science.gov (United States)

    Bertolami, Orfeu; Mariji, Hodjat

    2016-05-01

    An ungravity-inspired model is employed to examine the astrophysical parameters of white dwarf stars (WDs) using polytropic and degenerate gas approaches. Based on the observed properties such as mass, radius, and luminosity of selected WDs, namely, Sirius B and ɛ Reticulum, bounds on the characteristic length and scaling dimension of the ungravity (UG) model are estimated. The UG effect on the Chandrasekhar limit for WDs is shown. The UG model is examined in the study of ultramassive WDs, e.g., EUVE J1746-706. The UG-inspired model implies that a new location for some WDs on the Hertzsprung-Russell diagram is found.

  8. White dwarfs in an ungravity-inspired model

    CERN Document Server

    Bertolami, Orfeu

    2016-01-01

    An ungravity-inspired model is employed to examine the astrophysical parameters of white dwarf stars (WDs) using polytropic and degenerate gas approaches. Based on the observed properties such as mass, radius, and luminosity of selected WDs, namely, Sirius B and $\\epsilon$ Reticulum, bounds on the characteristic length and scaling dimension of the ungravity (UG) model are estimated. The UG effect on the Chandrasekhar limit for WDs is shown. The UG model is examined in the study of ultra-massive WDs, e.g., EUVE J1746-706. In the contact of UG inspired model, a new location for some WDs on the Hertzsprung-Russell diagram is found.

  9. Magnetic Fields and the Crystallization of White Dwarfs

    Science.gov (United States)

    Isern, J.; García-Berro, E.; Külebi, B.; Lorén-Aguilar, P.

    2017-03-01

    The evolution of white dwarfs can be described as a cooling process. When the temperature is low enough, the interior experiences a phase transition and crystallizes. Crystallization introduces two new sources of energy, latent heat and chemical sedimentation, and induces the formation of a convective mantle around the solid core. This structure, which is analogous to that of the Earth, could induce the formation of a magnetic field via dynamo mechanism. In this work we discuss the viability of such mechanism, and its use as a diagnostic tool of crystallization.

  10. Gravitational wave radiation from a double white dwarf system inside our galaxy: a potential method for seeking strange dwarfs

    Institute of Scientific and Technical Information of China (English)

    Zhan-Kui Lü; Shi-Wei Wu; Zhi-Cheng Zeng

    2009-01-01

    Like the investigation of double white dwarf (DWD) systems, strange dwarf (SD) - white dwarf (WD) system evolution in Laser Interferometer Space Antenna (LISA)'s absolute amplitude-frequency diagram is investigated. Since there is a strange quark core inside an SD, SDs' radii are significantly smaller than the value predicted by the standard WD model, which may strongly affect the gravitational wave (GW) signal in the mass-transferring phases of binary systems. We study how an SD-WD binary evolves across LISA's absolute amplitude-frequency diagram. In principle, we provide an executable way to detect SDs in the Galaxy's DWD systems by radically new windows offered by GW detectors.

  11. A stellar prominence in the white dwarf/red dwarf binary QS Vir: evidence for a detached system

    CERN Document Server

    Parsons, S G; Gänsicke, B T; Tappert, C

    2010-01-01

    Using high resolution UVES spectra of the eclipsing Post Common Envelope Binary QS Vir we detect material along the line of sight to the white dwarf at orbital phase $\\phi=0.16$. We ascribe this to a stellar prominence originating from the M dwarf secondary star which passes in front of the white dwarf at this phase. This creates sharp absorption features in the hydrogen Balmer series and Ca II H and K lines. The small size of the white dwarf allows us to place tight constraints on the column density of hydrogen in the n=2 level of log_(10)(N_2) = 14.10 +/- 0.03 cm^(-2) and, assuming local thermodynamical equilibrium, the temperature of the prominence material of ~9000K. The prominence material is at least 1.5 stellar radii from the surface of the M dwarf. The location of the prominence is consistent with emission features previously interpreted as evidence for Roche lobe overflow in the system. We also detect Mg II 4481A absorption from the white dwarf. The width of the Mg II line indicates that the white dw...

  12. The dynamical evolution of low-mass hydrogen-burning stars, brown dwarfs and planetary-mass objects formed through disc fragmentation

    CERN Document Server

    Li, Yun; Stamatellos, D; Goodwin, S P

    2015-01-01

    Theory and simulations suggest that it is possible to form low-mass hydrogen-burning stars, brown dwarfs and planetary-mass objects via disc fragmentation. As disc fragmentation results in the formation of several bodies at comparable distances to the host star, their orbits are generally unstable. Here, we study the dynamical evolution of these objects. We set up the initial conditions based on the outcomes of the SPH simulations of Stamatellos & Whitworth, and for comparison we also study the evolution of systems resulting from lower-mass fragmenting discs. We refer to these two sets of simulations as set 1 and set 2. At 10 Myr, approximately half of the host stars have one companion left, and approximately 22% (set 1) to 9.8% (set 2) of the host stars are single. Systems with multiple secondaries in relatively stable configurations are common (about 30% and 44%, respectively). The majority of the companions are ejected within 1 Myr with velocities mostly below 5 km/s, with some runaway escapers with ve...

  13. A Thermal Infrared Imaging Study of Very Low-Mass, Wide Separation Brown Dwarf Companions to Upper Scorpius Stars: Constraining Circumstellar Environments

    CERN Document Server

    Bailey, Vanessa; Currie, Thayne; Su, Kate Y L; Esposito, Simone; Hill, John M; Hoffmann, William F; Jones, Terry; Kim, Jihun; Leisenring, Jarron; Meyer, Michael; Murray-Clay, Ruth; Nelson, Matthew J; Pinna, Enrico; Puglisi, Alfio; Rieke, George; Rodigas, Timothy; Skemer, Andrew; Skrutskie, Michael F; Vaitheeswaran, Vidhya; Wilson, John C

    2013-01-01

    We present a 3-5um LBT/MMT adaptive optics imaging study of three Upper Scorpius stars with brown dwarf (BD) companions with very low-masses/mass ratios (M_BD < 25M_Jup; M_BD / M_star ~ 1-2%), and wide separations (300-700 AU): GSC 06214, 1RXS 1609, and HIP 78530. We combine these new thermal IR data with existing 1-4um and 24um photometry to constrain the properties of the BDs and identify evidence for circumprimary/secondary disks in these unusual systems. We confirm that GSC 06214B is surrounded by a disk, further showing this disk produces a broadband IR excess due to small dust near the dust sublimation radius. An unresolved 24um excess in the system may be explained by the contribution from this disk. 1RXS 1609B exhibits no 3-4um excess, nor does its primary; however, the system as a whole has a modest 24um excess, which may come from warm dust around the primary and/or BD. Neither object in the HIP 78530 system exhibits near- to mid-IR excesses. We additionally find that the 1-4um colors of HIP 7853...

  14. Supernovae from Direct Collisions of White Dwarfs and the Role of Helium Shell Ignition

    Science.gov (United States)

    Papish, Oded; Perets, Hagai Binyamin

    2016-05-01

    Models for supernovae (SNe) arising from thermonuclear explosions of white dwarfs (WDs) have been studied extensively over the past few decades, mostly focusing on the single-degenerate (accretion of material by a WD) and double-degenerate (WD-WD merger) scenarios. In recent years it was suggested that direct WD-WD collisions provide an additional channel for such explosions. Here we extend the studies of such explosions and explore the role of helium shells in affecting the thermonuclear explosions. We study the impact of both low-mass (˜0.01 M ⊙) and high-mass (≥0.1 M ⊙) helium shells. We find that detonation of the massive helium layers precedes the detonation of the WD carbon-oxygen (CO) bulk during the collision and can change the evolution of the explosion and the outcomes for the cases of high-mass He shells. In particular, the He shell detonation propagates on the WD surface and inefficiently burns material prior to the CO detonation that later follows in the central parts of the WD. Such evolution leads to greater production of intermediate elements, producing larger yields of 44Ti and 48Cr relative to the pure CO-CO WD collisions. Collisions of WDs with a low-mass He shell do not give rise to helium detonation, but helium burning does precede the CO bulk detonation. Such collisions eject at high velocity a low mass of burned material enriched with intermediate elements and produce smaller changes in the overall explosion outcomes. The various effects arising from the contribution of low-/high-mass He layers change the kinematics and the morphological structure of collision-induced SNe and may thereby provide unique observational signatures for such SNe and play a role in the chemical enrichment of galaxies and the production of intermediate elements and positrons from their longer-term decay.

  15. SGRs and AXPs: Massive Rotating White Dwarfs versus Magnetars

    CERN Document Server

    Malheiro, Manuel; Ruffini, Remo

    2011-01-01

    The recent observations of SGR 0418+5729 offer an authentic Rosetta Stone for deciphering the energy source of Soft Gamma Ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). The "magnetar" model, appeals to a yet untested new energy source in astrophysical systems: bulk magnetic energy. It leads for SGR 0418+5729 to results in contradiction with observations. It is shown how a consistent model for SGRs and AXPs can be expressed in terms of canonical physics and astrophysics within massive, fast rotating, and highly magnetized white dwarfs. The pioneering work of B. Paczynski (1990) on 1E 2259+586 is extended and further developed to describe the observed properties of all known SGRS and AXPs by assuming spin-down powered massive, fast rotating, and highly magnetized white dwarfs. Within this model, we obtain the theoretical prediction for the lower limit of the first time derivative of the rotational period of SGR 0418+5729, $\\dot{P} \\geq L_X P^3/(4\\pi^2 I)=1.18\\times 10^{-16}$ s/s being $L_X$ and $P$ th...

  16. Orbital periods and component masses of three double white dwarfs

    CERN Document Server

    Rebassa-Mansergas, A; Garcia-Berro, E; Gaensicke, B T; Schreiber, M R; Rybicka, M; Koester, D

    2016-01-01

    The merger of close double white dwarfs (CDWDs) is one of the favourite evolutionary channels for producing Type Ia supernovae (SN Ia). Unfortunately, current theories of the evolution and formation of CDWDs are still poorly constrained and have several serious uncertainties, which affect the predicted SN Ia rates. Moreover, current observational constraints on this evolutionary pathway for SN Ia mainly rely on only 18 double-lined and/or eclipsing CDWDs with measured orbital and stellar parameters for both white dwarfs. In this paper we present the orbital periods and the individual masses of three new double-lined CDWDs, derived using a new method. This method employs mass ratios, the Halpha core ratios and spectral model-fitting to constrain the masses of the components of the pair. The three CDWDs are WD0028-474 (Porb=9.350 +- 0.007 hours, M1=0.60 +- 0.06 Msun, M2=0.45 +- 0.04 Msun), HE0410-1137 (Porb = 12.208 +- 0.008 hours, M1= 0.51 +- 0.04 Msun, M2= 0.39 +- 0.03 Msun) and SDSSJ031813.25-010711.7 (Porb ...

  17. Near-UV absorption in very cool DA white dwarfs

    CERN Document Server

    Saumon, D; Kowalski, P M

    2014-01-01

    The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are challenging to models because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman alpha line of atomic H by collisions primarily with H2. For the purpose of validating this model more rigorously, we acquired Hubble Space Telescope STIS spectra of 8 very cool white dwarfs (5 DA and 3 DC stars). Combined with their known parallaxes, BVRIJHK and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (from 0.24 to 9.3 micron) with a large grid of model atmospheres and synthetic spectra. We f...

  18. Radio transients from accretion-induced collapse of white dwarfs

    CERN Document Server

    Moriya, Takashi J

    2016-01-01

    We investigate observational properties of accretion-induced collapse (AIC) of white dwarfs in radio frequencies. If AIC is triggered by accretion from a companion star, a dense circumstellar medium can be formed around the progenitor system. Then, the ejecta from AIC collide to the dense circumstellar medium, making a strong shock. The strong shock can produce synchrotron emission which can be observed in radio frequencies. Even if AIC occurs as a result of white dwarf mergers, we argue that AIC may cause fast radio bursts if a certain condition is satisfied. If AIC forms neutron stars which are so massive that rotation is required to support themselves (i.e., supramassive neutron stars), the supramassive neutron stars may immediately lose their rotational energy by the r-mode instability and collapse to black holes. If the collapsing supramassive neutron stars are strongly magnetized, they may emit fast radio bursts as previously suggested. The AIC radio transients from the single-degenerate systems may be ...

  19. On the formation of hot DQ white dwarfs

    CERN Document Server

    Althaus, L G; Córsico, A H; Bertolami, M M Miller; Romero, A D

    2009-01-01

    We present the first full evolutionary calculations aimed at exploring the origin of hot DQ white dwarfs. These calculations consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Our calculations provide strong support to the diffusive/convective-mixing picture for the formation of hot DQs. We find that the hot DQ stage is a short-lived stage and that the range of effective temperatures where hot DQ stars are found can be accounted for by different masses of residual helium and/or different initial stellar masses. In the frame of this scenario, a correlation between the effective temperature and the surface carbon abundance in DQs should be expected, with the largest carbon abundances expected in the hottest DQs. From our calculations, we suggest that most of the hot DQs could be the cooler descendants of some PG1159 stars characterized by He-rich envelopes markedly smaller than those predicted by the standard theory of stellar evolution. At least for one hot DQ, t...

  20. Planetary engulfment as a trigger for white dwarf pollution

    CERN Document Server

    Petrovich, Cristobal

    2016-01-01

    The presence of a planetary system can shield a planetesimal disk from the secular gravitational perturbations due to distant outer massive objects (planets or stellar companions). As the host star evolves off the main sequence to become a white dwarf, these planets can be engulfed, triggering secular instabilities and leading to the tidal disruptions of small rocky bodies. These disrupted bodies can feed the white dwarfs with rocky material and possibly explain the high-metallicity material in their atmospheres. We illustrate how this mechanism can operate when the gravitational perturbations are due to the Kozai-Lidov mechanism from a stellar binary companion. We show that this mechanism can explain the observed levels of accretion if: (1) the planetary engulfment happens fast compared to the secular timescale, which is generally the case for wide binaries ($>100$ AU) and planetary engulfment during the Asymptotic Giant Branch; (2) the planetesimal disk has a total mass of $\\sim10^{-4}-10^{-2}M_\\oplus$. We ...

  1. Explosion of white dwarfs harboring hybrid CONe cores

    CERN Document Server

    Bravo, E; Gutiérrez, J L; Doherty, C L

    2016-01-01

    Recently, it has been found that off-centre carbon burning in a subset of intermediate-mass stars does not propagate all the way to the center, resulting in a class of hybrid CONe cores. Here, we consider the possibility that stars hosting these hybrid CONe cores might belong to a close binary system and, eventually, become white dwarfs accreting from a non-degenerate companion at rates leading to a supernova explosion. We have computed the hydrodynamical phase of the explosion of Chandrasekhar-mass white dwarfs harboring hybrid cores, assuming that the explosion starts at the center, either as a detonation (as may be expected in some degenerate merging scenarios) or as a deflagration (that afterwards transitions into a delayed detonation). We assume these hybrid cores are made of a central CO volume, of mass M(CO), surrounded by an ONe shell. We show that, in case of a pure detonation, a medium-sized CO-rich region, M(CO)<0.4 Msun, results in the ejection of a small fraction of the mantle while leaving a ...

  2. DA WHITE DWARFS OBSERVED IN THE LAMOST PILOT SURVEY

    Energy Technology Data Exchange (ETDEWEB)

    Zhang Yueyang; Deng Licai; Liu Chao; Carrell, Kenneth; Yang Fan; Gao Shuang; Xu Yan; Li Jing; Zhang Haotong; Zhao Yongheng; Luo Ali; Bai Zhongrui; Yuan Hailong [Key Lab for Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China); Lepine, Sebastien [Department of Astrophysics, Division of Physical Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY (United States); Newberg, Heidi Jo; Carlin, Jeffrey L. [Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180 (United States); Jin Ge [University of Science and Technology of China, Hefei 230026 (China)

    2013-08-01

    A total of {approx}640, 000 objects from the LAMOST pilot survey have been publicly released. In this work, we present a catalog of DA white dwarfs (DAWDs) from the entire pilot survey. We outline a new algorithm for the selection of white dwarfs (WDs) by fitting Sersic profiles to the Balmer H{beta}, H{gamma}, and H{delta} lines of the spectra, and calculating the equivalent width of the Ca II K line. Two thousand nine hundred sixty-four candidates are selected by constraining the fitting parameters and the equivalent width of the Ca II K line. All the spectra of candidates are visually inspected. We identify 230 DAWDs (59 of which are already included in the Villanova and SDSS WD catalogs), 20 of which are DAWDs with non-degenerate companions. In addition, 128 candidates are classified as DAWDs/subdwarfs, which means the classifications are ambiguous. The result is consistent with the expected DAWD number estimated based on the LEGUE target selection algorithm.

  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. CARMENES science preparation: characterisation of M dwarfs with low-resolution spectroscopy and search for low-mass wide companions to young stars

    Science.gov (United States)

    Alonso-Floriano, F. J.

    2015-11-01

    This thesis is focused on the study of low-mass objects that can be targets of exoplanet searches with near-infrared spectrographs in general and CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs; see Quirrenbach et al. 2014) in particular. The CARMENES consortium comprises 11 institutions from Germany and Spain that are building a high-resolution spectrograph (R=82,000) with two channels, visible (0.55 - 1.05 um) and infrared (0.95 - 1.7 um), for the 3.5 m Calar Alto telescope. It will observe a sample of 300 M dwarfs in 600 nights of guaranteed time during at least three years, starting in January 2016. The final sample will be chosen from the 2200 M dwarfs included in the CARMENCITA input catalogue. For these stars, we have obtained and collected a large amount of data: spectral types, radial and rotational velocities, photometry in several bands, etc. Part of the e effort of the science preparation necessary for the final selection of targets for CARMENES and other near-infrared spectrographs has been collected in two publications, which are presented in this PhD thesis. In the first publication (Alonso-Floriano et al., 2015A&A...577A.128A), we obtained low-resolution spectra for 753 stars using the CAFOS spectrograph at the 2.2 m Calar Alto telescope. The main goal was to derive accurate spectral types, which are fundamental parameters for the sample selection. We used a grid of 49 standard stars, from spectral types K3V to M8V, together with a double least-square minimisation technique and 31 spectral indices previously defined by other authors. In addition, we quantified the surface gravity, metallicity and chromospheric activity of the sample, in order to detect low-gravity stars (giants and very young), metal-poor and very metal-poor stars (subdwarfs), and very active stars. In the second publication (Alonso-Floriano et al., 2015A&A...583A..85A), we searched for common proper

  5. Ultraviolet carbon lines in the spectrum of the white dwarf BPM 11668

    Science.gov (United States)

    Wegner, G.

    1983-01-01

    The southern hemisphere DC white dwarf BPM 11668 has been found to show strong ultraviolet lines of neutral carbon using observations from the IUE satellite. This star seems typical of the growing number of DC white dwarfs found to be of this type and appears to have a carbon abundance near C:He = 0.0001, with an effective temperature of 8500 K.

  6. New population synthesis model: Preliminary results for close double white dwarf populations

    CERN Document Server

    Toonen, Silvia; Zwart, Simon Portegies

    2011-01-01

    An update is presented to the software package SeBa for simulating single star and binary evolution in which new stellar evolution tracks have been implemented. SeBa is applied to study the population of close double white dwarf and the delay time distribution of double white dwarf mergers that may lead to Supernovae Type Ia.

  7. An Ultramassive 1.28 M$_\\odot$ White Dwarf in NGC 2099

    CERN Document Server

    Cummings, Jeffrey D; Tremblay, P -E; Ramirez-Ruiz, Enrico; Bergeron, P

    2016-01-01

    With the Keck I Low-Resolution Imaging Spectrometer we have observed nine white dwarf candidates in the very rich open cluster NGC 2099 (M37). The spectroscopy shows seven to be DA white dwarfs, one to be a DB white dwarf, and one to be a DZ white dwarf. Three of these DA white dwarfs are consistent with singly evolved cluster membership: an ultramassive (1.28$^{+0.05}_{-0.08}$ M$_\\odot$) and two intermediate-mass (0.70 and 0.75 M$_\\odot$) white dwarfs. Analysis of their cooling ages allows us to calculate their progenitor masses and establish new constraints on the initial-final mass relation. The intermediate-mass white dwarfs are in strong agreement with previous work over this mass regime. The ultramassive white dwarf has $V$ = 24.5, $\\sim$2 mag fainter than the other two remnants. The spectrum of this star has lower quality, so the derived stellar properties (e.g., T$_{\\rm eff}$, log g) have uncertainties that are several times higher than the brighter counterparts. We measure these uncertainties and est...

  8. GW Librae: a unique laboratory for pulsations in an accreting white dwarf

    Science.gov (United States)

    Toloza, O.; Gänsicke, B. T.; Hermes, J. J.; Townsley, D. M.; Schreiber, M. R.; Szkody, P.; Pala, A.; Beuermann, K.; Bildsten, L.; Breedt, E.; Cook, M.; Godon, P.; Henden, A. A.; Hubeny, I.; Knigge, C.; Long, K. S.; Marsh, T. R.; de Martino, D.; Mukadam, A. S.; Myers, G.; Nelson, P.; Oksanen, A.; Patterson, J.; Sion, E. M.; Zorotovic, M.

    2016-07-01

    Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of Hubble Space Telescope (HST) ultraviolet spectroscopy taken in 2002, 2010, and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in 2013 May, we obtained new HST/Cosmic Origin Spectrograph ultraviolet observations that displayed unexpected behaviour: besides showing variability at ≃275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits high-amplitude variability on an ≃4.4 h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to ≃30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.

  9. Strangelet dwarfs

    CERN Document Server

    Alford, Mark G; Reddy, Sanjay

    2011-01-01

    If the surface tension of quark matter is low enough, quark matter is not self bound. At sufficiently low pressure and temperature, it will take the form of a crystal of positively charged strangelets in a neutralizing background of electrons. In this case there will exist, in addition to the usual family of strange stars, a family of low-mass large-radius objects analogous to white dwarfs, which we call "strangelet dwarfs". Using a generic parametrization of the equation of state of quark matter, we calculate the mass-radius relationship of these objects.

  10. The evolution of white dwarfs resulting from helium-enhanced, low-metallicity progenitor stars

    CERN Document Server

    Althaus, Leandro G; Córsico, Alejandro H; Torres, Santiago; García--Berro, Enrique

    2016-01-01

    Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for $\\omega$ Centauri, which shows clear evidence of a helium- enriched sub-population characterized by a helium abundance as high as $Y= 0.4$. We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of helium-rich progenitors. Different issues of the white dwarf evolution and their helium-rich progenitors have been explored. In particular, the final mass of the remnants, the role of overshooting during the thermally-pulsing phase, and the cooling of the resulting white dwarfs differ markedly from the evolutionary predictions of progenitor stars with standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored. We find that, for the range of initial masses explored in this paper, the ...

  11. Axisymmetric Magnetic Fields, Electron Capture and Pycnonuclear Reactions in Magnetized White Dwarfs

    CERN Document Server

    Otoniel, Edson; Malheiro, Manuel; Schramm, Stefan; Weber, Fridolin

    2016-01-01

    In this work, we study the properties of magnetized white dwarfs taking into account possible instabilities due to electron capture and pycnonuclear fusion reactions in the cores of such objects. The structure of white dwarfs is obtained by solving the Einstein-Maxwell equations with a poloidal magnetic field in a fully general relativistic approach. The stellar interior is composed of a regular crystal lattice made of carbon ions immersed in a degenerate relativistic electron gas. The onsets of electron capture reactions and pycnonuclear reactions are determined with and without magnetic fields. We find that magnetized white dwarfs violate the standard Chandrasekhar mass limit significantly, even when electron capture and pycnonuclear instabilities are present in the stellar interior. We obtain a maximum white dwarf mass of around $2.12\\,M_{\\odot}$ with a central magnetic field of $\\sim 1.74\\times 10^{14}$G, which indicates that magnetized white dwarfs may be the progenitor candidates of superluminous type I...

  12. A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

    CERN Document Server

    García-Berro, Enrique; Althaus, Leandro G; Renedo, Isabel; Lorén-Aguilar, Pablo; Córsico, Alejandro H; Rohrmann, René D; Salaris, Maurizio; Isern, Jordi

    2010-01-01

    NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, solving the age discrepancy for NGC 6791.

  13. The WIRED Survey. IV. New Dust Disks from the McCook & Sion White Dwarf Catalog

    CERN Document Server

    Hoard, D W; Wachter, Stefanie; Leisawitz, David T; Cohen, Martin

    2013-01-01

    We have compiled photometric data from the Wide-field Infrared Survey Explorer All Sky Survey and other archival sources for the more than 2200 objects in the original McCook & Sion Catalog of Spectroscopically Identified White Dwarfs. We applied color-selection criteria to identify 28 targets whose infrared spectral energy distributions depart from the expectation for the white dwarf photosphere alone. Seven of these are previously known white dwarfs with circumstellar dust disks, five are known central stars of planetary nebulae, and six were excluded for being known binaries or having possible contamination of their infrared photometry. We fit white dwarf models to the spectral energy distributions of the remaining ten targets, and find seven new candidates with infrared excess suggesting the presence of a circumstellar dust disk. We compare the model dust disk properties for these new candidates with a comprehensive compilation of previously published parameters for known white dwarfs with dust disks....

  14. New Pulsating DB White Dwarf Stars from the Sloan Digital Sky Survey

    CERN Document Server

    Nitta, A; Krzesínski, J; Kepler, S O; Metcalfe, T S; Mukadam, Anjum S; Mullally, Fergal; Nather, R E; Sullivan, Denis J; Thompson, Susan E; Winget, D E

    2008-01-01

    We are searching for new He atmosphere white dwarf pulsators (DBVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better known cousins, the H atmosphere white dwarf pulsators (DAVs or ZZ Ceti stars). Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of DBVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest DBVs are thought to have neutrino luminosities exceeding their photon luminosities (Winget et al. 2004), a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. So far we have discovered nine new DBVs, doubling the number of previously known DBVs. Here we report the new pulsators' lightcurves and power spectra.

  15. The EBLM project. I. Physical and orbital parameters, including spin-orbit angles, of two low-mass eclipsing binaries on opposite sides of the brown dwarf limit

    Science.gov (United States)

    Triaud, A. H. M. J.; Hebb, L.; Anderson, D. R.; Cargile, P.; Collier Cameron, A.; Doyle, A. P.; Faedi, F.; Gillon, M.; Gomez Maqueo Chew, Y.; Hellier, C.; Jehin, E.; Maxted, P.; Naef, D.; Pepe, F.; Pollacco, D.; Queloz, D.; Ségransan, D.; Smalley, B.; Stassun, K.; Udry, S.; West, R. G.

    2013-01-01

    This paper introduces a series of papers aiming to study the dozens of low-mass eclipsing binaries (EBLM), with F, G, K primaries, that have been discovered in the course of the WASP survey. Our objects are mostly single-line binaries whose eclipses have been detected by WASP and were initially followed up as potential planetary transit candidates. These have bright primaries, which facilitates spectroscopic observations during transit and allows the study of the spin-orbit distribution of F, G, K+M eclipsing binaries through the Rossiter-McLaughlin effect. Here we report on the spin-orbit angle of WASP-30b, a transiting brown dwarf, and improve its orbital parameters. We also present the mass, radius, spin-orbit angle and orbital parameters of a new eclipsing binary, J1219-39b (1SWAPJ121921.03-395125.6, TYC 7760-484-1), which, with a mass of 95 ± 2 Mjup, is close to the limit between brown dwarfs and stars. We find that both objects have projected spin-orbit angles aligned with their primaries' rotation. Neither primaries are synchronous. J1219-39b has a modestly eccentric orbit and is in agreement with the theoretical mass-radius relationship, whereas WASP-30b lies above it. Using WASP-South photometric observations (Sutherland, South Africa) confirmed with radial velocity measurement from the CORALIE spectrograph, photometry from the EulerCam camera (both mounted on the Swiss 1.2 m Euler Telescope), radial velocities from the HARPS spectrograph on the ESO's 3.6 m Telescope (prog ID 085.C-0393), and photometry from the robotic 60 cm TRAPPIST telescope, all located at ESO, La Silla, Chile. The data is publicly available at the CDS Strasbourg and on demand to the main author.Tables A.1-A.3 are available in electronic form at http://www.aanda.orgPhotometry tables are 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/549/A18

  16. Testing Model Atmospheres for Young Very-low-mass Stars and Brown Dwarfs in the Infrared: Evidence for Significantly Underestimated Dust Opacities

    Science.gov (United States)

    Tottle, Jonathan; Mohanty, Subhanjoy

    2015-05-01

    We test state-of-the-art model atmospheres for young very-low-mass stars and brown dwarfs in the infrared, by comparing the predicted synthetic photometry over 1.2-24 μm to the observed photometry of M-type spectral templates in star-forming regions. We find that (1) in both early and late young M types, the model atmospheres imply effective temperatures ({{T}eff}) several hundred Kelvin lower than predicted by the standard pre-main sequence (PMS) spectral type-{{T}eff} conversion scale (based on theoretical evolutionary models). It is only in the mid-M types that the two temperature estimates agree. (2) The {{T}eff} discrepancy in the early M types (corresponding to stellar masses ≳ 0.4 {{M}⊙ } at ages of a few Myr) probably arises from remaining uncertainties in the treatment of atmospheric convection within the atmospheric models, whereas in the late M types it is likely due to an underestimation of dust opacity. (3) The empirical and model-atmosphere J-band bolometric corrections are both roughly flat, and similar to each other, over the M-type {{T}eff} range. Thus the model atmospheres yield reasonably accurate bolometric luminosities ({{L}bol}), but lead to underestimations of mass and age relative to evolutionary expectations (especially in the late M types) due to lower {{T}eff}. We demonstrate this for a large sample of young Cha I and Taurus sources. (4) The trends in the atmospheric model J-Ks colors, and their deviations from the data, are similar at PMS and main sequence ages, suggesting that the model dust opacity errors we postulate here for young ages also apply at field ages.

  17. Type Ia Supernovae from Merging White Dwarfs I. Prompt Detonations

    CERN Document Server

    Moll, Rainer; Kasen, Daniel; Woosley, Stan

    2013-01-01

    Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe Ia). While it is not entirely clear if and when an explosion is triggered in such systems, numerical models suggest that a detonation might be initiated before the stars have coalesced to form a single compact object. Here we study such "peri-merger" detonations by means of numerical simulations, modeling the disruption and nucleosynthesis of the stars until the ejecta reach the coasting phase. Synthetic light curves and spectra are generated for comparison with observations. Three models are considered with primary masses 0.96 Msun, 1.06 Msun, and 1.20 Msun. Of these, the 0.96 Msun dwarf merging with an 0.81 Msun companion, with a Ni56 yield of 0.58 Msun, is the most promising candidate for reproducing common SNe Ia. The more massive mergers produce unusually luminous SNe Ia with peak luminosities approaching those attributed to "super-Chandrasekhar" mass SNe Ia. While the synthetic light curves and spectra of some of the models resemb...

  18. White Dwarfs in Globular Clusters HST Observations of M4

    CERN Document Server

    Richer, H B; Ibata, R A; Pryor, C E; Bell, R A; Bolte, M; Bond, H E; Harris, W E; Hesser, J E; Holland, S; Ivanans, N; Mandushev, G I; Stetson, P B; Wood, M A; Richer, Harvey B.; Fahlman, Gregory G.; Ibata, Rodrigo A.; Pryor, Carlton; Bell, Roger A.; Bolte, Michael; Bond, Howard E.; Harris, William E.; Hesser, James E.; Holland, Steve; Ivanans, Nicholas; Mandushev, Georgi; Stetson, Peter B.; Wood, Matt A.

    1997-01-01

    Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed and sizeable WD populations were found in all three. The location of these WDs in the color-magnitude diagram, their mean mass of 0.51($ \\pm 0.03$)M$_{\\odot}$, and their luminosity function confirm basic tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are about 9 Gyr old, a level which is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turnoff ages of globular clusters a...

  19. Accreting Double White Dwarf Binaries: Implications for LISA

    Science.gov (United States)

    Kremer, Kyle; Breivik, Katelyn; Larson, Shane L.; Kalogera, Vassiliki

    2017-09-01

    We explore the long-term evolution of mass-transferring white dwarf (WD) binaries undergoing both direct-impact and disk accretion and explore implications of such systems to gravitational-wave (GW) astronomy. We cover a broad range of initial component masses and show that these systems, the majority of which lie within the Laser Interferometer Space Antenna (LISA) sensitivity range, exhibit prominent negative orbital frequency evolution (chirp) for a significant fraction of their lifetimes. Using a galactic population synthesis, we predict ∼2700 of these systems will be observable with a negative chirp of 0.1 yr‑2 by a space-based GW detector like LISA. We also show that detections of mass-transferring double WD systems by LISA may provide astronomers with unique ways of probing the physics governing close compact object binaries.

  20. Crystallization of Carbon Oxygen Mixtures in White Dwarf Stars

    CERN Document Server

    Horowitz, C J; Berry, D K

    2010-01-01

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

  1. The First Six Outbursting Cool DA White Dwarf Pulsators

    CERN Document Server

    Bell, Keaton J; Montgomery, M H; Winget, D E; Fusillo, N P Gentile; Raddi, R; Gänsicke, B T

    2016-01-01

    Extensive observations from the Kepler spacecraft have recently revealed a new outburst phenomenon operating in cool pulsating DA (hydrogen atmosphere) white dwarfs (DAVs). With the introduction of two new outbursting DAVs from K2 Fields 7 (EPIC 229228364) and 8 (EPIC 220453225) in these proceedings, we presently know of six total members of this class of object. We present the observational commonalities of the outbursting DAVs: (1) outbursts that increase the mean stellar flux by up to 15%, last many hours, and recur irregularly on timescales of days; (2) effective temperatures that locate them near the cool edge of the DAV instability strip; and (3) rich pulsation spectra with modes that are observed to wander in amplitude/frequency.

  2. Detectability of substellar companions around white dwarfs with Gaia

    CERN Document Server

    Silvotti, Roberto; Lattanzi, Mario; Morbidelli, Roberto

    2014-01-01

    To date not a single-bona fide planet has been identified orbiting a single white dwarf. In fact we are ignorant about the final configuration of >95% of planetary systems. Theoretical models predict a gap in the final distribution of orbital periods, due to the opposite effects of stellar mass loss (planets pushed outwards) and tidal interactions (planets pushed inwards) during the RGB and the AGB stellar expansions. Over its five year primary mission, Gaia is expected to astrometrically detect the first (few tens of) WD massive planets/BDs giving first evidence that WD planets exist, at least those in wide orbits. In this article we present preliminary results of our simulations of what Gaia should be able to find in this field.

  3. Infrared Opacities in Dense Atmospheres of Cool White Dwarf Stars

    CERN Document Server

    Kowalski, Piotr M; Dufour, Patrick

    2016-01-01

    Dense, He-rich atmospheres of cool white dwarfs represent a challenge to the modeling. This is because these atmospheres are constituted of a dense fluid in which strong multi-atomic interactions determine their physics and chemistry. Therefore, the ideal-gas-based description of absorption is no longer adequate, which makes the opacities of these atmospheres difficult to model. This is illustrated with severe problems in fitting the spectra of cool, He-rich stars. Good description of the infrared (IR) opacity is essential for proper assignment of the atmospheric parameters of these stars. Using methods of computational quantum chemistry we simulate the IR absorption of dense He/H media. We found a significant IR absorption from He atoms (He-He-He CIA opacity) and a strong pressure distortion of the H$_2$-He collision-induced absorption (CIA). We discuss the implication of these results for interpretation of the spectra of cool stars.

  4. On the possible observational signatures of white dwarf dynamical interactions

    CERN Document Server

    Aznar-Siguán, G; Magnien, M; Lorén-Aguilar, P

    2014-01-01

    We compute the possible observational signatures of white dwarf dynamical interactions in dense stellar environments. Specifically, we compute the emission of gravitational waves, and we compare it with the sensitivity curves of planned space-borne gravitational wave detectors. We also compute the light curves for those interactions in which a detonation occurs, and one of the stars is destroyed, as well as the corresponding neutrino luminosities. We find that for the three possible outcomes of these interactions - which are the formation of an eccentric binary system, a lateral collision in which several mass transfer episodes occur, and a direct one in which just a single mass transfer episode takes place - only those in which an eccentric binary are formed are likely to be detected by the planned gravitational wave mission eLISA, while more sensitive detectors would be able to detect the signals emitted in lateral collisions. On the other hand, the light curves (and the thermal neutrino emission) of these ...

  5. Dynamical Tides in Compact White Dwarf Binaries: Influence of Rotation

    CERN Document Server

    Fuller, Jim

    2014-01-01

    Tidal interactions play an important role in the evolution and ultimate fate of compact white dwarf (WD) binaries. Not only do tides affect the pre-merger state (such as temperature and rotation rate) of the WDs, but they may also determine which systems merge and which undergo stable mass transfer. In this paper, we attempt to quantify the effects of rotation on tidal angular momentum transport in binary stars, with specific calculations applied to WD stellar models. We incorporate the effect of rotation using the traditional approximation, in which the dynamically excited gravity waves within the WDs are transformed into gravito-inertial Hough waves. The Coriolis force has only a minor effect on prograde gravity waves, and previous results predicting the tidal spin-up and heating of inspiraling WDs are not significantly modified. However, rotation strongly alters retrograde gravity waves and inertial waves, with important consequences for the tidal spin-down of accreting WDs. We identify new dynamical tidal...

  6. Infrared Opacities in Dense Atmospheres of Cool White Dwarf Stars

    Science.gov (United States)

    Kowalski, P. M.; Blouin, S.; Dufour, P.

    2017-03-01

    Dense, He-rich atmospheres of cool white dwarfs represent a challenge to the modeling. This is because these atmospheres are constituted of a dense fluid in which strong multi-atomic interactions determine their physics and chemistry. Therefore, the ideal-gas-based description of absorption is no longer adequate, which makes the opacities of these atmospheres difficult to model. This is illustrated with severe problems in fitting the spectra of cool, He-rich stars. Good description of the infrared (IR) opacity is essential for proper assignment of the atmospheric parameters of these stars. Using methods of computational quantum chemistry we simulate the IR absorption of dense He/H media. We found a significant IR absorption from He atoms (He-He-He CIA opacity) and a strong pressure distortion of the H2-He collision-induced absorption (CIA). We discuss the implication of these results for the interpretation of the spectra of cool stars.

  7. Eccentric double white dwarfs as LISA sources in globular clusters

    CERN Document Server

    Willems, B; Vecchio, A; Ivanova, N; Rasio, F A; Fregeau, J M; Belczynski, K

    2007-01-01

    We consider the formation of double white dwarfs (DWDs) through dynamical interactions in globular clusters. Such interactions can readily give rise to eccentric DWDs, in contrast to the exclusively circular population that is expected to form in the Galactic disk. We show that for a 5-year Laser Interferometer Space Antenna (LISA) mission and distances as far as the Large Magellanic Cloud, multiple harmonics from eccentric DWDs can be detected at a signal-to-noise ratio higher than 8 for at least a handful of eccentric DWDs, given their formation rate and typical merger lifetimes estimated from current cluster simulations. Consequently the association of eccentricity with stellar-mass LISA sources does not uniquely involve neutron stars, as is usually assumed. Due to the difficulty of detecting these systems with present and planned electromagnetic observatories, LISA could provide unique dynamical identifications of eccentric DWDs in globular clusters.

  8. Remarkable spectral variability on the spin period of the accreting white dwarf in V455 And

    CERN Document Server

    Bloemen, S; De Smedt, K; Vos, J; Gänsicke, B T; Marsh, T R; Rodriguez-Gil, P

    2012-01-01

    We present spin-resolved spectroscopy of the accreting white dwarf binary V455 And. With a suggested spin period of only 67s, it has one of the fastest spinning white dwarfs known. To study the spectral variability on the spin period of the white dwarf, we observed V455 And with 2s integration times, which is significantly shorter than the spin rate of the white dwarf. To achieve this cadence, we used the blue arm of the ISIS spectrograph at the 4.2-m William Herschel Telescope, equipped with an electron multiplying CCD (EMCCD). Strong coherent signals were detected in our time series, which lead to a robust determination of the spin period of the white dwarf (Pspin=67.619 +/- 0.002 s). Folding the spectra on the white dwarf spin period uncovered very complex emission line variations in Hgamma, He I 4472 and He II 4686. We attribute the observed spin phase dependence of the emission line shape to the presence of magnetically controlled accretion onto the white dwarf via accretion curtains, consistent with an ...

  9. New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution

    CERN Document Server

    Althaus, Leandro G; Isern, Jordi; Córsico, Alejandro H; Bertolami, Marcelo M Miller

    2011-01-01

    Cool white dwarfs are reliable and independent stellar chronometers. The most common white dwarfs have carbon-oxygen dense cores. Consequently, the cooling ages of very cool white dwarfs sensitively depend on the adopted phase diagram of the carbon-oxygen binary mixture. A new phase diagram of dense carbon-oxygen mixtures appropriate for white dwarf interiors has been recently obtained using direct molecular dynamics simulations. In this paper, we explore the consequences of this phase diagram in the evolution of cool white dwarfs. To do this we employ a detailed stellar evolutionary code and accurate initial white dwarf configurations, derived from the full evolution of progenitor stars. We use two different phase diagrams, that of Horowitz et al. (2010), which presents an azeotrope, and the phase diagram of Segretain & Chabrier (1993), which is of the spindle form. We computed the evolution of 0.593 and 0.878M_sun white dwarf models during the crystallization phase, and we found that the energy released...

  10. HR2875 Spectroscopic discovery of the first B star + white dwarf binary

    CERN Document Server

    Burleigh, M R; Burleigh, Matt; Barstow, Martin

    1998-01-01

    We report the discovery, in an Extreme Ultraviolet Explorer (EUVE) short wavelength spectrum, of an unresolved hot white dwarf companion to the 5th-magnitude B5Vp star HR2875. This is the first time that a non-interacting white dwarf$+$ B star binary has been discovered; previously, the the earliest type star known with a white dwarf companion was Sirius (A1V). Since the white dwarf must have evolved from a main sequence progenitor with a mass greater than that of a B5V star ($\\geq$6.0M$_\\odot$), this places a lower limit on the maximum mass for white dwarf progenitors, with important implications for our knowledge of the initial-final mass relation. Assuming a pure-hydrogen atmospheric composition, we constrain the temperature of the white dwarf to be between 39,000K and 49,000K. We also argue that this degenerate star is likely to have mass significantly greater than the mean mass for white dwarf stars ($\\approx$0.55M$_\\odot$). Finally, we suggest that other bright B stars (e.g.\\ Field Camera and EUVE may a...

  11. A search for white dwarfs in the Galactic plane: the field and the open cluster population

    CERN Document Server

    Raddi, R; Gaensicke, B T; Hermes, J J; Napiwotzki, R; Koester, D; Tremblay, P -E; Barentsen, G; Farnhill, J H; Mohr-Smith, M; Drew, J E; Groot, P J; Guzman-Ramirez, L; Parker, Q A; Steeghs, D; Zijlstra, A

    2016-01-01

    We investigated the prospects for systematic searches of white dwarfs at low Galactic latitudes, using the VLT Survey Telescope (VST) H$\\alpha$ Photometric Survey of the Galactic plane and Bulge (VPHAS+). We targeted 17 white dwarf candidates along sightlines of known open clusters, aiming to identify potential cluster members. We confirmed all the 17 white dwarf candidates from blue/optical spectroscopy, and we suggest five of them to be likely cluster members. We estimated progenitor ages and masses for the candidate cluster members, and compared our findings to those for other cluster white dwarfs. A white dwarf in NGC 3532 is the most massive known cluster member (1.13 M$_{\\odot}$), likely with an oxygen-neon core, for which we estimate an $8.8_{-4.3}^{+1.2}$ M$_{\\odot}$ progenitor, close to the mass-divide between white dwarf and neutron star progenitors. A cluster member in Ruprecht 131 is a magnetic white dwarf, whose progenitor mass exceeded 2-3 M$_{\\odot}$. We stress that wider searches, and improved...

  12. The WIRED Survey. IV. New Dust Disks from the McCook & Sion White Dwarf Catalog

    Science.gov (United States)

    Hoard, D.W.; Debes, John H.; Wachter, Stefanie; Leisawitz, David T.; Cohen, Martin

    2013-01-01

    We have compiled photometric data from the Wide-field Infrared Survey Explorer All Sky Survey and other archival sources for the more than 2200 objects in the original McCook & Sion Catalog of Spectroscopically Identified White Dwarfs. We applied color-selection criteria to identify 28 targets whose infrared spectral energy distributions depart from the expectation for the white dwarf photosphere alone. Seven of these are previously known white dwarfs with circumstellar dust disks, five are known central stars of planetary nebulae, and six were excluded for being known binaries or having possible contamination of their infrared photometry. We fit white dwarf models to the spectral energy distributions of the remaining ten targets, and find seven new candidates with infrared excess suggesting the presence of a circumstellar dust disk. We compare the model dust disk properties for these new candidates with a comprehensive compilation of previously published parameters for known white dwarfs with dust disks. It is possible that the current census of white dwarfs with dust disks that produce an excess detectable at K-band and shorter wavelengths is close to complete for the entire sample of known WDs to the detection limits of existing near-IR all-sky surveys. The white dwarf dust disk candidates now being found using longer wavelength infrared data are drawn from a previously underrepresented region of parameter space, in which the dust disks are overall cooler, narrower in radial extent, and/or contain fewer emitting grains.

  13. THE HABITABILITY AND DETECTION OF EARTH-LIKE PLANETS ORBITING COOL WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Fossati, L.; Haswell, C. A.; Patel, M. R.; Busuttil, R. [Department of Physical Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA (United Kingdom); Bagnulo, S. [Armagh Observatory, College Hill, Armagh BT61 9DG (United Kingdom); Kowalski, P. M. [GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam (Germany); Shulyak, D. V. [Institute of Astrophysics, Georg-August-University, Friedrich-Hund-Platz 1, D-37077 Goettingen (Germany); Sterzik, M. F., E-mail: l.fossati@open.ac.uk, E-mail: C.A.Haswell@open.ac.uk, E-mail: M.R.Patel@open.ac.uk, E-mail: r.busuttil@open.ac.uk, E-mail: sba@arm.ac.uk, E-mail: kowalski@gfz-potsdam.de, E-mail: denis.shulyak@gmail.com, E-mail: msterzik@eso.org [European Southern Observatory, Casilla 19001, Santiago 19 (Chile)

    2012-09-20

    Since there are several ways planets can survive the giant phase of the host star, we examine the habitability and detection of planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU would remain in the continuous habitable zone (CHZ) for {approx}8 Gyr. We show that photosynthetic processes can be sustained on such planets. The DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, and hence non-magnetic white dwarfs are compatible with the persistence of complex life. Polarization due to a terrestrial planet in the CHZ of a cool white dwarf (CWD) is 10{sup 2} (10{sup 4}) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a viable way to detect close-in rocky planets around white dwarfs. Multi-band polarimetry would also allow us to reveal the presence of a planet atmosphere, providing a first characterization. Planets in the CHZ of a 0.6 M{sub Sun} white dwarf will be distorted by Roche geometry, and a Kepler-11d analog would overfill its Roche lobe. With current facilities a super-Earth-sized atmosphereless planet is detectable with polarimetry around the brightest known CWD. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue.

  14. 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).

  15. An Eccentric Binary Millisecond Pulsar with a Helium White Dwarf Companion in the Galactic field

    Science.gov (United States)

    Antoniadis, John; Kaplan, David L.; Stovall, Kevin; Freire, Paulo C. C.; Deneva, Julia S.; Koester, Detlev; Jenet, Fredrick; Martinez, Jose G.

    2016-10-01

    Low-mass white dwarfs (LMWDs) are believed to be exclusive products of binary evolution, as the universe is not old enough to produce them from single stars. Because of the strong tidal forces operating during the binary interaction phase, the remnant systems observed today are expected to have negligible eccentricities. Here, we report on the first unambiguous identification of an LMWD in an eccentric (e = 0.13) orbit around the millisecond pulsar PSR J2234+0511, which directly contradicts this picture. We use our spectra and radio-timing solution (derived elsewhere) to infer the WD temperature ({T}{{eff}}=8600+/- 190 K), and peculiar systemic velocity relative to the local standard of rest (≃ 31 km s-1). We also place model-independent constraints on the WD radius ({R}{{WD}}={0.024}-0.002+0.004 {R}⊙ ) and surface gravity ({log} g={7.11}-0.16+0.08 dex). The WD and kinematic properties are consistent with the expectations for low-mass X-ray binary evolution and disfavor a dynamic three-body formation channel. In the case of the high eccentricity being the result of a spontaneous phase transition, we infer a mass of ˜1.60 M ⊙ for the pulsar progenitor, which is too low for the quark-nova mechanism proposed by Jiang et al., and too high for the scenario of Freire & Tauris, in which a WD collapses into a neutron star via a rotationally delayed accretion-induced collapse. We find that eccentricity pumping via interaction with a circumbinary disk is consistent with our inferred parameters. Finally, we report tentative evidence for pulsations that, if confirmed, would transform the star into an unprecedented laboratory for WD physics.

  16. First detection of the white dwarf cooling sequence of the galactic bulge

    Energy Technology Data Exchange (ETDEWEB)

    Calamida, A.; Sahu, K. C.; Anderson, J.; Casertano, S.; Brown, T.; Sokol, J.; Bond, H. E.; Ferguson, H.; Livio, M.; Valenti, J. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Cassisi, S.; Buonanno, R.; Pietrinferni, A. [Osservatorio Astronomico di Teramo—INAF, Via M. Maggini, I-64100 Teramo (Italy); Salaris, M. [Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF (United Kingdom); Ferraro, I. [Osservatorio Astronomico di Roma—INAF, Via Frascati 33, I-00040 Monte Porzio Catone (Italy); Clarkson, W., E-mail: calamida@stsci.edu [University of Michigan-Dearborn, 4901 Evergreen Road, Dearborn, MI 48128 (United States)

    2014-08-01

    We present Hubble Space Telescope data of the low-reddening Sagittarius window in the Galactic bulge. The Sagittarius Window Eclipsing Extrasolar Planet Search field (∼3'× 3'), together with three more Advanced Camera for Surveys and eight Wide-Field Camera 3 fields, were observed in the F606W and F814W filters, approximately every two weeks for 2 yr, with the principal aim of detecting a hidden population of isolated black holes and neutron stars through astrometric microlensing. Proper motions were measured with an accuracy of ≈0.1 mas yr{sup –1} (≈4 km s{sup –1}) at F606W ≈ 25.5 mag, and better than ≈0.5 mas yr{sup –1} (≈20 km s{sup –1}) at F606W ≈ 28 mag, in both axes. Proper-motion measurements allowed us to separate disk and bulge stars and obtain a clean bulge color-magnitude diagram. We then identified for the first time a white dwarf (WD) cooling sequence in the Galactic bulge, together with a dozen candidate extreme horizontal branch stars. The comparison between theory and observations shows that a substantial fraction of the WDs (≈30%) are systematically redder than the cooling tracks for CO-core H-rich and He-rich envelope WDs. This evidence would suggest the presence of a significant number of low-mass WDs and WD-main-sequence binaries in the bulge. This hypothesis is further supported by the finding of two dwarf novae in outburst, two short-period (P ≲ 1 day) ellipsoidal variables, and a few candidate cataclysmic variables in the same field.

  17. Foretellings of Ragnar\\"ok: World-engulfing Asymptotic Giants and the Inheritance of White Dwarfs

    CERN Document Server

    Mustill, Alexander James

    2012-01-01

    The search for planets around White Dwarf stars, and evidence for dynamical instability around them in the form of atmospheric pollution and circumstellar discs, raises questions about the nature of planetary systems that can survive the vicissitudes of the Asymptotic Giant Branch (AGB). We study the competing effects, on planets at several AU from the star, of strong tidal forces arising from the star's large convective envelope, and of the planets' orbital expansion due to stellar mass-loss. We, for the first time, study the evolution of planets while following each thermal pulse on the AGB. For Jovian planets, tidal forces are strong, and can pull into the envelope planets initially at ~3 AU for a 1M_Sol star and ~5 AU for a 5M_Sol star. Lower-mass planets feel weaker tidal forces, and Terrestrial planets initially within 1.5-3 AU enter the stellar envelope. Thus, low-mass planets that begin inside the maximum stellar radius can survive, as their orbits expand due to mass-loss. The inclusion of a moderate ...

  18. The evolution and fate of super-Chandrasekhar mass white dwarf merger remnants

    Science.gov (United States)

    Schwab, Josiah; Quataert, Eliot; Kasen, Daniel

    2016-12-01

    We present stellar evolution calculations of the remnant of the merger of two carbon-oxygen white dwarfs (CO WDs). We focus on cases that have a total mass in excess of the Chandrasekhar mass. After the merger, the remnant manifests as an L ˜ 3× 10^4 L_{⊙} source for ˜104 yr. A dusty wind may develop, leading these sources to be self-obscured and to appear similar to extreme asymptotic giant branch (AGB) stars. Roughly ˜10 such objects should exist in the Milky Way and M31 at any time. As found in previous work, off-centre carbon fusion is ignited within the merger remnant and propagates inwards via a carbon flame, converting the WD to an oxygen-neon (ONe) composition. By following the evolution for longer than previous calculations, we demonstrate that after carbon-burning reaches the centre, neutrino-cooled Kelvin-Helmholtz contraction leads to off-centre neon ignition in remnants with masses ≥ 1.35 M_{⊙}. The resulting neon-oxygen flame converts the core to a silicon WD. Thus, super-Chandrasekhar WD merger remnants do not undergo electron-capture induced collapse as traditionally assumed. Instead, if the remnant mass remains above the Chandrasekhar mass, we expect that it will form a low-mass iron core and collapse to form a neutron star. Remnants that lose sufficient mass will end up as massive, isolated ONe or Si WDs.

  19. Discovery of the Eclipsing Detached Double White Dwarf Binary NLTT 11748

    CERN Document Server

    Steinfadt, Justin D R; Shporer, Avi; Bildsten, Lars; Howell, Steve B

    2010-01-01

    We report the discovery of the first eclipsing detached double white dwarf (WD) binary. In a pulsation search, the low-mass helium core WD NLTT 11748 was targeted for fast (approx 1-min) differential photometry with the Las Cumbres Observatory's Faulkes Telescope North. Rather than pulsations, we discovered approx 180-s 3-6% dips in the photometry. Subsequent radial velocity measurements of the primary from the Keck telescope found variations with a semi-amplitude K_1 = 271 +/- 3 km/s, and confirmed the dips as eclipses caused by an orbiting WD with a mass M_2 = 0.648-0.771 M_sun for M_1 = 0.1-0.2 M_sun. We detect both the primary and secondary eclipse during the P_orb = 5.64 hr orbit and measure the secondary's brightness to be 3.5 +/- 0.3% of the primary at SDSS-g'. Assuming that the secondary follows the mass-radius relation of a cold C/O WD and including the effects of microlensing in the binary, the primary eclipse yields a primary radius of R_1 = 0.043-0.039 R_sun for M_1 = 0.1-0.2 M_sun; consistent wit...

  20. Supernovae from direct collisions of white dwarfs and the role of helium shell ignition

    CERN Document Server

    Papish, Oded

    2015-01-01

    Models for supernovae (SNe) arising from thermonuclear explosions of white dwarfs (WDs) have been extensively studied over the last few decades, mostly focusing on the single degenerate (accretion of material of a WD) and double degenerate (WD-WD merger) scenarios. In recent years it was suggested that WD-WD direct collisions provide an additional channel for such explosions. Here we extend the studies of such explosions, and explore the role of Helium-shells in affecting the thermonuclear explosions. We study both the impact of low-mass helium ($\\sim0.01$ M$_{\\odot})$ shells, as well as high mass shells ($\\ge0.1$ M$_{\\odot}$). We find that detonation of the massive helium layers precede the detonation of the WD Carbon-Oxygen (CO) bulk during the collision and can change the explosive evolution and outcomes for the cases of high mass He-shells. In particular, the He-shell detonation propagates on the WD surface and inefficiently burns material prior to the CO detonation that later follows in the central parts...

  1. Illuminating Massive Black Holes With White Dwarfs: Orbital Dynamics and High Energy Transients from Tidal Interactions

    CERN Document Server

    MacLeod, Morgan; Ramirez-Ruiz, Enrico; Guillochon, James; Samsing, Johan

    2014-01-01

    White dwarfs (WDs) can be tidally disrupted only by massive black holes (MBHs) with masses less than approximately $10^5 M_\\odot$. 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 signpost to an otherwise quiescent MBH of relatively low mass. We show that the mass transfer history, and thus the lightcurve, are 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 and making it difficult to produce a Swift J1644+57-like lightcurve via this channel. We then examine the stellar dynamics of clusters surrounding these MBHs to show that single-passage WD disruptions are substantially more common than repeating encounters in eccentric orbits. The $10^{49}$ erg s$^{-1}$ peak luminosity of these events makes them visible to cosmologi...

  2. Diffusion and the occurrence of hydrogen shell flashes in helium white dwarf stars

    CERN Document Server

    Althaus, L G; Benvenuto, O G

    2000-01-01

    We investigate the effects of element diffusion on the structure and evolution of low-mass helium white dwarfs (WD). Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. Initial models from 0.406 to 0.161 solar masses are constructed by applying mass loss rates at different stages of the RGB evolution of a solar model. The multicomponent flow equations describing gravitational settling, and chemical and thermal diffusion are solved and the diffusion calculations are coupled to an evolutionary code. In addition, the same sequences are computed but neglecting diffusion. We find that element diffusion strongly affects the structure and cooling history of helium WD. In particular, diffusion induces the occurrence of hydrogen shell flashes in models with masses ranging from 0.18 to 0.41 solar masses, which is in sharp contrast from the situation when diffusion is neglected. In connection with the further evolution, these diffusion-induced flashes le...

  3. ON THE FORMATION OF ECCENTRIC MILLISECOND PULSARS WITH HELIUM WHITE-DWARF COMPANIONS

    Energy Technology Data Exchange (ETDEWEB)

    Antoniadis, John, E-mail: antoniadis@dunlap.utoronto.ca [Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn (Germany); Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4 (Canada)

    2014-12-20

    Millisecond pulsars (MSPs) orbiting helium white dwarfs (WDs) in eccentric orbits challenge the established binary-evolution paradigm that predicts efficient orbital circularization during the mass-transfer episode that spins up the pulsar. Freire and Tauris recently proposed that these binary MSPs may instead form from the rotationally delayed accretion-induced collapse of a massive WD. However, their hypothesis predicts that eccentric systems preferably host low-mass pulsars and travel with small systemic velocities—in tension with new observational constraints. Here, I show that a substantial growth in eccentricity may alternatively arise from the dynamical interaction of the binary with a circumbinary disk. Such a disk may form from ejected donor material during hydrogen flash episodes, when the neutron star is already an active radio pulsar and tidal forces can no longer circularize the binary. I demonstrate that a short-lived (10{sup 4}-10{sup 5} yr) disk can result in eccentricities of e ≅ 0.01-0.15 for orbital periods between 15 and 50 days. Finally, I propose that, more generally, the disk hypothesis may explain the lack of circular binary pulsars for the aforementioned orbital-period range.

  4. On the formation of eccentric millisecond pulsars with helium white-dwarf companions

    CERN Document Server

    Antoniadis, John

    2014-01-01

    Millisecond pulsars (MSPs) orbiting helium white-dwarfs (WD) in eccentric orbits challenge the established binary-evolution paradigm that predicts efficient orbital circularization during the mass-transfer episode that spins up the pulsar. Freire and Tauris (2014) recently proposed that these binary MSPs may instead form from the rotationally-delayed accretion-induced collapse of a massive WD. This scenario predicts that eccentric systems preferably host low-mass pulsars and travel with small systemic velocities -- in tension with new observational constraints. Here, I show that a substantial growth in eccentricity may alternatively arise from the dynamical interaction of the binary with a circumbinary disk. Such a disk may form from ejected donor material during hydrogen flash episodes, when the neutron star is already an active radio pulsar and tidal forces can no longer circularize the binary. I demonstrate that a short-lived (10^4-10^5 yrs disk can result to eccentricities of e ~ 0.01-0.15 for orbital per...

  5. Planetary systems and real planetary nebulae from planets destruction near white dwarfs

    CERN Document Server

    Bear, Ealeal

    2015-01-01

    We suggest that tidal destruction of Earth-like and icy planets near a white dwarf (WD) might lead to the formation of one or more low-mass planets in tight orbits around the WD. More massive planets contain hydrogen which will start burning on the surface of the WD and inflate an envelope, part of which be ejected to form a nebula. This nebula will be ionized and be observed as a planetary nebulae. The formation of the WD planetary system starts with a tidal break-up of icy or lower mass planets to planetesimals near their tidal radius of about 1Rsun. Internal stress forces keep the planetesimal from tidal break-up when their radius is less than about 100km. We suggest that the planetesimals then bind together to form new sub-Earth-like planets around the WD at a few solar radii. More massive planets that contain hydrogen will supply the WD with fresh nuclear fuel to reincarnate its stellar-giant phase. Some of the hydrogen will be inflated in a large envelope that will cause the planetesimal formed from the...

  6. On the Formation of Eccentric Millisecond Pulsars with Helium White-dwarf Companions

    Science.gov (United States)

    Antoniadis, John

    2014-12-01

    Millisecond pulsars (MSPs) orbiting helium white dwarfs (WDs) in eccentric orbits challenge the established binary-evolution paradigm that predicts efficient orbital circularization during the mass-transfer episode that spins up the pulsar. Freire & Tauris recently proposed that these binary MSPs may instead form from the rotationally delayed accretion-induced collapse of a massive WD. However, their hypothesis predicts that eccentric systems preferably host low-mass pulsars and travel with small systemic velocities—in tension with new observational constraints. Here, I show that a substantial growth in eccentricity may alternatively arise from the dynamical interaction of the binary with a circumbinary disk. Such a disk may form from ejected donor material during hydrogen flash episodes, when the neutron star is already an active radio pulsar and tidal forces can no longer circularize the binary. I demonstrate that a short-lived (104-105 yr) disk can result in eccentricities of e ~= 0.01-0.15 for orbital periods between 15 and 50 days. Finally, I propose that, more generally, the disk hypothesis may explain the lack of circular binary pulsars for the aforementioned orbital-period range.

  7. RX J0648.0--4418: the fastest-spinning white dwarf

    CERN Document Server

    Mereghetti, Sandro

    2013-01-01

    RX J0648.0-4418 is a post common-envelope X-ray binary composed of a hot subdwarf and one of the most massive white dwarfs with a dynamical mass measurement (1.28+/-0.05 M_sun). This white dwarf, with a spin period of 13.2 s, rotates more than twice faster than the white dwarf in the cataclysmic variable AE Aqr. The current properties of these two binaries, as well as their future evolution, are quite different, despite both contain a fast-spinning white dwarf. RX J0648.0-4418 could be the progenitor of either a Type Ia supernova or of a non-recycled millisecond pulsars.

  8. An upper limit to the secular variation of the gravitational constant from white dwarf stars

    CERN Document Server

    García-Berro, Enrique; Torres, Santiago; Althaus, Leandro G; Isern, Jordi

    2011-01-01

    A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.

  9. Spiral Disk Instability Can Drive Thermonuclear Explosions in Binary White Dwarf Mergers

    CERN Document Server

    Kashyap, Rahul; García-Berro, Enrique; Aznar-Siguán, Gabriela; Ji, Suoqing; Lorén-Aguilar, Pablo

    2015-01-01

    Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion of carbon-oxygen white dwarfs, and serve as standardizable cosmological candles. However, despite their importance, the nature of the progenitor systems which give rise to SNe Ia has not been hitherto elucidated. Observational evidence favors the double-degenerate channel, in which merging white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies exist between observations and theory, and to date, there has been no self-consistent merger model which yields a SNe Ia. Here we show that a spiral mode instability in the accretion disk formed during a binary white dwarf merger leads to a detonation on a dynamical timescale. This mechanism sheds light on how white dwarf mergers may frequently yield SNe Ia.

  10. White Dwarfs in the Metal-Rich Open Cluster NGC 6253

    CERN Document Server

    Jeffery, Elizabeth J; Romero, Alejandra; Kepler, S O

    2016-01-01

    We have obtained 53 images with the $g$ filter and 19 images with the $i$ filter, each with 600-second exposures of the super metal rich open cluster NGC 6253 with the Gemini-South telescope to create deep images of the cluster to observe the cluster white dwarfs for the first time. We will analyze the white dwarf luminosity function to measure the cluster's white dwarf age, search for any anomalous features (as has been seen in the similarly metal rich cluster NGC 6791), and constrain the initial-final mass relation at high metallicities. We present an update on these observations and our program to study the formation of white dwarfs in super high metallicity environments.

  11. Hydrogen in hot subdwarfs formed by double helium white dwarf mergers

    CERN Document Server

    Hall, Philip D

    2016-01-01

    Isolated hot subdwarfs might be formed by the merging of two helium-core white dwarfs. Before merging, helium-core white dwarfs have hydrogen-rich envelopes and some of this hydrogen may survive the merger. We calculate the mass of hydrogen that is present at the start of such mergers and, with the assumption that hydrogen is mixed throughout the disrupted white dwarf in the merger process, estimate how much can survive. We find a hydrogen mass of up to about $2 \\times 10^{-3}\\,\\mathrm{M}_{\\odot}$ in merger remnants. We make model merger remnants that include the hydrogen mass appropriate to their total mass and compare their atmospheric parameters with a sample of apparently isolated hot subdwarfs, hydrogen-rich sdBs. The majority of these stars can be explained as the remnants of double helium white dwarf mergers.

  12. WhiteDwarf.org - Establishing a permanent endowment for the Whole Earth Telescope

    CERN Document Server

    Metcalfe, T S

    2002-01-01

    White Dwarf Research Corporation is a 501(c)(3) non-profit organization dedicated to scientific research and public education on topics relevant to white dwarf stars. It was founded in 1999 in Austin, Texas to help fulfill the need for an alternative research center where scarce funding dollars could be used more efficiently, and to provide a direct link between astronomers who study white dwarf stars and the general public. Due to its administrative simplicity, WDRC can facilitate the funding of multi-institutional and international collaborations, provide seamless grant portability, minimize overhead rates, and actively seek non-governmental funding sources. I describe the motivation for, and current status of, one of the long-term goals of WDRC: to establish a permanent endowment for the operation of the Whole Earth Telescope. I pay particular attention to fund-raising efforts through the website at http://WhiteDwarf.org/donate/

  13. The polluted atmospheres of cool white dwarfs and the magnetic field connection

    CERN Document Server

    Kawka, A

    2014-01-01

    We present an analysis of X-Shooter spectra of the polluted hydrogen-rich white dwarfs (DAZ) NLTT 888 and NLTT 53908. The spectra of NLTT 53908 show strong, Zeeman-split calcium lines (CaII H&K and Ca I $\\lambda 4226$) and the star appears to be a close relative of the polluted magnetic white dwarf (DAZH) NLTT 10480, while the spectra of NLTT 888 show narrow lines of calcium and iron. A comparison of the DAZ NLTT 888 and the DAZH NLTT 53908 with other class members illustrates the diversity of environment and formation circumstances surrounding these objects. In particular, we find that the incidence of magnetism in old, polluted white dwarfs significantly exceeds that found in the general white dwarf population which suggests an hypothetical link between a crowded planetary system and magnetic field generation.

  14. The Effects of Element Diffusion on the Pulsational Properties of Variable DA White Dwarf Stars

    CERN Document Server

    Corsico, A H; Althaus, L G; Serenelli, A M

    2002-01-01

    We explore the effects of element diffusion due to gravitational settling and thermal and chemical diffusion on the pulsational properties of DA white dwarfs. To this end, we employ an updated evolutionary code coupled with a pulsational, finite difference code for computing the linear, non-radial g-modes in the adiabatic approximation. We follow the evolution of a 0.55 \\msun white dwarf model in a self-consistent way with the evolution of chemical abundance distribution as given by time dependent diffusion processes. Results are compared with the standard treatment of diffusive equilibrium in the trace element approximation. Appreciable differences are found between the two employed treatments. We conclude that time dependent element diffusion plays an important role in determining the whole oscillation pattern and the temporal derivative of the periods in DAV white dwarfs. In addition, we discuss the plausibility of the standard description employed in accounting for diffusion in most of white dwarf asteros...

  15. The Parentage of Magnetic White Dwarfs: Implications from Their Space Motions

    Science.gov (United States)

    Anselowitz, Tara; Wasatonic, Richard; Matthews, Karen; Sion, Edward M.; McCook, George P.

    1999-06-01

    We have examined the statistical properties, cooling ages, and vector components of the three-dimensional space motion U, V, W for the enlarged sample of 53 magnetic white dwarfs contained in the fourth edition of the Catalog of Spectroscopically Identified White Dwarfs (McCook & Sion). Their cooling ages range from 2 million years to 12.6 billion years. A comparison of the total kinematic samples of magnetics and DA stars over the same luminosity range 10.0near miss'' pulsar progenitors. We find preliminary evidence that the magnetic white dwarfs show a peculiar distribution in UV velocity space relative to other spectroscopic subgroups of white dwarfs. However, there is little evidence of a difference in velocity dispersion among the hot and cool magnetic degenerates, despite their having vastly different cooling ages. This may be taken as indirect evidence that the magnetics represent a sample of mixed evolutionary progeny.

  16. Understanding the Oldest White Dwarfs: Atmospheres of Cool WDs as Extreme Physics Laboratories

    CERN Document Server

    Kowalski, Piotr M

    2010-01-01

    Reliable modeling of the atmospheres of cool white dwarfs is crucial for understanding the atmospheric evolution of these stars and for accurate white dwarfs cosmochronology. Over the last decade {\\it ab initio} modeling entered many research fields and has been successful in predicting properties of various materials under extreme conditions. In many cases the investigated physical regimes are difficult or even impossible to access by experimental methods, and first principles quantum mechanical calculations are the only tools available for investigation. Using modern methods of computational chemistry and physics we investigate the atmospheres of helium-rich, old white dwarfs. Such atmospheres reach extreme, fluid like densities (up to grams per cm$^3$) and represent an excellent laboratory for high temperature and pressure physics and chemistry. We show our results for the stability and opacity of $\\rm H^-$ and $\\rm C_2$ in dense helium and the implications of our work for understanding cool white dwarfs.

  17. Understanding the Oldest White Dwarfs: Atmospheres of Cool WDs as Extreme Physics Laboratories

    Science.gov (United States)

    Kowalski, Piotr M.

    2010-11-01

    Reliable modeling of the atmospheres of cool white dwarfs is crucial for understanding the atmospheric evolution of these stars and for accurate white dwarfs cosmochronology. Over the last decade ab initio modeling entered many research fields and has been successful in predicting properties of various materials under extreme conditions. In many cases the investigated physical regimes are difficult or even impossible to access by experimental methods, and first principles quantum mechanical calculations are the only tools available for investigation. Using modern methods of computational chemistry and physics we investigate the atmospheres of helium-rich, old white dwarfs. Such atmospheres reach extreme, fluid like densities (up to grams per cm3) and represent an excellent laboratory for high temperature and pressure physics and chemistry. We show our results for the stability and opacity of H- and C2 in dense helium and the implications of our work for understanding cool white dwarfs.

  18. Enigmas from the Sloan Digital Sky Survey DR7 Kleinman White Dwarf Catalog

    CERN Document Server

    Liebert, James; Wickramasinghe, Dayal; Smith, Paul

    2015-01-01

    We report results from a continuation of our searches for high field magnetic white dwarfs paired in a detached binary with non degenerate companions. We made use of the Sloan Digital Sky Survey DR7 catalog of Kleinman et al. (2013) with 19,712 spectroscopically-identified white dwarfs. These include 1,735 white dwarf plus M dwarf detached pairs (almost 10\\% of the Kleinman at al.'s list). No new pairs were found, although we did recover the polar (AM~Herculis system) ST\\,LMi in a low state of accretion. With the larger sample the original situation reported ten years ago remains intact now at a much higher level of statistical significance: in the selected SDSS sample, high field magnetic white dwarfs are not found in an apparently-detached pairing with an M dwarf, unless they are a magnetic CV in a low state of accretion. This finding strengthens the case that the fields in the isolated high field magnetic white dwarfs are generated by stellar mergers but also raises questions on the nature of the progenito...

  19. New population synthesis model Preliminary results for close double white dwarf populations

    Science.gov (United States)

    Toonen, Silvia; Nelemans, Gijs; Portegies Zwart, Simon F.

    2010-11-01

    An update is presented to the software package SeBa (Portegies Zwart and Verbunt [1], Nelemans et al. [2]) for simulating single star and binary evolution in which new stellar evolution tracks (Hurley et al. [3]) have been implemented. SeBa is applied to study the population of close double white dwarf and the delay time distribution of double white dwarf mergers that may lead to Supernovae Type Ia.

  20. A New Population Synthesis Model: Preliminary Results for Close Double White Dwarf Populations

    Science.gov (United States)

    Toonen, Silvia; Nelemans, Gijs; Portegies Zwart, Simon F.

    2010-12-01

    An update is presented to the software package SeBa (Portegies Zwart and Verbunt [1], Nelemans et al. [2]) for simulating single star and binary evolution in which new stellar evolution tracks (Hurley et al. [3]) have been implemented. SeBa is applied to study the population of close double white dwarf and the delay time distribution of double white dwarf mergers that may lead to Supernovae Type Ia.

  1. On the formation of DA white dwarfs with low hydrogen contents: Preliminary Results

    CERN Document Server

    Bertolami, M M Miller; Córsico, A H

    2016-01-01

    Systematic photometric and asteroseismological studies in the last decade support the belief that white dwarfs in the solar neighborhood harbor a broad range of hydrogen-layer contents. The reasons behind this spread of hydrogen-layer masses are not understood and usually misunderstood. In this work we present, and review, the different mechanisms that can (or cannot) lead to the formation of white dwarfs with a broad range hydrogen contents.

  2. White dwarf mergers and the origin of R Coronae Borealis stars

    CERN Document Server

    Lorén-Aguilar, P; José, J; García-Berro, E; Althaus, L G; Isern, J

    2011-01-01

    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.

  3. Physical Properties of the Current Census of Northern White Dwarfs within 40 pc of the Sun

    CERN Document Server

    Limoges, M -M; Lépine, S

    2015-01-01

    We present a detailed description of the physical properties of our current census of white dwarfs within 40 pc of the Sun, based on an exhaustive spectroscopic survey of northern hemisphere candidates from the SUPERBLINK proper motion database. Our method for selecting white dwarf candidates is based on a combination of theoretical color-magnitude relations and reduced proper motion diagrams. We reported in an earlier publication the discovery of nearly 200 new white dwarfs, and we present here the discovery of an additional 133 new white dwarfs, among which we identify 96 DA, 3 DB, 24 DC, 3 DQ, and 7 DZ stars. We further identify 178 white dwarfs that lie within 40 pc of the Sun, representing a 40% increase of the current census, which now includes 492 objects. We estimate the completeness of our survey at between 66 and 78%, allowing for uncertainties in the distance estimates. We also perform a homogeneous model atmosphere analysis of this 40 pc sample and find a large fraction of massive white dwarfs, in...

  4. Cracking the Diamond: Testing White Dwarf Crystallization Theory with BPM 37093

    Science.gov (United States)

    Metcalfe, T. S.; Montgomery, M. H.; Kanaan, A.

    2003-12-01

    More than four decades have passed since it was predicted that the cores of the coolest white dwarf stars should theoretically crystallize. This effect is one of the largest sources of uncertainty in white dwarf cooling models, which are now routinely used to estimate the ages of stellar populations in both the galactic disk and the halo. We are attempting to minimize this source of uncertainty by calibrating the models, using observations of pulsating white dwarfs. In a typical mass white dwarf model, crystallization does not begin until the surface temperature reaches 6000-8000 K. In more massive white dwarf models the effect begins at higher surface temperatures, where pulsations are observed in the ZZ Ceti (DAV) stars. The most massive DAV white dwarf presently known is BPM 37093. We are using the observed pulsation periods of this star to probe the interior and determine the size of the crystallized core empirically. We will present preliminary results from our application of a genetic-algorithm-based fitting method to address this enormous computational problem. This research was partially supported by a grant from NASA administered by the American Astronomical Society.

  5. An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate.

    Science.gov (United States)

    Gilfanov, Marat; Bogdán, Akos

    2010-02-18

    There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10(7) years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of approximately 30-50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.

  6. The DB gap and a new class of pulsating white dwarfs

    Directory of Open Access Journals (Sweden)

    Shibahashi H.

    2013-03-01

    Full Text Available The recent systematic surveys providing enormously massive datasets of white dwarfs show that there is still a deficit of a factor of 2.5 in the DA/non-DA ratio within the temperature range of 30 000 K < Teff < 45 000 K, which has been regarded as the “DB gap” meaning a range with almost no helium atmosphere white dwarfs. Since all white dwarfs have to evolve through this temperature range along almost the identical sequence on the color-magnitude diagram, this implies that most of the helium atmosphere DO stars once evolve into hydrogen atmosphere hot DA stars in the temperature range of the DB gap and then back to helium atmosphere DB stars. Possible scenarios for this chameleon-like disguises of white dwarfs with helium dominant atmospheres are described and a new class of pulsating white dwarfs, named the hot-DAV stars, is predicted from these scenarios. One pulsating DA white dwarf, being consistent with the prediction, has been discovered indeed.

  7. A Search for Rocky Planets in Close Orbits around White Dwarfs

    Science.gov (United States)

    Debes, John; Sandhaus, Phoebe; Ely, Justin

    2015-12-01

    The search for transiting habitable exoplanets has broadened to include several types of stars that are smaller than the Sun in order to increase the observed transit depth and hence the atmospheric signal of the planet. Of all current spectral types, white dwarfs are the most favorable for this type of investigation. The fraction of white dwarfs that possess close-in rocky planets is unknown, but several large angle surveys of stars have the photometric precision and cadence to discover at least one if they are common. Ultraviolet observations of white dwarfs may allow for detection of molecular oxygen or ozone in the atmosphere of a terrestrial planet. We use archival Hubble Space Telescope data from the Cosmic Origins Spectrograph to search for transiting rocky planets around UV-bright white dwarfs. In the process, we discovered unusual variability in the pulsating white dwarf GD~133, which shows slow sinusoidal variations in the UV. While we detect no planets around our small sample of targets, we do place stringent limits on the possibility of transiting planets, down to sub-lunar radii. We also point out that non-transiting small planets in thermal equilibrium are detectable around hotter white dwarfs through infrared excesses, and identify two candidates.

  8. The white dwarf cooling sequence of NGC 6791: an unique tool for stellar astrophysics

    CERN Document Server

    García-Berro, E; Renedo, I; Camacho, J; Althaus, L G; Córsico, A H; Salaris, M; Isern, J

    2011-01-01

    NGC 6791 is a well-studied, metal-rich open cluster that is so close to us that can be imaged down to luminosities fainter than that of the termination of its white dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population. We use a Monte Carlo simulator that employs up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The cooling sequences for carbon-oxygen cores account for the delays introduced by both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase separation upon crystallization. We do not find evidence for a substantial fraction of helium-core white dwarfs, and hence our results support the suggestion that the origin of the bright peak of the white dwarf luminosity function can only be attributed to a population of unresolved binary white dwarfs. Moreover, our results indicate that the number distribution of secondary masses of the population of unresolved ...

  9. The connection between period spectra and constraints in white dwarf asteroseismology

    Science.gov (United States)

    Kim, Agnes

    2017-01-01

    White dwarfs are the end product of evolution for around 98% of the stars in our Galaxy. Buried in their interiors are the records of physical processes that take place during earlier stages in the life of the star. Not long after the discovery of the first pulsating white dwarf, the promise of asteroseismology to unveil the interior structure of white dwarfs and therefore help us constrain their prior evolution became apparent. In recent years, a well-established theory of non-radial oscillations, improved white dwarf models, year of expertise built up in the field of white dwarf asteroseismic fitting, and computing power have culminated in the asteroseismology finally delivering what it promised: a detailed map of the interior structure of white dwarfs. As always in science, new results raise new questions. We perform a number of numerical experiments to better understand the connection between a given set of periods varying in the number of periods and in the set of radial overtones and the quality of the constraints on interior structure one obtains from fitting these periods.

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

    CERN Document Server

    Jeffery, C Simon; 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 evolved to become the carbon-oxygen white dwarf. Thus the surface composition of the merged star is estimated as a function of the initial mass and metallicity of its progenitor. The question of whether additional nucleosynthesis occurs during the white dwarf merger has been examined. The high observed abundances of carbon and oxygen must either originate by dredge-up from the core of the carbon-oxygen white dwarf during a cold merger or be generated directly by alpha-burning during a hot merger. The presence of large quantiti...

  11. Transit surveys for Earths in the habitable zones of white dwarfs

    CERN Document Server

    Agol, Eric

    2011-01-01

    To date the search for habitable Earth-like planets has primarily focused on nuclear burning stars. I propose that this search should be expanded to cool white dwarf stars that have expended their nuclear fuel. I define the continuously habitable zone of white dwarfs, and show that it extends from ~0.005 to 0.02 AU for white dwarfs with masses from 0.4-0.9 solar masses, temperatures less than 10,000 K, and habitable durations of at least 3 Gyr. As they are similar in size to Earth, white dwarfs may be completely eclipsed by terrestrial planets that orbit edge-on, which can easily be detected with ground-based telescopes. If planets can migrate inward or reform near white dwarfs, I show that a global robotic telescope network could carry out a transit survey of nearby white dwarfs placing interesting constraints on the presence of habitable Earths. If planets were detected, I show that the survey would favor detection of planets similar to Earth: similar size, temperature, rotation period, and host star temper...

  12. The Gaia DR1 Mass-Radius Relation for White Dwarfs

    CERN Document Server

    Tremblay, P -E; Raddi, R; Jordan, S; Besson, C; Gaensicke, B T; Parsons, S G; Koester, D; Marsh, T; Bohlin, R; Kalirai, J

    2016-01-01

    The Gaia Data Release 1 (DR1) sample of white dwarf parallaxes is presented, including 6 directly observed degenerates and 46 white dwarfs in wide binaries. This data set is combined with spectroscopic atmospheric parameters to study the white dwarf mass-radius relationship (MRR). Gaia parallaxes and G magnitudes are used to derive model atmosphere dependent white dwarf radii, which can then be compared to the predictions of a theoretical MRR. We find a good agreement between Gaia DR1 parallaxes, published effective temperatures (Teff) and surface gravities (log g), and theoretical MRRs. As it was the case for Hipparcos, the precision of the data does not allow for the characterisation of hydrogen envelope masses. The uncertainties on the spectroscopic atmospheric parameters are found to dominate the error budget and current error estimates for well-known and bright white dwarfs may be slightly optimistic. With the much larger Gaia DR2 white dwarf sample it will be possible to explore the MRR over a much wide...

  13. The Gaia DR1 mass-radius relation for white dwarfs

    Science.gov (United States)

    Tremblay, P.-E.; Gentile-Fusillo, N.; Raddi, R.; Jordan, S.; Besson, C.; Gänsicke, B. T.; Parsons, S. G.; Koester, D.; Marsh, T.; Bohlin, R.; Kalirai, J.; Deustua, S.

    2017-03-01

    The Gaia Data Release 1 (DR1) sample of white dwarf parallaxes is presented, including six directly observed degenerates and 46 white dwarfs in wide binaries. This data set is combined with spectroscopic atmospheric parameters to study the white dwarf mass-radius relationship (MRR). Gaia parallaxes and G magnitudes are used to derive model atmosphere-dependent white dwarf radii, which can then be compared to the predictions of a theoretical MRR. We find a good agreement between Gaia DR1 parallaxes, published effective temperatures (Teff) and surface gravities (log g), and theoretical MRRs. As it was the case for Hipparcos, the precision of the data does not allow for the characterization of hydrogen envelope masses. The uncertainties on the spectroscopic atmospheric parameters are found to dominate the error budget and current error estimates for well-known and bright white dwarfs may be slightly optimistic. With the much larger Gaia DR2 white dwarf sample, it will be possible to explore the MRR over a much wider range of mass, Teff, and spectral types.

  14. Evolution and colors of helium-core white dwarf stars with high-metallicity progenitors

    CERN Document Server

    Althaus, L G; Romero, A D; Rohrmann, R D; Córsico, A H; García-Berro, E; Bertolami, M M Miller

    2009-01-01

    Motivated by the recent detection of single and binary He-core white dwarfs in metal-rich clusters, we present a full set of evolutionary calculations and colors appropriate for the study of such white dwarfs. The paper is also aimed at investigating whether stable hydrogen burning may constitute a main source of energy for massive He-core white dwarfs resulting from high-metallicity progenitors. White dwarf sequences are derived by taking into account the evolutionary history of progenitor stars with supersolar metallicities. We also incorporate a self-consistent, time-dependent treatment of gravitational settling and chemical diffusion, as well as of the residual nuclear burning. We find that the influence of residual nuclear burning during the late stages of white dwarf evolution is strongly dependent on the occurrence of chemical diffusion at the base of the hydrogen-rich envelope. When no diffusion is considered, residual hydrogen burning strongly influences the advanced stages of white dwarf cooling, in...

  15. SEISMOLOGY OF A MASSIVE PULSATING HYDROGEN ATMOSPHERE WHITE DWARF

    Energy Technology Data Exchange (ETDEWEB)

    Kepler, S. O.; Pelisoli, Ingrid; Pecanha, Viviane; Costa, J. E. S. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS (Brazil); Fraga, Luciano [Southern Observatory for Astrophysical Research, Casilla 603, La Serena (Chile); Hermes, J. J.; Winget, D. E.; Castanheira, Barbara [Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712-1083 (United States); Corsico, A. H.; Romero, A. D.; Althaus, Leandro [Facultad de Ciencias Astronomicas y Geofisicas, Universidad Nacional de La Plata (Argentina); Kleinman, S. J.; Nitta, A. [Gemini Observatory, Northern Operations Center, 670 North A' ohoku Place, Hilo, HI 96720 (United States); Koester, D. [Institut fuer Theoretische Physik und Astrophysik, Universitaet Kiel, D-24098 Kiel (Germany); Kuelebi, Baybars [Institut de Ciencies de L' Espai, Universitat Autonoma de Barcelon and Institute for Space Studies of Catalonia, c/Gran Capita 2-4, Edif. Nexus 104, E-08034 Barcelona (Spain); Jordan, Stefan [Astronomisches Rechen-Institut, ZAH, Moenchhofstr. 12-14, D-69120 Heidelberg (Germany); Kanaan, Antonio, E-mail: kepler@if.ufrgs.br [Departamento de Fisica, Universidade Federal de Santa Catarina, Florianopolis, SC (Brazil)

    2012-10-01

    We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1 m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1 m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M{sub *} = 0.88 {+-} 0.02 M{sub Sun} and T{sub eff} = 12, 100 {+-} 140 K. These values are consistent with those derived from the optical spectra and photometric colors.

  16. Dusty Disks around White Dwarfs I: Origin of Debris Disks

    CERN Document Server

    Dong, Ruobing; Lin, D N C; Liu, X -W

    2010-01-01

    A significant fraction of the mature FGK stars have cool dusty disks at least an orders of magnitudes brighter than the solar system's outer zodiacal light. Since such dusts must be continually replenished, they are generally assumed to be the collisional fragments of residual planetesimals analogous to the Kuiper Belt objects. At least 10% of solar type stars also bear gas giant planets. The fraction of stars with known gas giants or detectable debris disks (or both) appears to increase with the stellar mass. Here, we examine the dynamical evolution of systems of long-period gas giant planets and residual planetesimals as their host stars evolve off the main sequence, lose mass, and form planetary nebula around remnant white dwarf cores. The orbits of distant gas giant planets and super-km-size planetesimals expand adiabatically. During the most intense AGB mass loss phase, sub-meter-size particles migrate toward their host stars due to the strong hydrodynamical drag by the intense stellar wind. Along their ...

  17. White dwarf pollution by planets in stellar binaries

    Science.gov (United States)

    Hamers, Adrian S.; Portegies Zwart, Simon F.

    2016-10-01

    Approximately 0.2 ± 0.2 of white dwarfs (WDs) show signs of pollution by metals, which is likely due to the accretion of tidally disrupted planetary material. Models invoking planet-planet interactions after WD formation generally cannot explain pollution at cooling times of several Gyr. We consider a scenario in which a planet is perturbed by Lidov-Kozai oscillations induced by a binary companion and exacerbated by stellar mass-loss, explaining pollution at long cooling times. Our computed accretion rates are consistent with observations assuming planetary masses between ˜0.01 and 1 MMars, although non-gravitational effects may already be important for masses ≲0.3 MMars. The fraction of polluted WDs in our simulations, ˜0.05, is consistent with observations of WDs with intermediate cooling times between ˜0.1 and 1 Gyr. For cooling times ≲0.1 Gyr and ≳1 Gyr, our scenario cannot explain the high observed pollution fractions of up to 0.7. Nevertheless, our results motivate searches for companions around polluted WDs.

  18. 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.

  19. A DOUBLE WHITE-DWARF COOLING SEQUENCE IN {omega} CENTAURI

    Energy Technology Data Exchange (ETDEWEB)

    Bellini, A.; Anderson, J. [Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 (United States); Salaris, M. [Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD (United Kingdom); Cassisi, S. [Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Collurania, via Mentore Maggini, I-64100 Teramo (Italy); Bedin, L. R. [Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, v.co dell' Osservatorio 5, I-35122, Padova (Italy); Piotto, G. [Dipartimento di Fisica e Astronomia ' Galileo Galilei' , Universita di Padova, v.co dell' Osservatorio 3, I-35122, Padova (Italy); Bergeron, P., E-mail: bellini@stsci.edu [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada)

    2013-06-01

    We have applied our empirical-point-spread-function-based photometric techniques on a large number of calibration-related WFC3/UVIS UV-B exposures of the core of {omega} Cen, and found a well-defined split in the bright part of the white-dwarf cooling sequence (WDCS). The redder sequence is more populated by a factor of {approx}2. We can explain the separation of the two sequences and their number ratio in terms of the He-normal and He-rich subpopulations that had been previously identified along the cluster main sequence. The blue WDCS is populated by the evolved stars of the He-normal component ({approx}0.55 M{sub Sun} CO-core DA objects), while the red WDCS hosts the end products of the He-rich population ({approx}0.46 M{sub Sun} objects, and {approx}10% CO-core and {approx}90% He-core WDs). The He-core WDs correspond to He-rich stars that missed the central He ignition, and we estimate their fraction by analyzing the population ratios along the cluster horizontal branch.

  20. On the white dwarf cooling sequence with extremely large telescopes

    CERN Document Server

    Bono, G; Gilmozzi, R

    2012-01-01

    We present new diagnostics of white dwarf (WD) cooling sequences and luminosity functions (LFs) in the near-infrared (NIR) bands that will exploit the sensitivity and resolution of future extremely large telescopes. The collision-induced absorption (CIA) of molecular hydrogen causes a clearly defined blue turn-off along the WD (WDBTO) cooling sequences and a bright secondary maximum in the WD LFs. These features are independent of age over a broad age range and are minimally affected by metal abundance. This means that the NIR magnitudes of the WDBTO are very promising distance indicators. The interplay between the cooling time of progressively more massive WDs and the onset of CIA causes a red turn-off along the WD (WDRTO) cooling sequences and a well defined faint peak in the WD LFs. These features are very sensitive to the cluster age, and indeed the K-band magnitude of the faint peak increases by 0.2--0.25 mag/Gyr for ages between 10 and 14 Gyr. On the other hand, the faint peak in the optical WD LF incre...

  1. The Viscous Evolution of White Dwarf Merger Remnants

    CERN Document Server

    Schwab, Josiah; Quataert, Eliot; Dan, Marius; Rosswog, Stephan

    2012-01-01

    The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multi-dimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an $\\alpha$-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al. (2011), which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towards spherical states on time-scales of hours, even though a significant fraction of the mass is initially rotationally supported. The viscous evolution unbinds only a very small amount of mass $(< 10^{-5} M_\\odot)$. Viscous heating causes some of the systems we study with He WD secondaries to reach conditions of nearly dynamical burning. It is thus possible that the post-merger viscous phase triggers detonation of...

  2. High-Energy Neutrino Emission from White Dwarf Mergers

    CERN Document Server

    Xiao, Di; Murase, Kohta; Dai, Zi-gao

    2016-01-01

    The merger of two white dwarfs is expected to result in a central fast rotating core surrounded by a debris disk, in which magnetorotational instabilities give rise to a hot magnetized corona and a magnetized outflow. The dissipation of magnetic energy via reconnection could lead to the acceleration of cosmic-rays in the expanding material, which would result in high energy neutrinos. We discuss the possibility of using these neutrino signals as probes of the outflow dynamics, magnetic energy dissipation rate and cosmic-ray acceleration efficiency. Importantly, the accompanying high-energy gamma-rays are absorbed within these sources because of the large optical depth, so these neutrino sources can be regarded as hidden cosmic-ray accelerators that are consistent with the non-detection of gamma-rays with Fermi-LAT. While the cosmic-ray generation rate is highly uncertain, if it reaches $\\sim10^{45}\\,\\rm erg\\,Mpc^{-3}\\,yr^{-1}$, the diffuse neutrino flux could contribute a substantial fraction of the IceCube o...

  3. White Dwarf Mass Estimation with X-ray Spectroscopy

    Science.gov (United States)

    Hayashi, T.

    2017-03-01

    I present X-ray spectral modeling of intermediate polars (IPs) and its application to Suzaku satellite data. The intrinsic thermal X-rays are modeled by integrating the plasma emissions of various temperatures in the post-shock accretion column (PSAC). The physical quantity distributions for the thermal spectral model is calculated from quasi-one-dimensional hydrodynamics. The PSAC calculation includes especially the dipolar geometry and variation of the specific accretion rate. The X-ray reflection from the white dwarf (WD) is modeled by a Monte Carlo simulation. In this simulation, the PSAC irradiates a cool, neutral and spherical WD with the various thermal spectra from the corresponding positions in the PSAC according to the thermal spectral model. The coherent and incoherent scattering, the photoelectric absorption, and Kα and Kβ re-emission of iron and nickel are taken into account for the photons arriving at the WD. The constructed X-ray spectral model is applied to EX Hya and V1223 Sgr, finding 0.65+0.11 –0.12 M⊙ and 0.91+0.08–0.03 M⊙, respectively. Their specific accretion rates are estimated at 0.069+0.33–0.045 g cm–2 s-1 and > 2 g cm–2 s-1, respectively, while the reflecting angles are 78.0–1.6+1.4 deg and 66.2+2.5–2.3 deg, respectively.

  4. Multi-Messenger Astronomy: White Dwarf Binaries, LISA and GAIA

    Science.gov (United States)

    Bueno, Michael; Breivik, Katelyn; Larson, Shane L.

    2017-01-01

    The discovery of gravitational waves has ushered in a new era in astronomy. The low-frequency band covered by the future LISA detector provides unprecedented opportunities for multi-messenger astronomy. With the Global Astrometric Interferometer for Astrophysics (GAIA) mission, we expect to discover about 1,000 eclipsing binary systems composed of a WD and a main sequence star - a sizeable increase from the approximately 34 currently known binaries of this type. In advance of the first GAIA data release and the launch of LISA within the next decade, we used the Binary Stellar Evolution (BSE) code simulate the evolution of White Dwarf Binaries (WDB) in a fixed galaxy population of about 196,000 sources. Our goal is to assess the detectability of a WDB by LISA and GAIA using the parameters from our population synthesis, we calculate GW strength h, and apparent GAIA magnitude G. We can then use a scale factor to make a prediction of how many multi- messenger sources we expect to be detectable by both LISA and GAIA in a galaxy the size of the Milky Way. We create binaries 10 times to ensure randomness in distance assignment and average our results. We then determined whether or not astronomical chirp is the difference between the total chirp and the GW chirp. With Astronomical chirp and simulations of mass transfer and tides, we can gather more information about the internal astrophysics of stars in ultra-compact binary systems.

  5. The structure and fate of white dwarf merger remnants

    CERN Document Server

    Dan, M; Brueggen, M; Podsiadlowski, P

    2013-01-01

    We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored and we also probe the effect of different initial conditions. We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium-rich WD achieve the conditions to trigger a detonation. In contrast, for carbon-oxygen transferring systems only the most massive mergers with a total mass above ~2.1 solar masses detonate. Even systems with lower mass may detonate long after the merger if the remnant remains above the Chandrasekhar mass and carbon is ignited at the centre. Finally, our findings are discussed in the context of several possible observed ...

  6. White dwarf pollution by planets in stellar binaries

    CERN Document Server

    Hamers, Adrian S

    2016-01-01

    Approximately $0.2 \\pm 0.2$ of white dwarfs (WDs) show signs of pollution by metals, which is likely due to the accretion of tidally disrupted planetary material. Models invoking planet-planet interactions after WD formation generally cannot explain pollution at cooling times of several Gyr. We consider a scenario in which a planet is perturbed by Lidov-Kozai oscillations induced by a binary companion and exacerbated by stellar mass loss, explaining pollution at long cooling times. Our computed accretion rates are consistent with observations assuming planetary masses between $\\sim 0.01$ and $1\\,M_\\mathrm{Mars}$, although nongravitational effects may already be important for masses $\\lesssim 0.3 \\, M_\\mathrm{Mars}$. The fraction of polluted WDs in our simulations, $\\sim 0.05$, is consistent with observations of WDs with intermediate cooling times between $\\sim 0.1$ and 1 Gyr. For cooling times $\\lesssim 0.1$ Gyr and $\\gtrsim 1$ Gyr, our scenario cannot explain the high observed pollution fractions of up to 0....

  7. Mass-radius relations for helium white dwarfs

    CERN Document Server

    Civelek, R

    1998-01-01

    In recent years, white dwarf stars are receiving increasing attension. The recent studies on the relation of M-R for the WDs are those of Wood (1990), Vennes et al. (1995) and Althaus and Benvenuto (1997, 1998). Vennes et al. (1995) computed static M-R relation for masses between 0.4 $M_\\odot$ and 0.7 $M_\\odot$ assuming non zero temperature effects. They assumed the luminosity is proportional to the mass which works for cool WDs but their results are in the range of high effective temperatures. As Althaus and Benvenuto (1997) mentioned, luminosity is not proportional to a constant for hot WD interiors because of neutrino emission. The purpose of this study is to present the effect of neutrino emission at finite temperatures. We considered fully degenerate configuration for WDs with pure helium composition to obtain static mass-radius relation for masses greater than 0.4 $M_\\odot$ with luminosity mass ratio due to neutrino energy loss.

  8. Molecular Magnetic Dichroism in Spectra of White Dwarfs

    Science.gov (United States)

    Berdyugina, S. V.; Berdyugin, A. V.; Piirola, V.

    2007-08-01

    We present novel calculations of the magnetic dichroism appearing in molecular bands in the presence of a strong magnetic field, which perturbs the internal structure of the molecule and results in net polarization due to the Paschen-Back effect. Based on that, we analyze new spectropolarimetric observations of the cool magnetic helium-rich white dwarf G99-37, which shows strongly polarized molecular bands in its spectrum. In addition to previously known molecular bands of the C2 Swan and CH A-X systems, we find a firm evidence for the violet CH B-X bands at 390 nm and C2 Deslandres-d’Azambuja bands at 360 nm. Combining the polarimetric observations with our model calculations, we deduce a dipole magnetic field of 7.5±0.5MG with the positive pole pointing towards the Earth. We conclude that the developed technique is an excellent tool for studying magnetic fields on cool magnetic stars.

  9. Models of Vortices and Spirals in White Dwarf's Accretion Binaries

    Science.gov (United States)

    Boneva, Daniela

    2010-11-01

    The main aim in the current survey is to suggest models of the development of structures, such as vortices and spirals, in accretion white dwarf's binaries. On the base of hydrodynamical analytical considerations it is applied numerical methods and simulations. It is suggested in the theoretical model the perturbation's parameters of the accretion flow, caused by the influences of the tidal wave over the flux of accretion matter around the secondary star. To examine such disturbed flow, the numerical code has involved in the calculations. The results reveal us an appearing of structure with spiral shape due to the tidal interaction in the close binaries. Our further simulations give the solution, which expresses the formation of vortical configurations in the accretion disc's zone. The evolution of vortices in areas of the flow's interaction is explored using single vortex and composite vortex models. Gas in the disc matter is considered to be compressible and non-ideal. The longevity of all these structures is different and each depends of time period of the rotation, density and velocity of the accretion matter.

  10. Type Ia supernovae from exploding oxygen-neon white dwarfs

    CERN Document Server

    Marquardt, Kai S; Ruiter, Ashley J; Seitenzahl, Ivo R; Ohlmann, Sebastian T; Kromer, Markus; Pakmor, Ruediger; Roepke, Friedrich K

    2015-01-01

    The progenitor problem of Type Ia supernovae (SNe Ia) is still unsolved. Most of these events are thought to be explosions of carbon-oxygen (CO) white dwarfs (WDs), but for many of the explosion scenarios, particularly those involving the externally triggered detonation of a sub-Chandrasekhar mass WD (sub-M Ch WD), there is also a possibility of having an oxygen-neon (ONe) WD as progenitor. We simulate detonations of ONe WDs and calculate synthetic observables from these models. The results are compared with detonations in CO WDs of similar mass and observational data of SNe Ia. We perform hydrodynamic explosion simulations of detonations in initially hydrostatic ONe WDs for a range of masses below the Chandrasekhar mass (M Ch), followed by detailed nucleosynthetic postprocessing with a 384-isotope nuclear reaction network. The results are used to calculate synthetic spectra and light curves, which are then compared with observations of SNe Ia. We also perform binary evolution calculations to determine the nu...

  11. On the possibility of a helium white dwarf donor in the presumed ultracompact binary 2S 0918-549

    CERN Document Server

    in 't Zand, J J M; Van der Sluys, M V; Verbunt, F; Pols, O R

    2005-01-01

    2S 0918-549 is a low-mass X-ray binary (LMXB) with a low optical to X-ray flux ratio. Probably it is an ultracompact binary with an orbital period shorter than 60 min. Such binaries cannot harbor hydrogen rich donor stars. As with other (sometimes confirmed) ultracompact LMXBs, 2S 0918-549 is observed to have a high neon-to-oxygen abundance ratio (Juett et al. 2001) which has been used to argue that the companion star is a CO or ONe white dwarf. However, type-I X-ray bursts have been observed from several of these systems implying the presence of hydrogen or helium on the neutron star surface. In this paper, we argue that the companion star in 2S 0918-549 is a helium white dwarf We first present a Type I X-ray burst from 2S 0918-549 with a long duration of 40 minutes. We show that this burst is naturally explained by accretion of pure helium at the inferred accretion rate of ~0.01 times the Eddington accretion rate. At higher accretion rates of ~0.1 Eddington, hydrogen is required to explain long duration bur...

  12. The ultracool dwarf DENIS-P J104814.7-395606. Chromospheres and coronae at the low-mass end of the main-sequence

    CERN Document Server

    Stelzer, B; Biazzo, K; Ercolano, B; Crespo-Chacon, I; Lopez-Santiago, J; Martinez-Arnaiz, R; Schmitt, J H M M; Rigliaco, E; Leone, F; Cupani, G

    2011-01-01

    We have obtained an XMM-Newton observation and a broad-band spectrum from the ultraviolet to the near infrared with X-Shooter for one of the nearest M9 dwarfs, DENIS-P J1048-3956 (4pc). We integrate these data by a compilation of activity parameters for ultracool dwarfs from the literature with the aim to advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Our deep XMM-Newton observation has led to the first X-ray detection of DENIS-P J1048-3956 (log Lx = 25.1) as well as the first measurement of its V band brightness (V = 17.35mag). Flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreement of DENIS-P J1048-3956 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker as judged from their lower fluxes. The observed Balmer decrement...

  13. White dwarf-main sequence binaries from LAMOST: the DR1 catalogue

    CERN Document Server

    Ren, Juanjuan; Luo, Ali; Zhao, Yongheng; Xiang, Maosheng; Liu, Xiaowei; Zhao, Gang; Jin, Ge; Zhang, Yong

    2014-01-01

    Context. White dwarf-main sequence (WDMS) binaries are used to study several different important open problems in modern astrophysics. Aims. The Sloan Digital Sky Survey (SDSS) identified the largest catalogue of WDMS binaries currently known. However, this sample is seriously affected by selection effects and the population of systems containing cool white dwarfs and early-type companions is under-represented.Here we search for WDMS binaries within the spectroscopic data release 1 of the LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) survey. LAMOST and SDSS follow different target selection algorithms. Hence, LAMOST WDMS binaries may be drawn from a different parent population and thus help in overcoming the selection effects incorporated by SDSS on the current observed population. Methods. We develop a fast and efficient routine based on the wavelet transform to identify LAMOST WDMS binaries containing a DA white dwarf and a M dwarf companion, and apply a decomposition/fitting routine to...

  14. FURTHER DEFINING SPECTRAL TYPE 'Y' AND EXPLORING THE LOW-MASS END OF THE FIELD BROWN DWARF MASS FUNCTION

    Energy Technology Data Exchange (ETDEWEB)

    Davy Kirkpatrick, J.; Gelino, Christopher R.; Griffith, Roger L.; Marsh, Kenneth A. [Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA 91125 (United States); Cushing, Michael C. [Department of Physics and Astronomy, MS 111, University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606-3328 (United States); Mace, Gregory N.; Wright, Edward L.; McLean, Ian S. [Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095-1547 (United States); Skrutskie, Michael F. [Department of Astronomy, University of Virginia, Charlottesville, VA 22904 (United States); Eisenhardt, Peter R.; Mainzer, Amanda K. [NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States); Burgasser, Adam J. [Department of Physics, University of California, San Diego, CA 92093 (United States); Tinney, C. G.; Parker, Stephen; Salter, Graeme, E-mail: davy@ipac.caltech.edu [Department of Astrophysics, School of Physics, University of New South Wales, NSW 2052 (Australia)

    2012-07-10

    We present the discovery of another seven Y dwarfs from the Wide-field Infrared Survey Explorer (WISE). Using these objects, as well as the first six WISE Y dwarf discoveries from Cushing et al., we further explore the transition between spectral types T and Y. We find that the T/Y boundary roughly coincides with the spot where the J - H colors of brown dwarfs, as predicted by models, turn back to the red. Moreover, we use preliminary trigonometric parallax measurements to show that the T/Y boundary may also correspond to the point at which the absolute H (1.6 {mu}m) and W2 (4.6 {mu}m) magnitudes plummet. We use these discoveries and their preliminary distances to place them in the larger context of the solar neighborhood. We present a table that updates the entire stellar and substellar constituency within 8 pc of the Sun, and we show that the current census has hydrogen-burning stars outnumbering brown dwarfs by roughly a factor of six. This factor will decrease with time as more brown dwarfs are identified within this volume, but unless there is a vast reservoir of cold brown dwarfs invisible to WISE, the final space density of brown dwarfs is still expected to fall well below that of stars. We also use these new Y dwarf discoveries, along with newly discovered T dwarfs from WISE, to investigate the field substellar mass function. We find that the overall space density of late-T and early-Y dwarfs matches that from simulations describing the mass function as a power law with slope -0.5 < {alpha} < 0.0; however, a power law may provide a poor fit to the observed object counts as a function of spectral type because there are tantalizing hints that the number of brown dwarfs continues to rise from late-T to early-Y. More detailed monitoring and characterization of these Y dwarfs, along with dedicated searches aimed at identifying more examples, are certainly required.

  15. First results from the MADCASH Survey: A Faint Dwarf Galaxy Companion to the Low Mass Spiral Galaxy NGC 2403 at 3.2 Mpc

    CERN Document Server

    Carlin, Jeffrey L; Price, Paul; Willman, Beth; Karunakaran, Ananthan; Spekkens, Kristine; Bell, Eric F; Brodie, Jean P; Crnojević, Denija; Forbes, Duncan A; Hargis, Jonathan; Kirby, Evan; Lupton, Robert; Peter, Annika H G; Romanowsky, Aaron J; Strader, Jay

    2016-01-01

    We report the discovery of the faintest known dwarf galaxy satellite of an LMC stellar-mass host beyond the Local Group, based on deep imaging with Subaru/Hyper Suprime-Cam. MADCASH J074238+652501-dw lies $\\sim$35 kpc in projection from NGC 2403, a dwarf spiral galaxy at $D$$\\approx$3.2 Mpc. This new dwarf has $M_{g} = -7.4\\pm0.4$ and a half-light radius of $168\\pm70$ pc, at the calculated distance of $3.39\\pm0.41$ Mpc. The color-magnitude diagram reveals no evidence of young stellar populations, suggesting that MADCASH J074238+652501-dw is an old, metal-poor dwarf similar to low luminosity dwarfs in the Local Group. The lack of either detected HI gas ($M_{\\rm HI}/L_{V} < 0.69 M_\\odot/L_\\odot$, based on Green Bank Telescope observations) or $GALEX$ NUV/FUV flux enhancement is consistent with a lack of young stars. This is the first result from the MADCASH (Magellanic Analog Dwarf Companions And Stellar Halos) survey, which is conducting a census of the stellar substructure and faint satellites in the halos...

  16. White Dwarfs in SDSS DR9 and DR10

    Science.gov (United States)

    Gentile Fusillo, Nicola Pietro; Gänsicke, Boris; Koester, Detlev

    2015-06-01

    Currently the largest catalogue of spectroscopically identified WDs is based on the 7th Data Release (DR) of the Sloan Digital Sky Survey and contains over 20000 WDs (Kleinman et al. 2013). However, only a fraction of all WDs in the photometric footprint of SDSS have been spectroscopically followed up. Using DR7 spectroscopy as a training sample, we developed a method to select high confidence photometric WD candidates. The novelty of our selection is that it allows us to assign to any object with multi-colour and proper motion data a well-defined "probability of being a white dwarf" (or a contaminant). Exploiting this selection method we compiled a catalogue (Gentile Fusillo et al. in prep) which currently covers the entire photometric footprint of SDSS, 14555sq deg, with a limiting magnitude of g ≤ 19. The catalogue contains over 20000 high-confidence WDs and WD candidates 11500 of which have not yet been followed up with Sloan spectroscopy. Even though, so far, our catalogue relies only SDSS we plan to extend the sky coverage as additional deep multi-colour large area surveys become available. DR10 includes over 1.4 million spectra taken with the new BOSS spectrograph, which improves over the original SDSS spectograph in both resolution and wavelength coverage, but has so far not been systematically mined for WD science. As part of this project, we also inspected over 8000 BOSS spectra of bright (g ≤ 19) colour selected sources and classified 1765 new WDs. We used this independent, spectroscopically confirmed sample to further validate our selection method. Finally we discuss possible application of our catalogue , focusing on the selection and follow up of 9 new DZs which show strong pollution from elements other than Ca and IR excess emission emission consistent with the presence of debris discs.

  17. Disk Accretion of Tidally Disrupted Rocky Bodies onto White Dwarfs

    Science.gov (United States)

    Feng, Wanda; Desch, Steven; Turner, Neal; Kalyaan, Anusha

    2016-06-01

    About 1/3 of white dwarfs (WDs) are polluted with heavy elements (e.g., Koester et al., 2014; Zuckerman et al., 2010) that should sediment out of their atmospheres on astronomically short timescales unless replenished by accretion from a reservoir, at rates that for many WDs must exceed ~1010 g/s (Farihi et al., 2010). Direct accretion of planetesimals is too improbable and Poynting-Robertson drag of dust is too slow (due to the low luminosity of WDs) (Jura, 2003), so it is often assumed that WDs accrete from a disk of gas and solid particles, fed by tidal disruption of planeteismals inside the WD Roche limit (e.g. Debes et al., 2012; Rafikov, 2011a, 2011b). A few such gaseous disks have been directly observed, through emission from Ca II atoms in the disk (e.g. Manser et al., 2016; Wilson et al. 2014). Models successfully explain the accretion rates of metals onto the WD, provided the gaseous disk viscously spreads at rates consistent with a partially suppressed magnetorotational instability (Rafikov, 2011a, 2011b). However, these models currently do not explore the likely extent of the magnetorotational instability in disks by calculating the degree of ionization, or suppression by strong magnetic field.We present a 1-D model of a gaseous WD disk accretion, to assess the extent of the magnetorotational instability in WD disks. The composition of the disk, the ionization and recombination mechanisms, and the degree of ionization of the disk are explored. Magnetic field strengths consistent with WD dipolar magnetic fields are assumed. Elsasser numbers are calculated as a function of radius in the WD disk. The rate of viscous spreading is calculated, and the model of Rafikov (2011a, 2011b) updated to compute likely accretion rates of metals onto WDs.

  18. Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey

    CERN Document Server

    Harris, H C; Gyuk, G; Subba-Rao, M; Anderson, S F; Hall, P B; Munn, J A; Liebert, J; Knapp, G R; Bizyaev, D; Malanushenko, E; Malanushenko, V; Pan, K; Schneider, D P; Smith, J A

    2008-01-01

    We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system -- its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310-00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk.

  19. A second case of outbursts in a pulsating white dwarf observed by Kepler

    CERN Document Server

    Hermes, J J; Bell, Keaton J; Chote, P; Gaensicke, B T; Kawaler, Steven D; Clemens, J C; Dunlap, B H; Winget, D E; Armstrong, D J

    2015-01-01

    We present observations of a new phenomenon in pulsating white dwarf stars: large-amplitude outbursts at timescales much longer than the pulsation periods. The cool (Teff = 11,010 K), hydrogen-atmosphere pulsating white dwarf PG 1149+057 was observed nearly continuously for more than 78.8 d by the extended Kepler mission in K2 Campaign 1. The target showed 10 outburst events, recurring roughly every 8 d and lasting roughly 15 hr, with maximum flux excursions up to 45% in the Kepler bandpass. We demonstrate that the outbursts affect the pulsations and therefore must come from the white dwarf. Additionally, we argue that these events are not magnetic reconnection flares, and are most likely connected to the stellar pulsations and the relatively deep surface convection zone. PG 1149+057 is now the second cool pulsating white dwarf to show this outburst phenomenon, after the first variable white dwarf observed in the Kepler mission, KIC 4552982. Both stars have the same effective temperature, within the uncertain...

  20. Evolutionary calculations of carbon dredge-up in helium envelope white dwarfs

    CERN Document Server

    MacDonald, J; José, J; Donald, James Mac; Hernanz, Margarita; José, Jordi; Physics, Department of; Delaware, University of; Catalunya, Institut d'Estudis Espacials de; Unit, CSIC Research; Nexus-201, Edifici; Nuclear, Departament de Fí sica i Enginyeria; Balaguer, Avda. Ví ctor

    1998-01-01

    We investigate the evolution of cooling helium atmosphere white dwarfs using a full evolutionary code, specifically developed for following the effects of element diffusion and gravitational settling on white dwarf cooling. The major difference between this work and previous work is that we use more recent opacity data from the OPAL project. Since, in general, these opacities are higher than those available ten years ago, at a given effective temperature, convection zones go deeper than in models with older opacity data. Thus convective dredge-up of observationally detectable carbon in helium atmosphere white dwarfs can occur for thicker helium layers than found by Pelletier et al (1986). We find that the range of observed C to He ratios in different DQ white dwarfs of similar effective temperature is well explained by a range of initial helium layer mass between $10^{-3}$ and $10^{-2} M_{\\odot}$, in good agreement with stellar evolution theory, assuming a typical white dwarf mass of $0.6 M_{\\odot}$. We also ...