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Sample records for low-mass white dwarf

  1. The origin of low-mass white dwarfs

    SciTech Connect

    Rebassa-Mansergas, A.; Schreiber, M. R.; Gaensicke, B. T.; Girven, J.; Gomez-Moran, A. Nebot

    2010-11-23

    We present white dwarf mass distributions of a large sample of post common-envelope binaries and wide white dwarf main sequence binaries and demonstrate that these distributions are statistically independent. While the former contains a much larger fraction of low-mass white dwarfs, the latter is similar to single white dwarf mass distributions. Taking into account observational biases we also show that the majority of low-mass white dwarfs are formed in close binaries.

  2. Radius constraints from high-speed photometry of 20 low-mass white dwarf binaries

    SciTech Connect

    Hermes, J. J.; Brown, Warren R.; Kilic, Mukremin; Gianninas, A.; Chote, Paul; Sullivan, D. J.; Winget, D. E.; Bell, Keaton J.; Falcon, R. E.; Winget, K. I.; Harrold, Samuel T.; Montgomery, M. H.; Mason, Paul A.

    2014-09-01

    We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods of less than four hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here. We use these observations to place empirical constraints on the mass-radius relationship for extremely low-mass (≤0.30 M {sub ☉}) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms their high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.

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

    NASA Astrophysics Data System (ADS)

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

    NASA Astrophysics Data System (ADS)

    Córsico, A. H.; Romero, A. D.; Althaus, L. G.; Hermes, J. J.

    2012-11-01

    Context. In recent years, many low-mass (≲ 0.45 M⊙) white dwarf stars expected to harbor He cores have been detected in the field of the Milky Way and in several galactic globular and open clusters. Until recently, no objects of this kind showed pulsations. This situation has changed recently with the exciting discovery of SDSS J184037.78+642312.3, the first pulsating low-mass white dwarf star. Aims: Motivated by this extremely important finding, and in view of the very valuable asteroseismological potential of these objects, we present here a detailed pulsational study applied to low-mass He-core white dwarfs with masses ranging from 0.17 to 0.46 M⊙, based on full evolutionary models representative of these objects. This study is aimed to provide a theoretical basis from which to interpret future observations of variable low-mass white dwarfs. Methods: The background stellar models on which our pulsational analysis was carried out were derived by taking into account the complete evolutionary history of the progenitor stars, with special emphasis on the diffusion processes acting during the white dwarf cooling phase. We computed nonradial g-modes to assess the dependence of the pulsational properties of these objects with stellar parameters such as the stellar mass and the effective temperature, and also with element diffusion processes. We also performed a g- and p-mode pulsational stability analysis on our models and found well-defined blue edges of the instability domain, where these stars should start to exhibit pulsations. Results: We found substantial differences in the seismic properties of white dwarfs with M∗ ≳ 0.20 M⊙ and the extremely low-mass (ELM) white dwarfs (M∗ ≲ 0.20 M⊙). Specifically, g-mode pulsation modes in ELM white dwarfs mainly probe the core regions and are not dramatically affected by mode-trapping effects by the He/H interface, whereas the opposite is true for more massive He-core white dwarfs. We found that element

  5. LP 400-22, A Very Low Mass and High-Velocity White Dwarf

    NASA Technical Reports Server (NTRS)

    Kawka, Adela; Vennes, Stephane; Oswalt, Terry D.; Smith, J. Allyn; Silvestri, Nicole M.

    2006-01-01

    We report the identification of LP 400-22 (WD 2234+222) as a very low mass and high-velocity white dwarf. The ultraviolet GALEX and optical photometric colors and a spectral line analysis of LP 400-22 show this star to have an effective temperature of 11,080+/-140 K and a surface gravity of log g = 6.32 +/-0.08. Therefore, this is a helium-core white dwarf with a mass of 0.17 M,. The tangential velocity of this white dwarf is 414+/-43 km/s, making it one of the fastest moving white dwarfs known. We discuss probable evolutionary scenarios for this remarkable object.

  6. DISCOVERY OF A BRIGHT, EXTREMELY LOW MASS WHITE DWARF IN A CLOSE DOUBLE DEGENERATE SYSTEM

    SciTech Connect

    Vennes, S.; Kawka, A.; Nemeth, P.; Thorstensen, J. R.; Skinner, J. N.; Pigulski, A.; Steslicki, M.; Kolaczkowski, Z.; Srodka, P.

    2011-08-10

    We report the discovery of a bright (V {approx} 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{sub sun} and is the primary component of a close double degenerate system (P = 0.246137 days, K{sub 1} = 288 km s{sup -1}) comprising a fainter white dwarf secondary with M{sub 2} {approx} 0.9 M{sub sun}. Light curves phased with the orbital ephemeris show evidence of relativistic beaming and weaker ellipsoidal variations. The light curves also reveal secondary eclipses (depth {approx}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 sun), while the normal kinematics suggest a relatively recent formation history. Close binary evolutionary scenarios suggest that extremely low mass white dwarfs form via a common-envelope phase and possible Roche lobe overflow.

  7. Recent Advances in the Theoretical Modeling of Pulsating Low-mass He-core White Dwarfs

    NASA Astrophysics Data System (ADS)

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

    2017-03-01

    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.

  8. A SPITZER SEARCH FOR SUBSTELLAR COMPANIONS TO LOW-MASS WHITE DWARFS

    SciTech Connect

    Kilic, Mukremin; Brown, Warren R.; McLeod, B.

    2010-01-01

    The formation scenarios for single low-mass (M < 0.45 M{sub sun}) white dwarfs (WDs) include enhanced mass loss from a metal-rich progenitor star or a common envelope phase of a solar-like star with a close-in massive planet or a brown dwarf. Both scenarios suggest that low-mass WDs may have planets. Here, we present a Spitzer IRAC search for substellar and planetary mass companions to 14 low-mass WDs. One of our targets, HS 1653+7753, displays near- and mid-infrared flux excess. However, follow-up MMT observations show that this excess is due to a nearby resolved source, which is mostly likely a background object. Another target, PG 2257+162, shows flux excess compatible with a late-type stellar companion. We do not detect substellar companions to any of the remaining targets. In addition, eight of these stars do not show any radial velocity variations, ruling out stellar mass companions including other WDs. We conclude that a significant fraction of the low-mass WDs in our sample do not have stellar or massive brown dwarf companions.

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

    NASA Astrophysics Data System (ADS)

    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.

  10. VizieR Online Data Catalog: Low-mass helium white dwarfs evolutionary models (Istrate+, 2016)

    NASA Astrophysics Data System (ADS)

    Istrate, A.; Marchant, P.; Tauris, T. M.; Langer, N.; Stancliffe, R. J.; Grassitelli, L.

    2016-07-01

    Evolutionary models of low-mass helium white dwarfs including element diffusion and rotational mixing. The WDs are produced considering binary evolution through the LMXB channel, with final WDs masses between ~0.16-~0.44. The models are computed using MESA, for different metallicities: Z=0.02, 0.01, 0.001 and 0.0002. For each metallicity, the models are divided in three categories: (1) basic (no diffusion nor rotation are considered) (2) diffusion (element diffusion is considered) (3) rotation+diffusion (both element diffusion and rotational mixing are considered) (4 data files).

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

    SciTech Connect

    Nomoto, K.

    1986-01-01

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

  12. Two new pulsating low-mass pre-white dwarfs or SX Phoenicis stars?

    NASA Astrophysics Data System (ADS)

    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.

  13. A RADIO SEARCH FOR PULSAR COMPANIONS TO SLOAN DIGITAL SKY SURVEY LOW-MASS WHITE DWARFS

    SciTech Connect

    Agueeros, Marcel A.; Camilo, Fernando; Silvestri, Nicole M.; Anderson, Scott F.; Kleinman, S. J.; Liebert, James W.

    2009-05-20

    We have conducted a search for pulsar companions to 15 low-mass white dwarfs (LMWDs; M <0.4 M {sub 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 white dwarf (WD) or a neutron star (NS) as a companion. Indeed, evolutionary models of low-mass X-ray binaries, the precursors of millisecond pulsars (MSPs), produce significant numbers of LMWDs, suggesting that the SDSS LMWDs may have NS companions. No convincing pulsar signal is detected in our data. This is consistent with the findings of van Leeuwen et al., 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 'ordinary' WDs). We discuss the constraints our nondetections place on the probability P {sub MSP} that the companion to a given LMWD is a radio pulsar in the context of the luminosity and acceleration limits of our search; we find that P {sub MSP} < 10{sup +4} {sub -2}%.

  14. Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution

    NASA Astrophysics Data System (ADS)

    Althaus, L. G.; Córsico, A. H.; Gautschy, A.; Han, Z.; Serenelli, A. M.; Panei, J. A.

    2004-01-01

    The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we follow the evolution of a 0.33-Msolar white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. The evolutionary stages prior to the white dwarf formation are also fully accounted for by computing the conservative binary evolution of an initially 2.5-Msolar Population I star with a 1.25-Msolar companion, and with period Pi= 3 d. Evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. We find that chemical diffusion induces the occurrence of an additional hydrogen thermonuclear flash, which leads to stellar models with thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced stages of evolution. In addition, we explore the adiabatic pulsational properties of the resulting white dwarf models. As compared with their helium-core counterparts, low-mass oxygen-core white dwarfs are characterized by a pulsational spectrum much more featured, an aspect which could eventually be used for distinguishing both types of stars, if low-mass white dwarfs were in fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass white dwarf models.

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    We present the discovery of 15 extremely low-mass (5\\lt {log}g\\lt 7) white dwarf (WD) candidates, 9 of which are in ultra-compact double-degenerate binaries. Our targeted extremely low-mass 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 M⊙ mean and 0.25 M⊙ dispersion. Thus extremely low-mass WDs are found in binaries with a typical mass ratio of 1:4. Statistically speaking, 95% of the WD 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 WDs and stable mass transfer AM CVn binaries. Based on observations obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.

  16. 3D MODEL ATMOSPHERES FOR EXTREMELY LOW-MASS WHITE DWARFS

    SciTech Connect

    Tremblay, P.-E.; Gianninas, A.; Kilic, M.; Ludwig, H.-G.; Steffen, M.; Freytag, B.; Hermes, J. J.

    2015-08-20

    We present an extended grid of mean three-dimensional (3D) spectra for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD radiation-hydrodynamics 3D simulations covering T{sub eff} = 6000–11,500 K and log g = 5–6.5 (g in cm s{sup −2}) to derive analytical functions to convert spectroscopically determined 1D temperatures and surface gravities to 3D atmospheric parameters. Along with the previously published 3D models, the 1D to 3D corrections are now available for essentially all known convective DA WDs (i.e., log g = 5–9). For low-mass WDs, the correction in temperature is relatively small (a few percent at the most), but the surface gravities measured from the 3D models are lower by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the discrepancies seen in the radius and mass measurements for relatively cool ELM WDs in eclipsing double WD and WD + millisecond pulsar binary systems. We also use the 3D corrections to revise the boundaries of the ZZ Ceti instability strip, including the recently found ELM pulsators.

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

    SciTech Connect

    Córsico, A. H.; Althaus, L. G.

    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.

  18. Formation of millisecond pulsars with low-mass helium white dwarf companions in very compact binaries

    SciTech Connect

    Jia, Kun; Li, X.-D.

    2014-08-20

    Binary millisecond pulsars (BMSPs) are thought to have evolved from low-mass X-ray binaries (LMXBs). If the mass transfer in LMXBs is driven by nuclear evolution of the donor star, the final orbital period is predicted to be well correlated with the mass of the white dwarf (WD), which is the degenerate He core of the donor. Here we show that this relation can be extended to very small WD mass (∼0.14-0.17 M {sub ☉}) and narrow orbital period (about a few hours), depending mainly on the metallicities of the donor stars. There is also discontinuity in the relation, which is due to the temporary contraction of the donor when the H-burning shell crosses the hydrogen discontinuity. BMSPs with low-mass He WD companions in very compact binaries can be accounted for if the progenitor binary experienced very late Case A mass transfer. The WD companion of PSR J1738+0333 is likely to evolve from a Pop II star. For PSR J0348+0432, to explain its extreme compact orbit in the Roche-lobe-decoupling phase, even lower metallicity (Z = 0.0001) is required.

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

    NASA Astrophysics Data System (ADS)

    Córsico, A. H.; Althaus, L. G.; Serenelli, A. M.; Kepler, S. O.; Jeffery, C. S.; Corti, M. A.

    2016-04-01

    Context. Many low-mass (M⋆/M⊙ ≲ 0.45) and extremely low-mass (ELM, M⋆/M⊙ ≲ 0.18-0.20) white-dwarf stars are currently being found in the field of the Milky Way. Some of these stars exhibit long-period gravity-mode (g-mode) pulsations, and constitute the class of pulsating white dwarfs called ELMV stars. In addition, 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. Aims: Motivated by this finding, we present a detailed nonadiabatic pulsation study of such stars, employing full evolutionary sequences of low-mass He-core pre-white dwarf models. Methods: Our pulsation stability analysis is based on a set of low-mass He-core pre-white dwarf models with masses ranging from 0.1554 to 0.2724 M⊙, which were derived by computing the nonconservative evolution of a binary system consisting of an initially 1 M⊙ ZAMS star and a 1.4 M⊙ neutron star companion. We have considered models in which element diffusion is accounted for and also models in which it is neglected. Results: We confirm and explore in detail a new instability strip in the domain of low gravities and low effective temperatures of the Teff - 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 κ - γ 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 are unable to explain the pulsations observed in the two known pulsating pre-white dwarfs, suggesting that element diffusion might be inhibited at these stages of the pre-white dwarf evolution. Our nonadiabatic models without diffusion, on the other hand, naturally explain the existence and range of

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

    NASA Astrophysics Data System (ADS)

    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

  1. sdA in SDSS DR12 are Overwhelmingly Not Extremely Low-Mass (ELM) White Dwarfs

    NASA Astrophysics Data System (ADS)

    Hermes, J. J.; Gänsicke, B. T.; Breedt, E.

    2017-03-01

    In a search for new white dwarfs in DR12 of the Sloan Digital Sky Survey, Kepler et al. 2016 found atmospheric parameters for thousands of objects with effective temperatures below 20,000 K and surface gravities between 5.5 < log g < 6.5. They classified these objects as cool subdwarfs – sdA – and speculated that many may be extremely low-mass (ELM) white dwarfs (helium-core white dwarfs with masses below 0.3 M⊙). We present evidence – using radial velocities, photometric colors, and reduced proper motions – that the vast majority (>99%) of these objects are unlikely to be ELM white dwarfs. Their true identity remains an interesting question.

  2. THE MASS DISTRIBUTION OF COMPANIONS TO LOW-MASS WHITE DWARFS

    SciTech Connect

    Andrews, Jeff J.; Price-Whelan, Adrian M.; Agüeros, Marcel A.

    2014-12-20

    Measuring the masses of companions to single-line spectroscopic binary stars is (in general) not possible because of the unknown orbital plane inclination. Even when the mass of the visible star can be measured, only a lower limit can be placed on the mass of the unseen companion. However, since these inclination angles should be isotropically distributed, for a large enough, unbiased sample, the companion mass distribution can be deconvolved from the distribution of observables. In this work, we construct a hierarchical probabilistic model to infer properties of unseen companion stars given observations of the orbital period and projected radial velocity of the primary star. We apply this model to three mock samples of low-mass white dwarfs (LMWDs; M ≲ 0.45 M {sub ☉}) and a sample of post-common-envelope binaries. We use a mixture of two Gaussians to model the WD and neutron star (NS) companion mass distributions. Our model successfully recovers the initial parameters of these test data sets. We then apply our model to 55 WDs in the extremely low-mass (ELM) WD Survey. Our maximum a posteriori model for the WD companion population has a mean mass μ{sub WD} = 0.74 M {sub ☉}, with a standard deviation σ{sub WD} = 0.24 M {sub ☉}. Our model constrains the NS companion fraction f {sub NS} to be <16% at 68% confidence. We make samples from the posterior distribution publicly available so that future observational efforts may compute the NS probability for newly discovered LMWDs.

  3. THE ELM SURVEY. III. A SUCCESSFUL TARGETED SURVEY FOR EXTREMELY LOW MASS WHITE DWARFS

    SciTech Connect

    Brown, Warren R.; Kenyon, Scott J.; Kilic, Mukremin; Allende Prieto, Carlos E-mail: skenyon@cfa.harvard.edu E-mail: callende@iac.es

    2012-01-10

    Extremely low mass (ELM) white dwarfs (WDs) with masses < 0.25 M{sub Sun} are rare objects that result from compact binary evolution. Here, we present a targeted spectroscopic survey of ELM WD candidates selected by color. The survey is 71% complete and has uncovered 18 new ELM WDs. Of the seven ELM WDs with follow-up observations, six are short-period binaries and four have merger times less than 5 Gyr. The most intriguing object, J1741+6526, likely has either a pulsar companion or a massive WD companion making the system a possible supernova Type Ia or an Ia progenitor. The overall ELM survey has now identified 19 double degenerate binaries with <10 Gyr merger times. The significant absence of short orbital period ELM WDs at cool temperatures suggests that common envelope evolution creates ELM WDs directly in short period systems. At least one-third of the merging systems are halo objects, thus ELM WD binaries continue to form and merge in both the disk and the halo.

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

    NASA Astrophysics Data System (ADS)

    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).

  5. THE ELM SURVEY. I. A COMPLETE SAMPLE OF EXTREMELY LOW-MASS WHITE DWARFS

    SciTech Connect

    Brown, Warren R.; Kilic, Mukremin; Kenyon, Scott J.; Prieto, Carlos Allende E-mail: mkilic@cfa.harvard.ed E-mail: callende@iac.e

    2010-11-10

    We analyze radial velocity observations of the 12 extremely low-mass (ELM), with {<=}0.25 M{sub sun}, white dwarfs (WDs) in the MMT Hypervelocity Star Survey. Eleven of the twelve WDs are binaries with orbital periods shorter than 14 hr; the one non-variable WD is possibly a pole-on system among our non-kinematically selected targets. Our sample is unique: it is complete in a well-defined range of apparent magnitude and color. The orbital mass functions imply that the unseen companions are most likely other WDs, although neutron star companions cannot be excluded. Six of the eleven systems with orbital solutions will merge within a Hubble time due to the loss of angular momentum through gravitational wave radiation. The quickest merger is J0923+3028, a g = 15.7 ELM WD binary with a 1.08 hr orbital period and a {<=}130 Myr merger time. The chance of a supernova Ia event among our ELM WDs is only 1%-7%, however. Three binary systems (J0755+4906, J1233+1602, and J2119-0018) have extreme mass ratios and will most likely form stable mass-transfer AM CVn systems. Two of these objects, SDSS J1233+1602 and J2119-0018, are the lowest surface gravity WDs ever found; both show Ca II absorption likely from accretion of circumbinary material. We predict that at least one of our WDs is an eclipsing detached double WD system, important for constraining helium core WD models.

  6. The ELM Survey. I. A Complete Sample of Extremely Low-mass White Dwarfs

    NASA Astrophysics Data System (ADS)

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

    2010-11-01

    We analyze radial velocity observations of the 12 extremely low-mass (ELM), with <=0.25 M sun, white dwarfs (WDs) in the MMT Hypervelocity Star Survey. Eleven of the twelve WDs are binaries with orbital periods shorter than 14 hr; the one non-variable WD is possibly a pole-on system among our non-kinematically selected targets. Our sample is unique: it is complete in a well-defined range of apparent magnitude and color. The orbital mass functions imply that the unseen companions are most likely other WDs, although neutron star companions cannot be excluded. Six of the eleven systems with orbital solutions will merge within a Hubble time due to the loss of angular momentum through gravitational wave radiation. The quickest merger is J0923+3028, a g = 15.7 ELM WD binary with a 1.08 hr orbital period and a <=130 Myr merger time. The chance of a supernova Ia event among our ELM WDs is only 1%-7%, however. Three binary systems (J0755+4906, J1233+1602, and J2119-0018) have extreme mass ratios and will most likely form stable mass-transfer AM CVn systems. Two of these objects, SDSS J1233+1602 and J2119-0018, are the lowest surface gravity WDs ever found; both show Ca II absorption likely from accretion of circumbinary material. We predict that at least one of our WDs is an eclipsing detached double WD system, important for constraining helium core WD models. Based on observations obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.

  7. Discovery of four new low-mass white-dwarf companions in the Kepler data

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    We report the discovery of four new 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. These add to the 6 Kepler, and 19 non-Kepler, previously known short-period eclipsing dA+WD binaries.The discoveries were made through searching the light curves of bright Kepler stars for BEaming, Ellipsoidal and Reflection (BEER) modulations that are consistent with a compact companion, using the BEER search algorithm. This was followed by inspection of the search top hits, looking for eclipsing systems with a secondary eclipse deeper than the primary one, as expected for a WD that is hotter than the primary star. Follow-up spectroscopic radial-velocity (RV) observations confirmed the binarity of the systems. We derive the systems' parameters through analyses of the light curves' eclipses and phase modulations, combined with RV orbital solutions and stellar evolution models.The four systems' orbital periods of 1.17-3.82 days and WD masses of 0.19-0.22 Msun are similar to those reported for the previously known systems. These values are consistent with evolution models of such systems, that undergo a stable mass transfer from the WD progenitor to the current A star.For KIC 4169521 we derive a bloated WD radius of 0.09 Rsun that is well within the WD radius range of 0.04-0.43 Rsun of the known systems. For the remaining three systems we report WD radii of 0.026-0.035 Rsun, the smallest WD radii derived so far for short-period eclipsing dA+WD binaries.As suggested before, the previously known systems, together with KIC 4169521, all with hot and bloated WD secondaries, represent young systems probably at a proto-WD, or initial WD cooling track stage. The other three new systems - KOI-3818, KIC 2851474, and KIC 9285587, are probably positioned further along the WD cooling track, and extend the known population to older systems with cooler

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

    NASA Astrophysics Data System (ADS)

    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).

  9. Gravitational waves, pulsations, and more : high-speed photometry of low-mass, He-core white dwarfs

    NASA Astrophysics Data System (ADS)

    Hermes, J. J.

    2013-08-01

    This dissertation is an observational exploration of the exciting physics that can be enabled by high-speed photometric monitoring of extremely low-mass (< 0.25 Msun) white dwarf stars, which are found in some of the most compact binaries known. It includes the cleanest indirect detection of gravitational waves at visible wavelengths, the discovery of pulsations in He-core WDs, the strongest evidence for excited p-mode pulsations in a WD, the discovery of the first tidally distorted WDs and their use to constrain the low-end of the WD mass-radius relationship, and the strongest cases of Doppler beaming observed in a binary system. It is the result of the more than 220 nights spent at McDonald Observatory doing high-speed photometry with the Argos instrument on the 2.1 m Otto Struve telescope, which has led to a number of additional exciting results, including the discovery of an intermediate timescale in the evolution of cooling DA WDs and the discovery of the most massive pulsating WD, which should have an ONe-core and should be highly crystallized.

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

    NASA Astrophysics Data System (ADS)

    Istrate, A. G.; Marchant, P.; Tauris, T. M.; Langer, N.; Stancliffe, R. J.; Grassitelli, L.

    2016-10-01

    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 of Z = 0.02, 0.01, 0.001 and 0.0002, producing WDs with masses between 0.16-0.45 M⊙. Our results confirm that element diffusion plays a significant role in the evolution of proto-WDs that experience hydrogen shell flashes. The occurrence of these flashes produces a clear dichotomy in the cooling timescales of ELM WDs, which has important consequences e.g. for the age determination of binary millisecond pulsars. In addition, we confirm that the threshold mass at which this dichotomy occurs depends on metallicity. Rotational mixing is found to counteract the effect of gravitational settling in the surface layers of young, bloated ELM proto-WDs and therefore plays a key role in determining their surface chemical abundances, i.e. the observed presence of metals in their atmospheres. We predict that these proto-WDs have helium-rich envelopes through a significant part of their lifetime. This is of great importance as helium is a crucial ingredient in the driving of the κ-mechanism suggested for the newly observed ELM proto-WD pulsators. However, we find that the number of hydrogen shell flashes and, as a result, the hydrogen envelope mass at the beginning of

  11. Evolutionary sequences of very hot, low-mass, accreting white dwarfs with application to symbiotic variables and ultrasoft/supersoft low-luminosity x-ray sources

    NASA Technical Reports Server (NTRS)

    Sion, Edward M.; Starrfield, Sumner G.

    1994-01-01

    We present the first detailed model results of quasi-static evolutionary sequences of very hot low-mass white dwarfs accreting hydrogen-rich material at rates between 1 x 10(exp -7) and 1 x 10(exp -9) solar mass/yr. Most of the sequences were generated from starting models whose core thermal structures were not thermally relaxed in the thermal pulse cycle-averaged sense of an asymptotic giant branch stellar core. Hence, the evolution at constant accretion rate was not invariably characterized by series of identical shell flashes. Sequences exhibiting stable steady state nuclear burning at the accretion supply rate as well as sequences exhibiting recurrent thermonuclear shell flashes are presented and discussed. In some cases, the white dwarf accretors remain small (less than 10(exp 11) cm) and very hot even during the shell flash episode. They then experience continued but reduced hydrogen shell burning during the longer quiescent intervals while their surface temperatures increase both because of compressional heating and envelope structure readjustment in response to accretion over thousands of years. Both accretion and continued hydrogen burning power these models with luminosities of a few times 10(exp 37) ergs/s. We suggest that the physical properties of these model sequences are of considerable relevance to the observed outburst and quiescent behavior of those symbiotic variables and symbiotic novae containing low-mass white dwarfs. We also suggest that our models are relevant to the observational characteristics of the growing class of low-luminosity, supersoft/ultrasoft X-ray sources in globular clusters, and the Magellanic Clouds.

  12. Astrometric Binaries: White Dwarfs?

    NASA Astrophysics Data System (ADS)

    Oliversen, Nancy A.

    We propose to observe a selection of astrometric or spectroscopicastrometric binaries nearer than about 20 pc with unseen low mass companions. Systems of this type are important for determining the luminosity function of low mass stars (white dwarfs and very late main sequence M stars), and their contribution to the total mass of the galaxy. Systems of this type are also important because the low mass, invisible companions are potential candidates in the search for planets. Our target list is selected primarily from the list of 31 astrometric binaries near the sun by Lippincott (1978, Space Sci. Rev., 22, 153), with additional candidates from recent observations by Kamper. The elimination of stars with previous IUE observations, red companions resolved by infrared speckle interferometry, or primaries later than M1 (because if white dwarf companions are present they should have been detected in the visible region) reduces the list to 5 targets which need further information. IUE SWP low dispersion observations of these targets will show clearly whether the remaining unseen companions are white dwarfs, thus eliminating very cool main sequence stars or planets. This is also important in providing complete statistical information about the nearest stars. The discovery of a white dwarf in such a nearby system would provide important additional information about the masses of white dwarfs. Recent results by Greenstein (1986, A. J., 92, 859) from binary systems containing white dwarfs imply that 80% of such systems are as yet undetected. The preference of binaries for companions of approximately equal mass makes the Lippincott-Kamper list of A through K primaries with unseen companions a good one to use to search for white dwarfs. The mass and light dominance of the current primary over the white dwarf in the visible makes ultraviolet observations essential to obtain an accurate census of white dwarf binaries.

  13. The Instability Strip of ZZ Ceti White Dwarfs and Its Extension to the Extremely Low Mass Pulsators

    NASA Astrophysics Data System (ADS)

    Van Grootel, V.; Fontaine, G.; Brassard, P.; Dupret, M.-A.

    2013-12-01

    The determination of the location of the theoretical ZZ Ceti instability strip in the log g - Teff diagram has remained a challenge over the years, due to the lack of a suitable treatment for convection in these stars. We report here a detailed stability survey over the whole ZZ Ceti regime, including the very low masses where three pulsators have recently been found. With this in mind, we computed twenty-nine evolutionary sequences of DA models with various masses and chemical layering. These models are characterized by the so-called ML2/α = 1.0 convective efficiency and take into account the important feedback effect of convection on the atmospheric structure. We computed power spectra for these models with the Liège nonadiabatic pulsation code MAD, which is the only one to conveniently incorporate a full time-dependent convection treatment and, thus, provides the best available description of the blue edge of the instability strip. On the other hand, given the failure of all nonadiabatic codes to properly account for the red edge of the strip, including MAD, we tested the idea that the red edge is due to energy leakage through the atmosphere. Using this approach, we found that our theoretical ZZ Ceti instability strip accounts remarkably well for the boundaries of the empirical strip.

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

    NASA Astrophysics Data System (ADS)

    Hermes, J. J.; Gänsicke, B. T.; 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.

    2014-10-01

    Using the Hubble Space Telescope, we detail the first abundance analysis enabled by far-ultraviolet spectroscopy of a low-mass (≃0.19 M⊙) white dwarf (WD), GALEX J1717+6757, which is in a 5.9-h 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 many but not all of the elements present in the atmosphere of this WD; radiative levitation cannot, on its own, explain all of the observed abundance patterns, and additional mechanisms such as rotational mixing may be required. Finally, we detect both primary and secondary eclipses using ULTRACAM high-speed photometry, which we use to constrain the low-mass WD radius and rotation rate as well as update the ephemeris from the discovery observations of this WD+WD binary.

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

    SciTech Connect

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

    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.

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

    NASA Astrophysics Data System (ADS)

    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.

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

    SciTech Connect

    Gianninas, A.; Barber, Sara D.; Kilic, Mukremin; Hermes, J. J.; Harrold, Samuel T.; Brown, Warren R.; Kenyon, Scott J.; Dufour, P.

    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.

  18. Very Low-Mass Stars and Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Rebolo, Rafael; Rosa Zapatero-Osorio, Maria

    2001-02-01

    Part I. Searches in Clusters, Stellar Associations and the Field: 1. Open clusters after HIPPARCOS J. S. Mermilliod; 2. Proper motions of very low mass stars and brown dwarfs in open clusters N. C. Hambly; 3. Parallaxes for brown dwarfs in clusters C. G. Tinney; 4. Very low mass stars and brown dwarfs in the Belt of Orion S. J. Wolk and F. M. Walter; 5. Photometric surveys in open clusters M. R. Zapatero Osorio; 6. The mass function of the Pleiades R. F. Jameson et al.; 7. Brown dwarfs and the low-mass initial mass function in young clusters K. L. Luhman; 8. Very low mass stars in globular clusters I. R. King and G. Piotto; 9. The DENIS very low mass star and brown dwarf results X. Delfosse and T. Forveille; 10. Preliminary results from the 2MASS core project J. Liebert et al.; Part II. Spectroscopic Properties, Fundamental Parameters and Modelling: 11. Properties of M dwarfs in clusters and the field S. L. Hawley et al.; 12. Spectroscopy of very low mass stars and brown dwarfs in young clusters E. L. Martin; 13. High resolution spectra of L type stars and brown dwarfs G. Basri et al.; 14. Modelling very low mass stars and brown dwarf atmospheres F. Allard; 15. Dust in very cool dwarfs T. Tsuji; 16. On the interpretation of the optical spectra of very cool dwarfs Ya. V. Pavlenko; 17. Absolute dimensions for M type dwarfs A. Gimenez; 18. Theory of very low mass stars and brown dwarfs I. Baraffe; Part III. Convection, Rotation and Acitivity: 19. Convection in low mass stars F. D'Antona; 20. Rotation law and magnetic field in M dwarf models G. Rudiger and M. Kuker; 21. Doppler imaging of cool dwarf stars K. G. Strassmeier; 22. X-ray Emission from cool dwarfs in clusters S. Randich; 23. X-ray variability for dM stars G. Micela and A. Marino; 24. The coronae of AD Leo and EV Lac S. Sciortino et al.; 25. Prospects of vuture X-ray missions for low mass stars and cluster stars R. Pallavicini.

  19. White Dwarf Mass Distribution

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Koester, D.; Romero, A. D.; Ourique, G.; Pelisoli, I.

    2017-03-01

    We present the mass distribution for all S/N ≥ 15 DA white dwarfs detected in the Sloan Digital Sky Survey up to Data Release 12 in 2015, fitted with Koester models for ML2/α=0.8 (Teff≥ 10000 K), and for DBs with S/N ≥ 10, fitted with ML2/α=1.25, for Teff >16 000 K. These mass distributions are for logg≥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 M ⊙ but very distinct shapes.

  20. Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low-Mass White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kalirai, Jasonjot S.; Bergeron, P.; Hansen, Brad M. S.; Kelson, Daniel D.; Reitzel, David B.; Rich, R. Michael; Richer, Harvey B.

    2007-12-01

    We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H]~+0.4) open clusters in our Galaxy and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster using Keck/LRIS spectra suggests that most of these stars are undermassive, =0.43+/-0.06 Msolar, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791's evolved stars must have lost enough mass on the red giant branch to avoid the flash and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium-core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen-core stars, and therefore the corrected white dwarf cooling age is in fact >~7 Gyr, consistent with the well-measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high-metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Based on observations obtained at the

  1. White Dwarfs in Astrometric Binaries?

    NASA Astrophysics Data System (ADS)

    Oliversen, N. A.; Evans, N. R.; Feibelman, W. A.; Kamper, K. W.

    1993-12-01

    Lippincott (1978, Space Sci Rev, 22, 153) compiled a list of astrometric binaries with unseen companions typically within 20 pc of the sun. Red companions have been observed in a number of these systems (e.g. McCarthy, D. W. 1983, IAU Coll. # 76, p. 107). Unseen, low mass companions could also be white dwarfs. We have obtained IUE observations of stars on the list which have primaries with spectral types M1 or earlier (white dwarf companions of cooler primaries could be detected from the ground), and are brighter than 10 mag, which do not have known red companions. Preliminary reductions (comparison with standard stars of appropriate spectral types) indicate that there are no white dwarfs in the sample. Further processing is being done to determine limits on possible white dwarf temperatures.

  2. PTF1 J082340.04+081936.5: A Hot Subdwarf B Star with a Low-mass White Dwarf Companion in an 87-minute Orbit

    NASA Astrophysics Data System (ADS)

    Kupfer, Thomas; van Roestel, Jan; Brooks, Jared; Geier, Stephan; Marsh, Tom R.; Groot, Paul J.; Bloemen, Steven; Prince, Thomas A.; Bellm, Eric; Heber, Ulrich; Bildsten, Lars; Miller, Adam A.; Dyer, Martin J.; Dhillon, Vik S.; Green, Matthew; Irawati, Puji; Laher, Russ; Littlefair, Stuart P.; Shupe, David L.; Steidel, Charles C.; Rattansoon, Somsawat; Pettini, Max

    2017-02-01

    We present the discovery of the hot subdwarf B star (sdB) binary PTF1 J082340.04+081936.5. The system has an orbital period of {P}{orb} = 87.49668(1) minutes (0.060761584(10) days), making it the second-most compact sdB binary known. The light curve shows ellipsoidal variations. Under the assumption that the sdB primary is synchronized with the orbit, we find a mass of {M}{sdB}={0.45}-0.07+0.09 {M}ȯ , a companion white dwarf mass of {M}{WD}={0.46}-0.09+0.12 {M}ȯ , and a mass ratio of q=\\tfrac{{M}{WD}}{{M}{sdB}}={1.03}-0.08+0.10. The future evolution was calculated using the MESA stellar evolution code. Adopting a canonical sdB mass of {M}{sdB}=0.47 {M}ȯ , we find that the sdB still burns helium at the time it will fill its Roche lobe if the orbital period was less than 106 minutes at the exit from the last common envelope (CE) phase. For longer CE exit periods, the sdB will have stopped burning helium and turned into a C/O white dwarf at the time of contact. Comparing the spectroscopically derived {log}g and {T}{eff} with our MESA models, we find that an sdB model with a hydrogen envelope mass of 5× {10}-4 {M}ȯ matches the measurements at a post-CE age of 94 Myr, corresponding to a post-CE orbital period of 109 minutes, which is close to the limit to start accretion while the sdB is still burning helium.

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

    NASA Technical Reports Server (NTRS)

    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.

  4. Discovery of an Ultracool White Dwarf Companion

    NASA Astrophysics Data System (ADS)

    Farihi, J.

    2004-08-01

    The discovery of a low-luminosity common proper-motion companion to the white dwarf GD 392 at a wide separation of 46" is reported. BVRI photometry suggests a low temperature (Teff~4000 K), while JHK data strongly indicate suppressed flux at all near-infrared wavelengths. Thus, GD 392B is one of the few white dwarfs to show significant collision-induced absorption due to the presence of photospheric H2 and the first ultracool white dwarf detected as a companion to another star. Models fail to explain GD 392B as a normal-mass white dwarf. If correct, the cool companion may be explained as a low-mass white dwarf or unresolved double degenerate. The similarities of GD 392B to known ultracool degenerates are discussed, including some possible implications for the faint end of the white dwarf luminosity function.

  5. New White Dwarf-Brown Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Casewell, S. L.; Geier, S.; Lodieu, N.

    2017-03-01

    We present follow-up spectroscopy to 12 candidate white dwarf-brown dwarf binaries. We have confirmed that 8 objects do indeed have a white dwarf primary (7 DA, 1 DB) and two are hot subdwarfs. We have determined the Teff and log g for the white dwarfs and subdwarfs, and when combining these values with a model spectrum and the photometry, we have 3 probable white dwarf-substellar binaries with spectral types between M6 and L6.

  6. The Puzzling Atmospheres of Low-mass Stars, Brown Dwarfs and Exoplanets Revealed by the Discovery Channel Telescope

    NASA Astrophysics Data System (ADS)

    Muirhead, Philip Steven; Croll, Bryce; Dalba, Paul A.; Veyette, Mark; Han, Eunkyu; Kesseli, Aurora; Healy, Brian

    2017-01-01

    The Large Monolithic Imager (LMI) on the Discovery Channel Telescope (DCT) enables high-precision photometry with a scriptable interface and rapid cycling between photometric bands, all while guiding off-axis. Using LMI, scientists at Boston University have undertaken a number of investigations into low-mass stars, brown dwarfs and extrasolar planets. We will report on recent results from these investigations, including (1) measurements of transiting asteroids orbiting a white dwarf, (2) refined ephemerides for long-period transiting exoplanets, (3) investigations revealing biases in space-based exoplanet light curves, (4) investigations of the nature of activity in low-mass stars and brown dwarfs and (5) investigations of low-mass eclipsing binary stars. We will also propose future studies of low-mass stars, brown dwarfs and exoplanets using current and future DCT instrumentation.

  7. Low-mass Visual Companions to Nearby G-dwarfs

    NASA Astrophysics Data System (ADS)

    Tokovinin, Andrei

    2011-02-01

    A complete census of wide visual companions to nearby G-dwarf stars can be achieved by selecting candidates from the Two Micron All Sky Survey (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 a 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 an 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 toward reaching unbiased multiplicity statistics over the full range of orbital periods and, eventually, understanding the origin of multiple systems.

  8. LOW-MASS VISUAL COMPANIONS TO NEARBY G-DWARFS

    SciTech Connect

    Tokovinin, Andrei

    2011-02-15

    A complete census of wide visual companions to nearby G-dwarf stars can be achieved by selecting candidates from the Two Micron All Sky Survey (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 a 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 an 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 toward reaching unbiased multiplicity statistics over the full range of orbital periods and, eventually, understanding the origin of multiple systems.

  9. Asteroseismology of White Dwarf Stars

    NASA Technical Reports Server (NTRS)

    Hansen, Carl J.

    1997-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Allard, F.; Homeier, D.; Freytag, B.

    2011-12-01

    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 micro-turbulence velocity, the diffusion coefficient, and the advective mixing of molecules as a function of depth. This new model grid of atmospheres and synthetic spectra has been computed for 100,000 K > Teff > 400 K, 5.5 > logg > -0.5, and [M/H]= +0.5 to -1.5, and the reference solar abundances of Asplund et al. (2009). We found that the new solar abundances allow an improved (close to perfect) reproduction of the photometric and spectroscopic VLMs properties, and, for the first time, a smooth transition between stellar and substellar regimes -- unlike the transition between the NextGen models from Hauschildt et al. 1999a,b, and the AMES-Dusty models from Allard et al. 2001. In the BDs regime, the BT-Settl models propose an improved explanation for the M-L-T spectral transition. In this paper, we therefore present the new BT-Settl model atmosphere grid, which explains the entire transition from the stellar to planetary mass regimes.

  11. Multiplicity among Young Brown Dwarfs and Very Low Mass Stars

    NASA Astrophysics Data System (ADS)

    Ahmic, Mirza; Jayawardhana, Ray; Brandeker, Alexis; Scholz, Alexander; van Kerkwijk, Marten H.; Delgado-Donate, Eduardo; Froebrich, Dirk

    2007-12-01

    We report on a near-infrared adaptive optics imaging survey of 31 young brown dwarfs and very low mass (VLM) stars, 28 of which are in the Chamaeleon I star-forming region, using the ESO Very Large Telescope. We resolve the suspected 0.16'' (~26 AU) binary Cha Hα 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13'' (~20 AU) and 0.30'' (~50 AU), respectively; the latter is one of the widest VLM systems known. We find a binary frequency of 11+9-6%, thus confirming the trend for a lower binary frequency with decreasing mass. By combining our work with previous surveys, we arrive at the largest sample of young VLM objects (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLM objects in the field. Furthermore, we note that many field stellar binaries with lower binding energies and/or wider cross sections have survived dynamical evolution and that statistical models suggest tidal disruption by passing stars is unlikely to affect the binary properties of our systems. Thus, we argue that there is no significant evolution of multiplicity with age among brown dwarfs and VLM stars in OB and T associations between a few megayears to several gigayears. Instead, the observations so far suggest that VLM objects are either less likely to be born in fragile multiple systems than solar-mass stars or such systems are disrupted very early. We dedicate this paper to the memory of our coauthor, Eduardo Delgado-Donate, who died in a hiking accident in Tenerife earlier this year.

  12. White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.

    2014-10-01

    White dwarfs are the evolutionary endpoint for nearly 95% of all stars born in our Galaxy, the final stages of evolution of all low- and intermediate mass stars, i.e., main sequence stars with masses below (8.5± 1.5) M_{odot}, depending on metallicity of the progenitor, mass loss and core overshoot. Massive white dwarfs are intrinsically rare objects, tand produce a gap in the determination of the initial vs. final mass relation at the high mass end (e.g. Weidemann 2000 A&A, 363, 647; Kalirai et al. 2008, ApJ, 676, 594; Williams, Bolte & Koester 2009, ApJ, 693, 355). Main sequences stars with higher masses will explode as SNII (Smartt S. 2009 ARA&A, 47, 63), but the limit does depend on the metallicity of the progenitor. Massive white dwarfs are probably SNIa progenitors through accretion or merger. They are rare, being the final product of massive stars (less common) and have smaller radius (less luminous). Kepler et al. 2007 (MNRAS, 375, 1315), Kleinman et al. 2013 (ApJS, 204, 5) estimate only 1-2% white dwarfs have masses above 1 M_{odot}. The final stages of evolution after helium burning are a race between core growth and loss of the H-rich envelope in a stellar wind. When the burning shell is exposed, the star rapidly cools and burning ceases, leaving a white dwarf. As they cool down, the magnetic field freezes in, ranging from a few kilogauss to a gigagauss. Peculiar type Ia SN 2006gz, SN 2007if, SN 2009dc, SN 2003fg suggest progenitors in the range 2.4-2.8 M_{odot}, and Das U. & Mukhopadhyay B. (2012, Phys. Rev. D, 86, 042001) estimate that the Chandrasekhar limit increases to 2.3-2.6 M_{odot} for extremely high magnetic field stars, but differential rotation induced by accretion could also increase it, according to Hachisu I. et al. 2012 (ApJ, 744, 69). García-Berro et al. 2012, ApJ, 749, 25, for example, proposes double degenerate mergers are the progenitors of high-field magnetic white dwarfs. We propose magnetic fields enhance the line broadening in

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

    NASA Technical Reports Server (NTRS)

    Becklin, E. E.; Zuckerman, B.

    1988-01-01

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

  14. Convection in White Dwarfs

    NASA Astrophysics Data System (ADS)

    Provencal, Judith L.; Shipman, H.; Dalessio, J.; M, M.

    2012-01-01

    Convection is one of the largest sources of theoretical uncertainty in our understanding of stellar physics. Current studies of convective energy transport are based on the mixing length theory. Originally intended to depict turbulent flows in engineering situations, MLT enjoys moderate success in describing stellar convection. However, problems arising from MLT's incompleteness are apparent in studies ranging from determinations of the ages of massive stars, to understanding the structure F and early A stars, to predicting the pulsation periods of solar stars, to understanding the atmosphere of Titan. As an example for white dwarfs, Bergeron et al. (1995) show that model parameters such as flux, line profiles, energy distribution, color indices, and equivalent widths are extremely sensitive to the assumed MLT parameterization. The authors find systematic uncertainties ranging from 25% for effective temperatures to 11% for mass and radius. The WET is engaged in a long term project to empirically determine the physical properties of convection in the atmospheres of pulsating white dwarfs. The technique, outlined by Montgomery et al. (2010), uses information from nonlinear (non-sinusoidal) pulse shapes of the target star to empirically probe the physical properties of its convection zone. Approximately two thirds of all white dwarfs show nonlinear characteristics in their light curves. We present current results from WET targets in 2008-2011.

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

    NASA Astrophysics Data System (ADS)

    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.

  16. Occurrence rate of low-mass planets around nearby M dwarfs

    NASA Astrophysics Data System (ADS)

    Jones, Hugh

    2015-08-01

    We re-analyse archival radial velocities of nearby M dwarfs to constrain low-amplitude Keplerian signals. We apply a variety of signal detection criteria and photometric monitoring to assess the number of planet candidates in the sample. We use the estimated detection probability function to calculate the occurrence rate of low-mass planets around nearby M dwarfs. Our results indicate that M dwarfs are hosts to an abundance of low-mass planets and the occurrence rate of planets less massive than 10 Earth masses is of the order of one planet per star and that planets are common in the stellar habitable zones of M dwarfs.

  17. The white dwarf binary pathways survey - I. A sample of FGK stars with white dwarf companions

    NASA Astrophysics Data System (ADS)

    Parsons, S. G.; Rebassa-Mansergas, A.; Schreiber, M. R.; Gänsicke, B. T.; Zorotovic, M.; Ren, J. J.

    2016-12-01

    The number of spatially unresolved 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 survey in the Southern hemisphere and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope 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 a hot subdwarf or pre-helium white dwarf, demonstrating that this sample is very clean. We also address the potential of this sample to test binary evolution models and Type Ia supernovae formation channels.

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

    NASA Technical Reports Server (NTRS)

    Shipman, Harry L.

    1986-01-01

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

  19. White Dwarf Calibration

    NASA Astrophysics Data System (ADS)

    Colina, Luis

    1994-01-01

    As a result of last November calibration workshop, all parties agreed that the HST should be switched to the WD basis for absolute fluxes. This proposal implements that decision. A measurement of the absolute sensitivity of the FOS detectors will be performed using theoretical pure hydrogen model atmosphere calculations for three white dwarfs. The high resolution gratings will be used in the 1 arcsec aperture. A four stage peakup of the standard star provides centering in the aperture. Observations are requested for fall 94 with repeated observations about two months after.

  20. Solidification of carbon-oxygen white dwarfs

    NASA Technical Reports Server (NTRS)

    Schatzman, E.

    1982-01-01

    The internal structure of white dwarfs is discussed. Highly correlated plasmas are reviewed. Implications for phase separation in the core of cooling white dwarfs are considered. The consequences for evolution of white dwarfs are addressed.

  1. Kinematics of faint white dwarfs.

    PubMed

    Luyten, W J

    1978-10-01

    An analysis has been made for solar motion for 128 very faint white dwarfs of color class b or a. While about 40% of these stars may be high-velocity objects, it seems definitely indicated that the luminosity of all of them is considerably lower than that for the "normal" white dwarf of the same color.

  2. OGLE-2014-BLG-0257L: A Microlensing Brown Dwarf Orbiting a Low-mass M Dwarf

    NASA Astrophysics Data System (ADS)

    Han, C.; Jung, Y. K.; Udalski, A.; Gould, A.; Bozza, V.; Szymański, M. K.; Soszyński, I.; Poleski, R.; Kozłowski, S.; Pietrukowicz, P.; Skowron, J.; Ulaczyk, K.; Wyrzykowski, Ł.; OGLE Collaboration

    2016-05-01

    In this paper, we report the discovery of a binary composed of a brown dwarf (BD) and a low-mass M dwarf from observation of the microlensing event OGLE-2014-BLG-0257. The resolution of the very brief caustic crossing combined with the detection of subtle continuous deviation in the lensing light curve induced by the Earth’s orbital motion enable us to precisely measure both the Einstein radius {θ }{{E}} and the lens parallax {π }{{E}}, which are the two quantities needed to unambiguously determine the mass and distance to the lens. It is found that the companion is a substellar BD with a mass of 0.036+/- 0.005 {M}⊙ (37.7+/- 5.2 {M}{{J}}) and it is orbiting an M dwarf with a mass of 0.19+/- 0.02 {M}⊙ . The binary is located at a distance of 1.25 ± 0.13 kpc toward the Galactic bulge and the projected separation between the binary components is 0.61 ± 0.07 au. The separation scaled by the mass of the host is 3.2 {{au}}/{M}⊙ . Based on the assumption that separations scale with masses, the discovered BD is located in the BD desert. With the growing sample of BDs in various environments, microlensing will provide a powerful probe of BDs in the Galaxy.

  3. White Dwarf Stars

    NASA Technical Reports Server (NTRS)

    1999-01-01

    Peering deep inside a cluster of several hundred thousand stars, NASA's Hubble Space Telescope has uncovered the oldest burned-out stars in our Milky Way Galaxy, giving astronomers a fresh reading on the age of the universe.

    Located in the globular cluster M4, these small, burned-out stars -- called white dwarfs -- are about 12 to 13 billion years old. By adding the one billion years it took the cluster to form after the Big Bang, astronomers found that the age of the white dwarfs agrees with previous estimates that the universe is 13 to 14 billion years old.

    The images, including some taken by Hubble's Wide Field and Planetary Camera 2, are available online at

    http://oposite.stsci.edu/pubinfo/pr/2002/10/ or

    http://www.jpl.nasa.gov/images/wfpc .

    The camera was designed and built by NASA's Jet Propulsion Laboratory, Pasadena, Calif.

    In the top panel, a ground-based observatory snapped a panoramic view of the entire cluster, which contains several hundred thousand stars within a volume of 10 to 30 light-years across. The Kitt Peak National Observatory's .9-meter telescope took this picture in March 1995. The box at left indicates the region observed by the Hubble telescope.

    The Hubble telescope studied a small region of the cluster. A section of that region is seen in the picture at bottom left. A sampling of an even smaller region is shown at bottom right. This region is only about one light-year across. In this smaller region, Hubble pinpointed a number of faint white dwarfs. The blue circles indicate the dwarfs. It took nearly eight days of exposure time over a 67-day period to find these extremely faint stars.

    Globular clusters are among the oldest clusters of stars in the universe. The faintest and coolest white dwarfs within globular clusters can yield a globular cluster's age. Earlier Hubble observations showed that the first stars formed less than 1 billion years after the universe's birth in the big bang. So, finding the

  4. DETECTION OF A WHITE DWARF COMPANION TO THE WHITE DWARF SDSSJ125733.63+542850.5

    SciTech Connect

    Marsh, T. R.; Gaensicke, B. T.; Steeghs, D.; Southworth, J.; Koester, D.; Harris, V.; Merry, L.

    2011-08-01

    SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a compact white dwarf binary from the Sloan Digital Sky Survey that exhibits high-amplitude radial velocity variations on a period of 4.56 hr. While an initial analysis suggested the presence of a neutron star or black hole binary companion, a follow-up study concluded that the spectrum was better understood as a combination of two white dwarfs. Here we present optical spectroscopy and ultraviolet fluxes which directly reveal the presence of the second white dwarf in the system. SDSSJ1257+5428's spectrum is a composite, dominated by the narrow-lined spectrum from a cool, low-gravity white dwarf (T{sub eff} {approx_equal} 6300 K, log g = 5-6.6) with broad wings from a hotter, high-mass white dwarf companion (11, 000-14, 000 K; {approx}1 M{sub sun}). The high-mass white dwarf has unusual line profiles which lack the narrow central core to H{alpha} that is usually seen in white dwarfs. This is consistent with rapid rotation with vsin i = 500-1750 km s{sup -1}, although other broadening mechanisms such as magnetic fields, pulsations, or a helium-rich atmosphere could also be contributory factors. The cool component is a puzzle since no evolutionary model matches its combination of low gravity and temperature. Within the constraints set by our data, SDSSJ1257+5428 could have a total mass greater than the Chandrasekhar limit and thus be a potential Type Ia supernova progenitor. However, SDSSJ1257+5428's unusually low-mass ratio q {approx} 0.2 suggests that it is more likely that it will evolve into an accreting double white dwarf (AM CVn star).

  5. Low-mass spectroscopic binaries in the Hyades: a candidate brown dwarf companion

    NASA Astrophysics Data System (ADS)

    Reid, I. Neill; Mahoney, S.

    2000-08-01

    We have used the HIRES echelle spectrograph on the Keck I telescope to obtain high-resolution spectroscopy of 51 late-type M dwarfs in the Hyades cluster. Cross-correlating the calibrated data against spectra of white dwarfs allows us to determine heliocentric velocities with an accuracy of +/-0.3kms-1. 27 stars were observed at two epochs in 1997; two stars, RHy 42 and RHy 403, are confirmed spectroscopic binaries. RHy 42 is a double-lined, equal-mass system; RHy 403 is a single-lined, short-period binary, P~1.275d. RHy 403A has an absolute magnitude of MI=10.85, consistent with a mass of 0.15Msolar. The systemic mass function has a value M2sin(i)]3/(M1+M2)2 =0.0085, which, combined with the non-detection of a secondary peak in the cross-correlation function, implies 0.095>M2>0.07Msolar, and the strong possibility that the companion is the first Hyades brown dwarf to be identified. Unfortunately, the maximum expected angular separation in the system is only ~0.25mas. Five other low-mass Hyads are identified as possible spectroscopic binaries, based either on repeat observations or on a comparison between the observed radial velocity and the value expected for Hyades cluster members. Combined with HST imaging data, we infer a binary fraction between 23 and 30per cent. All of the stars are chromospherically active. RHy 281 was caught in mid-flare and, based on that detection, we estimate a flaring frequency of ~2.5per cent for low-mass Hyades stars. Nine stars have rotational velocities, vsin(i), exceeding 20kms-1, and most of the sample have detectable rotation. We examine the H&alpha emission characteristics of low-mass cluster members, and show that there is no evidence for a correlation with rotation.

  6. Rotation Velocities of White Dwarfs

    NASA Astrophysics Data System (ADS)

    Karl, C.; Napiwotzki, R.; Heber, U.; Dreizler, S.; Koester, D.; Reid, I. N.

    White dwarfs are the compact remnants of low and intermediate mass stars (M < 8Msolar). Due to the conservation of angular momentum white dwarfs should be very fast rotators, if a significant fraction of the angular momentum of the progenitor stars were preserved. The existence of sharp NLTE cores of the hydrogen Hα line in high resolution spectra (obtained at the Keck observatory) of DA white dwarfs allowed us to determine (projected) rotational velocities v sin i for white dwarfs. Among those of our targets lying close to the ZZ Ceti instability many show evidence for extra broadening similar to rotation, whereas stars at higher temperatures (and therefore younger ones) rotate more slowly or not at all. Our result based on a large sample is in accordance with previous results presented by Koester et al. (1998). We discuss possible explanations for this astonishing result.

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

    NASA Astrophysics Data System (ADS)

    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.

  8. TiNy Titans: The Role of Dwarf-Dwarf Interactions in Low-mass Galaxy Evolution

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

    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 spectroscopic portion of the Sloan Digital Sky Survey and caught in various stages of interaction. The TNT dwarf pairs span mass ratios of M*,1/M*,2 < 10, projected separations <50 kpc, and pair member masses of 7 < log({{M}*}/{{M}⊙ }) < 9.7. The dwarf-dwarf merger sequence, as defined by TNT at z = 0, demonstrates conclusively and for the first time that the star formation enhancement observed for massive galaxy pairs also extends to the dwarf mass range. Star formation is enhanced in paired dwarfs in otherwise isolated environments by a factor of 2.3 (±0.7) at pair separations <50 kpc relative to unpaired analogs. The enhancement decreases with increasing pair separation and extends out to pair separations as large as 100 kpc. Starbursts, defined by Hα EQW >100 Å, occur in 20% of the TNT dwarf pairs, regardless of environment, compared to only 6%-8% of the matched unpaired dwarfs. Starbursts can be triggered throughout the merger (i.e., out to large pair separations) and not just approaching coalescence. Despite their enhanced star formation and triggered starbursts, most TNT dwarf pairs have similar gas fractions relative to unpaired dwarfs of the same stellar mass. Thus, there may be significant reservoirs of diffuse, non-star-forming neutral 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\\lt 0.4) and the only dwarfs, either paired or unpaired, with Hα EQW < 2 Å are found near massive galaxy hosts. We conclude that dwarf-dwarf interactions are significant drivers of galaxy evolution at the low-mass end, but

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

    NASA Technical Reports Server (NTRS)

    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.

  10. The Dusty Accretion of Polluted White Dwarfs

    NASA Astrophysics Data System (ADS)

    Bonsor, A.; Farihi, J.; Wyatt, M. C.; van Lieshout, R.

    2017-03-01

    Infrared observations of polluted white dwarfs provide key insights into the accretion processes in action. The standard model for the observed infrared excesses is a flat, opaque, dust disc. The infrared observations are inconsistent with the presence of such a disc around all polluted white dwarfs. We discuss potential explanations for the absence of an infrared excess for many polluted white dwarfs.

  11. A Pulsar and White Dwarf in an Unexpected Orbit

    NASA Astrophysics Data System (ADS)

    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

  12. The Physics of White Dwarfs.

    ERIC Educational Resources Information Center

    Van Horn, Hugh M.

    1979-01-01

    Describes the current understanding of the structure and evolution of the white dwarf stars that was gained as a result of the increasingly sensitive and detailed astronomical observations coupled with calculations of the properties of matter under extreme conditions. (Author/GA)

  13. Investigating Low-Mass Binary Stars And Brown Dwarfs with Near-Infrared Spectroscopy

    NASA Astrophysics Data System (ADS)

    Mace, Gregory Nathan

    The mass of a star at formation determines its subsequent evolution and demise. Low-mass stars are the most common products of star formation and their long main-sequence lifetimes cause them to accumulate over time. Star formation also produces many substellar-mass objects known as brown dwarfs, which emerge from their natal molecular clouds and continually cool as they age, pervading the Milky Way. Low-mass stars and brown dwarfs exhibit a wide range of physical characteristics and their abundance make them ideal subjects for testing formation and evolution models. I have examined a pair of pre-main sequence spectroscopic binaries and used radial velocity variations to determine orbital solutions and mass ratios. Additionally, I have employed synthetic spectra to estimate their effective temperatures and place them on theoretical Hertzsprung-Russell diagrams. From this analysis I discuss the formation and evolution of young binary systems and place bounds on absolute masses and radii. I have also studied the late-type T dwarfs revealed by the Wide-field Infrared Survey Explorer (WISE). This includes the exemplar T8 subdwarf Wolf 1130C, which has the lowest inferred metallicity in the literature and spectroscopic traits consistent with old age. Comparison to synthetic spectra implies that the dispersion in near-infrared colors of late-type T dwarfs is a result of age and/or thin sulfide clouds. With the updated census of the L, T, and Y dwarfs we can now study specific brown dwarf subpopulations. Finally, I present a number of future studies that would develop our understanding of the physical qualities of T dwarf color outliers and disentangle the tracers of age and atmospheric properties.

  14. The atomic and molecular content of disks around very low-mass stars and brown dwarfs

    SciTech Connect

    Pascucci, I.; Herczeg, G.; Carr, J. S.; Bruderer, S.

    2013-12-20

    There is growing observational evidence that disk evolution is stellar-mass-dependent. Here, we show that these dependencies extend to the atomic and molecular content of disk atmospheres. We analyze a unique dataset of high-resolution Spitzer/IRS spectra from eight very low mass star and brown dwarf disks. We report the first detections of Ne{sup +}, H{sub 2}, CO{sub 2}, and tentative detections of H{sub 2}O toward these faint and low-mass disks. Two of our [Ne II] 12.81 μm emission lines likely trace the hot (≥5000 K) disk surface irradiated by X-ray photons from the central stellar/sub-stellar object. The H{sub 2} S(2) and S(1) fluxes are consistent with arising below the fully or partially ionized surface traced by the [Ne II] emission in gas at ∼600 K. We confirm the higher C{sub 2}H{sub 2}/HCN flux and column density ratio in brown dwarf disks previously noted from low-resolution IRS spectra. Our high-resolution spectra also show that the HCN/H{sub 2}O fluxes of brown dwarf disks are on average higher than those of T Tauri disks. Our LTE modeling hints that this difference extends to column density ratios if H{sub 2}O lines trace warm ≥600 K disk gas. These trends suggest that the inner regions of brown dwarf disks have a lower O/C ratio than those of T Tauri disks, which may result from a more efficient formation of non-migrating icy planetesimals. An O/C = 1, as inferred from our analysis, would have profound implications on the bulk composition of rocky planets that can form around very low mass stars and brown dwarfs.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Context. 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. Aims: We report on the analysis of a sample of 11 stars belonging to five different ultra-faint dwarf spheroidal galaxies (UfDSph) that is based on X-Shooter spectra obtained at the VLT. Methods: 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. Results: Considering all the stars as representative of the same population of low-mass galaxies, we found that the [α/Fe] ratios vs.s [Fe/H] decreases as the metallicity of the star increases in a way similar to that which is found for the population of stars that belong to dwarf spheroidal galaxies. The main difference is that the solar [α/Fe] is reached at a much lower metallicity for the UfDSph than for the dwarf spheroidal galaxies. We report for the first time the abundance of strontium in CVn II. The star we analyzed in this galaxy has a very high [Sr/Fe] and a very low upper limit of barium which makes it a star with an exceptionally high [Sr/Ba] ratio.

  16. ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. III. IRON, MAGNESIUM, AND SILICON

    SciTech Connect

    Visscher, Channon; Lodders, Katharina; Fegley, Bruce E-mail: lodders@wustl.ed

    2010-06-20

    We use thermochemical equilibrium calculations to model iron, magnesium, and silicon chemistry in the atmospheres of giant planets, brown dwarfs, extrasolar giant planets (EGPs), and low-mass stars. The behavior of individual Fe-, Mg-, and Si-bearing gases and condensates is determined as a function of temperature, pressure, and metallicity. Our equilibrium results are thus independent of any particular model atmosphere. The condensation of Fe metal strongly affects iron chemistry by efficiently removing Fe-bearing species from the gas phase. Monatomic Fe is the most abundant Fe-bearing gas throughout the atmospheres of EGPs and L dwarfs, and in the deep atmospheres of giant planets and T dwarfs. Mg- and Si-bearing gases are effectively removed from the atmosphere by forsterite (Mg{sub 2}SiO{sub 4}) and enstatite (MgSiO{sub 3}) cloud formation. Monatomic Mg is the dominant magnesium gas throughout the atmospheres of EGPs and L dwarfs and in the deep atmospheres of giant planets and T dwarfs. Silicon monoxide (SiO) is the most abundant Si-bearing gas in the deep atmospheres of brown dwarfs and EGPs, whereas SiH{sub 4} is dominant in the deep atmosphere of Jupiter and other gas giant planets. Several other Fe-, Mg-, and Si-bearing gases become increasingly important with decreasing effective temperature. In principle, a number of Fe, Mg, and Si gases are potential tracers of weather or diagnostic of temperature in substellar atmospheres.

  17. Evaporation and accretion of extrasolar comets following white dwarf kicks

    NASA Astrophysics Data System (ADS)

    Stone, Nicholas; Metzger, Brian D.; Loeb, Abraham

    2015-03-01

    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 an IR excess temporarily visible around newborn white dwarfs; examples of such discs may already have been seen in the Helix Nebula, and around several other young white dwarfs. Future observations with the James Webb Space Telescope may distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. Although competing hypotheses exist, the observation that ≳15 per cent of young white dwarfs possess such discs, if interpreted as indeed being cometary in origin, provides indirect evidence that low-mass gas giants (thought necessary to produce an Oort cloud) are common in the outer regions of extrasolar planetary systems. Hydrogen abundances in the atmospheres of older white dwarfs can, if sufficiently low, also be used to place constraints on the joint parameter space of natal kicks and exo-Oort cloud models.

  18. The physics of white dwarfs

    NASA Astrophysics Data System (ADS)

    Isern, Jordi; García-Berro, Enrique; Hernanz, Margarida; Mochkovitch, Robert

    1998-12-01

    White dwarfs are the final remnants of low- and intermediate-mass stars. Their evolution is essentially a cooling process that lasts for 0953-8984/10/49/015/img6 and allows one to obtain information about the age of the Galaxy as well as about the past stellar formation rate in the solar neighbourhood. Therefore, it is important to identify all of the relevant sources of energy as well as the mechanisms that control its flow to the space. We show in this paper that the inclusion of a detailed treatment of phase transitions in Coulomb plasmas made up of a mixture of different chemical species is crucial, since their redistribution can keep the white dwarf warm for 0.5 to 9 Ga depending on the chemical composition and physical assumptions adopted.

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

    NASA Technical Reports Server (NTRS)

    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.

  20. Characterizing Accreting White Dwarf Pulsators

    NASA Astrophysics Data System (ADS)

    Szkody, Paula; Mukadam, Anjum

    2014-02-01

    Understanding the population, mass distribution, and evolution of accreting white dwarfs impacts the entire realm of binary interaction, including the creation of Type Ia supernovae. We are concentrating on accreting white dwarf pulsators, as the pulsation properties allow us a view of how the accretion affects the interior of the star. Our ground- based photometry on 11 accreting pulsators with corresponding temperatures from HST UV spectra suggest a broad instability strip in the range of 10500 to 16000K. Additionally, tracking a post-outburst heated white dwarf as it cools and crosses the blue edge and resumes pulsation provides an independent method to locate the empirical instability strip. Determining a post-outburst cooling curve yields an estimate of the amount of heating and the accreted mass during the outburst. We request additional photometry of 2 objects that present unique properties: GW Lib which has not yet returned to its pre-outburst pulsation spectrum after 6 yrs, and EQ Lyn which returned to its pre- outburst pulsation after 3 yrs but is now turning on and off without ongoing outbursts. Following the pulsation spectrum changes over stretches of several nights in a row will provide specific knowledge of the stability of the observed modes.

  1. Identification and characterization of low mass stars and brown dwarfs using Virtual Observatory tools

    NASA Astrophysics Data System (ADS)

    Aberasturi, Miriam

    2015-11-01

    Context: Two thirds of the stars in our galactic neighborhood (d < 10 pc) are M-dwarfs which also constitute the most common stellar objects in the Milky Way. This property, combined with their small stellar masses and radii, increases the likelihood of detecting terrestrial planets through radial velocity and transit techniques, making them very adequate targets for the exoplanet hunting projects. Nevertheless, M dwarfs have associated different observational difficulties. They are cool objects whose emission radiation peaks at infrared wavelengths and, thus, with a low surface brightness in the optical range. Also, the photometric variability as well as the significant chromospheric activity hinder the radial velocity and transit determinations. It is necessary, therefore, to carry out a detailed characterization of M-dwarfs before building a shortlist with the best possible candidates for exoplanet searches. Brown dwarfs (BDs) are self-gravitating objects that do not get enough mass to maintain a sufficiently high temperature in their core for stable hydrogen fusion. They represent the link between low-mass stars and giant planets. Due to their low temperatures, BDs emit significant flux at mid-infrared wavelength which makes this range very adequate to look for this type of objects. The Virtual Observatory (VO) is an international initiative designed to help the astronomical community in the exploitation of the multi-wavelength information that resides in data archives. In the last years the Spanish Virtual Observatory is conducting a number of projects focused on the study of substellar objects taking advantage of Virtual Observatory tools for an easy data access and analysis of large area surveys. This is the framework where this thesis has been carried out. This dissertation addresses three problems in the framework of low-mass stars and brown dwarfs, namely, the search for brown dwarf candidates crossmatching catalogues (Chapter 4), the search for nearby

  2. Optical Spectroscopy of Low-Mass Stars and Brown Dwarfs in Orion

    NASA Astrophysics Data System (ADS)

    Riddick, F. C.; Roche, P. F.; Lucas, P. W.

    2006-06-01

    Using multi-object optical spectroscopy from the AAT and Gemini-North, 35 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified both by comparison with other previously classified young, low-mass sources in the Chamaeleon I star-forming region and by the use of spectral indices: narrowband indices which measure the strength of various highly temperature-sensitive molecular lines. The objects are all very likely cluster members, by analysis of the strength of the gravity-sensitive Na doublet, which is much weaker than in dwarfs for these very young objects. The spectral types obtained have been converted to effective temperatures using the temperature scale of Luhman et al. (2003b), which is intermediate between dwarf and giant scales and hence suitable for young pre-main sequence objects. In combination with the dereddened H band luminosities obtained from the photometry of Lucas & Roche (2000), the objects have been placed on an H-R diagram overlaid with the theoretical isochrones of Baraffe et al. (1998). The low-mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while the lower mass substellar objects are located well above this isochrone, probably due to selection effects. The average age of 1 Myr for the majority of the objects is in agreement with other age estimates for the region, but the lack of any objects older than 5 Myr is in contrast to the results of Slesnick et al. (2004) which show in addition an older population at 10 Myr. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and 18 of the objects are found to have substellar masses.

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

    PubMed

    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.

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

    PubMed Central

    Allard, F.; Homeier, D.; Freytag, B.

    2012-01-01

    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. PMID:22547243

  5. Are All Magnetic White Dwarf Stars Massive?

    NASA Astrophysics Data System (ADS)

    Nitta, A.; Kepler, S. O.; Kulebi, B.; Koester, D.; Kleinman, S. J.; Winget, D. E.; Castanheira, B. G.; Corsico, A. H.

    2017-03-01

    We obtained follow-up spectra on 25 white dwarf stars identified in our white dwarf catalog of Sloan Digital Sky Survey (SDSS) as massive or magnetic. We identified over 300 magnetic white dwarf stars from SDSS with some uncertainties due to the low S/N of the spectra. With much higher S/N Gemini data, our sample should be able to help us confirm accuracy of our determinations. We present here our results so far from the follow up observations.

  6. Dynamical Masses of Accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  7. Tuning Into Brown Dwarfs: Long-Term Radio Monitoring of Two Very Low Mass Dwarfs

    NASA Astrophysics Data System (ADS)

    Van Linge, Russell; Burgasser, Adam J.; Melis, Carl; Williams, Peter K. G.

    2017-01-01

    The very lowest-mass (VLM) stars and brown dwarfs, with effective temperatures T < 3000 K, exhibit mixed magnetic activity trends, with H-alpha and X-ray emission that declines rapidly beyond type M7/M8, but persistent radio emission in roughly 10-20% of sources. The dozen or so VLM radio emitters known show a broad range of emission characteristics and time-dependent behavior, including steady persistent emission, periodic oscillations, periodic polarized bursts, and aperiodic flares. Understanding the evolution of these variability patterns, and in particular whether they undergo solar-like cycles, requires long-term monitoring. We report the results of a long-term JVLA monitoring program of two magnetically-active VLM dwarf binaries, the young M7 2MASS 1314+1320AB and older L5 2MASS 1315-2649AB. On the bi-weekly cadence, 2MASS 1314 continues to show variability by revealing regular flaring while 2MASS 1315 continues to be a quiescent emitter. On the daily time scale, both sources show a mean flux density that can vary significantly just over a few days. These results suggest long-term radio behavior in radio-emitting VLM dwarfs is just as diverse and complex as short-term behavior.

  8. THE (DOUBLE) WHITE DWARF BINARY SDSS 1257+5428

    SciTech Connect

    Kulkarni, S. R.; Van Kerkwijk, M. H.

    2010-08-20

    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 2 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 is short, about 1 minute. Similarly rapid rotation is only seen in accreting white dwarfs that are magnetic; empirically, it appears that in non-magnetized white dwarfs, accreted angular momentum is lost by nova explosions before it can be transferred to the white dwarf. This suggests that the massive white dwarf in SDSS 1257+5428 is magnetic as well, with B {approx_equal} 10{sup 5} G. Alternatively, the broadening seen in the spectral lines could be due to a stronger magnetic field, of {approx}10{sup 6} G. The two models can be distinguished by further observations.

  9. Double White Dwarf Merger Rates

    NASA Astrophysics Data System (ADS)

    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.

  10. Testing low-mass stellar models with M-dwarf eclipsing binaries from SDSS Stripe 82

    NASA Astrophysics Data System (ADS)

    Bhatti, Waqas A.

    , and the binary systems themselves. From this sample, we identify six total M-dwarf eclipsing binary candidates for additional follow-up observations. For the brightest two targets, we obtain estimates of the absolute masses and radii of the stars in these systems and find that the measured radii are systematically larger than predictions generated by models of the low-mass stellar main sequence. Finally, we characterize the influence of tidally-induced magnetic fields on the measured radii of the M-dwarf components of these systems.

  11. Extremely Low Mass: The Circumstellar Envelope of a Potential Proto-Brown Dwarf

    NASA Technical Reports Server (NTRS)

    Wiseman, Jennifer

    2011-01-01

    What is the environment for planet formation around extremely low mass stars? Is the environment around brown dwarfs and extremely low mass stars conducive and sufficiently massive for planet production? The determining conditions may be set very early in the process of the host object's formation. IRAS 16253-2429, the source of the Wasp-Waist Nebula seen in Spitzer IRAC images, is an isolated, very low luminosity ("VeLLO") Class 0 protostar in the nearby rho Ophiuchi cloud. We present VLA ammonia mapping observations of the dense gas envelope feeding the central core accreting system. We find a flattened envelope perpendicular to the outflow axis, and gas cavities that appear to cradle the outflow lobes as though carved out by the flow and associated (apparently precessing) jet, indicating environmental disruption. Based on the NH3 (1,1) and (2,2) emission distribution, we derive the mass, velocity fields and temperature distribution for the envelope. We discuss the combined evidence for this source to be one of the youngest and lowest mass sources in formation yet known, and discuss the ramifications for planet formation potential in this extremely low mass system.

  12. White Dwarf Critical Tests for Modified Gravity.

    PubMed

    Jain, Rajeev Kumar; Kouvaris, Chris; Nielsen, Niklas Grønlund

    2016-04-15

    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 setup to test beyond Horndeski theories of G^{3} type. We obtain stringent and independent constraints on the parameter ϒ characterizing the deviations from Newtonian 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 ϒ than red and brown dwarfs.

  13. THE SURVEY OF H I IN EXTREMELY LOW-MASS DWARFS (SHIELD)

    SciTech Connect

    Cannon, John M.; Engstrom, Eric; Allan, John; Erny, Grace; Fliss, Palmer; Smith, AnnaLeigh

    2011-09-20

    We present first results from the Survey of H I in Extremely Low-mass Dwarfs (SHIELD), a multi-configuration Expanded Very Large Array (EVLA) study of the neutral gas contents and dynamics of galaxies with H I masses in the 10{sup 6}-10{sup 7} M{sub sun} range detected by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We describe the survey motivation and concept demonstration using Very Large Array imaging of six low-mass galaxies detected in early ALFALFA data products. We then describe the primary scientific goals of SHIELD and present preliminary EVLA and WIYN 3.5 m 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 one system have we detected H I column densities higher than 10{sup 21} cm{sup -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 that extremely low-mass galaxies are forming stars in conditions different from those found in more massive systems. While detailed dynamical analysis requires the completion of data acquisition, the most well-resolved system is amenable to meaningful position-velocity analysis. For AGC 749237, we find well-ordered rotation of 30 km s{sup -1} at {approx}40'' distance from the dynamical center. At the adopted distance of 3.2 Mpc, this implies the presence of a {approx}>1 x 10{sup 8} M{sub sun} dark matter halo and a baryon fraction {approx}<0.1.

  14. Magnetic White Dwarfs with Heavy Elements

    NASA Astrophysics Data System (ADS)

    Hardy, F.; Dufour, P.; Jordan, S.

    2017-03-01

    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.

  15. A white dwarf with an oxygen atmosphere.

    PubMed

    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.

  16. A white dwarf with an oxygen atmosphere

    NASA Astrophysics Data System (ADS)

    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.

  17. Properties of an eclipsing double white dwarf binary NLTT 11748

    SciTech Connect

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

    2014-01-10

    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 (<0.2 M {sub ☉}) helium-core white dwarf in a 5.6 hr orbit. To date, such extremely low-mass white dwarfs, which can have thin, stably burning outer layers, have been modeled via poorly constrained atmosphere and cooling calculations where uncertainties in the detailed structure can strongly influence the eventual fates of these systems when mass transfer begins. With precise (individual precision ≈1%), high-cadence (≈2 s), multicolor photometry of multiple primary and secondary eclipses spanning >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 M {sub ☉}) and surface gravity (log (g) = 6.32-6.38; radii are 0.0423-0.0433 R {sub ☉}) constraints do not agree with previous spectroscopic determinations. We use precise eclipse timing to detect the Rømer delay at 7σ significance, providing an additional weak constraint on the masses and limiting the eccentricity to ecos ω = (– 4 ± 5) × 10{sup –5}. Finally, we use multicolor data to constrain the secondary's effective temperature (7600 ± 120 K) and cooling age (1.6-1.7 Gyr).

  18. AR Sco: A Precessing White Dwarf Synchronar?

    NASA Astrophysics Data System (ADS)

    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.

  19. PREFACE: 16th European White Dwarfs Workshop

    NASA Astrophysics Data System (ADS)

    Garcia-Berro, Enrique; Hernanz, Margarita; Isern, Jordi; Torres, Santiago

    2009-07-01

    The 16th European Workshop on White Dwarfs was held in Barcelona, Spain, from 30 June to 4 July 2008 at the premises of the UPC. Almost 120 participants from Europe (France, Germany, United Kingdom, Italy, and several others), America (USA, Canada, Argentina, Brazil, and Chile), and other continents (Australia, South Africa, . . . ) attended the workshop. Among these participants were the most relevant specialists in the field. The topics covered by the conference were: White dwarf structure and evolution Progenitors and Planetary Nebulae White dwarfs in binaries: cataclysmic variables, double degenerates and other binaries White dwarfs, dust disks and planetary systems Atmospheres, chemical composition, magnetic fields Variable white dwarfs White dwarfs in stellar clusters and the halo White Dwarfs as SNIa progenitors The programme included 54 talks, and 45 posters. The oral presentations were distributed into the following sessions: Luminosity function, mass function and populations White dwarf structure and evolution White dwarf ages White dwarf catalogs and surveys Central stars of planetary nebulae Supernovae progenitors White dwarfs in novae and CVs Physical processes in white dwarfs and magnetic white dwarfs Disks, dust and planets around white dwarfs Pulsating white dwarfs Additionally we had a special open session about Spitzer and white dwarfs. The Proceedings of the 16th European Workshop on White Dwarfs are representative of the current state-of-the-art of the research field and include new and exciting results. We acknowledge the very positive attitude of the attendants to the workshop, which stimulated very fruitful discussions that took place in all the sessions and after the official schedule. Also, the meeting allowed new collaborations tp start that will undoubtedly result in significant advances in the research field. We also acknowledge the willingness of the participants to deliver their contributions before the final deadline. We sincerely

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

    NASA Astrophysics Data System (ADS)

    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

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

    SciTech Connect

    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 (≳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 to single

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

    SciTech Connect

    Cody, Ann Marie; Hillenbrand, Lynne A.

    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.

  3. Magnetars and white dwarf pulsars

    NASA Astrophysics Data System (ADS)

    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.

  4. The Effect of Feedback and Reionization on Star Formation in Low-mass Dwarf Galaxy Halos

    NASA Astrophysics Data System (ADS)

    Simpson, Christine M.; Bryan, G.; Johnston, K. V.; Smith, B. D.; Mac Low, M.; Sharma, S.; Tumlinson, J.

    2013-01-01

    I will present a set of high resolution simulations of a 109 M⊙ dark matter halo in a cosmological setting done with an adaptive-mesh refinement code as a mass analogue to local low-luminosity dwarf spheroidal galaxies. The primary goal of our simulations 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 (but not local) background, a simple prescription for self-shielding, star formation, and a simple model for supernova driven energetic feedback. 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 difference in the final stellar mass of the system. For our full physics run with reionization at z=9, we find a stellar mass of about 105 M⊙ at z=0, and a mass-to-light ratio within the half-light radius of approximately 130 M⊙/L⊙, consistent with observed low-luminosity dwarfs. However, the resulting median stellar metallicity is 0.06 Z⊙, considerably larger than observed systems. In addition, we find star formation is truncated between redshifts 4 and 7, at odds with the observed late time star formation in isolated dwarf systems but in agreement with Milky Way ultrafaint dwarf spheroidals. We investigate the efficacy of energetic feedback in our simple thermal-energy driven feedback scheme, and suggest that it may still suffer from excessive radiative losses, despite reaching stellar particle masses of about 100 M⊙, and a comoving spatial resolution of 11 pc. This has led us to pursue improvements in our supernova feedback model to include kinetic as well as thermal energy in

  5. SDSS DR7 WHITE DWARF CATALOG

    SciTech Connect

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

    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.

  6. An overview of white dwarf stars

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Brassard, P.; Charpinet, S.; Randall, S. K.; Van Grootel, V.

    2013-03-01

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

  7. Are white dwarfs born with a `KICK'?

    NASA Astrophysics Data System (ADS)

    Davis, Saul; Richer, H. B.; Coffey, J.; Anderson, J.; Brewer, J.; Fahlman, G. G.; Hansen, B. M.; Hurley, J.; Kalirai, J. S.; King, I. R.; Reitzel, D.; Rich, R. M.; Rich, M. R.; Shara, M. M.

    2006-12-01

    The unusually large kinetic energies possessed by some pulsars, as inferred from their observed velocities in excess of the escape speed of the Galaxy, imply that the violent explosions in which they are born impart some fraction of their energy into the motion of the pulsar. Does a similar, but less energetic process occur during the birth of a white dwarf? Two major Hubble Space Telescope imaging campaigns of the two nearest globular star clusters, NGC 6397 and Messier 4, yield the radial distribution of both white dwarfs and main-sequences. Because globular clusters are relaxed populations, the velocity dispersion, and hence radial distribution, for stars of a particular mass is directly dependent on that mass. To first approximation, all white dwarf s have a mass of 0.55 M⊙. If white dwarfs are not born with a kick, we expect white dwarf s of an age younger than a relaxation time to have a radial distribution similar to main-sequence stars of 0.8 M⊙, i.e. the mass of their progenitor. Conversely, if white dwarf s are born with a kick, the radial distribution of white dwarfs younger than the relaxation time should mimic that of main-sequence stars of lesser mass. By comparing the radial distributions of white dwarfs of various ages with those of main-sequence stars of various masses in these two globular clusters, we find that the radial distributions of young white dwarfs are most similar to that of main-sequence stars of 0.2 M⊙, implying a natal kick of >1.6 km/s.

  8. Observations and Theory of Pulsating Helium White Dwarfs

    NASA Astrophysics Data System (ADS)

    Steinfadt, Justin D.

    Average C/O-core white dwarf stars pulsate in observable normal modes of oscillation with amplitudes of a few percent and periods of 100-1,000 seconds. As of this dissertation, no WD of less than 0.5 M sun has been observed to pulsate. White dwarfs of this low mass likely possess a He core and are products of very different stellar evolution. In this dissertation, we have constructed very low mass He-core WD models and predict the parameter space in which they may be observed to pulsate. We have also observed 13 stars, most of which are He-core WDs, in a search for the first He-core WD pulsator. While we were unsuccessful in discovering a pulsator, our detection limits offer unique constraints on He-core WD pulsation parameter space. As a fortuitous result of our pulsation search, we have discovered two unique eclipsing binary systems. One of these is the first eclipsing detached double white dwarf binary system offering the first opportunity to make model independent constraints on He-core WD models and evolution.

  9. Empirical Determination of Convection in Pulsating White Dwarfs

    NASA Astrophysics Data System (ADS)

    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.

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

    NASA Astrophysics Data System (ADS)

    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.

  11. Keck Telescope Observations of Externally-Polluted White Dwarfs

    NASA Astrophysics Data System (ADS)

    Zuckerman, Ben M.; NASA, Research was Supported in Part by

    2013-01-01

    Beginning in the late 1990s the Keck telescope and HIRES echelle spectrometer have contributed mightily to investigations of white dwarf photospheres that contain elements heavier than helium that have been accreted from surrounding planetary systems. Today we report new Keck measurements of helium atmosphere (DB and DZ) white dwarfs, of Hyades white dwarfs, and of white dwarfs in binary systems.

  12. A circumbinary debris disk in a polluted white dwarf system

    NASA Astrophysics Data System (ADS)

    Farihi, J.; Parsons, S. G.; Gänsicke, B. T.

    2017-03-01

    Planetary systems commonly survive the evolution of single stars, as evidenced by terrestrial-like planetesimal debris observed orbiting and polluting the surfaces of white dwarfs 1,2 . Here, we report the identification of a circumbinary dust disk surrounding a white dwarf with a substellar companion in a 2.27 h orbit. The system bears the dual hallmarks of atmospheric metal pollution and infrared excess 3,4 ; however, the standard (flat and opaque) disk configuration is dynamically precluded by the binary. Instead, the detected reservoir of debris must lie well beyond the Roche limit in an optically thin configuration, where erosion by stellar irradiation is relatively rapid. This finding shows that rocky planetesimal formation is robust around close binaries, even those with low mass ratios.

  13. White Dwarfs, Neutron Stars and Black Holes

    ERIC Educational Resources Information Center

    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)

  14. Merging white dwarfs and thermonuclear supernovae.

    PubMed

    van Kerkwijk, M H

    2013-06-13

    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 the 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.

  15. Tidal Effects in Inspiraling Double White Dwarfs

    NASA Astrophysics Data System (ADS)

    Willems, B.; Kalogera, Vicky; Vecchio, A.; Ivanova, N.; Deloye, C.; Hansen, B.

    2006-12-01

    Despite the overwhelming abundance of double white dwarfs in the LISA gravitational wave frequency band, modeling of their waveforms has remained limited to the point-mass approximation in which gravitational radiation is the only source of systemic orbital angular momentum loss. As a significant fraction of these systems spirals in to periods as short as 5-10 minutes, tidal effects can, however, play an important role in modifying the gravitational wave frequency evolution. The strength of the tidal effects depends strongly on the energy dissipation mechanism damping the tides, which, for white dwarfs, is highly uncertain. In this poster, we present the first results of a systematic study of tidal dissipation in white dwarfs, and the impact of tides on the gravitational wave signal of close double white dwarfs.

  16. Transit probabilities for debris around white dwarfs

    NASA Astrophysics Data System (ADS)

    Lewis, John Arban; Johnson, John A.

    2017-01-01

    The discovery of WD 1145+017 (Vanderburg et al. 2015), a metal-polluted white dwarf with an infrared-excess and transits confirmed the long held theory that at least some metal-polluted white dwarfs are actively accreting material from crushed up planetesimals. A statistical understanding of WD 1145-like systems would inform us on the various pathways for metal-pollution and the end states of planetary systems around medium- to high-mass stars. However, we only have one example and there are presently no published studies of transit detection/discovery probabilities for white dwarfs within this interesting regime. We present a preliminary look at the transit probabilities for metal-polluted white dwarfs and their projected space density in the Solar Neighborhood, which will inform future searches for analogs to WD 1145+017.

  17. Pulsating White Dwarf Stars and Precision Asteroseismology

    NASA Astrophysics Data System (ADS)

    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.

  18. PHL 5038: a spatially resolved white dwarf + brown dwarf binary

    NASA Astrophysics Data System (ADS)

    Steele, P. R.; Burleigh, M. R.; Farihi, J.; Gänsicke, B. T.; Jameson, R. F.; Dobbie, P. D.; Barstow, M. A.

    2009-06-01

    A near-infrared excess is detected at the white dwarf PHL 5038 in UKIDSS photometry, consistent with the presence of a cool, substellar companion. We have obtained H- and K-grism spectra and images of PHL 5038 using NIRI on Gemini North. The target is spatially and spectrally resolved into two components: an 8000 K DA white dwarf, and a likely L8 brown dwarf companion, separated by 0.94 arcsec. The spectral type of the secondary was determined using standard spectral indices for late L and T dwarfs. The projected orbital separation of the binary is 55 AU, so it becomes only the second known wide WD+dL binary to be found after GD 165AB. This object could potentially be used as a benchmark for testing substellar evolutionary models at intermediate to older ages.

  19. ON THE EVOLUTION OF MAGNETIC WHITE DWARFS

    SciTech Connect

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

    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.

  20. Building Magnetic Fields in White Dwarfs

    NASA Astrophysics Data System (ADS)

    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

  1. THE INITIAL-FINAL MASS RELATION AMONG WHITE DWARFS IN WIDE BINARIES

    SciTech Connect

    Zhao, J. K.; Oswalt, T. D.; Willson, L. A.; Wang, Q.; Zhao, G. E-mail: toswalt@fit.edu E-mail: lwillson@iastate.edu

    2012-02-20

    We present the initial-final mass relation derived from 10 white dwarfs in wide binaries that consist of a main-sequence star and a white dwarf. The temperature and gravity of each white dwarf were measured by fitting theoretical model atmospheres to the observed spectrum using a {chi}{sup 2} fitting algorithm. The cooling time and mass were obtained using theoretical cooling tracks. The total age of each binary was estimated from the chromospheric activity of its main-sequence component to an uncertainty of about 0.17 dex in log t. The difference between the total age and white dwarf cooling time is taken as the main-sequence lifetime of each white dwarf. The initial mass of each white dwarf was then determined using stellar evolution tracks with a corresponding metallicity derived from spectra of their main-sequence companions, thus yielding the initial-final mass relation. Most of the initial masses of the white dwarf components are between 1 and 2 M{sub Sun }. Our results suggest a correlation between the metallicity of a white dwarf's progenitor and the amount of post-main-sequence mass loss it experiences-at least among progenitors with masses in the range of 1-2 M{sub Sun }. A comparison of our observations to theoretical models suggests that low-mass stars preferentially lose mass on the red giant branch.

  2. Angular Momentum Evolution of Young Very Low Mass Stars and Brown Dwarfs: The Orion Nebula Cluster

    NASA Astrophysics Data System (ADS)

    Rodríguez-Ledesma, M. V.; Mundt, R.; Eislöffel, J.; Herbst, W.

    2008-12-01

    The rotational periods of young late-type stars and brown dwarfs (BDs) can be derived from photometric light curves, due to the rotational brightness modulation by surface features (i.e. magnetic cool spots). These kind of studies give important constrains on certain aspects of the so-called angular momentum problem of star formation. We report the first results of an extensive rotational period study of young stellar objects (YSOs) down into the BD mass regime in the Orion Nebula Cluster (ONC, d=450pc, age ˜ 1Myr). Our results are based on an deep photometric monitoring campaign, using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2 meter telescope in La Silla, Chile. We found that 487 objects show detectable periodic light modulations, 377 of which are new detections. In addition 124 are potential BDs. This is by far the most extensive and complete rotational periods data set in the very low mass (VLM) star and BD regime. The spatial distribution of the variable objects, their rotational periods as well as the amplitude of the brightness modulation have been analyzed clearly indicating different stellar properties inside and outside the half-mass cluster radius of the ONC (R_{cluster} = 6.7'). In addition, we studied the dependence of the periodic brightness modulation on the magnitude (mass) of the objects and performed a comparison of the found period distribution with those of higher-mass objects in the ONC ( te{H2002}).

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

    NASA Astrophysics Data System (ADS)

    Baraffe, I.; Elbakyan, V. G.; Vorobyov, E. I.; Chabrier, G.

    2017-01-01

    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 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 accretion and its impact on the central stellar temperature, the growth of the stellar radiative core and the accretion of fresh Li from the accretion disk. Only consistent models could reveal such a subtle combination of effects. This new result could explain the recent, puzzling observations of Li-excess of fast rotators in the young cluster NGC 2264. Present self-consistent accreting models are available in electronic form.

  4. Confirmation and characterization of low-mass and brown dwarf candidate members of nearby young associations

    NASA Astrophysics Data System (ADS)

    Malo, Lison; Gagne, Jonathan; Doyon, Rene; Lafreniere, David; Artigau, Etienne; Chene, Andre-Nicolas; Faherty, Jackie; Albert, Loic; Naud, Marie-Eve

    2013-08-01

    Young nearby associations provide a crucial sample of stars for studying the local star formation history, investigating the early phases of planet formation, and searching for exoplanets through direct imaging. While new associations have been identified in the last decade, their members are mostly restricted to relatively massive (F,G,K) stars. Through a novel statistical analysis, we have identified 357 highly probable young low-mass and brown dwarfs members of BetaPictoris and ABDoradus moving groups, TWHydrae, Tucana-Horologium, Columba, Carina and Argus associations. A confirmation of their membership and their young age require, however, measurements of their radial velocity, parallax and spectroscopic age indicators. First, we propose to use Phoenix to measure radial velocity for 40 northern candidates. Previous time allocations have already enabled the confirmation of 39 new members. Second, we propose to confirm the kinematic age of 15 candidates by using Chiron to measure several spectroscopic youth indicators, such as CaH, lithium, Halpha and HeI line (accretion) and rotation. Finally, we proposed to continue our trigonometric parallax program, initiated in 2011, to measure precisely the fourth astrometric epoch of 19 candidates. Our program will significantly improve the census of young moving group members.

  5. Building an Unusual White-Dwarf Duo

    NASA Astrophysics Data System (ADS)

    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

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

    SciTech Connect

    Harvey, Paul M.; Evans, Neal J. II; Henning, Thomas; Liu Yao; Wolf, Sebastian; Menard, Francois; Pinte, Christophe; Pascucci, Ilaria E-mail: nje@astro.as.utexas.edu E-mail: wolf@astrophysik.uni-kiel.de E-mail: yliu@pmo.ac.cn E-mail: christophe.pinte@obs.ujf-grenoble.fr E-mail: pascucci@lpl.arizona.edu

    2012-08-10

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

  7. Brown dwarfs as close companions to white dwarfs

    NASA Technical Reports Server (NTRS)

    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.

  8. Direct measurements of the fundamental properties of low-mass stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Dupuy, Trent J.

    2010-10-01

    [approximate]2 at a given mass, which means that model-based substellar mass determinations (e.g., for directly imaged extrasolar planets and the low-mass initial mass function) may be systematically overestimating the masses. (3) We have employed our large sample of binary orbits to carry out a novel test of the earliest evolutionary stages, by using the distribution of orbital eccentricities to distinguish between competing models of brown dwarf formation.

  9. White dwarf stars with carbon atmospheres.

    PubMed

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

    2007-11-22

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

  10. THE MASSES OF POPULATION II WHITE DWARFS

    SciTech Connect

    Kalirai, Jason S.; Davis, D. Saul; Richer, Harvey B.; Bergeron, P.; Catelan, Marcio; Hansen, Brad M. S.; Michael Rich, R. E-mail: sdavis@astro.ubc.c E-mail: bergeron@astro.umontreal.c E-mail: hansen@astro.ucla.ed

    2009-11-01

    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{sub 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 Hubble Space Telescope imaging observations, rich white dwarf populations of four nearby Milky Way globular clusters have recently been uncovered, and are found to extend impressive 5-8 mag in the faint-blue region of the Hertzsprung-Russell 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 spectroscopic observations, our results indicate that 0.8 M{sub sun} population II main-sequence stars evolving today form 0.53 +- 0.01 M{sub sun} white dwarfs. We discuss the implications of this result as it relates to our understanding of stellar structure and evolution of population II stars and for the age of the Galactic halo, as measured with white dwarf cooling theory.

  11. Gravitational Interactions of White Dwarf Double Stars

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

  12. The Potential of White Dwarf Cosmochronology

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Brassard, P.; Bergeron, P.

    2001-04-01

    In the light of recent significant progress on both the observational and theoretical fronts, we review the status of white dwarf stars as cosmochronometers. These objects represent the end products of stellar evolution for the vast majority of stars and, as such, can be used to constrain the ages of various populations of evolved stars in the Galaxy. For example, the oldest white dwarfs in the solar neighborhood (the remnants of the very first generation of intermediate-mass stars in the Galactic disk) are still visible and can be used, in conjunction with cooling theory, to estimate the age of the disk. More recent observations suggest the tantalizing possibility that a population of very old white dwarfs inhabits the Galactic halo. Such a population may contribute significantly to baryonic ``dark'' matter in the Milky Way and may be used to obtain an independent estimate of the age of the halo. In addition, white dwarf cosmochronology is likely to play a very significant role in the coming era of giant 8-10 m telescopes when faint white dwarf populations should be routinely discovered and studied in open and globular clusters. Based, in part, on the C. S. Beals Lecture presented by G. Fontaine at the Annual General Meeting of the Canadian Astronomical Society held in Vancouver (2000 May).

  13. White Dwarf Stars: A Brief Overview

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Brassard, P.; Charpinet, S.; Randall, S. K.; Van Grootel, V.

    2013-12-01

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

  14. White Dwarfs in the GALEX Survey

    NASA Technical Reports Server (NTRS)

    Kawka, Adela; Vennes, Stephane

    2007-01-01

    We have cross-correlated the 2dF QSO Redshift Survey (2QZ) white dwarf catalog with the GALEX 2nd Data Release and the Sloan Digital Sky Survey (SDSS) data release 5 to obtain ultraviolet photometry (FUV, NUV) for approximately 700 objects and optical photometry (ugriz) for approximately 800 objects. We have compared the optical-ultraviolet colors to synthetic white dwarf colors to obtain temperature estimates for approximately 250 of these objects. These white dwarfs have effective temperatures ranging from 10 000 K (cooling age of about 1Gyr) up to about 40000 K (cooling age of about 3 Myrs), with a few that have even higher temperatures. We found that to distinguish white dwarfs from other stellar luminosity classes both optical and ultraviolet colors are necessary, in particular for the hotter objects where there is contamination from B and 0 main-sequence stars. Using this sample we build a luminosity function for the DA white dwarfs with Mv < 12 mag.

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

    NASA Astrophysics Data System (ADS)

    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

  16. BANYAN. VIII. New Low-mass Stars and Brown Dwarfs with Candidate Circumstellar Disks

    NASA Astrophysics Data System (ADS)

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

    2016-11-01

    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 Gagné 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: 4 are new and 9 were already known in the literature. The new candidates are 2MASS J05010082-4337102 (M4.5), J08561384-1342242 ({{M}}8γ ), J12474428-3816464 ({{M}}9γ ), and J02265658-5327032 ({{L}}0δ ); they are candidate members of the TW Hya (˜ 10+/- 3 Myr), Columba (˜{42}-4+6 Myr), and Tucana-Horologium (˜ 45+/- 4 Myr) associations, with masses of 120 and 13-18 {M}{Jup}. The M8-L0 objects in Columba and Tucana-Horologium are potentially among the first substellar disk systems aged ˜40 Myr. Estimates of the new candidates’ mean disk temperatures and fractional luminosities are in the ranges ˜135{--}520 {{K}} and 0.021{--}0.15, respectively. New optical spectroscopy of J0501-4337 reveals strong Hα emission, possibly indicating ongoing accretion, provides a detection of lithium absorption, and shows a radial velocity measurement that is consistent with a membership to Columba. We also present a near-infrared spectrum of J0226-5327 that reveals Paschen β emission and shows signs of low surface gravity, consistent with accretion from a disk and a young age.

  17. VizieR Online Data Catalog: Near-IR spectroscopy of low-mass binaries and brown dwarfs (Mace, 2014)

    NASA Astrophysics Data System (ADS)

    Mace, G. N.

    2014-05-01

    The mass of a star at formation determines its subsequent evolution and demise. Low-mass stars are the most common products of star formation and their long main-sequence lifetimes cause them to accumulate over time. Star formation also produces many substellar-mass objects known as brown dwarfs, which emerge from their natal molecular clouds and continually cool as they age, pervading the Milky Way. Low-mass stars and brown dwarfs exhibit a wide range of physical characteristics and their abundance make them ideal subjects for testing formation and evolution models. I have examined a pair of pre-main sequence spectroscopic binaries and used radial velocity variations to determine orbital solutions and mass ratios. Additionally, I have employed synthetic spectra to estimate their effective temperatures and place them on theoretical Hertzsprung-Russell diagrams. From this analysis I discuss the formation and evolution of young binary systems and place bounds on absolute masses and radii. I have also studied the late-type T dwarfs revealed by the Wide-field Infrared Survey Explorer (WISE). This includes the exemplar T8 subdwarf Wolf 1130C, which has the lowest inferred metallicity in the literature and spectroscopic traits consistent with old age. Comparison to synthetic spectra implies that the dispersion in near-infrared colors of late-type T dwarfs is a result of age and/or thin sul de clouds. With the updated census of the L, T, and Y dwarfs we can now study specific brown dwarf subpopulations. Finally, I present a number of future studies that would develop our understanding of the physical qualities of T dwarf color outliers and disentangle the tracers of age and atmospheric properties. The thesis is available at: http://www.astro.ucla.edu/~gmace/thesis.html (7 data files).

  18. Be stars with white dwarf companions

    NASA Astrophysics Data System (ADS)

    Orio, Marina; Luna, Gerardo; Zemko, Polina; Kotulla, Ralf; Gallagher, Jay; Harbeck, Daniel

    2016-07-01

    A handful of supersoft X-ray sources in the Magellanic Clouds that could not be identified with transient nova outbursts turned out to be mainly massive close binaries. Recently, we have clearly identified a Be binary in M31, and are currently collecting data for another candidate in that galaxy. Work is in progress to assess whether the compact object companion really is a hydrogen burning white dwarf (the alternative being a massive stellar-mass black hole). If we can prove that Be+white dwarf interacting close binaries are common, and that hydrogen is often ignited on the white dwarf in these systems, we have discovered a new promising channel towards the explosion of supernovae of type Ia in star forming regions, without invoking double degenerate systems

  19. White dwarfs, the Galaxy and Dirac's cosmology

    NASA Technical Reports Server (NTRS)

    Stothers, R.

    1976-01-01

    The additive and multiplicative versions of Dirac's cosmological hypothesis relating the gravitational constant variation with elapsed time and number of particles populating the universe is invoked to account for the deficiency or absence of white dwarfs fainter than about 0.0001 solar luminosity. An estimate is made of white dwarf luminosity in accordance with the two evolutionary models, and it is conjectured that some old white dwarfs with high space velocities may be on the verge of gravitational collapse. Lack of a special mechanism to produce the vast numbers of black holes or other dead stars accounting for 'missing matter' in the vicinity of the sun and in the galactic halo is noted in Dirac's multiplicative model. Results indicate that either Dirac's theory is untenable, or that radiation and heating are of some unknown nature, or that the process of creation of new matter requires a corresponding input of energy.

  20. Recombination energy in double white dwarf formation

    NASA Astrophysics Data System (ADS)

    Nandez, J. L. A.; Ivanova, N.; Lombardi, J. C.

    2015-06-01

    In this Letter, we investigate the role of recombination energy during a common envelope event. We confirm that taking this energy into account helps to avoid the formation of the circumbinary envelope commonly found in previous studies. For the first time, we can model a complete common envelope event, with a clean compact double white dwarf binary system formed at the end. The resulting binary orbit is almost perfectly circular. In addition to considering recombination energy, we also show that between 1/4 and 1/2 of the released orbital energy is taken away by the ejected material. We apply this new method to the case of the double white dwarf system WD 1101+364, and we find that the progenitor system at the start of the common envelope event consisted of an ˜1.5 M⊙ red giant star in an ˜30 d orbit with a white dwarf companion.

  1. White Dwarfs in Gaia Data Release 1

    NASA Astrophysics Data System (ADS)

    Jordan, S.

    2017-03-01

    On September 14, the Gaia archives opened for access to the Gaia DR1. The catalogue with more than one billion star positions and more than two million parallaxes and proper motions will have enormous influence on many topics in astronomy. However, due to their extremely blue colour, parallaxes and proper motions of only six white dwarfs were directly measured. Tremblay et al. used these data and those for 46 white dwarfs in binaries in order to construct an empirical mass-radius relation. As it was the case for Hipparcos, the precision of the data does not allow for the characterisation of hydrogen envelope masses. With Gaia DR2 coming in late 2017 the prospects for white dwarf research are much better.

  2. A SuperWASP Benchmark Eclipsing Binary with a Very Low-Mass Secondary in the Brown Dwarf Desert

    NASA Astrophysics Data System (ADS)

    Gomez Maqueo Chew, Yilen; Garcia-Melendo, Enrique; Hebb, Leslie; Faedi, Francesca; Lopez-Morales, Mercedes; Pollacco, Don

    2012-08-01

    We will obtain eclipse light curves of a newly discovered eclipsing binary composed of a Sun-like primary with a secondary companion which can be either a very low mass M-dwarf (less than ~0.15 Msun) or a brown dwarf. The objects orbit each other with a period of ~14.3 days in an eccentric orbit, which as been confirmed with a high- precision radial velocity curve for the system. Therefore, these eclipse light curves will allow us to constrain the radii of the eclipsing components and orbital inclination of the system. Furthermore, the depth of the secondary eclipse which can only be observed in the near-infrared, directly constrains the temperature ratio between the components. In combination with the the masses derived from the radial velocity curve, our light curve analysis will unveil the true nature of the secondary. Whether it is a very-low mass star or a brown dwarf, direct measurements of the fundamental properties (masses, radii and temperatures) of such objects are very scarce and will provide key tests to current evolutionary models. Thus, we request two nights with FLAMINGOS at the KPNO 2.1m to observe a complete secondary eclipse of the system at near-infrared wavelengths in order to fully characterize the very low-mass component of the system.

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

    PubMed Central

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

    2011-01-01

    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, 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. PMID:21300910

  4. White Dwarf Mergers on Adaptive Meshes

    NASA Astrophysics Data System (ADS)

    Katz, Maximilian Peter

    The mergers of binary white dwarf systems are potential progenitors of astrophysical explosions such as Type Ia supernovae. These white dwarfs can merge either by orbital decay through the emission of gravitational waves or by direct collisions as a result of orbital perturbations. The coalescence of the stars may ignite nuclear fusion, resulting in the destruction of both stars through a thermonuclear runaway and ensuing detonation. The goal of this dissertation is to simulate binary white dwarf systems using the techniques of computational fluid dynamics and therefore to understand what numerical techniques are necessary to obtain accurate dynamical evolution of the system, as well as to learn what conditions are necessary to enable a realistic detonation. For this purpose I have used software that solves the relevant fluid equations, the Poisson equation for self-gravity, and the systems governing nuclear reactions between atomic species. These equations are modeled on a computational domain that uses the technique of adaptive mesh refinement to have the highest spatial resolution in the areas of the domain that are most sensitive to the need for accurate numerical evolution. I have identified that the most important obstacles to accurate evolution are the numerical violation of conservation of energy and angular momentum in the system, and the development of numerically seeded thermonuclear detonations that do not bear resemblance to physically correct detonations. I then developed methods for ameliorating these problems, and determined what metrics can be used for judging whether a given white dwarf merger simulation is trustworthy. This involved the development of a number of algorithmic improvements to the simulation software, which I describe. Finally, I performed high-resolution simulations of typical cases of white dwarf mergers and head-on collisions to demonstrate the impacts of these choices. The results of these simulations and the corresponding

  5. Mystery of a Dimming White Dwarf

    NASA Astrophysics Data System (ADS)

    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

  6. White dwarf stars with chemically stratified atmospheres

    NASA Technical Reports Server (NTRS)

    Muchmore, D.

    1982-01-01

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

  7. Ultra-high precision white dwarf asteroseismology

    NASA Astrophysics Data System (ADS)

    Giammichele, Noemi; Charpinet, Stéphane; Fontaine, Gilles; Brassard, Pierre; Zong, Weikai

    We present a brief progress report in our quest for deriving seismic models of pulsating white dwarfs that can account simultaneously for all the observed periods at the precision of the observations. We point out that this is possible from a pratical point of view only if parametrized models are used to complement evolutionary models. We adopt a double optimization procedure that insures that the best possible model in parameter space is found objectively and automatically. Our ultimate goal is to be able to account for the exquisite period data gathered with Kepler and Kepler-2 on key pulsating white dwarfs of both the DA (ZZ Ceti) and DB (V777 Her) type.

  8. White Dwarf Pulsational Constraints on Stellar Evolution

    NASA Astrophysics Data System (ADS)

    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

  9. Pulsating White Dwarfs in Globular Clusters

    NASA Astrophysics Data System (ADS)

    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.

  10. The Field White Dwarf Mass Distribution

    NASA Astrophysics Data System (ADS)

    Tremblay, P.-E.; Cummings, J.; Kalirai, J. S.

    2017-03-01

    We study the white dwarf mass distributions for the volume-complete survey within 20 pc and the SDSS magnitude-limited sample. The observed mass distributions are modelled with Monte Carlo simulations. We find that under fixed standard assumptions for Galactic and stellar evolution, the predicted masses are in good qualitative agreement with the observed values. Nevertheless, the number of massive white dwarfs is overpredicted and we 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.

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

    NASA Astrophysics Data System (ADS)

    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

  12. 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

    NASA Astrophysics Data System (ADS)

    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.

  13. 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

    SciTech Connect

    Jiang Peng; Ge Jian; De Lee, Nathan; Fleming, Scott W.; Lee, Brian L.; Ma Bo; Wang, Ji; Cargile, Phillip; Hebb, Leslie; Stassun, Keivan G.; Crepp, Justin R.; Porto de Mello, Gustavo F.; Ferreira, Leticia D.; Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I.; Ghezzi, Luan; Wisniewski, John P.; Agol, Eric; 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.

  14. White Dwarfs in the Galaxy's Halo

    NASA Astrophysics Data System (ADS)

    Oppenheimer, B.; Murdin, P.

    2002-12-01

    The Galaxy's large spherical halo (see GALACTICMETAL-POOR HALO and HALO, GALACTIC) may harboras many as several hundred billion WHITE DWARFS, apopulation as large in number as the total number of stars in theGalaxy's disk (see DISK GALAXIES and GALACTIC THIN DISK). Although this assertion iscontroversial, several astronomical surveys provide strong support for it andthe implications affect fields ...

  15. SOAR + SMARTS Southern White Dwarf Survey

    NASA Astrophysics Data System (ADS)

    Subasavage, John P.; Lepine, S.

    2012-01-01

    We present early results from the SOAR + SMARTS Southern White Dwarf SURVEY (SSSWDS). Our initial sift of relatively bright (15 < V < 18), white dwarf candidates uses the technique of reduced proper motion with inputs from the SUPERBLINK proper motion database combined with photographic magnitudes. Crude distance estimates from the linear photographic magnitude-color relation of Oppenheimer et al. 2001 are obtained and permit prioritized follow-up. For confirmation of luminosity class, we use the SOAR telescope atop Cerro Pachon equipped with the Goodman Spectrograph and a moderate resolution grating. In tandem, we acquire multi-epoch, optical Johnson-Kron-Cousins BVRI photometry using the SMARTS 1.0m telescope atop CTIO. Combined with JHK from 2MASS, we compare the photometric SED to relevant white dwarf model atmospheres to estimate physical parameters (e.g., effective temperature, mass) and distance. For the nearest targets, specifically those within the RECONS (www.recons.org) horizon of 25 pc, we aim to obtain trigonometric parallaxes as part of the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI) project being conducted at the SMARTS 0.9m telescope. To date, we have confirmed 100 relatively bright, new white dwarfs in the southern hemisphere. Of those, 13 are estimated to be within our 25 pc horizon-of-interest, including two that are estimated to be within 15 pc. Ongoing observations will boost these figures by the end of the project.

  16. Photospheric composition and structure in white dwarfs

    NASA Astrophysics Data System (ADS)

    Barstow, M. A.

    1993-12-01

    One of the central mysteries of white dwarf studies has been the nature and abundance of trace elements in the atmospheres of these stars. It had been thought that the dominant trace element in otherwise pure hydrogen DA white dwarf atmospheres was helium. However, some spectroscopic and theoretical evidence suggested that, at least in some stars, heavier elements may be important. Prior to the launch of ROSAT the questions regarding the atmospheric composition of DA white dwarfs in general remained unresolved. The ROSAT mission has provided EUV and X-ray data for a large sample of DA white dwarfs with which we can study their photospheric composition and structure through the effect of trace opacity sources on the emergent fluxes. Contrary to expectations little (if any) helium is found and the main sources of opacity appear to be trace heavy elements. Support for these conclusions is found in recent EUV and far-UV spectra of several stars. However, photometric data do not allow us to determine the abundance of the individual elements and observations with the extreme ultraviolet explorer satellite (EUVE) spectrometers will be essential for detailed composition measurements.

  17. White dwarfs identified in LAMOST DR 2

    NASA Astrophysics Data System (ADS)

    Guo, Jincheng; Zhao, Jingkun; Tziamtzis, Anestis; Liu, Jifeng; Li, Lifang; Zhang, Yong; Hou, Yonghui; Wang, Yuefei

    2015-12-01

    Here we present a catalogue of 1056 spectroscopically identified hydrogen-dominated white dwarfs (DAWDs), 34 helium-dominated white dwarfs (DBWDs) and 276 white dwarf main sequence (WDMS) binaries from the Large sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey data release 2 (DR2). 383 DAWDs, 4 DBWDs and 138 WDMSs are new identifications after cross-match with literature. There are ˜4100 k spectra in total from DR 2. The low ratio of white dwarfs found in LAMOST is attributed to biased selection of LAMOST input catalogue and much brighter targets relative to stars observed in Sloan Digital Sky Survey. In this paper, a new DAWD selection method is adopted as a new attempt and supplement to the traditional methods. The effective temperature, surface gravity, mass, cooling age and distance of high signal-to-noise DAWDs are estimated. The peak of the mass distribution is found to be ˜0.6 M⊙, which is consistent with previous work. The parameters of WDMS binaries are also provided in this paper. As the foundation of our future work, which is to identify more WDs with debris disc, WDs found in LAMOST showed a lot of potential. Interesting infrared-excess WDs will be reported in our forthcoming paper.

  18. DA white dwarfs in the Kepler field

    NASA Astrophysics Data System (ADS)

    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.

  19. Theoretical Study of White Dwarf Double Stars

    NASA Astrophysics Data System (ADS)

    Hira, Ajit; Koetter, Ted; Rivera, Ruben; Diaz, Juan

    2015-04-01

    We continue our interest in the computational simulation of the astrophysical phenomena with a study of gravitationally-bound binary stars, composed of at least one white dwarf star. Of particular interest to astrophysicists are the conditions inside a white dwarf star in the time frame leading up to its explosive end as a Type Ia supernova, for an understanding of the massive stellar explosions. In addition, the studies of the evolution of white dwarfs could serve as promising probes of theories of gravitation. We developed FORTRAN computer programs to implement our models for white dwarfs and other stars. These codes allow for different sizes and masses of stars. Simulations were done in the mass interval from 0.1 to 2.0 solar masses. Our goal was to obtain both atmospheric and orbital parameters. The computational results thus obtained are compared with relevant observational data. The data are further analyzed to identify trends in terms of sizes and masses of stars. We hope to extend our computational studies to blue giant stars in the future. Research Supported by National Science Foundation.

  20. The 25 parsec local white dwarf population

    NASA Astrophysics Data System (ADS)

    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.

  1. Disintegrating Planetary Bodies Around a White Dwarf

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-02-01

    Several months ago, the discovery of WD 1145+017 was announced. This white dwarf appears to be orbited by planetary bodies that are actively disintegrating due to the strong gravitational pull of their host. A follow-up study now reveals that this system has dramatically evolved since its discovery.Signs of DisruptionPotential planetary bodies orbiting a white dwarf would be exposed to a particular risk: if their orbits were perturbed and they passed inside the white dwarfs tidal radius, they would be torn apart. Their material could then form a debris disk around the white dwarf and eventually be accreted.Interestingly, we have two pieces of evidence that this actually happens:Weve observed warm, dusty debris disks around ~4% of white dwarfs, andThe atmospheres of ~25-50% of white dwarfs are polluted by heavy elements that have likely accreted recently.But in spite of this indirect evidence of planet disintegration, wed never observed planetary bodies actively being disrupted around white dwarfs until recently.Unusual TransitsIn April 2015, observations by Keplers K2 mission revealed a strange transit signal around WD 1145+017, a white dwarf 570 light-years from Earth that has both a dusty debris disk and a polluted atmosphere. This signal was interpreted as the transit of at least one, and possibly several, disintegrating planetesimals.In a recent follow-up, a team of scientists led by Boris Gnsicke (University of Warwick) obtained high-speed photometry of WD 1145+017 using the ULTRASPEC camera on the 2.4m Thai National Telescope. These observations were taken in November and December of 2015 roughly seven months after the initial photometric observations of the system. They reveal that dramatic changes have occurred in this short time.Rapid EvolutionA sample light curve from TNT/ULTRASPEC, obtained in December 2015 over 3.9 hours. Many varied transits are evident (click for a better view!). Transits labeled in color appear across multiple nights. [Gnsicke et al

  2. Characterizing Companions to Low-Mass Stars: A Large-Scale, Volume-Limited Survey of Local M-dwarfs

    NASA Astrophysics Data System (ADS)

    Ward-Duong, Kimberly; Patience, J.; De Rosa, R.; Rajan, A.

    2013-01-01

    M-dwarfs constitute the major fraction of stars within both the solar neighborhood and nearby star-forming regions. However, key M-dwarf companion characteristics - including multiplicity fraction, mass ratios, and separation distributions - are less certain for field stars, due to limited sample sizes and non-uniform selection criteria. Studies of star-forming regions often compare results to solar-type field stars due to the extensive population statistics available for G-dwarfs, but field M-dwarfs represent a more analogous population for comparison due to their prevalence. We present results on a stellar and substellar companion study covering separations from ~1 - 10,000 AU, based on a volume-limited survey of ~300 M-dwarfs within 15 pc. Our study constrains the frequency of binary companions and the shape of the companion separation and mass ratio distributions. Diffraction-limited, mid-to-near infrared archival data were obtained from the Very Large Telescope, Hubble Space Telescope, and Canada-France-Hawaii Telescope, to detect nearby companions to M-dwarfs from ~1 to 100 AU. To supplement the high-resolution data, wide-field archival plates were searched for companions with separations of 100 to 10,000 AU. The all-sky survey data include multiple epochs, and follow up observations at higher resolution will allow us to confirm or reject the new companion candidates detected during our analysis. These multi-epoch observations provide confirmation of common proper motions, thereby minimizing background contamination and providing comprehensive statistics for M-star binaries. Preliminary analysis of an initial subset of the sample suggests a lower limit to the multiplicity of 23 ± 7% within the restricted separation range. Characterizations of the binary frequency for M-dwarfs provide crucial insights into the low-mass star formation environment, and hold additional implications for the frequency and evolutionary histories of their associated disks and

  3. White dwarf evolution - Cradle-to-grave constraints via pulsation

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.

    1990-01-01

    White dwarf evolution, particularly in the early phases, is not very strongly constrained by observation. Fortunately, white dwarfs undergo nonradial pulsation in three distinct regions of the H-R diagram. These pulsations provide accurate masses, surface compositional structure and rotation velocities, and help constrain other important physical properties. We demonstrate the application of the tools of stellar seismology to white dwarf evolution using the hot white dwarf star PG 1159-035 and the cool DAV (or ZZ Ceti) stars as examples. From pulsation studies, significant challenges to the theory of white dwarf evolution emerge.

  4. Precise atmospheric parameters for the shortest-period binary white dwarfs: gravitational waves, metals, and pulsations

    SciTech Connect

    Gianninas, A.; Kilic, Mukremin; Dufour, P.; Bergeron, P.; Brown, Warren R.; Hermes, J. J.

    2014-10-10

    We present a detailed spectroscopic analysis of 61 low-mass white dwarfs and provide precise atmospheric parameters, masses, and updated binary system parameters based on our new model atmosphere grids and the most recent evolutionary model calculations. For the first time, we measure systematic abundances of He, Ca, and Mg for metal-rich, extremely low mass white dwarfs and examine the distribution of these abundances as a function of effective temperature and mass. Based on our preliminary results, we discuss the possibility that shell flashes may be responsible for the presence of the observed He and metals. We compare stellar radii derived from our spectroscopic analysis to model-independent measurements and find good agreement except for white dwarfs with T {sub eff} ≲ 10,000 K. We also calculate the expected gravitational wave strain for each system and discuss their significance to the eLISA space-borne gravitational wave observatory. Finally, we provide an update on the instability strip of extremely low mass white dwarf pulsators.

  5. The Brown Dwarf Eclipsing Binary 2M0535-05: A Case Study for Activity Effects on Physical Properties of Low-Mass Stars and Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Stassun, K. G.

    2013-02-01

    2M0535-05 is a one-of-a-kind eclipsing binary (EB) comprising two brown dwarfs (BDs), and is an important benchmark for understanding the fundamental properties of BDs and low-mass stars. Because 2M0535-05 presents a peculiar reversal of temperatures with mass (the higher mass, magnetically active BD in the system is cooler than the lower mass companion BD), 2M0535-05 is particularly important as a case study for the effects of magnetic activity on the properties of low-mass objects. Using a large number of low-mass M-dwarfs and EBs in the field, we have developed empirical relations for determining the amount by which the temperatures and radii-and therefore the estimated masses-of low-mass stars and BDs are altered due to chromospheric activity. The relations link the amount by which an active object's temperature is suppressed, and its radius inflated, to the strength of its Hα emission. These relations are found to approximately preserve bolometric luminosity. Applying these relations to 2M0535-05 brings the activity-corrected radii and temperatures of 2M0535-05 into precise agreement with theoretical isochrones for inactive stars. The relations that we present are applicable to BDs and low-mass stars with masses below 0.8 M⊙ and for which the activity, as measured by Hα, is in the range - 4.6 < log LHα/Lbol < -3.3. We discuss implications of this work for determinations of young cluster IMFs, and discuss competing ideas for the physical mechanism by which magnetic fields alter the temperatures and radii of low-mass stars.

  6. A wide binary trigger for white dwarf pollution

    NASA Astrophysics Data System (ADS)

    Bonsor, Amy; Veras, Dimitri

    2015-11-01

    Metal pollution in white dwarf atmospheres is commonly assumed 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 on to 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 per cent of an observed sample of white dwarfs with wide binary companions, independent of white dwarf age. This age independence is the key difference between this wide binary mechanism and others mechanisms suggested in the literature to explain white dwarf pollution. Current observational samples are not large enough to assess whether this mechanism makes a significant contribution to the population of polluted white dwarfs, for which better constraints on the wide binary population are required, such as those that will be obtained in the near future with Gaia.

  7. Radial velocities of very low mass stars and candidate brown dwarf members of the Hyades and Pleiades

    NASA Technical Reports Server (NTRS)

    Stauffer, John R.; Liebert, James; Giampapa, Mark; Macintosh, Bruce; Reid, Neill; Hamilton, Donald

    1994-01-01

    We have determined H alpha equivalent widths and radial velocities with 1 sigma accuracies of approximately 5 km s(exp -1) for approximately 20 candidate very low mass members of the Hyades and Pleiades clusters. The radial velocities for the Hyades sample suggest that nearly all of these stars are indeed highly probable members of the Hyades. The faintest stars in the Hyades sample have masses of order 0.1 solar mass. We also obtained radial velocities for four candidate very low mass members of the Pleiades and two objects that are candidate BD Pleiads. All of these stars have apparent V magnitudes fainter than the Hyades stars we observed, and the resultant radial velocity accuracy is worse. We believe that the three brighter stars are indeed likely very low mass stellar members of the Pleiades, whereas the status of the two brown dwarf candidates is uncertain. The Hyades stars we have observed and the three Pleiades very low mass stars are the lowest mass members of any open cluster whose membership has been confirmed by radial velocities and whose chromospheric activity has been measured. We see no change in chromospheric activity at the boundary where stars are expected to become fully convective (M approximately equals 0.3 solar mass) in either cluster. In the Pleiades, however, there may be a decrease in chromospheric activity for stars with (V-I)(sub K) greater than 3.5 (M less than or equal to 0.1 solar mass).

  8. Spectroscopic Analysis of Hot (Pre-) White Dwarfs

    NASA Astrophysics Data System (ADS)

    Reindl, Nicole

    2015-03-01

    In this work, different kinds of hot (pre-) white dwarfs (WD) were analyzed by means of static and expanding non-LTE model atmospheres to obtain a better understanding of the late, hot stages of stellar evolution. In the first paper, Reindl et al. (2014c), we derived for the first time the temporal evolution of the atmospheric parameters of the unusually quick evolving, hydrogen-rich central star of planetary nebula (CSPN) SAO 244567. We confirm that SAO 244567 must be a low-mass star (M < 0.55 M ). The slow evolution of the respective canonical stellar evolutionary models is, however, in strong contradiction to the observed fast evolution and the young planetary nebula. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of a close-binary evolution. Then SAO 244567 would be a low-mass (0.35 M ) helium pre-WD after the common-envelope phase, during which the planetary nebula was ejected. The paper Reindl et al. (2014b) aimed to obtain clues about the nature of the exotic O(He) stars. The new optical spectra allowed a more precise determination of the atmospheric parameters of the two CSPNe, K 1- 27 and LoTr 4. Furthermore, upper limits for the mass-loss rates of K 1- 27, LoTr 4, HS 1522+6615, and HS 2209+8229 were derived. Finally, the atmospheric parameters of the O(He) stars were compared to those of other helium-dominated stars and stellar evolution calculations in order to explain a helium-dominated stellar evolution sequence. In Reindl et al. (2014a), we identified 24 DO WDs in the tenth data release of the Sloan Digital Sky Survey and analyzed them for the first time by means of non-LTE model atmospheres. Two of our objects are the coolest DO WDs ever discovered that still show a considerable amount of carbon in the atmosphere. This is in strong contradiction with diffusion calculations and we suggested that a weak mass-loss is present in DO WDs. The mass distribution of DO WDs beyond the wind limit

  9. REFINED METALLICITY INDICES FOR M DWARFS USING THE SLoWPoKES CATALOG OF WIDE, LOW-MASS BINARIES

    SciTech Connect

    Dhital, Saurav; Stassun, Keivan G.; Bastien, Fabienne A.; West, Andrew A.; Massey, Angela P.; Bochanski, John J.

    2012-03-15

    We report the results from spectroscopic observations of 113 ultra-wide, low-mass binary systems, largely composed of M0-M3 dwarfs, from the SLoWPoKES catalog of common proper motion pairs identified in the Sloan Digital Sky Survey. Radial velocities of each binary member were used to confirm that they are comoving and, consequently, to further validate the high fidelity of the SLoWPoKES catalog. Ten stars appear to be spectroscopic binaries based on broad or split spectral features, supporting previous findings that wide binaries are likely to be hierarchical systems. We measured the H{alpha} equivalent width of the stars in our sample and found that components of 81% of the observed pairs have similar H{alpha} levels. The difference in H{alpha} equivalent width among components with similar masses was smaller than the range of H{alpha} variability for individual objects. We confirm that the Lepine et al. {zeta}-index traces iso-metallicity loci for most of our sample of M dwarfs. However, we find a small systematic bias in {zeta}, especially in the early-type M dwarfs. We use our sample to recalibrate the definition of {zeta}. While representing a small change in the definition, the new {zeta} is a significantly better predictor of iso-metallicity for the higher-mass M dwarfs.

  10. HUBBLE PINPOINTS WHITE DWARFS IN GLOBULAR CLUSTER

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Peering deep inside a cluster of several hundred thousand stars, NASA's Hubble Space Telescope uncovered the oldest burned-out stars in our Milky Way Galaxy. Located in the globular cluster M4, these small, dying stars - called white dwarfs - are giving astronomers a fresh reading on one of the biggest questions in astronomy: How old is the universe? The ancient white dwarfs in M4 are about 12 to 13 billion years old. After accounting for the time it took the cluster to form after the big bang, astronomers found that the age of the white dwarfs agrees with previous estimates for the universe's age. In the top panel, a ground-based observatory snapped a panoramic view of the entire cluster, which contains several hundred thousand stars within a volume of 10 to 30 light-years across. The Kitt Peak National Observatory's 0.9-meter telescope took this picture in March 1995. The box at left indicates the region observed by the Hubble telescope. The Hubble telescope studied a small region of the cluster. A section of that region is seen in the picture at bottom left. A sampling of an even smaller region is shown at bottom right. This region is only about one light-year across. In this smaller region, Hubble pinpointed a number of faint white dwarfs. The blue circles pinpoint the dwarfs. It took nearly eight days of exposure time over a 67-day period to find these extremely faint stars. Globular clusters are among the oldest clusters of stars in the universe. The faintest and coolest white dwarfs within globular clusters can yield a globular cluster's age. Earlier Hubble observations showed that the first stars formed less than 1 billion years after the universe's birth in the big bang. So, finding the oldest stars puts astronomers within arm's reach of the universe's age. M4 is 7,000 light-years away in the constellation Scorpius. Hubble's Wide Field and Planetary Camera 2 made the observations from January through April 2001. These optical observations were combined to

  11. An expanded set of brown dwarf and very low mass star models

    NASA Technical Reports Server (NTRS)

    Burrows, A.; Hubbard, W. B.; Saumon, D.; Lunine, J. I.

    1993-01-01

    We present in this paper updated and improved theoretical models of brown dwarfs and late M dwarfs. The evolution and characteristics of objects between 0.01 and 0.2 solar mass are exhaustively investigated and special emphasis is placed on their properties at early ages. The dependence on the helium fraction, deuterium fraction, and metallicity of the masses, effective temperature and luminosities at the edge of the hydrogen main sequence are calculated. We derive luminosity functions for representative mass functions and compare our predictions to recent cluster data. We show that there are distinctive features in the theoretical luminosity functions that can serve as diagnostics of brown dwarf physics. A zero-metallicity model is presented as a bound to or approximation of a putative extreme halo population.

  12. The IMF of Low-Mass Stars and Brown Dwarfs in Taurus

    NASA Astrophysics Data System (ADS)

    Luhman, K.

    2001-05-01

    By combining deep optical imaging and infrared spectroscopy with data from the Two-Micron All-Sky Survey (2MASS) and from previous studies, I have measured the Initial Mass Function (IMF) for a reddening-limited sample in four fields in the Taurus star forming region. This IMF is representative of the young populations within these fields for masses above 0.02 Msun. Relative to the similarly derived IMF for the Trapezium Cluster (Luhman et al.), the IMF for Taurus exhibits a modest deficit of stars above one solar mass (i.e., steeper slope), the same turnover mass (0.8 Msun), and a significant deficit of brown dwarfs. If the IMF in Taurus were the same as that in the Trapezium, 12.8+/-1.8 brown dwarfs (>0.02 Msun) are expected in these Taurus fields where only one brown dwarf candidate is found. These results are used to test theories of the IMF.

  13. Pulsating White Dwarf Star GD99

    NASA Astrophysics Data System (ADS)

    Chynoweth, K. M.; Thompson, S.; Mullally, F.; Yeates, C.

    2004-12-01

    We present 15 hours of time-series photometry of the variable white dwarf star GD99. These data were obtained at the McDonald Observatory 2.1m Otto Struve Telescope in January 2003, using the Argos CCD photometer. We achieved a noise level as low as 0.07 %, as measured from the power spectrum of our first night. Our observations confirm that GD99 is a unique pulsating white dwarf whose modes show characteristics of both the hot and cold type of DA variable stars. Additionally, GD99 has a large number of modes, making it a good candidate for asteroseismological study. Our preliminary results indicate that this star merits further study to decipher its abundant set of unusual modes. With such a rich period structure, longer continuous data sets will be required to fully resolve the pulsation spectrum.

  14. The field white dwarf mass distribution

    NASA Astrophysics Data System (ADS)

    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.

  15. PHYSICAL PROPERTIES OF YOUNG BROWN DWARFS AND VERY LOW MASS STARS INFERRED FROM HIGH-RESOLUTION MODEL SPECTRA

    SciTech Connect

    Rice, Emily L.; Mclean, Ian S.; Barman, T.; Prato, L.; Kirkpatrick, J. Davy

    2010-01-01

    By comparing near-infrared spectra with atmospheric 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 {approx}M6 objects with ages ranging from <1 Myr to >1 Gyr. Spectra were obtained in the J band at medium (R {approx} 2000) and high (R {approx} 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 K I lines, and investigate deficiencies in the models, particularly missing FeH opacity, that will need to be addressed in order to extend our analysis to cooler objects. The precision that can be obtained for each parameter using medium- and high-resolution near-infrared spectra is estimated and the implications for future studies of very low mass stars and brown dwarfs are discussed.

  16. On the White Dwarf Mass Problem of Cataclysmic Variables

    NASA Astrophysics Data System (ADS)

    Liu, Wei-Min; Li, Xiang-Dong

    2016-11-01

    Recent observations show that 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 be 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 (AML) 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 AML is required.

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

    SciTech Connect

    Brown, Warren R.; Kenyon, Scott J.; Kilic, Mukremin; Gianninas, A.; Allende Prieto, Carlos E-mail: skenyon@cfa.harvard.edu E-mail: alexg@nhn.ou.edu

    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.

  18. White Dwarf Convection Preceding Type Ia Supernovae

    NASA Astrophysics Data System (ADS)

    Zingale, Michael; Almgren, A. S.; Bell, J. B.; Malone, C. M.; Nonaka, A.; Woosley, S. E.

    2010-01-01

    In the single degenerate scenario for Type Ia supernovae, a Chandrasekhar mass white dwarf `simmers' for centuries preceding the ultimate explosion. During this period, reactions near the center drive convection throughout most of the interior of the white dwarf. The details of this convective flow determine how the first flames in the white dwarf ignite. Simulating this phase is difficult because the flows are highly subsonic. Using the low Mach number hydrodynamics code, MAESTRO, we present 3-d, full star models of the final hours of this convective phase, up to the point of ignition of a Type Ia supernova. We discuss the details of the convective velocity field and the locations of the initial hot spots. Finally, we show some preliminary results with rotation. Support for this work came from the DOE/Office of Nuclear Physics, grant No. DE-FG02-06ER41448 (Stony Brook), the SciDAC Program of the DOE Office of Mathematics, Information, and Computational Sciences under the DOE under contract No. DE-AC02-05CH11231 (LBNL), and the DOE SciDAC program, under grant No. DE-FC02-06ER41438 (UCSC). We made use of the jaguar machine via a DOE INCITE allocation at the Oak Ridge Leadership Computational Facility.

  19. White dwarf cosmochronology in the solar neighborhood

    SciTech Connect

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

    2014-08-20

    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 that 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 {sub initial}/M {sub ☉} ≲ 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 < Age (Gyr) < 10. Furthermore, the observed total age of ∼10 Gyr for the oldest white dwarfs in the local sample is consistent with the early seminal studies that have determined the age of the Galactic disk from stellar remnants. The main shortcoming of our study is the small size of the local white dwarf sample. However, the presented technique can be applied to larger samples in the future.

  20. Open Science Project in White Dwarf Research

    NASA Astrophysics Data System (ADS)

    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.

  1. Making Sense Out of Pulsating Pre-ELM and ELM White Dwarfs

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Istrate, A.; Gianninas, A.; Brassard, P.; Van Grootel, V.

    2017-03-01

    We present a unified view of pulsations in both pre-ELM and ELM white dwarfs within the framework of state-of-the-art binary evolution calculations that take into account the combined effects of diffusion and rotational mixing. We find that rotational mixing is able to maintain against settling a sufficient amount of helium in the envelope in order to fuel pulsations through He II-He III ionization on the pre-ELM branch of the evolutionary track in the spectroscopic HR diagram. By the time such a low-mass white dwarf enters the ZZ Ceti instability strip on the cooling branch, settling has taken over rotational mixing and produced a pure H envelope. Such a star then pulsates again, but, this time, as a DA white dwarf of the ZZ Ceti type.

  2. From Accretion to Explosion and Beyond: Transforming White Dwarfs to Neutron Stars and Black Holes

    NASA Astrophysics Data System (ADS)

    Di Stefano, Rosanne; Harris, R.

    2010-03-01

    White dwarfs accreting at high rates can grow in mass, exhibiting episodes of supersoft-source activity. Some can achieve the Chandrasekhar mass and will either become Type Ia supernovae or else will collapse, becoming neutron stars. We consider white dwarfs with giant donors, computing the rates of both supernovae and collapses. For the collapses, we follow each system to the end of accretion. Some of these systems will appear as ultraluminous x-ray sources and some will go on to become low-mass black holes. This scenario should be fairly common in young stellar populations and links a wide range of astrophysical phenomena. Indeed, it is a veritable cornucopia for the high-energy astrophysicist, offering accreting white dwarfs, neutron stars, and black holes, Type Ia supernovae, gamma-ray bursts, supersoft sources, ultraluminous sources, and neutron star and black hole binaries in globular clusters.

  3. A Statistical Analysis of SEEDS and Other High-contrast Exoplanet Surveys: Massive Planets or Low-mass Brown Dwarfs?

    NASA Astrophysics Data System (ADS)

    Brandt, Timothy D.; McElwain, Michael W.; Turner, Edwin L.; Mede, Kyle; Spiegel, David S.; Kuzuhara, Masayuki; Schlieder, Joshua E.; Wisniewski, John P.; Abe, L.; Biller, B.; 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.

    2014-10-01

    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 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 J, with a single power-law distribution. We find that p(M, a)vpropM -0.65 ± 0.60 a -0.85 ± 0.39 (1σ errors) provides an adequate fit to our data, with 1.0%-3.1% (68% confidence) of stars hosting 5-70 M 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. Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

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

    SciTech Connect

    Brandt, Timothy D.; Spiegel, David S.; McElwain, Michael W.; Grady, C. A.; Turner, Edwin L.; Mede, Kyle; Kuzuhara, Masayuki; Schlieder, Joshua E.; Brandner, W.; Feldt, M.; Wisniewski, John P.; Abe, L.; Biller, B.; Carson, J.; Currie, T.; Egner, S.; Golota, T.; Guyon, O.; Goto, M.; Hashimoto, J.; 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.

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

    NASA Astrophysics Data System (ADS)

    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.

  6. The evolution of white dwarfs resulting from helium-enhanced, low-metallicity progenitor stars

    NASA Astrophysics Data System (ADS)

    Althaus, Leandro G.; De Gerónimo, Francisco; Córsico, Alejandro; Torres, Santiago; García-Berro, Enrique

    2017-01-01

    Context. Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for ω Centauri, which shows clear evidence of a helium-enriched subpopulation characterized by a helium abundance as high as Y = 0.4 Aims: We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage that is appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of helium-rich progenitors. Methods: We derived white dwarf sequences from progenitors with stellar mass ranging from 0.60 to 2.0 M⊙ and for an initial helium abundance of Y = 0.4. We adopted two values of metallicity: Z = 0.001 and Z = 0.0005. Results: We explored different issues of white dwarf evolution and their helium-rich progenitors. 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 the standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored. Conclusions: We find that, for the range of initial masses explored in this paper, the final mass of the helium-rich progenitors is markedly higher than the final mass expected from progenitors with the usual helium abundance. We also find that progenitors with initial mass lower than M ≃ 0.65 M⊙ evolve directly into helium-core white dwarfs in less than 14 Gyr, and that, for larger progenitor masses, the evolution of the resulting low-mass carbon-oxygen white dwarfs is dominated by residual nuclear burning. For helium-core white dwarfs, we find that they evolve markedly faster than their counterparts coming from standard progenitors. Also, in contrast with what occurs for white dwarfs resulting from progenitors with the standard helium abundance, the impact of residual burning on the cooling time of

  7. The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Taurus

    NASA Astrophysics Data System (ADS)

    Luhman, K. L.

    2000-12-01

    By combining deep optical imaging and infrared spectroscopy with data from the Two-Micron All-Sky Survey (2MASS) and from previous studies (e.g., Briceño et al.), I have measured the initial mass function (IMF) for a reddening-limited sample in four fields in the Taurus star-forming region. This IMF is representative of the young populations within these fields for masses above 0.02 Msolar. Relative to the similarly derived IMF for the Trapezium Cluster (Luhman et al.), the IMF for Taurus exhibits a modest deficit of stars above 1 solar mass (i.e., steeper slope), the same turnover mass (~0.8 Msolar), and a significant deficit of brown dwarfs. If the IMF in Taurus were the same as that in the Trapezium, 12.8+/-1.8 brown dwarfs (>0.02 Msolar) are expected in these Taurus fields where only one brown dwarf candidate is found. These results are used to test theories of the IMF. Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation.

  8. Radial velocities of very low mass stars and candidate brown dwarf members of the Hyades and Pleiades, 2

    NASA Technical Reports Server (NTRS)

    Stauffer, John R.; Liebert, James; Giampapa, Mark

    1995-01-01

    We have determined H alpha equivalent widths and radial velocities with 1 sigma accuracies of approximately 5 km/s for approximately 20 candidate very low mass members of the Pleiades cluster and for a few proposed very low mass members of the Hyades. Most of the Pleiades targets were selected from the recent Hambly, Hawkins, and Jameson proper motion survey, where they were identified as probable Pleiades brown dwarfs with an age spread from 3 to 70 Myr. Our spectroscopic data and a reinterpretation of the photometric data confirm that these objects are indeed likely Pleiades members; however, we believe that they more likely have masses slightly above the hydrogen burning mass limit and that there is no firm evidence for an age spread amongst these stars. All of the very low mass Pleiades and Hyades members show H alpha in emission. However, the ratio of H alpha flux to biometric flux in the Pleiades shows a maximum near M(sub Bol) approximately equal to 9.5 (M approximately equal to 0.3 solar mass) and a sharp decrease to lower masses. This break occurs at the approximate mass where low mass stars are expected to become fully convective, and it is tempting to assume that the decrease in H alpha flux is caused by some change in the behavior of stellar dynamos at this mass. We do not see a similar break in activity at this mass in the Hyades. We discuss possible evolutionary explanations for this difference in the H alpha activity between the two clusters.

  9. Physical Properties of White Dwarfs from Multi-Band Photometry

    NASA Astrophysics Data System (ADS)

    Raddi, R.

    2017-03-01

    We describe a hierarchical Bayesian model to measure the physical parameters (mass, cooling age, distance, interstellar extinction) of single white dwarfs using only multi-band UV to IR photometry. We test our model on a set of known white dwarfs with well-assessed atmospheric parameters, determined via optical spectroscopy. Looking forward to the results of the ESA Gaia mission, we derive the posterior distributions of white dwarf parameters in two different scenarios with known or unknown parallaxes.

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

    NASA Astrophysics Data System (ADS)

    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.

  11. THE BROWN DWARF KINEMATICS PROJECT. II. DETAILS ON NINE WIDE COMMON PROPER MOTION VERY LOW MASS COMPANIONS TO NEARBY STARS ,

    SciTech Connect

    Faherty, Jacqueline K.; Shara, Michael M.; Burgasser, Adam J.; West, Andrew A.; Bochanski, John J.; Cruz, Kelle L.; Walter, Frederick M.

    2010-01-15

    We report on nine wide common proper motion systems containing late-type M, L, or T companions. We confirm six previously reported companions, and identify three new systems. The ages of these systems are determined using diagnostics for both stellar primaries and low-mass secondaries and masses for the secondaries are inferred using evolutionary models. Of our three new discoveries, the M3+T6.5 pair G 204-39 and SDSS J1758+4633 has an age constrained to 0.5-1.5 Gyr making the secondary a potentially useful brown dwarf benchmark. The G5+L4 pair G 200-28 and SDSS J1416+5006 has a projected separation of {approx}25,000 AU making it one of the widest and lowest binding energy systems known to date. The system containing NLTT 2274 and SDSS J0041+1341 is an older M4+L0 (>4.5 Gyr) pair which shows H{alpha} activity in the secondary but not the primary making it a useful tracer of age/mass/activity trends. Two of the nine systems have discrepant component ages that emerge from stellar or ultracool diagnostics indicating possible shortcomings in our understanding of the age diagnostics of stars and brown dwarfs. We find a resolved binary frequency for widely separated (>100 AU) low-mass companions (i.e., at least a triple system) which is at least twice the frequency found for the field ultracool dwarf population. The ratio of triples to binaries and quadruples to binaries is also high for this sample: 3:5 and 1:4, respectively, compared to 8 pc sample values of 1:4 and 1:26. The additional components in these wide companion systems indicates a formation mechanism that requires a third or fourth component to maintain gravitational stability or facilitate the exchange of angular momentum. The binding energies for the nine multiples discussed in this text are among the lowest known for wide low-mass systems, suggesting that weakly bound, low-to-intermediate mass (0.2 M {sub sun} < M {sub tot}< 1.0 M {sub sun}) multiples can form and survive to exist in the field (1-8 Gyr)

  12. The fate of exomoons in white dwarf planetary systems

    NASA Astrophysics Data System (ADS)

    Payne, Matthew J.; Veras, Dimitri; Gänsicke, Boris T.; Holman, Matthew J.

    2017-01-01

    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.

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

    NASA Astrophysics Data System (ADS)

    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.

  14. GRMHD formulation of highly super-Chandrasekhar magnetized white dwarfs: stable configurations of non-spherical white dwarfs

    SciTech Connect

    Das, Upasana; Mukhopadhyay, Banibrata E-mail: bm@physics.iisc.ernet.in

    2015-05-01

    The topic of magnetized super-Chandrasekhar white dwarfs is in the limelight, particularly in the last few years, since our proposal of their existence. By full-scale general relativistic magnetohydrodynamic (GRMHD) numerical analysis, we confirm in this work the existence of stable, highly magnetized, significantly super-Chandrasekhar white dwarfs with mass more than 3 solar mass. While a poloidal field geometry renders the white dwarfs oblate, a toroidal field makes them prolate retaining an overall quasi-spherical shape, as speculated in our earlier work. These white dwarfs are expected to serve as the progenitors of over-luminous type Ia supernovae.

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

    NASA Astrophysics Data System (ADS)

    Visscher, Channon; Lodders, Katharina; Fegley, Bruce, Jr.

    2006-09-01

    Thermochemical equilibrium and kinetic calculations are used to model sulfur and phosphorus chemistry in giant planets, brown dwarfs, 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. The results are independent of particular model atmospheres, and in principle, the equilibrium composition along the pressure-temperature profile of any object can be determined. Hydrogen sulfide (H2S) is the dominant S-bearing gas throughout substellar atmospheres and approximately represents the atmospheric sulfur inventory. Silicon sulfide (SiS) is a potential tracer of weather in substellar atmospheres. Disequilibrium abundances of phosphine (PH3) approximately representative of the total atmospheric phosphorus inventory are expected to be mixed upward into the observable atmospheres of giant planets and T dwarfs. In hotter objects, several P-bearing gases (e.g., P2, PH3, PH 2, PH, and HCP) become increasingly important at high temperatures.

  16. Untangling the White Dwarf Luminosity Functions

    NASA Astrophysics Data System (ADS)

    Lam, M. C.

    2017-03-01

    The inversion of the white dwarf luminosity function provides an independent way to prove the past star formation history of the Milky Way independent of any cosmological models. In Rowell & Hambly (2011), the effective volume method uses the average properties of all the objects in a given bin, so a significant amount of information is lost in the early stage of the analysis. In this work, I explore the possibility of assigning objects individually in a probabilistic way using the generalised Schmidt density estimator (1/Vmax).

  17. Search for Higgs shifts in white dwarfs

    SciTech Connect

    Onofrio, Roberto; Wegner, Gary A. E-mail: gary.a.wegner@dartmouth.edu

    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.

  18. Deep HST Imaging In 47 Tuc And NGC 6397: Helium-core White Dwarfs In The Core Of NGC 6397

    NASA Astrophysics Data System (ADS)

    Goldsbury, Ryan; Woodley, K.; Anderson, J.; Dotter, A.; Fahlman, G.; Hansen, B.; Hurley, J.; Kalirai, J.; King, I.; Rich, R. M.; Richer, H.; Shara, M.; Stetson, P.; Zurek, D.

    2011-01-01

    We present a detailed analysis of a population of helium-core white dwarfs in the core of the Galactic globular cluster NGC 6397. We analyze the radial distribution of these objects compared to the distributions of various other populations of known mass within the this cluster. From this comparison we are able to determine the average mass of the helium-core white dwarfs and their possible binary companions. We find that their distribution is inconsistent with the expected mass range of low-mass white dwarfs, but may be explained by the presence of massive companions to these objects. We also analyze the spectral energy distributions of the He-core white dwarfs to place constraints on the nature of their unresolved partners.

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

    NASA Astrophysics Data System (ADS)

    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

  20. Diffusion of neon in white dwarf stars.

    PubMed

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

    2010-12-01

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

  1. Studying white dwarf merger remnants with FLASH

    NASA Astrophysics Data System (ADS)

    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.

  2. Unstable Planetary Systems Around White Dwarfs

    NASA Astrophysics Data System (ADS)

    Sigurdsson, S.; Debes, J. H.

    2001-12-01

    The presence of planets around solar-type stars suggests that many white dwarfs should have planetary systems. While planets closer than ~5 AU will most likely not survive the post-main sequence lifetime of their parent star, any planet > 5 AU will survive and its semimajor axis will increase as the central star loses mass. Since the stability of adjacent orbits to mutual perturbations depends on the ratio of the planet mass to the central star's mass, some planets in previously stable orbits around a star undergoing mass loss will become unstable. We show that when mass loss is slow, systems of two planets that are marginally stable can become unstable to close encounters, while for three planets the timescale for close approaches decreases with increasing mass ratio. These processes could explain the presence of anomalous IR excesses around white dwarfs that cannot be explained by close companions, such as G29-38. We find that this should also be an effect for planetary embryos gaining mass in protoplanetary disks.

  3. Freak waves in white dwarfs and magnetars

    SciTech Connect

    Sabry, R.; Moslem, W. M.; Shukla, P. K.

    2012-12-15

    We report properties of ion acoustic freak waves that propagate in a plasma composed of warm ions and ultrarelativistic electrons and positrons. The dynamics of the nonlinear freak waves is governed by the nonlinear Schroedinger equation. The possible region for the freak waves to exist is defined precisely for typical parameters of white dwarfs and magnetars corona. It is found that for low wave number, the nonlinear ion-acoustic wave packets are structurally stable in magnetars corona than in white dwarfs. However, for large wave numbers the situation is opposite. The critical wave number threshold (k{sub c}), which indicates where the modulational instability sets in, is defined for both applications. It is seen that near to k{sub c} the freak wave amplitude becomes high, but it decreases whenever we stepped away from k{sub c}. For the wave numbers close to k{sub c}, the increase of the unperturbed density ratio of positrons-to-electrons ({beta}) would lead to increase the freak wave amplitude, but for larger wave numbers the amplitude decreases with the increase of {beta}.

  4. Two new extremely hot pulsating white dwarfs

    NASA Technical Reports Server (NTRS)

    Bond, H. E.; Grauer, A. D.; Green, R. F.; Liebert, J. W.

    1984-01-01

    High speed photometry of the extremely hot, nearly degenerate stars PG 1707 + 427 and PG 2131 + 066 reveals that they are low-amplitude pulsating variables. Power spectral analysis shows both to be multiperiodic, with dominant periods of 7.5 and 6.4-6.9 minutes, respectively. Together with the known pulsators PG 1159 - 035 and the central star of the planetary nebula Kohoutek 1-16, these objects define a new pulsational instability strip at the hot edge of the H-R diagram. The variations of these objects closely resemble those of the much cooler pulsating ZZ Ceti DA white dwarfs; both groups are probably nonradial g-mode pulsators. Evolutionary contraction of the PG 1159 - 035 variables may lead to period changes that would be detectable in as little as 1 year. The optical and IUE spectra of the PG 1159 - 035 variables are characterized by absorption lines of C IV and other CNO ions, indicating radiative levitation of species heavier than helium. He II is also present in the spectra, but the hydrogen Balmer lines are absent. Effective temperatures near 100,000 K are required, and the He II 4686 A profiles indicate log g greater than 6. These helium-rich pulsators form the hottest known subgroup of the DO white dwarfs.

  5. Interpretation of the Spectra of Strongly Magnetised White Dwarfs

    NASA Astrophysics Data System (ADS)

    Wunner, G.

    Quite recently tremendous progress has been made in analysing the spectra of magnetic DA white dwarfs with field strengths above ≡50 Megagauss. One particular white dwarf has played the rôle of a "Rosetta Stone" in this development, and it is therefore worthwhile to briefly retell the story of this object.

  6. Near-infrared imaging of white dwarfs with candidate debris disks

    SciTech Connect

    Wang, Zhongxiang; Tziamtzis, Anestis; Wang, Xuebing

    2014-02-10

    We have carried out JHK{sub s} imaging of 12 white dwarf debris disk candidates from the WIRED Sloan Digital Sky Survey Data Release 7 catalog, aiming to confirm or rule out disks among these sources. On the basis of positional identification and the flux density spectra, we find that seven white dwarfs have excess infrared emission, but mostly at Wide-field Infrared Survey Explorer W1 and W2 bands. Four are due to nearby red objects consistent with background galaxies or very low mass dwarfs, and one exhibits excess emission at JHK{sub s} consistent with an unresolved L0 companion at the correct distance. While our photometry is not inconsistent with all seven excesses arising from disks, the stellar properties are distinct from the known population of debris disk white dwarfs, making the possibility questionable. In order to further investigate the nature of these infrared sources, warm Spitzer imaging is needed, which may help resolve galaxies from the white dwarfs and provide more accurate flux measurements.

  7. SEARCH FOR VERY LOW-MASS BROWN DWARFS AND FREE-FLOATING PLANETARY-MASS OBJECTS IN TAURUS

    SciTech Connect

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

    2010-01-01

    The number of low-mass brown dwarfs and even free floating planetary-mass objects in young nearby star-forming (SF) regions and associations is continuously increasing, offering the possibility to study the low-mass end of the initial mass function in greater detail. In this paper, we present six new candidates for (very) low-mass objects in the Taurus SF region one of which was recently discovered in parallel by Luhman et al. 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 4 mag deeper than the Two Micron All Sky Survey and covers currently approx1.5 deg{sup 2}. Complementary optical photometry from Sloan Digital Sky Survey were available for roughly 1.0 deg{sup 2}. 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 spectral type of L2 +- 0.5 for one object, making it the object with the latest spectral type in Taurus known today. From models, we find the effective temperature to be 2080 +- 140 K and the mass 5-15 Jupiter masses. For the second source, the J-band spectrum does not provide definite proof of the young, low-mass nature of the object, as the expected steep water vapor absorption at 1.33 mum is not present in the data. We discuss the probability that this object might be a background giant or carbon star. If it were a young Taurus member, however, a comparison to theoretical models suggests that it lies close to or even below the deuterium burning limit (<13 M{sub Jup}) as well. A first proper motion analysis for both objects shows that they are good candidates for being Taurus members.

  8. Search for Very Low-Mass Brown Dwarfs and Free-Floating Planetary-Mass Objects in Taurus

    NASA Astrophysics Data System (ADS)

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

    2010-01-01

    The number of low-mass brown dwarfs and even free floating planetary-mass objects in young nearby star-forming (SF) regions and associations is continuously increasing, offering the possibility to study the low-mass end of the initial mass function in greater detail. In this paper, we present six new candidates for (very) low-mass objects in the Taurus SF region one of which was recently discovered in parallel by Luhman et al. 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 4 mag deeper than the Two Micron All Sky Survey and covers currently ~1.5 deg2. Complementary optical photometry from Sloan Digital Sky Survey were available for roughly 1.0 deg2. 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 spectral type of L2 ± 0.5 for one object, making it the object with the latest spectral type in Taurus known today. From models, we find the effective temperature to be 2080 ± 140 K and the mass 5-15 Jupiter masses. For the second source, the J-band spectrum does not provide definite proof of the young, low-mass nature of the object, as the expected steep water vapor absorption at 1.33 μm is not present in the data. We discuss the probability that this object might be a background giant or carbon star. If it were a young Taurus member, however, a comparison to theoretical models suggests that it lies close to or even below the deuterium burning limit (<13 M Jup) as well. A first proper motion analysis for both objects shows that they are good candidates for being Taurus members. Based on observations made at the Calar Alto Observatory. Based on observations made with ESO Telescopes at the Paranal Observatories under program ID 278.C-5043A. This

  9. ROSAT Pointed Observations of Cool Magnetic White Dwarfs

    NASA Technical Reports Server (NTRS)

    Musielak, Z. E.; Porter, J. G.; Davis, J. M.

    1995-01-01

    Observational evidence for the existence of a chromosphere on the cool magnetic white dwarf GD 356 has been reported. In addition, there has been theoretical speculations that cool magnetic white dwarfs may be sources of coronal X-ray emission. This emission, if it exists, would be distinct from the two types of X-ray emission (deep photospheric and shocked wind) that have already been observed from hot white dwarfs. We have used the PSPC instrument on ROSAT to observe three of the most prominent DA white dwarf candidates for coronal X-ray emission: GD 356, KUV 2316+123, and GD 90. The data show no significant emission for these stars. The derived upper limits for the X-ray luminosities provide constraints for a revision of current theories of the generation of nonradiative energy in white dwarfs.

  10. Fate of accreting white dwarfs: Type I supernovae vs collapse

    SciTech Connect

    Nomoto, Ken'ichi

    1986-01-01

    The final fate of accreting C + O white dwarfs is either thermonuclear explosion or collapse, if the white dwarf mass grows to the Chandrasekhar mass. We discuss how the fate depends on the initial mass, age, composition of the white dwarf and the mass accretion rate. Relatively fast accretion leads to a carbon deflagration at low central density that gives rise to a Type Ia supernova. Slower accretion induces a helium detonation that could be observed as a Type Ib supernova. If the initial mass of the C + O white dwarf is larger than 1.2 Msub solar, a carbon deflagration starts at high central density and induces a collapse of the white dwarf to form a neutron star. We examine the critical condition for which a carbon deflagration leads to collapse, not explosion. For the case of explosion, we discuss to what extent the nucleosynthesis models are consistent with spectra of Type Ia and Ib supernovae. 61 refs., 18 figs.

  11. High Velocity White Dwarfs from Type Ia Supernovae

    NASA Astrophysics Data System (ADS)

    Hansen, B.

    2002-12-01

    The single degenerate scenario for Type Ia supernovae predicts the post-supernova release of the donor star with a space velocity determined by the original binary orbital velocity. The mass transfer criteria for successful supernova ignition also place constraints on the mass of the donor. The combination of these two factors means that the great majority of high velocity donor remnants will be white dwarfs. We present models of the Type Ia supernova progenitors and antecedents in the Galaxy, and examine the donor remnant white dwarf population in the light of the current interest in high velocity white dwarfs. One potential discriminant between donor remnants and normal high velocity white dwarfs (from a thick disk or spheroid stellar population) is a determination of the binary fraction. White dwarfs which have their origin in disrupted close binaries will always be single.

  12. Debris Disks around White Dwarfs: The DAZ Connection

    NASA Astrophysics Data System (ADS)

    Kilic, Mukremin; von Hippel, Ted; Leggett, S. K.; Winget, D. E.

    2006-07-01

    We present near-infrared spectroscopic observations of 20 previously known DAZ white dwarfs obtained at the NASA Infrared Telescope Facility. Two of these white dwarfs (G29-38 and GD 362) are known to display significant K-band excesses due to circumstellar debris disks. Here we report the discovery of excess K-band radiation from another DAZ white dwarf, WD 0408-041 (GD 56). Using spectroscopic observations, we show that the excess radiation cannot be explained by a stellar or substellar companion, and is likely to be caused by a warm debris disk. Our observations strengthen the connection between the debris disk phenomena and the observed metal abundances in cool DAZ white dwarfs. However, we do not find any excess infrared emission from the most metal rich DAZs with Teff=16,000-20,000 K. This suggests that the metal abundances in warmer DAZ white dwarfs may require another explanation.

  13. A radio-pulsing white dwarf binary star.

    PubMed

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

    2016-09-15

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

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

    NASA Astrophysics Data System (ADS)

    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.

  15. A DARK SPOT ON A MASSIVE WHITE DWARF

    SciTech Connect

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

    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.

  16. WHITE DWARF/M DWARF BINARIES AS SINGLE DEGENERATE PROGENITORS OF TYPE Ia SUPERNOVAE

    SciTech Connect

    Wheeler, J. Craig

    2012-10-20

    Limits on the companions of white dwarfs in the single-degenerate scenario for the origin of Type Ia supernovae (SNe Ia) have gotten increasingly tight, yet igniting a nearly Chandrasekhar mass C/O white dwarf from a condition of near hydrostatic equilibrium provides compelling agreement with observed spectral evolution. The only type of non-degenerate stars that survive the tight limits, M{sub V} {approx}> 8.4 on the SN Ia in SNR 0509-67.5 and M{sub V} {approx}> 9.5 in the remnant of SN 1572, are M dwarfs. While M dwarfs are observed in cataclysmic variables, they have special properties that have not been considered in most work on the progenitors of SNe Ia: they have small but finite magnetic fields and they flare frequently. These properties are explored in the context of SN Ia progenitors. White dwarf/M dwarf pairs may be sufficiently plentiful to provide, in principle, an adequate rate of explosions even with slow orbital evolution due to magnetic braking or gravitational radiation. Even modest magnetic fields on the white dwarf and M dwarf will yield adequate torques to lock the two stars together, resulting in a slowly rotating white dwarf, with the magnetic poles pointing at one another in the orbital plane. The mass loss will be channeled by a 'magnetic bottle' connecting the two stars, landing on a concentrated polar area on the white dwarf. This enhances the effective rate of accretion compared to spherical accretion. Luminosity from accretion and hydrogen burning on the surface of the white dwarf may induce self-excited mass transfer. The combined effects of self-excited mass loss, polar accretion, and magnetic inhibition of mixing of accretion layers give possible means to beat the 'nova limit' and grow the white dwarf to the Chandrasekhar mass even at rather moderate mass accretion rates.

  17. Cool Customers in the Stellar Graveyard. I. Limits to Extrasolar Planets Around the White Dwarf G29-38

    NASA Astrophysics Data System (ADS)

    Debes, John H.; Sigurdsson, Steinn; Woodgate, Bruce E.

    2005-11-01

    We present high-contrast images of the hydrogen white dwarf G29-38 taken in the near-infrared with the Hubble Space Telescope and the Gemini North Telescope as part of a high-contrast imaging search for substellar objects in orbit around nearby white dwarfs. We review the current limits on planetary companions for G29-38, the only nearby white dwarf with an infrared excess due to a dust disk. We add our recent observations to these limits to produce extremely tight constraints on the types of possible companions that could be present. No objects >6MJ are detected in our data at projected separations >12 AU, and no objects >16MJ are detected for separations from 3 to 12 AU, assuming a total system age of 1 Gyr. Limits for companions at separations <3 AU come from a combination of Two Micron All Sky Survey (2MASS) photometry and previous studies of G29-38's pulsations. Our imaging with Gemini cannot confirm a tentative claim for the presence of a low-mass brown dwarf. These observations demonstrate that a careful combination of several techniques can probe nearby white dwarfs for large planets and low-mass brown dwarfs.

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

    SciTech Connect

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

    2014-02-10

    We report on Chandra X-ray observations of four candidate low-mass black hole (M {sub bh} ≲ 10{sup 6} M {sub ☉}) active galactic nuclei (AGNs) that have the estimated Eddington ratios among the lowest (∼10{sup –2}) found for this class. The aims are to validate the nature of their AGNs and to confirm the low Eddington ratios that are derived from the broad Hα line, and to explore this poorly studied regime in the AGN parameter space. Among them, two objects with the lowest significance of the broad lines are also observed with the Multi-Mirror Telescope, and the high-quality optical spectra taken confirm them as Seyfert 1 AGNs and as having small black hole masses. X-ray emission is detected from the nuclei of two of the galaxies, which is variable on timescales of ∼10{sup 3} s, whereas no significant (or only marginal at best) detection is found for the remaining two. The X-ray luminosities are on the order of 10{sup 41} erg s{sup –1} or even lower, on the order of 10{sup 40} erg s{sup –1} for non-detections, which are among the lowest regimes ever probed for Seyfert galaxies. The low X-ray luminosities, compared to their black hole masses derived from Hα, confirm their low accretion rates assuming typical bolometric corrections. Our results hint at the existence of a possibly large population of under-luminous low-mass black holes in the local universe. An off-nucleus ultra-luminous X-ray source in one of the dwarf galaxies is detected serendipitously, with a luminosity (6-9)× 10{sup 39} erg s{sup –1} in 2-10 keV.

  19. X-Shooter study of accretion in ρ-Ophiucus: very low-mass stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Testi, L.; Natta, A.; Alcalá, J. M.

    2015-07-01

    We present new VLT/X-Shooter optical and near-infrared spectra of a sample of 17 candidate young low-mass stars and brown dwarfs located in the ρ-Ophiucus cluster. We derived the spectral type and extinction for all the targets, and then we determined their physical parameters. All the objects but one have M⋆≲0.6 M⊙, and eight have mass below or close to the hydrogen-burning limit. Using the intensity of various permitted emission lines present in their spectra, we determined the accretion luminosity and mass accretion rates (Ṁacc) 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, such as Lupus, 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 leads us to conclude that we do not find evidence for a different dependence of Ṁacc with M⋆ when comparing low-mass stars and brown dwarfs. Moreover, we find a similar small (≲1 dex) scatter in the Ṁacc-M⋆ relation as in some of our recent works in other star-forming regions, and no significant differences in Ṁacc due to different ages or properties of the regions. The latter result suffers, however, from low statistics and sample selection biases in the current studies. The small scatter in the Ṁacc-M⋆ correlation confirms that mass accretion rate measurements in the literature based on uncertain photospheric parameters and single accretion indicators, such as the Hα width, can lead to a scatter that is unphysically large. Our studies show that only broadband spectroscopic surveys coupled with a detailed analysis of the

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

    NASA Astrophysics Data System (ADS)

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

    SciTech Connect

    Law, Nicholas M.; Kraus, Adam L.; Street, Rachel; Fulton, Benjamin J.; Shporer, Avi; Lister, Tim; Hillenbrand, Lynne A.; Baranec, Christoph; Bui, Khanh; Davis, Jack T. C.; Dekany, Richard G.; Kulkarni, S. R.; Ofek, Eran O.; Bloom, Joshua S.; Cenko, S. Bradley; Filippenko, Alexei V.; Burse, Mahesh P.; Das, H. K.; Kasliwal, Mansi M.; Nugent, Peter; 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

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

    NASA Astrophysics Data System (ADS)

    Law, Nicholas M.; Kraus, Adam L.; 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 P.

    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 × 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 ⊙ (0.01 AU). The M-dwarfs have masses of approximately 0.35 M ⊙, and the white dwarfs have hydrogen-rich atmospheres with temperatures of around 8000 K and have masses of approximately 0.5 M ⊙. 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 ⊙ (95% confidence) in one of the systems. Accounting for our detection efficiency and geometric factors, we estimate that 0.08%^{+0.10%}_{-0.05%} (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 relatively large orbital radii. Similar eclipsing

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

    NASA Astrophysics Data System (ADS)

    Stone, Jordan M.; Skemer, Andrew J.; 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

    2016-02-01

    Recently, Gauza et al. 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 ˜300 Myr the A and B components each have a mass of {64.6}-2.0+0.8 {M}{Jup}, and the b component has a mass of {11.2}-1.8+9.7, making VHS 1256-1257 only the third brown dwarf triple system. There exists some tension between the spectrophotometric distance of 17.2 ± 2.6 pc and the parallax distance of 12.7 ± 1.0 pc. At 12.7 pc VHS 1256-1257 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 particular, the mass of the b component increases well above the deuterium burning limit to ˜ 35 {M}{Jup} and the mass of each binary component increases to {73}-17+20 {M}{Jup}. At 17.1 pc, the UVW kinematics of the system are consistent with membership in the AB Dor moving group. The architecture of the system resembles a hierarchical stellar multiple suggesting it formed via an extension of the star formation process to low masses. Continued astrometric monitoring will resolve this distance uncertainty and will provide dynamical masses for a new benchmark system.

  4. TIDAL NOVAE IN COMPACT BINARY WHITE DWARFS

    SciTech Connect

    Fuller, Jim; Lai Dong

    2012-09-01

    Compact binary white dwarfs (WDs) undergoing orbital decay due to gravitational radiation can experience significant tidal heating prior to merger. In these WDs, the dominant tidal effect involves the excitation of outgoing gravity waves in the inner stellar envelope and the dissipation of these waves in the outer envelope. As the binary orbit decays, the WDs are synchronized from outside in (with the envelope synchronized first, followed by the core). We examine the deposition of tidal heat in the envelope of a carbon-oxygen WD and study how such tidal heating affects the structure and evolution of the WD. We show that significant tidal heating can occur in the star's degenerate hydrogen layer. This layer heats up faster than it cools, triggering runaway nuclear fusion. Such 'tidal novae' may occur in all WD binaries containing a CO WD, at orbital periods between 5 minutes and 20 minutes, and precede the final merger by 10{sup 5}-10{sup 6} years.

  5. UBV photometry of hot white dwarf stars

    NASA Astrophysics Data System (ADS)

    Cheselka, Mathew; Holberg, J. B.; Watkins, Ron; Collins, James; Tweedy, R. W.

    1993-12-01

    Johnson UBV photometry has been obtained for a set of hot degenerate stars, primarily DA and DO white dwarfs from among those detected in the Palomar-Green survey of UV excess objects. Most of our program stars have estimated effective temperatures (Teff) in the range 22,000 to 80,000 K and have no previous photometry. Some objects selected are also x-ray and extreme ultraviolet sources from the ROSAT all sky survey. The importance of precise photometric measurements in the analysis of x-ray data is discussed. A discrepancy between the observed colors and predicted colors is noted, and possibly accounted for by difficulties in defining the atmospheric cutoff of the U band and a general lack of hot stars used to define the photometric transformation between theoretical and observed colors.

  6. Heavy Metals Resisting Gravity in White Dwarfs?

    NASA Astrophysics Data System (ADS)

    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.

  7. Optical spectroscopy of candidate Alpha Persei white dwarfs

    NASA Astrophysics Data System (ADS)

    Casewell, S. L.; Dobbie, P. D.; Geier, S.; Lodieu, N.; Hambly, N. C.

    2015-08-01

    As part of an investigation into the high-mass end of the initial mass-final mass relation we performed a search for new white dwarf members of the nearby (172.4 pc), young (80-90 Myr) α Persei open star cluster. The photometric and astrometric search using the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey and SuperCOSMOS sky surveys discovered 14 new white dwarf candidates. We have obtained medium resolution optical spectra of the brightest 11 candidates using the William Herschel Telescope and confirmed that while 7 are DA white dwarfs, 3 are DB white dwarfs and 1 is an sdOB star, only three have cooling ages within the cluster age, and from their position on the initial mass-final mass relation, it is likely none are cluster members. This result is disappointing, as recent work on the cluster mass function suggests that there should be at least one white dwarf member, even at this young age. It may be that any white dwarf members of α Per are hidden within binary systems, as is the case in the Hyades cluster, however the lack of high-mass stars within the cluster also makes this seem unlikely. One alternative is that a significant level of detection incompleteness in the legacy optical image survey data at this Galactic latitude has caused some white dwarf members to be overlooked. If this is the case, Gaia will find them.

  8. A SEARCH FOR ASTEROIDS, MOONS, AND RINGS ORBITING WHITE DWARFS

    SciTech Connect

    Di Stefano, Rosanne; Howell, Steve B.; Kawaler, Steven D.

    2010-03-20

    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 10 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/or the presence of rings around planets can also produce transits detectable by Kepler. The presence of moons and rings can significantly increase the probability that Kepler will discover planets orbiting white dwarfs, even while monitoring only a small number of them.

  9. Fingering Convection and its Consequences for Accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Vauclair, Sylvie; Vauclair, Gérard; Deal, Morgan; Wachlin, F. C.

    2015-06-01

    A number of white dwarf stars show absoption lines of heavy elements in their spectra. Many of them also exhibit infra-red excess in their spectral energy distribution. These observations prove that these white dwarfs are surrounded by an orbiting debris disk resulting from the disruption of rocky planetesimals, remnants of the primordial planetary system. Part of the material from the debris disk is accreted onto the white dwarfs, explaining the presence of heavy elements in their outer layers. Previous attempts to estimate the accretion rates have overlooked the importance of the fingering convection. The fingering convection is an instability triggered by the accumulation in the white dwarf outer layers of material heavier than the underlying H-rich (for the DA) or the He-rich (for the DB) composition. The fingering convection induces a deep mixing of the accreted material. Our preliminary simulations of the fingering convection show that the effect may be important in DA white dwarfs. The accretion rates needed in order to reproduce the observed heavy element abundances exceed by order of magnitudes the accretion rates estimated when this extra-mixing is ignored. By contrast, in the cases of the DB white dwarfs that we have considered in our simulations the fingering convection either does not occur or has very little effects on the derived accretion rates. We have undertaken a systematic exploration of the consequences of the fingering convection in accreting white dwarfs.

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

    NASA Astrophysics Data System (ADS)

    Di Stefano, Rosanne; Howell, Steve B.; Kawaler, Steven D.

    2010-03-01

    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 10 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/or the presence of rings around planets can also produce transits detectable by Kepler. The presence of moons and rings can significantly increase the probability that Kepler will discover planets orbiting white dwarfs, even while monitoring only a small number of them.

  11. RE 0044+09: A new K dwarf rapid rotator with a white dwarf companion

    NASA Technical Reports Server (NTRS)

    Kellett, Barry J.; Bromage, Gordon E.; Brown, Alexander; Jeffries, Robin D.; James, David J.; Kilkenny, David; Robb, Russell M.; Wonnacott, David; Lloyd, Christopher; Clayton, C.

    1995-01-01

    We report the discovery of a new K dwarf rapid rotator with a potential white dwarf companion. The white dwarf accounts for over 90% of the observed extreme ultraviolet flux detected from this system. Analysis of ROSAT Wide Field Camera (WFC) and IUE data both suggest a white dwarf temperature of approximately 28,700 K. Optical photometry and the IUE long wavelength prime (LWP) spectrum (with the white dwarf contribution removed) imply that the late-type star has a spectral type of K1-3 V, and a distance of 55 +/- 5 pc. Using this distance, the observed IUE SWP flux, and the best-fit temperature results in a white dwarf radius of 0.0088 solar radius. The estimated white dwarf mass is then approximately 0.91 solar mass; somewhat over-massive compared to field white dwarfs. Optical photometry of the K star reveals a 'spot' modulation period of approximately 10 hr (now observed over 3 yr). However, radial velocity observations have revealed no significant variations. Spectroscopic observations place a low limit on the lithium abundance, but do show rapid rotation with a v sin i of 90 +/- 10 km/s. The K star was detected as a radio source at 3.6 cm (on two occasions) and 6 cm by the Very Large Array (VLA). The most likely evolutionary scenario is that the K star and hot white dwarf from either a wide binary or common proper motion pair with an age of 0.1-0.1 Gyr-consistent with the evolutionary timescale of the white dwarf and the rapid rotation of the K star. However, from the proper motion of the K star, this system does not seem to be associated with any of the known young stellar groups.

  12. Helium Shells on Sub-Chandrasekhar White Dwarfs: Ignition and Convection

    NASA Astrophysics Data System (ADS)

    Jacobs, Adam M.; Zingale, Michael; Nonaka, Andrew; Almgren, Ann; Bell, John

    2015-01-01

    Sub-Chandrasekhar white dwarfs accreting an envelope of helium allow for a range of explosive phenomena that could yield a variety of observable transients. Helium novae, so-called "point" Ia supernovae (.Ia SNe), rapid decline type Ia, and normal type Ia supernovae are all potential outcomes of helium accretion onto sub-Chandrasekhar white dwarfs. In this talk we outline why these systems have received a great deal of scrutiny recently and present our 3D models of convective nuclear burning in the helium envelope. We focus on thin, low-mass envelopes that are the best candidates for yielding normal type Ia supernovae. The envelope is modeled with the low-Mach hydrodynamics code Maestro. Maestro is optimized for modeling sub-sonic convective flow over long timescales while still being able to capture local compressibility effects due to nuclear burning as well as large-scale adjustments of stellar hydrostatic equilibrium. With it we model the convective burning in low-mass helium shells for carbon/oxygen white dwarf cores of 0.8, 1.0, 1.1, and 1.2 solar masses. For a suite of models we characterize the geometry, timing, and thermodynamics of ignition as well as the envelope's convective properties. Finally, we discuss the implications of our results for the viability of these systems as transient progenitors with a focus on normal type Ia supernovae.

  13. THE WHITE DWARF IN EM CYGNI: BEYOND THE VEIL

    SciTech Connect

    Godon, Patrick; Sion, Edward M.; Barrett, Paul E.; Linnell, Albert P. E-mail: edward.sion@villanova.edu E-mail: linnell@astro.washington.edu

    2009-07-10

    We present a spectral analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of the eclipsing double-line spectroscopic binary EM Cygni (EM Cyg), a Z Cam DN system. The FUSE spectrum, obtained in quiescence, consists of four individual exposures (orbits): two exposures, at orbital phases {phi} {approx} 0.65 and {phi} {approx} 0.90, have a lower flux; and two exposures, at orbital phases {phi} = 0.15 and 0.45, have a relatively higher flux. The change of flux level as a function of the orbital phase is consistent with the stream material (flowing over and below the disk from the hot spot region to smaller radii) partially masking the white dwarf. We carry out a spectral analysis of the FUSE data, obtained at phase 0.45 (when the flux is maximal), using synthetic spectra generated with the codes TLUSTY and SYNSPEC. Using a single white dwarf spectral component, we obtain a white dwarf temperature of 40, 000 K {+-} 1000 K, rotating at 100 km s{sup -1}. The white dwarf, or conceivably, the material overflowing the disk rim, shows suprasolar abundances of silicon, sulphur, and possibly nitrogen. Using a white dwarf+disk composite model, we obtain that the white dwarf temperature could be even as high as 50,000 K, contributing more than 90% of the FUV flux, and the disk contributing less than 10% must have a mass accretion rate reaching 10{sup -10} M{sub sun} yr{sup -1}. The single white dwarf model fits the absorption lines better than the white dwarf+disk model, but the white dwarf+disk model fits better the continuum in the shorter wavelengths. In both cases, however, we obtain that the white dwarf temperature is much higher than previously estimated. We emphasize the importance of modeling the spectra of EM Cyg around phase {phi} < 0.5, when the white dwarf and disk are facing the observer, and we suggest that the discrepancy between the present analysis and previous spectral analysis might be due to the occulting effect of the stream veiling the white

  14. Using DA White Dwarfs to Calibrate Synthetic Photometry

    NASA Astrophysics Data System (ADS)

    Holberg, J. B.

    2007-04-01

    Four widely used photometric systems, namely the Johnson-Kron-Cousins UBVRI, the Strömgren uvby, the 2MASS JHKs and the Sloan Digital Sky Survey ugriz systems have been directly compared with the HST absolute photometric scale of Bohlin & Gilliland (2004). These comparisons are subsequently used to construct a large grid of accurate synthetic magnitudes for DA white dwarfs. This grid is, in turn, critically evaluated with respect to the observed photometry from substantial samples of actual white dwarfs. The advantages of DA white dwarfs as photometric stars are emphasized, and the prospects for extending the use of these stars into the near infrared are highlighted.

  15. On the Evolution of Hydrogen-Deficient White Dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  16. A radio-pulsing white dwarf binary star

    NASA Astrophysics Data System (ADS)

    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.

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

    PubMed

    Hodgkin; Oppenheimer; Hambly; Jameson; Smartt; Steele

    2000-01-06

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

  18. Hot White Dwarf Donors in Ultracompact X-Ray Binaries

    NASA Astrophysics Data System (ADS)

    Bildsten, Lars

    2002-09-01

    The discovery of two accreting millisecond X-ray pulsars in binaries with ~43 minute orbital periods allows for a new probe of the donor's structure. For XTE J1751-305, only a hot white dwarf (WD) can fill the Roche lobe. A cold He WD is a possible solution for XTE J0929-314, although I will show that evolutionary arguments make a hot WD more likely. In addition to being larger than the T=0 models, these finite entropy, low-mass (Mc<0.03 Msolar) WDs have a minimum mass for a fixed core temperature. If they remain hot as they lose mass and expand, they can ``evaporate'' to leave an isolated millisecond radio pulsar. They also adiabatically expand upon mass loss at a rate faster than the growth of the Roche radius if the angular momentum deposited in the disk is not returned to the donor. If the timescale of the resulting runaway mass transfer is shorter than the viscous timescale in the outer disk, then the mass transfer instability of Ruderman & Shaham for He WDs would be realized. However, my estimates of these timescales still make the instability unlikely for adiabatic responses. I close by noting the possible impact of finite temperature WDs on our understanding of AM CVn binaries.

  19. EVERY INTERACTING DOUBLE WHITE DWARF BINARY MAY MERGE

    SciTech Connect

    Shen, Ken J.

    2015-05-20

    Interacting double white dwarf (WD) binaries can give rise to a wide variety of astrophysical outcomes ranging from faint thermonuclear and Type Ia supernovae to the formation of neutron stars and stably accreting AM Canum Venaticorum systems. One key factor affecting the final outcome is whether mass transfer remains dynamically stable or instead diverges, leading to the tidal disruption of the donor and the merger of the binary. It is typically thought that for low ratios of the donor mass to the accretor mass, mass transfer remains stable, especially if accretion occurs via a disk. In this Letter, we examine low mass ratio double WD binaries and find that the initial phase of hydrogen-rich mass transfer leads to a classical nova-like outburst on the accretor. Dynamical friction within the expanding nova shell shrinks the orbit and causes the mass transfer rate to increase dramatically above the accretor's Eddington limit, possibly resulting in a binary merger. If the binary survives the first hydrogen-rich nova outbursts, dynamical friction within the subsequent helium-powered nova shells pushes the system even more strongly toward merger. While further calculations are necessary to confirm this outcome for the entire range of binaries previously thought to be dynamically stable, it appears likely that most, if not all, interacting double WD binaries will merge during the course of their evolution.

  20. Every Interacting Double White Dwarf Binary May Merge

    NASA Astrophysics Data System (ADS)

    Shen, Ken J.

    2015-05-01

    Interacting double white dwarf (WD) binaries can give rise to a wide variety of astrophysical outcomes ranging from faint thermonuclear and Type Ia supernovae to the formation of neutron stars and stably accreting AM Canum Venaticorum systems. One key factor affecting the final outcome is whether mass transfer remains dynamically stable or instead diverges, leading to the tidal disruption of the donor and the merger of the binary. It is typically thought that for low ratios of the donor mass to the accretor mass, mass transfer remains stable, especially if accretion occurs via a disk. In this Letter, we examine low mass ratio double WD binaries and find that the initial phase of hydrogen-rich mass transfer leads to a classical nova-like outburst on the accretor. Dynamical friction within the expanding nova shell shrinks the orbit and causes the mass transfer rate to increase dramatically above the accretor's Eddington limit, possibly resulting in a binary merger. If the binary survives the first hydrogen-rich nova outbursts, dynamical friction within the subsequent helium-powered nova shells pushes the system even more strongly toward merger. While further calculations are necessary to confirm this outcome for the entire range of binaries previously thought to be dynamically stable, it appears likely that most, if not all, interacting double WD binaries will merge during the course of their evolution.

  1. Time dependent white dwarf radiative shocks

    SciTech Connect

    Imamura, J.N.; Wolff, M.T.; Durisen, R.H.

    1985-01-01

    We study the oscillatory instability of white dwarf radiative accretion shocks discovered by Langer, Chanmugam, and Shaviv. We extend previous works by examining spherical shocks dominated by: (1) bremsstrahlung and Compton cooling; and (2) bremsstrahlung and Compton cooling when the effects of electron thermal conduction are not negligible. The results of our calculations allow us to delineate stability regimes as a function of the dwarf mass, M/sub d/, and the accretion rate, M/sup 0/. We parameterize M/sup 0/ in terms of the optical depth to electron scattering through the preshock flow, tau/sub es/. In the Compton cooling and bremsstrahlung case, the shocks are unstable to low order oscillation modes if M/sub d/ less than or equal to (0.7 +- 0.1) M/sub solar/ for tau/sub es/ = 14, and if M/sub d/ less than or equal to (0.9 +- 0.1) M/sub solar/ for tau/sub es/ = 1. When electron thermal conduction is added, low order oscillation modes are unstable only if M/sub d/ less than or equal to (0.3 +- 0.1) M/sub sun mass/. The unstable modes have approximate oscillation periods of 1.1 tau/sub br/ and 0.63 tau/sub br/, where tau/sub br/ is the bremsstrahlung cooling time scale of the postshock plasma. Our results can be scaled to magnetically funneled accretion flows as long as cyclotron emission contributes less than about 10% of the postshock cooling. 14 refs., 1 fig.

  2. Active states and structure transformations in accreting white dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  3. A YOUNG WHITE DWARF WITH AN INFRARED EXCESS

    SciTech Connect

    Xu, S.; Jura, M.; Klein, B.; Zuckerman, B.; Pantoja, B.; Su, K. Y. L.; Meng, H. Y. A. E-mail: jura@astro.ucla.edu

    2015-06-10

    Using observations of Spitzer/IRAC, we report the serendipitous discovery of excess infrared emission from a single white dwarf PG 0010+280. At a temperature of 27,220 K and a cooling age of 16 Myr, it is the hottest and youngest white dwarf to display an excess at 3–8 μm. The infrared excess can be fit by either an opaque dust disk within the tidal radius of the white dwarf or a 1300 K blackbody, possibly from an irradiated substellar object or a re-heated giant planet. PG 0010+280 has two unique properties that are different from white dwarfs with a dust disk: (i) relatively low emission at 8 μm and (ii) non-detection of heavy elements in its atmosphere from high-resolution spectroscopic observations with Keck/HIRES. The origin of the infrared excess remains unclear.

  4. The Local Population of White Dwarfs within 25 pc

    NASA Astrophysics Data System (ADS)

    Holberg, Jay B.; Oswalt, Terry D.; Sion, Edward M.

    2015-01-01

    We have extended the detailed survey of the local white dwarf population from 20 pc to 25 pc, effectively doubling the sample volume to now include 231 stars. The present 25 pc has an estimated completeness of 70% (the corresponding 20 pc sample is now 85% complete). The space density of white dwarfs remains at 4.8 ± 0.5 x 10-3 pc-3. There exists a curious excess of single stars in the sample 70% vs 30% in systems with one or more companions. A pronounced apparent deficiency remains between the eleven known Sirius-like systems present in the 20 pc sample and only a single such system presently known in the extended 25 pc sample. Also demonstrated, using explicit individual white dwarf cooling ages, is the feasibility of estimating the white dwarf birth rates over the last ~ 5 Gyr.This work is supported by NSF grant AST-1413537

  5. Outbursts in Two New Cool Pulsating DA White Dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  6. QUIESCENT NUCLEAR BURNING IN LOW-METALLICITY WHITE DWARFS

    SciTech Connect

    Miller Bertolami, Marcelo M.; Althaus, Leandro G.

    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.

  7. Two white dwarfs with oxygen-rich atmospheres.

    PubMed

    Gänsicke, B T; Koester, D; Girven, J; Marsh, T R; Steeghs, D

    2010-01-08

    Stars with masses ranging from 7 to 10 times the mass of the Sun end their lives either as massive white dwarfs or weak type II supernovae, but there are only limited observational constraints on either evolutionary channel. Here we report the detection of two white dwarfs with large photospheric oxygen abundances, implying that they are bare oxygen-neon cores and that they may have descended from the most massive progenitors that avoid core collapse.

  8. A disintegrating minor planet transiting a white dwarf.

    PubMed

    Vanderburg, Andrew; Johnson, John Asher; 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

    2015-10-22

    Most stars become white dwarfs after they have exhausted their nuclear fuel (the Sun will be one such). Between one-quarter and one-half of white dwarfs have elements heavier than helium in their atmospheres, even though these elements ought to sink rapidly into the stellar interiors (unless they are occasionally replenished). The abundance ratios of heavy elements in the atmospheres of white dwarfs are similar to the ratios in rocky bodies in the Solar System. This fact, together with the existence of warm, dusty debris disks surrounding about four per cent of white dwarfs, suggests that rocky debris from the planetary systems of white-dwarf progenitors occasionally pollutes the atmospheres of the stars. The total accreted mass of this debris is sometimes comparable to the mass of large asteroids in the Solar System. However, rocky, disintegrating bodies around a white dwarf have not yet been observed. Here we report observations of a white dwarf--WD 1145+017--being transited by at least one, and probably several, disintegrating planetesimals, with periods ranging from 4.5 hours to 4.9 hours. The strongest transit signals occur every 4.5 hours and exhibit varying depths (blocking up to 40 per cent of the star's brightness) and asymmetric profiles, indicative of a small object with a cometary tail of dusty effluent material. The star has a dusty debris disk, and the star's spectrum shows prominent lines from heavy elements such as magnesium, aluminium, silicon, calcium, iron, and nickel. This system provides further evidence that the pollution of white dwarfs by heavy elements might originate from disrupted rocky bodies such as asteroids and minor planets.

  9. The Montreal White Dwarf Database: A Tool for the Community

    NASA Astrophysics Data System (ADS)

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

    2017-03-01

    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.

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

    NASA Astrophysics Data System (ADS)

    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 < 6) and reach luminosities log (L/L⊙) > 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

  11. A Low-mass Exoplanet Candidate Detected by K2 Transiting the Praesepe M Dwarf JS 183

    NASA Astrophysics Data System (ADS)

    Pepper, Joshua; Gillen, Ed; Parviainen, Hannu; Hillenbrand, Lynne A.; Cody, Ann Marie; Aigrain, Suzanne; Stauffer, John; Vrba, Frederick J.; David, Trevor; Lillo-Box, Jorge; Stassun, Keivan G.; Conroy, Kyle E.; Pope, Benjamin J. S.; Barrado, David

    2017-04-01

    We report the discovery of a repeating photometric signal from a low-mass member of the Praesepe open cluster that we interpret as a Neptune-sized transiting planet. The star is JS 183 (HSHJ 163, EPIC 211916756), with T eff = 3325 ± 100 K, M * = 0.44 ± 0.04 M ⊙, R * = 0.44 ± 0.03 R ⊙, and {log}{g}* = 4.82+/- 0.06. The planet has an orbital period of 10.134588 days and a radius of R P = 0.32 ± 0.02 R J. Since the star is faint at V = 16.5 and J = 13.3, we are unable to obtain a measured radial velocity orbit, but we can constrain the companion mass to below about 1.7 M J, and thus well below the planetary boundary. JS 183b (since designated as K2-95b) is the second transiting planet found with K2 that resides in a several-hundred-megayear open cluster; both planets orbit mid-M dwarf stars and are approximately Neptune sized. With a well-determined stellar density from the planetary transit, and with an independently known metallicity from its cluster membership, JS 183 provides a particularly valuable test of stellar models at the fully convective boundary. We find that JS 183 is the lowest-density transit host known at the fully convective boundary, and that its very low density is consistent with current models of stars just above the fully convective boundary but in tension with the models just below the fully convective boundary.

  12. White dwarf models of supernovae and cataclysmic variables

    SciTech Connect

    Nomoto, K.; Hashimoto, M.

    1986-01-01

    If the accreting white dwarf increases its mass to the Chandrasekhar mass, it will either explode as a Type I supernova or collapse to form a neutron star. In fact, there is a good agreement between the exploding white dwarf model for Type I supernovae and observations. We describe various types of evolution of accreting white dwarfs as a function of binary parameters (i.e,. composition, mass, and age of the white dwarf, its companion star, and mass accretion rate), and discuss the conditions for the precursors of exploding or collapsing white dwarfs, and their relevance to cataclysmic variables. Particular attention is given to helium star cataclysmics which might be the precursors of some Type I supernovae or ultrashort period x-ray binaries. Finally we present new evolutionary calculations using the updated nuclear reaction rates for the formation of O+Ne+Mg white dwarfs, and discuss the composition structure and their relevance to the model for neon novae. 61 refs., 14 figs.

  13. The Dwarf Novae UZ Serpentis and SS Aurigae During Quiescence: Exposed White Dwarfs?

    NASA Astrophysics Data System (ADS)

    Lake, J.; Sion, E. M.

    2000-12-01

    UZ Serpentis and SS Aurigae are both U Geminorum-type dwarf novae with similar orbital periods, outburst amplitudes, and outburst recurrence times. Since dwarf novae above the period gap have higher accretion rates, their accretion disks may remain optically thick even during quiescence. Hence the detection of the white dwarf is more difficult. UZ Ser and SS Aur offer the possiblity of extending the range of systems for which the underlying white dwarf accreter has been analyzed with model atmospheres. We have applied the Massa-Fitzpatrick (2000) flux calibration correction to the archival IUE NEWSIPS SWP spectra of these two systems, obtained during dwarf nova quiescence. We have carried out high gravity model atmosphere using the codes TLUSTY195, SYNSPEC42, ROTIN and accretion disk synthetic spectra from the grid of Wade and Hubeny (1998). We have determined the physical properties of the white dwarf accreters, including temperature, gravity chemical abundances estimates, and the accretion rate during quiescence. We discuss our results in the context of the overall picture of accretion physics in dwarf novae and the effects of accretion on the white dwarf. This research was supported in part by NSF grant AST 99-01955, NASA ADP grant NAG5-8388 and by summer research funding from the NASA- Delaware Space Grant Colleges Consortium.

  14. Where the Wild Young M Dwarfs Are: the SUPERBLINK Proper Motion Survey and a Search for Low-mass Moving Group Candidates

    NASA Astrophysics Data System (ADS)

    Lépine, Sébastien

    2016-01-01

    The SUPERBLINK survey catalogs all stars brighter than R = 19 mag and with proper motions larger than 40 mas yr-1, down to a declination of -33○. The catalog inevitably includes a significant fraction of the presumed low-mass members of several nearby young moving groups (Beta Pic, AB Dor, Tuc-Hor, Argus), or low-mass escapees from the Hyades and Pleiades clusters. We discuss opportunities and challenges in identifying the missing M dwarf members of these moving groups. While rounding up the majority of the potential M dwarf members of these groups, such samples are significantly affected by co-moving field stars, both young and old, due to the heavy clumping of the local field population in velocity space.

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

    NASA Astrophysics Data System (ADS)

    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.

  16. A Dark Spot on a Massive White Dwarf

    NASA Astrophysics Data System (ADS)

    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 < 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. This work is based on observations obtained at the Gemini Observatory, McDonald Observatory, and the Apache Point Observatory 3.5-m telescope. The latter is owned and operated by the Astrophysical Research Consortium. Gemini Observatory 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), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

  17. TIDAL INTERACTIONS IN MERGING WHITE DWARF BINARIES

    SciTech Connect

    Piro, Anthony L.

    2011-10-20

    The recently discovered system J0651 is the tightest known detached white dwarf (WD) binary. Since it has not yet initiated Roche-lobe overflow, it provides a relatively clean environment for testing our understanding of tidal interactions. I investigate the tidal heating of each WD, parameterized in terms of its tidal Q parameter. Assuming that the heating can be radiated efficiently, the current luminosities are consistent with Q {sub 1} {approx} 7 x 10{sup 10} and Q {sub 2} {approx} 2 x 10{sup 7}, for the He and C/O WDs, respectively. Conversely, if the observed luminosities are merely from the cooling of the WDs, these estimated values of Q represent the upper limits. A large Q {sub 1} for the He WD means its spin velocity will be slower than that expected if it was tidally locked, which, since the binary is eclipsing, may be measurable via the Rossiter-McLaughlin effect. After one year, gravitational wave emission shifts the time of eclipses by 5.5 s, but tidal interactions cause the orbit to shrink more rapidly, changing the time by up to an additional 0.3 s after a year. Future eclipse timing measurements may therefore infer the degree of tidal locking.

  18. Dark-matter admixed white dwarfs

    NASA Astrophysics Data System (ADS)

    Leung, Shing Chi; Chu, Ming Chung; Lin, Lap Ming; Wong, Ka Wing

    2014-03-01

    We study the equilibrium structures of white dwarfs (WD) with dark matter cores formed by non-self-annihilating dark matter (DM) particles with masses ranging from 1 GeV to 100 GeV, assuming in form of an ideal degenerate Fermi gas inside the stars. For DM particles of mass 10 GeV and 100 GeV, we find that stable stellar models exist only if the mass of the DM core inside the star is less than O and -3)Msun , respectively. The global properties of these stars, and the corresponding Chandrasekhar mass (CM) limits, are essentially the same as those of traditional WD models without DM. Nevertheless, in the 10 GeV case, the gravitational attraction of the DM core is strong enough to squeeze the normal matter in the core region to densities above neutron drip. For the 1 GeV case, the DM core inside the star can be as massive as O and affects the global structure of the star significantly. The radius of a stellar model with DM can be about two times smaller than that of a traditional WD. Furthermore, the CM limit can be decreased by as much as 40%. Our results may have implications on the extent to which type Ia supernovae can be regarded as standard candles. This work is partially supported by a grant from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project No. 400910).

  19. Liberating exomoons in white dwarf planetary systems

    NASA Astrophysics Data System (ADS)

    Payne, Matthew J.; Veras, Dimitri; Holman, Matthew J.; Gänsicke, Boris T.

    2016-03-01

    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 106 yr. The short sinking time-scale for the pollutants indicates 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 amongst planets in WD systems easily trigger moon ejection. Repeated close encounters within tenths of 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 WD. Moreover, during intense scattering, planets themselves have pericentres with respect to the WD of only a fraction of an astronomical unit, causing extreme Hill-sphere contraction, and the liberation of moons into WD-grazing orbits. Many of our results are directly applicable to exomoons orbiting planets around main-sequence stars.

  20. DISCOVERY OF AN ULTRAMASSIVE PULSATING WHITE DWARF

    SciTech Connect

    Hermes, J. J.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, M. H.; Harrold, Samuel T.; Kepler, S. O.; Gianninas, A.; Brown, Warren R.

    2013-07-01

    We announce the discovery of the most massive pulsating hydrogen-atmosphere white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12, 030 {+-} 210 K WD with a log g =9.08 {+-} 0.06, which corresponds to a mass of 1.20 {+-} 0.03 M{sub Sun }. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains Type Ia supernovae progenitor systems.

  1. The binary white dwarf LHS 3236

    SciTech Connect

    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.; Dupuy, Trent J.; Liu, Michael C.; Hartkopf, William I.; Ireland, Michael J.; Leggett, S. K.

    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.

  2. Calibrating White Dwarf Asteroseismic Fitting Techniques

    NASA Astrophysics Data System (ADS)

    Castanheira, B. G.; Romero, A. D.; Bischoff-Kim, A.

    2017-03-01

    The main goal of looking for intrinsic variability in stars is the unique opportunity to study their internal structure. Once we have extracted independent modes from the data, it appears to be a simple matter of comparing the period spectrum with those from theoretical model grids to learn the inner structure of that star. However, asteroseismology is much more complicated than this simple description. We must account not only for observational uncertainties in period determination, but most importantly for the limitations of the model grids, coming from the uncertainties in the constitutive physics, and of the fitting techniques. In this work, we will discuss results of numerical experiments where we used different independently calculated model grids (white dwarf cooling models WDEC and fully evolutionary LPCODE-PUL) and fitting techniques to fit synthetic stars. The advantage of using synthetic stars is that we know the details of their interior structure so we can assess how well our models and fitting techniques are able to the recover the interior structure, as well as the stellar parameters.

  3. Effective geometry of a white dwarf

    SciTech Connect

    Bini, D.; Cherubini, C.; Filippi, S.

    2011-03-15

    The ''effective geometry'' formalism is used to study the perturbations of a white dwarf described as a self-gravitating fermion gas with a completely degenerate relativistic equation of state of barotropic type. The quantum nature of the system causes an absence of homological properties, manifested instead by polytropic stars, and requires a parametric study of the solutions both at the numerical and analytical level. We have explicitly derived a compact analytical parametric approximate solution of Pade type, which gives density curves and stellar radii in good accordance with already existing numerical results. After validation of this new type of approximate solutions, we use them to construct the effective acoustic metric governing general perturbations following Chebsch's formalism. Even in this quantum case, the stellar surface exhibits a curvature singularity due to the vanishing of density, as already evidenced in past studies on nonquantum self-gravitating polytropic stars. The equations of the theory are finally numerically integrated in the simpler case of irrotational spherical pulsating perturbations, including the effect of backreaction, in order to have a dynamical picture of the process occurring in the acoustic metric.

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

    NASA Technical Reports Server (NTRS)

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

    NASA Astrophysics Data System (ADS)

    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.

  6. Probing an Ancient Thermonuclear Runaway on a White Dwarf in a Dwarf Nova

    NASA Astrophysics Data System (ADS)

    Sion, Edward

    1999-07-01

    We unexpectedly discovered evidence, based upon two GHRS G160M spectra, of greatly elevated abundances of odd-numbered nuclei Phosphorus and Aluminum as well as a Nitrogen to Carbon ratio of 10 in the photosphere of the white dwarf in the dwarf nova VW Hy ons and determine the first chem ical abundances of many odd-numbered proton capture species; {2} determine a accurate mass for the white dwarf and; {3} probe changes in the accretion belt and surface abundances as a function of time since a superoutburst. This study will hold important

  7. Planets around Low-mass Stars. III. A Young Dusty L Dwarf Companion at the Deuterium-burning Limit

    NASA Astrophysics Data System (ADS)

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.

    2013-09-01

    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 μm 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 λ6707 absorption, and spectral shape of 2MASS 0122-2439 B suggest a range of ~10-120 Myr is possible. The age and luminosity of 2MASS 0122-2439 B fall in a strip where "hot-start" evolutionary model mass tracks overlap as a result of deuterium burning. Several known substellar companions also fall in this region (2MASS J0103-5515 ABb, AB Pic b, κ And b, G196-3 B, SDSS 2249+0044 B, LP 261-75 B, HD 203030 B, and HN Peg B), but their dual-valued mass predictions have largely been unrecognized. The implied mass of 2MASS 0122-2439 B is ≈12-13 M Jup or ≈22-27 M Jup if it is an AB Dor member, or possibly as low as 11 M Jup if the wider age range is adopted. Evolutionary models predict an effective temperature for 2MASS 0122-2439 B that corresponds to spectral types near the L/T transition (≈1300-1500 K) for field objects. However, we find a mid-L near-infrared spectral type, indicating that 2MASS 0122-2439 B represents another case of photospheric dust being retained to cooler temperatures at low surface gravities, as seen in the spectra of young (8-30 Myr) planetary

  8. PLANETS AROUND LOW-MASS STARS. III. A YOUNG DUSTY L DWARF COMPANION AT THE DEUTERIUM-BURNING LIMIT ,

    SciTech Connect

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.

    2013-09-01

    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 ( Almost-Equal-To 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 Almost-Equal-To 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 {approx}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 {approx}10-120 Myr is possible. The age and luminosity of 2MASS 0122-2439 B fall in a strip where ''hot-start'' evolutionary model mass tracks overlap as a result of deuterium burning. Several known substellar companions also fall in this region (2MASS J0103-5515 ABb, AB Pic b, {kappa} And b, G196-3 B, SDSS 2249+0044 B, LP 261-75 B, HD 203030 B, and HN Peg B), but their dual-valued mass predictions have largely been unrecognized. The implied mass of 2MASS 0122-2439 B is Almost-Equal-To 12-13 M{sub Jup} or Almost-Equal-To 22-27 M{sub Jup} if it is an AB Dor member, or possibly as low as 11 M{sub Jup} if the wider age range is adopted. Evolutionary models predict an effective temperature for 2MASS 0122-2439 B that corresponds to spectral types near the L/T transition ( Almost-Equal-To 1300-1500 K) for field objects. However, we find a mid-L near-infrared spectral type, indicating that 2MASS 0122-2439 B represents another case of photospheric dust being

  9. BANYAN. V. A SYSTEMATIC ALL-SKY SURVEY FOR NEW VERY LATE-TYPE LOW-MASS STARS AND BROWN DWARFS IN NEARBY YOUNG MOVING GROUPS

    SciTech Connect

    Gagné, Jonathan; Lafrenière, David; Doyon, René; Malo, Lison; Artigau, Étienne

    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.

  10. BANYAN. V. A Systematic All-sky Survey for New Very Late-type Low-mass Stars and Brown Dwarfs in Nearby Young Moving Groups

    NASA Astrophysics Data System (ADS)

    Gagné, Jonathan; Lafrenière, David; Doyon, René; Malo, Lison; Artigau, Étienne

    2015-01-01

    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-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.

  11. DISCOVERY OF A VERY LOW MASS TRIPLE WITH LATE-M AND T DWARF COMPONENTS: LP 704-48/SDSS J0006-0852AB

    SciTech Connect

    Burgasser, Adam J.; Luk, Christopher; Bardalez Gagliuffi, Daniella; Nicholls, Christine P.; Dhital, Saurav; Prato, L.; West, Andrew A.; Lepine, Sebastien

    2012-10-01

    We report the identification of the M9 dwarf SDSS J000649.16-085246.3 as a spectral binary and radial velocity (RV) variable with components straddling the hydrogen-burning mass limit. Low-resolution near-infrared spectroscopy reveals spectral features indicative of a T dwarf companion, and spectral template fitting yields component types of M8.5 {+-} 0.5 and T5 {+-} 1. High-resolution near-infrared spectroscopy with Keck/NIRSPEC reveals pronounced RV variations with a semi-amplitude of 8.2 {+-} 0.4 km s{sup -1}. From these we determine an orbital period of 147.6 {+-} 1.5 days and eccentricity of 0.10 {+-} 0.07, making SDSS J0006-0852AB the third tightest very low mass binary known. This system is also found to have a common proper motion companion, the inactive M7 dwarf LP 704-48, at a projected separation of 820 {+-} 120 AU. The lack of H{alpha} emission in both M dwarf components indicates that this system is relatively old, as confirmed by evolutionary model analysis of the tight binary. LP 704-48/SDSS J0006-0852AB is the lowest-mass confirmed triple identified to date, and one of only seven candidate and confirmed triples with total masses below 0.3 M{sub Sun} currently known. We show that current star and brown dwarf formation models cannot produce triple systems like LP 704-48/SDSS J0006-0852AB, and we rule out Kozai-Lidov perturbations and tidal circularization as a viable mechanism to shrink the inner orbit. The similarities between this system and the recently uncovered low-mass eclipsing triples NLTT 41135AB/41136 and LHS 6343ABC suggest that substellar tertiaries may be common in wide M dwarf pairs.

  12. The Binary White Dwarf LHS 3236

    NASA Astrophysics Data System (ADS)

    Harris, Hugh C.; Dahn, Conard C.; Dupuy, Trent J.; Canzian, Blaise; Guetter, Harry H.; Hartkopf, William I.; Ireland, Michael J.; Leggett, S. K.; Levine, Stephen E.; Liu, Michael C.; Luginbuhl, Christian B.; Monet, Alice K. B.; Stone, Ronald C.; Subasavage, John P.; Tilleman, Trudy; Walker, Richard L.

    2013-12-01

    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-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 ⊙ 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 ⊙. 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 ⊙) is well above the Chandrasekhar limit; however, the timescale for coalescence is long. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  13. Spin and Magnetism of White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kissin, Yevgeni; Thompson, Christopher

    2015-08-01

    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. The downward pumping of angular momentum in the convective envelope, in combination with the absorption of a planet or tidal spin-up from a binary companion, can trigger strong dynamo action near the core-envelope boundary. Several arguments point to the outer core as the source for a magnetic field in the WD remnant: the outer third of a ˜ 0.55 {M}⊙ WD is processed during the shell burning phase(s) of the progenitor; the 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 the intense radiation flux at the core boundary facilitates magnetic buoyancy within a relatively thick tachocline layer. The helicity flux into the growing core is driven by a dynamical imbalance with a latitude-dependent rotational stress. The magnetic field deposited in an isolated massive WD is concentrated in an outer shell of mass ≲ 0.1 {M}⊙ and can reach ˜10 MG. A buried toroidal field experiences moderate ohmic decay above an age ˜0.3 Gyr, which may lead to growth or decay of the external magnetic field. The final WD spin period is related to a critical spin rate below which magnetic activity shuts off and core and envelope decouple; it generally sits in the range of hours to days. WD periods ranging up to a year are possible if the envelope re-expands following a late thermal pulse.

  14. Merging white dwarfs and Type Ia supernovae

    NASA Astrophysics Data System (ADS)

    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.

  15. Stellar explosions from accreting white dwarfs

    NASA Astrophysics Data System (ADS)

    Moore, Kevin L.

    Unstable thermonuclear burning on accreting white dwarfs (WDs) can lead to a wide variety of outcomes, and induce shock waves in several contexts. In classical and recurrent novae, a WD accreting hydrogen-rich material from a binary companion can experience thermonuclear runaways, ejecting mass into the interstellar/circumbinary environment at ~1000 km/s. This highly supersonic ejecta drives shock waves into the interstellar gas which may be relevant for sweeping out gas from globular clusters or forming circumstellar absorption regions in interacting supernovae. While runaway nuclear burning in novae releases enough energy for these objects to brighten by a factor of ~10 4 over roughly a weeklong outburst, it does not become dynamically unstable. In contrast, certain helium accretion scenarios may allow for dynamical burning modes, in part due to the higher temperature sensitivity of helium burning reactions and larger accreted envelopes. The majority of this thesis involves such dynamical burning modes, specifically detonations - shock waves sustained by nuclear energy release behind the shock front. We investigate when steady-state detonations are realizable in accreted helium layers on WDs, and model their strength and burning products using both semi-analytic and numerical models. We find the minimum helium layer thickness that will sustain a steady laterally propagating detonation and show that it depends on the density and composition of the helium layer, specifically 12 C and 16O. Though gravitationally unbound, the ashes still have unburned helium (~80% in the thinnest cases) and only reach up to heavy elements such as 40Ca, 44Ti, 48Cr, and 52Fe. It is rare for these thin shells to generate large amounts of radioactive isotopes necessary to power light curves, such as 56Ni. This has important implications on whether the unbound helium burning ashes may create faint and fast peculiar supernovae or events with virtually no radioactivity, as well as on off

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

    NASA Astrophysics Data System (ADS)

    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.

  17. SpeX SPECTROSCOPY OF UNRESOLVED VERY LOW MASS BINARIES. I. IDENTIFICATION OF 17 CANDIDATE BINARIES STRADDLING THE L DWARF/T DWARF TRANSITION

    SciTech Connect

    Burgasser, Adam J.; Cruz, Kelle L.; Cushing, Michael; Looper, Dagny L.; Gelino, Christopher R.; Kirkpatrick, J. Davy; Faherty, Jacqueline K.; Reid, I. Neill

    2010-02-20

    We report the identification of 17 candidate brown dwarf binaries whose components straddle the L dwarf/T dwarf transition. These sources were culled from a large near-infrared spectral sample of L and T dwarfs observed with the Infrared Telescope Facility SpeX spectrograph. Candidates were selected on the basis of spectral ratios which segregate known (resolved) L dwarf/T dwarf pairs from presumably single sources. Composite templates, constructed by combining 13,581 pairs of absolute flux-calibrated spectra, are shown to provide statistically superior fits to the spectra of our 17 candidates as compared to single templates. Ten of these candidates appear to have secondary components that are significantly brighter than their primaries over the 1.0-1.3 {mu}m band, indicative of rapid condensate depletion at the L dwarf/T dwarf transition. Our results support prior indications of enhanced multiplicity amongst early-type T dwarfs; 53% +- 7% of the T0-T4 dwarfs in our spectral sample are found to be either resolved or unresolved (candidate) pairs, although this is consistent with an intrinsic (volume complete) brown dwarf binary fraction of only 15%. If verified, this sample of spectral binaries more than doubles the number of known L dwarf/T dwarf transition pairs, enabling a broader exploration of this poorly understood phase of brown dwarf atmospheric evolution.

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

    NASA Technical Reports Server (NTRS)

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

    1983-01-01

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

  19. A DEEPLY ECLIPSING DETACHED DOUBLE HELIUM WHITE DWARF BINARY

    SciTech Connect

    Parsons, S. G.; Marsh, T. R.; Gaensicke, B. T.; Drake, A. J.; Koester, D.

    2011-07-10

    Using Liverpool Telescope+RISE photometry we identify the 2.78 hr period binary star CSS 41177 as a detached eclipsing double white dwarf binary with a 21,100 K primary star and a 10,500 K secondary star. This makes CSS 41177 only the second known eclipsing double white dwarf binary after NLTT 11748. The 2 minute long primary eclipse is 40% deep and the secondary eclipse 10% deep. From Gemini+GMOS spectroscopy, we measure the radial velocities of both components of the binary from the H{alpha} absorption line cores. These measurements, combined with the light curve information, yield white dwarf masses of M{sub 1} = 0.283 {+-} 0.064 M{sub sun} and M{sub 2} = 0.274 {+-} 0.034 M{sub sun}, making them both helium core white dwarfs. As an eclipsing, double-lined spectroscopic binary, CSS 41177 is ideally suited to measuring precise, model-independent masses and radii. The two white dwarfs will merge in roughly 1.1 Gyr to form a single sdB star.

  20. Cool white dwarf companions to four millisecond pulsars

    NASA Astrophysics Data System (ADS)

    Bassa, C. G.; Antoniadis, J.; Camilo, F.; Cognard, I.; Koester, D.; Kramer, M.; Ransom, S. R.; Stappers, B. W.

    2016-02-01

    We report on photometric and spectroscopic observations of white dwarf companions to four binary radio millisecond pulsars, leading to the discovery of companions to PSRs J0614-3329, J1231-1411 and J2017+0603. We place limits on the brightness of the companion to PSR J0613-0200. Optical spectroscopy of the companion to PSR J0614-3329 identifies it as a DA-type white dwarf with a temperature of Teff = 6460 ± 80 K, a surface gravity log g = 7.0 ± 0.2 cgs and a mass of MWD = 0.24 ± 0.04 M⊙. We find that the distance to PSR J0614-3329 is smaller than previously estimated, removing the need for the pulsar to have an unrealistically high γ-ray efficiency. Comparing the photometry with predictions from white dwarf cooling models allows us to estimate temperatures and cooling ages of the companions to PSRs J0613-0200, J1231-1411 and J2017+0603. We find that the white dwarfs in these systems are cool Teff < 4000 K and old ≳ 5 Gyr. Thin hydrogen envelopes are required for these white dwarfs to cool to the observed temperatures, and we suggest that besides hydrogen shell flashes, irradiation driven mass loss by the pulsar may have been important.

  1. The shortest period detached binary white dwarf system

    NASA Astrophysics Data System (ADS)

    Kilic, Mukremin; Brown, Warren R.; Kenyon, S. J.; Allende Prieto, Carlos; Andrews, J.; Kleinman, S. J.; Winget, K. I.; Winget, D. E.; Hermes, J. J.

    2011-05-01

    We identify SDSS J010657.39-100003.3 (hereafter J0106-1000) as the shortest period detached binary white dwarf (WD) system currently known. We targeted J0106-1000 as part of our radial velocity programme to search for companions around known extremely low-mass (ELM; ˜0.2 M⊙) WDs using the 6.5-m Multiple Mirror Telescope. We detect peak-to-peak radial velocity variations of 740 km s-1 with an orbital period of 39.1 min. The mass function and optical photometry rule out a main-sequence star companion. Follow-up high-speed photometric observations obtained at the McDonald 2.1-m telescope reveal ellipsoidal variations from the distorted primary but no eclipses. This is the first example of a tidally distorted WD. Modelling the light curve, we constrain the inclination angle of the system to be 67°± 13°. J0106-1000 contains a pair of WDs (0.17 M⊙ primary + 0.43 M⊙ invisible secondary) at a separation of 0.32 R⊙. The two WDs will merge in 37 Myr and most likely form a core He-burning single subdwarf star. J0106-1000 is the shortest time-scale merger system currently known. The gravitational wave strain from J0106-1000 is at the detection limit of the Laser Interferometer Space Antenna (LISA). However, accurate ephemeris and orbital period measurements may enable LISA to detect J0106-1000 above the Galactic background noise. Based on observations obtained at the Multiple Mirror Telescope (MMT) Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.

  2. Wind-driven evolution of white dwarf binaries to type Ia supernovae

    SciTech Connect

    Ablimit, Iminhaji; Xu, Xiao-jie; Li, X.-D.

    2014-01-01

    In the single-degenerate scenario for the progenitors of Type Ia supernovae (SNe Ia), a white dwarf rapidly accretes hydrogen- or helium-rich material from its companion star and appears as a supersoft X-ray source. This picture has been challenged by the properties of the supersoft X-ray sources with very low mass companions and the observations of several nearby SNe Ia. It has been pointed out that the X-ray radiation or the wind from the accreting white dwarf can excite winds or strip mass from the companion star, thus significantly influencing the mass transfer processes. In this paper, we perform detailed calculations of the wind-driven evolution of white dwarf binaries. We present the parameter space for the possible SN Ia progenitors and for the surviving companions after the SNe. The results show that the ex-companion stars of SNe Ia have characteristics more compatible with the observations, compared with those in the traditional single-degenerate scenario.

  3. The initiation and propagation of helium detonations in white dwarf envelopes

    SciTech Connect

    Shen, Ken J.; Moore, Kevin

    2014-12-10

    Detonations in helium-rich envelopes surrounding white dwarfs have garnered attention as triggers of faint thermonuclear '.Ia' supernovae and double detonation Type Ia supernovae. However, recent studies have found that the minimum size of a hotspot that can lead to a helium detonation is comparable to, or even larger than, the white dwarf's pressure scale height, casting doubt on the successful ignition of helium detonations in these systems. In this paper, we examine the previously neglected effects of C/O pollution and a full nuclear reaction network, and we consider hotspots with spatially constant pressure in addition to constant density hotspots. We find that the inclusion of these effects significantly decreases the minimum hotspot size for helium-rich detonation ignition, making detonations far more plausible during turbulent shell convection or during double white dwarf mergers. The increase in burning rate also decreases the minimum shell mass in which a helium detonation can successfully propagate and alters the composition of the shell's burning products. The ashes of these low-mass shells consist primarily of silicon, calcium, and unburned helium and metals and may explain the high-velocity spectral features observed in most Type Ia supernovae.

  4. Spin-up and mixing in accreting white dwarfs

    SciTech Connect

    Livio, M.; Truran, J.W.

    1987-07-01

    It is demonstrated that existing theories of mixing in accreting white dwarfs encounter difficulties when confronted with observations of enrichments in nova ejecta. Arguments are presented, based on the Ekman spin-up process, which suggest that angular momentum transport from the accreted material to the white dwarf is more efficient than previously thought. This should lead to matter spreading over the entire white dwarf surface, as well as inward mixing. It is shown that when efficient transfer of angular momentum is taken into account, the gross features of nova outbursts can be reproduced, with the runaway occuring in a mixed layer. Some implications of the results for DQ Her, the hibernation model of novae, recurrent novae, and soft X-ray emission are discussed. 63 references.

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

    NASA Astrophysics Data System (ADS)

    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 (<9000 K) DZ white dwarfs with strong metal 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.

  6. Halo White Dwarfs, Thick Disks, and a Sanity Check

    NASA Astrophysics Data System (ADS)

    Hansen, Brad M. S.

    2001-09-01

    The recent discovery of a population of high proper-motion white dwarfs by Oppenheimer and coworkers has caused a lot of speculation as to the origin of these stars. I show that the age distribution of the white dwarfs offers a kind of sanity check in these discussions. In particular, the majority of the identified population appears to have a similar age distribution to those in the standard, thin-disk white dwarf population. This is not what is expected for either the halo or thick disk, which are thought to be old populations. A subset of the Oppenheimer ``halo'' sample does indeed possess an age distribution consistent with a halo origin, but the density is smaller and consistent with the results of Gould, Flynn, & Bahcall for a high-end mass function slope of -0.9.

  7. Atypical Thermonuclear Supernovae from Tidally Crushed White Dwarfs

    SciTech Connect

    Rosswog, S.; Ramirez-Ruiz, E.; Hix, William Raphael

    2008-01-01

    Suggestive evidence has accumulated that intermediate mass black holes (IMBHs) exist in some globular clusters. Some stars will inevitably wander sufficiently close to the hole to suffer a tidal disruption. IMBHs can disrupt not only solar-type stars but also compact white dwarf stars. We investigate the fate of white dwarfs that approach the hole close enough to be disrupted and compressed to such an extent that explosive nuclear burning is triggered. Based on a precise modeling of the gas dynamics together with the nuclear reactions, it is argued that thermonuclear ignition is a natural outcome for white dwarfs of all masses passing well within the tidal radius. A good fraction of the star is accreted, yielding high luminosities that persist for up to a year. A peculiar, underluminous thermonuclear explosion accompanied by a soft X-ray transient signal would, if detected, be a compelling testimony for the presence of an IMBH.

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

    NASA Technical Reports Server (NTRS)

    Boehm-Vitense, Erika

    1980-01-01

    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 International Ultraviolet Explorer (IUE) shows that the white dwarf has an effective temperature of 23,000 +/- 2,000 K. If log g = 8 the Ly(alpha) profile indicates an effective temperature around 24,500 K. Using the theoretical models, one finds a visual magnitude of m(sub v) is approximately 16.5. For T(sub eff) = 24,500 K one expects for a white dwarf a luminosity of log L/solar luminosity is approximately -1.3 and M(sub V) is approximately 10.67. This gives a distance modulus for the system of m(sub v) - M(sub V) = 5.83 and an absolute magnitude M(sub v) = 0.3 for the giant.

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

    NASA Astrophysics Data System (ADS)

    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.

  10. Calibration of Synthetic Photometry Using DA White Dwarfs

    NASA Astrophysics Data System (ADS)

    Holberg, J. B.; Bergeron, P.

    2005-12-01

    We have calibrated four major ground-based photometric systems with respect to the Hubble Space Telescope absolute flux scale, which is defined by Vega and four fundamental DA white dwarfs. These photometric systems include the Johnson-Kron-Cousins UBVRI, the Stromgren uvby filters, the 2MASS 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 which span a wide range in both of these parameters.

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

    NASA Technical Reports Server (NTRS)

    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.

  12. THE SPECTRAL EVOLUTION OF CONVECTIVE MIXING WHITE DWARFS, THE NON-DA GAP, AND WHITE DWARF COSMOCHRONOLOGY

    SciTech Connect

    Chen, Eugene Y.; Hansen, Brad M. S. E-mail: hansen@astro.ucla.edu

    2012-07-01

    The spectral distribution of field white dwarfs shows a feature called the 'non-DA gap'. As defined by Bergeron et al., this is a temperature range (5100-6100 K) where relatively few non-DA stars are found, even though such stars are abundant on either side of the gap. It is usually viewed as an indication that a significant fraction of white dwarfs switch their atmospheric compositions back and forth between hydrogen-rich and helium-rich as they cool. In this Letter, we present a Monte Carlo model of the Galactic disk white dwarf population, based on the spectral evolution model of Chen and Hansen. We find that the non-DA gap emerges naturally, even though our model only allows white dwarf atmospheres to evolve monotonically from hydrogen-rich to helium-rich through convective mixing. We conclude by discussing the effects of convective mixing on the white dwarf luminosity function and the use thereof for Cosmochronology.

  13. Anomalous Cooling of the Massive White Dwarf in U Geminorum Following a Narrow Dwarf Nova Outburst

    NASA Astrophysics Data System (ADS)

    Sion, Edward M.; Cheng, F. H.; Szkody, Paula; Sparks, Warren; Gänsicke, Boris; Huang, Min; Mattei, Janet

    1998-03-01

    We obtained Hubble Space Telescope Goddard High-Resolution Spectrograph medium-resolution (G160M grating), phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf nova quiescence, 13 days and 61 days following the end of a narrow outburst. The spectral wavelength ranges were centered on three different line regions: N V (1238 Å, 1242 Å), Si III (1300 Å), and He II (1640 Å). All of the quiescent spectra at both epochs are dominated by absorption lines and show no emission features. The Si III and He II absorption-line velocities versus orbital phase trace the orbital motion of the white dwarf, but the N V absorption velocities appear to deviate from the white dwarf motion. We confirm our previously reported low white dwarf rotational velocity, V sin i = 100 km s-1. We obtain a white dwarf orbital velocity semiamplitude K1 = 107 km s-1. Using the γ-velocity of Wade, we obtain an Einstein redshift of 80.4 km s-1 and hence a carbon core white dwarf mass of ~1.1 M⊙. We report the first subsolar chemical abundances of C and Si for U Gem with C/H = 0.05 times solar, almost certainly a result of C depletion due to thermonuclear processing. This C depletion is discussed within the framework of a weak thermonuclear runaway, contamination of the secondary during the common envelope phase, and mixing of C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova event. Remarkably, the Teff of the white dwarf 13 days after outburst is only 32,000 K, anomalously cooler than previous early postoutburst measurements. Extensive cooling during an extraordinarily long (210 days) quiescence followed by accretion onto an out-of-equilibrium cooled degenerate could explain the lower Teff. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

  14. An Update on the Quirks of Pulsating, Accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Szkody, Paula; Mukadam, Anjum S.; Gänsicke, Boris T.; Hermes, J. J.; Toloza, Odette

    2015-06-01

    At the 18th European White Dwarf Workshop, we reported results for several dwarf novae containing pulsating white dwarfs that had undergone an outburst in 2006-2007. HST and optical data on the white dwarfs in GW Lib, EQ Lyn and V455 And all showed different behaviors in the years following their outbursts. We continued to follow these objects for the last 2 years, providing timescales of 6-7 years past outburst. All three reached their optical quiescent values within 4 years but pulsational stability has not returned. EQ Lyn showed its pre-outburst pulsation period after 3 years, but it continues to show photometric variability that alternates between pulsation and disk superhump periods while remaining at quiescence. V455 And has almost reached its pre-outburst pulsation period, while GW Lib still remains heated and with a different pulsation spectrum than at quiescence. These results indicate that asteroseismology provides a unique picture of the effects of outburst heating on the white dwarf.

  15. The Chemical Abundances of White Dwarfs in CVS

    NASA Astrophysics Data System (ADS)

    Sion, Edward M.

    Dwarf novae and nova-like variables contain accreting white dwarfs which may have undergone numerous thermonuclear runaways as classical novae. In order to demonstrate their connection with novae however attempts have been made to detect ejected shells without success (references). However a new approach has recently emerged for systems in which the white dwarf photosphere has been detected spectroscopically. Sion et al. (1997) showed that the surface abundances of the white dwarf in VW Hydri during its quiescence manifests a direct evolutionary to a past thermonuclear event. This conclusion is based upon the presence of a large ratio of nitrogen to carbon abundance and the spectroscopic presence of odd-numbered proton-capture nuclei in abundances greatly elevated above solar. Both of these spectroscopic characteristics point to hot CNO processing as the source of the abundances. Other systems besides VW Hyi reveal further evidence of nova processing. This talk will review all of the determinations of surface chemical abundances of white dwarfs in cataclysmic variables both above and below the period gap and will discuss the implications for CV evolution and contributions to the heavy element content of the interstellar medium.

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

    SciTech Connect

    Córsico, A.H.; Althaus, L.G.; García-Berro, E. E-mail: althaus@fcaglp.unlp.edu.ar E-mail: kepler@if.ufrgs.br

    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.

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

    NASA Astrophysics Data System (ADS)

    Córsico, A. H.; Althaus, L. G.; Miller Bertolami, M. M.; Kepler, S. O.; García-Berro, E.

    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 (μν) 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 (Pi dot) 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 Pi dot 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 μν lesssim 10-11 μB. 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.

  18. FURTHER DEFINING SPECTRAL TYPE 'Y' AND EXPLORING THE LOW-MASS END OF THE FIELD BROWN DWARF MASS FUNCTION

    SciTech Connect

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

    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.

  19. Do Some X-ray Stars Have White Dwarf Companions?

    NASA Technical Reports Server (NTRS)

    McCollum, Bruce

    1995-01-01

    Some Be stars which are intermittent C-ray sources may have white dwarf companions rather than neutron stars. It is not possible to prove or rule out the existence of Be+WD systems using X-ray or optical data. However, the presence of a white dwarf could be established by the detection of its EUV continuum shortward of the Be star's continuum turnover at 1OOOA. Either the detection or the nondetection of Be+WD systems would have implications for models of Be star variability, models of Be binary system formation and evolution, and models of wind-fed accretion.

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

    NASA Astrophysics Data System (ADS)

    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.

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

    NASA Astrophysics Data System (ADS)

    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.

  2. Magnetic white dwarf stars in the Sloan Digital Sky Survey

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Pelisoli, I.; Jordan, S.; Kleinman, S. J.; Koester, D.; Külebi, B.; Peçanha, V.; Castanheira, B. G.; Nitta, A.; Costa, J. E. S.; Winget, D. E.; Kanaan, A.; Fraga, L.

    2013-03-01

    To obtain 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 to 733 MG, which amounts to 4 per cent 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.

  3. Testing energy non-additivity in white dwarfs

    NASA Astrophysics Data System (ADS)

    Carmona, J. M.; Cortés, J. L.; Gracia-Ruiz, R.; Loret, N.

    2014-03-01

    We consider a particular effect which can be expected in scenarios of deviations from special relativity induced by Planckian physics: the loss of additivity in the total energy of a system of particles. We argue about the necessity to introduce a length scale to control the effects of non-additivity for macroscopic objects and consider white dwarfs as an appropriate laboratory to test this kind of new physics. We study the sensitivity of the mass-radius relation of the Chandrasekhar model to these corrections by comparing the output of a simple phenomenological model to observational data of white dwarfs.

  4. Pulsations powered by hydrogen shell burning in white dwarfs

    NASA Astrophysics Data System (ADS)

    Camisassa, M. E.; Córsico, A. H.; Althaus, L. G.; Shibahashi, H.

    2016-10-01

    Context. 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. Aims: 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. Methods: We construct white-dwarf sequences from low-metallicity progenitors by means of full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally pulsing and the post-asymptotic giant-branch phases, and analyze their pulsation stability by solving the linear, nonadiabatic, nonradial pulsation equations for the models in the range of effective temperatures Teff 15 000-8000 K. Results: We demonstrate that, for white dwarf models with masses M⋆ ≲ 0.71 M⊙ and effective temperatures 8500 ≲ Teff ≲ 11 600 K that evolved from low-metallicity progenitors (Z = 0.0001, 0.0005, and 0.001), the dipole (ℓ = 1) and quadrupole (ℓ = 2) g1-modes are excited mostly as a result of the hydrogen-burning shell through the ɛ-mechanism, in addition to other g-modes driven by either the κ - γ or the convective driving mechanism. However, the ɛ mechanism is insufficient to drive these modes in white dwarfs evolved from solar-metallicity progenitors. Conclusions: We suggest that efforts should be made to observe the dipole g1-mode in white dwarfs associated with low-metallicity environments, such as globular clusters and/or the galactic halo, to place constraints on hydrogen shell burning in cool white dwarfs and the third dredge-up episode during the preceding asymptotic giant-branch phase.

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

    NASA Technical Reports Server (NTRS)

    Mccollum, Bruce

    1995-01-01

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

  6. FORETELLINGS OF RAGNAROeK: WORLD-ENGULFING ASYMPTOTIC GIANTS AND THE INHERITANCE OF WHITE DWARFS

    SciTech Connect

    Mustill, Alexander J.; Villaver, Eva

    2012-12-20

    The search for planets around white dwarf stars, and evidence for dynamical instability around them in the form of atmospheric pollution and circumstellar disks, 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 study, for the first time, 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 {approx}3 AU for a 1 M{sub Sun} star and {approx}5 AU for a 5 M{sub Sun} 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 planetary eccentricity slightly strengthens the tidal forces experienced by Jovian planets. Eccentric terrestrial planets are more at risk, since their eccentricity does not decay and their small pericenter takes them inside the stellar envelope. We also find the closest radii at which planets will be found around white dwarfs, assuming that any planet entering the stellar envelope is destroyed. Planets are in that case unlikely to be found inside {approx}1.5 AU of a white dwarf with a 1 M{sub Sun} progenitor and {approx}10 AU of a white dwarf with a 5 M{sub Sun} progenitor.

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

    SciTech Connect

    Bardalez Gagliuffi, Daniella C.; Burgasser, Adam J.; Nicholls, Christine P.; Gelino, Christopher R.; Looper, Dagny L.; Schmidt, Sarah J.; Cruz, Kelle; West, Andrew A.; Gizis, John E.; Metchev, Stanimir

    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.

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

    NASA Astrophysics Data System (ADS)

    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.

  9. Mass-Radius Relation of Strongly Magnetized White Dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  10. Asteroseismic constraints on diffusion in white dwarf envelopes

    NASA Astrophysics Data System (ADS)

    Bischoff-Kim, A.; Metcalfe, T. S.

    2011-06-01

    The asteroseismic analysis of white dwarfs allows us to peer below their photospheres and determine their internal structure. At ˜28 000 K EC20058-5234 is the hottest known pulsating helium atmosphere white dwarf. As such, it constitutes an important link in the evolution of white dwarfs down the cooling track. It is also astrophysically interesting because it is at a temperature where white dwarfs are expected to cool mainly through the emission of plasmon neutrinos. In the present work, we perform an asteroseismic analysis of EC20058-5234 and place the results in the context of stellar evolution and time-dependent diffusion calculations. We use a parallel genetic algorithm complemented with targeted grid searches to find the models that fit the observed periods best. Comparing our results with similar modelling of EC20058-5234's cooler cousin CBS114, we find a helium envelope thickness consistent with time-dependent diffusion calculations and obtain a precise mode identification for EC20058-5234.

  11. Mass-radius relation of strongly magnetized white dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  12. White dwarfs constraints on dark sector models with light particles

    SciTech Connect

    Ubaldi, Lorenzo

    2014-06-24

    The white dwarf luminosity function is well understood in terms of standard model physics and leaves little room for exotic cooling mechanisms related to the possible existence of new weakly interacting light particles. This puts significant constraints on the parameter space of models that contain a massive dark photon and light dark sector particles.

  13. Planetary Engulfment as a Trigger for White Dwarf Pollution

    NASA Astrophysics Data System (ADS)

    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.

  14. First Detection of Krypton and Xenon in a White Dwarf

    NASA Technical Reports Server (NTRS)

    Werner, Klaus; Rauch, Thomas; Ringat, Ellen; Kruk, Jeffrey W.

    2012-01-01

    We report on the first detection of the noble gases krypton (Z = 36) and xenon (54) in a white dwarf. About 20 KrVI-VII and Xe VI-VII lines were discovered in the ultraviolet spectrum of the hot DO-type white dwarf RE 0503-289. The observations, performed with the Far Ultraviolet Spectroscopic Explorer, also reveal highly ionized photospheric lines from other trans-iron group elements, namely Ga (31), Ge (32), As (33), Se (34), Mo (42), Sn (50), Te (52), and I (53), from which gallium and molybdenum are new discoveries in white dwarfs, too. For Kr and Xe, we performed an NLTE analysis and derived mass fractions of log Kr = -4.3 plus or minus 0.5 and log Xe = -4.2 plus or minus 0.6, corresponding to an enrichment by factors of 450 and 3800, respectively, relative to the Sun. The origin of the large overabundances is unclear. We discuss the roles of neutron-capture nucleosynthesis in the-precursor star and radiation-driven diffusion. It is possible that diffusion is insignificant and thaI the observed metal abundances constrain the evolutionary history of the star. Its hydrogen deficiency may be the consequence of a late helium-shell nash or a binary white dwarf merger.

  15. FIRST DETECTION OF KRYPTON AND XENON IN A WHITE DWARF

    SciTech Connect

    Werner, Klaus; Rauch, Thomas; Ringat, Ellen; Kruk, Jeffrey W.

    2012-07-01

    We report on the first detection of the noble gases krypton (Z = 36) and xenon (54) in a white dwarf. About 20 Kr VI- VII and Xe VI- VII lines were discovered in the ultraviolet spectrum of the hot DO-type white dwarf RE 0503-289. The observations, performed with the Far Ultraviolet Spectroscopic Explorer, also reveal highly ionized photospheric lines from other trans-iron group elements, namely Ga (31), Ge (32), As (33), Se (34), Mo (42), Sn (50), Te (52), and I (53), from which gallium and molybdenum are new discoveries in white dwarfs, too. For Kr and Xe, we performed an NLTE analysis and derived mass fractions of log Kr = -4.3 {+-} 0.5 and log Xe = -4.2 {+-} 0.6, corresponding to an enrichment by factors of 450 and 3800, respectively, relative to the Sun. The origin of the large overabundances is unclear. We discuss the roles of neutron-capture nucleosynthesis in the precursor star and radiation-driven diffusion. It is possible that diffusion is insignificant and that the observed metal abundances constrain the evolutionary history of the star. Its hydrogen deficiency may be the consequence of a late helium-shell flash or a binary white dwarf merger.

  16. Chandra grating spectroscopy of three hot white dwarfs

    NASA Astrophysics Data System (ADS)

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

    2012-10-01

    Context. 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 (LB 1919) and the other is a non-DA of spectral type PG 1159 (PG 1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). 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. LB 1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD 246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG 1159 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 LB 1919. 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 GD 246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG 1520+525 constrains the effective temperature to Teff = 150 000 ± 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GW Vir class (PG 1159 - 035) defines the location of the blue edge of the GW Vir

  17. Chandra Grating Spectroscopy of Three Hot White Dwarfs

    NASA Technical Reports Server (NTRS)

    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

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

    SciTech Connect

    Bochanski, Jr, John J.

    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.

  19. Observations of the Ultraviolet Spectra of Helium (DB) White Dwarfs and a Study of the Ultraviolet Spectra of White Dwarfs Containing Carbon

    NASA Technical Reports Server (NTRS)

    Wegner, G. A.

    1984-01-01

    Strong ultraviolet carbon lines were detected in the spectrum of the southern DC white dwarf BPM 11668. Observations of a number of hotter DB white dwarfs with IUE show no evidence of carbon features. Two additional DA white dwarfs were observed that have the strong unidentified absorption near 1400 A which now seems to be identified with another lower temperature feature as satellite lines to Lyman alpha radiation.

  20. Dark baryons not in ancient halo white dwarfs

    NASA Astrophysics Data System (ADS)

    Crézé, M.; Mohan, V.; Robin, A. C.; Reylé, C.; McCracken, H. J.; Cuillandre, J.-C.; Le Fèvre, O.; Mellier, Y.

    2004-10-01

    Having ruled out the possibility that stellar objects are the main contributor of the dark matter embedding galaxies, microlensing experiments cannot exclude the hypothesis that a significant fraction of the Milky Way dark halo might be made of MACHOs with masses in the range 0.5-0.8 M⊙. Ancient white dwarfs are generally considered the most plausible candidates for such MACHOs. We report the results of a search for such white dwarfs in a proper motion survey covering a 0.16 sq. deg. field at three epochs at high galactic latitude, and 0.938 sq. deg. at two epochs at intermediate galactic latitude (VIRMOS survey), using the CFH telescope. Both surveys are complete to I = 23, with detection efficiency fading to 0 at I = 24.2. Proper motion data are suitable to separate unambiguously halo white dwarfs identified as belonging to a non rotating system. No candidates were found within the colour-magnitude-proper motion volume where such objects can be safely discriminated from any standard population as well as from possible artefacts. In the same volume, we estimate the maximum white dwarf halo fraction compatible with this observation at different significance levels if the halo is at least 14 gigayears old and under different ad hoc initial mass functions. Our data alone rule out a halo fraction greater than 14 % at a 95% confidence level. Combined with two previous investigations exploring comparable volumes, this pushes the limit below 4 % (95% confidence level) or below 1 % (64% confidence), and implies that if baryonic dark matter is present in galaxy halos, it is not, or is only marginally in the form of faint hydrogen white dwarfs. Based on observations made at Canada-France-Hawaii Telescope (CFHT).

  1. Contrasting Accreting White Dwarf Pulsators with the ZZ Ceti Stars

    NASA Astrophysics Data System (ADS)

    Mukadam, A. S.; Szkody, P.; Gänsicke, B. T.; Pala, A.

    2017-03-01

    Understanding the similarities and differences between the accreting white dwarf pulsators and their non-interacting counterparts, the ZZ Ceti stars, will eventually help us deduce how accretion affects pulsations. ZZ Ceti stars pulsate in a narrow instability strip in the range 10800–12300 K due to H ionization in their pure H envelopes; their pulsation characteristics depend on their temperature and stellar mass. Models of accreting white dwarfs are found to be pulsationally unstable due to the H/HeI ionization zone, and even show a second instability strip around 15000 K due to HeII ionization. Both these strips are expected to merge for a He abundance higher than 0.48 to form a broad instability strip, which is consistent with the empirical determination of 10500–16000 K. Accreting pulsators undergo outbursts, during which the white dwarf is heated to temperatures well beyond the instability strip and is observed to cease pulsations. The white dwarf then cools to quiescence in a few years as its outer layers cool more than a million times faster than the evolutionary rate. This provides us with an exceptional opportunity to track the evolution of pulsations from the blue edge to quiescence in a few years, while ZZ Ceti stars evolve on Myr timescales. Some accreting pulsators have also been observed to cease pulsations without any apparent evidence of an outburst. This is a distinct difference between this class of pulsators and the non-interacting ZZ Ceti stars. While the ZZ Ceti instability strip is well sampled, the strip for the accreting white dwarfs is sparsely sampled and we hereby add two new potential discoveries to improve the statistics.

  2. Spectroscopic Analysis of Hybrid White Dwarf Spectra from the Sloan Digital Sky Survey

    NASA Astrophysics Data System (ADS)

    Manseau, P. M.; Bergeron, P.; Green, E. M.

    2017-03-01

    We present a model atmosphere analysis of hot (Teff> 30 000 K) white dwarf spectra from the Sloan Digital Sky Survey showing both hydrogen and helium lines, under the assumption of chemically homogeneous and stratified atmospheric compositions. We identify several hybrid white dwarfs in the SDSS that are better explained in terms of chemically stratified atmospheres, where a thin hydrogen atmosphere floats in diffusive equilibrium on top of a more massive helium envelope. We also present an updated analysis of PG 1305–017, the only stratified white dwarf identified in previous spectroscopic analyses of DAO white dwarfs. We interpret our results in the general context of the spectral evolution of white dwarfs.

  3. Spectroscopic observations of V443 Herculis - A symbiotic binary with a low mass white dwarf

    NASA Technical Reports Server (NTRS)

    Dobrzycka, Danuta; Kenyon, Scott J.; Mikolajewska, Joanna

    1993-01-01

    We present an analysis of new and existing photometric and spectroscopic observations of the symbiotic binary V443 Herculis. This binary system consists of a normal M5 giant and a hot compact star. These two objects have comparable luminosities: about 1500 solar for the M5 giant and about 1000 solar for the compact star. We identify three nebular regions in this binary: a small, highly ionized volume surrounding the hot component, a modestly ionized shell close to the red giant photosphere, and a less dense region of intermediate ionization encompassing both binary components. The system parameters for V443 Her suggest the hot component currently declines from a symbiotic nova eruption.

  4. A 1.05 M ⊙ Companion to PSR J2222-0137: The Coolest Known White Dwarf?

    NASA Astrophysics Data System (ADS)

    Kaplan, David L.; Boyles, Jason; Dunlap, Bart H.; Tendulkar, Shriharsh P.; Deller, Adam T.; Ransom, Scott M.; McLaughlin, Maura A.; Lorimer, Duncan R.; Stairs, Ingrid H.

    2014-07-01

    The recycled pulsar PSR J2222-0137 is one of the closest known neutron stars (NSs) with a parallax distance of 267_{-0.9}^{+1.2} pc and an edge-on orbit. We measure the Shapiro delay in the system through pulsar timing with the Green Bank Telescope, deriving a low pulsar mass (1.20 ± 0.14 M ⊙) and a high companion mass (1.05 ± 0.06 M ⊙) consistent with either a low-mass NS or a high-mass white dwarf. We can largely reject the NS hypothesis on the basis of the system's extremely low eccentricity (3 × 10-4)—too low to have been the product of two supernovae under normal circumstances. However, despite deep optical and near-infrared searches with Southern Astrophysical Research and the Keck telescopes we have not discovered the optical counterpart of the system. This is consistent with the white dwarf hypothesis only if the effective temperature is <3000 K, a limit that is robust to distance, mass, and atmosphere uncertainties. This would make the companion to PSR J2222-0137 one of the coolest white dwarfs ever observed. For the implied age to be consistent with the age of the Milky Way requires the white dwarf to have already crystallized and entered the faster Debye-cooling regime.

  5. A 1.05 M{sub ☉} companion to PSR J2222–0137: The coolest known white dwarf?

    SciTech Connect

    Kaplan, David L.; Boyles, Jason; McLaughlin, Maura A.; Lorimer, Duncan R.; Dunlap, Bart H.; Tendulkar, Shriharsh P.; Deller, Adam T.; Ransom, Scott M.; Stairs, Ingrid H.

    2014-07-10

    The recycled pulsar PSR J2222–0137 is one of the closest known neutron stars (NSs) with a parallax distance of 267{sub −0.9}{sup +1.2} pc and an edge-on orbit. We measure the Shapiro delay in the system through pulsar timing with the Green Bank Telescope, deriving a low pulsar mass (1.20 ± 0.14 M{sub ☉}) and a high companion mass (1.05 ± 0.06 M{sub ☉}) consistent with either a low-mass NS or a high-mass white dwarf. We can largely reject the NS hypothesis on the basis of the system's extremely low eccentricity (3 × 10{sup –4})—too low to have been the product of two supernovae under normal circumstances. However, despite deep optical and near-infrared searches with Southern Astrophysical Research and the Keck telescopes we have not discovered the optical counterpart of the system. This is consistent with the white dwarf hypothesis only if the effective temperature is <3000 K, a limit that is robust to distance, mass, and atmosphere uncertainties. This would make the companion to PSR J2222–0137 one of the coolest white dwarfs ever observed. For the implied age to be consistent with the age of the Milky Way requires the white dwarf to have already crystallized and entered the faster Debye-cooling regime.

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

    NASA Astrophysics Data System (ADS)

    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.

  7. The Dwarf Novae Ty Psc and V436 Cen During Quiescence: Exposed White Dwarfs?

    NASA Astrophysics Data System (ADS)

    Nadalin, I.; Sion, E. M.

    2000-12-01

    The dwarf novae TY Psc and V436 Cen are SU UMa systems with very similar orbital periods, similar recurrence times for normal outbursts ( 23 days) and superoutbursts ( ~ 340 days) and nearly identical outburst amplitudes. We have applied the Massa-Fitzpatrick (2000) flux calibration correction to the archival IUE NEWSIPS SWP spectra of these two systems, obtained during dwarf nova quiescence. We have carried out high gravity model atmosphere using the codes TLUSTY195, SYNSPEC42, ROTIN and accretion disk synthetic spectra from the grid of Wade and Hubeny (1998). We present our results on the physical properties of the underlying white dwarf accreters, including temperature, gravity, chemical abundances estimates, and the accretion rate during quiescence. We discuss our results in the context of the overall picture of accretion physics in dwarf novae and the effects of accretion on the white dwarf. This research was supported in part by NSF grant AST 99-01955, NASA ADP grant NAG5-8388 and by summer research funding from the NASA- Delaware Space Grant Colleges Consortium.

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

    SciTech Connect

    Li, Yun; Kouwenhoven, M. B. N.; Stamatellos, D.; Goodwin, S. P.

    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

  9. PLANETS AROUND LOW-MASS STARS (PALMS). I. A SUBSTELLAR COMPANION TO THE YOUNG M DWARF 1RXS J235133.3+312720

    SciTech Connect

    Bowler, Brendan P.; Liu, Michael C.; Cieza, Lucas A.; Kraus, Adam L.; Shkolnik, Evgenya L.; Dupuy, Trent J.; Tamura, Motohide

    2012-07-10

    We report the discovery of a brown dwarf companion to the young M dwarf 1RXS J235133.3+312720 as part of a high contrast imaging search for planets around nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2.''4 ({approx}120 AU) pair is confirmed to be comoving from two epochs of high-resolution imaging. Follow-up low- and moderate-resolution near-infrared spectroscopy of 1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of L0{sup +2}{sub -1}. The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV activity levels comparable to young moving group members with ages of {approx}10-100 Myr. UVW kinematics based the measured radial velocity of the primary and the system's photometric distance (50 {+-} 10 pc) indicate it is likely a member of the {approx}50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology shows subtle hints of intermediate surface gravity. The spectrum is also an excellent match to the {approx}200 Myr M9 brown dwarf LP 944-20. Assuming an age of 50-150 Myr, evolutionary models imply a mass of 32 {+-} 6 M{sub Jup} for the companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor moving group after the L4 companion CD-35 2722 B and one of the few benchmark brown dwarfs known at young ages.

  10. IDENTIFICATION OF A WIDE, LOW-MASS MULTIPLE SYSTEM CONTAINING THE BROWN DWARF 2MASS J0850359+105716

    SciTech Connect

    Faherty, Jacqueline K.; Burgasser, Adam J.; Bochanski, John J.; Looper, Dagny L.; West, Andrew A.; Van der Bliek, Nicole S.

    2011-03-15

    We report our discovery of NLTT 20346 as an M5+M6 companion system to the tight binary (or triple) L dwarf 2MASS J0850359+105716. This nearby ({approx}31 pc), widely separated ({approx}7700 AU) quadruple system was identified through a cross-match of proper motion catalogs. Follow-up imaging and spectroscopy of NLTT 20346 revealed it to be a magnetically active M5+M6 binary with components separated by {approx}2'' (50-80 AU). Optical spectroscopy of the components shows only moderate H{alpha} emission corresponding to a statistical age of {approx}5-7 Gyr for both M dwarfs. However, NLTT 20346 is associated with the XMM-Newton source J085018.9+105644, and based on X-ray activity the age of NLTT 20346 is between 250 and 450 Myr. Strong Li absorption in the optical spectrum of 2MASS J0850+1057 indicates an upper age limit of 0.8-1.5 Gyr, favoring the younger age for the primary. Using evolutionary models in combination with an adopted system age of 0.25-1.5 Gyr indicates a total mass for 2MASS J0850+1057 of 0.07 {+-} 0.02 M{sub sun}, if it is a binary. NLTT 20346/2MASS J0850+1057 joins a growing list of hierarchical systems containing brown dwarf binaries and is among the lowest binding energy associations found in the field. Formation simulations via gravitational fragmentation of massive extended disks have successfully produced a specific analog to this system.

  11. The optical emission from oscillating white dwarf radiative shock waves

    NASA Technical Reports Server (NTRS)

    Imamura, James N.; Rashed, Hussain; Wolff, Michael T.

    1991-01-01

    The hypothesis that quasi-periodic oscillations (QPOs) are due to the oscillatory instability of radiative shock waves discovered by Langer et al. (1981, 1092) is examined. The time-dependent optical spectra of oscillating radiative shocks produced by flows onto magnetic white dwarfs are calculated. The results are compared with the observations of the AM Her QPO sources V834 Cen, AN UMa, EF Eri, and VV Pup. It is found that the shock oscillation model has difficulties with aspects of the observations for each of the sources. For VV Pup, AN UMa, and V834 Cen, the cyclotron luminosities for the observed magnetic fields of these systems, based on our calculations, are large. The strong cyclotron emission probably stabilizes the shock oscillations. For EF Eri, the mass of the white dwarf based on hard X-ray observations is greater than 0.6 solar mass.

  12. s-Process Abundances in Binary Stars With White Dwarfs

    NASA Astrophysics Data System (ADS)

    Merle, T.; Jorissen, A.; Van Eck, S.; Masseron, T.; van Winckel, H.

    2015-12-01

    The enrichment of barium stars in s-process elements is known to be due to pollution by mass transfer from an asymptotic giant branch (AGB) companion star, now an extincted C-O white-dwarf (McClure et al. [4]; Gray et al. [1]). We investigate the relationship between the level of enrichment in s-process elements in the barium star and the mass of its white dwarf (WD) companion. It is expected that helium WDs, which have masses smaller than about 0.5 M⊙ and whose progenitor never reached the AGB phase, should not pollute with s-process elements their giant companion. That companion should thus never turn into a barium star. Our results conform to the expectation that binary systems with WD companions less massive than 0.5 M⊙ do not host barium stars.

  13. Chemical stratification in white dwarf atmospheres and envelopes

    NASA Technical Reports Server (NTRS)

    Koester, D.

    1989-01-01

    Theoretical arguments supporting a new mechanism maintaining a homogeneously mixed composition in white dwarf atmospheres with traces of helium are presented. Diffusion time scales, meridional circulation, mass loss, accretion of interstellar matter, convection, and radiative levitation are discussed. Theoretically, layered envelopes, with hydrogen on top of helium and an abundance profile in the transition layer determined by diffusion equilibrium, are expected. In cases with observed helium and hydrogen in the atmosphere this means that the total hydrogen mass must be very small. The empirical evidence for such atmospheres are assessed, using a new grid of model atmospheres with stratified element abundances and applying it to typical mixed abundance cases at the hot end of the white dwarf temperature sequence.

  14. A gaseous metal disk around a white dwarf.

    PubMed

    Gänsicke, B T; Marsh, T R; Southworth, J; Rebassa-Mansergas, A

    2006-12-22

    The destiny of planetary systems through the late evolution of their host stars is very uncertain. We report a metal-rich gas disk around a moderately hot and young white dwarf. A dynamical model of the double-peaked emission lines constrains the outer disk radius to just 1.2 solar radii. The likely origin of the disk is a tidally disrupted asteroid, which has been destabilized from its initial orbit at a distance of more than 1000 solar radii by the interaction with a relatively massive planetesimal object or a planet. The white dwarf mass of 0.77 solar mass implies that planetary systems may form around high-mass stars.

  15. Testing Common Envelopes on Double White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Nandez, Jose L. A.; Ivanova, Natalia; Lombardi, James C., Jr.

    2015-06-01

    The formation of a double white dwarf binary likely involves a common envelope (CE) event between a red giant and a white dwarf (WD) during the most recent episode of Roche lobe overflow mass transfer. We study the role of recombination energy with hydrodynamic simulations of such stellar interactions. We find that the recombination energy helps to expel the common envelope entirely, while if recombination energy is not taken into account, a significant fraction of the common envelope remains bound. We apply our numerical methods to constrain the progenitor system for WD 1101+364 - a double WD binary that has well-measured mass ratio of q=0.87±0.03 and an orbital period of 0.145 days. Our best-fit progenitor for the pre-common envelope donor is a 1.5 ⊙ red giant.

  16. The Pulsating, Accreting White Dwarf in GW Lib after Outburst

    NASA Astrophysics Data System (ADS)

    Szkody, Paula

    The first known pulsating white dwarf in an accreting close binary system (GW Lib) underwent an outburst in April, 2007. We aim to follow the pulsation spectrum as the white dwarf cools back to to its quiescent temperature from its heating due to the outburst which should take about 3 years. As it cools, it should re-enter the instability strip and we can witness changes in the driving mechanism and detect modes that are excited by the temperature changes. The higher pulse amplitude in UV vs optical and the available time-tag mode makes GALEX the instrument of choice. The data in 2008 will be combined with our DOT time in May, June 2007 to provide coverage of the largest cooling that takes place in the year following outburst. Since the few known systems only outburst every 20-30 yrs, this is the first opportunity to accomplish a study of this type."

  17. Discovery of five new massive pulsating white dwarf stars

    NASA Astrophysics Data System (ADS)

    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.

  18. Photometric Variability and Rotation in Magnetic White Dwarfs

    NASA Astrophysics Data System (ADS)

    Lawrie, K. A.; Burleigh, M. R.; Brinkworth, C. S.; Marsh, T. R.

    2010-11-01

    We present a search for long term (months-years) photometric variability in a sample of ten isolated magnetic white dwarfs using observations taken with the Liverpool Robotic Telescope between March 2005 and January 2007. These stars had previously been found to be photometrically stable on short (hours-one week) timescales [1]. We construct differential light curves for each target and then use CLEAN and Lomb-Scargle periodograms to determine any periodicity that may be present. Photometric variability is detected in two of the targets during the observed timescale-G 240-72 and G 227-28. We find no variability in the remaining eight targets above the 1% level. Finally, we search for any correlations between the spin periods and intrinsic physical properties of magnetic white dwarfs, such as the magnetic field strength, temperature, mass and age.

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

    NASA Astrophysics Data System (ADS)

    Zhang, Z. H.; Pinfield, D. J.; Gálvez-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.

    2017-01-01

    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, and ULAS J124425.75+102439.3). We examined the spectral type and metallicity classification of subclasses of known L subdwarfs. We summarized 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 and 2700 K, which are higher than similar-typed L dwarfs by around 100-400 K depending on different subclasses and subtypes. We constrained the metallicity ranges of subclasses of M, L, and T subdwarfs. We also discussed the spectral-type and absolute magnitude relationships for L and T subdwarfs.

  20. Mapping the Properties of Convection in Pulsating White Dwarfs

    NASA Astrophysics Data System (ADS)

    Dalessio, J.; Provencal, J. L.; Montgomery, M. H.; Shipman, H. L.

    2013-01-01

    Montgomery (2005) showed that the properties of a pulsating white dwarf's convection zone can be determined by fitting the observed lightcurves with model simulations. The Whole Earth Telescope (WET) and the Delaware Asteroseismic Research Center (DARC) are using this technique to map the properties of convection across the DA and DB instability strips. We present the current status of the project, including preliminary analysis of light curves and from recent WET campaigns.

  1. Planet-Planet Scattering and White Dwarf Pollution

    NASA Astrophysics Data System (ADS)

    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.

  2. The Theoretical Instability Strip of V777 Her White Dwarfs

    NASA Astrophysics Data System (ADS)

    Van Grootel, V.; Fontaine, G.; Brassard, P.; Dupret, M.-A.

    2017-03-01

    We present a new theoretical investigation of the instability strip of V777 Her (DBV) white dwarfs. We apply a time-dependent convection (TDC) treatment to cooling models of DB and DBA white dwarfs. Using the spectroscopic calibration for the convective efficiency, ML2/α=1.25, we find a wide strip covering the range of effective temperature from 30,000 K down to about 22,000 K at log g = 8.0. This accounts very well for the empirical instability strip derived from a new accurate and homogenous spectroscopic analysis of known pulsators. Our approach leads to an exact description of the blue edge and to a correct understanding of the onset and development of pulsational instabilities, similarly to our results of TDC applied to ZZ Ceti white dwarfs in the recent past. We propose that, contrarily to what is generally believed, there is practically no fuzziness on the boundaries of the V777 Her instability strip due to traces of hydrogen in the atmospheres of some of these helium-dominated-atmosphere stars. Contrary to the blue edge, the red edge provided by TDC computations is far too cool compared to the empirical one. A similar situation was observed for the ZZ Ceti stars as well. We hence test the energy leakage argument (i.e., the red edge occurs when the thermal timescale in the driving region becomes equal to the critical period beyond which gravity modes cease to exist), which was successful to correctly reproduce the red edge of ZZ Ceti white dwarfs. Based on this argument, the red edge is qualitatively well reproduced as indicated above. However, upon close inspection, it may be about 1000 K too cool compared to the empirical one, although the latter relies on a few objects only. We also test the hypothesis of including turbulent pressure in our TDC computations in order to provide an alternate physical mechanism to account for the red edge. First promising results are presented.

  3. The white dwarf affair: Chandrasekhar, Eddington and the limiting mass

    NASA Astrophysics Data System (ADS)

    Gooneratne, Sakura

    A thesis describing and analysing the controversy between Subrahmanyan Chandrasekhar and Arthur Stanley Eddington over the limiting mass of white dwarf stars. The aim of the thesis is to discover why the controversy occurred and to analyse the reasons behind Eddington's rejection of relativistic degeneracy and the limiting mass. The ultimate reason behind Eddington's attack on relativistic degeneracy was found to be Eddington's severe objection to singularities which was apparent long before Chandrasekhar's discovery of the limiting mass and occurred in three separate areas of research undertaken by Eddington during this period: astrophysics, cosmology, general relativity and Dirac's relativistic equation of the electron which led to Eddington's fundamental theory. The thesis will focus on the problem of the limiting mass of white dwarfs between 1929 and 1935 but will use the problem to analyse Eddington's view of singularities within the three different research areas spanning two decades from 1916 to 1936. The Chandrasekhar-Eddington controversy is set within Eddington's earlier controversies with James Jeans and Edward Arthur Milne who together with Eddington founded theoretical astrophysics during the 1920s. The thesis will examine the problem of white dwarfs within the context of the earlier controversies on stellar structure. As well as the technical analysis of the controversy, the thesis will also analyse the social dynamics and interactions within the astronomical community and their impact on the controversies. The aim of this thesis is to create a more complete picture of the Chandrasekhar-Eddington controversy by analysing Eddington's arguments for rejecting relativistic degeneracy, the limiting mass of white dwarfs and singularities not just within the context of astrophysics, but also cosmology, general relativity and quantum mechanics and to provide some new explanations as to why Eddington opposed relativistic degeneracy.

  4. Discovery of a low-mass brown dwarf companion of the young nearby star G 196-3

    PubMed

    Rebolo; Osorio; Madruga; Bejar; Arribas; Licandro

    1998-11-13

    A substellar-mass object in orbit at about 300 astronomical units from the young low-mass star G 196-3 was detected by direct imaging. Optical and infrared photometry and low- and intermediate-resolution spectroscopy of the faint companion, hereafter referred to as G 196-3B, confirm its cool atmosphere and allow its mass to be estimated at 25-10+15 Jupiter masses. The separation between the objects and their mass ratio suggest the fragmentation of a collapsing cloud as the most likely origin for G 196-3B, but alternatively it could have originated from a protoplanetary disc that has been dissipated. Whatever the formation process was, the young age of the primary star (about 100 million years) demonstrates that substellar companions can form on short time scales.

  5. Feige 7 - A hot, rotating magnetic white dwarf

    NASA Technical Reports Server (NTRS)

    Liebert, J.; Angel, J. R. P.; Stockman, H. S.; Spinrad, H.; Beaver, E. A.

    1977-01-01

    Results are reported for image-tube-scanner and digicon observations of Feige 7, a faint blue star identified as a probable white dwarf. It is found that this star is a magnetic white dwarf showing a very rich spectrum with Zeeman subcomponents of both hydrogen and neutral helium as well as periodic spectrum and circular-polarization variations. A polarization period of 2.2 hr is computed, and a surface magnetic-field strength of about 18 MG is determined by matching features of the absorption spectrum to Zeeman components. It is suggested that the only reasonable explanation for the periodic variations in circular polarization is an oblique rotator with the spin axis approximately in the plane of the sky and tilted by about 24 deg to the magnetic axis. An effective temperature in the range from 20,000 to 25,000 K is estimated, an absolute magnitude of about 10.5 is derived, and the atmosphere is shown to be helium-dominated. The evolution of Feige 7 is discussed in terms of possible magnetic-field effects on atmospheric composition, rotation velocity (5.5 km/s for a radius of 7000 km), and the origin of white-dwarf magnetic fields.

  6. Are There Unstable Planetary Systems around White Dwarfs?

    NASA Astrophysics Data System (ADS)

    Debes, John H.; Sigurdsson, Steinn

    2002-06-01

    The presence of planets around solar-type stars suggests that many white dwarfs should have relic planetary systems. While planets closer than ~5 AU will most likely not survive the post-main-sequence lifetime of their parent star, any planet with semimajor axis greater than 5 AU will survive, and its semimajor axis will increase as the central star loses mass. Since the stability of adjacent orbits to mutual planet-planet perturbations depends on the ratio of the planet mass to the central star's mass, some planets in previously stable orbits around a star undergoing mass loss will become unstable. We show that when mass loss is slow, systems of two planets that are marginally stable can become unstable to close encounters, while for three planets the timescale for close approaches decreases significantly with increasing mass ratio. These processes could explain the presence of anomalous IR excesses around white dwarfs that cannot be explained by close companions, such as G29-38, and may also be an important factor in explaining the existence of DAZ white dwarfs. The onset of instability through changing mass ratios will also be a significant effect for planetary embryos gaining mass in protoplanetary disks.

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

    NASA Astrophysics Data System (ADS)

    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.

  8. Regimes of Internal Rotation in Differentially Rotating White Dwarfs

    NASA Astrophysics Data System (ADS)

    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 < 0.1, we find both the low-viscosity Zahn regime with a non-monotonic 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 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. Stark Broadening Parameters For White Dwarf Atmospheres Research

    NASA Astrophysics Data System (ADS)

    Larbi-Terzi, N.; Sahal-Brechot, S.; Nessib, N. B.; Dimitrijevic, M. S.

    2010-07-01

    Stark broadening parameters of C II lines were determined within 3d-nf series using semiclassical perturbation method. The atomic energy levels needed for calculations were taken from TOPBASE as well as the oscillator strengths, which were additionally calculated using the method of Bates and Damgaard. The both results were compared and only insignificant differences were found. Calculations were performed for plasma conditions relevant for atmospheres of DQ white dwarfs and for a new type of white dwarfs, with surface composed mostly of carbon, discovered in 2007 by Dufour et al. The aim of this work is to provide accurate C II Stark broadening data, which are crucial for this type of white dwarf atmosphere modellisation. Obtained results will be included in STARK-B database (http://stark-b.obspm.fr/), entering in the FP7 project of European Virtual Atomic and Molecular Data Center VAMDC aiming at building an interoperable e- Infrastructure for the exchange of atomic and molecular data (http://www.vamdc.org/).

  10. Search for Planets around Pulsating White Dwarf Stars

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

    We present initial results from our search for planets around variable white dwarf stars. White dwarf stars are the end point of stellar evolution for 98% of main sequence stars. Theoretical calculations (Sackmann 1993; Duncan & Lissauer 1998) predict that planets further than 1 AU from their parent star will survive the red giant phase. When a hydrogen atmosphere white dwarf (DA) cools to about 12000K it becomes a variable star. A subset of these stars exhibit pulsational stability that rivals atomic clocks (˙ {P} ˜ 10-15; Kepler et al. 2005). The reflex orbital motion of the star around the center of mass of the system due to the presence of a planet changes the light travel time of these stable pulses and hence their observed arrival time on earth, providing a method to detect the planet. Because we are measuring change in distance to the star, planets in long period orbits are easier to detect, complementing the Doppler shift method. This work is supported by grant from the NASA Origins program, NAG5-13094 and performed in part under contract with JPL through the Michelson Fellowship Program.

  11. Infrared Signatures of Disrupted Minor Planets at White Dwarfs

    NASA Astrophysics Data System (ADS)

    Farihi, Jay; Jura, M.; Zuckerman, B.

    2009-01-01

    Spitzer IRAC and MIPS photometric observations are presented for 20 white dwarfs with T < 20,000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457-086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates above 3e8 g/s display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 1e6 g/s in the solar system zodiacal cloud and 1e10 g/s often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

  12. Infrared Signatures of Disrupted Minor Planets at White Dwarfs

    NASA Astrophysics Data System (ADS)

    Farihi, J.; Jura, M.; Zuckerman, B.

    2009-04-01

    Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T eff lsim 20, 000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457-086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal-accretion rates dM/dtgsim 3 × 108 g s-1 display a warm infrared excess from orbiting dust; the likely result of a tidally destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 106 g s-1 in the solar system zodiacal cloud and 1010 g s-1 often inferred for debris disks at main-sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

  13. An asteroseismic test of diffusion theory in white dwarfs

    NASA Astrophysics Data System (ADS)

    Metcalfe, T. S.; Nather, R. E.; Watson, T. K.; Kim, S.-L.; Park, B.-G.; Handler, G.

    2005-05-01

    The helium-atmosphere (DB) white dwarfs are commonly thought to be the descendants of the hotter PG 1159 stars, which initially have uniform He/C/O atmospheres. In this evolutionary scenario, diffusion builds a pure He surface layer which gradually thickens as the star cools. In the temperature range of the pulsating DB white dwarfs (T_eff ˜ 25 000 K) this transformation is still taking place, allowing asteroseismic tests of the theory. We have obtained dual-site observations of the pulsating DB star CBS 114, to complement existing observations of the slightly cooler star GD 358. We recover the 7 independent pulsation modes that were previously known, and we discover 4 new ones to provide additional constraints on the models. We perform objective global fitting of our updated double-layered envelope models to both sets of observations, leading to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory. These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory, linking the DB white dwarfs to PG 1159 stars.

  14. THE SPECTRAL TYPES OF WHITE DWARFS IN MESSIER 4

    SciTech Connect

    Davis, D. Saul; Richer, Harvey B.; Rich, R. Michael; Reitzel, David R.; Kalirai, Jason S.

    2009-11-01

    We present the spectra of 24 white dwarfs in the direction of the globular cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs. Determining the spectral types of the stars in this sample, we find 24 type DA and 0 type DB (i.e., atmospheres dominated by hydrogen and helium, respectively). Assuming the ratio of DA/DB observed in the field with effective temperature between 15,000-25,000 K, i.e., 4.2:1, holds for the cluster environment, the chance of finding no DBs in our sample simply due to statistical fluctuations is only 6 x 10{sup -3}. The spectral types of the approx100 white dwarfs previously identified in open clusters indicate that DB formation is strongly suppressed in that environment. Furthermore, all the approx10 white dwarfs previously identified in other globular clusters are exclusively type DA. In the context of these two facts, this finding suggests that DB formation is suppressed in the cluster environment in general. Though no satisfactory explanation for this phenomenon exists, we discuss several possibilities.

  15. New White Dwarfs and Cataclysmic Variables from the FBS

    NASA Astrophysics Data System (ADS)

    Mickaelian, Areg M.

    The Second part of the First Byurakan Survey (FBS) is the continuation of the Markarian Survey and is aimed at discovery of UVX stellar objects: QSOs Seyferts white dwarfs hot subdwarfs cataclysmic variables etc. +33o<δ<+45o and +61o<δ<+90o regions at |b|>15o has been covered so far. 1103 blue stellar objects have been selected including 716 new ones. Observations with the Byurakan 2.6m SAO (Russia) 6m and Haute-Provence 1.93m telescopes revealed more than 50 new white dwarfs and 7 cataclysmic variables including a new bright (V=12.6) novalike cataclysmic variable of SW Sex subclass RXS J16437+3402 found by cross-correlation of ROSAT/USNO objects and further inspection of the FBS spectra and having a period within the period ``gap"" for such objects. The white dwarfs are being studied to reveal pulsating ones (ZZ Ceti stars) magnetic WDs polars (AM Her type objects) planetary nebulae nuclei (DO stars PG 1159 type objects) etc. Polarimetric observations have been undertaken as well: FBS 1704+347 is found to be a possible polar and FBS 1815+381 a variable magnetic WD. The total number of WDs is estimated to be 270 in the whole sample (24%) and cataclysmic variables - 35 (3%)

  16. COS Spectroscopy of White Dwarf Companions to Blue Stragglers

    NASA Astrophysics Data System (ADS)

    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.

  17. Fundamental Physics from Observations of White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Bainbridge, M. B.; Barstow, M. A.; Reindl, N.; Barrow, J. D.; Webb, J. K.; Hu, J.; Preval, S. P.; Holberg, J. B.; Nave, G.; Tchang-Brillet, L.; Ayres, T. R.

    2017-03-01

    Variation in fundamental constants provide an important test of theories of grand unification. Potentially, white dwarf spectra allow us to directly observe variation in fundamental constants at locations of high gravitational potential. We study hot, metal polluted white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling and fundamental physics, in the search for variation in the fine structure constant. This registers as small but measurable shifts in the observed wavelengths of highly ionized Fe and Ni lines when compared to laboratory wavelengths. Measurements of these shifts were performed by Berengut et al (2013) using high-resolution STIS spectra of G191-B2B, demonstrating the validity of the method. We have extended this work by; (a) using new (high precision) laboratory wavelengths, (b) refining the analysis methodology (incorporating robust techniques from previous studies towards quasars), and (c) enlarging the sample of white dwarf spectra. A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. We describe our approach and present preliminary results.

  18. Calibration of Synthetic Photometry Using DA White Dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  19. Strong Radio Emission from a Hyperactive L Dwarf: A Low-Mass Oddball or a Rosetta Stone for Ultracool Dwarf Activity?

    NASA Astrophysics Data System (ADS)

    Burgasser, Adam J.; Melis, C.; Zauderer, B. A.; Berger, E.

    2013-01-01

    We report the detection of radio emission at 5.5 GHz from the unusually active L5 + T7 binary 2MASS J13153094-2649513AB, based on observations conducted with the Australian Telescope Compact Array. An unresolved source at the proper-motion-corrected position of 2MASS J1315-2649AB was detected with a continuum flux of 0.37+/-0.05 mJy, corresponding to a radio luminosity L_rad = (9+/-3)x10^23 erg/s or log(L_rad/L_bol) = -5.24+/-0.22. While we cannot resolve the emission to one or both components, its strength strongly favors the L5 primary, making this component the latest-type L dwarf to be detected in the radio. No detection is made at 9.0 GHz to a 5-sigma limit of 0.29 mJy, consistent with a declining power law spectrum scaling as nu^-0.5 or steeper. The emission is quiescent, with no evidence of variability or bursts over 3 hours, and no measurable polarization (V/I < 34%). 2MASS J1315-2649AB is one of the most radio-luminous ultracool dwarfs detected in quiescent emission to date, comparable in strength to other ultracool dwarfs detected while in outburst. Its combination of strong and persistent H-alpha and radio emission is unique among L dwarfs, but we find no evidence of interaction between primary and secondary. We suggest further observations that may reveal whether 2MASS J1315-2649AB is a true oddball or a benchmark for understanding the origins of activity in the coldest stars and brown dwarfs.

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

    PubMed

    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. NO NEUTRON STAR COMPANION TO THE LOWEST MASS SDSS WHITE DWARF

    SciTech Connect

    Agueeros, Marcel A.; Camilo, Fernando; Heinke, Craig; Kilic, Mukremin; Anderson, Scott F.; Silvestri, Nicole M.; Freire, Paulo; Kleinman, Scot J.; Liebert, James W.

    2009-08-01

    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 {approx} 0.17 M{sub sun}). 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{sub sun}. 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.

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

    NASA Astrophysics Data System (ADS)

    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. Enhanced Low-temperature Triple-alpha and Helium-accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Connolly, Ryan; Brown, Edward

    2015-10-01

    The triple-alpha reaction is of critical importance to a variety of astrophysical phenomena. Despite this relevance, the non-resonant contribution to the reaction rate at temperatures below 108 K remains uncertain, with calculations by different groups spanning over 20 orders of magnitude around 107 K Recently, Nguyen et al. (2012) showed that their calculation of the reaction rate, although enhanced at low temperatures compared to the standard NACRE rate, remains consistent with post-main-sequence evolution and the well-observed red giant branch. Nevertheless, there are other astrophysical scenarios where an enhancement of the triple-alpha rate at low temperatures may have observable consequences. One example is AM CVn systems, in which a white dwarf accretes helium-rich material from a low-mass companion in a tight binary. As the white dwarf accretes, runaway helium burning may ignite at the base of the envelope, resulting in a ``helium nova.'' Using the MESA stellar evolution code, we find that for the most energetic outbursts the new triple-alpha rate increases both the time delay and mass of the helium envelope at ignition by a factor of two or more, which may affect the observable frequency and energetics of these explosive events in future surveys.

  4. EPISODIC ACCRETION AT EARLY STAGES OF EVOLUTION OF LOW-MASS STARS AND BROWN DWARFS: A SOLUTION FOR THE OBSERVED LUMINOSITY SPREAD IN H-R DIAGRAMS?

    SciTech Connect

    Baraffe, I.; Chabrier, G.; Gallardo, J. E-mail: chabrier@ens-lyon.fr

    2009-09-01

    We present evolutionary models for young low-mass stars and brown dwarfs taking into account episodic phases of accretion at early stages of the evolution, a scenario supported by recent large surveys of embedded protostars. An evolution including short episodes of vigorous accretion followed by longer quiescent phases can explain the observed luminosity spread in H-R diagrams of star-forming regions at ages of a few Myr, for objects ranging from a few Jupiter masses to a few tenths of a solar mass. The gravitational contraction of these accreting objects strongly departs from the standard Hayashi track at constant T{sub eff}. The best agreement with the observed luminosity scatter is obtained if most of the accretion shock energy is radiated away. The obtained luminosity spread at 1 Myr in the H-R diagram is equivalent to what can be misinterpreted as an {approx}10 Myr age spread for non-accreting objects. We also predict a significant spread in radius at a given T{sub eff}, as suggested by recent observations. These calculations bear important consequences for our understanding of star formation and early stages of evolution and on the determination of the initial mass function for young ({<=} a few Myr) clusters. Our results also show that the concept of a stellar birthline for low-mass objects has no valid support.

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

    SciTech Connect

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

  6. The dwarfs beyond: The stellar-to-halo mass relation for a new sample of intermediate redshift low-mass galaxies

    SciTech Connect

    Miller, Sarah H.; Ellis, Richard S.; Newman, Andrew B.; Benson, Andrew

    2014-02-20

    A number of recent challenges to the standard ΛCDM paradigm relate to discrepancies that arise in comparing the abundance and kinematics of local dwarf galaxies with the predictions of numerical simulations. Such arguments rely heavily on the assumption that the Local Volume's dwarf and satellite galaxies form a representative distribution in terms of their stellar-to-halo mass ratios. To address this question, we present new, deep spectroscopy using DEIMOS on Keck for 82 low-mass (10{sup 7}-10{sup 9} M {sub ☉}), star-forming galaxies at intermediate redshift (0.2 < z < 1). For 50% of these we are able to determine resolved rotation curves using nebular emission lines and thereby construct the stellar mass Tully-Fisher relation to masses as low as 10{sup 7} M {sub ☉}. Using scaling relations determined from weak lensing data, we convert this to a stellar-to-halo mass relation for comparison with abundance matching predictions. We find a discrepancy between our observations and the predictions from abundance matching in the sense that we observe 3-12 times more stellar mass at a given halo mass. We suggest possible reasons for this discrepancy, as well as improved tests for the future.

  7. Peculiar variations of white dwarf pulsation frequencies and maestro

    NASA Astrophysics Data System (ADS)

    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

  8. X-ray spectroscopy of hot white dwarfs

    NASA Astrophysics Data System (ADS)

    Adamczak, Jens

    2010-10-01

    X-ray spectra of two hot white dwarfs observed by the Chandra satellite have been analyzed. The first is a white dwarf of spectral class DA with an almost pure hydrogen atmosphere. Contrary to that, the atmosphere of the second object, a PG 1159 star, is basically hydrogen free. The reason for the different composition can be found in the differing evolution of these objects. Some DA white dwarfs show much smaller metallicities than predicted by the mechanism of radiative levitation. Many spectral lines of the heavy elements that are the key to the explanation to the unusual metal poorness are located in the X-ray wavelength range. Some PG 1159 stars are non-radial g-mode pulsators. The pulsations depend amongst others on the abundances of the elements in the atmosphere, log g, and T eff. The soft X-ray range is particularly temperature sensitive and allows to constrain the temperature of a non-pulsating PG 1159 star with respect to its pulsating spectroscopic twin. Detailed analysis of X-ray spectra of single white dwarfs do not yet exist. The aim of this thesis was to analyze spectra of the DA white dwarfs LB 1919 and GD 246 in different wavelength ranges in order to find out if the metals in the atmospheres of these objects are homogeneously mixed or chemically stratified. This helps to identify or exclude possible unexpected mechanisms that might disturb the equilibrium between gravitational and radiative forces in the atmosphere. For LB 1919 an additional aim was to identify photospheric features of several elements and determine their abundances for the first time. It was further intended to determine the temperature of the non-pulsating PG 1159 star PG 1520+525 precisely. The spectra of LB 1919 and GD 246 ranging from X-ray to optical wavelengths were analyzed with advanced homogeneous and stratified Non-LTE model atmospheres. The Chandra spectrum of the PG 1159 star PG 1520+525 was analyzed with homogeneous Non-LTE model atmospheres only since no

  9. AN OPTICALLY OBSCURED AGN IN A LOW MASS, IRREGULAR DWARF GALAXY: A MULTI-WAVELENGTH ANALYSIS OF J1329+3234

    SciTech Connect

    Secrest, N. J.; Satyapal, S.; Gliozzi, M.; Rothberg, B.; Mowry, W. S.; Rosenberg, J. L.; Ellison, S. L.; Fischer, J.; Schmitt, H.

    2015-01-01

    Supermassive black holes (SMBHs) are found ubiquitously in large, bulge-dominated galaxies throughout the local universe, yet little is known about their presence and properties in bulgeless and low-mass galaxies. This is a significant deficiency, since the mass distribution and occupation fraction of nonstellar black holes provide important observational constraints on SMBH seed formation theories and many dwarf galaxies have not undergone major mergers that would erase information on their original black hole population. Using data from the Wide-field Infrared Survey Explorer, we discovered hundreds of bulgeless and dwarf galaxies that display mid-infrared signatures of extremely hot dust highly suggestive of powerful accreting massive black holes, despite having no signatures of black hole activity at optical wavelengths. Here we report, in our first follow-up X-ray investigation of this population, that the irregular dwarf galaxy J132932.41+323417.0 (z = 0.0156) contains a hard, unresolved X-ray source detected by XMM-Newton with luminosity L {sub 2-10} {sub keV} = 2.4 × 10{sup 40} erg s{sup –1}, over two orders of magnitude greater than that expected from star formation, strongly suggestive of the presence of an accreting massive black hole. While enhanced X-ray emission and hot dust can be produced in extremely low metallicity environments, J132932.41+323417.0 is not extremely metal poor (≈40% solar). With a stellar mass of 2.0 × 10{sup 8} M {sub ☉}, this galaxy is similar in mass to the Small Magellanic Cloud, and is one of the lowest mass galaxies with evidence for a massive nuclear black hole currently known.

  10. A Survey of Helium Accreting White Dwarfs

    NASA Technical Reports Server (NTRS)

    Steeghs, Daniel

    2005-01-01

    We have initiated a survey of the X-ray spectral properties of double white binaries using XMM-Newton. Three of our sources were indeed observed with XMM during AO-3. We have analyzed these data using the latest data reduction software and have also extracted several archival data sets of similar systems from the XMM archive. The first paper presenting the X-ray spectral and variability properties of four of these binary systems has been submitted in January 2005. We have also secured some optical ground-based spectroscopy and are currently analyzing the spectral signatures in more detail. We are also continuing this survey with additional approved XMM observations during AO-4 and anticipate additional publications in the near future.

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

    NASA Technical Reports Server (NTRS)

    Shipman, Harry L.

    1989-01-01

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

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

    SciTech Connect

    Han, C.; Jung, Y. K.; Udalski, A.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Soszyński, I.; Skowron, J.; Kozłowski, S.; Poleski, R.; Ulaczyk, K.; Wyrzykowski, Ł.; Pietrukowicz, P.; Sumi, T.; Gaudi, B. S.; Gould, A.; Bennett, D. P.; Tsapras, Y.; Abe, F.; Bond, I. A.; 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.

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

    SciTech Connect

    Bodenheimer, Peter; Fortney, Jonathan J.; Saumon, Didier E-mail: gennaro.dangelo@nasa.gov E-mail: jfortney@ucolick.org

    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.

  14. A Model of White Dwarf Pulsar AR Scorpii

    NASA Astrophysics Data System (ADS)

    Geng, Jin-Jun; Zhang, Bing; Huang, Yong-Feng

    2016-11-01

    A 3.56 hr 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 broadband (from radio to X-rays) spectral energy distribution of the system.

  15. Planets around Low-mass Stars (PALMS). I. A Substellar Companion to the Young M Dwarf 1RXS J235133.3+312720

    NASA Astrophysics Data System (ADS)

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.; Cieza, Lucas A.; Kraus, Adam L.; Tamura, Motohide

    2012-07-01

    We report the discovery of a brown dwarf companion to the young M dwarf 1RXS J235133.3+312720 as part of a high contrast imaging search for planets around nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2farcs4 (~120 AU) pair is confirmed to be comoving from two epochs of high-resolution imaging. Follow-up low- and moderate-resolution near-infrared spectroscopy of 1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of L0+2 -1. The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV activity levels comparable to young moving group members with ages of ~10-100 Myr. UVW kinematics based the measured radial velocity of the primary and the system's photometric distance (50 ± 10 pc) indicate it is likely a member of the ~50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology shows subtle hints of intermediate surface gravity. The spectrum is also an excellent match to the ~200 Myr M9 brown dwarf LP 944-20. Assuming an age of 50-150 Myr, evolutionary models imply a mass of 32 ± 6 M Jup for the companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor moving group after the L4 companion CD-35 2722 B and one of the few benchmark brown dwarfs known at young ages. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

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

    NASA Astrophysics Data System (ADS)

    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.

  17. Trigonometric parallaxes of high velocity halo white dwarf candidates

    NASA Astrophysics Data System (ADS)

    Ducourant, C.; Teixeira, R.; Hambly, N. C.; Oppenheimer, B. R.; Hawkins, M. R. S.; Rapaport, M.; Modolo, J.; Lecampion, J. F.

    2007-07-01

    Context: The status of 38 halo white dwarf candidates identified by Oppenheimer et al. (2001, Science, 292, 698) has been intensively discussed by various authors. In analyses undertaken to date, trigonometric parallaxes are crucial missing data. Distance measurements are mandatory to kinematically segregate halo object from disk objects and hence enable a more reliable estimate of the local density of halo dark matter residing in such objects. Aims: We present trigonometric parallax measurements for 15 candidate halo white dwarfs (WDs) selected from the Oppenheimer et al. (2001) list. Methods: We observed the stars using the ESO 1.56-m Danish Telescope and ESO 2.2-m telescope from August 2001 to July 2004. Results: Parallaxes with accuracies of 1-2 mas were determined yielding relative errors on distances of ~5% for 6 objects, ~12% for 3 objects, and ~20% for two more objects. Four stars appear to be too distant (probably farther than 100 pc) to have measurable parallaxes in our observations. Conclusions: Distances, absolute magnitudes and revised space velocities were derived for the 15 halo WDs from the Oppenheimer et al. (2001) list. Halo membership is confirmed unambiguously for 6 objects while 5 objects may be thick disk members and 4 objects are too distant to draw any conclusion based solely on kinematics. Comparing our trigonometric parallaxes with photometric parallaxes used in previous work reveals an overestimation of distance as derived from photometric techniques. This new data set can be used to revise the halo white dwarf space density, and that analysis will be presented in a subsequent publication. Based on observations collected at the European Southern Observatory, Chile (067.D-0107, 069.D-0054, 070.D-0028, 071.D-0005, 072.D-0153, 073.D-0028).

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

    NASA Technical Reports Server (NTRS)

    Whitney, Barbara

    1989-01-01

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

  19. Search for cool white dwarfs with GSC2

    NASA Astrophysics Data System (ADS)

    Carollo, D.; Spagna, A.; Lattanzi, M. G.; Smart, R. L.; Hodgkin, S. T.; Terranegra, L.; McLean, B.

    2004-07-01

    Microlensing experiments have suggested that a significant part of the dark halo of the Milky Way could be composed of matter in the form of massive compact halo objects (MACHOs). Cool ancient white dwarfs (WDs) are the natural candidates. Here we present a new survey of halo WDs and evaluate the local space density using an accurate kinematic analysis. A comparison to a revaluation of the Oppenheimer et al. result is also provided. The local space density estimated for the two independent samples is about ˜ 10-5 M⊙ pc-3.

  20. The GSC2 Survey of Cool White Dwarfs

    NASA Astrophysics Data System (ADS)

    Carollo, D.; Spagna, A.; Lattanzi, M. G.; Smart, R. L.; Bucciarelli, B.; Terranegra, L.; Hodgkin, S. T.; McLean, B.

    2005-07-01

    Microlensing experiments (Alcock et al. 2000) have suggested that a significant part of the dark halo of the Milky Way could be composed of matter in the form of massive compact halo objects (MACHOs). Cool ancient white dwarfs (WDs) are the natural candidates. Here we present a new survey of halo WDs and evaluate the local space density using an accurate kinematic analysis. A comparison to a re-derivation of the Oppenheimer et al. (2001) results is also provided. The local space density estimated from the two independent samples is only about ˜ 10-5M⊙pc-3 corresponding to 0.1%-0.2% of the local dark matter.

  1. The White Dwarf in EM Cygni: Beyond the Veil

    DTIC Science & Technology

    2009-07-10

    reserved. Printed in the U.S.A. THE WHITE DWARF IN EM CYGNI: BEYOND THE VEIL∗ Patrick Godon1,4, Edward M. Sion1, Paul E. Barrett2, and Albert P. Linnell3 1...However, more recent full three-dimensional hydrodynami- cal simulations, e.g., (Blondin 1998; Kunze et al. 2001; Bisikalo et al. 2003), have followed...deflection is believed to cause X-ray absorption in CVs (and LMXBs) around orbital phase 0.7, if the inclination is at least 65◦ ( Kunze et al. 2001

  2. Magnetic Fields and the Crystallization of White Dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  3. A continuing census of Galactic white dwarfs to 40 parsecs of the Sun

    NASA Astrophysics Data System (ADS)

    Limoges, Marie-Michele; Lepine, Sebastien; Bergeron, Pierre

    2010-08-01

    We propose to obtain spectra for 170 new white dwarf candidates suspected to be within 40 parsecs of the Sun, as a continuation of our survey of white dwarfs in the Solar Neiborhood. Spectral analysis will confirm the white dwarf status and provide reliable distance estimates for all objects. The census of nearby white dwarfs is currently complete only to about 20 parsecs of the Sun, a volume which includes only a little over 100 white dwarfs. The main limitation of the current census is that it is largely based on follow-up investigations of stars with very large proper motion. Our own survey investigates stars down to a much smaller proper motion limit (40 mas/yr) which minimizes the kinematic bias and provides a statistically complete census of white dwarfs to a significantly larger distance. Our follow-up spectroscopic program, initiated in the past two semesters, strikingly demonstrates the effectiveness of our white dwarf selection method, with a sucess rate exceeding 96%. We have now obtained spectra for 65% of our initial list of 350 white dwarf candidates, and request additional nights to observe the remaining 170. In particular, our survey has been efficient in locating faint, cool white dwarfs, which will help in the study of the cool end of the white dwarf luminosity function. Finally, objects of great astrophysical interest were found, such as the very cool DQ LSPM J0902+2010, the highly magnetic PM I11322+2809, and the extremely contaminated DA+dM binary system PM I04586+6209. We expect additional objects of interest to show up in our extended program, from which we expect to obtain a near-complete census of white dwarfs to a distance of 40 parsecs.

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

    NASA Astrophysics Data System (ADS)

    Bailey, Vanessa; Hinz, Philip M.; 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.

    2013-04-01

    We present a 3-5 μm LBT/MMT adaptive optics imaging study of three Upper Scorpius stars with brown dwarf (BD) companions with very low masses/mass ratios (M BD <25 M Jup; M BD/M sstarf ≈ 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 μm and 24 μm 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 μm excess in the system may be explained by the contribution from this disk. 1RXS 1609B exhibits no 3-4 μm excess, nor does its primary; however, the system as a whole has a modest 24 μm 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 μm 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 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. Observations reported here were obtained at the LBT and MMT Observatories. The MMT Observatory is a joint facility of the University of Arizona and the Smithsonian Institution. The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT

  5. A Study of the SDSS White Dwarf Component in the LSPM Proper Motion Survey

    NASA Astrophysics Data System (ADS)

    Darveau-Bernier, A.; Bergeron, P.; Lépine, S.

    2017-03-01

    We present a model atmosphere analysis of the white dwarfs identified in the Sloan Digital Sky Survey with proper motions measured in the LSPM proper motion survey of Lépine et al. We rely on reduced proper motion diagrams to build a sample of white dwarfs in the SDSS footprint, and cross correlate this sample with the SDSS spectroscopic database to understand the systematics related to completeness, contamination, WD+M dwarf binaries, reddening, etc. We then determine a white dwarf luminosity function for this sample using various methods.

  6. NEW EXTINCTION AND MASS ESTIMATES FROM OPTICAL PHOTOMETRY OF THE VERY LOW MASS BROWN DWARF COMPANION CT CHAMAELEONTIS B WITH THE MAGELLAN AO SYSTEM

    SciTech Connect

    Wu, Ya-Lin; Close, Laird M.; Males, Jared R.; Morzinski, Katie M.; Follette, Katherine B.; Bailey, Vanessa; Rodigas, Timothy J.; Hinz, Philip; Barman, Travis S.; Puglisi, Alfio; Xompero, Marco; Briguglio, Runa

    2015-03-01

    We used the Magellan adaptive optics system and its VisAO CCD camera to image the young low mass brown dwarf companion CT Chamaeleontis B for the first time at visible wavelengths. We detect it at r', i', z', and Y{sub S}. With our new photometry and T {sub eff} ∼ 2500 K derived from the shape of its K-band spectrum, we find that CT Cha B has A{sub V} = 3.4 ± 1.1 mag, and a mass of 14-24 M{sub J} according to the DUSTY evolutionary tracks and its 1-5 Myr age. The overluminosity of our r' detection indicates that the companion has significant Hα emission and a mass accretion rate ∼6 × 10{sup –10} M {sub ☉} yr{sup –1}, similar to some substellar companions. Proper motion analysis shows that another point source within 2'' of CT Cha A is not physical. This paper demonstrates how visible wavelength adaptive optics photometry (r', i', z', Y{sub S}) allows for a better estimate of extinction, luminosity, and mass accretion rate of young substellar companions.

  7. Thermonuclear Supernova Explosions From Hybrid White Dwarf Progenitors

    NASA Astrophysics Data System (ADS)

    Willcox, Donald E.; Townsley, Dean; Calder, Alan; Denissenkov, Pavel; Herwig, Falk

    2016-01-01

    Motivated by recent results in stellar evolution in which convective boundary mixing in SAGB stars can give rise to hybrid white dwarf (WD) stars with a C-O core inside an O-Ne shell, we simulate thermonuclear (Type Ia) supernovae from these hybrid progenitors. We use the FLASH code to perform multidimensional simulations in the deflagration to detonation transition (DDT) explosion paradigm from progenitor models produced with the MESA stellar evolution code that include the thermal energetics of the Urca process. We performed a suite of DDT simulations over a range of ignition conditions and compare to previous results from a suite of C-O white dwarfs. Despite significant variability within each suite, distinguishing trends are apparent in their Ni-56 yields and the kinetic properties of their ejecta. We comment on the feasibility of these hybrid WD explosions as the source of some classes of observed subluminous events. This research was supported in part by the U.S. Department of Energy under grant DE-FG02-87ER40317 and by resources at the Institute for Advanced Computational Science at Stony Brook University. The software used in this work was in part developed by the DOE-supported ASC/Alliances Center for Astrophysical Thermonuclear Flashes at the University of Chicago.

  8. Limb-darkening Coefficients for Eclipsing White Dwarfs

    NASA Astrophysics Data System (ADS)

    Gianninas, A.; Strickland, B. D.; Kilic, Mukremin; Bergeron, P.

    2013-03-01

    We present extensive calculations of linear and nonlinear 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, and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The LSST, for example, may find ~105 eclipsing white dwarfs. The limb-darkening calculations presented here will be an essential part of the detailed analysis of all of these systems.

  9. Evolved Planetary Systems around Very Cool and Old White Dwarfs

    NASA Astrophysics Data System (ADS)

    Hollands, Mark; Gänsicke, Boris; Koester, Detlev

    2015-06-01

    We have spectroscopically identified 61 very cool (below 9000 K) and old (1-7 Gyr) DZ white dwarfs from the Sloan Digital Sky Survey (SDSS). These stars have evaded prior detection as the extremely broad Ca H/K lines in the blue part of their spectra dramatically alter their colours, mixing them into the colour-space of intermediate redshift quasars. In most of these stars we detect photospheric Ca, Mg, Fe and Na. The coolest of these has Teff ≲ 5000 K corresponding to a cooling age of ˜ 7 Gyr. The only mechanism that can explain the large amounts of metal in the convection zones of these white dwarfs is accretion of planetary debris. Hence, these stars provide a lower limit on the onset of the formation of rocky material within the Milky Way, and, more generally, insight into the formation of early terrestrial planets. Additionally, we identify several of these DZ to have strong (0.6-10 MG) magnetic fields leading to an observed incidence of magnetism of 13 %.

  10. Prevention of accretion onto white dwarfs by stellar winds

    NASA Technical Reports Server (NTRS)

    Macdonald, James

    1992-01-01

    There is indirect observational evidence that hot white dwarfs may have weak stellar winds. In this paper, the interaction between such a wind and the flow of ISM material in the gravitational field of the white dwarf is investigated with the aim of finding limits on the mass-loss rate and terminal velocity of winds capable of preventing accretion from the ISM. The limiting cases of no relative motion of the star and the ISM and supersonic relative motion of the star through ISM are separately investigated. Each case is treated by generalizing models for the interaction between the solar wind and the local ISM to include the effects of gravity. It is found that, for wind velocities expected for radiatively driven winds, mass-loss rates as low as 10 exp -21 solar mass/yr are sufficient to prevent accretion from the hot phase of the ISM. To prevent accretion during passages through cold clouds, wind mass-loss rates of order 10 exp -18 to 10 exp -17 are required.

  11. DA WHITE DWARFS OBSERVED IN THE LAMOST PILOT SURVEY

    SciTech Connect

    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; Lepine, Sebastien; Newberg, Heidi Jo; Carlin, Jeffrey L.; Jin Ge

    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.

  12. Neutron stars and white dwarfs in galactic halos

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Olive, Keith A.; Silk, Joseph

    1989-01-01

    The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

  13. Neutron stars and white dwarfs in galactic halos?

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Olive, Keith A.; Silk, Joseph

    1990-01-01

    The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

  14. Ultraviolet carbon lines in the spectrum of the white dwarf BPM 11668

    NASA Technical Reports Server (NTRS)

    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.

  15. Sensitivity Analysis of Hierarchical Models for the Ages of Galactic Halo White Dwarfs

    NASA Astrophysics Data System (ADS)

    Si, S.; van Dyk, D. A.; von Hippel, T.

    2017-03-01

    The ages of white dwarfs are of great importance in stellar evolution. Si et al. developed a novel approach to increase the precision of such estimates by combining multiple white dwarfs in a Bayesian hierarchical model. In this paper, we further investigate the robustness of the Bayesian hierarchical model by performing a simulation study.

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

    NASA Astrophysics Data System (ADS)

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

    2014-11-01

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

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

    PubMed

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

    2014-11-06

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

  18. New halo white dwarf candidates in the Sloan Digital Sky Survey

    NASA Astrophysics Data System (ADS)

    Dame, Kyra; Gianninas, A.; Kilic, Mukremin; Munn, Jeffrey A.; Brown, Warren R.; Williams, Kurtis A.; von Hippel, Ted; Harris, Hugh C.

    2016-12-01

    We present optical spectroscopy and near-infrared photometry of 57 faint (g = 19-22) high proper motion white dwarfs identified through repeat imaging of ≈3100 deg2 of the Sloan Digital Sky Survey footprint by Munn et al. We use ugriz and JH photometry to perform a model atmosphere analysis, and identify 10 ultracool white dwarfs with Teff < 4000 K, including the coolest pure H atmosphere white dwarf currently known, J1657+2638, with Teff = 3550 ± 100 K. The majority of the objects with cooling ages larger than 9 Gyr display thick disc kinematics and constrain the age of the thick disc to ≥11 Gyr. There are four white dwarfs in our sample with large tangential velocities (vtan > 120 km s-1) and UVW velocities that are more consistent with the halo than the Galactic disc. For typical 0.6 M⊙ white dwarfs, the cooling ages for these halo candidates range from 2.3 to 8.5 Gyr. However, the total main-sequence+white dwarf cooling ages of these stars would be consistent with the Galactic halo if they are slightly undermassive. Given the magnitude limits of the current large-scale surveys, many of the coolest and oldest white dwarfs remain undiscovered in the solar neighbourhood, but upcoming surveys such as Gaia and the Large Synoptic Survey Telescope should find many of these elusive thick disc and halo white dwarfs.

  19. 40 Eridani: The Vulcan Sun as a Benchmark for the Evolutionary Properties of White Dwarfs and Cool Stars

    NASA Astrophysics Data System (ADS)

    Ballouz, Ronald-Louis; Guinan, E. F.; Wasatonic, R.; Engle, S. G.

    2010-01-01

    40 Eridani (omicron-2 Eri) is a bright nearby (d=5.04+/-0.02 pc) triple star system. 40 Eri A is a 4.4-mag K1 V star and its two more distant ( 400 AU) companion stars, 40 Eri B and 40 Eri C (which form an astrometric binary system), are a 9th mag DA4 white dwarf & an 11th mag M4.5 star. 40 Eri A is well known (in science fiction circles) as the probable host star to the planet Vulcan; however, 40 Eri is also well known in astronomy as containing the first identified white dwarf , only a handful of which have well-determine dynamical masses. Utilizing archival X-ray, ultraviolet (IUE) data, and Ca II HK emission measures, we determined (or improved) the properties of the system members. Using calibrated age-rotation-activity relations developed by us and others, we determine an age of 5.2+/-1.2 Gyr for 40 Eri A (and thus the system). It appears that 40 Eri A has similar activity levels (and thus age) to Alpha Centauri B - a middle-age ( 5.5+/-0.5 Gyr) K1 V star. This age is in good agreement with the age estimated from white dwarf component of 5.0+/-1 Gyr [main-sequence + cooling time (0.1Gyr)]. The accurate age, evolution, and mass of the hot white dwarf star provide a firm benchmark for calibrating ages/cooling times of lower mass white dwarfs. The implications of this new age determination on the evolution & cooling times of low mass white dwarfs (like 40 Eri B) and cosmochronology are discussed. Furthermore, we are carrying out high precision photometry of 40 Eri A with the aim of determining its rotation period (from star spot modulations) and thus its age using rotation-age relations for dK stars. This work is partially supported with grants from NSF/RUI & NASA/FUSE programs.

  20. CARMENES science preparation: characterisation of M dwarfs with low-resolution spectroscopy and search for low-mass wide companions to young stars

    NASA Astrophysics Data System (ADS)

    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

  1. Search for and study of photometric variability in magnetic white dwarfs

    NASA Astrophysics Data System (ADS)

    Valeev, A. F.; Antonyuk, K. A.; Pit, N. V.; Moskvitin, A. S.; Grauzhanina, A. O.; Gadelshin, D. R.; Kolesnikov, S. V.; Zhuzhulina, E. A.; Burlakova, T. E.; Galazutdinov, G. A.; Gutaev, A. G.; Zhuchkov, R. Ya.; Ikhsanova, A. I.; Zhukov, D. G.; Joshi, Arti; Pandey, J. C.; Kholtygin, A. F.; Valyavin, G. G.

    2017-01-01

    We report the results of photometric observations of a number of magnetic white dwarfs in order to search for photometric variability in these stars. These V-band observations revealed significant variability in the classical highly magnetized white dwarf GRW+70°8247 with a likely period from several days to several dozen days and a half-amplitude of about 0. m 04. Our observations also revealed the variability of the well-known white dwarf GD229. The half amplitude of its photometric variability is equal to about 0. m 005, and the likely period of this degenerate star lies in the 10-20 day interval. This variability is most likely due to the rotation of the stars considered.We also discuss the peculiarities of the photometric variability in a number of other white dwarfs. We present the updated "magnetic field-rotation period" diagram for the white dwarfs.

  2. HST Spectra of White Dwarfs and the Mass-Radius Relation

    NASA Astrophysics Data System (ADS)

    Joyce, S. R. G.; Barstow, M. A.; Casewell, S. L.; Holberg, J. B.; Bond, H. E.

    2017-03-01

    We present HST spectra of the white dwarf HR1358b which is one of a number of white dwarfs we are studying in Sirius-like binaries. These binaries give us the best chance yet to test the mass-radius relation through observations of the Balmer lines of white dwarfs. These observations can be used to measure the white dwarf mass using both the spectroscopic method and the gravitational red-shift method. One of the major challenges for this type of observation is scattered light from the main sequence star contaminating the white dwarf spectrum. We will discuss methods that have been developed to correct for scattered light in the spectrum of HR1358b and the effect this correction can have on the mass-radius measurements.

  3. A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes.

    PubMed

    García-Berro, Enrique; Torres, Santiago; Althaus, Leandro G; Renedo, Isabel; Lorén-Aguilar, Pablo; Córsico, Alejandro H; Rohrmann, René D; Salaris, Maurizio; Isern, Jordi

    2010-05-13

    NGC 6791 is a well studied open cluster that it is so close to us that can be imaged down to very faint luminosities. The main-sequence turn-off age ( approximately 8 Gyr) and the age derived from the termination of the white dwarf cooling sequence ( approximately 6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, (22)Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf ((12)C and (16)O) is expected to occur. This sequence of events is expected to introduce long 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, resolving the age discrepancy for NGC 6791.

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

    NASA Technical Reports Server (NTRS)

    Boehm-Vitense, E.

    1984-01-01

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

  5. An irradiated brown-dwarf companion to an accreting white dwarf.

    PubMed

    Santisteban, Juan V Hernández; Knigge, Christian; Littlefair, Stuart P; Breton, Rene P; Dhillon, Vikram S; Gänsicke, Boris T; Marsh, Thomas R; Pretorius, Magaretha L; Southworth, John; Hauschildt, Peter H

    2016-05-19

    Interacting compact binary systems provide a natural laboratory in which to study irradiated substellar objects. As the mass-losing secondary (donor) in these systems makes a transition from the stellar to the substellar regime, it is also irradiated by the primary (compact accretor). The internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. The atmospheric properties of donors are largely unknown, but could be modified by the irradiation. To constrain models of donor atmospheres, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures and albedos). Here we report the spectroscopic detection and characterization of an irradiated substellar donor in an accreting white-dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of 0.055 ± 0.008 solar masses and an average spectral type of L1 ± 1, supporting both theoretical predictions and model-dependent observational constraints that suggest that the donor is a brown dwarf. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the dayside and nightside of the substellar donor (57 kelvin) and the maximum difference between the hottest and coolest parts of its surface (200 kelvin). The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of less than 0.54 at the 2σ confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the atmosphere of the brown-dwarf donor. These results add to our knowledge of binary evolution, in that the donor has survived the transition from the stellar to the substellar regime, and of substellar atmospheres, in that we have been able to test a regime in which the irradiation and the internal energy of a brown dwarf are

  6. HOT WHITE DWARF SHINES IN YOUNG STAR CLUSTER

    NASA Technical Reports Server (NTRS)

    2002-01-01

    A dazzling 'jewel-box' collection of over 20,000 stars can be seen in crystal clarity in this NASA Hubble Space Telescope image, taken with the Wide Field and Planetary Camera 2. The young (40 million year old) cluster, called NGC 1818, is 164,000 light-years away in the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way. The LMC, a site of vigorous current star formation, is an ideal nearby laboratory for studying stellar evolution. In the cluster, astronomers have found a young white dwarf star, which has only very recently formed following the burnout of a red giant. Based on this observation astronomers conclude that the red giant progenitor star was 7.6 times the mass of our Sun. Previously, astronomers have estimated that stars anywhere from 6 to 10 solar masses would not just quietly fade away as white dwarfs but abruptly self-destruct in torrential explosions. Hubble can easily resolve the star in the crowded cluster, and detect its intense blue-white glow from a sizzling surface temperature of 50,000 degrees Fahrenheit. IMAGE DATA Date taken: December 1995 Wavelength: natural color reconstruction from three filters (I,B,U) Field of view: 100 light-years, 2.2 arc minutes TARGET DATA Name: NGC 1818 Distance: 164,000 light-years Constellation: Dorado Age: 40 million years Class: Rich star cluster Apparent magnitude: 9.7 Apparent diameter: 7 arc minutes Credit: Rebecca Elson and Richard Sword, Cambridge UK, and NASA (Original WFPC2 image courtesy J. Westphal, Caltech) Image files are available electronically via the World Wide Web at: http://oposite.stsci.edu/pubinfo/1998/16 and via links in http://oposite.stsci.edu/pubinfo/latest.html or http://oposite.stsci.edu/pubinfo/pictures.html. GIF and JPEG images are available via anonymous ftp to oposite.stsci.edu in /pubinfo/GIF/9816.GIF and /pubinfo/JPEG/9816.jpg.

  7. DISCOVERY OF THE ECLIPSING DETACHED DOUBLE WHITE DWARF BINARY NLTT 11748

    SciTech Connect

    Steinfadt, Justin D. R.; Shporer, Avi; Bildsten, Lars; Kaplan, David L.; Howell, Steve B.

    2010-06-20

    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 minute) 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 white dwarf from the Keck telescope found variations with a semi-amplitude K{sub 1} = 271 {+-} 3 km s{sup -1} and confirmed the dips as eclipses caused by an orbiting WD with a mass M{sub 2} = 0.648-0.771 M{sub sun} for M{sub 1} = 0.1-0.2 M{sub sun}. We detect both the primary and secondary eclipses during the P{sub 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{sub 1} = 0.043-0.039 R{sub sun} for M{sub 1} = 0.1-0.2 M{sub sun}, consistent with the theoretically expected values for a helium core WD with a thick, stably burning hydrogen envelope. Though nearby (at {approx}150 pc), the gravitational wave strain from NLTT 11748 is likely not adequate for direct detection by the Laser Interferometer Space Antenna. Future observational efforts will determine M{sub 1}, yielding accurate WD mass-radius measurement of both components, as well as a clearer indication of the binary's fate once contact is reached.

  8. Explosion of white dwarfs harboring hybrid CONe cores

    NASA Astrophysics Data System (ADS)

    Bravo, E.; Gil-Pons, P.; Gutiérrez, J. L.; Doherty, C. L.

    2016-05-01

    Recently, it has been found that off-center 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. The implications of a significant presence of carbon in the resulting massive degenerate cores have not been thoroughly explored so far. 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 nondegenerate companion at rates leading to a supernova explosion. We computed the hydrodynamical phase of the explosion of Chandrasekhar-mass white dwarfs harboring hybrid cores, assuming that the explosion starts at the center; this explosion occurs either as a detonation, as may be expected in some degenerate merging scenarios, or as a deflagration that afterward transitions into a delayed detonation. We assume these hybrid cores are made of a central CO volume, of mass MCO, surrounded by an ONe shell. We show that, in the case of a pure detonation, a medium-sized carbon-rich region, MCO (<0.4 M⊙), results in the ejection of a small fraction of the mantle while leaving a massive bound remnant. Part of this remnant is made of the products of the detonation, that is, Fe-group nuclei, but they are buried in its inner regions unless convection is activated during the ensuing cooling and shrinking phase of the remnant. In contrast, and somehow paradoxically, delayed detonations do not leave remnants other than for the minimum MCO we explored of MCO = 0.2 M⊙, and even in this case the remnant is as small as 0.13 M⊙. The ejecta produced by these delayed detonations are characterized by slightly smaller masses of 56Ni and substantially smaller kinetic energies than the ejecta obtained for a delayed detonation of a "normal" CO white dwarf. The optical emission expected from these explosions most likely do not match the observational properties of typical Type Ia supernovae

  9. EX-111 Thermal Emission from Hot White Dwarfs: The Suggested He Abundance-Temperature Correlation. EX-112: The Unique Emission Line White Dwarf Star GD 356

    NASA Technical Reports Server (NTRS)

    Shipman, H. L.

    1986-01-01

    Progress in the EXOSAT data analysis program is reported. EXOSAT observations for four white dwarfs (WD1031-115, WD0004+330, WD1615-154, and WD0109-264) were obtained. Counting rates were unexpectedly low, indicating that these objects have a substantial amount of x-ray absorbing matter in their photosheres. In addition, soft x-ray pulsations characterized by a 9.25 minute cycle were discovered in the DA white dwarf V471 Tauri. A residual x-ray flux from the K dwarf companion can be seen during the white dwarf eclipse at orbital phase 0.0. Pronounced dips in the soft x-ray light curve occur at orbital phases 0.15, 0.18, and 0.85. The dips may be correlated with the triangular Lagrangian points of the binary orbit. Smaller dips at phases near the eclipse may be associated with cool loops in the K star corona. Data for the white dwarf H1504+65 was also analyzed. This object is particularly unusual in that its photoshere is devoid of hydrogen and helium. Finally, existing data on the white dwarf Sirius B were analyzed to see what constraints from other data can be placed on the properties of this star. Interrelationships between radius, rotational velocity, and effective temperature were derived.

  10. 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

    NASA Astrophysics Data System (ADS)

    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

  11. THE KEPLER LIGHT CURVE OF THE UNIQUE DA WHITE DWARF BOKS 53856

    SciTech Connect

    Holberg, J. B.; Howell, Steve B. E-mail: howell@noao.edu

    2011-08-15

    The faint (g = 16.9) hot white dwarf BOKS 53856 was observed by the Kepler Mission in short cadence mode during mid-2009. Analysis of these observations reveals a highly stable modulation with a period of 6.1375 hr and a 2.46% half-amplitude. The folded light curve has an unusual shape that is difficult to explain in terms of a binary system containing an unseen companion more luminous than an L0 brown dwarf. Optical spectra of BOKS 53856 show a T{sub eff} = 34,000 K, log g = 8.0 DA white dwarf. There are few, if any, known white dwarfs in this temperature range exhibiting photometric variations similar to those we describe. A magnetic spin-modulated white dwarf model can in principle explain the light curve, an interpretation supported by spectral observations of the H{alpha} line showing evidence of Zeeman splitting.

  12. THE FREQUENCY OF DEBRIS DISKS AT WHITE DWARFS

    SciTech Connect

    Barber, Sara D.; Patterson, Adam J.; Kilic, Mukremin; Leggett, S. K.; Dufour, P.; Bloom, J. S.; Starr, D. L.

    2012-11-20

    We present near- and mid-infrared photometry and spectroscopy from PAIRITEL, IRTF, and Spitzer of a metallicity-unbiased sample of 117 cool, hydrogen-atmosphere white dwarfs (WDs) from the Palomar-Green survey and find five with excess radiation in the infrared, translating to a 4.3{sup +2.7} {sub -1.2}% frequency of debris disks. This is slightly higher than, but consistent with the results of previous surveys. Using an initial-final mass relation, we apply this result to the progenitor stars of our sample and conclude that 1-7 M {sub Sun} stars have at least a 4.3% chance of hosting planets; an indirect probe of the intermediate-mass regime eluding conventional exoplanetary detection methods. Alternatively, we interpret this result as a limit on accretion timescales as a fraction of WD cooling ages; WDs accrete debris from several generations of disks for {approx}10 Myr. The average total mass accreted by these stars ranges from that of 200 km asteroids to Ceres-sized objects, indicating that WDs accrete moons and dwarf planets as well as solar system asteroid analogs.

  13. The collapse of white dwarfs to neutron stars

    NASA Technical Reports Server (NTRS)

    Woosley, S. E.; Baron, E.

    1992-01-01

    The observable consequences of an accreting white dwarf collapsing directly to a neutron star are considered. The outcome depends critically upon the nature of the wind that is driven by neutrino absorption in the surface layers as the dwarf collapses. Unlike previous calculations which either ignored mass loss or employed inadequate zoning to resolve it, a characteristic mass-loss rate of about 0.005 solar mass/s and an energy input of 5 x 10 exp 50 ergs/s is found. Such a large mass-loss rate almost completely obscures any prompt electromagnetic display and certainly rules out the production by this model of gamma-ray bursts situated at cosmological distances. The occurrence of such collapses with the Milky Way Galaxy might, however, be detected and limited by their nucleosynthesis and gamma-ray line emission. To avoid the overproduction of rare neutron-rich isotopes heavier than iron, such events must be very infrequent, probably happening no more than once every thousand years.

  14. Infrared Opacities in Dense Atmospheres of Cool White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    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.

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

    PubMed

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

    2010-06-11

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

  16. Instability of g-mode oscillations in white dwarf stars

    NASA Technical Reports Server (NTRS)

    Keeley, D. A.

    1979-01-01

    A white dwarf model with M = 6 solar masses, Te = 12,000 K, and L = 1.2 x 10 to the 31st erg/sec provided by Cox has been tested for linear stability of radial oscillations. The radial mode instability first reported for this model by Cox, et al. (1979) has been confirmed. The growth rates obtained are comparable to the rates found by Cox. A sequence of l = 2 g-modes has also been found to be unstable. The e-folding times range from around 10 to the 11th periods for a 137 second mode (1 radial node) to less than 100 periods for a 629 second mode (17 nodes). It is likely that the latter rate is too high because the eigenfunction has been forced to vanish at the non-zero inner radius of the model, at which the Brunt-Vaisala frequency is barely less than the mode frequency.

  17. Molecular magnetic dichroism in spectra of white dwarfs.

    PubMed

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

    2007-08-31

    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+/-0.5 MG 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.

  18. The First Six Outbursting Cool DA White Dwarf Pulsators

    NASA Astrophysics Data System (ADS)

    Bell, K. J.; Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Gentile Fusillo, N. P.; Raddi, R.; Gänsicke, B. T.

    2017-03-01

    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.

  19. Unraveling the Internal Chemical Composition of Kepler White Dwarf Pulsators

    NASA Astrophysics Data System (ADS)

    Giammichele, N.; Charpinet, S.; Fontaine, G.; Brassard, P.

    2017-03-01

    We present the results of the asteroseismic analysis of a selected sample of white dwarf stars in the Kepler and Kepler-2 fields. Our seismic procedure using the forward method based on physically sound, static models, includes a new core parameterization leading us to reproduce the periods of these stars at the precision of the observations. These new fits outperform current state-of-the-art standards by orders of magnitude. We precisely establish the internal structures of these stars and unravel the inner C/O stratification of their core. By studying their internal chemical compositions, and more precisely the C/O profiles, this opens up interesting perspectives on better constraining key processes in stellar physics such as nuclear burning, convection, and mixing, that shape this stratification over time.

  20. Seismology of a Massive Pulsating Hydrogen Atmosphere White Dwarf

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Pelisoli, Ingrid; Peçanha, Viviane; Costa, J. E. S.; Fraga, Luciano; Hermes, J. J.; Winget, D. E.; Castanheira, Barbara; Córsico, A. H.; Romero, A. D.; Althaus, Leandro; Kleinman, S. J.; Nitta, A.; Koester, D.; Külebi, Baybars; Jordan, Stefan; Kanaan, Antonio

    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 sstarf = 0.88 ± 0.02 M ⊙ and T eff = 12, 100 ± 140 K. These values are consistent with those derived from the optical spectra and photometric colors.

  1. SEISMOLOGY OF A MASSIVE PULSATING HYDROGEN ATMOSPHERE WHITE DWARF

    SciTech Connect

    Kepler, S. O.; Pelisoli, Ingrid; Pecanha, Viviane; Costa, J. E. S.; Fraga, Luciano; Hermes, J. J.; Winget, D. E.; Castanheira, Barbara; Corsico, A. H.; Romero, A. D.; Althaus, Leandro; Kleinman, S. J.; Nitta, A.; Koester, D.; Kuelebi, Baybars; Kanaan, Antonio

    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.

  2. Crystallization of Carbon-Oxygen Mixtures in White Dwarf Stars

    NASA Astrophysics Data System (ADS)

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

    2010-06-01

    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 from Medin and Cumming and gives lower melting temperatures than Segretain et al. Observations of WD crystallization in the globular cluster NGC 6397 by Winget et al. suggest that the melting temperature of WD cores is close to that for pure carbon. If this is true, our phase diagram implies that the central oxygen abundance in these stars is less than about 60%. This constraint, along with assumptions about convection in stellar evolution models, limits the effective S factor for the C12(α,γ)O16 reaction to S300≤170keVb.

  3. HST observations of the pulsating white dwarf GD 358

    NASA Astrophysics Data System (ADS)

    Castanheira, B. G.; Nitta, A.; Kepler, S. O.; Winget, D. E.; Koester, D.

    2005-03-01

    We used time-resolved ultraviolet spectroscopy obtained with the FOS and STIS spectrographs of the Hubble Space Telescope (HST), together with archival IUE observations to measure the effective temperature (Teff), surface gravity (log g) and distance (d) of the pulsating DB white dwarf GD 358 with unprecedented accuracy, and to show that the temperature did not change during the 1996 sforzando, when the star changed basically to a single mode pulsator. We also measured for the first time for a DBV the spherical harmonic degree (ℓ) for two modes, with k=8 and k=9, which was only possible because the stellar light curve was dominated by a single mode in 1996. The independent spectra provide the following values: Teff=24 100± 400 K, log g=7.91±0.26 and d=42.7±2.5 pc. The ultraviolet spectroscopic distance is in better agreement with the seismological value, than the one derived by parallax.

  4. DUSTY DISKS AROUND WHITE DWARFS. I. ORIGIN OF DEBRIS DISKS

    SciTech Connect

    Dong Ruobing; Wang Yan; Lin, D. N. C.; Liu, X.-W. E-mail: yuw123@psu.ed E-mail: liuxw@bac.pku.edu.c

    2010-06-01

    A significant fraction of the mature FGK stars have cool dusty disks at least an order of magnitude 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 asymptotic giant branch 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 migration paths, gas giant planets capture and sweep up sub-km-size planetesimals onto their mean-motion resonances. These planetesimals also acquire modest eccentricities which are determined by the mass of the perturbing planets, and the rate and speed of stellar mass loss. The swept-up planetesimals undergo disruptive collisions which lead to the production of grains with an extended size range. The radiation drag on these particles is ineffective against the planets' resonant barrier and they form 30-50 AU size rings which can effectively reprocess the stellar irradiation in the form of FIR continuum. We identify the recently discovered dust ring around the white dwarf WD 2226-210 at the center of the Helix nebula as a prototype of such disks and suggest such rings may be common.

  5. R Coronae Borealis Stars formed from Double White Dwarf Mergers

    NASA Astrophysics Data System (ADS)

    Staff, Jan E.; Herwig, F.; Menon, A.; Even, W.; Tohline, J.; Clayton, G.; Motl, P.; Fryer, C.; Geballe, T.

    2011-01-01

    R Coronae Borealis (RCB) stars are hydrogen-deficient variable stars that suddenly fade by several magnitudes at irregular intervals whereafter they gradually return to their original brightness over a period of some months. The origin of RCBs remain a mystery. It is often thought that they are the result of the merger of a He and a CO white dwarf, while the fading is thought to be due to the formation of dust blocking light from the star. We are working on revealing the secrets behind the origin of RCBs. Here we present the results of 3 dimensional hydrodynamic simulations of the merger of a double white dwarf system where total mass is 0.9 M⊙ and initial mass ratio is q=0.7. We use a zero-temperature plus ideal gas equation of state that allows for heating through shocks. These simulations allow us to follow the evolution of the system for 10-20 initial orbital periods (1000-2000 seconds), from the onset of mass-transfer to a point after merger when the combined object has settled into a nearly axisymmetric, rotationally flattened configuration. The final merged object from the hydrodynamics simulation is then used as input for a stellar evolution code where the object's evolution can be followed over a much longer (thermal and/or nuclear) timescale. A preliminary post-merger stellar evolution simulation shows how an initial configuration of a 0.7 CO WD surrounded by 0.3 M⊙ of dynamically accreted He evolves on a time scale of 105 years to the location of the RCB stars in the H-R diagram at an effective temperature Teff<7000 K and log L 4. We acknowledge support from NASA Astrophysics Theory Program grant number NNX10AC72G.

  6. Trace Hydrogen in Helium Atmosphere White Dwarfs as a Possible Signature of Water Accretion

    NASA Astrophysics Data System (ADS)

    Gentile-Fusillo, N. P.; Gänsicke, B. T.; Farihi, J.; Koester, D.; Schreiber, M. R.; Pala, A. F.

    2017-03-01

    A handful of white dwarfs with helium-dominated atmospheres contain exceptionally large masses of hydrogen in their convection zones, with the metal-polluted white dwarf GD 16 being one of the earliest recognised examples. We report the discovery of a similar star: the white dwarf coincidentally named GD 17. We obtained medium-resolution spectroscopy of both GD 16 and GD 17 and calculated accretion rates and abundances of photospheric H, Mg, Ca, Ti, Fe and Ni. The metal abundance ratios indicate that the two stars recently accreted debris which is Mg-poor compared to the composition of bulk Earth. However, unlike the metal pollutants, H never diffuses out of the atmosphere of white dwarfs and we propose that the exceptionally high atmospheric H content of GD 16 and GD 17 (2.2× 1024g and 2.9× 1024g respectively) could result from previous accretion of water bearing planetesimals. Comparing the detection of trace H and metal pollution among 729 helium atmosphere white dwarfs, we find that the presence of H is nearly twice as common in metal-polluted white dwarfs compared to their metal-free counterparts. This statistically highly significant correlation indicates that a significant amount of H is accreted alongside the metal pollutants in many He atmosphere white dwarfs (including GD 16 and GD 17). We argue that H is most likely accreted in the form of water which must therefore be commonly present in systems with rocky planetesimals.

  7. TRANSIT SURVEYS FOR EARTHS IN THE HABITABLE ZONES OF WHITE DWARFS

    SciTech Connect

    Agol, Eric

    2011-04-20

    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 {approx}0.005 to 0.02 AU for white dwarfs with masses from 0.4 to 0.9 M{sub sun}, temperatures less than {approx}10{sup 4} K, and habitable durations of at least 3 Gyr. As they are similar in size to Earth, white dwarfs may be deeply 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, and rotation period, and host star temperatures similar to the Sun. The Large Synoptic Survey Telescope could place even tighter constraints on the frequency of habitable Earths around white dwarfs. The confirmation and characterization of these planets might be carried out with large ground and space telescopes.

  8. The WIRED Survey. IV. New Dust Disks from the McCook & Sion White Dwarf Catalog

    NASA Technical Reports Server (NTRS)

    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.

  9. MOST Photometry and DDO Spectroscopy of the Eclipsing (White Dwarf + Red Dwarf) Binary V471 Tau

    NASA Astrophysics Data System (ADS)

    Kamiński, Krzysztof Z.; Ruciński, Slavek M.; Matthews, Jaymie M.; Kuschnig, Rainer; Rowe, Jason F.; Guenther, David B.; Moffat, Anthony F. J.; Sasselov, Dimitar; Walker, Gordon A. H.; Weiss, Werner W.

    2007-09-01

    The Hyades K2 V + WD system 471 Tau is a prototype post-common envelope system and a likely cataclysmic binary progenitor. We present 10 days of nearly continuous optical photometry by the MOST (Microvariability and Oscillations of Stars) satellite and partly simultaneous optical spectroscopy from DDO (David Dunlap Observatory) of the binary. The photometric data indicate that the spot coverage of the K dwarf component was less than observed in the past, suggesting that we monitored the star close to a minimum in its activity cycle. Despite the low spot activity, we still detected seven flarelike events whose estimated energies are among the highest ever observed in V471 Tau and whose times of occurrence do not correlate with the binary orbital phase. A detailed O - C analysis of the times of eclipse over the last ~35 years reveals timing variations which could be explained in several ways, including perturbations by an as-yet-undetected third body in the system or by a small orbital eccentricity inducing slow apsidal motion. The DDO spectra result in improved determinations of the K dwarf projected rotation velocity, VK sin i = 92 km s-1, and the orbital amplitude, KK = 150.5 km s-1. The spectra also allow us to measure changes in Hα emission strength and radial velocity variations. We measure a larger Hα velocity amplitude than found previously, suggesting that the source of the emission in V471 Tau was less concentrated around the sub-white dwarf point on the K star than had been observed in previous studies. Based on data from the MOST satellite, a Canadian Space Agency mission jointly operated by Dynacon, Inc., the University of Toronto Institute for Aerospace Studies, and the University of British Columbia, with the assistance of the University of Vienna, and on data obtained at the David Dunlap Observatory, University of Toronto.

  10. The helium white dwarf in two pulsars: too cool in PSR J0751+1807 and too hot in PSR J1012+5307?

    NASA Astrophysics Data System (ADS)

    Ergma, Ene; Sarna, Marek J.; Gerškevitš-Antipova, Jelena

    2001-02-01

    We discuss the cooling history of the low-mass, helium-core white dwarfs in the short orbital period millisecond pulsars PSR J0751+1807 and PSR J1012+5307. The revised cooling age estimated in 1996 by Alberts et al. agrees with the age estimation for PSR J1012+5307, removing the discrepancy between the spin-down age and the cooling age. However, if we accept this model, then the helium white dwarf in the binary pulsar system PSR J0751+1807 must be much hotter than is observed. We propose that this discrepancy may be resolved if, after detachment of the secondary star from its Roche lobe in PSR J0751+1807, the star loses its hydrogen envelope as a result of pulsar irradiation. When hydrogen burning stops, the white dwarf will cool down much more quickly than in the case of a thick hydrogen envelope with a hydrogen-burning shell. We discuss several possibilities to explain the different cooling histories of the white dwarfs in the two systems.

  11. Recent Progress Characterizing Convection Using White Dwarf Light Curves from the Whole Earth Telescope

    NASA Astrophysics Data System (ADS)

    Provencal, J. L.; Dalessio, J.; Montgomery, M. H.; WET Team

    2013-01-01

    Convection remains of the largest sources of theoretical uncertainties in our understanding of stars and other natural phenomena. Montgomery (2005b) shows how precise observations of white dwarf light curves are used to observationally determine the depth of the pulsator's convection zone and its sensitivity to changes in temperature. The Whole Earth Telescope (WET) and the Delaware Asteroseismic Research Center (DARC) are currently conducting a project to map convection across the white dwarf instability strips. We present preliminary results for the DA BPM 31594 and the DB EC 04207-4748 and show the current status of our project mapping convection across the hydrogen white dwarf instability strip.

  12. When flux standards go wild: white dwarfs in the age of Kepler

    NASA Astrophysics Data System (ADS)

    Hermes, JJ

    2017-01-01

    White dwarfs have been used as flux standards for decades, thanks to their staid simplicity. We have tested their photometric stability by analyzing the light curves of several hundred spectroscopically confirmed white dwarfs, through K2 Campaign 8. We find that the vast majority (>90%) are <1% stable in the Kepler bandpass on 0.04-d to 10-d timescales, confirming that these stellar remnants are generally excellent flux standards. From the cases that do exhibit significant (several percent) variability, we caution that binarity and magnetism are two extremely important attributes to rule out when establishing a white dwarf as a photometric standard.

  13. A search for a new class of pulsating DA white dwarf stars in the DB gap

    NASA Astrophysics Data System (ADS)

    Kurtz, D. W.; Shibahashi, H.; Dhillon, V. S.; Marsh, T. R.; Littlefair, S. P.

    2008-10-01

    While white dwarf stars are classified into many subgroups based on the appearance of hydrogen, helium, carbon, oxygen and other spectral lines - or even pure continuum with no lines in the case of the DC stars - the vast majority fall into two major subgroups: those with hydrogen atmospheres (the DA white dwarfs), and those with helium atmospheres (the DO and DB white dwarfs). Remarkably, in the range 45000 >= Teff >= 30000K there are only a few white dwarfs with helium atmospheres to be found - the vast majority are DAs in this temperature range - although white dwarfs with helium atmospheres are found at both hotter (DO) and cooler (DB) effective temperatures. This dearth of helium atmosphere white dwarfs in this temperature range is known as the `DB gap' and is understood in terms convective mixing of the outer atmospheres at the hot and cool ends of the gap, while radiative stability allows the lighter hydrogen to float to the top in the DB gap, so the stars are seen to be DA hydrogen atmosphere white dwarfs. Asteroseismology is an important tool for probing stellar interiors, and white dwarf stars are the most successfully studied group using this technique. In a stability analysis of the stars in the DB gap, Shibahashi has recently predicted the existence of a new class of pulsating white dwarf stars. He finds from models that DA white dwarfs near the red edge of the DB gap have convectively stable outer atmospheres because of a steep mean molecular weight gradient, yet nevertheless have a superadiabatic layer that renders them pulsationally unstable due to radiative heat exchange. There have been very few observational tests for pulsation among stars of this type. We have initiated a survey to search for the predicted pulsators and report here our first observations of five stars with the South African Astronomical Observatory 1.9-m telescope and University of Cape Town CCD photometer, and two stars with the William Herschel Telescope 4.2-m telescope and

  14. THE HABITABILITY AND DETECTION OF EARTH-LIKE PLANETS ORBITING COOL WHITE DWARFS

    SciTech Connect

    Fossati, L.; Haswell, C. A.; Patel, M. R.; Busuttil, R.; Bagnulo, S.; Kowalski, P. M.; Shulyak, D. V.; Sterzik, M. F. E-mail: C.A.Haswell@open.ac.uk E-mail: r.busuttil@open.ac.uk E-mail: kowalski@gfz-potsdam.de E-mail: msterzik@eso.org

    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.

  15. An Eccentric Binary Millisecond Pulsar with a Helium White Dwarf Companion in the Galactic field

    NASA Astrophysics Data System (ADS)

    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. The White Dwarf in SS Cygni and Related Topics: FUSE + HST Spectral Analysis

    NASA Astrophysics Data System (ADS)

    Sion, Edward M.; Godon, Patrick; Myszka, Janine; Blair, William P.

    2010-11-01

    We have carried out a combined Hubble Space Telescope (HST/GHRS) and Far Ultraviolet Spectroscopic Explorer (FUSE) analysis of the prototype dwarf nova SS Cygni during quiescence. The FUSE and HST spectra were obtained at comparable times after outburst and have matching flux levels where the two spectra overlap. From the best-fit model solutions to the combined HST +FUSE spectral energy distribution, we find that the white dwarf is reaching a temperature Teff~45-55,000 K in quiescence, assuming log(g) = 8.3 with a solar composition accreted atmosphere. We discuss two challenges to understanding the cooling of a white dwarf in response to heating by a dwarf nova accretion event. We present the most recent distribution of white dwarf temperatures versus orbital period in the context of the time-averaged accretion rate and long term compressional heating models.

  17. First Detection of the White Dwarf Cooling Sequence of the Galactic Bulge

    NASA Astrophysics Data System (ADS)

    Calamida, A.; Sahu, K. C.; Anderson, J.; Casertano, S.; Cassisi, S.; Salaris, M.; Brown, T.; Sokol, J.; Bond, H. E.; Ferraro, I.; Ferguson, H.; Livio, M.; Valenti, J.; Buonanno, R.; Clarkson, W.; Pietrinferni, A.

    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-1 (≈4 km s-1) at F606W ≈ 25.5 mag, and better than ≈0.5 mas yr-1 (≈20 km s-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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained by the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

  18. First detection of the white dwarf cooling sequence of the galactic bulge

    SciTech Connect

    Calamida, A.; Sahu, K. C.; Anderson, J.; Casertano, S.; Brown, T.; Sokol, J.; Bond, H. E.; Ferguson, H.; Livio, M.; Valenti, J.; Cassisi, S.; Buonanno, R.; Pietrinferni, A.; Salaris, M.; Ferraro, I.; Clarkson, W.

    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.

  19. MULTI-DIMENSIONAL MODELS FOR DOUBLE DETONATION IN SUB-CHANDRASEKHAR MASS WHITE DWARFS

    SciTech Connect

    Moll, R.; Woosley, S. E.

    2013-09-10

    Using two-dimensional and three-dimensional simulations, 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, then 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.

  20. The evolution and fate of super-Chandrasekhar mass white dwarf merger remnants

    NASA Astrophysics Data System (ADS)

    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.

  1. Planetary systems and real planetary nebulae from planet destruction near white dwarfs

    NASA Astrophysics Data System (ADS)

    Bear, Ealeal; Soker, Noam

    2015-07-01

    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 - Earth-like and lighter - planets in tight orbits around the WD. The formation of the new WD planetary system starts with a tidal breakup of the parent planet to planetesimals near the tidal radius of about 1 R⊙. Internal stress forces keep the planetesimal from further tidal breakup when their radius is less than about 100 km. We speculate that the planetesimals then bind together to form new sub-Earth daughter-planets at a few solar radii around the WD. More massive planets that contain hydrogen supply the WD with fresh nuclear fuel to reincarnate its stellar-giant phase. Some of the hydrogen will be inflated in a large envelope. The envelope blows a wind to form a nebula that is later (after the entire envelope is lost) ionized by the hot WD. We term this glowing ionized nebula that originated from a planet a real planetary nebula (RPN). This preliminary study of daughter-planets from a planet and the RPN scenarios are of speculative nature. More detailed studies must follow to establish whether the suggested scenarios can indeed take place.

  2. On the Formation of Eccentric Millisecond Pulsars with Helium White-dwarf Companions

    NASA Astrophysics Data System (ADS)

    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.

  3. Evidence for Neutron Star Formation from Accretion Induced Collapse of a White Dwarf

    NASA Technical Reports Server (NTRS)

    Paradijis, J. Van; VanDenHeuvel, E. P. J.; Kouveliotou, C.; Fishman, G. J.; Finger, M. H.; Lewin, W. H. G.

    1997-01-01

    The orbital parameters of the recently discovered transient burster/pulsar GRO J1744-28 indicate that this system is a low-mass X-ray binary in an advanced stage of its mass transfer, with several tenths of a solar mass already transferred from the donor to the compact star. All neutron stars known to have accreted such an amount have very weak magnetic fields, and this has led to the idea that the magnetic fields of neutron stars decay as a result of accretion. The observation of a strongly magnetized neutron star in GRO J1744-28 then suggests that this neutron star was formed recently as a result of the collapse of a white dwarf during an earlier stage of the current phase of mass transfer. It is shown that this model can consistently explain the observed characteristics of GRO J1744-28. Attractive progenitors for such an evolution are the luminous supersoft X-ray sources detected with ROSAT.

  4. A STRANGE STAR SCENARIO FOR THE FORMATION OF ECCENTRIC MILLISECOND PULSAR/HELIUM WHITE DWARF BINARIES

    SciTech Connect

    Jiang, Long; Li, Xiang-Dong; Dey, Jishnu; Dey, Mira

    2015-07-01

    According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during binary evolution, as observed in most binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are the PSRs J2234+06, J1946+3417, and J1950+2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf (WD) companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive WDs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars (NSs) in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from NSs to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the NS during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X–1, which also possesses a low-field compact star in an eccentric orbit.

  5. ON THE FORMATION OF ECCENTRIC MILLISECOND PULSARS WITH HELIUM WHITE-DWARF COMPANIONS

    SciTech Connect

    Antoniadis, John

    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.

  6. KNOW YOUR NEIGHBORHOOD: A DETAILED MODEL ATMOSPHERE ANALYSIS OF NEARBY WHITE DWARFS

    SciTech Connect

    Giammichele, N.; Bergeron, P.; Dufour, P. E-mail: pierre.bergeron@astro.umontreal.ca

    2012-04-01

    We present improved atmospheric parameters of nearby white dwarfs lying within 20 pc of the Sun. The aim of the current study is to obtain the best statistical model of the least-biased sample of the white dwarf population. A homogeneous analysis of the local population is performed combining detailed spectroscopic and photometric analyses based on improved model atmosphere calculations for various spectral types including DA, DB, DC, DQ, and DZ stars. The spectroscopic technique is applied to all stars in our sample for which optical spectra are available. Photometric energy distributions, when available, are also combined to trigonometric parallax measurements to derive effective temperatures, stellar radii, as well as atmospheric compositions. A revised catalog of white dwarfs in the solar neighborhood is presented. We provide, for the first time, a comprehensive analysis of the mass distribution and the chemical distribution of white dwarf stars in a volume-limited sample.

  7. Discovery of High Proper-Motion Ancient White Dwarfs: Nearby Massive Compact Halo Objects?

    PubMed

    Ibata; Irwin; Bienaymé; Scholz; Guibert

    2000-03-20

    We present the discovery and spectroscopic identification of two very high proper-motion ancient white dwarf stars, found in a systematic proper-motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool halo white dwarf WD 0346+246. Canonical stellar halo models predict a white dwarf volume density that is 2 orders of magnitude less than the rho approximately 7x10-4 M middle dot in circle pc-3 inferred from this survey. With the caveat that the sample size is very small, it appears that a significant fraction, approximately 10%, of the local dark matter halo is in the form of very old, cool, white dwarfs.

  8. Hydrogen in hot subdwarfs formed by double helium white dwarf mergers

    NASA Astrophysics Data System (ADS)

    Hall, Philip D.; Jeffery, C. Simon

    2016-12-01

    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 × 10-3 M⊙ 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.

  9. SPIRAL INSTABILITY CAN DRIVE THERMONUCLEAR EXPLOSIONS IN BINARY WHITE DWARF MERGERS

    SciTech Connect

    Kashyap, Rahul; Fisher, Robert; García-Berro, Enrique; Aznar-Siguán, Gabriela; Lorén-Aguilar, Pablo

    2015-02-10

    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 that 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 that 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

    NASA Astrophysics Data System (ADS)

    Jeffery, E. J.; Campos, F.; Romero, A.; Kepler, S. O.

    2017-03-01

    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. LIMITS ON UNRESOLVED PLANETARY COMPANIONS TO WHITE DWARF REMNANTS OF 14 INTERMEDIATE-MASS STARS

    SciTech Connect

    Kilic, Mukremin; Gould, Andrew; Koester, Detlev

    2009-11-10

    We present Spitzer IRAC photometry of white dwarf remnants of 14 stars with M = 3-5 M{sub sun}. We do not detect mid-infrared excess around any of our targets. By demanding a 3sigma photometric excess at 4.5 mum for unresolved companions, we rule out planetary mass companions down to 5, 7, or 10 M {sub J} for 13 of our targets based on the Burrows et al. substellar cooling models. Combined with previous IRAC observations of white dwarf remnants of intermediate-mass stars, we rule out >=10M {sub J} companions around 40 white dwarfs and >=5M {sub J} companions around 10 white dwarfs.

  12. Understanding the Oldest White Dwarfs: Atmospheres of Cool WDs as Extreme Physics Laboratories

    NASA Astrophysics Data System (ADS)

    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.

  13. White Dwarf WD-1145+17 "Zombie Star" Consumes Asteroid

    NASA Astrophysics Data System (ADS)

    Kaye, Thomas G.; Gary, Bruce L.; Rappaport, Saul A.; Foote, Jerry, Benni, Paul

    2016-05-01

    It has long been suspected that white dwarfs accrete asteroid debris as evidenced by heavy metals in many white dwarf spectra. WD1145 was initially detected in Kepler data as an exoplanet candidate with a repeating 1.3% dip over the course of the Jul-Sep 2014 observing season. Follow-up ground based observations were conducted with professional telescopes during March through May of 2015, and these showed that the Kepler dip must likely consist of deeper and shorter dips which come and go with an uncertain pattern. It was hypothesized that the observations were due to an asteroid in a 4.5 hour orbit. In anticipation of its return to nighttime visibility, major observatories scheduled time starting in 2016 Feb. A pro/am collaboration was formed in late 2015 for amateur observations prior to the 2016 Feb professional observations in order to determine an ephemeris for fade activity for the purpose of scheduling relatively short observations with professional telescopes. The amateur observations began in 2015 Nov, sooner than requested, and they showed that the fade activity level had exploded, becoming 20 times the level measured by Kepler. As many as 13 different fades per 4.5-hour orbit were measured, and these varied in depth from night to night. The amateur project turned into a full assault on the star with as many as 4 amateur telescopes observing on the same night. Continuous monitoring mysteriously showed that the clouds drifted in phase with respect to the dominant period i.e., they have a shorter period than measured by Kepler; this would imply that the orbiting dust clouds were located inside the orbit of the parent planetesimal. The best model indicated that the parent planetesimal was releasing fragments from inside its Hill sphere at the L1 Lagrange point, causing them to fall into an inner orbit. New astrophysics was described for the first time when the team used the diameter of the planetesimal orbit, and the diameter of the drift fragment orbit, to

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

    NASA Technical Reports Server (NTRS)

    Peery, Benjamin F., Jr.

    1986-01-01

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

  15. On the Formation of DA White Dwarfs with low Hydrogen Contents: Preliminary Results

    NASA Astrophysics Data System (ADS)

    Miller Bertolami, M. M.; Althaus, L. G.; Córsico, A. H.

    2017-03-01

    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.

  16. New population synthesis model Preliminary results for close double white dwarf populations

    NASA Astrophysics Data System (ADS)

    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.

  17. A New Population Synthesis Model: Preliminary Results for Close Double White Dwarf Populations

    NASA Astrophysics Data System (ADS)

    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.

  18. The Impact of Radiative Atmospheres on Spectroscopic and Photometric Analyses of Cool White Dwarfs

    NASA Astrophysics Data System (ADS)

    Lecavalier-Hurtubise, É.; Bergeron, P.

    2017-03-01

    It has been recently suggested in the literature that convective energy transport can be seriously impeded by magnetic fields, in particular in the photosphere of white dwarf stars. We push this idea to the extreme and explore the consequences of using purely radiative atmosphere models for the spectroscopic and photometric analyses of cool (Teff<12,000 K) DA white dwarfs. We also perform a similar analysis for known magnetic DA stars with weak sub-megagauss magnetic fields.

  19. Outbursts from Cool Pulsating White Dwarfs in Kepler and K2

    NASA Astrophysics Data System (ADS)

    Bell, Keaton J.; Hermes, J. J.; Montgomery, Michael H.; Winget, Donald E.

    2017-01-01

    Data from the Kepler and K2 missions have captured the signatures of a new pulsation-related phenomenon in hydrogen atmosphere white dwarfs. Some pulsating white dwarfs within 500 K of the empirical cool edge of the ZZ Ceti instability strip exhibit outburst-like brightness enhancements of up to 15% that last many hours and recur irregularly on timescales on days. In this thesis talk, I summarize the observational characteristics of this new outbursting class of ZZ Ceti.

  20. Temperature and Gravity Dependence of Trace Element Abundances in Hot DA White Dwarfs (94-EUVE-094)

    NASA Technical Reports Server (NTRS)

    Finley, David S.

    1998-01-01

    EUV spectroscopy has shown that DA white dwarfs hotter than about 45,000 K may contain trace heavy elements, while those hotter than about 50,000 K almost always have significant abundances of trace heavy elements. One of our continuing challenges is to identify and determine the abundances of these trace constituents, and then to relate the observed abundance patterns to the present conditions and previous evolutionary histories of the hot DA white dwarfs.

  1. The Gaia DR1 mass-radius relation for white dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  2. An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate.

    PubMed

    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.

  3. Using self-organizing maps to identify potential halo white dwarfs.

    PubMed

    García-Berro, Enrique; Torres, Santiago; Isern, Jordi

    2003-01-01

    We present the results of an unsupervised classification of the disk and halo white dwarf populations in the solar neighborhood. The classification is done by merging the results of detailed Monte Carlo (MC) simulations, which reproduce very well the characteristics of the white dwarf populations in the solar neighborhood, with a catalogue of real stars. The resulting composite catalogue is analyzed using a competitive learning algorithm. In particular we have used the so-called self-organized map. The MC simulated stars are used as tracers and help in identifying the resulting clusters. The results of such an strategy turn out to be quite satisfactory, suggesting that this approach can provide an useful framework for analyzing large databases of white dwarfs with well determined kinematical, spatial and photometric properties once they become available in the next decade. Moreover, the results are of astrophysical interest as well, since a straightforward interpretation of several recent astronomical observations, like the detected microlensing events in the direction of the Magellanic Clouds, the possible detection of high proper motion white dwarfs in the Hubble Deep Field and the discovery of high velocity white dwarfs in the solar neighborhood, suggests that a fraction of the baryonic dark matter component of our galaxy could be in the form of old and dim halo white dwarfs.

  4. The connection between period spectra and constraints in white dwarf asteroseismology

    NASA Astrophysics Data System (ADS)

    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.

  5. THE HYADES CLUSTER: IDENTIFICATION OF A PLANETARY SYSTEM AND ESCAPING WHITE DWARFS

    SciTech Connect

    Zuckerman, B.; Xu, S.; Klein, B.; Jura, M. E-mail: sxu@astro.ucla.edu E-mail: jura@astro.ucla.edu

    2013-06-20

    Recently, some hot DA-type white dwarfs have been proposed to plausibly be escaping members of the Hyades. We used hydrogen Balmer lines to measure the radial velocities of seven such stars and confirm that three, and perhaps two others, are/were indeed cluster members and one is not. The other candidate Hyad is strongly magnetic and its membership status remains uncertain. The photospheres of at least one quarter of field white dwarf stars are ''polluted'' by elements heavier than helium that have been accreted. These stars are orbited by extended planetary systems that contain both debris belts and major planets. We surveyed the seven classical single Hyades white dwarfs and the newly identified (escaping) Hyades white dwarfs and found calcium in the photosphere of LP 475-242 of type DBA (now DBAZ), thus implying the presence of an orbiting planetary system. The spectrum of white dwarf GD 31, which may be, but probably is not, an escaping member of the Hyades, displays calcium absorption lines; these originate either from the interstellar medium or, less likely, from a gaseous circumstellar disk. If GD 31 was once a Hyades member, then it would be the first identified white dwarf Hyad with a cooling age >340 Myr.

  6. Seismic evidence for the loss of stellar angular momentum before the white-dwarf stage.

    PubMed

    Charpinet, S; Fontaine, G; Brassard, P

    2009-09-24

    White-dwarf stars represent the final products of the evolution of some 95% of all stars. If stars were to keep their angular momentum throughout their evolution, their white-dwarf descendants, owing to their compact nature, should all rotate relatively rapidly, with typical periods of the order of a few seconds. Observations of their photospheres show, in contrast, that they rotate much more slowly, with periods ranging from hours to tens of years. It is not known, however, whether a white dwarf could 'hide' some of its original angular momentum below the superficial layers, perhaps spinning much more rapidly inside than at its surface. Here we report a determination of the internal rotation profile of a white dwarf using a method based on asteroseismology. We show that the pulsating white dwarf PG 1159-035 rotates as a solid body (encompassing more than 97.5% of its mass) with the relatively long period of 33.61 +/- 0.59 h. This implies that it has lost essentially all of its angular momentum, thus favouring theories which suggest important angular momentum transfer and loss in evolutionary phases before the white-dwarf stage.

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

    NASA Technical Reports Server (NTRS)

    Wood, M. A.

    1990-01-01

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

  8. Pulsation properties of DB white dwarfs: A preliminary analysis

    SciTech Connect

    Winget, D.E.; Van Horn, H.M.; Tassoul, M.; Hansen, C.J.; Fontaine, G.

    1983-05-01

    We report preliminary results of a numerical investigation of the nonradial g-mode pulsation properties of evolutionary DB white dwarf models. We have solved the fully nonadiabatic equations for modes corresponding to spherical harmonic index l = 1 through 3. For each of the sequences of models we have examined (M/sub asterisk/ = 0.6 M/sub sun/; and helium layer masses of 10/sup -6/ M/sub asterisk/ and 10/sup -4/ M/sub asterisk/), we find a nonradial g-mode instability strip about 3000 K wide. For models with standard ML1 convection, this strip lies in the effective temperature range 19,000 K> or approx. =T/sub e/> or approx. =16,000 K. The boundaries of the instability strip are extremely sensitive to the assumed efficiency of convection, however, and for sequences with more efficient (ML3) convection, we find the instability strip to be in the range 29,000 K> or approx. =T/sub e/> or approx. = 26,000 K. Extrapolation of our calculations to 0.4 M/sub sun/ and 0.9 M/sub sun/ indicates that that the instability strip boundaries are insensitive to uncertainties in the total stellar mass. The most unstable modes have e-folding times of the order of days.

  9. High-energy Neutrino Emission from White Dwarf Mergers

    NASA Astrophysics Data System (ADS)

    Xiao, Di; Mészáros, Peter; Murase, Kohta; Dai, Zi-Gao

    2016-11-01

    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 (CRs) 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 CR 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 CR generation rate is highly uncertain, if it reaches ∼ {10}45 {erg} {{Mpc}}-3 {{yr}}-1, the diffuse neutrino flux could contribute a substantial fraction of the IceCube observations. We also evaluate the prospect of observing individual merger events, which provides a means for testing such sources in the future.

  10. Radio Transients from Accretion-induced Collapse of White Dwarfs

    NASA Astrophysics Data System (ADS)

    Moriya, Takashi J.

    2016-10-01

    We investigate observational properties of accretion-induced collapse (AIC) of white dwarfs (WDs) 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 with the dense circumstellar medium, creating a strong shock. The strong shock can produce synchrotron emission that can be observed in radio frequencies. Even if AIC occurs as a result of WD mergers, we argue that AIC may cause fast radio bursts (FRBs) if a certain condition is satisfied. If AIC forms neutron stars (NSs) that are so massive that rotation is required to support themselves (i.e., supramassive NSs), the supramassive NSs may immediately lose their rotational energy by the r-mode instability and collapse to black holes. If the collapsing supramassive NSs are strongly magnetized, they may emit FRBs, as previously proposed. The AIC radio transients from single-degenerate systems may be detected in future radio transient surveys like the Very Large Array Sky Survey or the Square Kilometer Array transient survey. Because AIC has been proposed as a source of gravitational waves (GWs), GWs from AIC may be accompanied by radio-bright transients that can be used to confirm the AIC origin of observed GWs.

  11. MASS CONSTRAINTS FROM ECLIPSE TIMING IN DOUBLE WHITE DWARF BINARIES

    SciTech Connect

    Kaplan, David L.

    2010-07-10

    I demonstrate that an effect similar to the Roemer delay, familiar from timing radio pulsars, should be detectable in the first eclipsing double white dwarf (WD) binary, NLTT 11748. By measuring the difference of the time between the secondary and primary eclipses from one-half period (4.6 s), one can determine the physical size of the orbit and hence constrain the masses of the individual WDs. A measurement with uncertainty <0.1 s-possible with modern large telescopes-will determine the individual masses to {+-}0.02 M{sub sun} when combined with good-quality (<1 km s{sup -1}) radial velocity data, although the eccentricity must also be known to high accuracy ({+-}10{sup -3}). Mass constraints improve as P {sup -1/2} (where P is the orbital period), so this works best in wide binaries and should be detectable even for non-degenerate stars, but such constraints require the mass ratio to differ from 1, as well as undistorted orbits.

  12. A 12 MINUTE ORBITAL PERIOD DETACHED WHITE DWARF ECLIPSING BINARY

    SciTech Connect

    Brown, Warren R.; Kilic, Mukremin; Kenyon, Scott J.; Hermes, J. J.; Winget, D. E.; Prieto, Carlos Allende E-mail: mkilic@cfa.harvard.edu E-mail: jjhermes@astro.as.utexas.edu E-mail: callende@iac.es

    2011-08-10

    We have discovered a detached pair of white dwarfs (WDs) with a 12.75 minute orbital period and a 1315 km s{sup -1} radial velocity amplitude. We measure the full orbital parameters of the system using its light curve, which shows ellipsoidal variations, Doppler boosting, and primary and secondary eclipses. The primary is a 0.25 M{sub sun} tidally distorted helium WD, only the second tidally distorted WD known. The unseen secondary is a 0.55 M{sub sun} carbon-oxygen WD. The two WDs will come into contact in 0.9 Myr due to loss of energy and angular momentum via gravitational wave radiation. Upon contact the systems may merge (yielding a rapidly spinning massive WD), form a stable interacting binary, or possibly explode as an underluminous Type Ia supernova. The system currently has a gravitational wave strain of 10{sup -22}, about 10,000 times larger than the Hulse-Taylor pulsar; this system would be detected by the proposed Laser Interferometer Space Antenna gravitational wave mission in the first week of operation. This system's rapid change in orbital period will provide a fundamental test of general relativity.

  13. TURBULENT MIXING ON HELIUM-ACCRETING WHITE DWARFS

    SciTech Connect

    Piro, Anthony L.

    2015-03-10

    An attractive scenario for producing Type Ia supernovae (SNe Ia) is a double detonation, where detonation of an accreted helium layer triggers ignition of a C/O core. Whether or not such a mechanism can explain some or most SNe Ia depends on the properties of the helium burning, which in turn is set by the composition of the surface material. Using a combination of semi-analytic and simple numerical models, I explore when turbulent mixing due to hydrodynamic instabilities during the accretion process can mix C/O core material up into the accreted helium. Mixing is strongest at high accretion rates, large white dwarf (WD) masses, and slow spin rates. The mixing would result in subsequent helium burning that better matches the observed properties of SNe Ia. In some cases, there is considerable mixing that can lead to more than 50% C/O in the accreted layer at the time of ignition. These results will hopefully motivate future theoretical studies of such strongly mixed conditions. Mixing also has implications for other types of WD surface explosions, including the so-called .Ia supernovae, the calcium-rich transients (if they arise from accreting WDs), and metal-enriched classical novae.

  14. Laboratory measurements of white dwarf photospheric spectral lines: Hβ

    DOE PAGES

    Falcon, Ross Edward; Rochau, Gregory A.; Bailey, James E.; ...

    2015-06-18

    We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf (WD) atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating WD photospheric conditions. We also present time-resolved measurements of Hβ and fit this line using different theoretical line profiles to diagnose electron density, ne, and n = 2 level population, n2. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer amore » continuous range of electron densities increasing from ne ~ 4 to ~30 × 1016 cm-3 throughout a 120-ns evolution of our plasma. Also, we observe n2 to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within ~55 ns to become consistent with LTE. This also supports our electron-temperature determination of Te ~ 1.3 eV (~15,000 K) after this time. At ne≲ 1017 cm-3, we find that computer-simulation-based line-profile calculations provide better fits (lower reduced χ2) than the line profiles currently used in the WD astronomy community. The inferred conditions, however, are in good quantitative agreement. Lastly, this work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.« less

  15. White Dwarf Mass Estimation with X-ray Spectroscopy

    NASA Astrophysics Data System (ADS)

    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.

  16. White dwarf pollution by planets in stellar binaries

    NASA Astrophysics Data System (ADS)

    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.

  17. HST Observations of the Pulsating White Dwarf GD 358

    NASA Astrophysics Data System (ADS)

    Castanheira, B. G.; Kepler, S. O.; Nitta, A.; Winget, D. E.; Koester, D.

    2005-07-01

    We used time-resolved ultraviolet spectroscopy obtained with the FOS and STIS spectrographs of the Hubble Space Telescope (HST), together with archival IUE observations to measure the effective temperature (Teff}), surface gravity (log g) and distance (d) of the pulsating DB white dwarf GD 358 with unprecedented accuracy, and to show the temperature did not change during the 1996 sforzando, when the star changed basically to a single mode pulsator. We also measured, for the first time for a DBV, the spherical harmonic degree (ℓ) for two modes, with k=8 and k=9, which was only possible because the stellar light curve was dominated by a single mode in 1996. In addition, we constrain ℓ to be 1 or 2 for the main pulsations in the normal multiperiodic state. The spectra are best fit for Teff}=24 100± 400 K, log g=7.91±0.26 and d=42.7±2.5 pc. The ultraviolet spectroscopic distance is in better agreement with the seismological value, than the one derived by parallax.

  18. A DOUBLE WHITE-DWARF COOLING SEQUENCE IN {omega} CENTAURI

    SciTech Connect

    Bellini, A.; Anderson, J.; Salaris, M.; Cassisi, S.; Bedin, L. R.; Piotto, G.; Bergeron, P.

    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.