Science.gov

Sample records for cool white dwarfs

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  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. Towards an Understanding of the Atmospheres of Cool White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kowalski, P. M.; Saumon, D.; Holberg, J.; Leggett, S.

    2013-01-01

    Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars that existed in our Galaxy. Their atmospheres, when properly characterized, can provide valuable information on white dwarf evolution and ultimately star formation through the history of the Milky Way. Understanding the atmospheres of these stars requires joined observational effort and reliable atmosphere modeling. We discuss and analyze recent observations of the near-ultraviolet (UV) and near-infrared (IR) spectrum of several cool white dwarfs including DQ/DQp stars showing carbon in their spectra. We present fits to the entire spectral energy distribution (SED) of selected cool stars, showing that the current pure-hydrogen atmosphere models are quite reliable, especially in the near-UV spectral region. Recently, we also performed an analysis of the coolest known DQ/DQp stars investigating further the origin of the rm C2 Swan bands-like spectral features that characterize the DQp stars. We show that the carbon abundances derived for DQp stars fit the trend of carbon abundance with Teff seen in normal cool DQ stars. This further supports the recent conclusion of Kowalski (2010a) that DQp stars are DQ stars with pressure distorted Swan bands. However, we encounter some difficulties in reproducing the IR part of the SED of stars having a mixed He/H atmosphere. This indicates limitations in current models of the opacity in dense He/H fluids.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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 %.

  12. 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.

  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. 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.

  15. 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.

  16. Near-UV absorption in very cool DA white dwarfs

    SciTech Connect

    Saumon, D.; Holberg, J. B.; Kowalski, P. M. E-mail: holberg@argus.lpl.arizona.edu

    2014-07-20

    The atmospheres of very cool, hydrogen-rich white dwarfs (WDs) (T{sub eff} < 6000 K) are challenging to model because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman α line of atomic H by collisions primarily with H{sub 2}. For the purpose of validating this model more rigorously, we acquired Hubble Space Telescope STIS spectra of eight very cool WDs (five DA and three DC stars). Combined with their known parallaxes, BVRIJHK, and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (from 0.24 to 9.3 μm) with a large grid of model atmospheres and synthetic spectra. We find that the red wing of the Lyman α line reproduces the rapidly decreasing near-UV flux of these very cool stars very well. We determine better constrained values of T{sub eff} and gravity as well as upper limits to the helium abundance in their atmospheres.

  17. 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.

  18. The white-dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution

    NASA Astrophysics Data System (ADS)

    García-Berro, E.; Torres, S.; Renedo, I.; Camacho, J.; Althaus, L. G.; Córsico, A. H.; Salaris, M.; Isern, J.

    2011-09-01

    Context. NGC 6791 is a well-studied, metal-rich open cluster that is so close to us that it can be imaged down to luminosities fainter than that of the termination of its white-dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population. Aims: White dwarfs carry important information about the history of the cluster. We use observations of the white-dwarf cooling sequence to constrain important properties of the cluster stellar population, such as the existence of a putative population of massive helium-core white dwarfs, and the properties of a large population of unresolved binary white dwarfs. We also investigate the use of white dwarfs to disclose the presence of cluster subpopulations with a different initial chemical composition, and we obtain an upper bound to the fraction of hydrogen-deficient white dwarfs. Methods: We use a Monte Carlo simulator that employs up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The cooling sequences for carbon-oxygen cores account for the delays introduced by both 22Ne sedimentation in the liquid phase and by carbon-oxygen phase separation upon crystallization. Results: We do not find evidence for a substantial fraction of helium-core white dwarfs, and hence our results support the suggestion that the origin of the bright peak of the white-dwarf luminosity function can only be attributed to a population of unresolved binary white dwarfs. Moreover, our results indicate that if this hypothesis is at the origin of the bright peak, the number distribution of secondary masses of the population of unresolved binaries has to increase with increasing mass ratio between the secondary and primary components of the progenitor system. We also find that the observed cooling sequence appears to be able to constrain the presence of progenitor subpopulations with different chemical compositions and the fraction of

  19. 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. The blue WDCS is populated by the evolved stars of the He-normal component ({approx}0.55 M{sub Sun} CO-core DA objects), while the red WDCS hosts the end products of the He-rich population ({approx}0.46 M{sub Sun} objects, and {approx}10% CO-core and {approx}90% He-core WDs). The He-core WDs correspond to He-rich stars that missed the central He ignition, and we estimate their fraction by analyzing the population ratios along the cluster horizontal branch.

  20. 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.

  1. 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.

  2. 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.

  3. 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.

  4. A New Generation of Cool White Dwarf Atmosphere Models Using Ab Initio Calculations

    NASA Astrophysics Data System (ADS)

    Blouin, S.; Dufour, P.; Kowalski, P. M.

    2017-03-01

    Due to their high photospheric density, cool helium-rich white dwarfs (particularly DZ, DQpec and ultracool) are often poorly described by current atmosphere models. As part of our ongoing efforts to design atmosphere models suitable for all cool white dwarfs, we investigate how the ionization ratio of heavy elements and the H2-He collision-induced absorption (CIA) spectrum are altered under fluid-like densities. For the conditions encountered at the photosphere of cool helium-rich white dwarfs, our ab initio calculations show that the ionization of most metals is inhibited and that the H2-He CIA spectrum is significantly distorted for densities higher than 0.1 g/cm3.

  5. On the Interpretation of Cool, Helium-rich White Dwarfs Showing Traces of Hydrogen

    NASA Astrophysics Data System (ADS)

    Rolland, B.; Bergeron, P.; Fontaine, G.

    2017-03-01

    We present a spectroscopic analysis of cool (Teff<12,000 K) helium-rich white dwarfs — drawn mostly from the Sloan Digital Sky Survey (SDSS) — also showing spectroscopic traces of hydrogen. We then perform numerical simulations and attempt to interpret these objects (1) in terms of DA white dwarfs whose thin hydrogen atmospheres have been convectively mixed with the deeper and more massive helium envelope, or (2) in terms of DB or DBA stars that have simply cooled off, taking into account the possible accretion of hydrogen from the interstellar medium or other external sources. We also investigate the origin of DBA stars.

  6. Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey

    NASA Astrophysics Data System (ADS)

    Leggett, S. K.; Lodieu, N.; Tremblay, P.-E.; Bergeron, P.; Nitta, A.

    2011-07-01

    We present the results of a search for cool white dwarfs in the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky Survey to identify cool hydrogen-rich white dwarf candidates by their neutral optical colors and blue near-infrared colors, as well as faint reduced proper motion magnitudes. Optical spectroscopy was obtained at Gemini Observatory and showed the majority of the candidates to be newly identified cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants. Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with effective temperature T eff ≈ 6000 K. The current follow-up of 1400 deg2 of sky has produced 13 new white dwarfs. Model fits to the photometry show that seven of the newly identified white dwarfs have 4120 K <=T eff <= 4480 K, and cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km s-1 <= v tan <= 85 km s-1 and are likely to be thick disk 10-11 Gyr-old objects. The other half of the sample has 4610 K <=T eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr, and 60 km s-1 <= v tan <= 100 km s-1. These are either thin disk remnants with unusually high velocities, or lower-mass remnants of thick disk or halo late-F or G stars.

  7. COOL WHITE DWARFS FOUND IN THE UKIRT INFRARED DEEP SKY SURVEY

    SciTech Connect

    Leggett, S. K.; Nitta, A.; Lodieu, N.

    2011-07-01

    We present the results of a search for cool white dwarfs in the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky Survey to identify cool hydrogen-rich white dwarf candidates by their neutral optical colors and blue near-infrared colors, as well as faint reduced proper motion magnitudes. Optical spectroscopy was obtained at Gemini Observatory and showed the majority of the candidates to be newly identified cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants. Our initial search of 280 deg{sup 2} of sky resulted in seven new white dwarfs with effective temperature T{sub eff} {approx} 6000 K. The current follow-up of 1400 deg{sup 2} of sky has produced 13 new white dwarfs. Model fits to the photometry show that seven of the newly identified white dwarfs have 4120 K {<=}T{sub eff} {<=} 4480 K, and cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km s{sup -1} {<=} v{sub tan} {<=} 85 km s{sup -1} and are likely to be thick disk 10-11 Gyr-old objects. The other half of the sample has 4610 K {<=}T{sub eff} {<=} 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr, and 60 km s{sup -1} {<=} v{sub tan} {<=} 100 km s{sup -1}. These are either thin disk remnants with unusually high velocities, or lower-mass remnants of thick disk or halo late-F or G stars.

  8. A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356

    NASA Technical Reports Server (NTRS)

    Weisskopf, Martin C.; Wu, Kinwah; Trimble, Virginia; ODell, Stephen L.; Elsner, Ronald F.; Zavlin, Vyacheslav E.; Kouveliotou, Chryssa

    2006-01-01

    We report observations with the Chandra X-ray Observatory of the single, cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray observations of single white dwarfs, we also re-analyzed archival ROSAT data for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ 339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD 358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting the most restrictive upper limit to the X-ray luminosity from any cool white dwarf - Lx less than 6.0 x 10(exp 25) erg s(sup -1), at 99.7% confidence, for a 1- keV thermal-bremsstrahlung spectrum. The corresponding limit to the electron density is no less than 4.4x10(exp 11) per cubic centimeter. Our re-analysis of the archival data confirmed the non-detections reported by the original investigators. We discuss the implications of our and prior observations on models for coronal emission from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent constraints imposed by cyclotron radiation. In addition, we describe (in an appendix) a statistical methodology for detecting a source and for constraining the strength of a source, which applies even when the number of source or background events is small.

  9. AN EMPIRICAL MEASURE OF THE RATE OF WHITE DWARF COOLING IN 47 TUCANAE

    SciTech Connect

    Goldsbury, R.; Heyl, J.; Richer, H. B.; Woodley, K. A. E-mail: heyl@phas.ubc.ca E-mail: kwoodley@phas.ubc.ca; and others

    2012-11-20

    We present an empirical determination of the white dwarf cooling sequence in the globular cluster 47 Tucanae. Using spectral models, we determine temperatures for 887 objects from Wide Field Camera 3 data, as well as 292 objects from data taken with the Advanced Camera for Surveys. We make the assumption that the rate of white dwarf formation in the cluster is constant. Stellar evolution models are then used to determine the rate at which objects are leaving the main sequence, which must be the same as the rate at which objects are arriving on the white dwarf sequence in our field. The result is an empirically derived relation between temperature (T {sub eff}) and time (t) on the white dwarf cooling sequence. Comparing this result to theoretical cooling models, we find general agreement with the expected slopes between 20,000 K and 30,000 K and between 6000 K and 20,000 K, but the transition to the Mestel cooling rate of T {sub eff}{proportional_to}t {sup -0.4} is found to occur at hotter temperatures, and more abruptly than is predicted by any of these models.

  10. Discovery of a Possible Cool White Dwarf Companion from the AllWISE Motion Survey

    NASA Astrophysics Data System (ADS)

    Fajardo-Acosta, Sergio B.; Kirkpatrick, J. Davy; Schneider, Adam C.; Cushing, Michael C.; Stern, Daniel; Gelino, Christopher R.; Bardalez-Gagliuffi, Daniella C.; Kellogg, Kendra; Wright, Edward L.

    2016-11-01

    We present optical and near-infrared spectroscopy of WISEA J061543.91-124726.8, which we rediscovered as a high motion object in the AllWISE survey. The spectra of this object are unusual; while the red optical (λ > 7000 Å) and near-infrared spectra exhibit characteristic TiO, VO, and H2O bands of a late-M dwarf, the blue portion of its optical spectrum shows a significant excess of emission relative to late-M-type templates. The excess emission is relatively featureless, with the exception of a prominent and very broad Na i D doublet. We find that no single, ordinary star can reproduce these spectral characteristics. The most likely explanation is an unresolved binary system of an M7 dwarf and a cool white dwarf. The flux of a cool white dwarf drops in the optical red and near-infrared, due to collision-induced absorption, thus allowing the flux of a late-M dwarf to show through. This scenario, however, does not explain the Na D feature, which is unlike that of any known white dwarf, but which could perhaps be explained via unusual abundance or pressure conditions.

  11. Discovery of a Cool, Massive, and Metal-rich DAZ White Dwarf

    NASA Astrophysics Data System (ADS)

    Gianninas, A.; Dufour, P.; Bergeron, P.

    2004-12-01

    We report the discovery of a new metal-rich DAZ white dwarf, GD 362. High signal-to-noise ratio optical spectroscopy reveals the presence of spectral lines from hydrogen as well as Ca I, Ca II, Mg I, and Fe I. A detailed model atmosphere analysis of this star yields an effective temperature of Teff=9740+/-50 K, a surface gravity of logg=9.12+/-0.07, and photospheric abundances of log(Ca/H)=-5.2+/-0.1, log(Mg/H)=-4.8+/-0.1, and log(Fe/H)=-4.5+/-0.1. White dwarf cooling models are used to derive a mass of 1.24 Msolar for GD 362, making it the most massive and metal-rich DAZ star uncovered to date. The problems related to the presence of such large metal abundances in a nearby (d~25 pc) white dwarf in terms of an accretion scenario are briefly discussed.

  12. Constraining Neutrino Cooling Using the Hot White Dwarf Luminosity Function in the Globular Cluster 47 Tucanae

    NASA Astrophysics Data System (ADS)

    Hansen, Brad M. S.; Richer, Harvey; Kalirai, Jason; Goldsbury, Ryan; Frewen, Shane; Heyl, Jeremy

    2015-08-01

    We present Hubble Space Telescope observations of the upper part ({T}{eff}\\gt {10}4 K) of the white dwarf cooling sequence in the globular cluster 47 Tucanae and measure a luminosity function of hot white dwarfs. Comparison with previous determinations from large-scale field surveys indicates that the previously determined plateau at high effective temperatures is likely a selection effect, as no such feature is seen in this sample. Comparison with theoretical models suggests that the current estimates of white dwarf neutrino emission (primarily by the plasmon channel) are accurate, and variations are restricted to no more than a factor of two globally, at 95% confidence. We use these constraints to place limits on various proposed exotic emission mechanisms, including a nonzero neutrino magnetic moment, formation of axions, and emission of Kaluza-Klein modes into extra dimensions.

  13. Cool White Dwarfs from the SuperCOSMOS and Sloan Digital Sky Surveys

    NASA Astrophysics Data System (ADS)

    Hambly, N. C.; Digby, A. P.; Oppenheimer, B. R.

    2005-07-01

    We have used datamining techniques in the SuperCOSMOS Science Archive (http://surveys.roe.ac.uk/ssa) to obtain a large, well defined proper motion and magnitude selected sample of cool white dwarfs. Using accurate 5-colour photometry from the Sloan Digital Sky Survey DR1 and SuperCOSMOS Sky Survey photometry and astrometry, we demonstrate the power of reduced proper motion in obtaining a sample of >700 white dwarfs. We examine the characteristics of these objects in various two-colour diagrams in conjunction with new model atmosphere predictions recently computed in the SDSS photometric system. Ultimately, we intend to analyse these data with techniques similar to those already used to examine the subdwarf luminosity function (Digby et al. 2003). In this way, we aim to decompose the contribution of thin disk, thick disk and spheroid white dwarfs in the sample to enable computation of accurate luminosity functions for those respective populations.

  14. 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.

  15. 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.

  16. Asymmetry in the triplet 3p-4s Mg lines in cool DZ white dwarfs

    NASA Astrophysics Data System (ADS)

    Allard, N. F.; Leininger, T.; Gadéa, F. X.; Brousseau-Couture, V.; Dufour, P.

    2016-04-01

    The purpose of the present work is to make an exhaustive study of the line shape of the triplet 3p-4s Mg line (Mgb triplet), which is perturbed by He in the extreme physical conditions found in the cool atmosphere of DZ white dwarfs. This study is undertaken by inferring both a unified theory of spectral line broadening and ab initio potential energies. Cool white dwarfs require a specific treatment for line broadening owing to the high helium densities that are involved. Beyond the conventional symmetrical Lorentzian core at low density, we show that the line profiles are asymmetrical and have significant additional contributions on the short wavelength side. This blue asymmetry is a consequence of low maxima in the corresponding Mg-He potential energy difference curves at short and intermediate internuclear distances. The new profiles are shown to provide a good fit to an SDSS (Sloan Digital Sky Survey) observation.

  17. Habitable planets around white and brown dwarfs: the perils of a cooling primary.

    PubMed

    Barnes, Rory; Heller, René

    2013-03-01

    White and brown dwarfs are astrophysical objects that are bright enough to support an insolation habitable zone (IHZ). Unlike hydrogen-burning stars, they cool and become less luminous with time; hence their IHZ moves in with time. The inner edge of the IHZ is defined as the orbital radius at which a planet may enter a moist or runaway greenhouse, phenomena that can remove a planet's surface water forever. Thus, as the IHZ moves in, planets that enter it may no longer have any water and are still uninhabitable. Additionally, the close proximity of the IHZ to the primary leads to concern that tidal heating may also be strong enough to trigger a runaway greenhouse, even for orbital eccentricities as small as 10(-6). Water loss occurs due to photolyzation by UV photons in the planetary stratosphere, followed by hydrogen escape. Young white dwarfs emit a large amount of these photons, as their surface temperatures are over 10(4) K. The situation is less clear for brown dwarfs, as observational data do not constrain their early activity and UV emission very well. Nonetheless, both types of planets are at risk of never achieving habitable conditions, but planets orbiting white dwarfs may be less likely to sustain life than those orbiting brown dwarfs. We consider the future habitability of the planet candidates KOI 55.01 and 55.02 in these terms and find they are unlikely to become habitable.

  18. Comparing the white dwarf cooling sequences in 47 Tuc and NGC 6397

    SciTech Connect

    Richer, Harvey B.; Goldsbury, Ryan; Heyl, Jeremy; Hurley, Jarrod; Dotter, Aaron; Kalirai, Jason S.; Woodley, Kristin A.; Fahlman, Gregory G.; Rich, R. Michael; Shara, Michael M. E-mail: rgoldsb@phas.ubc.ca E-mail: jhurley@swin.edu.au E-mail: jkalirai@stsci.edu E-mail: greg.fahlman@nrc-cnrc.gc.ca E-mail: mshara@amnh.org

    2013-12-01

    Using deep Hubble Space Telescope imaging, color-magnitude diagrams are constructed for the globular clusters 47 Tuc and NGC 6397. As expected, because of its lower metal abundance, the main sequence of NGC 6397 lies well to the blue of that of 47 Tuc. A comparison of the white dwarf cooling sequences of the two clusters, however, demonstrates that these sequences are indistinguishable over most of their loci—a consequence of the settling out of heavy elements in the dense white dwarf atmosphere and the near equality of their masses. Lower quality data on M4 continues this trend to a third cluster whose metallicity is intermediate between these two. While the path of the white dwarfs in the color-magnitude diagram is nearly identical in 47 Tuc and NGC 6397, the numbers of white dwarfs along the path are not. This results from the relatively rapid relaxation in NGC 6397 compared to 47 Tuc and provides a cautionary note that simply counting objects in star clusters in random locations as a method of testing stellar evolutionary theory is likely dangerous unless dynamical considerations are included.

  19. VizieR Online Data Catalog: White dwarf cooling sequences (Bono+, 2013)

    NASA Astrophysics Data System (ADS)

    Bono, G.; Salaris, M.; Gilmozzi, R.

    2013-01-01

    We present new diagnostics of white dwarf (WD) cooling sequences and luminosity functions (LFs) in the near-infrared (NIR) bands that will exploit the sensitivity and resolution of future extremely large telescopes. The collision-induced absorption (CIA) of molecular hydrogen causes a clearly defined blue turn-off along the WD (WDBTO) cooling sequences and a bright secondary maximum in the WD LFs. These features are independent of age over a broad age range and are minimally affected by metal abundance. This means that the NIR magnitudes of the WDBTO are very promising distance indicators. (3 data files).

  20. The white dwarf luminosity function

    NASA Astrophysics Data System (ADS)

    García-Berro, Enrique; Oswalt, Terry D.

    2016-06-01

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

  1. 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.

  2. The cooling of the white dwarf in U Geminorum following heating by two different outbursts

    NASA Technical Reports Server (NTRS)

    Huang, Min; Sion, Edward M.; Szkody, Paula

    1995-01-01

    We present the analysis of 13 orbital phase-resolved IUE (large and small aperture) SWP spectra, sampling the quiescent intervals of the proptotype dwarf nova U Geminorum following two individual outbursts, the outburst of 1992, 29 August-14 September, and the outburst of 1993, 19 March-5 April. The quiescent interval following the outburst 29 August 1992 is the longest ever recorded. During quiescence, the photospheric radiation of the exposed white dwarf dominates the far ultrviolet. The variations in absorption line strengths and continum flux levels are analyzed as a function of both orbital phase and elapsed time, since the return to optical quiescence, by fitting the IUE spectra with a grid of high gravity, LTE, model atmospheres with solar composition constructed with TLUSTY and SYNSPEC (Hubeny, I. Tlusty and Synnspec: A Users Guide (1994)). We present evidence from both absorption line variations and continuum variations, as a function of time since the outbursts, that the white dwarf photosphere has cooled by several thousand degrees. Within the signal-to-noise limitations, we find no evidence of a difference in heating and cooling between the two outbursts.

  3. Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary

    PubMed Central

    Heller, René

    2013-01-01

    Abstract White and brown dwarfs are astrophysical objects that are bright enough to support an insolation habitable zone (IHZ). Unlike hydrogen-burning stars, they cool and become less luminous with time; hence their IHZ moves in with time. The inner edge of the IHZ is defined as the orbital radius at which a planet may enter a moist or runaway greenhouse, phenomena that can remove a planet's surface water forever. Thus, as the IHZ moves in, planets that enter it may no longer have any water and are still uninhabitable. Additionally, the close proximity of the IHZ to the primary leads to concern that tidal heating may also be strong enough to trigger a runaway greenhouse, even for orbital eccentricities as small as 10−6. Water loss occurs due to photolyzation by UV photons in the planetary stratosphere, followed by hydrogen escape. Young white dwarfs emit a large amount of these photons, as their surface temperatures are over 104 K. The situation is less clear for brown dwarfs, as observational data do not constrain their early activity and UV emission very well. Nonetheless, both types of planets are at risk of never achieving habitable conditions, but planets orbiting white dwarfs may be less likely to sustain life than those orbiting brown dwarfs. We consider the future habitability of the planet candidates KOI 55.01 and 55.02 in these terms and find they are unlikely to become habitable. Key Words: Extrasolar terrestrial planets—Habitability—Habitable zone—Tides—Exoplanets. Astrobiology 13, 279–291. PMID:23537137

  4. Astrophysics with white dwarfs

    NASA Astrophysics Data System (ADS)

    Kalirai, Jasonjot Singh

    2004-10-01

    telescopes to simultaneously obtain spectra for 22 white dwarfs in the rich cluster NGC 2099. This work represents a planned follow-up study of the white dwarfs in the richest clusters that we identified in the CFHT Open Star Cluster Survey, and has produced several interesting results. First, all white dwarfs in this cluster are hydrogen rich suggesting perhaps that the ratio of hydrogen to helium white dwarfs is different in clusters than in the field, or that all massive white dwarfs are hydrogen rich. Secondly, the NGC 2099 white dwarfs provide the first ever confirmation of a white dwarf cooling age for a star cluster. Thirdly, with just this one cluster, we are able to almost double the number of white dwarfs that exist on the initial-final mass plane, and provide very strong, tight constraints on a key part of the initial- final mass relationship. The previous constraints on this relationship, which show a large scatter, had taken over 30 years to establish. Our findings directly show that stars with masses between 2.8-3.5 [Special characters omitted.] lose 75% of their mass through stellar evolution. Additionally, for the first time, we are beginning to see the effects of metallicity on the initial-final mass relationship. Finally, we are now in a position to obtain further spectroscopy of white dwarfs in the other rich clusters that we have imaged with CFHT , and, in the very near future, plan to put over 100 data points on the initial-final mass plane.

  5. Extreme abundance ratios in the polluted atmosphere of the cool white dwarf NLTT 19868

    NASA Astrophysics Data System (ADS)

    Kawka, Adela; Vennes, Stéphane

    2016-05-01

    We present an analysis of intermediate-dispersion spectra and photometric data of the newly identified cool, polluted white dwarf NLTT 19868. The spectra obtained with X-shooter on the Very Large Telescope-Melipal show strong lines of calcium, and several lines of magnesium, aluminium and iron. We use these spectra and the optical-to-near-infrared spectral energy distribution to constrain the atmospheric parameters of NLTT 19868. Our analysis shows that NLTT 19868 is iron poor with respect to aluminium and calcium. A comparison with other cool, polluted white dwarfs shows that the Fe to Ca abundance ratio (Fe/Ca) varies by up to approximately two orders of magnitudes over a narrow temperature range with NLTT 19868 at one extremum in the Fe/Ca ratio and, in contrast, NLTT 888 at the other extremum. The sample shows evidence of extreme diversity in the composition of the accreted material: in the case of NLTT 888, the inferred composition of the accreted matter is akin to iron-rich planetary core composition, while in the case of NLTT 19868 it is close to mantle composition depleted by subsequent chemical separation at the bottom of the convection zone.

  6. Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

    NASA Technical Reports Server (NTRS)

    Shipman, Harry; Barnhill, Maurice; Provencal, Judi; Roby, Scott; Bues, Irmela; Cordova, France; Hammond, Gordon; Hintzen, Paul; Koester, Detlev; Liebert, James

    1995-01-01

    Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

  7. Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

    NASA Technical Reports Server (NTRS)

    Shipman, Harry; Barnhill, Maurice; Provencal, Judi; Roby, Scott; Bues, Irmela; Cordova, France; Hammond, Gordon; Hintzen, Paul; Koester, Detlev; Liebert, James

    1995-01-01

    Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

  8. Get Ready for Gaia: Cool White Dwarfs in Common Proper Motion with Tycho Stars

    NASA Astrophysics Data System (ADS)

    Hambly, N.; Rowell, N.; Lam, M.

    2017-03-01

    We discuss the Gaia Data Release 1 (September 2016) and preliminary work on maximising the benefit for cool white dwarf (WD) science in advance of the full parallax catalogue which will appear around one year later in DR2. The Tycho catalogue is used in conjunction with the all–sky ground based astrometric/ photometric SuperCOSMOS Sky Survey in order to identify candidate faint common proper motion objects to the Tycho stars. Gaia DR1 is supplemented by the Tycho–Gaia Astrometric Solution catalogue containing some 2 million parallaxes with Hipparcos–like precision for Tycho stars. While hotter, brighter WDs are present in Tycho, cooler examples are much rarer (if present at all) and CPM offers one method to infer precision distances for a statistically useful sample of these very faint WDs.

  9. The Temperature and Cooling Age of the White Dwarf Companion to the Millisecond Pulsar PSR B1855+09.

    PubMed

    van Kerkwijk MH; Bell; Kaspi; Kulkarni

    2000-02-10

    We report on Keck and Hubble Space Telescope observations of the binary millisecond pulsar PSR B1855+09. We detect its white dwarf companion and measure mF555W=25.90+/-0.12 and mF814W=24.19+/-0.11 (Vega system). From the reddening-corrected color, (mF555W-mF814W&parr0;0=1.06+/-0.21, we infer a temperature Teff=4800+/-800 K. The white dwarf mass is known accurately from measurements of the Shapiro delay of the pulsar signal, MC=0.258+0.028-0.016 M middle dot in circle. Hence, given a cooling model, one can use the measured temperature to determine the cooling age. The main uncertainty in the cooling models for such low-mass white dwarfs is the amount of residual nuclear burning, which is set by the thickness of the hydrogen layer surrounding the helium core. From the properties of similar systems, it has been inferred that helium white dwarfs form with thick hydrogen layers, with mass greater, similar3x10-3 M middle dot in circle, which leads to significant additional heating. This is consistent with expectations from simple evolutionary models of the preceding binary evolution. For PSR B1855+09, though, such models lead to a cooling age of approximately 10 Gyr, which is twice the spin-down age of the pulsar. It could be that the spin-down age were incorrect, which would call the standard vacuum dipole braking model into question. For two other pulsar companions, however, ages well over 10 Gyr are inferred, indicating that the problem may lie with the cooling models. There is no age discrepancy for models in which the white dwarfs are formed with thinner hydrogen layers ( less, similar3x10-4 M middle dot in circle).

  10. 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.

  11. 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.

  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 LARGE STELLAR EVOLUTION DATABASE FOR POPULATION SYNTHESIS STUDIES. VI. WHITE DWARF COOLING SEQUENCES

    SciTech Connect

    Salaris, M.; Cassisi, S.; Pietrinferni, A.; Kowalski, P. M.; Isern, J. E-mail: cassisi@oa-teramo.inaf.i

    2010-06-20

    We present a new set of cooling models and isochrones for both H- and He-atmosphere white dwarfs (WDs), incorporating accurate boundary conditions from detailed model atmosphere calculations, and carbon-oxygen chemical abundance profiles based on updated stellar evolution calculations from the BaSTI stellar evolution archive-a theoretical data center for the Virtual Observatory. We discuss and quantify the uncertainties in the cooling times predicted by the models, arising from the treatment of mixing during the central H- and He-burning phases, the number of thermal pulses experienced by the progenitors, progenitor metallicity, and the {sup 12}C({alpha}, {gamma}){sup 16}O reaction rate. The largest sources of uncertainty turn out to be related to the treatment of convection during the last stages of the progenitor central He-burning phase and the {sup 12}C({alpha}, {gamma}){sup 16}O reaction rate. We compare our new models to previous calculations performed with the same stellar evolution code, and discuss their application to the estimate of the age of the solar neighborhood and the interpretation of the observed number ratios between H- and He-atmosphere WDs. The new WD sequences and an extensive set of WD isochrones that cover a large range of ages and progenitor metallicities are made publicly available at the official BaSTI Web site.

  14. HST AND OPTICAL DATA REVEAL WHITE DWARF COOLING, SPIN, AND PERIODICITIES IN GW LIBRAE 3-4 YEARS AFTER OUTBURST

    SciTech Connect

    Szkody, Paula; Mukadam, Anjum S.; Gaensicke, Boris T. E-mail: mukadam@astro.washington.edu; and others

    2012-07-10

    Since the large amplitude 2007 outburst which heated its accreting, pulsating white dwarf, the dwarf nova system GW Librae has been cooling to its quiescent temperature. Our Hubble Space Telescope ultraviolet spectra combined with ground-based optical coverage during the third and fourth year after outburst show that the fluxes and temperatures are still higher than quiescence (T = 19,700 K and 17,300 K versus 16,000 K pre-outburst for a log g = 8.7 and d = 100 pc). The K{sub wd} of 7.6 {+-} 0.8 km s{sup -1} determined from the C I {lambda}1463 absorption line, as well as the gravitational redshift implies a white dwarf mass of 0.79 {+-} 0.08 M{sub Sun }. The widths of the UV lines imply a white dwarf rotation velocity v sin i of 40 km s{sup -1} and a spin period of 209 s (for an inclination of 11 deg and a white dwarf radius of 7 Multiplication-Sign 10{sup 8} cm). Light curves produced from the UV spectra in both years show a prominent multiplet near 290 s, with higher amplitude in the UV compared to the optical, and increased amplitude in 2011 versus 2010. As the presence of this set of periods is intermittent in the optical on weekly timescales, it is unclear how this relates to the non-radial pulsations evident during quiescence.

  15. Pulsating white dwarfs

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Romero, Alejandra D.

    2017-09-01

    The Sloan Digital Sky Survey has allowed us to increase the number of known white dwarfs by a factor of five and consequently the number of known pulsating white dwarfs also by a factor of five. It has also led to the discovery of new types of variable white dwarfs, as the variable hot DQs, and the pulsating Extremely Low Mass white dwarfs. With the Kepler Mission, it has been possible to discover new phenomena, the outbursts present in a few pulsating white dwarfs.

  16. Deep HST Imaging in 47 Tuc and NGC 6397: The White Dwarf Cooling Sequence of 47 Tuc

    NASA Astrophysics Data System (ADS)

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

    2011-01-01

    In Cycle 17 we were awarded 121 orbits with HST to search for the faintest stellar populations (the coolest white dwarfs, the lowest mass main sequence stars and possibly the brown dwarfs) in 47 Tucanae. It took 10 months to secure all the data with exquisite care taken to minimize the effects of charge transfer and saturation spikes. The ACS stared at a single field for all 121 orbits but the roll angle of the telescope was varied through 180 degrees for the associated parallel fields observed with WFC3. Archival data were employed to proper motion clean the images allowing virtually complete separation of field stars and those in the background Small Magellanic Cloud from those in the cluster. In this poster, we present the resultant color-magnitude diagram for this important cluster which is a proxy for the Galactic bulge. A rich white dwarf cooling sequence is revealed which will be used to determine a cooling age for the cluster for comparison with the turnoff age (see associated poster by A. Dotter et al.). Multicolor data in other ACS filters as well as four filters with WFC3 are used to examine the spectral energy distributions of the cluster white dwarfs.

  17. New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve

    NASA Technical Reports Server (NTRS)

    Brown, T. M.; Lanz, T.; Sweigart, A. V.; Cracraft, Misty; Hubeny, Ivan; Landsman, W. B.

    2011-01-01

    Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters w Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear sub luminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).

  18. New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve

    NASA Astrophysics Data System (ADS)

    Brown, T. M.; Lanz, T.; Sweigart, A. V.; Cracraft, M.; Hubeny, I.; Landsman, W. B.

    2012-03-01

    Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters ω Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This “flash mixing” produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear subluminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).

  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. 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.

  1. Flash Mixing of the White Dwarf Cooling Curve Spectroscopic Confirmation in NGC 2808

    NASA Technical Reports Server (NTRS)

    Brown, Thomas M.; Lanz, Thierry; Sweigart, Allen V.; Cracraft, Misty; Hubeny, Ivan; Landsman, Wayne B.

    2009-01-01

    We present new HST far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of "blue-hook" stars. The blue-hook stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; the hotter temperatures and associated decrease in the hydrogen opacity shortward of the Lyman limit makes the stars brighter in the extreme UV but appear sub luminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the blue-hook stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the blue-hook stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the blue-hook and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in C, Si, and the Fe-peak elements indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 K to 50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our blue-hook sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen.

  2. Gaia photometry for white dwarfs

    NASA Astrophysics Data System (ADS)

    Carrasco, J. M.; Catalán, S.; Jordi, C.; Tremblay, P.-E.; Napiwotzki, R.; Luri, X.; Robin, A. C.; Kowalski, P. M.

    2014-05-01

    Context. White dwarfs can be used to study the structure and evolution of the Galaxy by analysing their luminosity function and initial mass function. Among them, the very cool white dwarfs provide the information for the early ages of each population. Because white dwarfs are intrinsically faint only the nearby (~ 20 pc) sample is reasonably complete. The Gaia space mission will drastically increase the sample of known white dwarfs through its 5-6 years survey of the whole sky up to magnitude V = 20-25. Aims: We provide a characterisation of Gaia photometry for white dwarfs to better prepare for the analysis of the scientific output of the mission. Transformations between some of the most common photometric systems and Gaia passbands are derived. We also give estimates of the number of white dwarfs of the different galactic populations that will be observed. Methods: Using synthetic spectral energy distributions and the most recent Gaia transmission curves, we computed colours of three different types of white dwarfs (pure hydrogen, pure helium, and mixed composition with H/He = 0.1). With these colours we derived transformations to other common photometric systems (Johnson-Cousins, Sloan Digital Sky Survey, and 2MASS). We also present numbers of white dwarfs predicted to be observed by Gaia. Results: We provide relationships and colour-colour diagrams among different photometric systems to allow the prediction and/or study of the Gaia white dwarf colours. We also include estimates of the number of sources expected in every galactic population and with a maximum parallax error. Gaia will increase the sample of known white dwarfs tenfold to about 200 000. Gaia will be able to observe thousands of very cool white dwarfs for the first time, which will greatly improve our understanding of these stars and early phases of star formation in our Galaxy. Tables 6 and 7 are available in electronic form at http://www.aanda.orgFull Tables 3-5 are available at the CDS via

  3. FLASH MIXING ON THE WHITE DWARF COOLING CURVE: SPECTROSCOPIC CONFIRMATION IN NGC 2808

    SciTech Connect

    Brown, Thomas M.; Cracraft, Misty; Lanz, Thierry; Sweigart, Allen V.; Hubeny, Ivan; Landsman, Wayne B. E-mail: cracraft@stsci.edu E-mail: allen.v.sweigart@nasa.gov E-mail: wayne.b.landsman@nasa.gov

    2012-04-01

    We present new Hubble Space Telescope far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of 'blue-hook' (BHk) stars. The BHk stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the H-R diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the extreme UV while appearing subluminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000-50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly discovered pulsating subdwarfs in {omega} Cen. In addition to the normal hot HB and BHk stars, we also obtain spectra of five blue HB stars, a post-HB star, and three unclassified

  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. 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.

  6. New model atmospheres for very cool white dwarfs with mixed H/He and pure He compositions

    NASA Technical Reports Server (NTRS)

    Bergeron, P.; Saumon, D.; Wesemael, F.

    1995-01-01

    New model atmosphere calculations for very cool white dwarfs with mixed H/He and pure He compositions are presented. The hydrogen-rich models incorporate improved cross section calculations of the collision-induced absorption by molecular hydrogen due to collisions with H2, H, and He. The effects associated with variations in the effective temperature (T(sub eff) greater than or equal to 4000 and less than or equal to 10,000 K), the surface gravity (log g greater than or equal to 7.5 and less than or equal to 9.5), and the chemical composition (N(He)/N(H) greater than or equal to O and less than or equal to 100) are investigated. Results from earlier calculations are confirmed qualitatively, but a more detailed comparison reveals large quantitative deviations. Cool white dwarfs with mixed H/He chemical compositions are shown to be easily recognizable from their predicted strong infrared flux deficiency. Pure helium model calculations are described as well. These include a modified version of the recently developed equation of state of D. Saumon and G. Chabrier. Nonideal effects brought about by various equations of state are explored in detail. For the purpose of this analysis, a model of pressure ionization based on an accurate description of the interactions in a mostly atomic helium fluid is developed. The effects of pressure ionization are shown to be the most important issue in the model calculations. A critical discussion of previous generations of pure helium model calculations is presented. Finally, broadband color indices are provided for the complete model grid.

  7. New model atmospheres for very cool white dwarfs with mixed H/He and pure He compositions

    NASA Technical Reports Server (NTRS)

    Bergeron, P.; Saumon, D.; Wesemael, F.

    1995-01-01

    New model atmosphere calculations for very cool white dwarfs with mixed H/He and pure He compositions are presented. The hydrogen-rich models incorporate improved cross section calculations of the collision-induced absorption by molecular hydrogen due to collisions with H2, H, and He. The effects associated with variations in the effective temperature (T(sub eff) greater than or equal to 4000 and less than or equal to 10,000 K), the surface gravity (log g greater than or equal to 7.5 and less than or equal to 9.5), and the chemical composition (N(He)/N(H) greater than or equal to O and less than or equal to 100) are investigated. Results from earlier calculations are confirmed qualitatively, but a more detailed comparison reveals large quantitative deviations. Cool white dwarfs with mixed H/He chemical compositions are shown to be easily recognizable from their predicted strong infrared flux deficiency. Pure helium model calculations are described as well. These include a modified version of the recently developed equation of state of D. Saumon and G. Chabrier. Nonideal effects brought about by various equations of state are explored in detail. For the purpose of this analysis, a model of pressure ionization based on an accurate description of the interactions in a mostly atomic helium fluid is developed. The effects of pressure ionization are shown to be the most important issue in the model calculations. A critical discussion of previous generations of pure helium model calculations is presented. Finally, broadband color indices are provided for the complete model grid.

  8. 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.

  9. 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.

  10. 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.

  11. Flash Mixing on the White-Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

    NASA Technical Reports Server (NTRS)

    Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Landsman, Wayne B.; Hubeny, Ivan; Fisher, Richard R. (Technical Monitor)

    2001-01-01

    We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data, obtained with the Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC 2808 harbors a significant population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our theoretical modeling indicates that the location of these subluminous stars in the CMD, as well as the high temperature gap along the HB of NGC 2808, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convective zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances that, in turn, leads to a discontinuous increase in the HB effective temperatures. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

  12. Flash Mixing on the White-Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

    NASA Technical Reports Server (NTRS)

    Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Landsman, Wayne B.; Hubeny, Ivan; Fisher, Richard R. (Technical Monitor)

    2001-01-01

    We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data, obtained with the Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC 2808 harbors a significant population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our theoretical modeling indicates that the location of these subluminous stars in the CMD, as well as the high temperature gap along the HB of NGC 2808, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convective zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances that, in turn, leads to a discontinuous increase in the HB effective temperatures. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

  13. 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.

  14. 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.

  15. White Dwarfs population as seen by Gaia

    NASA Astrophysics Data System (ADS)

    Carrasco, J. M.; Catalán, S.; Jordi, C.; Tremblay, P.-E.; Napiwotzki, R.; Luri, X.; Robin, A. C.; Kowalski, P. M.; Reylé, C.

    2013-05-01

    The launch of Gaia satellite of ESA is approaching (scheduled in 2013) and the scientific community is preparing for the maximal scientific return. As white dwarfs are very faint (especially in the very cool regime, Teff ≤ 5,000 K), they are very hard to detect and only the closest ones have been observed until now. Gaia, through its 5--6 years survey of the whole sky up to magnitude 20-25 (depending on the colour of the source), will drastically increase the sample of known white dwarfs and provide a lot of new science in this field. Using synthetic spectral energy distribution libraries and the most recent Gaia transmission curves, we derive colours of three different kinds of white dwarfs (pure hydrogen, pure helium and mixed composition with H/He=0.1). With these colours we derive transformations to other common photometric systems (Johnson-Cousins, Sloan Digital Sky Survey and 2MASS). Different relationships have to be considered for different white dwarfs compositions. Pure-He white dwarfs show an unique behaviour valid at different temperatures, but pure-H white dwarfs need to be analysed in two different temperature regimes, as their behaviour changes around Teff =5,000 K. We also compare the estimations of number of white dwarfs as predicted by the Gaia Universe Model Snapshot and by a different model of white dwarfs population (Napiwotzky's simulations). Among all white dwarfs observed, the most interesting ones will be those in the very cool regime. According to our simulations, Gaia will be able to observe thousands of them for the first time.

  16. 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.

  17. Collapsing white dwarfs

    NASA Technical Reports Server (NTRS)

    Baron, E.; Cooperstein, J.; Kahana, S.; Nomoto, K.

    1987-01-01

    The results of the hydrodynamic collapse of an accreting C + O white dwarf are presented. Collapse is induced by electron captures in the iron core behind a conductive deflagration front. The shock wave produced by the hydrodynamic bounce of the iron core stalls at about 115 km, and thus a neutron star formed in such a model would be formed as an optically quiet event.

  18. 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.

  19. 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.

  20. 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.

  1. 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

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. Analysis of cool DO-type white dwarfs from the Sloan Digital Sky Survey data release 10

    NASA Astrophysics Data System (ADS)

    Reindl, N.; Rauch, T.; Werner, K.; Kepler, S. O.; Gänsicke, B. T.; Gentile Fusillo, N. P.

    2014-12-01

    We report on the identification of 22 new cool DO-type white dwarfs (WDs) detected in data release 10 (DR10) of the Sloan Digital Sky Survey (SDSS). Among them, we found one more member of the so-called hot-wind DO WDs, which show ultrahigh excitation absorption lines. Our non-LTE model atmosphere analyses of these objects and two not previously analyzed hot-wind DO WDs, revealed effective temperatures and gravities in the ranges Teff = 45-80 kK and log g = 7.50-8.75. In eight of the spectra we found traces of C (0.001-0.01, by mass). Two of these are the coolest DO WDs ever discovered that still show a considerable amount of C in their atmospheres. This is in strong contradiction with diffusion calculations, and probably, similar to what is proposed for DB WDs, a weak mass-loss is present in DO WDs. One object is the most massive DO WD discovered so far with a mass of 1.07 M⊙ if it is an ONe-WD or 1.09 M⊙ if it is a CO-WD. We furthermore present the mass distribution of all known hot non-DA (pre-) WDs and derive the hot DA to non-DA ratio for the SDSS DR7 spectroscopic sample. The mass distribution of DO WDs beyond the wind limit strongly deviates from the mass distribution of the objects before the wind limit. We address this phenomenon by applying different evolutionary input channels. We argue that the DO WD channel may be fed by about 13% by post-extreme-horizontal branch stars and that PG 1159 stars and O(He) stars may contribute in a similar extent to the non-DA WD channel.

  7. The CI pressure shift and gravitational redshift of the cool DBQA5 white dwarf LDS678A

    NASA Technical Reports Server (NTRS)

    Sion, E. M.; Vauclair, G.; Oswalt, T. D.; Hammond, G.; Liebert, J.; Koester, D.; Wegner, G.; Marcum, P.

    1990-01-01

    A high resolution ultraviolet spectrum of the helium rich degenerate LDS 678A, obtained with the International Ultraviolet Explorer (IUE) satellite is presented. LDS 678A is the coolest metallic line generate (DQ or DZ) yet observed with the IUE scale. These observations provide a detailed line profile of the strong C I 2479 absorption line with equivalent width (W sub 2479 = 2.35 plus or minus 0.06 angstroms) from which theoretical line profile fits yield a C abundance (log C/He = 6.4). The presence of carbon in a helium rich atmosphere lends credence to the notion that LDS 678A is a transitional case between the DB white dwarfs with nearly pure helium atmospheres and the helium rich DQ white dwarfs which exhibit carbon bands. Corrected for an inferred pressure shift for the C I line, a gravitational redshift is deduced from which a most probable mass of 0.55 solar mass is derived.

  8. 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

  9. 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

  10. 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.

  11. 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.

  12. 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.

  13. Throwing Icebergs at White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2017-08-01

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

  14. Imaging planets around white dwarfs

    NASA Astrophysics Data System (ADS)

    Burleigh, Matt; Clarke, Fraser; Hodgkin, Simon

    White dwarfs should retain planetary systems in wide orbits (>≅5AU). Evolutionary models for Jovian planets show that infra-red imaging of suitable nearby white dwarfs should allow us to resolve and detect companions >≅5 Mtiny JUP. We have instigated programmes with both the 8m Gemini North (using NIRI), Gemini South (using Flamingos) and with the NAOMI Adaptive Optics system on the 4.2m William Herschel Telescope to search for such objects, which will share the large proper motions of their white dwarf hosts.

  15. 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).

  16. 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).

  17. 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.

  18. 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.

  19. 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.

  20. Rapid Rotation of a Heavy White Dwarf

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2017-05-01

    New Kepler observations of a pulsating white dwarf have revealed clues about the rotation of intermediate-mass stars.Learning About ProgenitorsStars weighing in at under 8 solar masses generally end their lives as slowly cooling white dwarfs. By studying the rotation of white dwarfs, therefore, we are able to learn about the final stages of angular momentum evolution in these progenitor stars.Most isolated field white dwarfs cluster in mass around 0.62 solar masses, which corresponds to a progenitor mass of around 2.2 solar masses. This abundance means that weve already learned a good deal about the final rotation of low-mass (13 solar-mass) stars. Our knowledge about the angular momentum of intermediate-mass (38 solar-mass) stars, on the other hand, remains fairly limited.Fourier transform of the pulsations from SDSSJ0837+1856. The six frequencies of stellar variability, marked with red dots, reveal a rotation period of 1.13 hours. [Hermes et al. 2017]Record-Breaking FindA newly discovered white dwarf, SDSSJ0837+1856, is now helping to shed light on this mass range. SDSSJ0837+1856 appears to be unusually massive: its measured at 0.87 solar masses, which corresponds to a progenitor mass of roughly 4.0 solar masses. Determining the rotation of this white dwarf would therefore tell us about the final stages of angular momentum in an intermediate-mass star.In a new study led by J.J. Hermes (Hubble Fellow at University of North Carolina, Chapel Hill), a team of scientists presents a series of measurements of SDSSJ0837+1856 that suggest its the highest-mass and fastest-rotating isolated pulsating white dwarf known.Histogram of rotation rates determined from the asteroseismology of pulsating white dwarfs (marked in red). SDSSJ0837+1856 (indicated in black) is more massive and rotates faster than any other known pulsating white dwarf. [Hermes et al. 2017]Rotation from PulsationsWhy pulsating? In the absence of measurable spots and other surface features, the way we

  1. White dwarf kinematics versus mass

    NASA Astrophysics Data System (ADS)

    Wegg, Christopher; Phinney, E. Sterl

    2012-10-01

    We investigated the relationship between the kinematics and mass of young (<3 × 108 yr) white dwarfs using proper motions. Our sample is taken from the colour-selected catalogues of the Sloan Digital Sky Survey and the Palomar-Green Survey, both of which have spectroscopic temperature and gravity determinations. We find that the dispersion decreases with increasing white dwarf mass. This can be explained as a result of less scattering by objects in the Galactic disc during the shorter lifetime of their more massive progenitors. A direct result of this is that white dwarfs with high mass have a reduced scale height, and hence their local density is enhanced over their less massive counterparts. In addition, we have investigated whether the kinematics of the highest mass white dwarfs (>0.95 M⊙) are consistent with the expected relative contributions of single star evolution and mergers. We find that the kinematics are consistent with the majority of high-mass white dwarfs being formed through single star evolution.

  2. The True Incidence of Magnetism Among Field White Dwarfs

    NASA Astrophysics Data System (ADS)

    Liebert, James; Bergeron, P.; Holberg, J. B.

    2003-01-01

    We study the incidence of magnetism in white dwarfs from three large and well-observed samples of hot, cool, and nearby white dwarfs in order to test whether the fraction of magnetic degenerates is biased and whether it varies with effective temperature, cooling age, or distance. The magnetic fraction is considerably higher for the cool sample of Bergeron, Ruiz, & Leggett and the Holberg, Oswalt, & Sion sample of local white dwarfs than it is for the generally hotter white dwarfs of the Palomar-Green survey. We show that the mean mass of magnetic white dwarfs in this survey is 0.93 Msolar or more, so there may be a strong bias against their selection in the magnitude-limited Palomar-Green survey. We argue that this bias is not as important in the samples of cool and nearby white dwarfs. However, this bias may not account for all of the difference in the magnetic fractions of these samples. It is not clear that the magnetic white dwarfs in the cool and local samples are drawn from the same population as the hotter PG stars. In particular, two or three of the cool sample are low-mass white dwarfs in unresolved binary systems. Moreover, there is a suggestion from the local sample that the fractional incidence may increase with decreasing temperature, luminosity, and/or cooling age. Overall, the true incidence of magnetism at the level of ~2 MG or greater is at least ~10%, and it could be higher. Limited studies capable of detecting lower field strengths down to ~10 kG suggest by implication that the total fraction may be substantially higher then 10%.

  3. 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

  4. 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.

  5. 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.

  6. NUCLEAR CONDENSATE AND HELIUM WHITE DWARFS

    SciTech Connect

    Bedaque, Paulo F.; Berkowitz, Evan; Cherman, Aleksey E-mail: evanb@umd.edu

    2012-04-10

    We consider a high-density region of the helium phase diagram, where the nuclei form a Bose-Einstein condensate rather than a classical plasma or a crystal. Helium in this phase may be present in helium-core white dwarfs. We show that in this regime there is a new gapless quasiparticle not previously noticed, arising when the constraints imposed by gauge symmetry are taken into account. The contribution of this quasiparticle to the specific heat of a white dwarf core turns out to be comparable in a range of temperatures to the contribution from the particle-hole excitations of the degenerate electrons. The specific heat in the condensed phase is two orders of magnitude smaller than in the uncondensed plasma phase, which is the ground state at higher temperatures, and four orders of magnitude smaller than the specific heat that an ion lattice would provide, if formed. Since the specific heat of the core is an important input for setting the rate of cooling of a white dwarf star, it may turn out that such a change in the thermal properties of the cores of helium white dwarfs has observable implications.

  7. 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.

  8. 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.

  9. 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.

  10. 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)

  11. 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)

  12. 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.

  13. Hubble Space Telescope observations of the Kepler-field cluster NGC 6819 - I. The bottom of the white dwarf cooling sequence

    NASA Astrophysics Data System (ADS)

    Bedin, L. R.; Salaris, M.; Anderson, J.; Cassisi, S.; Milone, A. P.; Piotto, G.; King, I. R.; Bergeron, P.

    2015-04-01

    We use Hubble Space Telescope (HST) to reach the end of the white dwarf (WD) cooling sequence (CS) in the solar-metallicity open cluster NGC 6819. Our photometry and completeness tests show a sharp drop in the number of WDs along the CS at magnitudes fainter than mF606W = 26.050 ± 0.075. This implies an age of 2.25 ± 0.20 Gyr, consistent with the age of 2.25 ± 0.30 Gyr obtained from fits to the main-sequence turn-off. The use of different WD cooling models and initial-final-mass relations have a minor impact the WD age estimate, at the level of ˜0.1 Gyr. As an important by-product of this investigation we also release, in electronic format, both the catalogue of all the detected sources and the atlases of the region (in two filters). Indeed, this patch of sky studied by HST (of size ˜70 arcmin2) is entirely within the main Kepler-mission field, so the high-resolution images and deep catalogues will be particularly useful.

  14. 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.

  15. Helium runaways in white dwarfs

    NASA Technical Reports Server (NTRS)

    Taam, R. E.

    1979-01-01

    The long term evolution of an accreting carbon white dwarf was studied from the onset of accretion to the ignition of helium. The variations in the details of the helium shell flash examined with respect to variations in mass accretion rate. For intermediate rates the helium flash is potentially explosive whereas for high rates the shell flash is relatively weak. The results are discussed in the context of the long term evolution of novae.

  16. Project 1640 observations of the white dwarf HD 114174 B

    NASA Astrophysics Data System (ADS)

    Bacchus, E.; Parry, I. R.; Oppenheimer, R.; Aguilar, J.; Beichman, C.; Brenner, D.; Burruss, R.; Cady, E.; Luszcz-Cook, S.; Crepp, J.; Dekany, R.; Gianninas, A.; Hillenbrand, L.; Kilic, M.; King, D.; Lockhart, T. G.; Matthews, C. T.; Nilsson, R.; Pueyo, L.; Rice, E. L.; Roberts, L. C.; Sivaramakrishnan, A.; Soummer, R.; Vasisht, G.; Veicht, A.; Zhai, C.; Zimmerman, N. T.

    2017-08-01

    We present the first near infrared spectrum of the faint white dwarf companion HD 114174 B, obtained with Project 1640. Our spectrum, covering the Y, J and H bands, combined with previous TaRgetting bENchmark-objects with Doppler Spectroscopy (TRENDS) photometry measurements, allows us to place further constraints on this companion. We suggest two possible scenarios; either this object is an old, low-mass, cool H atmosphere white dwarf with Teff ˜ 3800 K or a high-mass white dwarf with Teff > 6000 K, potentially with an associated cool (Teff ˜ 700 K) brown dwarf or debris disc resulting in an infrared excess in the L΄ band. We also provide an additional astrometry point for 2014 June 12 and use the modelled companion mass combined with the radial velocity and direct imaging data to place constraints on the orbital parameters for this companion.

  17. 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

  18. 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.

  19. 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.

  20. 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.

  1. 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

  2. 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

  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. 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. Effects of magnetic fields in white dwarfs

    NASA Astrophysics Data System (ADS)

    Franzon, Bruno; Schramm, Stefan

    2017-06-01

    We perform calculations of white dwarfs endowed with strong magnetic fields. White dwarfs are the progenitors of supernova Type Ia explosions and they are widely used as candles to show that the Universe is expanding and accelerating. However, observations of ultraluminous supernovae have suggested that the progenitor of such an explosion should be a white dwarf with mass above the well-known Chandrasekhar limit ~ 1.4 M⊙. In corroboration with other works, but by using a fully general relativistic framework, we obtained also strongly magnetized white dwarfs with masses M ~ 2.0 M⊙.

  8. White dwarfs and the interstellar medium

    NASA Technical Reports Server (NTRS)

    Dupree, A. K.; Raymond, J. C.

    1983-01-01

    Radiation emanating from hot (T greater than 40,000 K) white dwarfs can create large volumes of ionized material containing substantial column densities of highly ionized species, in particular Si IV and C IV. The ions N V and O VI can also be produced by hot, hydrogen-rich white dwarfs. These ionization spheres may be detectable around the nearby dwarfs. The relatively high space motions of these stars coupled with long recombination times in the interstellar medium suggest that a white dwarf leaves a region of ionized material - a fossil Stroemgren trail - that marks its progress through the galaxy. White dwarfs create a patchy substrate of ionized gas in the galactic plane and lead to extended ionized regions out of the plane. The spatial frequency of hot white dwarfs indicates that they contribute a radiative energy comparable to that provided by nondegenerate stars and by supernovae and capable of affecting the ionization balance of the interstellar medium.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. Helium runaways in white dwarfs

    NASA Technical Reports Server (NTRS)

    Taam, R. E.

    1980-01-01

    The long-term evolution of a carbon-accreting white dwarf (M = 0.5 solar mass) has been studied from the onset of accretion to the ignition of helium. The variations in the details of the helium-shell flash have been examined with respect to variations in mass accretion rate. For intermediate rates (10 to the -9th to 10 to the -8th solar mass/yr) the helium flash is potentially explosive, whereas for high rates (5 x 10 to the -8th solar mass/yr) the shell flash is relatively weak. The results are discussed in the context of the long-term evolution of novae.

  14. Theories of white dwarf oscillations

    NASA Technical Reports Server (NTRS)

    Vanhorn, H. M.

    1980-01-01

    The current status of theoretical understanding of the oscillations observed in the ZZ Ceti stars and cataclysmic variables is briefly reviewed. Nonradial g-mode oscillations appear to provide a satisfactory explanation for the low amplitude variables such as R548, with periods in the range of approximately 200 to 300 seconds, but for the longer period (800 to 1000 seconds) oscillators, the situation is still unclear. Rotation may play an important role in this problem, and the effects of both slow and fast rotation upon the mode structure are discussed. In the cataclysmic variables, both accretion and thermonuclear burning may act to excite oscillations of the white dwarf.

  15. A Multiwavelength Study of Nearby Millisecond Pulsar PSR J1400‑1431: Improved Astrometry and an Optical Detection of Its Cool White Dwarf Companion

    NASA Astrophysics Data System (ADS)

    Swiggum, J. K.; Kaplan, D. L.; McLaughlin, M. A.; Lorimer, D. R.; Bogdanov, S.; Ray, P. S.; Lynch, R.; Gentile, P.; Rosen, R.; Heatherly, S. A.; Barlow, B. N.; Hegedus, R. J.; Vasquez Soto, A.; Clancy, P.; Kondratiev, V. I.; Stovall, K.; Istrate, A.; Penprase, B.; Bellm, E. C.

    2017-09-01

    In 2012, five high-school students involved in the Pulsar Search Collaboratory discovered the millisecond pulsar (MSP) PSR J1400‑1431, and initial timing parameters were published in Rosen et al. a year later. Since then, we have obtained a phase-connected timing solution spanning five years, resolving a significant position discrepancy and measuring \\dot{P}, proper motion, parallax, and a monotonic slope in dispersion measure over time. Due to PSR J1400‑1431’s proximity and significant proper motion, we use the Shklovskii effect and other priors to determine a 95% confidence interval for PSR J1400‑1431’s distance, d={270}-80+130 pc. With an improved timing position, we present the first detection of the pulsar’s low-mass white dwarf (WD) companion using the Goodman Spectrograph on the 4.1 m SOAR telescope. Deeper imaging suggests that it is a cool DA-type WD with {T}{eff}=3000+/- 100 K and R/{R}ȯ =(2.19+/- 0.03)× {10}-2 (d/270 {pc}). We show a convincing association between PSR J1400‑1431 and a γ-ray point source, 3FGL J1400.5‑1437, but only weak (3.3σ) evidence of pulsations after folding γ-ray photons using our radio timing model. We detect an X-ray counterpart with XMM-Newton, but the measured X-ray luminosity (1×1029 erg s‑1) makes PSR J1400‑1431 the least X-ray luminous rotation-powered MSP detected to date. Together, our findings present a consistent picture of a nearby (d≈ 230 pc) MSP in a 9.5-day orbit around a cool ∼0.3 M ⊙ WD companion, with orbital inclination i≳ 60^\\circ .

  16. Chandra Observations of Magnetic White Dwarfs and Their Theoretical Implications

    NASA Technical Reports Server (NTRS)

    Musielak, Z. E.; Noble, M.; Porter, J. G.; Winget, D. E.; Six, N. Frank (Technical Monitor)

    2002-01-01

    Observations of cool DA and DB white dwarfs have not yet been successful in detecting coronal X-ray emission but observations of late-type dwarfs and giants show that coronae are common for these stars. To produce coronal X-rays, a star must have dynamo-generated surface magnetic fields and a well-developed convection zone. There is strong observational evidence that the DA star LHS 1038 and the DB star GD 358 have weak and variable surface magnetic fields. Since these fields are likely to be generated by dynamo action and since both stars have well-developed convection zones, theory predicts detectable levels of coronal X-rays from these white dwarfs. However, we present analysis of Chandra observations of both stars showing no detectable X-ray emission. The derived upper limits for the X-ray fluxes provide strong constraints on theories of formation of coronae around magnetic white dwarfs.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  1. 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. Copyright © 2016, American Association for the Advancement of Science.

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. Throwing Icebergs at White Dwarfs

    NASA Astrophysics Data System (ADS)

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

    2017-08-01

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

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. Imaging Extrasolar Planets Around Nearby White Dwarfs

    NASA Astrophysics Data System (ADS)

    Burleigh, M.; Clarke, F.; Hodgkin, S.

    White dwarfs should retain planetary systems in wide orbits (greater than about 5 AU). Evolutionary models for jovian planets show that infrared imaging of suitable nearby white dwarfs should allow us to resolve and detect companions of mass greater than about 5 Jupiter masses. We have instigated programs with both the 8m Gemini North (using NIRI), Gemini South (using Flamingos) and with the NAOMI Adaptive Optics system on the 4.2m William Herschel Telescope to search for such objects, which will share the large proper motions of their white dwarf hosts.

  12. Imaging planets around nearby white dwarfs

    NASA Astrophysics Data System (ADS)

    Burleigh, M. R.; Clarke, F. J.; Hodgkin, S. T.

    2002-04-01

    We suggest that Jovian planets will survive the late stages of stellar evolution, and that white dwarfs will retain planetary systems in wide orbits (>~5au). Utilizing evolutionary models for Jovian planets, we show that infrared imaging with 8-m class telescopes of suitable nearby white dwarfs should allow us to resolve and detect companions >~3M JUP . Detection of massive planetary companions to nearby white dwarfs would prove that such objects can survive the final stages of stellar evolution, place constraints on the frequency of main-sequence stars with planetary systems dynamically similar to our own and allow direct spectroscopic investigation of their composition and structure.

  13. WHITE DWARF-RED DWARF SYSTEMS RESOLVED WITH THE HUBBLE SPACE TELESCOPE. II. FULL SNAPSHOT SURVEY RESULTS

    SciTech Connect

    Farihi, J.; Hoard, D. W.; Wachter, S.

    2010-10-15

    Results are presented for a Hubble Space Telescope Advanced Camera for Surveys high-resolution imaging campaign of 90 white dwarfs with known or suspected low-mass stellar and substellar companions. Of the 72 targets that remain candidate and confirmed white dwarfs with near-infrared excess, 43 are spatially resolved into two or more components, and a total of 12 systems are potentially triples. For 68 systems where a comparison is possible, 50% have significant photometric distance mismatches between their white dwarf and M dwarf components, suggesting that white dwarf parameters derived spectroscopically are often biased due to the cool companion. Interestingly, 9 of the 30 binaries known to have emission lines are found to be visual pairs and hence widely separated, indicating an intrinsically active cool star and not irradiation from the white dwarf. There is a possible, slight deficit of earlier spectral types (bluer colors) among the spatially unresolved companions, exactly the opposite of expectations if significant mass is transferred to the companion during the common envelope phase. Using the best available distance estimates, the low-mass companions to white dwarfs exhibit a bimodal distribution in projected separation. This result supports the hypothesis that during the giant phases of the white dwarf progenitor, any unevolved companions either migrate inward to short periods of hours to days, or outward to periods of hundreds to thousands of years. No intermediate projected separations of a few to several AU are found among these pairs. However, a few double M dwarfs (within triples) are spatially resolved in this range, empirically demonstrating that such separations were readily detectable among the binaries with white dwarfs. A straightforward and testable prediction emerges: all spatially unresolved, low-mass stellar and substellar companions to white dwarfs should be in short-period orbits. This result has implications for substellar companion and

  14. 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.

  15. New Ultracool and Halo White Dwarf Candidates in SDSS Stripe 82

    DTIC Science & Technology

    2007-01-01

    identified a sample of ∼6000 cool white dwarfs from SDSS Data Release 3 ( DR3 ) using reduced proper motions (RPMs), based on SDSS and USNO-B combined data...sample of 33 halo white dwarf candi- dates from their study employing the SDSS DR3 and USNO-B cat- alogues. The combination of their larger sky

  16. 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.

  17. 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)

  18. 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.

  19. 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.

  20. 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.

  1. WHITE DWARFS IN LOCAL STAR STREAMS

    SciTech Connect

    Fuchs, Burkhard; Dettbarn, Christian

    2011-01-15

    We have studied the fine structure of the phase space distribution of white dwarfs in the solar neighborhood. White dwarfs have kinematics that are typical for the stellar population of the old thin disk of the Milky Way. Using a projection of the space velocities of stars onto vertical angular momentum components and eccentricities of the stellar orbits we demonstrate that stellar streams can be identified in the phase space distribution of the white dwarfs. These correspond to the well-known Sirius, Pleiades, and Hercules star streams. Membership of white dwarfs, which represent the oldest population in the Galaxy, in these streams lends support to the interpretation that the streams owe their existence to dynamical resonance effects of the stars with Galactic spiral arms or the Galactic bar, because these indiscriminately affect all stellar populations.

  2. 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)

  3. 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.

  4. 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."

  5. UBV photometry of hot DA white dwarfs

    NASA Technical Reports Server (NTRS)

    Kidder, K. M.; Holberg, J. B.; Mason, Paul A.

    1991-01-01

    Johnson UBV photometry has been obtained photoelectrically for a set of DA white dwarfs with effective temperatures greater than 20,000 K and for the AM Her type binary HO538 + 608. Most of the white dwarfs lie within existing Einstein IPC or EXOSAT LE soft X-ray fields, therefore they are of interest as potential serendipitous soft X-ray sources. In addition, high dispersion spectroscopy has been used to differentiate seven of these objects to be subdwarfs.

  6. Hot DQ white dwarfs: a pulsational test of the mixing scenario for their formation

    NASA Astrophysics Data System (ADS)

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

    2010-11-01

    Hot DQ white dwarfs constitute a new class of white dwarf stars, uncovered recently within the framework of SDSS project. There exist nine of them, out of a total of several thousands white dwarfs spectroscopically identified. Recently, three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. In this contribution, we presented the results of a non-adiabatic pulsation analysis of the recently discovered carbon-rich hot DQ white dwarf stars. Our study relies on the full evolutionary models of hot DQ white dwarfs recently developed by Althaus et al. (2009), that consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Specifically, we performed a stability analysis on white dwarf models from stages before the blue edge of the DBV instability strip (Teff ≈ 30000 K) until the domain of the hot DQ white dwarfs (18000-24000 K), including the transition DB→hot DQ white dwarf. We explore evolutionary models with M*= 0.585M⊙ and M* = 0.87M⊙, and two values of thickness of the He-rich envelope (MHe = 2 × 10-7M* and MHe = 10-8M*).

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. The brightest pure-H ultracool white dwarf

    NASA Astrophysics Data System (ADS)

    Catalán, S.; Tremblay, P.-E.; Pinfield, D. J.; Smith, L. C.; Zhang, Z. H.; Napiwotzki, R.; Marocco, F.; Day-Jones, A. C.; Gomes, J.; Forde, K. P.; Lucas, P. W.; Jones, H. R. A.

    2012-10-01

    We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high reduced proper motion value in the J band (HJ = 21.87). We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low contamination. LSR J0745+2627 is also detected in the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer (WISE). We have spectroscopically confirmed this object as a cool white dwarf using X-Shooter on the Very Large Telescope. A detailed analysis of its spectral energy distribution reveals that its atmosphere is compatible with a pure-H composition model with an effective temperature of 3880 ± 90 K. This object is the brightest pure-H ultracool white dwarf (Teff < 4000 K) ever identified. We have constrained the distance (24-45 pc), space velocities and age considering different surface gravities. The results obtained suggest that LSR J0745+2627 belongs to the thick-disk/halo population and is also one of the closest ultracool white dwarfs. Based on observations made with ESO telescopes at the Paranal Observatory under programme ID 088.C-0048(B).FITS version of the reduced spectrum is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/546/L3

  13. 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.

  14. 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.

  15. 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

  16. 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.

  17. UV observations of the cool DBQA5 white dwarf LDS 678A - Limits on the atmospheric composition, pressure shift, and gravitational redshift derived from C I 2479

    NASA Technical Reports Server (NTRS)

    Oswalt, Terry D.; Sion, Edward M.; Hammond, Gordon; Vauclair, Gerard; Liebert, James W.

    1991-01-01

    A high-resolution ultraviolet spectrum of the helium-rich degenerate LDS 678A, obtained with the International Ultraviolet Explorer (IUE) satellite, is presented. LDS 678A is the coolest metallic line degenerate (DQ or DZ) yet observed with the IUE echelle. These observations provide a detailed line profile of the strong C I 2479 absorption line and equivalent width W2479 = 2.35 + or - 0.06 A from which theoretical profile fits yield a carbon abundance of log C/He = (-6.7 + or- 0.2). The presence of carbon in a He-rich atmosphere lends credence to the notion that LDS 678A is a transitional case between the DB white dwarfs with nearly pure helium atmospheres and the helium-rich DQ white dwarfs which exhibit carbon bands. Corrected for an inferred pressure shift Vp = + 38 + or - 4 km/s for the C I 2479 line, a gravitational redshift of Vrs = + 26 + or - 13 km/s is deduced from which a most probable mass of 0.55 solar mass is derived.

  18. UV observations of the cool DBQA5 white dwarf LDS 678A - Limits on the atmospheric composition, pressure shift, and gravitational redshift derived from C I 2479

    NASA Technical Reports Server (NTRS)

    Oswalt, Terry D.; Sion, Edward M.; Hammond, Gordon; Vauclair, Gerard; Liebert, James W.

    1991-01-01

    A high-resolution ultraviolet spectrum of the helium-rich degenerate LDS 678A, obtained with the International Ultraviolet Explorer (IUE) satellite, is presented. LDS 678A is the coolest metallic line degenerate (DQ or DZ) yet observed with the IUE echelle. These observations provide a detailed line profile of the strong C I 2479 absorption line and equivalent width W2479 = 2.35 + or - 0.06 A from which theoretical profile fits yield a carbon abundance of log C/He = (-6.7 + or- 0.2). The presence of carbon in a He-rich atmosphere lends credence to the notion that LDS 678A is a transitional case between the DB white dwarfs with nearly pure helium atmospheres and the helium-rich DQ white dwarfs which exhibit carbon bands. Corrected for an inferred pressure shift Vp = + 38 + or - 4 km/s for the C I 2479 line, a gravitational redshift of Vrs = + 26 + or - 13 km/s is deduced from which a most probable mass of 0.55 solar mass is derived.

  19. 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.

  20. 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.

  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. 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).

  3. FIRST DIRECT EVIDENCE THAT BARIUM DWARFS HAVE WHITE DWARF COMPANIONS

    SciTech Connect

    Gray, R. O.; McGahee, C. E.; Griffin, R. E. M.; Corbally, C. J. E-mail: cmcgahe@g.clemson.edu E-mail: corbally@as.arizona.edu

    2011-05-15

    Barium II (Ba) stars are chemically peculiar F-, G-, and K-type objects that show enhanced abundances of s-process elements. Since s-process nucleosynthesis is unlikely to take place in stars prior to the advanced asymptotic giant branch (AGB) stage, the prevailing hypothesis is that each present Ba star was contaminated by an AGB companion which is now a white dwarf (WD). Unless the initial mass ratio of such a binary was fairly close to unity, the receiving star is thus at least as likely to be a dwarf as a giant. So although most known Ba stars appear to be giants, the hypothesis requires that Ba dwarfs be comparably plentiful and moreover that they should all have WD companions. However, despite dedicated searches with the IUE satellite, no WD companions have been directly detected to date among the classical Ba dwarfs, even though some 90% of those stars are spectroscopic binaries, so the contamination hypothesis is therefore presently in some jeopardy. In this paper, we analyze recent deep, near-UV and far-UV Galaxy Evolution Explorer (GALEX) exposures of four of the brightest of the class (HD 2454, 15360, 26367, and 221531), together with archived GALEX data for two newly recognized Ba dwarfs: HD 34654 and HD 114520 (which also prove to be spectroscopic binaries). The GALEX observations of the Ba dwarfs as a group show a significant far-UV excess compared to a control sample of normal F-type dwarfs. We suggest that this ensemble far-UV excess constitutes the first direct evidence that Ba dwarfs have WD companions.

  4. 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.

  5. 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.

  6. 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

  7. 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.

  8. 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.

  9. THE LINK BETWEEN PLANETARY SYSTEMS, DUSTY WHITE DWARFS, AND METAL-POLLUTED WHITE DWARFS

    SciTech Connect

    Debes, John H.; Walsh, Kevin J.; Stark, Christopher

    2012-03-10

    It has long been suspected that metal-polluted white dwarfs (types DAZ, DBZ, and DZ) and white dwarfs with dusty disks possess planetary systems, but a specific physical mechanism by which planetesimals are perturbed close to a white dwarf has not yet been fully posited. In this paper, we demonstrate that mass loss from a central star during post-main-sequence evolution can sweep planetesimals into interior mean motion resonances with a single giant planet. These planetesimals are slowly removed through chaotic excursions of eccentricity that in time create radial orbits capable of tidally disrupting the planetesimal. Numerical N-body simulations of the solar system show that a sufficient number of planetesimals are perturbed to explain white dwarfs with both dust and metal pollution, provided other white dwarfs have more massive relic asteroid belts. Our scenario requires only one Jupiter-sized planet and a sufficient number of asteroids near its 2:1 interior mean motion resonance. Finally, we show that once a planetesimal is perturbed into a tidal crossing orbit, it will become disrupted after the first pass of the white dwarf, where a highly eccentric stream of debris forms the main reservoir for dust-producing collisions. These simulations, in concert with observations of white dwarfs, place interesting limits on the frequency of planetary systems around main-sequence stars, the frequency of planetesimal belts, and the probability that dust may obscure future terrestrial planet finding missions.

  10. 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.

  11. 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.

  12. 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

  13. General relativistic calculations for white dwarfs

    NASA Astrophysics Data System (ADS)

    Mathew, Arun; Nandy, Malay K.

    2017-05-01

    The mass-radius relations for white dwarfs are investigated by solving the Newtonian as well as Tolman-Oppenheimer-Volkoff (TOV) equations for hydrostatic equilibrium assuming the electron gas to be non-interacting. We find that the Newtonian limiting mass of 1.4562{M}⊙ is modified to 1.4166{M}⊙ in the general relativistic case for {}_2^4{{He}} (and {}_612{{C}}) white dwarfs. Using the same general relativistic treatment, the critical mass for {}2656{{Fe}} white dwarfs is obtained as 1.2230{M}⊙ . In addition, departure from the ideal degenerate equation of state (EoS) is accounted for by considering Salpeter’s EoS along with the TOV equation, yielding slightly lower values for the critical masses, namely 1.4081{M}⊙ for {}_2^4{{He}}, 1.3916{M}⊙ for {}_612{{C}} and 1.1565{M}⊙ for {}2656{{Fe}} white dwarfs. We also compare the critical densities for gravitational instability with the neutronization threshold densities to find that {}_2^4{{He}} and {}_612{{C}} white dwarfs are stable against neutronization with the critical values of 1.4081{M}⊙ and 1.3916{M}⊙ , respectively. However, the critical masses for {}_816{{O}}, {}1020{{Ne}}, {}1224{{Mg}}, {}1428{{Si}}, {}1632{{S}} and {}2656{{Fe}} white dwarfs are lower due to neutronization. Corresponding to their central densities for neutronization thresholds, we obtain their maximum stable masses due to neutronization by solving the TOV equation coupled with the Salpeter EoS.

  14. 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

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  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. 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. Chandra Observations of Magnetic White Dwarfs and their Theoretical Implications

    NASA Technical Reports Server (NTRS)

    Musielak, Z. E.; Noble, M.; Porter, J. G.; Winget, D. E.

    2003-01-01

    Observations of cool DA and DB white dwarfs have not yet been successful in detecting coronal X-ray emission, but observations of late-type dwarfs and giants show that coronae are common for these stars. To produce coronal X-rays, a star must have dynamo-generated surface magnetic fields and a well-developed convection zone. There is some observational evidence that the DA star LHS 1038 and the DB star GD 358 have weak and variable surface magnetic fields. It has been suggested that such fields can be generated by dynamo action, and since both stars have well-developed convection zones, theory predicts detectable levels of coronal X-rays from these white dwarfs. However, we present analysis of Chandra observations of both stars showing no detectable X-ray emission. The derived upper limits for the X-ray fluxes provide strong constraints on theories of formation of coronae around magnetic white dwarfs. Another important implication of our negative Chandra observations is the possibility that the magnetic fields of LHS 1038 and GD 358 are fossil fields.

  4. 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.

  5. 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 ...

  6. 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.

  7. 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.

  8. 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.

  9. 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

  10. A Very Cool Pair of Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    2011-03-01

    Observations with the European Southern Observatory's Very Large Telescope, along with two other telescopes, have shown that there is a new candidate for the coldest known star: a brown dwarf in a double system with about the same temperature as a freshly made cup of tea - hot in human terms, but extraordinarily cold for the surface of a star. This object is cool enough to begin crossing the blurred line dividing small cold stars from big hot planets. Brown dwarfs are essentially failed stars: they lack enough mass for gravity to trigger the nuclear reactions that make stars shine. The newly discovered brown dwarf, identified as CFBDSIR 1458+10B, is the dimmer member of a binary brown dwarf system located just 75 light-years from Earth [1]. The powerful X-shooter spectrograph on ESO's Very Large Telescope (VLT) was used to show that the composite object was very cool by brown dwarf standards. "We were very excited to see that this object had such a low temperature, but we couldn't have guessed that it would turn out to be a double system and have an even more interesting, even colder component," said Philippe Delorme of the Institut de planétologie et d'astrophysique de Grenoble (CNRS/Université Joseph Fourier), a co-author of the paper. CFBDSIR 1458+10 is the coolest brown dwarf binary found to date. The dimmer of the two dwarfs has now been found to have a temperature of about 100 degrees Celsius - the boiling point of water, and not much different from the temperature inside a sauna [2]. "At such temperatures we expect the brown dwarf to have properties that are different from previously known brown dwarfs and much closer to those of giant exoplanets - it could even have water clouds in its atmosphere," said Michael Liu of the University of Hawaii's Institute for Astronomy, who is lead author of the paper describing this new work. "In fact, once we start taking images of gas-giant planets around Sun-like stars in the near future, I expect that many of them

  11. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2009-11-01

    Context: Hot DQ white dwarfs are a new class of white dwarf stars that were discovered recently within the framework of the SDSS project. There are nine known hot DQ stars, out of a total of several thousands white dwarfs spectroscopically identified. Three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. Aims: We present a nonadiabatic pulsation analysis of carbon-rich hot DQ white dwarf stars. One of our main aims is to test the convective-mixing scenario for the origin of hot DQs by studying their pulsational properties. Methods: Our pulsation study is based on the full evolutionary models of hot DQ white dwarfs developed by Althaus and collaborators, which consistently cover the entire evolution from the born-again stage to the white dwarf cooling track. Specifically, we present a stability analysis of white dwarf models from stages before the blue edge of the DBV instability strip (Teff ≈ 30 000 K), until the domain of the hot DQ white dwarfs (18 000-24 000 K), including the transition DB→hot DQ white dwarf. We explore evolutionary models with M* = 0.585 M⊙ and M* = 0.87 M⊙, and two values of the thickness of the He-rich envelope (MHe = 2×10-7 M* and MHe = 10-8 M_*). These envelopes are 4-5 orders of magnitude thinner than those of standard DB white dwarf models resulting from canonical stellar evolution computations. Results: We found that at evolutionary phases in which the models are characterized by He-dominated atmospheres, they exhibit unstable g-mode pulsations typical of DBV stars, and when the models become DQ white dwarfs with carbon-dominated atmospheres, they continue being pulsationally unstable with characteristics similar to DB models, and in agreement with the periods detected in variable hot DQ white dwarfs. In particular, for models with MHe = 10-8 M_*, a narrow gap exists separating the DB from the DQ instability domains. Conclusions: Our calculations provide

  14. 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

  15. 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

  16. Parallax measurements of cool brown dwarfs

    NASA Astrophysics Data System (ADS)

    Manjavacas, E.; Goldman, B.; Reffert, S.; Henning, T.

    2013-12-01

    Context. Accurate parallax measurements allow us to determine physical properties of brown dwarfs and help us constrain evolutionary and atmospheric models, break age-mass degeneracy, and reveal unresolved binaries. Aims: We measured absolute trigonometric parallaxes and proper motions of six cool brown dwarfs using background galaxies to establish an absolute reference frame. We derive the absolute J-band magnitude. The six T brown dwarfs in our sample have spectral types between T2.5 and T8 and magnitudes between 13.9 and 18.0 in the Two Micron All Sky Survey (2MASS) with photometric distances below 25 pc. Methods: The observations were taken in the J-band with the Omega-2000 camera on the 3.5 m telescope at Calar Alto during a time period of 27 months between March 2011 and June 2013. The number of epochs varied between 11 and 12 depending on the object. The reduction of the astrometric measurements was carried out with respect to the field stars. The relative parallax and proper motions were transformed into absolute measurements using the background galaxies in our fields. Results: We obtained absolute parallaxes for our six brown dwarfs with a precision between 3 and 6 mas. We compared our results in a color-magnitude diagram with other brown dwarfs with determined parallax and with the BT-Settl 2012 atmospheric models. For four of the six targets, we found a good agreement in luminosity with objects of similar spectral types. We obtained an improved accuracy in the parallaxes and proper motions in comparison to previous works. The object 2MASS J11061197+2754225 is more than 1 mag overluminous in all bands, which point to binarity or high order multiplicity. Based on observations taken with Omega-2000 at the 3.5 m telescope at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated by the Max Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC).Appendix A is available in electronic form at http://www.aanda.org

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  1. 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.

  2. White dwarfs in Be star binary systems

    NASA Technical Reports Server (NTRS)

    Apparao, K. M. V.

    1991-01-01

    An evaluation is made of possible reasons for the persistent inability to identify white dwarf stars in the Be binary systems. It is noted that many Be stars exhibiting large optical enhancements may be Be + WD and Be + He systems, and that observations of pulsations in the H-alpha emission, as well as observation of time delays between enhancements of optical line and continuum, can identify such systems.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. The White Dwarf Mass in Interacting Binaries

    NASA Astrophysics Data System (ADS)

    Mukai, Koji

    We are undertaking a comprehensive study of accreting white dwarfs in two broad types of interacting binaries, cataclysmic variables (CVs) and symbiotic stars, using X-ray and multi-wavelength data. Our goal is to understand the detailed accretion and X-ray emission processes in these binaries, and therefore determine what information can be extracted from X-ray observations of these systems. In paritular, we must measure the current masses of white dwarfs in CVs and symbiotic stars and understand if they gain or lose mass over time. We believe that these are all worthy objectives by themselves, with added interest in the context of Type Ia supernova progenitor models and the apparently diffuse Galactic ridge and bulge X-ray emission. For both these issues, we need surveys with well-understood selection effects to measure the space density of CVs and symbiotic stars, and X-ray surveys will likely play a key role. With these long-term goals in mind, we are undertaking several interlinked projects with overlapping sets of objectives and collaborators. In this proposal, we seek support for a subset of our overall research program, thematically linked to one of the most important parameters in any CVs and symbiotic stars: the white dwarf mass (Mwd). The depth of the gravitational potential of the white dwarf sets the maximum temperature that the accreting plasma can reach; therefore, by measuring the maximum temperature in the X-ray spectra of CVs and symbiotic stars, one can infer Mwd. This method has long been applied to magnetic CVs; we believe that it is also applicable to non-magnetic cases. We propose an empirical confirmation of this method for quiescent dwarf novae, and investigate any systematic uncertainties that may be inherent in this method. We already know that CVs and symbiotic stars with strong hard (>10 keV) X-ray emission harbor massive white dwarfs, and have used this fact to study the population such systems detected in INTEGRAL and Swift BAT

  8. 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

  9. 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.

  10. Trace hydrogen in helium atmosphere white dwarfs as a possible signature of water accretion

    NASA Astrophysics Data System (ADS)

    Gentile Fusillo, Nicola Pietro; Gänsicke, Boris T.; Farihi, Jay; Koester, Detlev; Schreiber, Matthias R.; Pala, Anna F.

    2017-06-01

    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 recognized 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 abundances and accretion rates 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 × 1024 and 2.9 × 1024 g, 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 highly significant correlation indicates that, over the cooling age of the white dwarfs, at least some fraction of the H detected in many He atmospheres (including GD 16 and GD 17) is accreted alongside metal pollutants, where the most plausible source is water. In this scenario, water must be common in systems with rocky planetesimals.

  11. 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.

  12. 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.

  13. Probing white dwarf interiors with LISA: periastron precession in eccentric double white dwarfs.

    PubMed

    Willems, B; Vecchio, A; Kalogera, V

    2008-02-01

    In globular clusters, dynamical interactions give rise to a population of eccentric double white dwarfs detectable by the Laser Interferometer Space Antenna (LISA) up to the Large Magellanic Cloud. In this Letter, we explore the detectability of periastron precession in these systems with LISA. Unlike previous investigations, we consider contributions due to tidal and rotational distortions of the binary components in addition to general relativistic contributions to the periastron precession. At orbital frequencies above a few mHz, we find that tides and stellar rotation dominate, opening up a possibly unique window to the study of the interior and structure of white dwarfs.

  14. ECHOES OF A DECAYING PLANETARY SYSTEM: THE GASEOUS AND DUSTY DISKS SURROUNDING THREE WHITE DWARFS

    SciTech Connect

    Melis, C.; Jura, M.; Klein, B.; Zuckerman, B.; Albert, L.

    2010-10-20

    We have performed a comprehensive ground-based observational program aimed at characterizing the circumstellar material orbiting three single white dwarf stars previously known to possess gaseous disks. Near-infrared imaging unambiguously detects excess infrared emission toward Ton 345 and allows us to refine models for the circumstellar dust around two of the three white dwarf stars. We find that each white dwarf hosts gaseous and dusty disks that are roughly spatially coincident, a result that is consistent with a scenario in which dusty and gaseous material has its origin in remnant parent bodies of the white dwarfs' planetary systems. We briefly describe a new model for the gas disk heating mechanism in which the gaseous material behaves like a 'Z II' region. In this Z II region, gas primarily composed of metals is photoionized by ultraviolet light and cools through optically thick allowed Ca II-line emission.

  15. Echoes of a Decaying Planetary System: The Gaseous and Dusty Disks Surrounding Three White Dwarfs

    NASA Astrophysics Data System (ADS)

    Melis, C.; Jura, M.; Albert, L.; Klein, B.; Zuckerman, B.

    2010-10-01

    We have performed a comprehensive ground-based observational program aimed at characterizing the circumstellar material orbiting three single white dwarf stars previously known to possess gaseous disks. Near-infrared imaging unambiguously detects excess infrared emission toward Ton 345 and allows us to refine models for the circumstellar dust around two of the three white dwarf stars. We find that each white dwarf hosts gaseous and dusty disks that are roughly spatially coincident, a result that is consistent with a scenario in which dusty and gaseous material has its origin in remnant parent bodies of the white dwarfs' planetary systems. We briefly describe a new model for the gas disk heating mechanism in which the gaseous material behaves like a "Z II" region. In this Z II region, gas primarily composed of metals is photoionized by ultraviolet light and cools through optically thick allowed Ca II-line emission.

  16. 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.

  17. Exoplanet recycling in massive white-dwarf debris discs

    NASA Astrophysics Data System (ADS)

    Van Lieshout, Rik

    2017-06-01

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

  18. 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.

  19. Polarimetry as a tool to find and characterise habitable planets orbiting white dwarfs

    NASA Astrophysics Data System (ADS)

    Fossati, Luca; Bagnulo, Stefano; Haswell, Carole A.; Patel, Manish R.; Busuttil, Richard; Kowalski, Piotr M.; Shukyak, Denis V.; Sterzik, Michael F.; Valyavin, Gennady

    2015-10-01

    There are several ways planets can survive the giant phase of the host star, hence one can consider the case of Earth-like planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU from the star would remain in the continuous habitable zone (CHZ) for about 8 Gyr. Polarisation due to a terrestrial planet in the CHZ of a cool white dwarf (CWD) is 102 (104) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a powerful tool to detect close-in planets around white dwarfs. Multi-band polarimetry would also allow one to reveal the presence of a planet atmosphere, even providing a first characterisation. 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. Preliminary habitability study show also that photosynthetic processes can be sustained on Earth-like planets orbiting CWDs and that the DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, hence white dwarfs are compatible with the persistence of complex life from the perspective of UV irradiation.

  20. Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs

    NASA Astrophysics Data System (ADS)

    Dufour, P.; Bergeron, P.; Fontaine, G.

    2005-07-01

    We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the First Data Release of the Sloan Digital Sky Survey (SDSS). Our analysis confirms that effective temperatures (Teff) derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly, the mean mass of the trigonometric parallax sample, =0.62Msolar, is significantly lower than that obtained from pure helium models, =0.73Msolar, and more consistent with the spectroscopic mean mass of DB stars, =0.59Msolar, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well-defined sequence in a carbon abundance versus effective temperature diagram; below Teff~10,000 K, carbon pollution decreases monotonically with decreasing effective temperature. Improved evolutionary models including diffusion and connecting to the PG 1159 phase are used to infer a typical value for the thickness of the helium layer MHe/M* between 10-3 and 10-2, compatible with the predictions of post-AGB models. Several DQ stars in our sample, however, show larger than average carbon abundances. We argue that these DQ stars are all massive white dwarfs and could represent the high-mass tail of the white dwarf mass distribution, with their hotter counterparts corresponding to the hot DQ stars reported recently by Liebert et al. The number distribution of DQ white dwarfs as a function of effective temperature clearly shows a sudden drop at about Teff~7000 K and an abrupt cutoff at Teff~6000 K. The existence of this cutoff is now statistically more significant

  1. 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.

  2. The Effect of 22NE Diffusion in the Evolution and Pulsational Properties of White Dwarfs with Solar Metallicity Progenitors

    NASA Astrophysics Data System (ADS)

    Camisassa, María E.; Althaus, Leandro G.; Córsico, Alejandro H.; Vinyoles, Núria; Serenelli, Aldo M.; Isern, Jordi; Miller Bertolami, Marcelo M.; García–Berro, Enrique

    2016-06-01

    Because of the large neutron excess of 22Ne, sedimentation of this isotope occurs rapidly in the interior of white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of 22Ne in the inner regions of the star modifies the Brunt-Väisälä frequency, thus altering the pulsational properties of these stars. In this work we discuss the impact of 22Ne sedimentation in white dwarfs resulting from solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs with masses of 0.528, 0.576, 0.657, and 0.833 {M}⊙ derived from full evolutionary computations of their progenitor stars, starting at the zero-age main sequence all the way through the central hydrogen and helium burning, the thermally pulsing asymptotic giant branch (AGB), and post-AGB phases. Our computations show that at low luminosities ({log}(L/{L}⊙ )≲ -4.25), 22Ne sedimentation delays the cooling of white dwarfs with solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of 22Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that 22Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.

  3. 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.

  4. 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).

  5. 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

  6. 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

  7. 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.

  8. White-dwarf-white-dwarf galactic background in the LISA data

    SciTech Connect

    Edlund, Jeffrey A.; Tinto, Massimo; Krolak, Andrzej; Nelemans, Gijs

    2005-06-15

    LISA (Laser Interferometer Space Antenna) is a proposed space mission, which will use coherent laser beams exchanged between three remote spacecraft to detect and study low-frequency cosmic gravitational radiation. In the low part of its frequency band, the LISA strain sensitivity will be dominated by the incoherent superposition of hundreds of millions of gravitational wave signals radiated by inspiraling white-dwarf binaries present in our own Galaxy. In order to estimate the magnitude of the LISA response to this background, we have simulated a synthesized population that recently appeared in the literature. Our approach relies on entirely analytic expressions of the LISA time-delay interferometric responses to the gravitational radiation emitted by such systems, which allows us to implement a computationally efficient and accurate simulation of the background in the LISA data. We find the amplitude of the galactic white-dwarf binary background in the LISA data to be modulated in time, reaching a minimum equal to about twice that of the LISA noise for a period of about two months around the time when the Sun-LISA direction is roughly oriented towards the Autumn equinox. This suggests that, during this time period, LISA could search for other gravitational wave signals incoming from directions that are away from the galactic plane. Since the galactic white-dwarf background will be observed by LISA not as a stationary but rather as a cyclostationary random process with a period of 1 yr, we summarize the theory of cyclostationary random processes, present the corresponding generalized spectral method needed to characterize such process, and make a comparison between our analytic results and those obtained by applying our method to the simulated data. We find that, by measuring the generalized spectral components of the white-dwarf background, LISA will be able to infer properties of the distribution of the white-dwarf binary systems present in our Galaxy.

  9. 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.

  10. Hunting For Wild Brown Dwarf Companions To White Dwarfs In UKIDSS And SDSS

    NASA Astrophysics Data System (ADS)

    Day-Jones, Avril; Pinfield, D. J.; Jones, H. R. A.; Napiwotzki, R.; Burningham, B.; Jenkins, J. S.; UKIDSS Cool Dwarf Science Working Group

    2008-03-01

    We present findings from our search of the latest releases of SDSS and UKIDSS LAS for very widely separated white dwarf - ultracool dwarf binaries. Ultracool dwarfs found in such binary systems could be used as benchmark objects, whose properties, such as age and distance can be inferred indirectly from the white dwarf primary (with no need to refer to atmospheric models) and can provide a test bed for theoretical models, they can therefore be used observationally pin down how physical properties affect ultracool dwarf spectra.

  11. 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}.

  12. Understanding the White Dwarfs in Intermediate Polars

    NASA Astrophysics Data System (ADS)

    Belle, Kunegunda E.; Sion, E. M.

    2006-12-01

    The temperature of a white dwarf (WD) in a cataclysmic variable (CV) can tell you much about the accretion history of the binary system, and hence the evolution of the system. Best seen in the ultraviolet, a white dwarf's temperature is typically measured through modeling of a UV spectrum. This approach has been taken for numerous CVs modeling the spectrum as a combination of accretion disk and WD photosphere and also for polars, the strongly magnetic (B 10-80 MG) subset of CVs that contain magnetically controlled accretion streams rather than a disk. The task of extracting information about the WD in an intermediate polar (IP, magnetic CVs with B < 5 MG) is a bit trickier as they contain both magnetically controlled accretion streams and a truncated accretion disk. The fact that there are only five IPs for which WD temperatures have been estimated is a testament to the difficulty of modeling these systems. Here we present initial results from our project of determining the temperatures of WDs in IPs. We show that traditional methods of obtaining an inner accretion disk radius via optical emission line wings is likely inaccurate for most IPs, and that in some cases, the truncation radius is so large that the accretion disk does not contribute UV flux. Finally, we show that IPs follow the general trend of magnetic CVs containing cooler WDs than non-magnetic CVs. This work is supported in part by NSF grant AST0507514 and by NASA grant NNG04GE78G.

  13. Circumstellar features in hot DA white dwarfs

    NASA Astrophysics Data System (ADS)

    Bannister, N. P.; Barstow, M. A.; Holberg, J. B.; Bruhweiler, F. C.

    2003-05-01

    We present a phenomenological study of highly ionized, non-photospheric absorption features in high spectral resolution vacuum ultraviolet spectra of 23 hot DA white dwarfs. Prior to this study, four of the survey objects (Feige 24, REJ 0457-281, G191-B2B and REJ 1614-085) were known to possess these features. We find four new objects with multiple components in one or more of the principal resonance lines: REJ 1738+665, Ton 021, REJ 0558-373 and WD 2218+706. A fifth object, REJ 2156-546, also shows some evidence of multiple components, though further observations are required to confirm the detection. We discuss possible origins for these features including ionization of the local interstellar environment, the presence of material inside the gravitational well of the white dwarf, mass loss in a stellar wind and the existence of material in an ancient planetary nebula around the star. We propose ionization of the local interstellar medium as the origin of these features in G191-B2B and REJ 1738+665, and demonstrate the need for higher-resolution spectroscopy of the sample, to detect multiple interstellar medium velocity components and to identify circumstellar features that may lie close to the photospheric velocity.

  14. Radial pulsations in DB white dwarfs?

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.

    1993-01-01

    Theoretical models of DB white dwarfs are unstable against radial pulsation at effective temperatures near 20,000-30,000 K. Many high-overtone modes are unstable, with periods ranging from 12 s down to the acoustic cutoff period of approximately 0.1 s. The blue edge for radial instability lies at slightly higher effective temperatures than for nonradial pulsations, with the temperature of the blue edge dependent on the assumed efficiency of convection. Models with increased convective efficiency have radial blue edges that are increasingly closer to the nonradial blue edge; in all models the instability persists into the nonradial instability strip. Radial pulsations therefore may exist in the hottest DB stars that lie below the DB gap; the greatest chance for detection would be observations in the ultraviolet. These models also explain why searches for radial pulsations in DA white dwarfs have failed: the efficient convection needed to explain the blue edge for nonradial DA pulsation means that the radial instability strip is 1000 K cooler than found in previous investigations. The multiperiodic nature of the expected pulsations can be used to advantage to identify very low amplitude modes using the uniform spacing of the modes in frequency. This frequency spacing is a direct indicator of the mass of the star.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. DISCOVERY OF MOLECULAR HYDROGEN IN WHITE DWARF ATMOSPHERES

    SciTech Connect

    Xu, S.; Jura, M.; Klein, B.; Zuckerman, B.; Koester, D. E-mail: jura@astro.ucla.edu E-mail: ben@astro.ucla.edu

    2013-04-01

    With the Cosmic Origins Spectrograph on board the Hubble Space Telescope, we have detected molecular hydrogen in the atmospheres of three white dwarfs with effective temperatures below 14,000 K, G29-38, GD 133, and GD 31. This discovery provides new independent constraints on the stellar temperature and surface gravity of white dwarfs.

  20. 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.

  1. 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.

  2. Conditions for accretion-induced collapse of white dwarfs

    NASA Technical Reports Server (NTRS)

    Nomoto, Ken'ichi; Kondo, Yoji

    1991-01-01

    Recent discovery of an unexpectedly large number of low-mass binary pulsars (LMBPs) in globular clusters has instigated active discussions on the evolutionary origin of binary pulsars. Prompted by the possibility that at least some of LMBPs originate from accretion-induced collapse (AIC) of white dwarfs, a reexamination is conducted as to whether or not AIC occurs for the new models of O + Ne + Mg white dwarfs and solid C + O white dwarfs that can ignite explosive nuclear burning at significantly lower central densities than in the previous models. Even with low critical densities, AIC is still much more likely than explosion for both types of white dwarfs. Possible regions for AIC are presented in a diagram of mass accretion rate vs initial mass of the white dwarfs.

  3. Wave energy in white dwarf atmospheres. I - Magnetohydrodynamic energy spectra for homogeneous DB and layered DA stars

    NASA Technical Reports Server (NTRS)

    Musielak, Zdzislaw E.

    1987-01-01

    The radiative damping of acoustic and MHD waves that propagate through white dwarf photospheric layers is studied, and other damping processes that may be important for the propagation of the MHD waves are calculated. The amount of energy remaining after the damping processes have occurred in different types of waves is estimated. The results show that lower acoustic fluxes should be expected in layered DA and homogeneous DB white dwarfs than had previously been estimated. Acoustic emission manifests itself in an enhancement of the quadrupole term, but this term may become comparable to or even lower than the dipole term for cool white dwarfs. Energy carried by the acoustic waves is significantly dissipated in deep photospheric layers, mainly because of radiative damping. Acoustically heated corona cannot exist around DA and DB white dwarfs in a range T(eff) = 10,000-30,000 K and for log g = 7 and 8. However, relatively hot and massive white dwarfs could be exceptions.

  4. Wave energy in white dwarf atmospheres. I - Magnetohydrodynamic energy spectra for homogeneous DB and layered DA stars

    NASA Technical Reports Server (NTRS)

    Musielak, Zdzislaw E.

    1987-01-01

    The radiative damping of acoustic and MHD waves that propagate through white dwarf photospheric layers is studied, and other damping processes that may be important for the propagation of the MHD waves are calculated. The amount of energy remaining after the damping processes have occurred in different types of waves is estimated. The results show that lower acoustic fluxes should be expected in layered DA and homogeneous DB white dwarfs than had previously been estimated. Acoustic emission manifests itself in an enhancement of the quadrupole term, but this term may become comparable to or even lower than the dipole term for cool white dwarfs. Energy carried by the acoustic waves is significantly dissipated in deep photospheric layers, mainly because of radiative damping. Acoustically heated corona cannot exist around DA and DB white dwarfs in a range T(eff) = 10,000-30,000 K and for log g = 7 and 8. However, relatively hot and massive white dwarfs could be exceptions.

  5. 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.

  6. PG1258+593 and its common proper motion magnetic white dwarf counterpart

    NASA Astrophysics Data System (ADS)

    Girven, J.; Gänsicke, B. T.; Külebi, B.; Steeghs, D.; Jordan, S.; Marsh, T. R.; Koester, D.

    2010-05-01

    We confirm SDSSJ130033.48+590407.0 as a common proper motion companion to the well-studied hydrogen-atmosphere (DA) white dwarf PG1258+593 (GD322). The system lies at a distance of 68 +/- 3pc, where the angular separation of 16.1 +/- 0.1arcsec corresponds to a minimum binary separation of 1091 +/- 7au. SDSSJ1300+5904 is a cool (Teff = 6300 +/- 300K) magnetic white dwarf (B ~= 6mG). PG1258+593 is a DA white dwarf with Teff = 14790 +/- 77K and logg = 7.87 +/- 0.02. Using the white dwarf mass-radius relation implies the masses of SDSSJ1300+5904 and PG1258+593 are 0.54 +/- 0.06 and 0.54 +/- 0.01Msolar, respectively, and therefore a cooling age difference of 1.67 +/- 0.05Gyr. Adopting main-sequence lifetimes from stellar models, we derive an upper limit of 2.2Msolar for the mass of the progenitor of PG1258+593. A plausible range of initial masses is 1.4-1.8 Msolar for PG1258+593 and 2-3 Msolar for SDSSJ1300+5904. Our analysis shows that white dwarf common proper motion binaries can potentially constrain the white dwarf initial mass-final mass relation and the formation mechanism for magnetic white dwarfs. The magnetic field of SDSSJ1300+5904 is consistent with an Ap progenitor star. A common envelope origin of the system cannot be excluded, but requires a triple system as progenitor.

  7. Accurate Mass Determination of the Ancient White Dwarf ER 8 Through Astrometric Microlensing

    NASA Astrophysics Data System (ADS)

    Sahu, Kailash

    2005-07-01

    We propose to determine the mass of the very cool white dwarf ER 8 through astrometric microlensing. We have predicted that ER 8 will pass very close to a 15th-mag background star in January 2006, with an impact parameter of less than 0.05 arcsec. As it passes in front, it will cause a deflection of the background star's image by >8 milliarcsec, an amount easily detectable with HST/FGS. The gravitational deflection angle depends only on the distances and relative positions of the stars, and on the mass of the white dwarf. Since the distances and positions can be determined precisely before the event, the astrometric measurement offers a unique and direct method to measure the mass of the white dwarf to high accuracy {<5%}. Unlike all other stellar mass determinations, this technique works for single stars {but only if they are nearby and of sufficient mass}. The mass of ER 8 is of special interest because it is a member of the Galactic halo, and appears to be the oldest known field white dwarf. This object can thus set a lower limit on the age of the Galactic halo, but since white-dwarf cooling rates depend on their masses, the mass is a necessary ingredient in the age determination. As a byproduct, we will obtain an accurate parallax for ER 8, and thus its luminosity and {from its effective temperature} its radius. Such quantities are at present rather poorly known for the coolest white dwarfs, and will provide strong constraints on white-dwarf physics.

  8. Accurate Mass Determination of the Ancient White Dwarf ER 8 Through Astrometric Microlensing

    NASA Astrophysics Data System (ADS)

    Sahu, Kailash

    2004-07-01

    We propose to determine the mass of the very cool white dwarf ER 8 through astrometric microlensing. We have predicted that ER 8 will pass very close to a 15th-mag background star in January 2006, with an impact parameter of less than 0.05 arcsec. As it passes in front, it will cause a deflection of the background star's image by >8 milliarcsec, an amount easily detectable with HST/FGS. The gravitational deflection angle depends only on the distances and relative positions of the stars, and on the mass of the white dwarf. Since the distances and positions can be determined precisely before the event, the astrometric measurement offers a unique and direct method to measure the mass of the white dwarf to high accuracy {<5%}. Unlike all other stellar mass determinations, this technique works for single stars {but only if they are nearby and of sufficient mass}. The mass of ER 8 is of special interest because it is a member of the Galactic halo, and appears to be the oldest known field white dwarf. This object can thus set a lower limit on the age of the Galactic halo, but since white-dwarf cooling rates depend on their masses, the mass is a necessary ingredient in the age determination. As a byproduct, we will obtain an accurate parallax for ER 8, and thus its luminosity and {from its effective temperature} its radius. Such quantities are at present rather poorly known for the coolest white dwarfs, and will provide strong constraints on white-dwarf physics.

  9. 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.

  10. Population Synthesis Studies of the White Dwarfs of the Galactic Disk and Halo

    NASA Astrophysics Data System (ADS)

    Cojocaru, Elena-Ruxandra

    2016-09-01

    White dwarfs are fossil stars that can encode valuable information about the formation, evolution and other properties of the different Galactic stellar populations. They are the direct descendants of main-sequence stars with masses ranging from ∼0.8 M⊙ to ∼10 M⊙, which means that over 95% of the stars in our Galaxy will eventually become white dwarfs. This fact, correlated with the excellent quality of modern white dwarf cooling models, clearly marks their potential as cosmic clocks for estimating the ages of Galactic stellar populations, as well as place white dwarfs as privileged objects in understanding several actual astrophysical problems. Stellar population synthesis methods (Tinsley, 1968) use theoretical evolutionary sequences to reproduce luminosities, temperatures and other parameters building up to a synthetic population that can be readily compared to an observed sample of stars. Such techniques are perfect for the study of the different white dwarf populations in our Galaxy and their strength has only grown in recent years, fueled both by improved evolutionary sequences and detailed cooling tracks and also by the ever growing samples of white dwarfs identified through modern survey missions. In particular, the work presented in this thesis uses an updated population synthesis code based on previous versions of the code from our group (García-Berro et al., 1999; Torres et al., 2002; García-Berro et al., 2004; Torres et al., 2005; Camacho et al., 2014). Our synthetic population code, based on Monte Carlo statistical techniques, has been extensively used in the study of the disk (García-Berro et al., 1! 999; Torres et al., 2001; Torres & García-Berro, 2016) and halo (Torres et al., 2002; García-Berro et al., 2004) single white-dwarf population, white dwarf plus main sequence stars (Camacho et al., 2014), as well as open clusters such as NGC 6791 (García-Berro et al., 2010; García-Berro et al., 2011) or globular clusters, as 47 Tuc (Garc

  11. Likely detection of water-rich asteroid debris in a metal-polluted white dwarf

    NASA Astrophysics Data System (ADS)

    Raddi, R.; Gänsicke, B. T.; Koester, D.; Farihi, J.; Hermes, J. J.; Scaringi, S.; Breedt, E.; Girven, J.

    2015-06-01

    The cool white dwarf SDSS J124231.07+522626.6 exhibits photospheric absorption lines of eight distinct heavy elements in medium resolution optical spectra, notably including oxygen. The Teff = 13 000 K atmosphere is helium-dominated, but the convection zone contains significant amounts of hydrogen and oxygen. The four most common rock-forming elements (O, Mg, Si, and Fe) account for almost all the accreted mass, totalling at least 1.2 × 1024 g, similar to the mass of Ceres. The time-averaged accretion rate is 2 × 1010 g s-1, one of the highest rates inferred among all known metal-polluted white dwarfs. We note a large oxygen excess, with respect to the most common metal oxides, suggesting that the white dwarf accreted planetary debris with a water content of ≈38 per cent by mass. This star, together with GD 61, GD 16, and GD 362, form a small group of outliers from the known population of evolved planetary systems accreting predominantly dry, rocky debris. This result strengthens the hypothesis that, integrated over the cooling ages of white dwarfs, accretion of water-rich debris from disrupted planetesimals may significantly contribute to the build-up of trace hydrogen observed in a large fraction of helium-dominated white dwarf atmospheres.

  12. Consequence of total lepton number violation in strongly magnetized iron white dwarfs

    NASA Astrophysics Data System (ADS)

    Belyaev, V. B.; Ricci, P.; Šimkovic, F.; Adam, J.; Tater, M.; Truhlík, E.

    2015-05-01

    The influence of a neutrinoless electron to positron conversion on a cooling of strongly magnetized iron white dwarfs is studied. It is shown that they can be good candidates for soft gamma-ray repeaters and anomalous X-ray pulsars.

  13. 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.

  14. HELIUM CORE WHITE DWARFS IN CATACLYSMIC VARIABLES

    SciTech Connect

    Shen, Ken J.; Bildsten, Lars; Idan, Irit

    2009-11-01

    Binary evolution predicts a population of helium core (M < 0.5 M{sub sun}) white dwarfs (WDs) that are slowly accreting hydrogen-rich material from low-mass main-sequence or brown dwarf donors with orbital periods less than 4 hr. Four binaries are presently known in the Milky Way that will reach such a mass-transferring state in a few Gyr. Despite these predictions and observations of progenitor binaries, there are still no secure cases of helium core WDs among the mass-transferring cataclysmic variables. This led us to calculate the fate of He WDs once accretion begins at a rate M-dot<10{sup -10}M-odot yr{sup -1} set by angular momentum losses. We show here that the cold He core temperatures (T{sub c} < 10{sup 7} K) and low M-dot thermonuclear runaway. Shara and collaborators noted that these large accumulated masses may lead to exceptionally long classical nova (CN) events. For a typical donor star of 0.2 M{sub sun}, such binaries will only yield a few hundred CNe, making these events rare among all CNe. We calculate the reheating of the accreting WD, allowing a comparison to the measured WD effective temperatures in quiescent dwarf novae and raising the possibility that WD seismology may be the best way to confirm the presence of a He WD. We also find that a very long (>1000 yr) stable burning phase occurs after the CN outburst, potentially explaining enigmatic short orbital period supersoft sources like RX J0537-7034 (P{sub orb} = 3.5 hr) and 1E 0035.4-7230 (P{sub orb} = 4.1 hr).

  15. 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.

  16. 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.

  17. 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.

  18. The g-modes of white dwarfs

    NASA Technical Reports Server (NTRS)

    Sobouti, Y.; Khajehpour, M. R. H.; Dixit, V. V.

    1980-01-01

    The neutral g-modes of a degenerate fluid at zero temperature are analyzed. The g-modes of a degenerate fluid at finite but small temperatures are then expanded in terms of those of the zero temperature fluid. For nonrelativistic degenerate fluids it is found that (1) the g-eigenvalues are proportional to T mu(6)sub e mu(-1)sub i, where T is the internal temperature of the fluid, mu sub e and mu sub i are the mean molecular weights of electrons and ions, respectively; (2) the ion pressure is solely responsible for driving the g-modes. For white dwarfs of about a solar mass, the periods of the g-oscillations are in the range of a few hundredths of seconds.

  19. REMNANTS OF BINARY WHITE DWARF MERGERS

    SciTech Connect

    Raskin, Cody; Scannapieco, Evan; Timmes, F. X.; Fryer, Chris; Rockefeller, Gabriel

    2012-02-10

    We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

  20. 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.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. The stellar seismology of hot white dwarfs and planetary nebula nuclei

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.

    1987-01-01

    The pulsation properties of hot white dwarfs make it possible to determine their mass, surface composition, rotation, and rate of evolution, and provide constraints on their internal structure. Period spacings are sensitive measures of stellar mass and indicate surface layer structure. Measurement of the rate of period change for these stars provide a way to determine their cooling rates. Attention is also given to how well (or poorly) models of excitation of the pulsations fit within current models of planetary nebula nuclei and hot white dwarfs.

  6. A Nearby Old Halo White Dwarf Candidate from the Sloan Digital Sky Survey

    DTIC Science & Technology

    2008-07-01

    2008 May 27 ABSTRACT We report the discovery of a nearby old halo white dwarf (WD) candidate from the Sloan Digital Sky Survey ( SDSS ). SDSS J110217.48...411315.4 has a proper motion of 1′′.75 yr−1 and redder optical colors than all other known featureless (type DC) WDs. We present SDSS imaging and...complicated physics of the dense atmospheres of cool WDs. Key words: stars: individual ( SDSS J110217.48+411315.4) – white dwarfs Online-only material

  7. 70 DA WHITE DWARFS IDENTIFIED IN LAMOST PILOT SURVEY

    SciTech Connect

    Zhao, J. K.; Luo, A. L.; Zhao, G.; Oswalt, T. D. E-mail: gzhao@bao.ac.cn E-mail: toswalt@fit.edu

    2013-06-01

    We present a spectroscopically identified catalog of 70 DA white dwarfs (WDs) from the LAMOST pilot survey. Thirty-five are found to be new identifications after cross-correlation with the Eisenstein et al. and Villanova catalogs. The effective temperature and gravity of these WDs are estimated by Balmer lines fitting. Most of them are hot WDs. The cooling times and masses of these WDs are estimated by interpolation in theoretical evolution tracks. The peak of the mass distribution is found to be {approx}0.6 M {sub Sun }, which is consistent with prior work in the literature. The distances of these WDs are estimated using the method of synthetic spectral distances. All of these WDs are found to be in the Galactic disk from our analysis of space motions. Our sample supports the expectation that WDs with high mass are concentrated near the plane of the Galactic disk.

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

    NASA Astrophysics Data System (ADS)

    Godon, Patrick; Sion, Edward M.; Barrett, Paul E.; Linnell, Albert P.

    2009-07-01

    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 ~ 0.65 and phi ~ 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-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 10-10 M sun yr-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 dwarf and disk. Based on observations made

  9. 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

  10. Albus 1: A Very Bright White Dwarf Candidate

    NASA Astrophysics Data System (ADS)

    Caballero, José Antonio; Solano, Enrique

    2007-08-01

    We have serendipitously discovered a previously unknown, bright source (BT=11.75+/-0.07 mag) with a very blue VT-Ks color, which we have named Albus 1. A photometric and astrometric study using Virtual Observatory tools has shown that it possesses an appreciable proper motion and magnitudes and colors very similar to those of the well-known white dwarf G191-B2B. We consider Albus 1 as a DA-type white dwarf located at about 40 pc. If its nature is confirmed, Albus 1 would be the sixth brightest isolated white dwarf in the sky, which would make it an excellent spectrophotometric standard.

  11. Simulating AXAF Grating Spectra of Accreting White Dwarfs

    NASA Technical Reports Server (NTRS)

    Tennant, Allyn F.; Wu, Kinwah; ODell, Stephen L.; Weisskopf, Martin C.

    1998-01-01

    We present simulated AXAF spectra of accreting white dwarfs, using parameters appropriate for magnetic cataclysmic variables. The very high spectral resolution that can be obtained with the High-Energy Transmission Grating of AXAF can resolve the keV X-ray emission lines that characterize the temperature, density and velocity profiles of the shock-heated emission regions of these systems. These simulations demonstrate that actual spectra will allow us to place constraints on the white-dwarf mass and the accretion rate of the systems. The high-resolution spectra also allow the measurement of the velocity of the accretion flow in regions close to the white-dwarf surface.

  12. 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.

  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. K2 Ultracool Dwarfs Survey. II. The White Light Flare Rate of Young Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Gizis, John E.; Paudel, Rishi R.; Mullan, Dermott; Schmidt, Sarah J.; Burgasser, Adam J.; Williams, Peter K. G.

    2017-08-01

    We use Kepler K2 Campaign 4 short-cadence (one-minute) photometry to measure white light flares in the young, moving group brown dwarfs 2MASS J03350208+2342356 (2M0335+23) and 2MASS J03552337+1133437 (2M0355+11), and report on long-cadence (thirty-minute) photometry of a superflare in the Pleiades M8 brown dwarf CFHT-PL-17. The rotation period (5.24 hr) and projected rotational velocity (45 km s-1) confirm 2M0335+23 is inflated (R≥slant 0.20 {R}⊙ ) as predicted for a 0.06 {M}⊙ , 24 Myr old brown dwarf βPic moving group member. We detect 22 white light flares on 2M0335+23. The flare frequency distribution follows a power-law distribution with slope -α =-1.8+/- 0.2 over the range 1031 to 1033 erg. This slope is similar to that observed in the Sun and warmer flare stars, and is consistent with lower-energy flares in previous work on M6-M8 very-low-mass stars; taking the two data sets together, the flare frequency distribution for ultracool dwarfs is a power law over 4.3 orders of magnitude. The superflare (2.6× {10}34 erg) on CFHT-PL-17 shows higher-energy flares are possible. We detect no flares down to a limit of 2× {10}30 erg in the nearby L5γ AB Dor moving group brown dwarf 2M0355+11, consistent with the view that fast magnetic reconnection is suppressed in cool atmospheres. We discuss two multi-peaked flares observed in 2M0335+23, and argue that these complex flares can be understood as sympathetic flares, in which fast-mode magnetohydrodynamic waves similar to extreme-ultraviolet waves in the Sun trigger magnetic reconnection in different active regions.

  15. Physical Properties of the Current Census of Northern White Dwarfs within 40 pc of the Sun

    NASA Astrophysics Data System (ADS)

    Limoges, M.-M.; Bergeron, P.; Lépine, S.

    2015-08-01

    We present a detailed description of the physical properties of our current census of white dwarfs within 40 pc of the Sun, based on an exhaustive spectroscopic survey of northern hemisphere candidates from the SUPERBLINK proper motion database. Our method for selecting white dwarf candidates is based on a combination of theoretical color-magnitude relations and reduced proper motion diagrams. We reported in an earlier publication the discovery of nearly 200 new white dwarfs, and we present here the discovery of an additional 133 new white dwarfs, among which we identify 96 DA, 3 DB, 24 DC, 3 DQ, and 7 DZ stars. We further identify 178 white dwarfs that lie within 40 pc of the Sun, representing a 40% increase of the current census, which now includes 492 objects. We estimate the completeness of our survey at between 66% and 78%, allowing for uncertainties in the distance estimates. We also perform a homogeneous model atmosphere analysis of this 40 pc sample and find a large fraction of massive white dwarfs, indicating that we are successfully recovering the more massive, and less luminous objects often missed in other surveys. We also show that the 40 pc sample is dominated by cool and old white dwarfs, which populate the faint end of the luminosity function, although trigonometric parallaxes will be needed to shape this part of the luminosity function more accurately. Finally, we identify 4 probable members of the 20 pc sample, 4 suspected double degenerate binaries, and we also report the discovery of two new ZZ Ceti pulsators.

  16. PHYSICAL PROPERTIES OF THE CURRENT CENSUS OF NORTHERN WHITE DWARFS WITHIN 40 pc OF THE SUN

    SciTech Connect

    Limoges, M.-M.; Bergeron, P.; Lépine, S. E-mail: bergeron@astro.umontreal.ca

    2015-08-15

    We present a detailed description of the physical properties of our current census of white dwarfs within 40 pc of the Sun, based on an exhaustive spectroscopic survey of northern hemisphere candidates from the SUPERBLINK proper motion database. Our method for selecting white dwarf candidates is based on a combination of theoretical color–magnitude relations and reduced proper motion diagrams. We reported in an earlier publication the discovery of nearly 200 new white dwarfs, and we present here the discovery of an additional 133 new white dwarfs, among which we identify 96 DA, 3 DB, 24 DC, 3 DQ, and 7 DZ stars. We further identify 178 white dwarfs that lie within 40 pc of the Sun, representing a 40% increase of the current census, which now includes 492 objects. We estimate the completeness of our survey at between 66% and 78%, allowing for uncertainties in the distance estimates. We also perform a homogeneous model atmosphere analysis of this 40 pc sample and find a large fraction of massive white dwarfs, indicating that we are successfully recovering the more massive, and less luminous objects often missed in other surveys. We also show that the 40 pc sample is dominated by cool and old white dwarfs, which populate the faint end of the luminosity function, although trigonometric parallaxes will be needed to shape this part of the luminosity function more accurately. Finally, we identify 4 probable members of the 20 pc sample, 4 suspected double degenerate binaries, and we also report the discovery of two new ZZ Ceti pulsators.

  17. 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.

  18. Activity and Kinematics of Cool and Ultracool Dwarfs

    NASA Astrophysics Data System (ADS)

    Schmidt, Sarah Jane

    The ages of cool and ultracool dwarfs are particularly important. For cool M dwarfs, accurate ages combined with their ubiquity in the stellar disk could lead to a new level of precision in age dating the Galaxy. A better understanding of the chromospheres of M dwarfs could provide important clues about the relationship between activity and age in these low mass stars. Ultracool (late-M and L) dwarfs have the distinction of including both warm, young brown dwarfs and stars with mean ages more representative of the stellar disk. Kinematics are a source of mean ages and could provide or confirm discriminating features between stars and brown dwarfs. This thesis is composed of several different projects, each investigating the activity or kinematics of cool or ultracool dwarfs. First, a sample of nearly 500 L dwarfs selected from SDSS DR7 photometry and spectroscopy is examined; we discovered 200 new L dwarfs and found evidence of a bias towards red J - KS colors in the entire population of previously known L dwarfs. Using the three-dimensional kinematics of 300 SDSS DR7 L dwarfs, we find that their kinematics are consistent with those of the stellar disk and include a previously undetected thick disk component. We also confirmed a relationship between age and J - KS color (due to our large sample of UVW motions and unbiased J - KS colors), with blue L dwarfs having hotter kinematics (consistent with older ages) and redder L dwarfs having colder, younger kinematics. The DR7 L dwarf sample showed no distinct kinematic difference between young brown dwarfs and disk-age stars, perhaps due to a bias towards early spectral types. In order to probe the kinematic distribution of L dwarfs in a volume-limited sample, we began a survey of radial velocities of nearby (d<20pc) L dwarfs using the TripleSpec instrument on the ARC 3.5-m telescope at APO. While several reduction packages were tested on the TripleSpec data, none were found to provide reductions of sufficient quality

  19. A fast spinning magnetic white dwarf in the double degenerate, super-Chandrasekhar system NLTT 12758

    NASA Astrophysics Data System (ADS)

    Kawka, Adela; Briggs, Gordon P.; Vennes, Stéphane; Ferrario, Lilia; Paunzen, Ernst; Wickramasinghe, Dayal T.

    2017-04-01

    We present an analysis of the close double degenerate NLTT 12758, which is comprised of a magnetic white dwarf with a field of about 3.1 MG and an apparently non-magnetic white dwarf. We measured an orbital period of 1.154 d and found that the magnetic white dwarf is spinning around its axis with a period of 23 min. An analysis of the atmospheric parameters has revealed that the cooling ages of the two white dwarfs are comparable, suggesting that they formed within a short period of time from each other. Our modelling indicates that the non-magnetic white dwarf is more massive (M = 0.83 M⊙) than its magnetic companion (M = 0.69 M⊙) and that the total mass of the system is higher than the Chandrasekhar mass. Although the stars will not come into contact over a Hubble time, when they do come into contact, dynamically unstable mass transfer will take place leading to either an accretion induced collapse into a rapidly spinning neutron star or a Type Ia supernova.

  20. WHITE-DWARF-MAIN-SEQUENCE BINARIES IDENTIFIED FROM THE LAMOST PILOT SURVEY

    SciTech Connect

    Ren Juanjuan; Luo Ali; Li Yinbi; Wei Peng; Zhao Jingkun; Zhao Yongheng; Song Yihan; Zhao Gang E-mail: lal@nao.cas.cn

    2013-10-01

    We present a set of white-dwarf-main-sequence (WDMS) binaries identified spectroscopically from the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST, also called the Guo Shou Jing Telescope) pilot survey. We develop a color selection criteria based on what is so far the largest and most complete Sloan Digital Sky Survey (SDSS) DR7 WDMS binary catalog and identify 28 WDMS binaries within the LAMOST pilot survey. The primaries in our binary sample are mostly DA white dwarfs except for one DB white dwarf. We derive the stellar atmospheric parameters, masses, and radii for the two components of 10 of our binaries. We also provide cooling ages for the white dwarf primaries as well as the spectral types for the companion stars of these 10 WDMS binaries. These binaries tend to contain hot white dwarfs and early-type companions. Through cross-identification, we note that nine binaries in our sample have been published in the SDSS DR7 WDMS binary catalog. Nineteen spectroscopic WDMS binaries identified by the LAMOST pilot survey are new. Using the 3{sigma} radial velocity variation as a criterion, we find two post-common-envelope binary candidates from our WDMS binary sample.

  1. ANCIENT PLANETARY SYSTEMS ARE ORBITING A LARGE FRACTION OF WHITE DWARF STARS

    SciTech Connect

    Zuckerman, B.; Melis, C.; Klein, B.; Jura, M.; Koester, D. E-mail: cmelis@ucsd.ed E-mail: jura@astro.ucla.ed

    2010-10-10

    Infrared studies have revealed debris likely related to planet formation in orbit around {approx}30% of youthful, intermediate mass, main-sequence stars. We present evidence, based on atmospheric pollution by various elements heavier than helium, that a comparable fraction of the white dwarf descendants of such main-sequence stars are orbited by planetary systems. These systems have survived, at least in part, through all stages of stellar evolution that precede the white dwarf. During the time interval ({approx}200 million years) that a typical polluted white dwarf in our sample has been cooling it has accreted from its planetary system the mass of one of the largest asteroids in our solar system (e.g., Vesta or Ceres). Usually, this accreted mass will be only a fraction of the total mass of rocky material that orbits these white dwarfs; for plausible planetary system configurations we estimate that this total mass is likely to be at least equal to that of the Sun's asteroid belt, and perhaps much larger. We report abundances of a suite of eight elements detected in the little studied star G241-6 that we find to be among the most heavily polluted of all moderately bright white dwarfs.

  2. 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.

  3. White Dwarf Distance and Precision Age for Globular Clusters

    NASA Astrophysics Data System (ADS)

    Bragaglia, Angela

    1996-07-01

    This proposal is the natural complement of our Cycle 4 and 5 programs aimed at using the White Dwarf cooling sequence for the calibration of the globular cluster distance scale, the determination of accurate distances being a critical prerequisite for the estimate of reliable cluster ages. In Cycle 4 we obtained data on NGC6752, while in Cycle 5 we got images of 47Tuc. These two clusters will test whether there really exists a measurable age difference between the metal poor and the metal rich clusters, as has been inferred from some horizontal branch luminosity- metallicity calibration. While reducing our Cycle 4 data we have realized that a few short exposures would greatly help to accurately subtract the wings of the heavily saturated images in the field of view, hence to improve the accuracy of the faint star fotometry in the crowded field of the cluster. We therefore apply for one orbit for each of the two clusters in order to get shallow images of our Cycle 4 and 5 fields. The observed cooling sequences for the clusters {for DA and non-DA WDs} will then be fit to the corresponding FCSs to determine the cluster distance moduli. Moreover, directly fitting to each other the WD cooling sequences of the two clusters will provide very accurate relative distances {hence ages}.

  4. 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.

  5. Hard X-ray Emission from White Dwarfs

    NASA Technical Reports Server (NTRS)

    Chu, You-Hua; Gruendl, Robert

    2004-01-01

    Hot white dwarfs may exhibit photospheric emission at X-ray wavelengths, but their X- ray emission should be soft, mutch less than 0.5 keV. Hard X-ray emission, at approx. 1 keV, is not expected from white dwarfs, unless they are in binary systems and the hard X-ray emission is produced by a late-type companion's coronal activity or by accretion of a companion's material onto the surface of the white dwarf. We proposed to use the ROSAT archive to search for hard X-ray emission from white dwarfs in order to determine whether hard X-ray emission may provide a sensitive diagnostic for the existence of a binary companion.

  6. 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.

  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. 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.

  11. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2017-09-01

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

  13. 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.

  14. 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.

  15. 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).

  16. 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.

  17. 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).

  18. RXJ2130.6+4710 - an eclipsing white dwarf-M-dwarf binary star

    NASA Astrophysics Data System (ADS)

    Maxted, P. F. L.; Marsh, T. R.; Morales-Rueda, L.; Barstow, M. A.; Dobbie, P. D.; Schreiber, M. R.; Dhillon, V. S.; Brinkworth, C. S.

    2004-12-01

    We report the detection of eclipses in the close white-dwarf-M-dwarf binary star RXJ2130.6+4710. We present light curves in the B, V and I bands and fast photometry obtained with the three-channel CCD photometer Ultracam of the eclipse in the u', g' and r' bands. The depth of the eclipse varies from 3.0 mag in the u' band to less than 0.1 mag in the I band. The times of mid-eclipse are given by the ephemeris BJD(mid-eclipse) = 2452785.681876(2) + 0.521035625(3) E, where figures in parentheses denote uncertainties in the final digit. We present medium-resolution spectroscopy from which we have measured the spectroscopic orbits of the M dwarf and white dwarf. We estimate that the spectral type of the M dwarf is M3.5Ve or M4Ve, although the data on which this is based are not ideal for spectral classification. We have compared the spectra of the white dwarf with synthetic spectra from pure hydrogen model atmospheres to estimate that the effective temperature of the white dwarf is Teff= 18000 +/- 1000 K. We have used the width of the primary eclipse and duration of totality measured precisely from the Ultracam u' data combined with the amplitude of the ellipsoidal effect in the I band and the semi-amplitudes of the spectroscopic orbits to derive masses and radii for the M dwarf and white dwarf. The M dwarf has a mass of 0.555 +/- 0.023 Msolar and a radius of 0.534 +/- 0.053 Rsolar, which is a typical radius for stars of this mass. The mass of the white dwarf is 0.554 +/- 0.017 Msolar and its radius is 0.0137 +/- 0.0014 Rsolar, which is the radius expected for a carbon-oxygen white dwarf of this mass and effective temperature. The light curves are affected by frequent flares from the M dwarf and the associated dark spots on its surface can be detected from the distortions to the light curves and radial velocities. RXJ2130.6+4710 is a rare example of a pre-cataclysmic variable star that will start mass transfer at a period above the period gap for cataclysmic variables.

  19. DE Canum Venaticorum: a bright, eclipsing red dwarf-white dwarf binary

    NASA Astrophysics Data System (ADS)

    van den Besselaar, E. J. M.; Greimel, R.; Morales-Rueda, L.; Nelemans, G.; Thorstensen, J. R.; Marsh, T. R.; Dhillon, V. S.; Robb, R. M.; Balam, D. D.; Guenther, E. W.; Kemp, J.; Augusteijn, T.; Groot, P. J.

    2007-05-01

    Context: Close white dwarf-red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf-red dwarf binary with a relatively short (~8.7 h) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study. Aims: From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters that we discuss in the framework of common-envelope evolution. Methods: Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average low-resolution spectrum of DE CVn, we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system. Results: The derived ephemeris is HJDmin = 2 452 784.5533(1) + 0.3641394(2) × E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8 000 K. The inclination of the system is 86+3°-2 and the mass and radius of the red dwarf are 0.41± 0.06 M⊙ and 0.37+0.06-0.007 R⊙, respectively, and the mass and radius of the white dwarf are 0.51+0.06-0.02 M⊙ and 0.0136+0.0008-0.0002 R⊙, respectively. Conclusions: We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively low-mass star (M≤ 1.6~M⊙). The current age of this system is 3.3-7.3× 109 years, while it will take longer than the Hubble time for DE CVn to evolve into a semi-detached system.

  20. On the occurrence and detectability of Bose-Einstein condensation in helium white dwarfs

    SciTech Connect

    Benvenuto, O.G.; Vito, M.A. De E-mail: adevito@fcaglp.unlp.edu.ar

    2011-02-01

    It has been recently proposed that helium white dwarfs may provide promising conditions for the occurrence of the Bose-Einstein condensation. The argument supporting this expectation is that in some conditions attained in the core of these objects, the typical De Broglie wavelength associated with helium nuclei is of the order of the mean distance between neighboring nuclei. In these conditions the system should depart from classical behavior showing quantum effects. As helium nuclei are bosons, they are expected to condense. In order to explore the possibility of detecting the Bose-Einstein condensation in the evolution of helium white dwarfs we have computed a set of models for a variety of stellar masses and values of the condensation temperature. We do not perform a detailed treatment of the condensation process but mimic it by suppressing the nuclei contribution to the equation of state by applying an adequate function. As the cooling of white dwarfs depends on average properties of the whole stellar interior, this procedure should be suitable for exploring the departure of the cooling process from that predicted by the standard treatment. We find that the Bose-Einstein condensation has noticeable, but not dramatic effects on the cooling process only for the most massive white dwarfs compatible with a helium dominated interior ( ≈ 0.50M{sub s}un) and very low luminosities (say, Log(L/L{sub s}un) < −4.0). These facts lead us to conclude that it seems extremely difficult to find observable signals of the Bose-Einstein condensation. Recently, it has been suggested that the population of helium white dwarfs detected in the globular cluster NGC 6397 is a good candidate for detecting signals of the Bose-Einstein condensation. We find that these stars have masses too low and are too bright to have an already condensed interior.

  1. Hot DAVs: a probable new class of pulsating white dwarf stars

    NASA Astrophysics Data System (ADS)

    Kurtz, D. W.; Shibahashi, H.; Dhillon, V. S.; Marsh, T. R.; Littlefair, S. P.; Copperwheat, C. M.; Gänsicke, B. T.; Parsons, S. G.

    2013-06-01

    We have discovered a pulsating DA white dwarf at the lower end of the temperature range 45 000-30 000 K where a few helium atmosphere white dwarfs are known. There are now three such pulsators known, suggesting that a new class of theoretically predicted pulsating white dwarf stars exists. We name them the hot DAV stars. From high-speed photometric observations with the ULTRACAM photometer on the 4.2-m William Herschel Telescope, we show that the hydrogen atmosphere white dwarf star WD1017-138 pulsates in at least one mode with a frequency of 1.62 mHz (a period of 624 s). The amplitude of that mode was near 1 mmag at a 10σ confidence level on one night of observation and an 8.4σ confidence level on a second night. The combined data have a confidence level of 11.8σ. This supports the two other detections of hot DAV stars previously reported. From three Very Large Telescope Ultraviolet and Visual Echelle Spectrograph spectra we confirm also that WD1017-138 is a hydrogen atmosphere white dwarf with no trace of helium or metals with Teff = 32 600 K, log g = 7.8 (cgs) and M = 0.55 M⊙. The existence of pulsations in these DA white dwarfs at the cool edge of the 45 000-30 000 K temperature range supports the thin hydrogen layer model for the deficit of helium atmosphere white dwarfs in this range. DA white dwarfs with thick hydrogen layers do not have the superadiabatic, chemically inhomogeneous (μ-gradient) zone that drives pulsation in this temperature range. The potential for higher amplitude hot DAV stars exists; their discovery would open the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars. A search for pulsation in a further 22 candidates with ULTRACAM on the European Southern Observatory New Technology Telescope gave null results for pulsation at precisions in the range 0.5-3 mmag, suggesting that the pulsation

  2. 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

  3. Very low mass stars and white dwarfs in NGC 6397

    NASA Technical Reports Server (NTRS)

    Paresce, Francesco; De Marchi, Guido; Romaniello, Martino

    1995-01-01

    Deep Wide Field/Planetary Camera 2 (WFPC2) images in wide bands centered at 606 and 802 nm were taken with the Hubble Space Telescope (HST) 4.6 min from the center of the galactic globular cluster NGC 6397. The images were used to accurately position approximately 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m(sub 814) approximately = m(sub I) approximately = 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m(sub 814) = 18.5 to m(sub 814) = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m(sub 814) approximately = 20 and m(sub 814) approximately = 22.5 are evident. The corresponding luminosity function increases slowly from M(sub 814) approximately = 6.5 to 8.5 are expected from ground-based observations but then drops sharply from there dwon to the measurement limit. The corresponding mass function obtained bu using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between approximately 0.25 and approximately 0.15 solar mass, but drops toward the expected mass limit of the normal hydrogen burning main sequence at approximately 0.1 solar mass. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 solar mass at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition. Contamination from unresolved galaxies, which cannot be

  4. 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.

  5. 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

  6. 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.

  7. A numerical study of the stability of radiative shocks. [in accretion flows onto white dwarf stars

    NASA Technical Reports Server (NTRS)

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

    1984-01-01

    Attention is given to the oscillatory instability of optically thin radiative shocks in time-dependent numerical calculations of accretion flows onto degenerate dwarfs. The present nonlinear calculations yield good quantitative agreement with the linear results obtained for oscillation frequencies, damping rates, and critical alpha-values. The fundamental mode and the first overtone in the shock radius and luminosity variations can be clearly identified, and evidence is sometimes seen for the second overtone. Time-dependent calculations are also performed which include additional physics relevant to degenerate dwarf accretion, such as electron thermal conduction, unequal electron and ion temperatures, Compton cooling, and relativistic corrections to the bremsstrahlung cooling law. All oscillatory modes are found to be damped, and hence stable, in the case of a 1-solar mass white dwarf accreting in spherical symmetry.

  8. 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.

  9. White dwarfs in the UKIRT infrared deep sky survey data release

    SciTech Connect

    Tremblay, P.-E.; Kalirai, J. S.; Leggett, S. K.; Lodieu, N.; Bergeron, P.; Ludwig, H.-G.

    2014-06-20

    We have identified 8 to 10 new cool white dwarfs from the Large Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired with the Sloan Digital Sky Survey to obtain proper motions and a broad ugrizYJHK wavelength coverage. Optical spectroscopic observations were secured at Gemini Observatory and confirm the degenerate status for eight of our targets. The final sample includes two additional white dwarf candidates with no spectroscopic observations. We rely on improved one-dimensional model atmospheres and new multi-dimensional simulations with CO5BOLD to review the stellar parameters of the published LAS white dwarf sample along with our additional discoveries. Most of the new objects possess very cool atmospheres with effective temperatures below 5000 K, including two pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km s{sup –1} ≤v {sub tan} ≤ 60 km s{sup –1}. They are likely thick disk 10-11 Gyr old objects. In addition, we find a resolved double degenerate system with v {sub tan} ∼ 155 km s{sup –1} and a cooling age between 3.0 and 5.0 Gyr. These white dwarfs could be disk remnants with a very high velocity or former halo G stars. We also compare the LAS sample with earlier studies of very cool degenerates and observe a similar deficit of helium-dominated atmospheres in the range 5000 < T {sub eff} (K) < 6000. We review the possible explanations for the spectral evolution from helium-dominated toward hydrogen-rich atmospheres at low temperatures.

  10. White Dwarfs in the UKIRT Infrared Deep Sky Survey Data Release 9

    NASA Astrophysics Data System (ADS)

    Tremblay, P.-E.; Leggett, S. K.; Lodieu, N.; Freytag, B.; Bergeron, P.; Kalirai, J. S.; Ludwig, H.-G.

    2014-06-01

    We have identified 8 to 10 new cool white dwarfs from the Large Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired with the Sloan Digital Sky Survey to obtain proper motions and a broad ugrizYJHK wavelength coverage. Optical spectroscopic observations were secured at Gemini Observatory and confirm the degenerate status for eight of our targets. The final sample includes two additional white dwarf candidates with no spectroscopic observations. We rely on improved one-dimensional model atmospheres and new multi-dimensional simulations with CO5BOLD to review the stellar parameters of the published LAS white dwarf sample along with our additional discoveries. Most of the new objects possess very cool atmospheres with effective temperatures below 5000 K, including two pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km s-1 <=v tan <= 60 km s-1. They are likely thick disk 10-11 Gyr old objects. In addition, we find a resolved double degenerate system with v tan ~ 155 km s-1 and a cooling age between 3.0 and 5.0 Gyr. These white dwarfs could be disk remnants with a very high velocity or former halo G stars. We also compare the LAS sample with earlier studies of very cool degenerates and observe a similar deficit of helium-dominated atmospheres in the range 5000 < T eff (K) < 6000. We review the possible explanations for the spectral evolution from helium-dominated toward hydrogen-rich atmospheres at low temperatures.

  11. A COMPREHENSIVE SPECTROSCOPIC ANALYSIS OF DB WHITE DWARFS

    SciTech Connect

    Bergeron, P.; Wesemael, F.; Dufour, Pierre; Beauchamp, A.; Hunter, C.; Gianninas, A.; Limoges, M.-M.; Dufour, Patrick; Fontaine, G.; Saffer, Rex A.; Ruiz, M. T.; Liebert, James E-mail: wesemael@astro.umontreal.ca E-mail: limoges@astro.umontreal.ca E-mail: fontaine@astro.umontreal.ca E-mail: chris.hunter@yale.edu E-mail: mtruiz@das.uchile.cl

    2011-08-10

    We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 M{sub sun} for our sample, with a dispersion of only 0.09 M{sub sun}. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M < 0.5 M{sub sun}) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green Survey. We show that 20% of all white dwarfs in the temperature range of interest are DB stars, although the fraction drops to half this value above T{sub eff} {approx} 20,000 K. We also show that the persistence of DB stars with no hydrogen features at low temperatures is difficult to reconcile with a scenario involving accretion from the interstellar medium, often invoked to account for the observed hydrogen abundances in DBA stars. We present evidence for the existence of two different evolutionary channels that produce DB white dwarfs: the standard model where DA stars are transformed into DB stars through the convective dilution of a thin hydrogen layer and a second channel where DB stars retain a helium atmosphere throughout their evolution. We finally demonstrate that the instability strip of pulsating V777 Her white dwarfs contains no non-variables, if the hydrogen content of these stars is properly accounted for.

  12. 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

  13. 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.

  14. 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.

  15. 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.

  16. Physical properties and evolution of the two white dwarfs in the Sanduleak-Pesch binary

    NASA Astrophysics Data System (ADS)

    Greenstein, J. L.; Dolez, N.; Vauclair, G.

    1983-10-01

    An important new binary white dwarf has been found by Sanduleak and Pesch. The stars are analyzed with the data from the Palomar double CCD spectrograph, using continuum fluxes, lines profiles, and Balmer decrements. They have hydrogen atmospheres, are young Population I, age ≈5×108 yr, temperatures of 12500K and 9500K, and the same visual magnitude. The cooler and less luminous star, B, has the larger radius and lower mass; B started its degenerate cooling, more recently, as the brighter of the pair. The estimated cooling times differ by approximately 108 yr. The white dwarfs, with masses 0.80 and 0.43 m_sun;, are descended from progenitors of 8 and 4 m_sun; (or 5 and 3.5 m_sun;).

  17. Effects of Fe/C phase separation on the ages of white dwarfs

    NASA Technical Reports Server (NTRS)

    Xu, Z. W.; Van Horn, H. M.

    1992-01-01

    The energy release associated with the phase separation of Fe from C in a predominantly C white dwarf is calculated. The total gravitational-plus-internal energy differences between models of homogeneous compositions and those wth Fe-enriched cores are computed. In the unlikely case where the core is pure Fe, a substantial extension of the white dwarf cooling times is found, even with the small cosmic abundance of this element. For the more realistic core compositions that result if the Fe/C phase diagram is either of the spindle or of the azeotropic type, the energy release is still sufficient to prolong the cooling times by about 0.6 Gyr, comparable to that produced by C/O phase separation. Phase separation is found to produce an appreciable 'bump' in the luminosity function, although not one large enough to exceed the observational errors at low luminosities.

  18. 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.

  19. Accreting Pulsating White Dwarfs: Hotter than Single DAVs

    NASA Astrophysics Data System (ADS)

    Szkody, Paula; Mukadam, A.; Gaensicke, B. T.; Woudt, P. A.; Solheim, J.; Sion, E. M.; Nitta, A.; Warner, B.; Sahu, D. K.; Prabhu, T.; Henden, A.

    2006-12-01

    Single DAV pulsating white dwarfs are known to show non-radial g-mode pulsations with periods around 50-1400s and have temperatures in the specific range of 11,000-12,500K. With the discovery of several pulsating white dwarfs in the accreting close binary systems of cataclysmic variables, it is possible to probe the effects of mass transfer and accretion, external heat input, He enriched envelopes and fast rotation on the location of the instability strip. Our UV observations of three pulsating accreting white dwarfs in the cataclysmic variables SDSSJ013132-0901, SDSSJ161033-0102 and SDSSJ220553+1155 with the Solar Blind Channel on HST show enhanced pulsation amplitudes over the optical and white dwarf temperatures near 15,000K. Combined with temperatures of two other known accreting pulsators (GW Lib and HS2331+3905), it appears that there is a wide range in the instability strip for accreting pulsators. This range may be due to different white dwarf masses or compositions compared to single DAVs. This research was supported by NASA grant GO-10233.01A from STScI.

  20. 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.

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

    NASA Astrophysics Data System (ADS)

    Fenn, D.; Plewa, T.

    2017-06-01

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

  2. The ELM Survey: Finding the Shortest Period Binary White Dwarfs

    NASA Astrophysics Data System (ADS)

    Canton, Paul; Gianninas, Alexandros; Kilic, Mukremin; Brown, Warren; Kenyon, Scott

    2014-08-01

    A new discovery space for short period binary white dwarfs has opened up with the availability of 14,600 deg^2 of SDSS Data Release 9 photometry. The Extremely Low-Mass (ELM) Survey takes advantage of this photometry and SDSS spectroscopy to identify compact systems with 1 hour or shorter orbital periods. To significantly increase the number of merging white dwarf systems known, we have proposed to obtain follow- up spectroscopic observations of all candidates with g ≤ 19 mag and photometric colors consistent with extremely low-mass (≤ 0.3 M_⊙) white dwarfs. Most of our 2012A Hale and 2012B KP 4m observing runs were lost to weather, yet we managed to identify at least one new short period binary. Our 2013A run on the KP 4m was successful in identifying many new ELM white dwarfs, and in our 2013B follow-up run we observed two new 3 hour binaries while also obtaining further data on a number of other merging systems. Here we propose to continue our program by observing our fall targets on the KP 4m telescope to constrain their binary orbital periods. Our two major science goals are to discover detached gravitational wave sources for fundamental tests of general relativity, and to constrain the formation rate and space density of merging white dwarfs. The latter is important for constraining the contribution of double degenerates to Type Ia and underluminous supernovae.

  3. White Dwarfs in HETDEX: Preparation for the Survey

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

  4. Characterizing K2 Planetary Systems Orbiting Cool Dwarfs

    NASA Astrophysics Data System (ADS)

    Dressing, Courtney D.; Newton, Elisabeth R.; Schlieder, Joshua; Vanderburg, Andrew; Charbonneau, David; Knutson, Heather; K2C2

    2017-01-01

    The NASA K2 mission is using the repurposed Kepler spacecraft to search for transiting planets in multiple fields along the ecliptic plane. K2 observes 10,000 - 30,000 stars in each field for roughly 80 days, which is too short to observe multiple transits of planets in the habitable zones of Sun-like stars, but long enough to detect potentially habitable planets orbiting low-mass dwarfs. Accordingly, M and K dwarfs are frequently nominated as K2 Guest Observer targets and K2 has already observed significantly more low-mass stars than the original Kepler mission. While the K2 data are therefore an enticing resource for studying the properties and frequency of planetary systems orbiting low-mass stars, many K2 cool dwarfs are not well-characterized. We are refining the properties of K2 planetary systems orbiting cool dwarfs by acquiring medium-resolution NIR spectra with SpeX on the IRTF and TripleSpec on the Palomar 200". In our initial sample of 144 potential cool dwarfs hosting candidate planetary systems detected by K2, we noted a high contamination rate from giants (16%) and reddened hotter dwarfs (31%). After employing empirically-based relations to determine the temperatures, radii, masses, luminosities, and metallicities of K2 planet candidate host stars, we found that our new cool dwarf radius estimates were 10-40% larger than the initial values, indicating that the radii of the associated planet candidates were also underestimated. Refining the stellar parameters allows us to identify astrophysical false positives and better constrain the radii and insolation flux environments of bona fide transiting planets. I will present our resulting catalog of system properties and highlight the most attractive K2 planets for radial velocity mass measurement and atmospheric characterization with Spitzer, HST, JWST, and the next generation of extremely large ground- and space-based telescopes. We gratefully acknowledge funding from the NASA Sagan Fellowship Program

  5. 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

  6. 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.

  7. 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.

  8. A catalogue of white dwarf candidates in VST ATLAS

    NASA Astrophysics Data System (ADS)

    Gentile Fusillo, Nicola Pietro; Raddi, Roberto; Gänsicke, Boris T.; Hermes, J. J.; Pala, Anna F.; Fuchs, Joshua T.; Chehade, Ben; Metcalfe, Nigel; Shanks, Tom

    2017-07-01

    The Sloan Digital Sky Survey (SDSS) has created a knowledge gap between the Northern and the Southern hemispheres, which is very marked for white dwarfs: Only ≃15 per cent of the known white dwarfs are south of the equator. Here, we make use of the VLT Survey Telescope (VST) ATLAS survey, one of the first surveys obtaining deep, optical, multiband photometry over a large area of the southern skies, to remedy this situation. Applying the colour and proper-motion selection developed in our previous work on SDSS to the most recent internal data release (2016 April 25) of VST ATLAS, we created a catalogue of ≃4200 moderately bright (g ≤ 19), high-confidence southern white dwarf candidates, which can be followed up individually with both the large array of southern telescopes or in bulk with ESO's forthcoming multi-object spectrograph 4MOST.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. Characterization of High-Energy Emissions of GKM Stars using Wide Binaries with White Dwarfs

    NASA Astrophysics Data System (ADS)

    Catalán, S.; Garcés, A.; Ribas, I.

    2011-12-01

    The definition of an age calibration for main-sequence late-type stars has multiple applications, e.g., in the fields of galactic evolution, stellar dynamos, theories of angular momentum loss and planetary atmospheres. In the latter, the characterization of the time-evolution of stellar high-energy emissions can help us understand the influence on planetary atmospheres and their potential habitability. A key element for this characterization is a reliable age determination. For this purpose we have studied a sample of late G, K, and M stars. To cover the age window up to 0.7 Gyr we have used stars belonging to open clusters, while for ages above this limit we use wide binaries containing white dwarfs. Since the evolution of white dwarfs can be understood as a cooling process, which is relatively well known at the moment, we can use them as age calibrators. Wide binary members are supposed to have been born simultaneously and with the same chemical composition. Since they are well separated (100-1000 AU aprox.) we can assume that no interaction has occurred between them in the past and they have evolved as single stars. So, from the white dwarf age we can infer the age of the system. We present our current results based in a sample of 30 binaries from the NLTT catalogue comprised by a DA white dwarf (showing only H absorption lines) and a G, K or M star.

  15. Magnetic field evolution in white dwarfs: The hall effect and complexity of the field

    NASA Technical Reports Server (NTRS)

    Muslimov, A. G.; Van Horn, H. M.; Wood, M. A.

    1995-01-01

    We calculate the evolution of the magnetic fields in white dwarfs, taking into account the Hall effect. Because this effect depends nonlinearly upon the magnetic field strength B, the time dependences of the various multipole field components are coupled. The evolution of the field is thus significantly more complicated than has been indicated by previous investigations. Our calculations employ recent white dwarf evolutionary sequences computed for stars with masses 0.4, 0.6, 0.8, and 1.0 solar mass. We show that in the presence of a strong (up to approximately 10(exp 9) G) internal toroidal magnetic field; the evolution of even the lowest order poloidal modes can be substantially changed by the Hall effect. As an example, we compute the evolution of an initially weak quadrupole component, which we take arbitrarily to be approximately 0.1%-1% of the strength of a dominant dipole field. We find that coupling provided by the Hall effect can produce growth of the ratio of the quadrupole to the dipole component of the surface value of the magnetic field strength by more than a factor of 10 over the 10(exp 9) to 10(exp 10) year cooling lifetime of the white dwarf. Some consequences of these results for the process of magnetic-field evolution in white dwarfs are briefly discussed.

  16. Magnetic field evolution in white dwarfs: The hall effect and complexity of the field

    NASA Technical Reports Server (NTRS)

    Muslimov, A. G.; Van Horn, H. M.; Wood, M. A.

    1995-01-01

    We calculate the evolution of the magnetic fields in white dwarfs, taking into account the Hall effect. Because this effect depends nonlinearly upon the magnetic field strength B, the time dependences of the various multipole field components are coupled. The evolution of the field is thus significantly more complicated than has been indicated by previous investigations. Our calculations employ recent white dwarf evolutionary sequences computed for stars with masses 0.4, 0.6, 0.8, and 1.0 solar mass. We show that in the presence of a strong (up to approximately 10(exp 9) G) internal toroidal magnetic field; the evolution of even the lowest order poloidal modes can be substantially changed by the Hall effect. As an example, we compute the evolution of an initially weak quadrupole component, which we take arbitrarily to be approximately 0.1%-1% of the strength of a dominant dipole field. We find that coupling provided by the Hall effect can produce growth of the ratio of the quadrupole to the dipole component of the surface value of the magnetic field strength by more than a factor of 10 over the 10(exp 9) to 10(exp 10) year cooling lifetime of the white dwarf. Some consequences of these results for the process of magnetic-field evolution in white dwarfs are briefly discussed.

  17. 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.

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

    NASA Astrophysics Data System (ADS)

    McDonald, I.; Zijlstra, A. A.

    2015-01-01

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

  19. 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.

  20. Hubble Space Telescope high resolution spectroscoy of the exposed white dwarf in the dwarf nova VW Hydri in quiescence: A rapidly rotating white dwarf

    NASA Technical Reports Server (NTRS)

    Sion, Edward M.; Huang, Min; Szkody, Paula; Cheng, Fu-Hua

    1995-01-01

    We obtained a far-ultraviolet spectrum of the dwarf nova VW Hyi in quiescence, with the Hubble Space Telescope Goddard High Resolution Spectrograph covering the region of the Si iv lambda(lambda)1393, 1402 resonance doublet. The broad, shallow Si iv doublet feature is fully resolved, has a total equivalent width of 2.8 A, and is the first metal absorption feature to be clearly detected in the exposed white dwarf. Our synthetic spectral analysis, using a model grid constructed with the code TLUSTY, resulted in a reasonable fit to a white dwarf photosphere with T(sub eff) = 22,000 +/- 2000 K, log g = 8.0 +/- 0.3, an approximately solar Si/H abundance, and a rotational velocity, v sin i approximately equal to 600 km/s. This rotation rate, while not definitive because it is based upon just one line transition, is 20% of the Keplerian (breakup) velocity of the white dwarf and hence does not account for the unexpectedly low boundary-layer luminosity inferred from the soft-X-ray/extreme ultra-violet bands where most of the boundary-layer luminosity should be radiated. The predicted boundary-layer luminosity for a 0.6 solar mass white dwarf accreting at the rate 10(exp -10) solar mass/yr and rotating at 600 km/s, corresponding to VW Hyi in quiescence, is 2 x 10(exp 32) ergs/s when proper account is taken of the rotational kinetic energy going into spinning up the white dwarf. If the boundary-layer area is equal to that of the white dwarf, then T(sub bl) = 24,000 K. This is essentially identical to the photspheric luminosity and temperature determined in far-ultraviolet photospheric analyses. If the boundary-layer area is 10(exp -3) of the white dwarf surface area, then T(sub bl) = 136,000 K.

  1. 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.

  2. 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.

  3. Constraints on modified gravity models from white dwarfs

    NASA Astrophysics Data System (ADS)

    Banerjee, Srimanta; Shankar, Swapnil; Singh, Tejinder P.

    2017-10-01

    Modified gravity theories can introduce modifications to the Poisson equation in the Newtonian limit. As a result, we expect to see interesting features of these modifications inside stellar objects. White dwarf stars are one of the most well studied stars in stellar astrophysics. We explore the effect of modified gravity theories inside white dwarfs. We derive the modified stellar structure equations and solve them to study the mass-radius relationships for various modified gravity theories. We also constrain the parameter space of these theories from observations.

  4. 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.

  5. 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.

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

    NASA Technical Reports Server (NTRS)

    Van Horn, Hugh M.

    1991-01-01

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

  7. 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.

  8. 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

  9. 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.

  10. The Thermal History of the Magnetic White Dwarf in V471 Tauri

    NASA Astrophysics Data System (ADS)

    Henry, C. K.; Sion, E. M.

    2000-05-01

    We consider the thermal history of the magnetic white dwarf in the Hyades, pre-cataclysmic, eclipsing-spectroscopic binary, V471 Tauri. With Teff=35,000K, (Mwd = 0.8 Msun), its cooling age is only 7.8x 106 years compared with the mean cooling age of Hyades single white dwarfs of 9x 107 years. If its formation was essentially coeval with the single Hyades degenerates, then V471 Tauri is ~13,000K hotter than their average Teff (19,680K). This amount of heating by accretion is in the same regime as that experienced, on average, by white dwarfs in cataclysmic variables (Sion 1999, PASP, 111, 532). We compare the positions of the Hyades degenerates in the HRD with a Bondi-Hoyle locus of constant accretion, as discussed by Castellani and Panagia (1971). We consider several possible phyical explanations for the puzzling cooling history of the V471 Tauri degenerate. These include the possiblity of an ancient nova with limit cycle evolution, steady proton-proton nuclear burning providing its surface luminosity, and radial accretion heating maintaining its observed Teff. For steady hydrogen burning, an accretion rate ˙ {M} = 10-11 Ls/Xh = 2.3x 10-12 Msun/yr is sufficient to account for the present Teff. As a test of two of these three scenarios, we are carrying out a quasi-static evolutionary calculation of radial accretion onto the white dwarf. This research is supported by NSF grant AST99-09155 to Villanova University and by NASA ADP grant NAG5-8388.

  11. FUSE Observation of Silicon in the Photosphere of the DBA White Dwarf GD 61

    NASA Astrophysics Data System (ADS)

    Wesemael, F.; Petitclerc, N.; Chayer, P.; Kruk, J. W.; Pesant, S.; Tardif, B.

    2006-06-01

    FUSE observations of the cool (Teff˜ 17,000 K) DBA white dwarf reveal that the carbon features observed in several hotter DB stars are absent, but that photospheric transitions associated with the Si III ion are present. This may be the first indication that the mechanism which allows carbon to remain conspicuous in hotter objects, postulated to be a stellar wind, may well have turned off by the time a DB star cools down to 17,000 K. The photospheres of cooler DB stars are likely to be dominated instead by accretion from the ISM and, at even cooler temperatures, by convective dredge-up of carbon.

  12. 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

  13. An independent test of the photometric selection of white dwarf candidates using LAMOST DR3

    NASA Astrophysics Data System (ADS)

    Gentile Fusillo, N. P.; Rebassa-Mansergas, A.; Gänsicke, B. T.; Liu, X.-W.; Ren, J. J.; Koester, D.; Zhan, Y.; Hou, Y.; Wang, Y.; Yang, M.

    2015-09-01

    In previous work by Gentile Fusillo et al., we developed a selection method for white dwarf candidates which makes use of photometry, colours and proper motions to calculate a probability of being a white dwarf (PWD). The application of our method to the Sloan Digital Sky Survey (SDSS) data release 10 resulted in ≃66 000 photometrically selected objects with a derived PWD, approximately ≃21 000 of which are high-confidence white dwarf candidates. Here, we present an independent test of our selection method based on a sample of spectroscopically confirmed white dwarfs from the Large Sky Area Multi-Fiber Spectroscopic Telescope (LAMOST) survey. We do this by cross-matching all our ≃66 000 SDSS photometric white dwarf candidates with the over 4 million spectra available in the third data release of LAMOST. This results in 1673 white dwarf candidates with no previous SDSS spectroscopy, but with available LAMOST spectra. Among these objects, we identify 309 genuine white dwarfs. We find that our PWD can efficiently discriminate between confirmed LAMOST white dwarfs and contaminants. Our white dwarf candidate selection method can be applied to any multiband photometric survey and in this work we conclusively confirm its reliability in selecting white dwarfs without recourse to spectroscopy. We also discuss the spectroscopic completeness of white dwarfs in LAMOST, as well as deriving effective temperatures, surface gravities and masses for the hydrogen-rich atmosphere white dwarfs in the newly identified LAMOST sample.

  14. Deposition of steeply infalling debris around white dwarf stars

    NASA Astrophysics Data System (ADS)

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

    2017-06-01

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

  15. Limits on detectability of mass loss from cool dwarfs

    NASA Technical Reports Server (NTRS)

    Mullan, D. J.; Doyle, J. G.; Redman, R. O.; Mathioudakis, M.

    1992-01-01

    Recent spectroscopic evidence supports the theoretical expectation that certain cool dwarfs may have stellar winds with M-dot values several orders of magnitude larger than the solar rate. For large enough values of M-dot, the emission from the wind is expected to have a spectrum which, at low enough frequencies, becomes a power law, S(v) about v exp alpha with alpha about 0.7. Data from IRAS and VLA suggest that such a spectrum may in fact occur in certain M dwarfs: a key test of the wind spectrum would be provided if the stars could be detected at lambda about 1 mm. We show that the M-dot required to ensure power-law emission is a few times 10 exp -10 solar mass/yr. With M-dot of this order, fluxes at lambda about 1 mm would be tens of mJy. Using the James Clerk Maxwell Telescope, we have tested this prediction on several stars: the data are suggestive but are near the limits of detection. Confirmation of our estimates will be important for evolution and for interstellar medium (ISM) physics: if even a few percent of all M dwarfs are losing mass at the above rates, the mass balance of the ISM will be dominated by M dwarfs.

  16. 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.

  17. 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.

  18. 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.

  19. Imaging Extra-Solar Planets around White Dwarfs

    NASA Astrophysics Data System (ADS)

    Burleigh, Matt R.; Clarke, Fraser

    In Burleigh Clarke and Hodgkin (2002 MNRAS 331 L41) we showed that Jovian planets in initially wide orbits (>5AU) should survive the final stages of stellar evolution and migrate outwards to orbit the resultant white dwarfs at separations of up to >100AU. Using evolutionary models for massive Jupiters we also showed that such planets should be detectable around a variety of suitable white dwarfs within 20pc of the Sun. Based on these predictions we have been searching for planets >5Mjup around nearby white dwarfs using direct imaging in the near infra-red. We will present results from our observational programmes with the two 8m Gemini Telescopes and with the NAOMI Adaptive Optics programme on the 4m William Herschell Telescope on La Palma. Detection of massive planetary companions to nearby white dwarfs would prove that such objects can survive the final stages of stellar evolution place constraints on the frequency of main sequence stars with planetary systems dynamically similar to our own and allow direct spectroscopic investigation of their composition and structure.

  20. 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.

  1. White Dwarf Stars in the HET Dark Energy Experiment

    NASA Astrophysics Data System (ADS)

    Castanheira, Barbara; Winget, D.; Gebhardt, K.; Allende Prieto, C.; Shetrone, M.; Odewahn, S.; Montgomery, M. H.

    2012-01-01

    In this poster, we present the project that will survey all white dwarf stars observed in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) and the Visible Integral-field Replicable Unit Spectrograph (VIRUS) observations in parallel mode. The final product will be a unique magnitude-limited catalog of as many as 10,000 stars. Since we will use data from an Integral-field Units, our survey will be free of the selection biases that plagued preceding surveys, e.g. the Sloan Digital Sky Survey (SDSS). The critical advantages of our program are our ability to produce a white dwarf luminosity function five magnitudes fainter than the one derived from the Palomar-Green survey and with a similar number of faint stars as the one from SDSS. Our project will help to derive a more precise age of the Galactic disk, and will provide fundamental information about the white dwarf population and the star formation history of the Milky Way, impacting the white dwarf field and many other fields of astronomy.

  2. White dwarf dynamical interactions and fast optical transients

    NASA Astrophysics Data System (ADS)

    García-Berro, Enrique; Badenes, Carles; Aznar-Siguán, Gabriela; Lorén-Aguilar, Pablo

    2017-07-01

    Recent advances in time-domain astronomy have uncovered a new class of optical transients with time-scales shorter than typical supernovae and a wide range of peak luminosities. Several subtypes have been identified within this broad class, including Ca-rich transients, Type Ia supernovae (SNe Ia) and fast/bright transients. We examine the predictions from a state-of-the-art grid of three-dimensional simulations of dynamical white dwarf interactions in the context of these fast optical transients. We find that for collisions involving carbon-oxygen or oxygen-neon white dwarfs, the peak luminosities and durations of the light curves in our models are in good agreement with the properties of fast/bright transients. When one of the colliding white dwarfs is made of helium, the properties of the light curves are similar to those of Ca-rich gap transients. The model light curves from our white dwarf collisions are too slow to reproduce those of SNe Ia, and too fast to match any normal or peculiar SNe Ia.

  3. High-Resolution EUV Spectroscopy of White Dwarfs

    NASA Astrophysics Data System (ADS)

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

    2014-01-01

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

  4. 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.

  5. Central Magnetic Field of a Magnetic White Dwarf Star

    NASA Astrophysics Data System (ADS)

    Shah, Hridaya; Sebastian, Kunnat

    2017-07-01

    Observations of over-luminous Type 1a supernovae have prompted researchers to come up with various hypotheses in order to explain them. One hypothesis is based on the explosion of a progenitor super-massive magnetic white dwarf star. These stars are assumed to have very strong magnetic fields inside of them. However, there is a lack of analytic proof of the existence of such magnetic fields in the magnetic white dwarf stars. In this work, we plan to address an analytic proof of the existence of very strong magnetic fields in the center of these magnetic white dwarfs. We will see that for a one Landau-level white dwarf star, with central density {10}9{--}{10}11 {{g}} {{cm}}-3, it is possible to have central magnetic fields of the order of {10}13{--}{10}15G at least. In the presence of strong magnetic fields, the threshold densities chosen for this work that correspond to instabilities due to general relativity and pycnonuclear reactions have been found to increase so that the matter does not acquire instability at such central densities.

  6. UV Spectroscopy of a Peculiar White Dwarf Supernova

    NASA Astrophysics Data System (ADS)

    McCully, Curtis

    2012-10-01

    While type Ia supernovae {SNe Ia} have been extremely useful for studying the cosmic expansion history, their explosion mechanism and progenitor system remain unsolved problems. Moreover, as large samples of SNe are observed, the diversity among these explosions has grown: not all exploding white dwarfs look like normal SNe Ia. Understanding why these "peculiar" objects are different from the normal ones can help explain the standard SN Ia scenario, as well as give us a better understanding of the many endpoints of stellar evolution. Connecting observations to physical models has been difficult for both normal and peculiar white dwarf supernovae. The ultraviolet is an unexplored wavelength region for peculiar SNe Ia; the high opacity in the UV from typical thermonuclear burning products means that the UV flux in white dwarf supernovae is very sensitive to the outermost layers of ejecta. This material is the least processed, and is thus an ideal place to look for clues to progenitors and explosion mechanisms. Here we propose target-of-opportunity UV spectroscopy of a peculiar white dwarf SN using the STIS NUV-MAMA instrument to add a unique piece of the puzzle connecting peculiar SNe Ia and their progenitors.

  7. Metal Accretion onto White Dwarfs. I. The Approximate Approach Based on Estimates of Diffusion Timescales

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Brassard, P.; Dufour, P.; Tremblay, P.-E.

    2015-06-01

    The accretion-diffusion picture is the model par excellence for describing the presence of planetary debris polluting the atmospheres of relatively cool white dwarfs. Some important insights into the process may be derived using an approximate approach which combines static stellar models with estimates of diffusion timescales at the base of the outer convection zone or, in its absence, at the photosphere. Until recently, and to our knowledge, values of diffusion timescales in white dwarfs have all been obtained on the basis of the same physics as that developed initially by Paquette et al., including their diffusion coefficients and thermal diffusion coefficients. In view of the recent exciting discoveries of a plethora of metals (including some never seen before) polluting the atmospheres of an increasing number of cool white dwarfs, we felt that a new look at the estimates of settling timescales would be worthwhile. We thus provide improved estimates of diffusion timescales for all 27 elements from Li to Cu in the periodic table in a wide range of the surface gravity-effective temperature domain and for both DA and non-DA stars.

  8. 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

  9. 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

  10. Sirius B - A still mysterious white dwarf

    NASA Astrophysics Data System (ADS)

    Wesemael, F.; Fontaine, G.

    1982-02-01

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

  11. 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).

  12. Atmospheric Modeling and Retrieval of Cool Y Dwarfs

    NASA Astrophysics Data System (ADS)

    Zalesky, Joseph; Line, Michael R.; Schneider, Adam

    2017-06-01

    Brown dwarfs' near-infrared spectra contain a wealth of information which can reveal the physical and chemical processes that occur in their atmospheres. Using a recently developed atmospheric retrieval model, we are able to constrain various molecular abundances, along with photometric radius, gravity, cloud optical depths, and temperature profiles for a set of ultral-cool (T8-Y1) dwarfs observed with the Hubble Space Telescope Wide Field Camera 3. From these spectra, we are able to constrain the abundances of water, methane, ammonia, CO, CO_2, H_2S, and Na+K. Using the retrieved abundances of water and methane, we are able to determine the atmospheric carbon-to-oxygen ratio and metallicity for these objects. We also identify a continuing trend of alkali metal depletion towards cooler effective temperatures likely due to the formation of optically thin Na2S and KCl clouds.

  13. The long term period stability of the hot DBV white dwarf EC20058-5234

    NASA Astrophysics Data System (ADS)

    Sullivan, Denis J.

    2009-06-01

    Since its discovery over a decade ago, the helium atmosphere pulsator EC 20058-5234 (QU Tel) has been studied both spectroscopically, and also extensively using the techniques of time-series photometry. Model atmosphere fits to quality spectra obtained with one of the Magellan telescopes have confirmed its status as the hottest know DBV, so it currently defines the blue edge of the DBV instability strip. Extensive time-series photometry (primarily from Mt John in NZ but also including a Whole Earth Telescope run) clearly demonstrates that this white dwarf is a very stable low amplitude pulsator. This is consistent with its position at or near the blue edge of the DBV instability strip. However, of perhaps greater significance is the possibility of employing this period stability to look for a period change that can be sourced to the predicted neutrino-dominated cooling of the hot white dwarfs. This paper provides an update on this work.

  14. 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.

  15. FIRST UNAMBIGUOUS DETECTION OF THE RETURN OF PULSATIONS IN THE ACCRETING WHITE DWARF SDSS J074531.92+453829.6 AFTER AN OUTBURST

    SciTech Connect

    Mukadam, Anjum S.; Szkody, P.; Townsley, D. M.; Gaensicke, B. T.; Winget, D. E.; Hermes, J. J.; Howell, Steve B.; Teske, J.; Patterson, Joseph; Armstrong, Eve; Kemp, Jonathan

    2011-02-20

    The primary white dwarf of the cataclysmic variable SDSS J074531.92+453829.6 was discovered to exhibit non-radial pulsations in 2006 January. This accreting white dwarf underwent its first recorded dwarf nova outburst in 2006 October, during which its brightness increased by more than 5 mag. A Hubble Space Telescope (HST) ultraviolet spectrum, obtained one year after the outburst, revealed a white dwarf temperature of 16,500 K, hotter than all other known accreting white dwarf pulsators. This implies that the accreting primary white dwarf of SDSS J074531.92+453829.6 was heated to temperatures beyond the instability strip during the outburst. Optical observations acquired a year after the outburst did not reveal any evidence of pulsations, suggesting that the white dwarf had not cooled to quiescence by then. We recently acquired optical high-speed time-series photometry on this cataclysmic variable SDSS J074531.92+453829.6 more than three years after its outburst to find that pulsations have now returned to the primary white dwarf. Moreover, the observed pulsation periods agree with pre-outburst periods within the uncertainties of a few seconds. This discovery is significant because it indicates that the outburst did not affect the interior stellar structure, which governs the observed pulsation frequencies. It also suggests that the surface of the white dwarf has now cooled to quiescence. Using this discovery in addition to the prior HST temperature measurement of 16,500 K, we have been able to constrain the matter accreted during the 2006 outburst. This is the first time an accreting white dwarf was unambiguously observed to resume pulsating after an outburst.

  16. 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.

  17. THE MASS OF THE WHITE DWARF IN GW LIBRA

    SciTech Connect

    Van Spaandonk, L.; Steeghs, D.; Marsh, T. R.; Parsons, S. G.

    2010-06-01

    We report a mass and rotational broadening (vsin i) for the pulsating white dwarf (WD) component of the WZ Sge type Dwarf Nova GW Lib based on high-resolution Very Large Telescope spectroscopy that resolves the Mg II 4481 A absorption feature. Its gravitational redshift combined with WD mass-radius models provides us with a direct measurement of the WD mass of M {sub 1} = 0.84 {+-} 0.02 M {sub sun}. The line is clearly resolved and if associated with rotational broadening gives vsin i = 87.0 {+-} 3.4 km s{sup -1}, equivalent to a spin period of 97 {+-} 12 s.

  18. 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.

  19. Deep HST Imaging in 47 Tuc and NGC 6397: White Dwarfs and Brown Dwarfs in the Cluster and Field of NGC 6397

    NASA Astrophysics Data System (ADS)

    Rich, Robert Michael; Anderson, J.; Dotter, A.; Hansen, B.; Richer, H.; Fahlman, G.; Hurley, J.; Kalirai, J.; King, I.; Reitzel, D.; Shara, M.; Stetson, P.

    2011-01-01

    We report analysis of second epoch imaging of the very deep 126 orbit ACS integration in NGC 6397, yielding the deepest proper-motion selected color-magnitude diagram of a globular cluster. We reach past the faint end of the white dwarf cooling sequence and confirm the "blue hook" caused by H2 collision-induced absorption opacity. We also confirm a suspected population of faint red proper-motion members of NGC6397, many of which lie both fainter and redder than the theoretical limit for the hydrogen burning. Other proper motion members of NGC 6397 (with unusual colors) may be white dwarf/red dwarf binaries. Our reduced proper motion plot for the field reveals (for the first time) an extended white dwarf cooling sequence in the thick disk population, along with a well populated sequence of extremely faint red stars, many of which are also fainter and redder than the theoretical hydrogen burning limit. Supported by AURA STScI GO-11633 (R. M. Rich, PI)

  20. Probing dark matter crests with white dwarfs and IMBHs

    NASA Astrophysics Data System (ADS)

    Amaro-Seoane, P.; Casanellas, J.; Schödel, R.; Davidson, E.; Cuadra, J.

    2016-06-01

    White dwarfs (WDs) are the most promising captors of dark matter (DM) particles in the crests that are expected to build up in the cores of dense stellar clusters. The DM particles could reach sufficient densities in WD cores to liberate energy through self-annihilation. The extinction associated with our Galactic Centre makes it impossible to detect the potential-associated luminosities, contrary to smaller stellar systems which are close enough to us and not heavily extincted, such as -Cen. We investigate the prospects of detection of DM-burning WDs in a stellar cluster harbouring an intermediate-mass black hole (IMBH), which leads to higher densities of DM at the centre. We calculate the capture rate and estimate the luminosity that a WD would emit depending on its distance to the centre of the cluster. Direct-summation N-body simulations of -Cen yield a non-negligible number of WDs in the range of radii of interest. We apply our assumption to published Hubble Space Telescope/Advanced Camera for Surveys observations of stars in the centre of -Cen and, although we are not able to identify any evident candidate, we proof that their bunching up at high luminosities would be unique. We predict that DM burning will lead to a truncation of the cooling sequence at the faint end. The detection of DM burning in future observations of dense stellar clusters could allow us to probe different models of DM distributions and characteristics. On the other hand, if DM-burning WDs really exist, their number and properties could give hints to the existence of IMBHs.

  1. Dynamical tides in compact white dwarf binaries: helium core white dwarfs, tidal heating and observational signatures

    NASA Astrophysics Data System (ADS)

    Fuller, Jim; Lai, Dong

    2013-03-01

    Tidal dissipation in compact white dwarf (WD) binary systems significantly influences the physical conditions (such as surface temperature and rotation rate) of the WDs prior to mass transfer or merger. In these systems, the dominant tidal effects involve the excitation of gravity waves and their dissipation in the outer envelope of the star. We calculate the amplitude of tidally excited gravity waves in low-mass (0.3 M⊙) helium (He) core WDs as a function of the tidal forcing frequency ω. Like carbon-oxygen (CO) WDs studied in our previous paper, we find that the dimensionless tidal torque F(ω) (inversely proportional to the effective tidal quality factor) depends on ω in an erratic way. On average, F(ω) scales approximately as ω6, and is several orders of magnitude smaller for He WDs than for CO WDs. We find that tidal torques can begin to synchronize the WD rotation when the orbital period is less than about an hour, although a nearly constant asynchronization is maintained even at small periods. We examine where the tidally excited gravity waves experience non-linear breaking or resonant absorption at a critical layer, allowing us to estimate the location and magnitude of tidal heating in the WD envelope. We then incorporate tidal heating in the MESA stellar evolution code, calculating the physical conditions of the WD as a function of orbital period for different WD models. We find that tidal heating makes a significant contribution to the WD luminosity for short-period (˜10 min) systems such as SDSS J0651+2844. We also find that for WDs containing a hydrogen envelope, tidal heating can trigger runaway hydrogen shell burning, leading to a nova-like event before the onset of mass transfer.

  2. Pan-Planets: Searching for hot Jupiters around cool dwarfs

    NASA Astrophysics Data System (ADS)

    Obermeier, C.; Koppenhoefer, J.; Saglia, R. P.; Henning, Th.; Bender, R.; Kodric, M.; Deacon, N.; Riffeser, A.; Burgett, W.; Chambers, K. C.; Draper, P. W.; Flewelling, H.; Hodapp, K. W.; Kaiser, N.; Kudritzki, R.-P.; Magnier, E. A.; Metcalfe, N.; Price, P. A.; Sweeney, W.; Wainscoat, R. J.; Waters, C.

    2016-03-01

    The Pan-Planets survey observed an area of 42 sq deg. in the galactic disk for about 165 h. The main scientific goal of the project is the detection of transiting planets around M dwarfs. We establish an efficient procedure for determining the stellar parameters Teff and log g of all sources using a method based on SED fitting, utilizing a three-dimensional dust map and proper motion information. In this way we identify more than 60 000 M dwarfs, which is by far the largest sample of low-mass stars observed in a transit survey to date. We present several planet candidates around M dwarfs and hotter stars that are currently being followed up. Using Monte Carlo simulations we calculate the detection efficiency of the Pan-Planets survey for different stellar and planetary populations. We expect to find 3.0+3.3-1.6 hot Jupiters around F, G, and K dwarfs with periods lower than 10 days based on the planet occurrence rates derived in previous surveys. For M dwarfs, the percentage of stars with a hot Jupiter is under debate. Theoretical models expect a lower occurrence rate than for larger main sequence stars. However, radial velocity surveys find upper limits of about 1% due to their small sample, while the Kepler survey finds a occurrence rate that we estimate to be at least 0.17b(+0.67-0.04) %, making it even higher than the determined fraction from OGLE-III for F, G and K stellar types, 0.14 (+0.15-0.076) %. With the large sample size of Pan-Planets, we are able to determine an occurrence rate of 0.11 (+0.37-0.02) % in case one of our candidates turns out to be a real detection. If, however, none of our candidates turn out to be true planets, we are able to put an upper limit of 0.34% with a 95% confidence on the hot Jupiter occurrence rate of M dwarfs. This limit is a significant improvement over previous estimates where the lowest limit published so far is 1.1% found in the WFCAM Transit Survey. Therefore we cannot yet confirm the theoretical prediction of a lower

  3. 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.

  4. Mass loss, levitation, accretion, and the sharp-lined features in hot white dwarfs

    NASA Technical Reports Server (NTRS)

    Bruhweiler, F. C.; Kondo, Y.

    1983-01-01

    A study has been conducted of eight white dwarfs, including seven DA and one He-rich types. The study is based on high-resolution observations conducted with the aid of the International Ultraviolet Explorer. Four of the dwarfs show features related to heavy elements which are not interstellar in origin. It is tentatively suggested that, at least in the hottest low-gravity DA white dwarfs, the observed narrow-lined features are formed in expanding halos or winds associated with the white dwarfs. Theoretically, stable white dwarf halos should actually be coronae with temperatures in excess of 1,000,000 K. However, the observed narrow-lined features do not suggest such high temperatures. The observed radial velocities suggest weak stellar winds in two hot white dwarfs, namely, G191-B2B and 2111+49. It is tentatively proposed that radiative levitation can explain the appearance of the observed metallic lines in the hot DA white dwarfs.

  5. Polluting white dwarfs with perturbed exo-comets

    NASA Astrophysics Data System (ADS)

    Caiazzo, Ilaria; Heyl, Jeremy S.

    2017-08-01

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

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. Discovery of Photospheric Germanium in Hot DA White Dwarfs

    NASA Astrophysics Data System (ADS)

    Vennes, Stéphane; Chayer, Pierre; Dupuis, Jean

    2005-04-01

    We report the identification of Ge IV resonance lines in ultraviolet spectra of the hot DA white dwarfs Feige 24, G191-B2B, and GD 246. The lines originate in the stellar photosphere, and we measure low Ge/H abundance ratios ranging between -8.0 and -8.7. We also tentatively identify a resonance line of Sn IV blended with an Fe V line in the spectrum of G191-B2B. The presence of germanium extends our knowledge of the abundance pattern in hot white dwarfs beyond the iron group. The abundance ratio appears nearly solar, which implies either that the germanium abundance mixture in these stars has remained unaltered since leaving the main sequence or that diffusion processes (e.g., selective radiation pressure) are coincidentally reproducing a solar Ge/H ratio.

  13. 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.

  14. The pi line polarization in magnetic white dwarfs

    NASA Astrophysics Data System (ADS)

    Achilleos, N.; Wickramasinghe, D. T.

    1995-01-01

    The pi subcomponents of hydrogen line transitions only absorb light that is linearly polarized parallel to a local magnetic field. In the spectra of magnetic white dwarfs, the circular polarization in these pi lines is usually expected and often observed to be reduced in magnitude relative to the neighbouring continuum. However, observations of some strongly magnetized white dwarfs show some pi lines that have circular polarization equal to that of the adjacent continuum. We show that this is a consequence of the effect of magnetic anomalous dispersion (magneto-optical effects) on radiative transfer. The observed equality between continuum and pi line polarization is obtained, in theory, at adequately high fields and for special field geometries.

  15. 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.

  16. THE DISCOVERY OF THE MOST METAL-RICH WHITE DWARF: COMPOSITION OF A TIDALLY DISRUPTED EXTRASOLAR DWARF PLANET

    SciTech Connect

    Dufour, P.; Fontaine, G.; Bergeron, P.; Lachapelle, F.-R.; Kilic, M.; Kleinman, S. J.; Leggett, S. K.

    2010-08-10

    Cool white dwarf stars are usually found to have an outer atmosphere that is practically pure in hydrogen or helium. However, a small fraction have traces of heavy elements that must originate from the accretion of extrinsic material, most probably circumstellar matter. Upon examining thousands of Sloan Digital Sky Survey (SDSS) spectra, we discovered that the helium-atmosphere white dwarf SDSS J073842.56+183509.6 shows the most severe metal pollution ever seen in the outermost layers of such stars. We present here a quantitative analysis of this exciting star by combining high signal-to-noise ratio follow-up spectroscopic and photometric observations with model atmospheres and evolutionary models. We determine the global structural properties of our target star, as well as the abundances of the most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca, and Fe. The relative abundances of these elements imply that the source of the accreted material has a composition similar to that of Bulk Earth. We also report the signature of a circumstellar disk revealed through a large infrared excess in JHK photometry. Combined with our inferred estimate of the mass of the accreted material, this strongly suggests that we are witnessing the remains of a tidally disrupted extrasolar body that was as large as Ceres.

  17. STRENGTHENING THE CASE FOR ASTEROIDAL ACCRETION: EVIDENCE FOR SUBTLE AND DIVERSE DISKS AT WHITE DWARFS

    SciTech Connect

    Farihi, J.; Jura, M.; Zuckerman, B.; Lee, J.-E. E-mail: jura@astro.ucla.ed E-mail: jelee@sejong.ac.k

    2010-05-10

    Spitzer Space Telescope IRAC 3-8 {mu}m and AKARI IRC 2-4 {mu}m photometry are reported for 10 white dwarfs with photospheric heavy elements; nine relatively cool stars with photospheric calcium and one hotter star with a peculiar high carbon abundance. A substantial infrared excess is detected at HE 2221-1630, while modest excess emissions are identified at HE 0106-3253 and HE 0307+0746, implying these latter two stars have relatively narrow ({Delta}r < 0.1 R{sub sun}) rings of circumstellar dust. A likely 7.9 {mu}m excess is found at PG 1225-079 and may represent, together with G166-58, a sub-class of dust ring with a large inner hole. The existence of attenuated disks at white dwarfs substantiates the connection between their photospheric heavy elements and the accretion of disrupted minor planets, indicating many polluted white dwarfs may harbor orbiting dust, even those lacking an obvious infrared excess.

  18. A new detailed examination of white dwarfs in NGC 3532 and NGC 2287

    NASA Astrophysics Data System (ADS)

    Dobbie, P. D.; Napiwotzki, R.; Burleigh, M. R.; Williams, K. A.; Sharp, R.; Barstow, M. A.; Casewell, S. L.; Hubeny, I.

    2009-06-01

    We present the results of a photometric and spectroscopic study of the white dwarf candidate members of the intermediate age open clusters NGC 3532 and NGC 2287. Of the nine objects investigated, it is determined that six are probable members of the clusters, four in NGC 3532 and two in NGC 2287. For these six white dwarfs, we use our estimates of their cooling times together with the cluster ages to constrain the lifetimes and masses of their progenitor stars. We examine the location of these objects in initial mass-final mass space and find that they now provide no evidence for substantial scatter in initial mass-final mass relation (IFMR) as suggested by previous investigations. Instead, we demonstrate that, when combined with current data from other solar metallicity open clusters and the Sirius binary system, they hint at an IFMR that is steeper in the initial mass range 3 Msolar <~ Minit <~ 4 Msolar than at progenitor masses immediately lower and higher than this. This form is generally consistent with the predictions of stellar evolutionary models and can aid population synthesis models in reproducing the relatively sharp drop observed at the high mass end of the main peak in the mass distribution of white dwarfs. * Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme IDs 079.D-0490(A) and 080.D-0654(A). E-mail: pdd@aao.gov.au

  19. Four new massive pulsating white dwarfs including an ultramassive DAV

    NASA Astrophysics Data System (ADS)

    Curd, Brandon; Gianninas, A.; Bell, Keaton J.; Kilic, Mukremin; Romero, A. D.; Allende Prieto, Carlos; Winget, D. E.; Winget, K. I.

    2017-06-01

    We report the discovery of four massive (M > 0.8 M⊙) ZZ Ceti white dwarfs, including an ultramassive 1.16 M⊙ star. We obtained ground-based, time series photometry for 13 white dwarfs from the Sloan Digital Sky Survey Data Release 7 and Data Release 10 whose atmospheric parameters place them within the ZZ Ceti instability strip. We detect monoperiodic pulsations in three of our targets (J1015, J1554 and J2038) and identify three periods of pulsation in J0840 (173, 327 and 797 s). Fourier analysis of the remaining nine objects does not indicate variability above the 4 detection threshold. Our preliminary asteroseismic analysis of J0840 yields a stellar mass M = 1.14 ± 0.01 M⊙, hydrogen and helium envelope masses of MH = 5.8 × 10-7 M⊙ and MHe = 4.5 × 10-4 M⊙ and an expected core crystallized mass ratio of 50-70 per cent. J1015, J1554 and J2038 have masses in the range 0.84-0.91 M⊙ and are expected to have a CO core; however, the core of J0840 could consist of highly crystallized CO or ONeMg given its high mass. These newly discovered massive pulsators represent a significant increase in the number of known ZZ Ceti white dwarfs with mass M > 0.85 M⊙, and detailed asteroseismic modelling of J0840 will allow for significant tests of crystallization theory in CO and ONeMg core white dwarfs.

  20. 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.

  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. 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.

  3. On the ultraviolet iron spectrum of pre-white dwarfs

    NASA Technical Reports Server (NTRS)

    Schoenberner, D.; Drilling, J. S.

    1985-01-01

    Numerous lines of Fe V, Fe VI, and Fe VII have been detected in high-resolution IUE spectra of two central stars and a number of very hot subdwarfs. The strengths of these lines vary widely from star to star, suggesting that the abundance of iron in the atmospheres of the immediate progenitors of white dwarfs is strongly variable. A possible explanation is elemental separation by gravity and radiation pressure.

  4. 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.

  5. On the maximum mass of magnetized white dwarfs

    NASA Astrophysics Data System (ADS)

    Chatterjee, D.; Fantina, A. F.; Chamel, N.; Novak, J.; Oertel, M.

    2017-07-01

    We develop a detailed and self-consistent numerical model, assuming a poloidal magnetic field, for extremely magnetized white dwarfs, which have been proposed as progenitors of overluminous Type Ia supernovae. This model can describe fully consistent equilibria of magnetic stars in axial symmetry, with rotation, general relativistic effects and realistic equations of state (including electrostatic interactions and taking into account Landau quantization of electrons due to the magnetic field). We study the influence of each of these ingredients on to the white dwarf structure and, in particular, on their maximum mass. We perform an extensive stability analysis of such objects, with their highest surface magnetic fields reaching ˜1013 G (at which point the star adopts a torus-like shape). We confirm previous speculations that, although very massive strongly magnetized white dwarfs, could potentially exist, the onset of electron captures and pycnonuclear reactions may severely limit their stability. Finally, the emission of gravitational waves by these objects is addressed - our results suggest that detection by the currently planned space-based detector Laser Interferometer Space Antenna is unlikely.

  6. Deflagrating white dwarfs: a Type Ia supernova model

    NASA Astrophysics Data System (ADS)

    Calder, A. C.; Plewa, T.; Vladimirova, N.; Brown, E. F.; Lamb, D. Q.; Robinson, K.; Truran, J. W.

    2003-12-01

    We present multidimensional simulations of deflagrating Chandrasekhar-mass C/O white dwarfs for both centered and off-centered ignition. Type Ia supernovae are thought to be thermonuclear explosions of Chandrasekhar-mass C/O white dwarfs. These events are used as ``standard candles" for investigating the structure of the Universe and play a fundamental role in galactic chemical evolution. Despite their importance, the exact nature of the explosion mechanism remains unsolved. Observations of intermediate mass elements such as Si, S, Mg indicate that the explosion begins as a deflagration. Our simulations utilize a custom implementation of the ``thick flame" model of Khokhlov for the sub-grid-scale nuclear flame. We present results of an extensive 1- and 2-dimensional parameter study of these events. We present three-dimensional whole-star deflagrations investigating the effect of centered vs. off-center ignition. Our results indicate that a slightly off-center ignition leads to a result that is not compatible with observations, providing constraints on conditions in the central region of the white dwarf prior to ignition. This work was supported by the DOE ASCI/Alliances program at the University of Chicago under grant No. B341495.

  7. 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/).

  8. 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%)

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. ROCKY EXTRASOLAR PLANETARY COMPOSITIONS DERIVED FROM EXTERNALLY POLLUTED WHITE DWARFS

    SciTech Connect

    Klein, B.; Jura, M.; Zuckerman, B.; Koester, D. E-mail: jura@astro.ucla.edu E-mail: koester@astrophysik.uni-kiel.de

    2011-11-01

    We report Keck High Resolution Echelle Spectrometer data and model atmosphere analysis of two helium-dominated white dwarfs, PG1225-079 and HS2253+8023, whose heavy pollutions most likely derive from the accretion of terrestrial-type planet(esimal)s. For each system, the minimum accreted mass is {approx}10{sup 22} g, that of a large asteroid. In PG1225-079, Mg, Cr, Mn, Fe, and Ni have abundance ratios similar to bulk Earth values, while we measure four refractory elements, Ca, Sc, Ti, and V, all at a factor of {approx}2-3 higher abundance than in the bulk Earth. For HS2253+8023 the swallowed material was compositionally similar to bulk Earth in being more than 85% by mass in the major element species, O, Mg, Si, and Fe, and with abundances in the distinctive proportions of mineral oxides-compelling evidence for an origin in a rocky parent body. Including previous studies we now know of four heavily polluted white dwarfs where the measured oxygen and hydrogen are consistent with the view that the parents' bodies formed with little ice, interior to any snow line in their nebular environments. The growing handful of polluted white dwarf systems with comprehensive abundance measurements form a baseline for characterizing rocky exoplanet compositions that can be compared with bulk Earth.

  14. Laminar flame acceleration by neon enrichment in white dwarf supernovae

    NASA Astrophysics Data System (ADS)

    Chamulak, David; Brown, Edward F.; Timmes, Francis X.

    We explore how the laminar flame speed of degenerate C/O thermonuclear burning during a type Ia supernova depends on the composition of the white dwarf. Type Ia supernovae are currently the premier standard candle for measuring distances to redshift 1.6. The currently favored scenario for this supernovae is the thermonuclear incineration of a C/O white dwarf. Recent observations suggest that there may be more than one population of progenitor, and it has been suggested the peak luminosity may depend on the composition of the progenitor white dwarf. Of particular interest is 22 Ne, which is formed from CNO elements during core He burning of the progenitor star and therefore reflects the metallicity of the progenitor. We find that the laminar flame speed of a C/O mixture increases linearly with the abundance of 22 Ne when the abundance of 22 Ne is small. The faster and narrower laminar flame enlarges the lengthscale at which turbulent eddies can disrupt the burn. As a result, the addition of 22 Ne might lower the density at which a transition to distributed burning occurs.

  15. 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.

  16. 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.

  17. 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.

  18. Convection Destroys the Core/Mantle Structure in Hybrid C/O/Ne White Dwarfs

    NASA Astrophysics Data System (ADS)

    Brooks, Jared; Schwab, Josiah; Bildsten, Lars; Quataert, Eliot; Paxton, Bill

    2017-01-01

    A hybrid C/O/Ne white dwarf (WD)—an unburned C/O core surrounded by an O/Ne/Na mantle—can be formed if the carbon flame is quenched in a super-AGB star or white dwarf merger remnant. We show that this segregated hybrid structure becomes unstable to rapid mixing within 2000 years of the onset of WD cooling. Carbon burning includes a weak reaction that removes electrons, resulting in a lower electron-to-baryon ratio ({Y}{{e}}) in the regions processed by carbon burning compared to the unburned C/O core, making the O/Ne mantle denser than the C/O core as the WD cools. This is unstable to efficient mixing. We use the results of {\\mathtt{MESA}} models with different size C/O cores to quantify the rate at which the cores mix with the mantle as they cool. In all cases, we find that the WDs undergo significant core/mantle mixing on timescales shorter than the time available to grow the WD to the Chandrasekhar mass (MCh) by accretion. As a result, hybrid WDs that reach MCh due to later accretion will have lower central carbon fractions than assumed thus far. We briefly discuss the implications of these results for the possibility of SNe Ia from hybrid WDs.

  19. TIDALLY INDUCED APSIDAL PRECESSION IN DOUBLE WHITE DWARFS: A NEW MASS MEASUREMENT TOOL WITH LISA

    SciTech Connect

    Valsecchi, F.; Farr, W. M.; Willems, B.; Deloye, C. J.; Kalogera, V.

    2012-02-01

    Galactic interacting double white dwarfs (DWDs) are guaranteed gravitational wave (GW) sources for the Laser Interferometer Space Antenna GW detector, with more than 10{sup 4} binaries expected to be detected over the mission's lifetime. Part of this population is expected to be eccentric, and here we investigate the potential for constraining the white dwarf (WD) properties through apsidal precession in these binaries. We analyze the tidal, rotational, and general relativistic contributions to apsidal precession by using detailed He WD models, where the evolution of the star's interior is followed throughout the cooling phase. In agreement with previous studies of zero-temperature WDs, we find that apsidal precession in eccentric DWDs can lead to a detectable shift in the emitted GW signal when binaries with cool (old) components are considered. This shift increases significantly for hot (young) WDs. We find that apsidal motion in hot (cool) DWDs is dominated by tides at orbital frequencies above {approx}> 10{sup -4} Hz (10{sup -3} Hz). The analysis of apsidal precession in these sources while ignoring the tidal component would lead to an extreme bias in the mass determination, and could lead us to misidentify WDs as neutron stars or black holes. We use the detailed WD models to show that for older, cold WDs, there is a unique relationship that ties the radius and apsidal precession constant to the WD masses, therefore allowing tides to be used as a tool to constrain the source masses.

  20. Additional Ultracool White Dwarfs Found In The Sloan Digital Sky Survey

    DTIC Science & Technology

    2008-05-20

    the Sloan Digital Sky Survey ( SDSS ). The SDSS pho- tometry, spectra, and proper motions are presented, and additional BVRI data are given for these and...models for very cool white dwarfs. One of the new stars ( SDSS J1251+44) exhibits strong collision- induced absorption (CIA) in its spectra, while the...spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries ( SDSS J2239+00A) is part of a binary system—its

  1. The discontinuity near 1600 A in the spectra of DA white dwarfs

    NASA Technical Reports Server (NTRS)

    Wegner, G.

    1984-01-01

    Ultraviolet spectroscopic observations of two relatively cool DA white dwarfs, L481 - 60 (= WD 1544 - 37) and BPM 1266 ( = WD 2105 - 82), with the International Ultraviolet Explorer (IUE) satellite show a strong drop in their spectral energy distributions below 1600 A. Published model atmospheres and thier visual spectra suggest that these two stars have effective temperatures in the vicinity of 9,000-10,000 K, and it is proposed that the 1600 A feature could be due to the 342(1S) 3s2(1S) photoionization edge of Mg I.

  2. The discontinuity near 1600 A in the spectra of DA white dwarfs

    NASA Technical Reports Server (NTRS)

    Wegner, G.

    1984-01-01

    Ultraviolet spectroscopic observations of two relatively cool DA white dwarfs, L481 - 60 (= WD 1544 - 37) and BPM 1266 ( = WD 2105 - 82), with the International Ultraviolet Explorer (IUE) satellite show a strong drop in their spectral energy distributions below 1600 A. Published model atmospheres and thier visual spectra suggest that these two stars have effective temperatures in the vicinity of 9,000-10,000 K, and it is proposed that the 1600 A feature could be due to the 342(1S) 3s2(1S) photoionization edge of Mg I.

  3. Coronal X rays from single, magnetic white dwarfs - A search and probable detection

    NASA Technical Reports Server (NTRS)

    Arnaud, Keith A.; Zhelezniakov, Vladimir V.; Trimble, Virginia

    1992-01-01

    X-ray emission from a sample of five nearby (6-20 pc), strongly magnetic (10-200 MG), relatively cool (6000-14,000 K), single white dwarfs, two of which may possess coronae was explored. One star (GR 290) at better than 99-percent confidence is detected and upper limits from Einstein Observatory IPC data for four others are given. The detected luminosities and limits are in the range 1.4-12.5 x 10 exp 27 ergs/s.

  4. 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

  5. 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.

  6. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-03-01

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

  8. MagAO Imaging of Long-period Objects (MILO). II. A Puzzling White Dwarf around the Sun-like Star HD 11112

    NASA Astrophysics Data System (ADS)

    Rodigas, Timothy J.; Bergeron, P.; Simon, Amélie; Arriagada, Pamela; Faherty, Jacqueline K.; Anglada-Escudé, Guillem; Mamajek, Eric E.; Weinberger, Alycia; Butler, R. Paul; Males, Jared R.; Morzinski, Katie; Close, Laird M.; Hinz, Philip M.; Bailey, Jeremy; Carter, Brad; Jenkins, James S.; Jones, Hugh; O'Toole, Simon; Tinney, C. G.; Wittenmyer, Rob; Debes, John

    2016-11-01

    HD 11112 is an old, Sun-like star that has a long-term radial velocity (RV) trend indicative of a massive companion on a wide orbit. Here we present direct images of the source responsible for the trend using the Magellan Adaptive Optics system. We detect the object (HD 11112B) at a separation of 2.″2 (100 au) at multiple wavelengths spanning 0.6-4 μm and show that it is most likely a gravitationally bound cool white dwarf. Modeling its spectral energy distribution suggests that its mass is 0.9-1.1 M ⊙, which corresponds to very high eccentricity, near edge-on orbits from a Markov chain Monte Carlo analysis of the RV and imaging data together. The total age of the white dwarf is >2σ, which is discrepant with that of the primary star under most assumptions. The problem can be resolved if the white dwarf progenitor was initially a double white dwarf binary that then merged into the observed high-mass white dwarf. HD 11112B is a unique and intriguing benchmark object that can be used to calibrate atmospheric and evolutionary models of cool white dwarfs and should thus continue to be monitored by RV and direct imaging over the coming years.

  9. Hubble Space Telescope/STIS Spectroscopy of the White Dwarfs in the Short-Period Dwarf Novae LL Andromedae and EF Pegasi

    NASA Astrophysics Data System (ADS)

    Howell, Steve B.; Gänsicke, Boris T.; Szkody, Paula; Sion, Edward M.

    2002-08-01

    We present new Hubble Space Telescope Space Telescope Imaging Spectrograph observations of the short-period dwarf novae LL And and EF Peg during deep quiescence. We fit stellar models to the UV spectra and use optical and IR observations to determine the physical parameters of the white dwarfs in the systems, the distances to the binaries, and the properties of the secondary stars. Both white dwarfs are relatively cool, having Teff near 15,000 K, and consistent with a mass of 0.6Msolar. The white dwarf in LL And appears to be of solar abundance or slightly lower, while that in EF Peg is near 0.1-0.3 times solar. LL And is found to be 760 pc away, while EF Peg is closer, at 380 pc. EF Peg appears to have an ~M5 V secondary star, consistent with that expected for its orbital period, while the secondary object in LL And remains a mystery. Based on observations made with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the AURA under NASA contract NAS 5-26555 and with the Apache Point Observatory (APO) 3.5 m telescope, which is operated by the Astrophysical Research Consortium (ARC).

  10. 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.

  11. GASEOUS MATERIAL ORBITING THE POLLUTED, DUSTY WHITE DWARF HE 1349-2305

    SciTech Connect

    Melis, Carl; Burgasser, Adam J.; Dufour, P.; Farihi, J.; Bochanski, J.; Parsons, S. G.; Gaensicke, B. T.; Koester, D.; Swift, Brandon J.

    2012-05-20

    We present new spectroscopic observations of the polluted, dusty, helium-dominated atmosphere white dwarf star HE 1349-2305. Optical spectroscopy reveals weak Ca II infrared triplet emission indicating that metallic gas debris orbits and is accreted by the white dwarf. Atmospheric abundances are measured for magnesium and silicon while upper limits for iron and oxygen are derived from the available optical spectroscopy. HE 1349-2305 is the first gas disk-hosting white dwarf star identified among previously known polluted white dwarfs. Further characterization of the parent body polluting this star will require ultraviolet spectroscopy.

  12. An analysis of DA white dwarfs from the Hamburg Quasar Survey

    NASA Astrophysics Data System (ADS)

    Homeier, D.; Koester, D.; Hagen, H.-J.; Jordan, S.; Heber, U.; Engels, D.; Reimers, D.; Dreizler, S.

    1998-10-01

    Follow-up spectroscopy of several hundred hot stars detected by the Hamburg Quasar Survey (HQS) has been carried out between 1989 and 1996. We present the analysis of 80 DA white dwarfs using model atmospheres and theoretical cooling tracks to derive the atmospheric parameters T_eff and log g, masses and absolute magnitudes. The HQS turned out to be sensitive to the detection of hydrogen-rich white dwarfs in a wide temperature range, from 10 000 K upwards. Star counts within four HQS fields for magnitudes B<= 16fm 4 exceed those from the Palomar Green survey by about 50%. The more recent observation campaigns emphasized the detection of very hot degenerates, yielding a large fraction of DA stars with T_eff > 50 000 K compared to other surveys. The mean mass of our DA sample is M=0.61{M}_{\\odot}, with three massive DA stars exceeding 1{M}_{\\odot} and three DA stars with masses significantly below the assummed lower mass limit for single white dwarf evolution of 0.45{M}_{\\odot}. Among the cool DA stars, thirteen are potential ZZ Ceti candidates because their effective temperatures lie close to the instability strip. Based on observations collected at the German-Spanish Astronomical Center (DSAZ), Calar Alto, operated by the Max-Plank-Institut für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy; with the International Ultraviolet Explorer satellite (IUE) collected at Villafranca, Spain, and with the ROSAT X-ray telescope.

  13. On the Detection and Characterization of Polluted White Dwarfs

    NASA Astrophysics Data System (ADS)

    Steele, Amy; Debes, John H.; Deming, Drake

    2017-06-01

    There is evidence of circumstellar material around main sequence, giant, and white dwarf stars. What happens to this material after the main sequence? With this work, we focus on the characterization of the material around WD 1145+017. The goals are to monitor the white dwarf—which has a transiting, disintegrating planetesimal and determine the composition of the evaporated material for that same white dwarf by looking at high-resolution spectra. We also present preliminary results of follow-up photometric observations of known polluted WDs. If rocky bodies survive red giant branch evolution, then the material raining down on a WD atmosphere is a direct probe of main sequence cosmochemistry. If rocky bodies do not survive the evolution, then this informs the degree of post-main-sequence processing. These case studies will provide the community with further insight about debris disk modeling, the degree of post-main-sequence processing of circumstellar material, and the composition of a disintegrating planetesimal.

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

    NASA Astrophysics Data System (ADS)

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

    2017-08-01

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

  15. 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).

  16. Heavy-element abundance patterns in hot DA white dwarfs

    NASA Astrophysics Data System (ADS)

    Barstow, M. A.; Good, S. A.; Holberg, J. B.; Hubeny, I.; Bannister, N. P.; Bruhweiler, F. C.; Burleigh, M. R.; Napiwotzki, R.

    2003-05-01

    We present a series of systematic abundance measurements for 25 hot DA white dwarfs in the temperature range ~20 000-110 000 K, based on far-ultraviolet spectroscopy with the Space Telescope Imaging Spectrograph (STIS)/Goddard High Resolution Spectrograph (GHRS) on-board Hubble Space Telescope, IUE and FUSE. Using our latest heavy-element blanketed non-local thermodynamic equilibrium (non-LTE) stellar atmosphere calculations we have addressed the heavy-element abundance patterns, making completely objective measurements of abundance values and upper limits using a χ2 fitting technique to determine the uncertainties in the abundance measurements, which can be related to the formal upper limits in those stars where particular elements are not detected. We find that the presence or absence of heavy elements in the hot DA white dwarfs largely reflects what would be expected if radiative levitation is the supporting mechanism, although the measured abundances do not match the predicted values very well, as reported by other authors in the past. Almost all stars hotter than ~50 000 K contain heavy elements. For most of these the spread in element abundances is quite narrow and similar to the abundances measured in G191-B2B. However, there is an unexplained dichotomy at lower temperatures with some stars having apparently pure H envelopes and others having detectable quantities of heavy elements. The heavy elements present in these cooler stars are often stratified, lying in the outermost layers of the envelope. A few strong temperature/evolutionary effects are seen in the abundance measurements. There is a decreasing Si abundance with temperature, the N abundance pattern splits into two groups at lower temperature and there is a sharp decline in Fe and Ni abundance to zero, below ~50 000 K. When detected, the Fe and Ni abundances maintain an approximately constant ratio, close to the cosmic value of ~20. For the hottest white dwarfs observed by STIS, the strongest

  17. 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.

  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. 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

  20. High Resolution EUV & FUV Spectroscopy of DA White Dwarfs

    NASA Astrophysics Data System (ADS)

    Barstow, M. A.; Good, S. A.; Bannister, N. P.; Burleigh, M. R.; Holberg, J. B.; Bruhweiler, F. C.; Napiwotzki, R.; Cruddace, R. G.; Kowalski, M. P.

    We report on recent results from a high-resolution spectroscopic survey of hot DA white dwarfs, based on IUE, FUSE and HST observations. For the first time, we address the measurement of element abundances in a completely objective manner with a spectroscopic model fitting technique, which allows us to consider formally the limits that can be placed on abundances in stars where no heavy elements are detected. We also include our latest analysis of the high resolution EUV spectrum of G191-B2B recorded by J-PEX.

  1. Ultraviolet spectrum synthesis of the helium white dwarf Ross 640

    NASA Technical Reports Server (NTRS)

    Cottrell, P. L.; Greenstein, J. L.

    1980-01-01

    The UV spectrum of the He-rich white dwarf Ross 640 was used to derive Mg, Si, and Fe abundances by a spectrum synthesis technique. The values obtained were: Mg/He = 6.8 x 10 to the -8th; Si/He = 3.2 x 10 to the -8th; Ca/He = 7.6 x 10 to the -10th; and Fe/He = 2.0 x 10 to the -9th. The abundances indicate the operation of diffusion processes within a helium-rich atmosphere, provided some mechanism exists to deposit metals in the observable layers.

  2. Properties of quasi-periodic oscillations in accreting magnetic white dwarfs

    NASA Technical Reports Server (NTRS)

    Wu, Kinwah; Chanmugam, G.; Shaviv, G.

    1992-01-01

    Previous studies of time-dependent accretion onto magnetic white dwarfs, in which the cooling was assumed to be due to bremsstrahlung emission, have shown that the accretion shock undergoes oscillations. However, when cyclotron cooling is also included, the oscillations are damped for sufficiently strong magnetic fields. Here we demonstrate that the oscillations can be sustained by accretion-fluctuation-induced excitations. The frequency of the QPOs are shown to increase quadratically with the magnetic field strength. We interpret the oscillations as a two-phase process in which bremsstrahlung cooling dominates in one half-cycle and cyclotron cooling in the other. Such a process may have very different consequences compared to a single-phase process where the functional form of the cooling is essentially the same throughout the cycle. If in the two-phase process damping occurs mainly in the cyclotron cooling half-cycle, there will be a universal effective damping factor which tends to suppress all oscillation modes indiscriminately. The oscillations of the accretion shock also could be a limit cycle process in which the system vacillates between two branches.

  3. 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

  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. Toward High-precision Seismic Studies of White Dwarf Stars: Parametrization of the Core and Tests of Accuracy

    NASA Astrophysics Data System (ADS)

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

    2017-01-01

    We present a prescription for parametrizing the chemical profile in the core of white dwarfs in light of the recent discovery that pulsation modes may sometimes be deeply confined in some cool pulsating white dwarfs. Such modes may be used as unique probes of the complicated chemical stratification that results from several processes that occurred in previous evolutionary phases of intermediate-mass stars. This effort is part of our ongoing quest for more credible and realistic seismic models of white dwarfs using static, parametrized equilibrium structures. Inspired by successful techniques developed in design optimization fields (such as aerodynamics), we exploit Akima splines for the tracing of the chemical profile of oxygen (carbon) in the core of a white dwarf model. A series of tests are then presented to better seize the precision and significance of the results that can be obtained in an asteroseismological context. We also show that the new parametrization passes an essential basic test, as it successfully reproduces the chemical stratification of a full evolutionary model.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. Mass transfer in white dwarf-neutron star binaries

    NASA Astrophysics Data System (ADS)

    Bobrick, Alexey; Davies, Melvyn B.; Church, Ross P.

    2017-05-01

    We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs (WDs) with masses less than a critical mass of MWD, crit = 0.2 M⊙ undergo stable mass transfer and evolve into ultracompact X-ray binaries. Systems with higher mass WDs experience unstable mass transfer, which leads to tidal disruption of the WD. Our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-Eddington mass-transfer rates. We find that the eccentricities expected for WD-NS binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. Our results are broadly consistent with the observed numbers of both ultracompact X-ray binaries and radio pulsars with WD companions. The observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass WDs.

  14. LIMB-DARKENING COEFFICIENTS FOR ECLIPSING WHITE DWARFS

    SciTech Connect

    Gianninas, A.; Strickland, B. D.; Kilic, Mukremin; Bergeron, P. E-mail: benstrickland@ou.edu E-mail: bergeron@astro.umontreal.ca

    2013-03-20

    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 {approx}10{sup 5} eclipsing white dwarfs. The limb-darkening calculations presented here will be an essential part of the detailed analysis of all of these systems.

  15. 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.

  16. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2017-10-01

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

  18. A search for variable white dwarfs in large-area time-domain surveys: a pilot study in SDSS Stripe 82

    NASA Astrophysics Data System (ADS)

    Gentile Fusillo, Nicola Pietro; Hermes, J. J.; Gänsicke, Boris T.

    2016-01-01

    We present a method to reliably select variable white dwarfs from large-area time-domain surveys and apply this method in a pilot study to search for pulsating white dwarfs in the Sloan Digital Sky Survey Stripe 82. From a sample 400 high-confidence white dwarf candidates, we identify 24 which show significant variability in their multi-epoch Stripe 82 data. Using colours, we further selected a sample of pulsating white dwarf (ZZ Ceti) candidates and obtained high-cadence follow-up for six targets. We confirm five of our candidates as cool ZZ Cetis, three of which are new discoveries. Among our 24 candidates we also identify: one eclipsing binary, two magnetic white dwarfs and one pulsating PG1159 star. Finally, we discuss the possible causes for the variability detected in the remaining targets. Even with sparse multi-epoch data over the limited area of Stripe 82, we demonstrate that our selection method can successfully identify various types of variable white dwarfs and efficiently select high-confidence ZZ Ceti candidates.

  19. White dwarf models for type 1 supernovae and quiet supernovae, and presupernova evolution

    NASA Technical Reports Server (NTRS)

    Nomoto, K.

    1980-01-01

    Supernova mechanisms in accreting white dwarfs are considered with emphasis on deflagration as a plausible mechanism for producing Type I supernovae and electron captures to form quiet supernovae leaving neutron stars. These outcomes depend on accretion rate of helium, initial mass and composition of the white dwarf. The various types of hydrogen shell burning in the presupernova stage are also discussed.

  20. 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.

  1. 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.

  2. 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.

  3. GW Librae: a unique laboratory for pulsations in an accreting wh