Sample records for cool white dwarfs

  1. Cooling Models for Old White Dwarfs

    NASA Astrophysics Data System (ADS)

    Hansen, Brad M. S.

    1999-08-01

    We present new white dwarf cooling models that incorporate an accurate outer boundary condition based on new opacity and detailed radiative transfer calculations. We find that helium-atmosphere dwarfs cool considerably faster than has previously been claimed, while old hydrogen-atmosphere dwarfs will deviate significantly from blackbody appearance. We use our new models to derive age limits for the Galactic disk. We find that the Liebert, Dahn, & Monet luminosity function yields an age of only 6 Gyr if it is complete to stated limits. However, age estimates of individual dwarfs and the luminosity function of Oswalt et al. are both consistent with disk ages as large as ~11 Gyr. We have also used our models to place constraints on white dwarf dark matter in the Galactic halo. We find that previous attempts using inadequate cooling models were too severe and that direct detection limits allow a halo that is 11 Gyr old. If the halo is composed solely of helium-atmosphere dwarfs, the lower age limit is only 7.5 Gyr. We also demonstrate the importance of studying the cooling sequences of white dwarfs in globular clusters.

  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. 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.

  4. 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.

  5. 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.

  6. The Continuing Search for Variability Among Cool White Dwarfs

    NASA Astrophysics Data System (ADS)

    Schaefer, J. J.; Oswalt, T. D.; Johnston, K. B.; Rudkin, M.; Heinz, T.

    2002-12-01

    The Continuing Search for Variability Among Cool White Dwarfs Justin J. Schaefer University of Wyoming Department of Physics and Astronomy P.O. Box 3905 Laramie, Wyoming 82071 USA (schaefju@uwyo.edu) Terry D. Oswalt, Kyle Johnston, Merissa Rudkin, Tamalyn Heinz Florida Institute of Technology and the SARA Observatory Department of Physics & Space Sciences 150 West University Boulevard Melbourne, Florida 32901 USA (oswalt@luyten.astro.fit.edu, kyjohnst@fit.edu, mrudkin@astro.fit.edu, theinz@fit.edu) ABSTRACT We present BVRI photometry of eleven binaries with white dwarf (WD) components. The observations were obtained at the SARA 0.9-meter telescope on Kitt Peak during the summer of 2002. Standard system (B-V), (V-R) and (R-I) color indices of four white dwarfs were determined. This data will be used to estimate the WD cooling ages in wide WD+dM binaries, as part of our ongoing research program to determine the chromospheric activity-age relation for M dwarf stars. Time-series differential photometry was also collected for eight cool white dwarfs as part of a program to explore the variability in the low luminosity, low temperature regime of the WD cooling track. We failed to detect any variability greater than ~0.04 magnitudes in these stars. Several nights of differential photometry data were collected on the DAO WD + K dwarf short-period variable HS1136+6646. From the light variations we determined a likely orbital period of 0.825 +/-0.009 days. Strong evidence is presented for two other possible periods within this light curve, possibly indicative of rotational modulation by the WD component. We gratefully acknowledge support from the National Science Foundation, which funds the SARA Research Experiences for Undergraduates program via grant AST-0097616 to Florida Tech. One of us (TDO) also acknowledges partial support for this work from NASA (subcontract Y701296) and the NSF (AST 0206115).

  7. 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

  8. Cool DZ white dwarfs II: compositions and evolution of old remnant planetary systems

    NASA Astrophysics Data System (ADS)

    Hollands, M. A.; Gänsicke, B. T.; Koester, D.

    2018-06-01

    In a previous study, we analysed the spectra of 230 cool (Teff < 9000 K) white dwarfs exhibiting strong metal contamination, measuring abundances for Ca, Mg, Fe and in some cases Na, Cr, Ti, or Ni. Here, we interpret these abundances in terms of the accretion of debris from extrasolar planetesimals, and infer parent body compositions ranging from crust-like (rich in Ca and Ti) to core-like (rich in Fe and Ni). In particular, two white dwarfs, SDSS J0823+0546 and SDSS J0741+3146, which show log [Fe/Ca] > 1.9 dex, and Fe to Ni ratios similar to the bulk Earth, have accreted by far the most core-like exoplanetesimals discovered to date. With cooling ages in the range 1-8 Gyr, these white dwarfs are among the oldest stellar remnants in the Milky Way, making it possible to probe the long-term evolution of their ancient planetary systems. From the decrease in maximum abundances as a function of cooling age, we find evidence that the arrival rate of material on to the white dwarfs decreases by three orders of magnitude over a ≃ 6.5 Gyr span in white dwarf cooling ages, indicating that the mass-reservoirs of post-main sequence planetary systems are depleted on a ≃ 1 Gyr e-folding time-scale. Finally, we find that two white dwarfs in our sample are members of wide binaries, and both exhibit atypically high abundances, thus providing strong evidence that distant binary companions can dynamically perturb white dwarf planetary systems.

  9. A Search for Variability in Warm and Cool C-rich DQ White Dwarfs

    NASA Astrophysics Data System (ADS)

    Dupuis, Christopher Michael; Williams, Kurtis A.

    2018-01-01

    Hot DQ white dwarfs are a rare class of white dwarfs that have atmospheres dominated by carbon with little to no hydrogen or helium. Recently it has been found that the majority of these stars are photometrically variable likely due to rapid rotation with star spots. The cool progeny of the hot DQs are expected to also be rapidly rotating as no strong braking mechanisms should be present. We present the time-series photometry of multiple warm and cool C-rich DQ white dwarfs as part of an ongoing search for variability in hot DQ white dwarfs and their progeny. This program will permit us to confirm rotation as the source of variability, compare the distribution of rotation rates to those of more common white dwarf spectral types, and constrain the evolutionary rates of hot DQ rotation. These data are one way to better understand the formation scenarios of these stars.

  10. 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.

  11. 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.

  12. Luminosity and cooling of highly magnetized white dwarfs: suppression of luminosity by strong magnetic fields

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Mukul; Mukhopadhyay, Banibrata; Mukerjee, Subroto

    2018-06-01

    We investigate the luminosity and cooling of highly magnetized white dwarfs with electron-degenerate cores and non-degenerate surface layers where cooling occurs by diffusion of photons. We find the temperature and density profiles in the surface layers or envelope of white dwarfs by solving the magnetostatic equilibrium and photon diffusion equations in a Newtonian framework. We also obtain the properties of white dwarfs at the core-envelope interface, when the core is assumed to be practically isothermal. With the increase in magnetic field, the interface temperature increases whereas the interface radius decreases. For a given age of the white dwarf and for fixed interface radius or interface temperature, we find that the luminosity decreases significantly from about 10-6 to 10-9 L⊙ as the magnetic field strength increases from about 109 to 1012 G at the interface and hence the envelope. This is remarkable because it argues that magnetized white dwarfs are fainter and can be practically hidden in an observed Hertzsprung-Russell diagram. We also find the cooling rates corresponding to these luminosities. Interestingly, the decrease in temperature with time, for the fields under consideration, is not found to be appreciable.

  13. 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.

  14. The Cool White Dwarf Luminosity Function and the Age of the Galactic Disk

    NASA Astrophysics Data System (ADS)

    Leggett, S. K.; Ruiz, Maria Teresa; Bergeron, P.

    1998-04-01

    We present new optical and infrared data for the cool white dwarfs in the proper motion sample of Liebert, Dahn, & Monet. Stellar properties--surface chemical composition, effective temperature, radius, surface gravity, mass, and luminosity--are determined from these data by using the model atmospheres of Bergeron, Saumon, & Wesemael. The space density contribution is calculated for each star and the luminosity function (LF) for cool white dwarfs is determined. Comparing the LF to the most recent cooling sequences by Wood implies that the age of the local region of the Galactic disk is 8 +/- 1.5 Gyr. This result is consistent with the younger ages now being derived for the globular clusters and the universe itself.

  15. DISCOVERY OF A POSSIBLE COOL WHITE DWARF COMPANION FROM THE AllWISE MOTION SURVEY

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Fajardo-Acosta, Sergio B.; Kirkpatrick, J. Davy; Gelino, Christopher R.

    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 H{sub 2}O 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.more » 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.« less

  16. 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.

  17. 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.

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Richer, Harvey B.; Goldsbury, Ryan; Heyl, Jeremy

    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 clustermore » 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.« less

  19. 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.

  20. Building Magnetic Fields in White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2017-03-01

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

  1. ON THE EVOLUTION OF MAGNETIC WHITE DWARFS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

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

    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 entirelymore » 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.« less

  2. 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).

  3. Is EG 50 a White or Strange Dwarf?

    NASA Astrophysics Data System (ADS)

    Hajyan, G. S.; Vartanyan, Yu. L.

    2017-12-01

    The time dependences of the luminosity of a white dwarf and four strange dwarfs with masses of 0.5 M (the mass of the white dwarf EG 50 with a surface temperature of 2.1·104 K) are determined taking neutrino energy losses into account. It was assumed that these configurations radiate only at the expense of thermal energy reserves. It is shown that the sources of thermal energy owing to nonequilibrium b-processes and the phenomenon of crystallization of electron-nuclear matter are insignificant in determining the cooling time of white and strange dwarfs with masses of 0.5 M⨀. It is shown that in this approximation the time dependences of the luminosity of white and strange dwarfs with masses of 0.5 M⨀ differ significantly only for surface temperatures TR≥7·104 K, so it is impossible to determine whether EG 50 is a white or strange dwarf based on the cooling time.

  4. 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.

  5. The population of single and binary white dwarfs of the Galactic bulge

    NASA Astrophysics Data System (ADS)

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

    2018-05-01

    Recent Hubble Space Telescope observations have unveiled the white dwarf cooling sequence of the Galactic bulge. Although the degenerate sequence can be well fitted employing the most up-to-date theoretical cooling sequences, observations show a systematic excess of red objects that cannot be explained by the theoretical models of single carbon-oxygen white dwarfs of the appropriate masses. Here, we present a population synthesis study of the white dwarf cooling sequence of the Galactic bulge that takes into account the populations of both single white dwarfs and binary systems containing at least one white dwarf. These calculations incorporate state-of-the-art 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 evolutionary history of binary systems. Our Monte Carlo simulator also incorporates all the known observational biases. This allows us to model with a high degree of realism the white dwarf population of the Galactic bulge. We find that the observed excess of red stars can be partially attributed to white dwarf plus main sequence binaries, and to cataclysmic variables or dwarf novae. Our best fit is obtained with a higher binary fraction and an initial mass function slope steeper than standard values, as well as with the inclusion of differential reddening and blending. Our results also show that the possible contribution of double degenerate systems or young and thick-discbulge stars is negligible.

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

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

    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.more » 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.« less

  7. 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.

  8. A Search for a Surviving White Dwarf Companion in SN 1006

    NASA Astrophysics Data System (ADS)

    Kerzendorf, W. E.; Strampelli, G.; Shen, K. J.; Schwab, J.; Pakmor, R.; Do, T.; Buchner, J.; Rest, A.

    2018-05-01

    Multiple channels have been proposed to produce Type Ia supernovae, with many scenarios suggesting that the exploding white dwarf accretes from a binary companion pre-explosion. In almost all cases, theory suggests that this companion will survive. However, no such companion has been unambiguously identified in ancient supernova remnants - possibly falsifying the accretion scenario. Existing surveys, however, have only looked for stars as faint as ≈0.1L⊙ and thus might have missed a surviving white dwarf companion. In this work, we present very deep DECAM imaging (u, g, r, z) of the Type Ia supernova remnant SN 1006 specifically to search for a potential surviving white dwarf companion. We find no object that is consistent with a relatively young cooling white dwarf within the inner half of the SN 1006 remnant. We find that if there is a companion white dwarf, it must be redder than the standard white dwarf cooling track, or it must have formed long ago and cooled undisturbed for >108 yr. We conclude that our findings are consistent with the complete destruction of the secondary (such as in a merger) or an anomalously red or very dim surviving companion white dwarf.

  9. 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

  10. 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.

  11. The Origin and Evolution of the White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Clemens, J. C.

    1994-05-01

    The secret of how white dwarf stars form and evolve is hidden in their interiors. There, gravity separates the constituent elements into layers; the lighter elements float to the top and the heavier ones sink. Consequently, a white dwarf's structure depends on the quantity of the elements present. Measuring that structure can tell us about the processes which formed white dwarfs and allow us to calculate how fast they cool. The latter is indispensable for measuring the age of our galaxy using the oldest white dwarfs as chronometers. Because some white dwarfs pulsate, we can exploit the resulting luminosity variations to measure their internal structure using asteroseismology. Exploring white dwarf structure via asteroseismology poses a difficult observational task: acquiring essentially uninterrupted time series measurements of the brightness changes of pulsating white dwarf stars. We have accomplished this task using an instrument we call the Whole Earth Telescope (WET). By combining data from the WET with published measurements, we have detected a common pattern in the pulsation spectra of all the variable, hydrogen spectra white dwarfs (DAVs), implying that they have similar surface hydrogen layer masses. Because we have identified the degree (l) and the radial overtone (k) of the modes in the pattern detected, we have been able to compare their periods to published pulsation models to find the mass of the hydrogen layer; it is about 10(-4) times the total stellar mass. This result will require adjustments to published estimates of the age of the galaxy which use theoretical cooling times of the oldest white dwarfs as a time standard; the theoretical models typically assume much thinner hydrogen layers. We have also investigated the two classes of pulsating helium spectra white dwarfs (DOVs and DBVs). From their pulsation properties, and the mass of the hydrogen layer measured for the DAVs, we have concluded that the helium surface white dwarfs do not form via

  12. 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

  13. Thermal evolution of old white dwarfs

    NASA Astrophysics Data System (ADS)

    Kozhberov, Andrew

    2017-11-01

    This work is devoted to a description of thermodynamic properties of Coulomb crystals which are expected to form in white dwarf interiors. Effects of magnetic field, isotopic impurities, polarization of the electron background and crystal lattice type on the thermal evolution of white dwarfs are discussed. It is shown that the electron polarization could play a noticeable role in the cooling process. While other parameters in concern do not make a significant impact.

  14. White dwarfs in the Gaia era

    NASA Astrophysics Data System (ADS)

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

    2018-04-01

    The vast majority of stars will become white dwarfs at the end of the stellar life cycle. These remnants are precise cosmic clocks owing to their well constrained cooling rates. Gaia Data Release 2 is expected to discover hundreds of thousands of white dwarfs, which can then be observed spectroscopically with WEAVE and 4MOST. By employing spectroscopically derived atmospheric parameters combined with Gaia parallaxes, white dwarfs can constrain the stellar formation history in the early developing phases of the Milky Way, the initial mass function in the 1.5 to 8 M ⊙ range, and the stellar mass loss as well as the state of planetary systems during the post main-sequence evolution.

  15. 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

  16. NEW INSIGHTS INTO THE PROBLEM OF THE SURFACE GRAVITY DISTRIBUTION OF COOL DA WHITE DWARFS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Tremblay, P.-E.; Bergeron, P.; Gianninas, A.

    2010-04-01

    We review at length the longstanding problem in the spectroscopic analysis of cool hydrogen-line (DA) white dwarfs (T{sub eff}< 13,000 K) where gravities are significantly higher than those found in hotter DA stars. The first solution that has been proposed for this problem is a mild and systematic helium contamination from convective mixing that would mimic the high gravities. We constrain this scenario by determining the helium abundances in six cool DA white dwarfs using high-resolution spectra from the Keck I 10 m telescope. We obtain no detections, with upper limits as low as He/H = 0.04 in some cases.more » This allows us to put this scenario to rest for good. We also extend our model grid to lower temperatures using improved Stark profiles with non-ideal gas effects from Tremblay and Bergeron and find that the gravity distribution of cool objects remains suspiciously high. Finally, we find that photometric masses are, on average, in agreement with expected values, and that the high-log g problem is so far unique to the spectroscopic approach.« less

  17. The Origin and Evolution of the White-Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Clemens, J. C.

    1994-12-01

    The secret of how white dwarf stars form and evolve is hidden in their interiors. There, gravity separates the constituent elements into layers; the lighter elements float to the top and the heavier ones sink. Consequently, a white dwarf's structure depends on the quantity of the elements present. Measuring that structure can tell us about the processes which formed white dwarfs and allow us to calculate how fast they cool. The latter is indispensable for measuring the age of our galaxy using the oldest white dwarfs as chronometers. Because some white dwarfs pulsate, we can exploit the resulting luminosity variations to measure their internal structure using "asteroseismology," a procedure analogous to terrestrial seismology. Exploring white dwarf structure via asteroseismology poses a difficult observational task: acquiring essentially uninterrupted time series measurements of the brightness changes of pulsating white dwarf stars. We have accomplished this task using an instrument we developed for this purpose, the Whole Earth Telescope. By combining data from the Whole Earth Telescope with published measurements, we have detected a common pattern in the pulsation spectra of all the variable, hydrogen spectra white dwarfs (DAVs), implying that they have similar surface hydrogen layer masses. Because we have identified the degree (l) and the radial overtone (k) of the modes in the pattern detected, we have been able to compare their periods to published pulsation models to find the mass of the hydrogen layer; it is about 10^-4 times the total stellar mass. This result will require adjustments to published estimates of the age of the galaxy which use theoretical cooling times of the oldest white dwarfs as a time standard; the theoretical models typically assume much thinner hydrogen layers. We have also investigated the two classes of pulsating helium spectra white dwarfs (DOVs and DBVs). From their pulsation properties, and the mass of the hydrogen layer measured

  18. The origin and evolution of the white dwarf stars

    NASA Astrophysics Data System (ADS)

    Clemens, James Christopher

    1994-01-01

    The secret of how white dwarf stars form and evolve is hidden in their interiors. There, gravity separates the constituent elements into layers; the lighter elements float to the top and the heavier ones sink. Consequently, a white dwarf's structure depends on the quantity of the elements present. Measuring that structure can tell Us about the processes which formed white dwarfs and allow us to calculate how fast they cool. The latter is indispensable for measuring the age of our galaxy using the oldest white dwarfs as chronometers. Because some white dwarfs pulsate, we can exploit the resulting luminosity variations to measure their internal structure using 'asteroseismology', a procedure analogous to terrestrial seismology. Exploring white dwarf structure via asteroseismology poses a difficult observational task: acquiring essentially uninterrupted time series measurements of the brightness changes of pulsating white dwarf stars. We have accomplished this task using an instrument we developed for this purpose, the Whole Earth Telescope. By combining data from the Whole Earth Telescope with published measurements, we have detected a common pattern in the pulsation spectra of all the variable, hydrogen spectra white dwarfs (DAVs), implying that they have similar surface hydrogen layer masses. Because we have identified the degree (l) and the radial overtone (k) of the modes in the pattern detected, we have been able to compare their periods to published pulsation models to find the mass of the hydrogen layer, it is about 10-4 times the total stellar mass. This result will require adjustments to published estimates of the age of the galaxy which use theoretical cooling times of the oldest white dwarfs as a time standard; the theoretical models typically assume much thinner hydrogen layers. We have also investigated the two classes of pulsating helium spectra white dwarfs (DOVs and DBVs). From their pulsation properties and the mass of the hydrogen layer measured for

  19. Observations of the Ultraviolet Spectra of Carbon White Dwarfs

    NASA Technical Reports Server (NTRS)

    Wagner, G. A.

    1982-01-01

    Strong ultraviolet carbon lines were detected in additional white DC (continuous visual spectra) dwarfs using the IUE. These lines are not seen in the ultraviolet spectrum of the cool DC star Stein 2051 B. The bright DA white dwarf LB 3303 has a strong unidentified absorption near lambda 1400.

  20. Wolf 1130: A Nearby Triple System Containing a Cool, Ultramassive White Dwarf

    NASA Astrophysics Data System (ADS)

    Mace, Gregory N.; Mann, Andrew W.; Skiff, Brian A.; Sneden, Christopher; Kirkpatrick, J. Davy; Schneider, Adam C.; Kidder, Benjamin; Gosnell, Natalie M.; Kim, Hwihyun; Mulligan, Brian W.; Prato, L.; Jaffe, Daniel

    2018-02-01

    Following the discovery of the T8 subdwarf WISE J200520.38+542433.9 (Wolf 1130C), which has a proper motion in common with a binary (Wolf 1130AB) consisting of an M subdwarf and a white dwarf, we set out to learn more about the old binary in the system. We find that the A and B components of Wolf 1130 are tidally locked, which is revealed by the coherence of more than a year of V-band photometry phase-folded to the derived orbital period of 0.4967 days. Forty new high-resolution, near-infrared spectra obtained with the Immersion Grating Infrared Spectrometer provide radial velocities and a projected rotational velocity (v sin i) of 14.7 ± 0.7 {km} {{{s}}}-1 for the M subdwarf. In tandem with a Gaia parallax-derived radius and verified tidal locking, we calculate an inclination of i = 29° ± 2°. From the single-lined orbital solution and the inclination we derive an absolute mass for the unseen primary ({1.24}-0.15+0.19 M ⊙). Its non-detection between 0.2 and 2.5 μm implies that it is an old (>3.7 Gyr) and cool (T eff < 7000 K) ONe white dwarf. This is the first ultramassive white dwarf within 25 pc. The evolution of Wolf 1130AB into a cataclysmic variable is inevitable, making it a potential SN Ia progenitor. The formation of a triple system with a primary mass >100 times the tertiary mass and the survival of the system through the common-envelope phase, where ∼80% of the system mass was lost, is remarkable. Our analysis of Wolf 1130 allows us to infer its formation and evolutionary history, which has unique implications for understanding low-mass star and brown dwarf formation around intermediate-mass stars.

  1. 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.

  2. 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.

  3. 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.

  4. The interacting binary white dwarf systems

    NASA Astrophysics Data System (ADS)

    Provencal, Judith Lucille

    1994-01-01

    helium white dwarfs. If this hypothesis is correct, these systems represent a second entry point onto the white dwarf cooling curve.

  5. Stationary radiation hydrodynamics of accreting magnetic white dwarfs.

    NASA Astrophysics Data System (ADS)

    Woelk, U.; Beuermann, K.

    1996-02-01

    Using an artificial viscosity, we solved the one-dimensional time-independent two-fluid hydrodynamic equations simultaneously to the fully frequency and angle dependent radiation transport in an accretion flow directed towards the surface of a magnetic white dwarf. We consider energy transfer from ions to electrons by Coulomb encounters and cooling by bremsstrahlung and by cyclotron radiation in fields between B=5 and 70MG. Electron and ion temperatures relax in the post-shock regime and the cooling flow settles onto the white dwarf surface. For high mass flow rates ˙(m) (in g/cm^2^/s), cooling takes place mainly by bremsstrahlung and the solutions approach the non-magnetic case. For low ˙(m) and high B, cooling is dominated by cyclotron radiation which causes the thickness of the cooling region to collapse by 1-2 orders of magnitude compared to the non-magnetic case. The electron temperature behind the shock drops from a few 10^8^ to a few 10^7^K and the ratio of cyclotron vs. total radiative flux approaches unity. For high ˙(m) and low B values, bremsstrahlung dominates, but cyclotron losses can never be neglected. We find a smooth transition from particle-heated to shock-heated atmospheres in the maximum electron temperature and also in the thickness of the heated layer. With these results, the stationary radiation-hydrodynamics of accreting magnetic white dwarfs with cyclotron and bremsstrahlung cooling has been solved for the whole range of observed mass flow rates and field strengths.

  6. The frequency of planetary debris around young white dwarfs

    NASA Astrophysics Data System (ADS)

    Koester, D.; Gänsicke, B. T.; Farihi, J.

    2014-06-01

    Context. Heavy metals in the atmospheres of white dwarfs are thought in many cases to be accreted from a circumstellar debris disk, which was formed by the tidal disruption of a rocky planetary body within the Roche radius of the star. The abundance analysis of photospheric elements and conclusions about the chemical composition of the accreted matter are a new and promising method of studying the composition of extrasolar planetary systems. However, ground-based searches for metal-polluted white dwarfs that rely primarily on the detection of the Ca ii K line become insensitive at Teff > 15 000 K because this ionization state depopulates. Aims: We present the results of the first unbiased survey for metal pollution among hydrogen-atmosphere (DA type) white dwarfs with cooling ages in the range 20-200 Myr and 17 000 K white dwarfs studied, or 56% show traces of heavy elements. In 25 stars (showing only Si and occasionally C), the elements can be explained by radiative levitation alone, although we argue that accretion has very likely occurred recently. The remaining 23 white dwarfs (27%), however, must be currently accreting. Together with previous studies from the ground and adopting bulk Earth abundances for the debris, accretion rates range from a few 105 g s-1 to a few 108 g s-1, with no evident trend in cooling age from ≈40 Myr to ≈2 Gyr. Only a single, modest case of metal pollution (Ṁ < 106 g s-1) is found among ten white dwarfs with Teff > 23 000 K, in excellent agreement

  7. 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.

  8. Can Oort clouds pollute their parent stars after they become white dwarfs?

    NASA Astrophysics Data System (ADS)

    Veras, D.; Shannon, A.; Gänsicke, B. T.

    2017-09-01

    Comets impact the Sun frequently. In fact, coronographs like those which are part of Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph Experiment (LASCO) reveal that a comet grazes the Sun every few days, with a total of about 2400 grazers from 1996 to 2008. This frequency underscores an outstanding question in the quest to understand planetary systems: what types of small bodies - pebbles, asteroids, comets or moons - are the primary polluter of white dwarfs? We determine how often remnant exo-Oort clouds, freshly excited from post-main-sequence stellar mass loss, dynamically inject comets inside the white dwarf's Roche radius. We improve upon previous studies by considering a representative range of single white dwarf masses (0.52-1.00 M⊙) and incorporating different cloud architectures, giant branch stellar mass loss, stellar flybys, Galactic tides and a realistic escape ellipsoid in self-consistent numerical simulations that integrate beyond 8 Gyr ages of white dwarf cooling. We find that ˜10^(-5) of the material in an exo-Oort cloud is typically amassed onto the white dwarf, and that hydrogen deposits accumulate even as the cloud dissipates. This accumulation may account for the relatively large amount of trace hydrogen, 10^(22) -10^(25) g, that is determined frequently among white dwarfs with cooling ages ≥1 Gyr. Our results also reaffirm the notion that exo-Oort cloud comets are not the primary agents of the metal budgets observed in polluted white dwarf atmospheres.

  9. Genesis of magnetic fields in isolated white dwarfs

    NASA Astrophysics Data System (ADS)

    Briggs, Gordon P.; Ferrario, Lilia; Tout, Christopher A.; Wickramasinghe, Dayal T.

    2018-05-01

    A dynamo mechanism driven by differential rotation when stars merge has been proposed to explain the presence of strong fields in certain classes of magnetic stars. In the case of the high field magnetic white dwarfs (HFMWDs), the site of the differential rotation has been variously thought to be the common envelope, the hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently accreted by a degenerate core. We have shown previously that the observed incidence of magnetism and the mass distribution in HFMWDs are consistent with the hypothesis that they are the result of merging binaries during common envelope evolution. Here we calculate the magnetic field strengths generated by common envelope interactions for synthetic populations using a simple prescription for the generation of fields and find that the observed magnetic field distribution is also consistent with the stellar merging hypothesis. We use the Kolmogorov-Smirnov test to study the correlation between the calculated and the observed field strengths and find that it is consistent for low envelope ejection efficiency. We also suggest that field generation by the plunging of a giant gaseous planet on to a white dwarf may explain why magnetism among cool white dwarfs (including DZ white dwarfs) is higher than among hot white dwarfs. In this picture a super-Jupiter residing in the outer regions of the white dwarf's planetary system is perturbed into a highly eccentric orbit by a close stellar encounter and is later accreted by the white dwarf.

  10. Genesis of magnetic fields in isolated white dwarfs

    NASA Astrophysics Data System (ADS)

    Briggs, Gordon P.; Ferrario, Lilia; Tout, Christopher A.; Wickramasinghe, Dayal T.

    2018-07-01

    A dynamo mechanism driven by differential rotation when stars merge has been proposed to explain the presence of strong fields in certain classes of magnetic stars. In the case of the high-field magnetic white dwarfs (HFMWDs), the site of the differential rotation has been variously thought to be the common envelope, the hot outer regions of a merged degenerate core or an accretion disc are formed by a tidally disrupted companion that is subsequently accreted by a degenerate core. We have shown previously that the observed incidence of magnetism and the mass distribution in HFMWDs are consistent with the hypothesis that they are the result of merging binaries during common envelope evolution. Here, we calculate the magnetic field strengths generated by common envelope interactions for synthetic populations using a simple prescription for the generation of fields and find that the observed magnetic field distribution is also consistent with the stellar merging hypothesis. We use the Kolmogorov-Smirnov test to study the correlation between the calculated and the observed field strengths and find that it is consistent for low envelope ejection efficiency. We also suggest that the field generation by the plunging of a giant gaseous planet on to a white dwarf may explain why magnetism among cool white dwarfs (including DZ white dwarfs) is higher than among hot white dwarfs. In this picture, a super-Jupiter residing in the outer regions of the white dwarf's planetary system is perturbed into a highly eccentric orbit by a close stellar encounter and is later accreted by the white dwarf.

  11. White Dwarfs

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Wesemael, F.; Murdin, P.

    2000-11-01

    White dwarf stars, also known as degenerate dwarfs, represent the endpoint of the evolution of stars with initial masses ranging from about 0.08 to about 8 solar masses. This large range encompasses the vast majority of stars formed in our Galaxy and thus white dwarf stars represent the most common endpoint of STELLAR EVOLUTION. It is believed that over 95% of the stars of our Galaxy will eventu...

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Law, Nicholas M.; Kraus, Adam L.; Street, Rachel

    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 decomposemore » 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

  13. 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

  14. 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.

  15. IUE spectrophotometry of the DA4 primary in the short-period white dwarf-red dwarf spectroscopic binary Case 1

    NASA Technical Reports Server (NTRS)

    Sion, E. M.; Guinan, E. F.; Wesemael, F.

    1984-01-01

    Low-resolution ultraviolet International Ultraviolet Explorer spectra of the DA white dwarf Case 1 are presented. The spectra show the presence of the 1400 A feature, already discovered in several other DA stars, and of a shallower trough in the 1550-1700 A range. A model atmosphere analysis of the ultraviolet energy distribution of the Ly-alpha red wing yields T(e) = 13,000 + or - 500 K. Possible interpretations of the 1400 A feature are reviewed. Case 1 is the coolest white dwarf found in a short-period, detached white dwarf-red dwarf binary, and its cooling time is consistent with estimates of the efficiency of angular momentum removal mechanisms in the phases subsequent to common envelope binary evolution.

  16. Helium shell flashes and evolution of accreting white dwarfs

    NASA Astrophysics Data System (ADS)

    Fujimoto, M. Y.; Sugimoto, D.

    1982-06-01

    The evolution of accreting white dwarfs is investigated from the onset of accretion through the helium shell flash. Properties of the helium shell flashes are studied by means of a generalized theory of shell flash and by numerical computations, and it is found that the shell flash grows up to the strength of a supernova explosion when the mass of the helium zone is large enough on a massive white dwarf. Although accretion onto a hot white dwarf causes a weaker shell flash than those onto cool ones, a strong tendency exists for the strength to be determined mainly by the accretion rate. For fast accretion, the shell flashes are weak and triggered recurrently, while for slow accretion the helium shell flash, once triggered, develops into a detonation supernova.

  17. 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.

  18. 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.

  19. 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.

  20. The 25 parsec local white dwarf population

    NASA Astrophysics Data System (ADS)

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

    2016-11-01

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

  1. 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.

  2. On the Spectral Evolution of Helium-atmosphere White Dwarfs Showing Traces of Hydrogen

    NASA Astrophysics Data System (ADS)

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

    2018-04-01

    We present a detailed spectroscopic analysis of 115 helium-line (DB) and 28 cool, He-rich hydrogen-line (DA) white dwarfs based on atmosphere fits to optical spectroscopy and photometry. We find that 63% of our DB population show hydrogen lines, making them DBA stars. We also demonstrate the persistence of pure DB white dwarfs with no detectable hydrogen feature at low effective temperatures. Using state-of-the art envelope models, we next compute the total quantity of hydrogen, M H, that is contained in the outer convection zone as a function of effective temperature and atmospheric H/He ratio. We find that some (T eff, M H) pairs cannot physically exist as a homogeneously mixed structure; such a combination can only occur as stratified objects of the DA spectral type. On that basis, we show that the values of M H inferred for the bulk of the DBA stars are too large and incompatible with the convective dilution scenario. We also present evidence that the hydrogen abundances measured in DBA and cool, helium-rich white dwarfs cannot be globally accounted for by any kind of accretion mechanism onto a pure DB star. We suggest that cool, He-rich DA white dwarfs are most likely created by the convective mixing of a DA star with a thin hydrogen envelope; they are not cooled down DBAs. We finally explore several scenarios that could account for the presence of hydrogen in DBA stars.

  3. White dwarf-main sequence binaries from LAMOST: the DR5 catalogue

    NASA Astrophysics Data System (ADS)

    Ren, J.-J.; Rebassa-Mansergas, A.; Parsons, S. G.; Liu, X.-W.; Luo, A.-L.; Kong, X.; Zhang, H.-T.

    2018-03-01

    We present the data release (DR) 5 catalogue of white dwarf-main sequence (WDMS) binaries from the Large Area Multi-Object fiber Spectroscopic Telescope (LAMOST). The catalogue contains 876 WDMS binaries, of which 757 are additions to our previous LAMOST DR1 sample and 357 are systems that have not been published before. We also describe a LAMOST-dedicated survey that aims at obtaining spectra of photometrically-selected WDMS binaries from the Sloan Digital Sky Survey (SDSS) that are expected to contain cool white dwarfs and/or early type M dwarf companions. This is a population under-represented in previous SDSS WDMS binary catalogues. We determine the stellar parameters (white dwarf effective temperatures, surface gravities and masses, and M dwarf spectral types) of the LAMOST DR5 WDMS binaries and make use of the parameter distributions to analyse the properties of the sample. We find that, despite our efforts, systems containing cool white dwarfs remain under-represented. Moreover, we make use of LAMOST DR5 and SDSS DR14 (when available) spectra to measure the Na I λλ 8183.27, 8194.81 absorption doublet and/or Hα emission radial velocities of our systems. This allows identifying 128 binaries displaying significant radial velocity variations, 76 of which are new. Finally, we cross-match our catalogue with the Catalina Surveys and identify 57 systems displaying light curve variations. These include 16 eclipsing systems, two of which are new, and nine binaries that are new eclipsing candidates. We calculate periodograms from the photometric data and measure (estimate) the orbital periods of 30 (15) WDMS binaries.

  4. White Dwarfs in Wide Binaries and the Age of the Galaxy

    NASA Astrophysics Data System (ADS)

    Smith, John Allyn

    A comprehensive study of common proper binary systems suspected of containing white dwarf stars is being conducted by Oswalt and collaborators (Oswalt et al. 1988). These systems usually contain a white dwarf and a main sequence star. In the present study, we use the white dwarf luminosity function to determine the age of the local Galactic disk as well as the local space density of white dwarfs. We obtained BVRI photometry of approximately 475 systems (of 512) which were found to contain about 325 white dwarfs. Of these white dwarfs, 152 met the selection criteria for our study and were used in the final analysis. Using this largest sample of cool white dwarfs in binary systems observed to date, we have determined an age for the Galactic disk of 9.7-0.8+0.9 Gyr which yields a lower limit age for the Universe of about 11.7 Gyr. Recent globular cluster studies agree to within ±1σ for the Galaxy age derived from our Disk age. The latest cosmologically derived age for the Universe, modified for the recently released Hipparcos data, is now in accordance with our age estimates for the Universe, for H o (69 km s-1 Mpc-1) and an inflationary cosmology. Further, our age is in accord with the ages derived for the Galaxy from nucleocomsochronology and meteoritic sample analyses. As a part of this work, we have determined the white dwarf space density to be 4.5 ± 1.0 10-3 pc-3, in accord with the results previously reported by Liebert, Dahn & Monet (1987). This space density corresponds to a white dwarf birthrate of 4.65 × 10-13 yr-1 pc-3. This research also details a unique approach to calculating and correcting for the incompleteness of a proper motion and magnitude selected stellar sample.

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Isern, Jordi; Külebi, Baybars; García-Berro, Enrique

    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, {sup 12}C and {sup 16}O, and of the impuritiesmore » 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.« less

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Kaplan, David L.; Walker, Arielle N.; Marsh, Thomas 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 eclipsesmore » 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).« less

  7. 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

  8. White dwarf-main sequence binaries from LAMOST: the DR5 catalogue

    NASA Astrophysics Data System (ADS)

    Ren, J.-J.; Rebassa-Mansergas, A.; Parsons, S. G.; Liu, X.-W.; Luo, A.-L.; Kong, X.; Zhang, H.-T.

    2018-07-01

    We present the data release (DR) 5 catalogue of white dwarf-main sequence (WDMS) binaries from the Large sky Area Multi-Object fibre Spectroscopic Telescope (LAMOST). The catalogue contains 876 WDMS binaries, of which 757 are additions to our previous LAMOST DR1 sample and 357 are systems that have not been published before. We also describe a LAMOST-dedicated survey that aims at obtaining spectra of photometrically selected WDMS binaries from the Sloan Digital Sky Survey (SDSS) that are expected to contain cool white dwarfs and/or early-type M dwarf companions. This is a population under-represented in previous SDSS WDMS binary catalogues. We determine the stellar parameters (white dwarf effective temperatures, surface gravities and masses, and M dwarf spectral types) of the LAMOST DR5 WDMS binaries and make use of the parameter distributions to analyse the properties of the sample. We find that, despite our efforts, systems containing cool white dwarfs remain under-represented. Moreover, we make use of LAMOST DR5 and SDSS DR14 (when available) spectra to measure the Na I λλ 8183.27, 8194.81 absorption doublet and/or Hα emission radial velocities of our systems. This allows identifying 128 binaries displaying significant radial velocity variations, 76 of which are new. Finally, we cross-match our catalogue with the Catalina Surveys and identify 57 systems displaying light-curve variations. These include 16 eclipsing systems, two of which are new, and nine binaries that are new eclipsing candidates. We calculate periodograms from the photometric data and measure (estimate) the orbital periods of 30 (15) WDMS binaries.

  9. 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

  10. 2MASS J06562998+3002455: Not a Cool White Dwarf Candidate, but a Population II Halo Star

    NASA Astrophysics Data System (ADS)

    de la Fuente Marcos, Raúl; de la Fuente Marcos, Carlos

    2018-06-01

    2MASS J06562998+3002455 or PSS 309-6 is a high proper-motion star that was discovered during a survey with the 2.1 m telescope at Kitt Peak National Observatory. Here, we reevaluate the status of this interesting star using Gaia DR2. Our results strongly suggest that PSS 309-6 could be a Population II star as the value of its V component is close to -220 km/s, which is typical for halo stars in the immediate solar neighborhood. Kapteyn's star is the nearest known halo star and PSS 309-6 exhibits similar kinematic and photometric signatures. Its properties also resemble those of 2MASS J15484023-3544254, which was once thought to be the nearest cool white dwarf but was later reclassified as K-type subdwarf. Although it is virtually certain that PSS 309-6 is not a nearby white dwarf but a more distant Population II subdwarf, further spectroscopic information, including radial velocity measurements, is necessary to fully characterize this probable member of the Galactic halo.

  11. First detection of the white dwarf cooling sequence of the galactic bulge

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Calamida, A.; Sahu, K. C.; Anderson, J.

    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{supmore » –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.« less

  12. The white dwarf luminosity function - A possible probe of the galactic halo

    NASA Technical Reports Server (NTRS)

    Tamanaha, Christopher M.; Silk, Joseph; Wood, M. A.; Winget, D. E.

    1990-01-01

    The dynamically inferred dark halo mass density, amounting to above 0.01 solar masses/cu pc at the sun's Galactocentric radius, can be composed of faint white dwarfs provided that the halo formed in a sufficiently early burst of star formation. The model is constrained by the observed disk white dwarf luminosity function which falls off below log (L/solar L) = -4.4, due to the onset of star formation in the disk. By using a narrow range for the initial mass function and an exponentially decaying halo star formation rate with an e-folding time equal to the free-fall time, all the halo dark matter is allowed to be in cool white dwarfs which lie beyond the falloff in the disk luminosity function. Although it is unlikely that all the dark matter is in these dim white dwarfs, a definite signature in the low-luminosity end of the white dwarf luminosity function is predicted even if they comprise only 1 percent of the dark matter. Current CCD surveys should answer the question of the existence of this population within the next few years.

  13. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2018-02-01

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

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

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

    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.

  16. 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.

  17. Gaia Reveals Evidence for Merged White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kilic, Mukremin; Hambly, N. C.; Bergeron, P.; Genest-Beaulieu, C.; Rowell, N.

    2018-06-01

    We use Gaia Data Release 2 to identify 13,928 white dwarfs within 100 pc of the Sun. The exquisite astrometry from Gaia reveals for the first time a bifurcation in the observed white dwarf sequence in both Gaia and the Sloan Digital Sky Survey (SDSS) passbands. The latter is easily explained by a helium atmosphere white dwarf fraction of 36%. However, the bifurcation in the Gaia colour-magnitude diagram depends on both the atmospheric composition and the mass distribution. We simulate theoretical colour-magnitude diagrams for single and binary white dwarfs using a population synthesis approach and demonstrate that there is a significant contribution from relatively massive white dwarfs that likely formed through mergers. These include white dwarf remnants of main-sequence (blue stragglers) and post-main sequence mergers. The mass distribution of the SDSS subsample, including the spectroscopically confirmed white dwarfs, also shows this massive bump. This is the first direct detection of such a population in a volume-limited sample.

  18. Using White Dwarf Companions of Blue Stragglers to Constrain Mass Transfer Physics

    NASA Astrophysics Data System (ADS)

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

    2018-06-01

    Complete membership studies of old open clusters reveal that 25% of the evolved stars follow pathways in stellar evolution that are impacted by binary evolution. Recent studies show that the majority of blue straggler stars, traditionally defined to be stars brighter and bluer than the corresponding main sequence turnoff, are formed through mass transfer from a giant star onto a main sequence companion, resulting in a white dwarf in a binary system with a blue straggler. We will present constraints on the histories and mass transfer efficiencies for two blue straggler-white dwarf binaries in open cluster NGC 188. The constraints are a result of measuring white dwarf cooling temperatures and surface gravities with HST COS far-ultraviolet spectroscopy. This information sets both the timeline for mass transfer and the stellar masses in the pre-mass transfer binary, allowing us to constrain aspects of the mass transfer physics. One system is formed through Case C mass transfer, leaving a CO-core white dwarf, and provides an interesting test case for mass transfer from an asymptotic giant branch star in an eccentric system. The other system formed through Case B mass transfer, leaving a He-core white dwarf, and challenges our current understanding of the expected regimes for stable mass transfer from red giant branch stars.

  19. Skylab ultraviolet stellar spectra - A new white dwarf, HD 149499 B

    NASA Technical Reports Server (NTRS)

    Parsons, S. B.; Wray, J. D.; Benedict, G. F.; Henize, K. G.; Laget, M.

    1976-01-01

    The letter reports the discovery of a cool star with excess brightness in the vacuum ultraviolet on an objective-prism photograph obtained during the second Skylab mission. This star, HD 149499, is of type K0 V and has a companion with an apparent magnitude of about 11.8; the relatively flat UV spectrum observed at the position of HD 149499 is characteristic of a 10th or 11th magnitude unreddened O- or early B-type star. It is shown that the excess VUV brightness is due to the companion, HD 149499B, which probably lies in the region of the H-R diagram occupied by the hot white dwarfs. Inspection of white dwarf lists indicates that this star is the sixth or seventh brightest white dwarf known. A maximum orbital motion of 0.025 arcsec/yr is estimated along with a period of just under 500 yr.

  20. 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

  1. A NEAR-INFRARED SPECTROSCOPIC SURVEY OF COOL WHITE DWARFS IN THE SLOAN DIGITAL SKY SURVEY

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Kilic, Mukremin; Kowalski, Piotr M.; Von Hippel, Ted

    2009-07-15

    We present near-infrared photometric observations of 15 and spectroscopic observations of 38 cool white dwarfs (WDs). This is the largest near-infrared spectroscopic survey of cool WDs to date. Combining the Sloan Digital Sky Survey photometry and our near-infrared data, we perform a detailed model atmosphere analysis. The spectral energy distributions of our objects are explained fairly well by model atmospheres with temperatures ranging from 6300 K down to 4200 K. Two WDs show significant absorption in the infrared, and are best explained with mixed H/He atmosphere models. Based on the up-to-date model atmosphere calculations by Kowalski and Saumon, we findmore » that the majority of the stars in our sample have hydrogen-rich atmospheres. We do not find any pure helium atmosphere WDs below 5000 K, and we find a trend of increasing hydrogen to helium ratio with decreasing temperature. These findings present an important challenge to understanding the spectral evolution of WDs.« less

  2. 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.

  3. 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.

  4. White Dwarfs in the SDSS Photometric Footprint

    NASA Astrophysics Data System (ADS)

    Gentile Fusillo, N. P.; Girven, J.; Gänsicke, B.

    2013-01-01

    Attempts to create a homogeneous catalogue of white dwarfs have always been faced with the challenge posed by the intrinsic faintness of these objects. In recent years, thanks to large area surveys like the Sloan Digital Sky Survey (SDSS), the size of the known white dwarf population has increased dramatically, but, in order to carry out a statical study on the population of white dwarfs, it is necessary to have a reliable and well-defined selection method. We present a method which uses cuts in colour-colour space to select from DR7 16785 bright (g ≤ 19) photometric DA white dwarf candidates (Girven et al. 2011). The selection is 62% efficient in returning DA white dwarfs and produces a DA sample which is 95% complete for Teff > 8000 K. This sample contains 4636 spectroscopically confirmed DA white dwarfs; i.e. a ˜70% increase compared to Eisenstein et al.'s sample. As a first application of the SDSS DR7 DA candidates sample we cross correlated it with Data Release 8 of UKIDSS Large Area Survey with the aim of identifying white dwarfs which exhibit an infrared excess consistent with the presence of low mass stellar companions or dusty debris discs. Our current work aims to extend the photometric selection to all types of white dwarfs, using reduced proper motion as a further constrain. We expect to find a total of ˜20 000 photometric white dwarf candidates with g ≤ 19 in the footprint of SDSS DR8.

  5. Building an Unusual White-Dwarf Duo

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-09-01

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

  6. White Dwarfs in Star Clusters: The Initial-Final Mass Relation for Stars from 0.85 to 8 M$_\\odot$

    NASA Astrophysics Data System (ADS)

    Cummings, Jeffrey; Kalirai, Jason; Tremblay, P.-E.; Ramírez-Ruiz, Enrico

    2018-01-01

    The spectroscopic study of white dwarfs provides both their mass, cooling age, and intrinsic photometric properties. For white dwarfs in the field of well-studied star clusters, this intrinsic photometry can be used to determine if they are members of that star cluster. Comparison of a member white dwarf's cooling age to its total cluster's age provides the evolutionary timescale of its progenitor star, and hence the mass. This is the initial-final mass relation (IFMR) for stars, which gives critical information on how a progenitor star evolves and loses mass throughout its lifetime, and how this changes with progenitor mass. Our work, for the first time, presents a uniform analysis of 85 white dwarf cluster members spanning from progenitor masses of 0.85 to 8 M$_\\odot$. Comparison of our work to theoretical IFMRs shows remarkable consistency in their shape but differences remain. We will discuss possible explanations for these differences, including the effects of stellar rotation.

  7. 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.

  8. White Dwarf/M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae

    NASA Astrophysics Data System (ADS)

    Wheeler, J. Craig

    2012-10-01

    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, MV >~ 8.4 on the SN Ia in SNR 0509-67.5 and MV >~ 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.

  9. 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.

  10. 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.

  11. Atmospheric studies of C2 white dwarfs

    NASA Astrophysics Data System (ADS)

    Swanson, Steven Roger

    Model atmosphere and line formation calculations for the delta nu = + 1 Swan bands of the C2 molecule are presented for seven white dwarfs and are compared to high resolution optical spectra. Limits on the C-12 to C-13 ratio are computed for highly pressure broadened lines and are used to analyze the observed spectra for any sign of absorption by the (C-12)(C-13) molecule. The metal abundances in cool white dwarf atmospheres and the usefulness of the determination of the C-12 to C-13 ratio are discussed. The line center shift and the pressure broadening are used to determine a value for the van der Waals interaction constant, C6. This is done using a detailed line modelling program which explicitly includes approximately 2000 rotational transition lines within the vibrational bands, in conjunction with atmospheric models calculated by the LUCIFER atmosphere modelling program. The isotopic shift of the vibrational and rotational lines is also included in the model to compare the detectability of various C-12 to C-13 ratios. The line models fit the observed spectra with varying degrees of accuracy. One star, WD0548-001, shows an unusually small pressure shift and broadening for the high pressures that the atmospheric model predicts. The results show that only in the hottest stars with the least pressure broadened lines in this study can the isotopic effect be seen. With the data available, the best limit on the C-12 to C-13 ratio is a minimum of 40 for WD0856 + 331. The models show that even for very high signal to noise data, the isotopic shift in the Swan bands in very cool white dwarfs would be difficult to separate from the pressure broadening effects. It is shown that the isotopic ratio is high enough to rule out the possibility that the carbon is a relic from previous CNO burning.

  12. Accretion Flows in Magnetic White Dwarf Systems

    NASA Technical Reports Server (NTRS)

    Imamura, James N.

    2005-01-01

    We received Type A and B funding under the NASA Astrophysics Data Program for the analysis and interpretation of hard x-ray data obtained by the Rossi X-ray Timing Explorer and other NASA sponsored missions for Intermediate Polars (IPS) and Polars. For some targets, optical data was available. We reduced and analyzed the X-ray spectra and the X-ray and optical (obtained at the Cerro Tololo Inter-American Observatory) timing data using detailed shock models (which we constructed) to place constraints on the properties of the accreting white dwarfs, the high energy emission mechanisms of white dwarfs, and the large-scale accretion flows of Polars and IPS. IPS and Polars are white dwarf mass-transfer binaries, members of the larger class of cata,clysmic variables. They differ from the bulk of the cataclysmic variables in that they contain strongly magnetic white dwarfs; the white dwarfs in Polars have B, = 7 to 230 MG and those in IPS have B, less than 10 MG. The IPS and Polars are both examples of funneled accretion flows in strong magnetic field systems. The IPS are similar to x-ray pulsars in that accretion disks form in the systems which are disrupted by the strong stellar magnetic fields of the white dwarfs near the stellar surface from where the plasma is funneled to the surface of the white dwarf. The localized hot spots formed at the footpoints of the funnels coupled with the rotation of the white dwarf leads to coherent pulsed x-ray emission. The Polars offer an example of a different accretion topology; the magnetic field of the white dwarf controls the accretion flow from near the inner Lagrangian point of the system directly to the stellar surface. Accretion disks do not form. The strong magnetic coupling generally leads to synchronous orbital/rotational motion in the Polars. The physical system in this sense resembles the Io/Jupiter system. In both IPS and Polars, pulsed emission from the infrared to x-rays is produced as the funneled flows merge onto the

  13. MagAO IMAGING OF LONG-PERIOD OBJECTS (MILO). II. A PUZZLING WHITE DWARF AROUND THE SUN-LIKE STAR HD 11112

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Rodigas, Timothy J.; Arriagada, Pamela; Faherty, Jacqueline K.

    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 μ m 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 {sub ⊙}, which corresponds to very high eccentricity, near edge-on orbits from amore » Markov chain Monte Carlo analysis of the RV and imaging data together. The total age of the white dwarf is >2 σ , 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.« less

  14. 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.

  15. 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.

  16. 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.

  17. 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

  18. WHITE DWARFS IN LOCAL STAR STREAMS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    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 thesemore » 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.« less

  19. A DARK SPOT ON A MASSIVE WHITE DWARF

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Kilic, Mukremin; Gianninas, Alexandros; Curd, Brandon

    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 mustmore » 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.« less

  20. 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.

  1. Cool Companions of White Dwarfs from 2MASS

    NASA Astrophysics Data System (ADS)

    Hoard, D. W.; Wachter, S.; Sturch, L. K.; Widhalm, A. M.; Weiler, K. P.; Wellhouse, J. W.; Gibiansky, M.

    2006-12-01

    Detecting low mass stellar companions to white dwarfs (WDs) offers many advantages compared to main sequence primaries. In the latter case, faint low mass companions are often hidden in the glare of the more luminous main sequence primary, and radial velocity variations are small and, therefore, difficult to detect. Since WDs are less luminous than main sequence stars, the brightness contrast compared to a potential faint companion is significantly reduced. Most importantly, the markedly different spectral energy distributions of the WDs and their low mass companions makes the detection and separation of the two components relatively straightforward even with simple broad-band multi-color photometry. We have shown in Wachter et al. (2003) that the 2MASS near-IR color-color diagram can easily and efficiently identify candidates for unresolved WD + red dwarf binaries. Our follow-up observations (e.g., Farihi et al. 2006) have shown that a large fraction of these candidates are confirmed as previously unknown binary stars. Here, we present results from our full survey of the 2235 WDs from the McCook & Sion (1999) Catalog using the 2MASS All-Sky Data Release. We have identified an additional large sample of candidate WD + red dwarf binaries, as well as a number of systems that may contain extremely low mass stellar or substellar companions. Support for this work was provided by the National Aeronautics and Space Administration (NASA) under an Astrophysics Data Program grant issued through the Office of Space Science. This research made use of the NASA/Infrared Processing and Analysis Center (IPAC) Infrared Science Archive, which is operated by the Jet Propulsion Laboratory/California Institute of Technology (CIT), under contract with NASA, and data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and IPAC/CIT, funded by NASA and the National Science Foundation.

  2. 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.

  3. Connecting Variability and Metals in White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kilic, Mukremin

    2016-10-01

    The Kepler and K2 missions have revealed that about half of the observed white dwarfs with sufficient signal-to-noise ratio light curves have low-level photometric variations at hour to day timescales. Potential explanations for the observed variability include the relativistic beaming effect, ellipsodial variations, eclipses, and reflection off of giant planets in close orbits. However, these are all rare events. Roughly 10% of white dwarfs are magnetic, and magnetic fields can explain part of this puzzle. However, the high incidence (50%) of variability is currently unexplained. HST COS spectroscopy of nearby white dwarfs show that about half of them have metals on their surface. Hence, we propose that the observed variability is due to the rotation of the star coupled with an inhomogeneous surface distribution of accreted metals. We have recently discovered an ideal system to test this hypothesis. J1529 is an apparently non-magnetic white dwarf that shows 5.9% photometric dips in the optical every 38 min. We propose to obtain COS TIME-TAG spectroscopy of J1529 over 4 orbits to search for surface abundance differences throughout the orbit and look for the flux redistribution effect in the optical. These observations will confirm or rule out the idea that inhomogeneous metal accretion on white dwarfs can explain the high incidence of variability. We predict that the LSST will identify 100,000 variable white dwarfs. Hence, understanding the source of variability in white dwarfs has implications for the current and future transient surveys.

  4. The temperature and radius of the white dwarf Stein 2051B

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Liebert, J.

    1976-12-15

    The temperature, radius, and other atmospheric parameters are derived for the cool DC white dwarf Stein 2051B (=G175-34B=EG 180), whose mass was recently determined by Strand. New spectrophotometric scans of this star and its dwarf M companion are discussed; these and existing Stroemgren photometry are fitted to model atmospheres with hydrogen/metal deficient compositions, and a temperature of 7050 +- 400 K is determined. The resulting radius of 0.0111 +- 0.0015 R/sub sun/ is marginally smaller than that of 40 Eri B. (AIP)

  5. 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.

  6. Thermonuclear Explosions from Hybrid C/O/Ne White Dwarf Progenitors Ignited Centrally After Interior Mixing

    NASA Astrophysics Data System (ADS)

    Augustine, Carlyn

    2018-01-01

    Type Ia Supernovae are thermonuclear explosions of white dwarf (WD) stars. Past studies predict the existence of "hybrid" white dwarfs, made of a C/O/Ne core with a O/Ne shell, and that these are viable progenitors for supernovae. More recent work found that the C/O core is mixed with the surrounding O/Ne while the WD cools. Inspired by this scenario, we performed simulations of thermonuclear supernovae in the single degenerate paradigm from these hybrid progenitors. Our investigation began by constructing a hybrid white dwarf model with the one-dimensional stellar evolution code MESA. The model was allowed to go through unstable interior mixing ignite carbon burning centrally. The MESA model was then mapped to a two-dimensional initial condition and an explosion simulated from that with FLASH. For comparison, a similar simulation of an explosion was performed from a traditional C/O progenitor WD. Comparing the yields produced by explosion simulations allows us to determine which model produces more 56Ni, and therefore brighter events, and how explosions from these models differ from explosions from previous models without the mixing during the WD cooling.

  7. Analyzing the Effects of Stellar Evolution on White Dwarf Ages

    NASA Astrophysics Data System (ADS)

    Moss, Adam; Von Hippel, Ted, Dr.

    2018-01-01

    White dwarfs are among the oldest objects in our Galaxy, thus if we can determine their ages, we can derive the star formation history of our Galaxy. As part of a larger project that will use Gaia parallaxes to derive the ages of tens of thousands of white dwarfs, we explore the impact on the total white dwarf age of various modern models of main sequence and red giant branch stellar evolution, as well as uncertainties in progenitor metallicity. In addition, we study the effect on white dwarf ages caused by uncertainties in the Initial Final Mass Relation, which is the mapping between zero age main sequence and white dwarf masses. We find that for old and high mass white dwarfs, uncertainties in these factors have little effect on the total white dwarf age.

  8. Studies of Binary Pulsar Evolution Through Hubble Space Telescope Imaging of White Dwarf Companions

    NASA Astrophysics Data System (ADS)

    Lundgren, S. C.; Foster, R. S.; Camilo, F.

    1995-12-01

    In observations of six binary millisecond pulsars with the Hubble Space Telescope, we have discovered white dwarf companions to PSRs J0034-0534, J1022+1001, and J1713+0747 and improved photometry on PSRs J1640+2224 and J2145-0750. The companion to PSR J2019+2425 was not detected down to m_I=25.4. For the five companions detected, effective temperatures were estimated for the colors measured. Two of the white dwarfs, J0034-0534 and J1713+0747, are among the coolest and oldest known. Using distance estimates to the pulsars, the absolute luminosities were determined. Constrains on the masses and cooling times were obtained from the luminosities and temperatures. The results for each pulsar were related to expectations based on models for white dwarf cooling, Roche lobe overflow in the preceding low-mass X-ray binary phase, and mass accretion rate/neutron star spin period relations. Precision pulsar astrophysics at the Naval Research Laboratory is supported by the Office of Naval Research. SL is supported by a post-doctoral fellowship through the National Research Council. FC acknowledges support from NSF grant AST 91-15103 and a fellowship under the auspices of the European Commission.

  9. 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.

  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. Gravitational Interactions of White Dwarf Double Stars

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

  12. Probing LSST's Ability to Detect Planets Around White Dwarfs

    NASA Astrophysics Data System (ADS)

    Cortes, Jorge; Kipping, David

    2018-01-01

    Over the last four years more than 2,000 planets outside our solar system have been discovered, motivating us to search for and characterize potentially habitable worlds. Most planets orbit Sun-like stars, but more exotic stars can also host planets. Debris disks and disintegrating planetary bodies have been detected around white dwarf stars, the inert, Earth-sized cores of once-thriving stars like our Sun. These detections are clues that planets may exist around white dwarfs. Due to the faintness of white dwarfs and the potential rarity of planets around them, a vast survey is required to have a chance at detecting these planetary systems. The Large Synoptic Survey Telescope (LSST), scheduled to commence operations in 2023, will image the entire southern sky every few nights for 10 years, providing our first real opportunity to detect planets around white dwarfs. We characterized LSST’s ability to detect planets around white dwarfs through simulations that incorporate realistic models for LSST’s observing strategy and the white dwarf distribution within the Milky Way galaxy. This was done through the use of LSST's Operations Simulator (OpSim) and Catalog Simulator (CatSim). Our preliminary results indicate that, if all white dwarfs were to possess a planet, LSST would yield a detection for every 100 observed white dwarfs. In the future, a larger set of ongoing simulations will help us quantify the number of planets LSST could potentially find.

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Limoges, M.-M.; Bergeron, P.; Lépine, S., E-mail: limoges@astro.umontreal.ca, E-mail: bergeron@astro.umontreal.ca, E-mail: slepine@chara.gsu.edu

    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 7more » 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.« less

  14. Highly magnetized super-Chandrasekhar white dwarfs and their consequences

    NASA Astrophysics Data System (ADS)

    Mukhopadhyay, B.; Das, U.; Rao, A. R.

    2018-01-01

    Since 2012, we have been exploring possible existence of highly magnetized significantly super-Chandrasekhar white dwarfs with a new mass-limit. This explains several observations, e.g. peculiar over-luminous type Ia supernovae, some white dwarf pulsars, soft gamma-ray repeaters and anomalous X-ray pulsars, which otherwise puzzled us enormously. We have proceeded to uncover the underlying issues by exploiting the enormous potential in quantum, classical and relativistic effects lying with magnetic fields present in white dwarfs. We have also explored the issues related to the stability and gravitational radiation of these white dwarfs.

  15. Magnetic white dwarfs: Observations, theory and future prospects

    NASA Astrophysics Data System (ADS)

    García-Berro, Enrique; Kilic, Mukremin; Kepler, Souza Oliveira

    2016-01-01

    Isolated magnetic white dwarfs have field strengths ranging from 103G to 109G, and constitute an interesting class of objects. The origin of the magnetic field is still the subject of a hot debate. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of the progenitor of white dwarfs, or on the contrary, are the result of binary interactions or, finally, other physical mechanisms that could produce such large magnetic fields during the evolution of the white dwarf itself, remains to be elucidated. In this work, we review the current status and paradigms of magnetic fields in white dwarfs, from both the theoretical and observational points of view.

  16. A Dark Spot on a Massive White Dwarf

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    We present the serendipitous discovery of eclipse-like events around the massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We selected J1529+2928 for time-series photometry based on its spectroscopic temperature and surface gravity, which place it near the ZZ Ceti instability strip. Instead of pulsations, we detect photometric dips from this white dwarf every 38 minutes. Follow-up optical spectroscopy observations with Gemini reveal no significant radial velocity variations, ruling out stellar and brown dwarf companions. A disintegrating planet around this white dwarf cannot explain the observed light curves in different filters. Given the short period, the source of the photometric dips must be a dark spot that comes into view every 38 minutes due to the rotation of the white dwarf. Our optical spectroscopy does not show any evidence of Zeeman splitting of the Balmer lines, limiting the magnetic field strength to B < 70 kG. Since up to 15% of white dwarfs display kG magnetic fields, such eclipse-like events should be common around white dwarfs. We discuss the potential implications of this discovery on transient surveys targeting white dwarfs, like the K2 mission and the Large Synoptic Survey Telescope. This work is based on observations obtained at the Gemini Observatory, McDonald Observatory, and the Apache Point Observatory 3.5-m telescope. The latter is owned and operated by the Astrophysical Research Consortium. Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

  17. 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.

  18. 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.

  19. 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.

  20. TOWARD HIGH-PRECISION SEISMIC STUDIES OF WHITE DWARF STARS: PARAMETRIZATION OF THE CORE AND TESTS OF ACCURACY

    DOE Office of Scientific and Technical Information (OSTI.GOV)

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

    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 Akimamore » 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.« less

  1. 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.

  2. The Post-merger Magnetized Evolution of White Dwarf Binaries: The Double-degenerate Channel of Sub-Chandrasekhar Type Ia Supernovae and the Formation of Magnetized White Dwarfs

    NASA Astrophysics Data System (ADS)

    Ji, Suoqing; Fisher, Robert T.; García-Berro, Enrique; Tzeferacos, Petros; Jordan, George; Lee, Dongwook; Lorén-Aguilar, Pablo; Cremer, Pascal; Behrends, Jan

    2013-08-01

    Type Ia supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though the nature of their progenitors is a subject of active investigation. Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. Additionally, recent theoretical work suggests that mergers which fail to detonate may produce magnetized, rapidly rotating white dwarfs. In this paper, we present the first multidimensional simulations of the post-merger evolution of white dwarf binaries to include the effect of the magnetic field. In these systems, the two white dwarfs complete a final merger on a dynamical timescale, and are tidally disrupted, producing a rapidly rotating white dwarf merger surrounded by a hot corona and a thick, differentially rotating disk. The disk is strongly susceptible to the magnetorotational instability (MRI), and we demonstrate that this leads to the rapid growth of an initially dynamically weak magnetic field in the disk, the spin-down of the white dwarf merger, and to the subsequent central ignition of the white dwarf merger. Additionally, these magnetized models exhibit new features not present in prior hydrodynamic studies of white dwarf mergers, including the development of MRI turbulence in the hot disk, magnetized outflows carrying a significant fraction of the disk mass, and the magnetization of the white dwarf merger to field strengths ~2 × 108 G. We discuss the impact of our findings on the origins, circumstellar media, and observed properties of SNe Ia and magnetized white dwarfs.

  3. The properties and origin of magnetic fields in white dwarfs

    NASA Astrophysics Data System (ADS)

    Kawka, A.

    2018-01-01

    A significant fraction of white dwarfs harbour a magnetic field with strengths ranging from a few kG up to about 1000 MG. The fraction appears to depend on the specific class of white dwarfs being investigated and may hold some clues to the origin of their magnetic field. The number of white dwarfs with variable fields as a function of their rotation phase have revealed a large field structure diversity, from a simple offset dipole to structures with spots or multipoles. A review of the current challenges in modelling white dwarf atmospheres in the presence of a magnetic field is presented, and the proposed scenarios for the formation of magnetic fields in white dwarfs are examined.

  4. The Spin Evolution of Fast-rotating, Magnetized Super-Chandrasekhar White Dwarfs in the Aftermath of White Dwarf Mergers

    NASA Astrophysics Data System (ADS)

    Becerra, L.; Rueda, J. A.; Lorén-Aguilar, P.; García-Berro, E.

    2018-04-01

    The evolution of the remnant of the merger of two white dwarfs is still an open problem. Furthermore, few studies have addressed the case in which the remnant is a magnetic white dwarf with a mass larger than the Chandrasekhar limiting mass. Angular momentum losses might bring the remnant of the merger to the physical conditions suitable for developing a thermonuclear explosion. Alternatively, the remnant may be prone to gravitational or rotational instabilities, depending on the initial conditions reached after the coalescence. Dipole magnetic braking is one of the mechanisms that can drive such losses of angular momentum. However, the timescale on which these losses occur depends on several parameters, like the strength of the magnetic field. In addition, the coalescence leaves a surrounding Keplerian disk that can be accreted by the newly formed white dwarf. Here we compute the post-merger evolution of a super-Chandrasekhar magnetized white dwarf taking into account all the relevant physical processes. These include magnetic torques acting on the star, accretion from the Keplerian disk, the threading of the magnetic field lines through the disk, and the thermal evolution of the white dwarf core. We find that the central remnant can reach the conditions suitable to develop a thermonuclear explosion before other instabilities (such as the inverse beta-decay instability or the secular axisymmetric instability) are reached, which would instead lead to gravitational collapse of the magnetized remnant.

  5. Searching for white dwarfs candidates in Sloan Digital Sky Survey Data

    NASA Astrophysics Data System (ADS)

    Należyty, Mirosław; Majczyna, Agnieszka; Ciechanowska, Anna; Madej, Jerzy

    2009-06-01

    Large amount of observational spectroscopic data are recently available from different observational projects, like Sloan Digital Sky Survey. It's become more urgent to identify white dwarfs stars based on data itself i.e. without modelling white dwarf atmospheres. In particular, existing methods of white dwarfs identification presented in Kleinman et al. (2004) and in Eisenstein et al. (2006) did not allow to find all the white dwarfs in examined data. We intend to test various criteria of searching for white dwarf candidates, based on photometric and spectral features.

  6. Polluted White Dwarf (Artist's Concept)

    NASA Image and Video Library

    2017-11-01

    This artist's concept shows an exoplanet and debris disk orbiting a polluted white dwarf. White dwarfs are dim, dense remnants of stars similar to the Sun that have exhausted their nuclear fuel and blown off their outer layers. By "pollution," astronomers mean heavy elements invading the photospheres -- the outer atmospheres -- of these stars. The leading explanation is that exoplanets could push small rocky bodies toward the star, whose powerful gravity would pulverize them into dust. That dust, containing heavy elements from the torn-apart body, would then fall on the star. NASA's Spitzer Space Telescope has been instrumental in expanding the field of polluted white dwarfs orbited by hot, dusty disks. Since launch in 2004, Spitzer has confirmed about 40 of these special stars. Another space telescope, NASA's Wide-field Infrared Survey Explorer (WISE), also detected a handful, bringing the total up to about four dozen known today. Because these objects are so faint, infrared light is crucial to identifying them. https://photojournal.jpl.nasa.gov/catalog/PIA22084

  7. White dwarf stars exceeding the Chandrasekhar mass limit

    NASA Astrophysics Data System (ADS)

    Tomaschitz, Roman

    2018-01-01

    The effect of nonlinear ultra-relativistic electron dispersion on the mass-radius relation of high-mass white dwarfs is studied. The dispersion is described by a permeability tensor in the Dirac equation, generated by the ionized high-density stellar matter, which constitutes the neutralizing background of the nearly degenerate electron plasma. The electron dispersion results in a stable mass-radius relation for high-mass white dwarfs, in contrast to a mass limit in the case of vacuum permeabilities. In the ultra-relativistic regime, the dispersion relation is a power law whose amplitude and scaling exponent is inferred from mass and radius estimates of two high-mass white dwarfs, Sirius B and LHS 4033. Evidence for the existence of super-Chandrasekhar mass white dwarfs is provided by several Type Ia supernovae (e.g., SN 2013cv, SN 2003fg, SN 2007if and SN 2009dc), whose mass ejecta exceed the Chandrasekhar limit by up to a factor of two. The dispersive mass-radius relation is used to estimate the radii, central densities, Fermi temperatures, bulk and compression moduli and sound velocities of their white dwarf progenitors.

  8. 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.

  9. White dwarf variability with gPhoton: pulsators

    NASA Astrophysics Data System (ADS)

    Tucker, Michael A.; Fleming, Scott W.; Pelisoli, Ingrid; Romero, Alejandra; Bell, Keaton J.; Kepler, S. O.; Caton, Daniel B.; Debes, John; Montgomery, Michael H.; Thompson, Susan E.; Koester, Detlev; Million, Chase; Shiao, Bernie

    2018-04-01

    We present results from a search for short time-scale white dwarf variability using gPhoton, a time-tagged data base of GALEX photon events and associated software package. We conducted a survey of 320 white dwarf stars in the McCook-Sion catalogue, inspecting each for photometric variability with particular emphasis on variability over time-scales less than ˜30 min. From that survey, we present the discovery of a new pulsating white dwarf: WD 2246-069. A Ca II K line is found in archival ESO spectra and an IR excess is seen in WISE W1 and W2 bands. Its independent modes are identified in follow-up optical photometry and used to model its interior structure. Additionally, we detect UV pulsations in four previously known pulsating ZZ Ceti-type (DAVs). Included in this group is the simultaneous fitting of the pulsations of WD 1401-147 in optical, near-ultraviolet and far-ultraviolet bands using nearly concurrent Whole Earth Telescope and GALEX data, providing observational insight into the wavelength dependence of white dwarf pulsation amplitudes.

  10. THE POST-MERGER MAGNETIZED EVOLUTION OF WHITE DWARF BINARIES: THE DOUBLE-DEGENERATE CHANNEL OF SUB-CHANDRASEKHAR TYPE Ia SUPERNOVAE AND THE FORMATION OF MAGNETIZED WHITE DWARFS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Ji Suoqing; Fisher, Robert T.; Garcia-Berro, Enrique

    2013-08-20

    Type Ia supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though the nature of their progenitors is a subject of active investigation. Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. Additionally, recent theoretical work suggests that mergers which fail to detonate may produce magnetized, rapidly rotating white dwarfs. In this paper, we present the first multidimensional simulations of the post-merger evolution of white dwarf binaries to include the effect of the magnetic field. In these systems, the twomore » white dwarfs complete a final merger on a dynamical timescale, and are tidally disrupted, producing a rapidly rotating white dwarf merger surrounded by a hot corona and a thick, differentially rotating disk. The disk is strongly susceptible to the magnetorotational instability (MRI), and we demonstrate that this leads to the rapid growth of an initially dynamically weak magnetic field in the disk, the spin-down of the white dwarf merger, and to the subsequent central ignition of the white dwarf merger. Additionally, these magnetized models exhibit new features not present in prior hydrodynamic studies of white dwarf mergers, including the development of MRI turbulence in the hot disk, magnetized outflows carrying a significant fraction of the disk mass, and the magnetization of the white dwarf merger to field strengths {approx}2 Multiplication-Sign 10{sup 8} G. We discuss the impact of our findings on the origins, circumstellar media, and observed properties of SNe Ia and magnetized white dwarfs.« less

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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).

  16. 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

  17. White Dwarfs in Cataclysmic Variables: An Update

    PubMed Central

    Sion, Edward M.; Godon, Patrick

    2018-01-01

    In this review, we summarize what is currently known about the surface temperatures of accreting white dwarfs in non-magnetic and magnetic cataclysmic variables (CVs) based upon synthetic spectral analyses of far ultraviolet data. We focus only on white dwarf surface temperatures, since in the area of chemical abundances, rotation rates, WD masses and accretion rates, relatively little has changed since our last review, pending the results of a large HST GO program involving 48 CVs of different CV types. The surface temperature of the white dwarf in SS Cygni is re-examined in the light of its revised distance. We also discuss new HST spectra of the recurrent nova T Pyxidis as it transitioned into quiescence following its April 2011 nova outburst. PMID:29505036

  18. 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.

  19. White Dwarfs in the HET Dark Energy Experiment

    NASA Astrophysics Data System (ADS)

    Castanheira, B. G.; Winget, D. E.; Williams, K.; Montgomery, M. H.; Falcon, R. E.; Hermes, J. J.

    2010-11-01

    In the past decades, large scale surveys have discovered a large number of white dwarfs. For example, the Sloan Digital Sky Survey (SDSS) Data Release 7 [5] lists about 20 000 spectroscopically confirmed new white dwarfs. More than just a number, the new discoveries revealed different flavors of white dwarfs, including a new class of pulsators [7] and a larger percentage of stars with a magnetic field [4]. The HET Dark Energy Experiment (HETDEX) will use the 9.2 m Hobby-Eberly Telescope at McDonald Observatory and a set of 150 spectrographs to map the three-dimensional positions of one million galaxies. The main goal of the survey is to probe dark energy by observing the recent universe (2<=z<=4). However, this unique, magnitude-limited survey (V<=22) will also provide a variety of by-products. We expect to obtain spectra for about 10 000 white dwarfs in the next 3 to 4 years.

  20. Ultraviolet Spectra of Two Magnetic White Dwarfs and Ultraviolet Spectra of Subluminous Objects Found in the Kiso Schmidt Survey

    NASA Technical Reports Server (NTRS)

    Wegner, Gary A.

    1987-01-01

    Low resolution International Ultraviolet Explorer (IUE) spectroscopic observations of two magnetic white dwarfs BPM25114 and K813-14 were obtained using both the SWP and LWP cameras. The first object has an observed magnetic field of 4 x 10(7) Gauss and the second has one of 3 x 10(7) Gauss. Both objects have overall spectral energy distributions appropriate for cool DA white dwarfs with T(eff) near 10,000 K and accordingly show strong lambda lambda 1400 and 1600 absorption in their spectra. Compared to non-magnetic DA white dwarfs of comparable effective temperature, there are some differences in the profiles, presumably produced by the magnetic fields in these objects. In addition, the ultraviolet spectra of a number of hot subluminous stars in the Kiso Schmidt survey were observed.

  1. Metal Lines in DA White Dwarfs

    NASA Astrophysics Data System (ADS)

    Zuckerman, B.; Koester, D.; Reid, I. N.; Hünsch, M.

    2003-10-01

    We report Keck telescope HIRES echelle observations of DA white dwarfs in a continuation of an extensive search for metals. These spectra are supplemented with new JHK magnitudes that are used to determine improved atmospheric parameters. Of the DA white dwarfs not in binary or common proper motion systems, about 25% show Ca II lines. For these, Ca abundances are determined from comparison with theoretical equivalent widths from model atmosphere calculations; in a few cases we also obtain Mg, Fe, Si, and Al abundances. If Ca is not observed, we generally determine very stringent upper limits. We compare the data to predictions of previously published models involving the accretion/diffusion of interstellar matter and of comets. The derived abundances are not obviously compatible with the predictions of either model, which up to now could only be tested with traces of metals in helium-rich white dwarfs. By modifying certain assumptions in the published interstellar accretion model we are able to match the distribution of the elements in the white dwarf atmospheres, but, even so, tests of other expectations from this scenario are less successful. Because comet accretion appears unlikely to be the primary cause of the DAZ phenomenon, the data suggest that no more than about 20% of F-type main-sequence stars are accompanied by Oort-like comet clouds. This represents the first observational estimate of this fraction. A plausible alternative to the accretion of cometary or interstellar matter is disruption and accretion of asteroidal material, a model first suggested in 1990 to explain excess near-infrared emission from the DAZ G29-38. An asteroidal debris model to account for the general DAZ phenomenon does not presently disagree with the HIRES data, but neither is there any compelling evidence in support of such a model. The HIRES data indicate that in close red dwarf/white dwarf binaries not known to be cataclysmic variables there is, nonetheless, significant mass

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

    PubMed

    Hause, Connor; Sion, Edward M; Godon, Patrick; Boris, T Gänsicke; 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 in Cataclysmic Variables. We use 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 white dwarf mass of 0.8 M ⊙ , our results indicate the presence of a 22-27,000 K white dwarf and a low mass accretion rate [Formula: see text], for a derived distance o ~200 to ~300 pc.

  3. Actively Disintegrating Astroids around a White Dwarf

    NASA Astrophysics Data System (ADS)

    Xu, Siyi

    2017-08-01

    Recent studies show that planetary systems can be widespread around white dwarfs. It has been proposed that planetary systems are responsible for the pollution observed in a white dwarf's atmosphere and the excess infrared radiation. This scenario is greatly strengthened by the recent discovery of actively disintegrating bodies orbiting around the white dwarf WD 1145+017. In addition, this system has a heavily polluted atmosphere, a dust disk, and circumstellar gas. Our team has been monitoring this system since its discovery and our recent COS data have revealed many new surprises. We propose to continue studying this system for the next two cycles and further constrain the evolution of the disintegrating bodies: what are the main mechanisms responsible for its destruction? How is circumstellar gas produced and maintained?

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

    NASA Astrophysics Data System (ADS)

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

    2017-11-01

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

  5. 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.

  6. Evidence for halo kinematics among cool carbon-rich dwarfs

    NASA Astrophysics Data System (ADS)

    Farihi, J.; Arendt, A. R.; Machado, H. S.; Whitehouse, L. J.

    2018-07-01

    This paper reports preliminary, yet compelling, kinematical inferences for N≳ 600 carbon-rich dwarf stars that demonstrate around 30-60 per cent are members of the Galactic halo. The study uses a spectroscopically and non-kinematically selected sample of stars from the SDSS, and cross-correlates these data with three proper motion catalogues based on Gaia DR1 astrometry to generate estimates of their 3D space velocities. The fraction of stars with halo-like kinematics is roughly 30 per cent for distances based on a limited number of parallax measurements, with the remainder dominated by the thick disc, but close to 60 per cent of the sample lies below an old, metal-poor disc isochrone in reduced proper motion. An ancient population is consistent with an extrinsic origin for C/O >1 in cool dwarfs, where a fixed mass of carbon pollution more readily surmounts lower oxygen abundance and with a lack of detectable ultraviolet-blue flux from younger white dwarf companions. For an initial stellar mass function that favours low-mass stars as in the Galactic disc, the dC stars are likely to be the dominant source of carbon-enhanced, metal-poor stars in the Galaxy.

  7. Evidence for halo kinematics among cool carbon-rich dwarfs

    NASA Astrophysics Data System (ADS)

    Farihi, J.; Arendt, A. R.; Machado, H. S.; Whitehouse, L. J.

    2018-04-01

    This paper reports preliminary yet compelling kinematical inferences for N ≳ 600 carbon-rich dwarf stars that demonstrate around 30% to 60% are members of the Galactic halo. The study uses a spectroscopically and non-kinematically selected sample of stars from the SDSS, and cross-correlates these data with three proper motion catalogs based on Gaia DR1 astrometry to generate estimates of their 3-D space velocities. The fraction of stars with halo-like kinematics is roughly 30% for distances based on a limited number of parallax measurements, with the remainder dominated by the thick disk, but close to 60% of the sample lie below an old, metal-poor disk isochrone in reduced proper motion. An ancient population is consistent with an extrinsic origin for C/O >1 in cool dwarfs, where a fixed mass of carbon pollution more readily surmounts lower oxygen abundances, and with a lack of detectable ultraviolet-blue flux from younger white dwarf companions. For an initial stellar mass function that favors low-mass stars as in the Galactic disk, the dC stars are likely to be the dominant source of carbon-enhanced, metal-poor stars in the Galaxy.

  8. 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.

  9. 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.

  10. 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.

  11. Testing the white dwarf mass-radius relationship with eclipsing binaries

    NASA Astrophysics Data System (ADS)

    Parsons, S. G.; Gänsicke, B. T.; Marsh, T. R.; Ashley, R. P.; Bours, M. C. P.; Breedt, E.; Burleigh, M. R.; Copperwheat, C. M.; Dhillon, V. S.; Green, M.; Hardy, L. K.; Hermes, J. J.; Irawati, P.; Kerry, P.; Littlefair, S. P.; McAllister, M. J.; Rattanasoon, S.; Rebassa-Mansergas, A.; Sahman, D. I.; Schreiber, M. R.

    2017-10-01

    We present high-precision, model-independent, mass and radius measurements for 16 white dwarfs in detached eclipsing binaries and combine these with previously published data to test the theoretical white dwarf mass-radius relationship. We reach a mean precision of 2.4 per cent in mass and 2.7 per cent in radius, with our best measurements reaching a precision of 0.3 per cent in mass and 0.5 per cent in radius. We find excellent agreement between the measured and predicted radii across a wide range of masses and temperatures. We also find the radii of all white dwarfs with masses less than 0.48 M⊙ to be fully consistent with helium core models, but they are on average 9 per cent larger than those of carbon-oxygen core models. In contrast, white dwarfs with masses larger than 0.52 M⊙ all have radii consistent with carbon-oxygen core models. Moreover, we find that all but one of the white dwarfs in our sample have radii consistent with possessing thick surface hydrogen envelopes (10-5 ≥ MH/MWD ≥ 10-4), implying that the surface hydrogen layers of these white dwarfs are not obviously affected by common envelope evolution.

  12. AR Sco as a possible seed of highly magnetized white dwarf

    NASA Astrophysics Data System (ADS)

    Mukhopadhyay, Banibrata; Rao, A. R.; Bhatia, Tanayveer Singh

    2017-12-01

    We explore the possibility that the recently discovered white dwarf pulsar AR Sco acquired its high spin and magnetic field due to repeated episodes of accretion and spin-down. An accreting white dwarf can lead to a larger mass and consequently a smaller radius thus causing an enhanced rotation period and a magnetic field. This spinning magnetic white dwarf temporarily can inhibit accretion, spin down and eventually, the accretion can start again due to the shrinking of the binary period by gravitational radiation. A repetition of the above cycle can eventually lead to a high magnetic field white dwarf, recently postulated to be the reason for overluminous type Ia supernovae. We also point out that these high magnetic field spinning white dwarfs are attractive sites for gravitational radiation.

  13. 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

  14. A Refined Search for Pulsations in White Dwarf Companions to Millisecond Pulsars

    NASA Astrophysics Data System (ADS)

    Kilic, Mukremin; Hermes, J. J.; Córsico, A. H.; Kosakowski, Alekzander; Brown, Warren R.; Antoniadis, John; Calcaferro, Leila M.; Gianninas, A.; Althaus, Leandro G.; Green, M. J.

    2018-06-01

    We present optical high-speed photometry of three millisecond pulsars with low-mass (<0.3 M⊙) white dwarf companions, bringing the total number of such systems with follow-up time-series photometry to five. We confirm the detection of pulsations in one system, the white dwarf companion to PSR J1738+0333, and show that the pulsation frequencies and amplitudes are variable over many months. A full asteroseismic analysis for this star is under-constrained, but the mode periods we observe are consistent with expectations for a M⋆ = 0.16 - 0.19M⊙ white dwarf, as suggested from spectroscopy. We also present the empirical boundaries of the instability strip for low-mass white dwarfs based on the full sample of white dwarfs, and discuss the distinction between pulsating low-mass white dwarfs and subdwarf A/F stars.

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Skinner, Julie N.; Morgan, Dylan P.; West, Andrew A.

    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 H α 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 amore » 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.« less

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  1. The end of the White Dwarf Cooling Sequence of NGC 6752

    NASA Astrophysics Data System (ADS)

    Bedin, Luigi

    2017-08-01

    We propose to study the last HST-accessible white dwarf (WD) cooling sequence (CS) for a nearby globular cluster (GC), the chemically complex, extreme blue horizontal branch cluster NGC 6752. Over 97% of stars end their lives as WDs, and the WD CS provides constraints not only on the age, but also potentially the star formation history of a GC. The CS of WDs also lies in the least-explored region of the color-magnitude diagram of old stellar populations. Recent deep imaging with HST has successfully reached the end of the WD CS in only three classical old GCs, M4, NGC 6397 and 47 Tuc, and reveals an unexpectedly complex, and double-peaked, WD CS in the metal rich old open cluster NGC 6791. One more investigation is in progress on the massive globular Omega Centauri, where over 14 sub-populations are known to exist.While almost every cluster is known to host multiple populations, every single cluster is unique. NGC 6752 is a bridge between the relatively simple globular clusters, and Omega Cen, the most complex globular cluster known. NGC 6752 has an extended blue horizontal branch, a collapsed core and 3 chemically distinct populations. It is our last chance to add diversity to our very limited sample of WD CS, so far containing only 3 globular clusters, one old open cluster, and the complex Omega Cen system. We need to undertake this investigation while HST is still operational, as there is no foreseeable opportunity in the post-HST era to have one extra WD CS in the homogeneus optical photometric system of HST.

  2. 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.

  3. Using self-organizing maps to identify potential halo white dwarfs.

    PubMed

    García-Berro, Enrique; Torres, Santiago; Isern, Jordi

    2003-01-01

    We present the results of an unsupervised classification of the disk and halo white dwarf populations in the solar neighborhood. The classification is done by merging the results of detailed Monte Carlo (MC) simulations, which reproduce very well the characteristics of the white dwarf populations in the solar neighborhood, with a catalogue of real stars. The resulting composite catalogue is analyzed using a competitive learning algorithm. In particular we have used the so-called self-organized map. The MC simulated stars are used as tracers and help in identifying the resulting clusters. The results of such an strategy turn out to be quite satisfactory, suggesting that this approach can provide an useful framework for analyzing large databases of white dwarfs with well determined kinematical, spatial and photometric properties once they become available in the next decade. Moreover, the results are of astrophysical interest as well, since a straightforward interpretation of several recent astronomical observations, like the detected microlensing events in the direction of the Magellanic Clouds, the possible detection of high proper motion white dwarfs in the Hubble Deep Field and the discovery of high velocity white dwarfs in the solar neighborhood, suggests that a fraction of the baryonic dark matter component of our galaxy could be in the form of old and dim halo white dwarfs.

  4. 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.

  5. Magnetically gated accretion in an accreting 'non-magnetic' white dwarf.

    PubMed

    Scaringi, S; Maccarone, T J; D'Angelo, C; Knigge, C; Groot, P J

    2017-12-13

    White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 10 6 gauss or more) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as 'non-magnetic', because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the 'non-magnetic' accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 10 4 gauss and 1 × 10 5 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified.

  6. 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.

  7. Nonvalidity of I-Love-Q Relations for Hot White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Boshkayev, K.; Quevedo, H.

    2018-05-01

    The equilibrium configurations of uniformly rotating white dwarfs at finite temperatures are investigated, exploiting the Chandrasekhar equation of state for different isothermal cores. The Hartle-Thorne formalism is applied to construct white dwarf configurations in the framework of Newtonian physics. The equations of structure are considered in the slow rotation approximation and all basic parameters of rotating hot white dwarfs are computed to test the so-called moment of inertia, tidal Love number and quadrupole moment (I-Love-Q) relations. It is shown that even within the same equation of state the I-Love-Q relations are not universal for white dwarfs at finite temperatures.

  8. Hubble COS Spectroscopy of the Dwarf Nova CW Mon: The White Dwarf in Quiescence?1

    PubMed Central

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

    2018-01-01

    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 in Cataclysmic Variables. We use 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 white dwarf mass of 0.8M⊙, our results indicate the presence of a 22–27,000 K white dwarf and a low mass accretion rate (M˙≲10−10M⊙/yr), for a derived distance o ~200 to ~300 pc. PMID:29430023

  9. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

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

  11. Prospective EUV observations of hot DA white dwarfs with the EUV Explorer

    NASA Technical Reports Server (NTRS)

    Finley, David S.; Malina, Roger F.; Bowyer, Stuart

    1987-01-01

    The Extreme Ultraviolet Explorer (EUVE) will perform a high sensitivity EUV all-sky survey. A major category of sources which will be detected with the EUVE instruments consists of hot white dwarfs. Detailed preliminary studies of synthetic EUV observations of white dwarfs have been carried out using the predicted EUVE instrumental response functions. Using available information regarding space densities of white dwarfs and the distribution of neutral hydrogen in the interstellar medium, the numbers of DA white dwarfs which will be detectable in the different EUV bandpasses have been estimated.

  12. 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.

  13. 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

  14. AR Sco: the first white dwarf pulsar?

    NASA Astrophysics Data System (ADS)

    Gaensicke, Boris

    2015-10-01

    AR Sco was mis-classified in 1971 as a pulsating delta-Scuti star, and has received little attention until now. In May this year, we became aware of the truly unique nature of this object: besides a two-magnitude modulation on the 3.56h orbital period, we detected a coherent 2min variability from the optical into the radio, and a spectacular infrared excess across the WISE bands. Our optical spectroscopy reveals a late-type companion star, clearly identifying AR Sco as a compact binary. While most of its observational characteristics are reminiscent of neutron star or black hole binaries, the 2min modulation is archetypical of the spin period of a strongly magnetic white dwarf. We believe that AR Sco is the first white dwarf radio pulsar, where the combination of a large field and rapid rotation results in the acceleration of relativisitic particles that blast the inner hemisphere of the M-dwarf companion, akin to the well-known milli-second pulsars. The ultimate proof of our hypothesis relies on the unambiguous identification of the white dwarf, which will be achieved through the detection of Zeeman-split Ly alpha absorption in the requested COS/G140L observations.

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

    PubMed Central

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

    2011-01-01

    The origin of highly magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star, is engulfed by a post-main-sequence giant, gravitational torques in the envelope of the giant lead to a reduction of the companion’s orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute to the origin of magnetar fields. PMID:21300910

  16. Axions and the luminosity function of white dwarfs. The thin and thick disks, and the halo

    NASA Astrophysics Data System (ADS)

    Isern, J.; García-Berro, E.; Torres, S.; Cojocaru, R.; Catalán, S.

    2018-05-01

    The evolution of white dwarfs is a simple gravothermal process of cooling. Since the shape of their luminosity function is sensitive to the characteristic cooling time, it is possible to use its slope to test the existence of additional sources or sinks of energy, such as those predicted by alternative physical theories. The aim of this paper is to study if the changes in the slope of the white dwarf luminosity function around bolometric magnitudes ranging from 8 to 10 and previously attributed to axion emission are, effectively, a consequence of the existence of axions and not an artifact introduced by the star formation rate. We compute theoretical luminosity functions of the thin and thick disk, and of the stellar halo including axion emission and we compare them with the existing observed luminosity functions. Since these stellar populations have different star formation histories, the slope change should be present in all of them at the same place if it is due to axions or any other intrinsic cooling mechanism. The signature of an unexpected cooling seems to be present in the luminosity functions of the thin and thick disks, as well as in the halo luminosity function. This additional cooling is compatible with axion emission, thus supporting to the idea that DFSZ axions, with a mass in the range of 4 to 10 meV, could exist. If this were the case, these axions could be detected by the future solar axioscope IAXO.

  17. 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

  18. A search for cataclysmic binaries containing strongly magnetic white dwarfs

    NASA Technical Reports Server (NTRS)

    Bond, H. E.; Chanmugam, G.

    1982-01-01

    The AM Herculis type binaries which contain accreting white dwarfs with surface magnetic fields of a few times 10 to the seventh power gauss were studied. If white dwarfs in cataclysmic binaries have a range of field strengths similar to that among single white dwarfs. AM Her like systems should exist with fields as high as 3 x 10 to the eighth power gauss. It is suggested that such objects will not have the strong optical polarization of the AM Her variables; however, they exhibit high harmonic cyclotron emission, making them spectacular UV sources. We made IUE observations of seven candidate cataclysmic variables selected for optical similarity to AM Her binaries. Although all seven objects were detected in the UV, none display unusually strong UV continua. It is suggested that the distribution of magnetic field strengths among single white dwarfs may be different from that among binaries.

  19. 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

  20. 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

  1. White dwarfs and revelations

    NASA Astrophysics Data System (ADS)

    Saltas, Ippocratis D.; Sawicki, Ignacy; Lopes, Ilidio

    2018-05-01

    We use the most recent, complete and independent measurements of masses and radii of white dwarfs in binaries to bound the class of non-trivial modified gravity theories, viable after GW170817/GRB170817, using its effect on the mass-radius relation of the stars. We show that the uncertainty in the latest data is sufficiently small that residual evolutionary effects, most notably the effect of core composition, finite temperature and envelope structure, must now accounted for if correct conclusions about the nature of gravity are to be made. We model corrections resulting from finite temperature and envelopes to a base Hamada-Salpeter cold equation of state and derive consistent bounds on the possible modifications of gravity in the stars' interiors, finding that the parameter quantifying the strength of the modification Y< 0.14 at 95% confidence, an improvement of a factor of three with respect to previous bounds. Finally, our analysis reveals some fundamental degeneracies between the theory of gravity and the precise chemical makeup of white dwarfs.

  2. 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)

  3. Stellar equilibrium configurations of white dwarfs in the f( R, T) gravity

    NASA Astrophysics Data System (ADS)

    Carvalho, G. A.; Lobato, R. V.; Moraes, P. H. R. S.; Arbañil, José D. V.; Otoniel, E.; Marinho, R. M.; Malheiro, M.

    2017-12-01

    In this work we investigate the equilibrium configurations of white dwarfs in a modified gravity theory, namely, f( R, T) gravity, for which R and T stand for the Ricci scalar and trace of the energy-momentum tensor, respectively. Considering the functional form f(R,T)=R+2λ T, with λ being a constant, we obtain the hydrostatic equilibrium equation for the theory. Some physical properties of white dwarfs, such as: mass, radius, pressure and energy density, as well as their dependence on the parameter λ are derived. More massive and larger white dwarfs are found for negative values of λ when it decreases. The equilibrium configurations predict a maximum mass limit for white dwarfs slightly above the Chandrasekhar limit, with larger radii and lower central densities when compared to standard gravity outcomes. The most important effect of f( R, T) theory for massive white dwarfs is the increase of the radius in comparison with GR and also f( R) results. By comparing our results with some observational data of massive white dwarfs we also find a lower limit for λ , namely, λ >- 3× 10^{-4}.

  4. A detection of the evolutionary time scale of the DA white dwarf G117 - B15A with the Whole Earth Telescope

    NASA Technical Reports Server (NTRS)

    Kepler, S. O.; Fontaine, G.; Bergeron, P.; Winget, D. E.; Nather, R. E.; Bradley, P. A.; Claver, C. F.; Grauer, A. D.; Vauclair, G.; Marar, T. M. K.

    1991-01-01

    The time rate of change for the main pulsation period of the 13,000 K DA white dwarf G117 - B15A has been detected using the Whole Earth Telescope (WET). The observed rate of period change, P(dot) = (12.0 + or - 3.5) x 10 to the -15th s/s, is somewhat larger than the published theoretical calculations of the rate of period change due to cooling, based on carbon core white dwarf models. Other effects that could contribute to the observed rate of period change are discussed.

  5. Magnetically gated accretion in an accreting ‘non-magnetic’ white dwarf

    NASA Astrophysics Data System (ADS)

    Scaringi, S.; Maccarone, T. J.; D’Angelo, C.; Knigge, C.; Groot, P. J.

    2017-12-01

    White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 106 gauss or more) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as ‘non-magnetic’, because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the ‘non-magnetic’ accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 104 gauss and 1 × 105 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified.

  6. Two-phase ultraviolet spectrophotometry of the pulsating white dwarf ZZ Piscium

    NASA Technical Reports Server (NTRS)

    Bond, H. E.; Kemper, E.; Grauer, A. D.; Holm, A. V.; Panek, R. J.; Schiffer, F. H., III

    1985-01-01

    Spectra of the pulsating white dwarf ZZ Psc (= G29-38) were obtained using the International Ultraviolet Explorer. By using a multiple-exposure technique in conjunction with simultaneous ground-based exposure-metering photometry, it was possible to obtain mean on-pulse and off-pulse spectra in the 1950-1310 A wavelength range. The ratio of the time-averaged on-pulse to off-pulse spectra is best fitted by a temperature variation that is in phase with the optical light variation. This result is consistent with the hypothesis that the observed variation is due to a high-order nonradial pulsation. Conventional ultraviolet spectra of ZZ Psc showed broad absorption features at 1390 and 1600 A. These features are also found in the spectra of the cool DA-type white dwarfs G226-29 and G67-23, and appear to increase in strength with decreasing temperature. A possible explanation for the 1600 A feature is absorption by the satellite band of resonance-broadened hydrogen Ly-alpha. Such absorption would also help explain a discrepancy between the observed pulsation amplitude shortward of 1650 A and the predicted amplitudes based on model atmospheres.

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

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

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

  8. SPIRAL INSTABILITY CAN DRIVE THERMONUCLEAR EXPLOSIONS IN BINARY WHITE DWARF MERGERS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Kashyap, Rahul; Fisher, Robert; García-Berro, Enrique

    2015-02-10

    Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion of carbon–oxygen white dwarfs, and serve as standardizable cosmological candles. However, despite their importance, the nature of the progenitor systems that give rise to SNe Ia has not been hitherto elucidated. Observational evidence favors the double-degenerate channel in which merging white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies exist between observations and theory, and to date, there has been no self-consistent merger model that yields a SNe Ia. Here we show that a spiral mode instability in the accretion disk formed during a binary white dwarf mergermore » leads to a detonation on a dynamical timescale. This mechanism sheds light on how white dwarf mergers may frequently yield SNe Ia.« less

  9. 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.

  10. A Catalog of Cool Dwarf Targets for the Transiting Exoplanet Survey Satellite

    NASA Astrophysics Data System (ADS)

    Muirhead, Philip S.; Dressing, Courtney D.; Mann, Andrew W.; Rojas-Ayala, Bárbara; Lépine, Sébastien; Paegert, Martin; De Lee, Nathan; Oelkers, Ryan

    2018-04-01

    We present a catalog of cool dwarf targets (V-J> 2.7, T eff ≲ 4000 K) and their stellar properties for the upcoming Transiting Exoplanet Survey Satellite (TESS), for the purpose of determining which cool dwarfs should be observed using two minute observations. TESS has the opportunity to search tens of thousands of nearby, cool, late K- and M-type dwarfs for transiting exoplanets, an order of magnitude more than current or previous transiting exoplanet surveys, such as Kepler, K2, and ground-based programs. This necessitates a new approach to choosing cool dwarf targets. Cool dwarfs are chosen by collating parallax and proper motion catalogs from the literature and subjecting them to a variety of selection criteria. We calculate stellar parameters and TESS magnitudes using the best possible relations from the literature while maintaining uniformity of methods for the sake of reproducibility. We estimate the expected planet yield from TESS observations using statistical results from the Kepler mission, and use these results to choose the best targets for two minute observations, optimizing for small planets for which masses can conceivably be measured using follow-up Doppler spectroscopy by current and future Doppler spectrometers. The catalog is available in machine readable format and is incorporated into the TESS Input Catalog and TESS Candidate Target List until a more complete and accurate cool dwarf catalog identified by ESA’s Gaia mission can be incorporated.

  11. General relativistic effects in the structure of massive white dwarfs

    NASA Astrophysics Data System (ADS)

    Carvalho, G. A.; Marinho, R. M.; Malheiro, M.

    2018-04-01

    In this work we investigate the structure of white dwarfs using the Tolman-Oppenheimer-Volkoff equations and compare our results with those obtained from Newtonian equations of gravitation in order to put in evidence the importance of general relativity (GR) for the structure of such stars. We consider in this work for the matter inside white dwarfs two equations of state, frequently found in the literature, namely, the Chandrasekhar and Salpeter equations of state. We find that using Newtonian equilibrium equations, the radii of massive white dwarfs (M>1.3M_{⊙ }) are overestimated in comparison with GR outcomes. For a mass of 1.415M_{⊙ } the white dwarf radius predicted by GR is about 33% smaller than the Newtonian one. Hence, in this case, for the surface gravity the difference between the general relativistic and Newtonian outcomes is about 65%. We depict the general relativistic mass-radius diagrams as M/M_{⊙ }=R/(a+bR+cR^2+dR^3+kR^4), where a, b, c and d are parameters obtained from a fitting procedure of the numerical results and k=(2.08× 10^{-6}R_{⊙ })^{-1}, being R_{⊙ } the radius of the Sun in km. Lastly, we point out that GR plays an important role to determine any physical quantity that depends, simultaneously, on the mass and radius of massive white dwarfs.

  12. Left Behind: A Bound Remnant from a White Dwarf Supernova?

    NASA Astrophysics Data System (ADS)

    Jha, Saurabh

    2017-08-01

    Type Ia supernovae (SN Ia) have enormous importance to cosmology and astrophysics, but their progenitors and explosion mechanisms are not understood in detail. Recently, observations and theoretical models have suggested that not all thermonuclear white-dwarf supernova explosions are normal SN Ia. In particular, type Iax supernovae (peculiar cousins to SN Ia), are thought to be exploding white dwarfs that are not completely disrupted, leaving behind a bound remnant. In deep and serendipitous HST pre-explosion data, we have discovered a luminous, blue progenitor system for the type Iax SN 2012Z in NGC 1309, which we interpret as a helium-star donor to the exploding white dwarf. HST observations of SN 2012Z in 2016, when the supernova light was expected to have faded away, still show a source at the location, as expected in our model where the pre-explosion flux was coming from the companion. However, the 2016 data also show a surprise: an excess flux compared to the progenitor system. Our proposed observations here will help unravel the mystery of that excess flux: is it from the bound ex-white dwarf remnant? Or is it from the shocked companion star that has been bombarded by supernova ejecta? Either of these possibilities would provide key new evidence as to the nature of these white dwarf supernovae.

  13. Constraints on modified gravity models from white dwarfs

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Banerjee, Srimanta; Singh, Tejinder P.; Shankar, Swapnil, E-mail: srimanta.banerjee@tifr.res.in, E-mail: swapnil.shankar@cbs.ac.in, E-mail: tpsingh@tifr.res.in

    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.

  14. 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.

  15. Spectroscopic survey of southern hemisphere white dwarfs. II. Spectroscopic data for forty-one southern white dwarfs

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Wegner, G.

    New spectroscopic data on 41 southern white dwarfs are presented. Most of these stars have not teen previously observed spectroscopically. Spectral types, as well as equivalent widths and line profiles for a few selected lines, are given. (auth)

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

    NASA Astrophysics Data System (ADS)

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

    2017-11-01

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

  17. 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.

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

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

    2011-08-15

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

  19. Follow-up spectroscopic observations of HD 107148 B: A new white dwarf companion of an exoplanet host star

    NASA Astrophysics Data System (ADS)

    Mugrauer, M.; Dinçel, B.

    2016-06-01

    We report on our follow-up spectroscopy of HD 1071478 B, a recently detected faint co-moving companion of the exoplanet host star HD 107148 A. The companion is separated from its primary star by about 35 arcsec (or 1790 AU of projected separation) and its optical and near infrared photometry is consistent with a white dwarf, located at the distance of HD 107148 A. In order to confirm the white dwarf nature of the co-moving companion, we obtained follow-up spectroscopic observations of HD 107148 B with CAFOS at the CAHA 2.2 m telescope. According to our CAFOS spectroscopy HD 107148 B is a DA white dwarf with an effective temperature in the range between 5900 and 6400 K. The properties of HD 107148 B can further be constrained with the derived effective temperature and the known visual and infrared photometry of the companion, using evolutionary models of DA white dwarfs. We obtain for HD 107148 B a mass of 0.56±0.05 M_⊙, a luminosity of (2.0±0.2)×10-4 L_⊙, log g [cm s-2])=7.95±0.09, and a cooling age of 2100±270 Myr. With its white dwarf companion the exoplanet host star HD 107148 A forms an evolved stellar system, which hosts at least one exoplanet. So far, only few of these evolved systems are known, which represent only about 5 % of all known exoplanet host multiple stellar systems. HD 107148 B is the second confirmed white dwarf companion of an exoplanet host star with a projected separation to its primary star of more than 1000 AU. Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC).

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

    NASA Astrophysics Data System (ADS)

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

    2018-01-01

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

  1. Evolutionary Calculations of Phase Separation in Crystallizing White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Montgomery, M. H.; Klumpe, E. W.; Winget, D. E.; Wood, M. A.

    1999-11-01

    We present an exploration of the significance of carbon/oxygen phase separation in white dwarf stars in the context of self-consistent evolutionary calculations. Because phase separation can potentially increase the calculated ages of the oldest white dwarfs, it can affect the age of the Galactic disk as derived from the downturn in the white dwarf luminosity function. We find that the largest possible increase in ages due to phase separation is ~1.5 Gyr, with a most likely value of approximately 0.6 Gyr, depending on the parameters of our white dwarf models. The most important factors influencing the size of this delay are the total stellar mass, the initial composition profile, and the phase diagram assumed for crystallization. We find a maximum age delay in models with masses of ~0.6 Msolar, which is near the peak in the observed white dwarf mass distribution. In addition, we note that the prescription that we have adopted for the mixing during crystallization provides an upper bound for the efficiency of this process, and hence a maximum for the age delays. More realistic treatments of the mixing process may reduce the size of this effect. We find that varying the opacities (via the metallicity) has little effect on the calculated age delays. In the context of Galactic evolution, age estimates for the oldest Galactic globular clusters range from 11.5 to 16 Gyr and depend on a variety of parameters. In addition, a 4-6 Gyr delay is expected between the formation of the globular clusters and the formation of the Galactic thin disk, while the observed white dwarf luminosity function gives an age estimate for the thin disk of 9.5+1.1-0.8 Gyr, without including the effect of phase separation. Using the above numbers, we see that phase separation could add between 0 and 3 Gyr to the white dwarf ages and still be consistent with the overall picture of Galaxy formation. Our calculated maximum value of <~1.5 Gyr fits within these bounds, as does our best-guess value of

  2. VizieR Online Data Catalog: Gaia photometry for white dwarfs (Carrasco+, 2014)

    NASA Astrophysics Data System (ADS)

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

    2014-03-01

    The Gaia space mission, through its 5-6 years survey of the whole sky up to magnitude V=20-25, will drastically increase the sample of known white dwarfs allowing to address new science questions. In this paper we provide a characterisation of Gaia photometry for the case of white dwarfs to better prepare for the analysis of the scientific output of the mission including relationships among colours involving Gaia magnitudes (white light G, blue GBP, red GRP and GRVS passbands) and colours from other commonly used photometric systems (Johnson-Cousins, SDSS and 2MASS). We also present numbers of white dwarfs predicted by the Gaia Universe Model Snapshot and compare them with an alternative simulation calibrated with the local white dwarfs sample. In these online tables we provide the values used to fit the relationships in the paper, especially useful for those cases where the deviation from the established relationships is large. The most recent Gaia transmission curves and three different compositions for white dwarfs were considered here (pure hydrogen, pure helium and mixed composition with H/He=0.1). (3 data files).

  3. White Dwarf Model Atmospheres: Synthetic Spectra for Supersoft Sources

    NASA Astrophysics Data System (ADS)

    Rauch, Thomas

    2013-01-01

    The Tübingen NLTE Model-Atmosphere Package (TMAP) calculates fully metal-line blanketed white dwarf model atmospheres and spectral energy distributions (SEDs) at a high level of sophistication. Such SEDs are easily accessible via the German Astrophysical Virtual Observatory (GAVO) service TheoSSA. We discuss applications of TMAP models to (pre) white dwarfs during the hottest stages of their stellar evolution, e.g. in the parameter range of novae and supersoft sources.

  4. Super-Nyquist White Dwarf Pulsations in K2 Long-Cadence Data

    NASA Astrophysics Data System (ADS)

    Bell, Keaton J.; Hermes, JJ; Montgomery, Michael H.; Vanderbosch, Zach

    2017-06-01

    The Kepler and K2 missions have recently revolutionized the field of white dwarf asteroseismology. Since white dwarfs pulsate on timescales of order 10 minutes, we aim to observe these objects at K2’s short cadence (1 minute). Occasionally we find signatures of pulsations in white dwarf targets that were only observed by K2 at long cadence (30 minute). These signals suffer extreme aliasing since the intrinsic frequencies exceed the Nyquist sampling limit. We present our work to recover accurate frequency determinations for these targets, guided by a limited amount of supplementary, ground-based photometry from McDonald Observatory.

  5. CALIBRATION OF THE MIXING-LENGTH THEORY FOR CONVECTIVE WHITE DWARF ENVELOPES

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Tremblay, P.-E.; Ludwig, H.-G.; Freytag, B.

    2015-02-01

    A calibration of the mixing-length parameter in the local mixing-length theory (MLT) is presented for the lower part of the convection zone in pure-hydrogen-atmosphere white dwarfs. The parameterization is performed from a comparison of three-dimensional (3D) CO5BOLD simulations with a grid of one-dimensional (1D) envelopes with a varying mixing-length parameter. In many instances, the 3D simulations are restricted to the upper part of the convection zone. The hydrodynamical calculations suggest, in those cases, that the entropy of the upflows does not change significantly from the bottom of the convection zone to regions immediately below the photosphere. We rely on thismore » asymptotic entropy value, characteristic of the deep and adiabatically stratified layers, to calibrate 1D envelopes. The calibration encompasses the convective hydrogen-line (DA) white dwarfs in the effective temperature range 6000 ≤ T {sub eff} (K) ≤15, 000 and the surface gravity range 7.0 ≤ log g ≤ 9.0. It is established that the local MLT is unable to reproduce simultaneously the thermodynamical, flux, and dynamical properties of the 3D simulations. We therefore propose three different parameterizations for these quantities. The resulting calibration can be applied to structure and envelope calculations, in particular for pulsation, chemical diffusion, and convective mixing studies. On the other hand, convection has no effect on the white dwarf cooling rates until there is a convective coupling with the degenerate core below T {sub eff} ∼ 5000 K. In this regime, the 1D structures are insensitive to the MLT parameterization and converge to the mean 3D results, hence they remain fully appropriate for age determinations.« less

  6. The kinematics of the white dwarf population from the SDSS DR12

    NASA Astrophysics Data System (ADS)

    Anguiano, B.; Rebassa-Mansergas, A.; García-Berro, E.; Torres, S.; Freeman, K. C.; Zwitter, T.

    2017-08-01

    We use the Sloan Digital Sky Survey Data Release 12, which is the largest available white dwarf catalogue to date, to study the evolution of the kinematical properties of the population of white dwarfs in the Galactic disc. We derive masses, ages, photometric distances and radial velocities for all white dwarfs with hydrogen-rich atmospheres. For those stars for which proper motions from the USNO-B1 catalogue are available, the true three-dimensional components of the stellar space velocity are obtained. This subset of the original sample comprises 20 247 objects, making it the largest sample of white dwarfs with measured three-dimensional velocities. Furthermore, the volume probed by our sample is large, allowing us to obtain relevant kinematical information. In particular, our sample extends from a Galactocentric radial distance RG = 7.8 to 9.3 kpc, and vertical distances from the Galactic plane ranging from Z = -0.5 to 0.5 kpc. We examine the mean components of the stellar three-dimensional velocities, as well as their dispersions with respect to the Galactocentric and vertical distances. We confirm the existence of a mean Galactocentric radial velocity gradient, ∂/∂RG = -3 ± 5 km s-1 kpc-1. We also confirm north-south differences in . Specifically, we find that white dwarfs with Z > 0 (in the North Galactic hemisphere) have < 0, while the reverse is true for white dwarfs with Z < 0. The age-velocity dispersion relation derived from the present sample indicates that the Galactic population of white dwarfs may have experienced an additional source of heating, which adds to the secular evolution of the Galactic disc.

  7. The kinematics of the white dwarf population from the SDSS DR12

    NASA Astrophysics Data System (ADS)

    Anguiano, B.; Rebassa-Mansergas, A.; García-Berro, E.; Torres, S.; Freeman, K.; Zwitter, T.

    2018-04-01

    We use the Sloan Digital Sky Survey Data Release 12, which is the largest available white dwarf catalog to date, to study the evolution of the kinematical properties of the population of white dwarfs in the Galactic disc. We derive masses, ages, photometric distances and radial velocities for all white dwarfs with hydrogen-rich atmospheres. For those stars for which proper motions from the USNO-B1 catalog are available the true three-dimensional components of the stellar space velocity are obtained. This subset of the original sample comprises 20,247 objects, making it the largest sample of white dwarfs with measured three-dimensional velocities. Furthermore, the volume probed by our sample is large, allowing us to obtain relevant kinematical information. In particular, our sample extends from a Galactocentric radial distance R G = 7.8 kpc to 9.3 kpc, and vertical distances from the Galactic plane ranging from Z = -0.5 kpc to 0.5 kpc. We examine the mean components of the stellar three-dimensional velocities, as well as their dispersions with respect to the Galactocentric and vertical distances. We confirm the existence of a mean Galactocentric radial velocity gradient, ∂/∂R G = -3 +/- 5 km s-1 kpc-1. We also confirm North-South differences in . Specifically, we find that white dwarfs with Z > 0 (in the North Galactic hemisphere) have < 0, while the reverse is true for white dwarfs with Z < 0. The age-velocity dispersion relation derived from the present sample indicates that the Galactic population of white dwarfs may have experienced an additional source of heating, which adds to the secular evolution of the Galactic disc.

  8. 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.

  9. Maximizing JWST Science for Dusty White Dwarfs

    NASA Astrophysics Data System (ADS)

    Farihi, Jay; Dennihy, Erik; Gentile Fusillo, Nicola; Debes, John; Gaensicke, Boris

    2018-05-01

    We propose a small program to increase the number of dusty white dwarfs that can be studied in detail by the James Webb Space Telescope. Currently, there are 8 systems for which MIRI MRS spectroscopy can be carried out in less than a few hours per target, and here we propose to double this number. Using cross-correlation of AllWISE photometry with Southern Hemisphere surveys such as Edinburgh-Cape and ATLAS, we have selected the strongest 22 potential dusty white dwarf candidates. We propose to use warm IRAC imaging photometry as the ultimate discriminant between dust and common photometric contaminants. This program has immediate legacy value via detailed mineralogical studies of debris disks using JWST.

  10. Accretional Heating by Periodic Dwarf Nova Outburst Events

    NASA Astrophysics Data System (ADS)

    Godon, P.; Sion, E. M.

    2001-12-01

    We carry out simulations of evolutionary models of accreting white dwarfs in dwarf novae to assess the combined effect of boundary layer irradiation and compressional heating on the accreting star. We focus on the behavior of the surface observables of the accreting white dwarf for different value of the mass accretion rate and accretor mass. Outburst of days to weeks are followed by a shut off of the radial infall during quiescences lasting weeks to months. Preliminary results indicate that after a long evolution time of many accretion cycles, the effective surface temperature of the white dwarf will increase substantially. The purpose of this work is to generate a grid of models that will then be used to compared with observations of white dwarf heating and cooling in dwarf nova systems. This work is supported by NASA HST grant GO-8139 and in part by NSF grant AST99-01955 and NASA grant NAG5-8388.

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

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

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

  12. 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.

  13. VizieR Online Data Catalog: SDSS DR7 white dwarf catalog (Kleinman+, 2013)

    NASA Astrophysics Data System (ADS)

    Kleinman, S. J.; Kepler, S. O.; Koester, D.; Pelisoli, I.; Pecanha, V.; Nitta, A.; Costa, J. E. S.; Krzesinski, J.; Dufour, P.; Lachapelle, F.-R.; Bergeron, P.; Yip, C.-W.; Harris, H. C.; Eisenstein, D. J.; Althaus, L.; Corsico, A.

    2013-01-01

    Here, we report on the white dwarf catalog built from the SDSS DR7 (Cat. II/294). We have applied automated techniques supplemented by complete, consistent human identifications of each candidate white dwarf spectrum. We make use of the latest SDSS reductions and white dwarf model atmosphere improvements in our spectral fits, providing logg and Teff determinations for each identified clean DA and DB where we use the word "clean" to identify spectra that show only features of non-magnetic, nonmixed, DA or DB stars. Our catalog includes all white dwarf stars from the earlier Kleinman et al. (2004, Cat. J/ApJ/607/426) and Eisenstein et al. (2006, Cat. J/ApJS/167/40) catalogs, although occasionally with different identifications. (1 data file).

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Battich, T.; Córsico, A.H.; Althaus, L.G.

    The Peccei-Quinn mechanism proposed to solve the CP problem of Quantum Chromodynamics has as consequence the existence of axions, hypothetical weakly interacting particles whose mass is constrained to be on the sub-eV range. If these particles exist and interact with electrons, they would be emitted from the dense interior of white dwarfs, becoming an important energy sink for the star. Due to their well known physics, white dwarfs are good laboratories to study the properties of fundamental particles such as the axions. We study the general effect of axion emission on the evolution of helium-rich white dwarfs and on theirmore » pulsational properties. To this aim, we calculate evolutionary helium-rich white dwarf models with axion emission, and assess the pulsational properties of this models. Our results indicate that the rates of change of pulsation periods are significantly affected by the existence of axions. We are able for the first time to independently constrain the mass of the axion from the study of pulsating helium-rich white dwarfs. To do this, we use an estimation of the rate of change of period of the pulsating white dwarf PG 1351+489 corresponding to the dominant pulsation period. From an asteroseismological model of PG 1351+489 we obtain g {sub ae} < 3.3 × 10{sup -13} for the axion-electron coupling constant, or m {sub a} cos{sup 2}β ∼< 11.5 meV for the axion mass. This constraint is relaxed to g {sub ae} < 5.5 × 10{sup -13} ( m {sub a} cos{sup 2}β ∼< 19.5 meV), when no detailed asteroseismological model is adopted for the comparison with observations.« less

  15. Transit probabilities for debris around white dwarfs in Kepler/K2 up to C13

    NASA Astrophysics Data System (ADS)

    Lewis, John Arban; Johnson, John Asher

    2018-01-01

    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. At present, there is a single example. We study all white dwarfs observed during the K2 mission to identify white dwarfs with either transits or light curve features similar to WD 1145+017. We correct for contamination using J.J. Hermes' list of high probability white dwarfs (available at k2wd.org ).

  16. Seismic evidence for the loss of stellar angular momentum before the white-dwarf stage.

    PubMed

    Charpinet, S; Fontaine, G; Brassard, P

    2009-09-24

    White-dwarf stars represent the final products of the evolution of some 95% of all stars. If stars were to keep their angular momentum throughout their evolution, their white-dwarf descendants, owing to their compact nature, should all rotate relatively rapidly, with typical periods of the order of a few seconds. Observations of their photospheres show, in contrast, that they rotate much more slowly, with periods ranging from hours to tens of years. It is not known, however, whether a white dwarf could 'hide' some of its original angular momentum below the superficial layers, perhaps spinning much more rapidly inside than at its surface. Here we report a determination of the internal rotation profile of a white dwarf using a method based on asteroseismology. We show that the pulsating white dwarf PG 1159-035 rotates as a solid body (encompassing more than 97.5% of its mass) with the relatively long period of 33.61 +/- 0.59 h. This implies that it has lost essentially all of its angular momentum, thus favouring theories which suggest important angular momentum transfer and loss in evolutionary phases before the white-dwarf stage.

  17. 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.

  18. White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8

    NASA Astrophysics Data System (ADS)

    Hermes, J. J.; Gänsicke, B. T.; Kawaler, Steven D.; Greiss, S.; Tremblay, P.-E.; Gentile Fusillo, N. P.; Raddi, R.; Fanale, S. M.; Bell, Keaton J.; Dennihy, E.; Fuchs, J. T.; Dunlap, B. H.; Clemens, J. C.; Montgomery, M. H.; Winget, D. E.; Chote, P.; Marsh, T. R.; Redfield, S.

    2017-10-01

    We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere white dwarfs (DAVs; a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The space-based photometry reveals pulsation properties previously inaccessible to ground-based observations. We observe a sharp dichotomy in oscillation mode line widths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode line widths, more reminiscent of a damped harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also permits extensive mode identification: we identify the spherical degree of 87 out of 201 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4 m-class telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra, we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low- and intermediate-mass stars. We find that 0.51-0.73 M ⊙ white dwarfs, which evolved from 1.7-3.0 M ⊙ ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Finally, we announce an online repository for our Kepler data and follow-up spectroscopy, which we collect at http://k2wd.org.

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

    NASA Astrophysics Data System (ADS)

    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.

  20. 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.

  1. 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.

  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 binary Feige 24 - The mass, radius, and gravitational redshift of the DA white dwarf

    NASA Technical Reports Server (NTRS)

    Vennes, Stephane; Shipman, Harry L.; Thorstensen, John R.; Thejll, Peter

    1991-01-01

    Observations are reported which refine the binary ephemeris of the Feige 24 system, which contains a peculiar hot DA white dwarf and an M dwarf with an atmosphere illuminated by extreme ultraviolet radiation from the white dwarf. With the new ephemeris and a set of IUE high-dispersion spectra, showing phase-dependent redshifted C IV, N V, and Si IV resonance lines, the orbital velocity, and hence the mass (0.54 + or - 0.20 solar masses), and the gravitational redshift of the white dwarf (14.1 + or - 5.2 km/s) are determined independently. It is shown that the measured Einstein redshift is consistent with an estimated radius for the white dwarf obtained from a model atmosphere solid angle and a parallax measurement. This radius is twice the Hamada-Salpeter radius for the given mass and offers a prospect to investigate the presence of a massive hydrogen envelope in that white dwarf star.

  4. Characterization of the Mysteriously Cool Brown Dwarf HD 4113

    NASA Astrophysics Data System (ADS)

    Ednie, Michaela; Follette, Katherine; Ward-Duong, Kimberly

    2018-01-01

    Characterizing the physical properties of brown dwarfs is necessary to expand and improve our understanding of low mass companions, including exoplanets. Systems with both close radial velocity companions and distant directly imaged companions are particularly powerful in understanding planet formation mechanisms. Early in 2017, members of the SPHERE team discovered a companion brown dwarf in the HD 4113 system, which also contains a known RV planet. Atmospheric model fits to the Y and J-band spectra and H2/H3 photometry of the brown dwarf suggested it is unusually cool. We obtained new Magellan data in the Z and K’ bands in mid-2017. This data will help us to complete a more detailed atmospheric and astrometric characterization of this unusually cool companion. Broader wavelength coverage will help in accurate spectral typing and estimations of luminosity, temperature, surface gravity, radius, and composition. Additionally, a second astrometric epoch will help constrain the architecture of the system.

  5. Hubble Witnesses Massive Comet-Like Object Pollute Atmosphere of a White Dwarf

    NASA Image and Video Library

    2017-12-08

    For the first time, scientists using NASA’s Hubble Space Telescope have witnessed a massive object with the makeup of a comet being ripped apart and scattered in the atmosphere of a white dwarf, the burned-out remains of a compact star. The object has a chemical composition similar to Halley’s Comet, but it is 100,000 times more massive and has a much higher amount of water. It is also rich in the elements essential for life, including nitrogen, carbon, oxygen, and sulfur. These findings are evidence for a belt of comet-like bodies orbiting the white dwarf, similar to our solar system’s Kuiper Belt. These icy bodies apparently survived the star’s evolution as it became a bloated red giant and then collapsed to a small, dense white dwarf. Caption: This artist's concept shows a massive, comet-like object falling toward a white dwarf. New Hubble Space Telescope findings are evidence for a belt of comet-like bodies orbiting the white dwarf, similar to our solar system's Kuiper Belt. The findings also suggest the presence of one or more unseen surviving planets around the white dwarf, which may have perturbed the belt to hurl icy objects into the burned-out star. Credits: NASA, ESA, and Z. Levay (STScI) NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

  6. Photospheric soft X-ray emission from hot DA white dwarfs

    NASA Technical Reports Server (NTRS)

    Wesemael, F.; Raymond, J. C.; Kahn, S. M.; Liebert, J.; Steiner, J. E.; Shipman, H. L.

    1984-01-01

    The Einstein Observatory's imaging proportional counter has detected 150-eV soft X-ray radiation from the four hot DA white dwarfs EG 187, Gr 288 and 289, and LB 1663. The observed pulse height spectra suggest that the emission is generated by hot photospheres whose T(eff) lie in the 30,000-60,000 K range. The IUE spacecraft UV spectra and H-beta line profiles for the four stars have been fitted, along with the X-ray fluxes, with a grid of hot, high gravity, homogeneous model atmospheres of mixed H-He composition. In all cases, the data require the presence of some X-ray opacity in the photosphere. Attention is given to the implications of this result in the context of white dwarf surface layer diffusion theories. Also noted are the limits imposed on the hot white dwarf population by the Einstein Medium Sensitivity Survey.

  7. A predicted astrometric microlensing event by a nearby white dwarf

    NASA Astrophysics Data System (ADS)

    McGill, Peter; Smith, Leigh C.; Evans, N. Wyn; Belokurov, Vasily; Smart, R. L.

    2018-07-01

    We used the Tycho-Gaia Astrometric Solution catalogue, part of Gaia Data Release 1, to search for candidate astrometric microlensing events expected to occur within the remaining lifetime of the Gaia satellite. Our search yielded one promising candidate. We predict that the nearby DQ type white dwarf LAWD 37 (WD 1142-645) will lens a background star and will reach closest approach on 2019 November 11 (±4 d) with impact parameter 380 ± 10 mas. This will produce an apparent maximum deviation of the source position of 2.8 ± 0.1 mas. In the most propitious circumstance, Gaia will be able to determine the mass of LAWD 37 to {˜ }3 per cent. This mass determination will provide an independent check on atmospheric models of white dwarfs with helium rich atmospheres, as well as tests of white dwarf mass radius relationships and evolutionary theory.

  8. A Predicted Astrometric Microlensing Event by a Nearby White Dwarf

    NASA Astrophysics Data System (ADS)

    McGill, Peter; Smith, Leigh C.; Wyn Evans, N.; Belokurov, Vasily; Smart, R. L.

    2018-04-01

    We used the Tycho-Gaia Astrometric Solution catalogue, part of Gaia Data Release 1, to search for candidate astrometric microlensing events expected to occur within the remaining lifetime of the Gaia satellite. Our search yielded one promising candidate. We predict that the nearby DQ type white dwarf LAWD 37 (WD 1142-645) will lens a background star and will reach closest approach on November 11th 2019 (± 4 days) with impact parameter 380 ± 10 mas. This will produce an apparent maximum deviation of the source position of 2.8 ± 0.1 mas. In the most propitious circumstance, Gaia will be able to determine the mass of LAWD 37 to ˜3%. This mass determination will provide an independent check on atmospheric models of white dwarfs with helium rich atmospheres, as well as tests of white dwarf mass radius relationships and evolutionary theory.

  9. AR Scorpii and possible gravitational wave radiation from pulsar white dwarfs

    NASA Astrophysics Data System (ADS)

    Franzon, B.; Schramm, S.

    2017-06-01

    In view of the new recent observation and measurement of the rotating and highly magnetized white dwarf AR Scorpii, we determine bounds of its moment of inertia, magnetic fields and radius. Moreover, we investigate the possibility of fast rotating and/or magnetized white dwarfs to be sources of detectable gravitational wave (GW) emission. Numerical stellar models at different baryon masses are constructed. For each star configuration, we compute self-consistent relativistic solutions for white dwarfs endowed with poloidal magnetic fields by solving the Einstein-Maxwell field equations in a self-consistent way. The magnetic field supplies an anisotropic pressure, leading to the braking of the spherical symmetry of the star. In this case, we compute the quadrupole moment of the mass distribution. Next, we perform an estimate of the GW of such objects. Finally, we show that the new recent observation and measurement pulsar white dwarf AR Scorpii, as well as other stellar models, might generate GW radiation that lies in the bandwidth of the discussed next generation of space-based GW detectors DECI-hertz Interferometer Gravitational wave Observatory (DECIGO) and Big Bang Observer (BBO).

  10. White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler  Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Hermes, J. J.; Fanale, S. M.; Dennihy, E.

    We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere white dwarfs (DAVs; a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The space-based photometry reveals pulsation properties previously inaccessible to ground-based observations. We observe a sharp dichotomy in oscillation mode line widths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode line widths, more reminiscent of a damped harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also permits extensive modemore » identification: we identify the spherical degree of 87 out of 201 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4 m-class telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra, we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low- and intermediate-mass stars. We find that 0.51–0.73 M {sub ⊙} white dwarfs, which evolved from 1.7–3.0 M {sub ⊙} ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Finally, we announce an online repository for our Kepler data and follow-up spectroscopy, which we collect at http://k2wd.org.« less

  11. A Spectroscopic Survey and Analysis of Bright, Hydrogen-rich White Dwarfs

    NASA Astrophysics Data System (ADS)

    Gianninas, A.; Bergeron, P.; Ruiz, M. T.

    2011-12-01

    We have conducted a spectroscopic survey of over 1300 bright (V <= 17.5), hydrogen-rich white dwarfs based largely on the last published version of the McCook & Sion catalog. The complete results from our survey, including the spectroscopic analysis of over 1100 DA white dwarfs, are presented. High signal-to-noise ratio optical spectra were obtained for each star and were subsequently analyzed using our standard spectroscopic technique where the observed Balmer line profiles are compared to synthetic spectra computed from the latest generation of model atmospheres appropriate for these stars. First, we present the spectroscopic content of our sample, which includes many misclassifications as well as several DAB, DAZ, and magnetic white dwarfs. Next, we look at how the new Stark broadening profiles affect the determination of the atmospheric parameters. When necessary, specific models and analysis techniques are used to derive the most accurate atmospheric parameters possible. In particular, we employ M dwarf templates to obtain better estimates of the atmospheric parameters for those white dwarfs that are in DA+dM binary systems. Certain unique white dwarfs and double-degenerate binary systems are also analyzed in greater detail. We then examine the global properties of our sample including the mass distribution and their distribution as a function of temperature. We then proceed to test the accuracy and robustness of our method by comparing our results to those of other surveys such as SPY and Sloan Digital Sky Survey. Finally, we revisit the ZZ Ceti instability strip and examine how the determination of its empirical boundaries is affected by the latest line profile calculations. Based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under program ID 078.D-0824(A).

  12. DA white dwarfs in Sloan Digital Sky Survey Data Release 7 and a search for infrared excess emission

    NASA Astrophysics Data System (ADS)

    Girven, J.; Gänsicke, B. T.; Steeghs, D.; Koester, D.

    2011-10-01

    We present a method which uses colour-colour cuts on the Sloan Digital Sky Survey (SDSS) photometry to select white dwarfs with hydrogen-rich (DA) atmospheres without the recourse to spectroscopy. This method results in a sample of DA white dwarfs that is 95 per cent complete at an efficiency of returning a true DA white dwarf of 62 per cent. The approach was applied to SDSS Data Release 7 for objects with and without SDSS spectroscopy. This led to 4636 spectroscopicially confirmed DA white dwarfs with g≤ 19; a ˜70 per cent increase compared to Eisenstein et al.'s 2006 sample. Including the photometric-only objects, we estimate a factor of 3 increase in DA white dwarfs. We find that the SDSS spectroscopic follow-up is 44 per cent complete for DA white dwarfs with Teff≳ 8000 K. We further cross-correlated the SDSS sample with Data Release 8 of the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Large Area Survey. The spectral energy distributions (SED) of both subsets, with and without SDSS spectroscopy, were fitted with white dwarf models to determine the fraction of DA white dwarfs with low-mass stellar companions or dusty debris discs via the detection of excess near-infrared emission. From the spectroscopic sample we find that 2.0 per cent of white dwarfs have an excess consistent with a brown dwarf type companion, with a firm lower limit of 0.8 per cent. From the white dwarfs with photometry only, we find that 1.8 per cent are candidates for having brown dwarf companions. Similarly, both samples show that ˜1 per cent of white dwarfs are candidates for having a dusty debris disc.

  13. 2MASS Photometry of the Hot DA White Dwarf Stars in the Palomar Green Survey

    NASA Astrophysics Data System (ADS)

    Holberg, J. B.; Magargal, K.

    2003-12-01

    The Palomar Green (PG) Survey is a complete, magnitude limited survey of UV excess objects that continues to provide well-defined sample populations for many types of objects, in particular hot white dwarf stars. The 2MASS All-Sky Survey limiting JHK magnitudes are reasonably well matched to the B magnitude limits of the PG survey. The 2MASS survey, therefore, constitutes an excellent source of uniform, high-quality of photometry, that can be used in conjunction with the PG Survey. The 2MASS Point Source Catalog in the All-Sky Data Release was searched for over 340 hot DA white dwarfs in the PG sample. The resulting JHK colors and apparent magnitudes are used to determine photometric distances for these stars and to place limits on the existence of possible cool binary companions. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

  14. The coolest extremely low-mass white dwarfs

    NASA Astrophysics Data System (ADS)

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

    2018-06-01

    Context. Extremely low-mass white dwarf (ELM WD; M⋆ ≲ 0.18-0.20 M⊙) stars are thought to be formed in binary systems via stable or unstable mass transfer. Although stable mass transfer predicts the formation of ELM WDs with thick hydrogen (H) envelopes that are characterized by dominant residual nuclear burning along the cooling branch, the formation of ELM WDs with thinner H envelopes from unstable mass loss cannot be discarded. Aims: We compute new evolutionary sequences for helium (He) core WD stars with thin H envelopes with the main aim of assessing the lowest Teff that could be reached by this type of stars. Methods: We generate a new grid of evolutionary sequences of He-core WD stars with thin H envelopes in the mass range from 0.1554 to 0.2025 M⊙, and assess the changes in both the cooling times and surface gravity induced by a reduction of the H envelope. We also determine, taking into account the predictions of progenitor evolution, the lowest Teff reached by the resulting ELM WDs. Results: We find that a slight reduction in the H envelope yields a significant increase in the cooling rate of ELM WDs. Because of this, ELM WDs with thin H envelopes could cool down to 2500 K, in contrast to their canonical counterparts that cool down to 7000 K. In addition, we find that a reduction of the thickness of the H envelope markedly increases the surface gravity (g) of these stars. Conclusions: If ELM WDs are formed with thin H envelopes, they could be detected at very low Teff. The detection of such cool ELM WDs would be indicative that they were formed with thin H envelopes, thus opening the possibility of placing constraints on the possible mechanisms of formation of this type of star. Last but not least, the increase in g due to the reduction of the H envelope leads to consequences in the spectroscopic determinations of these stars.

  15. 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.

  16. 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.

  17. Non-LTE spectral analyses of the lately discovered DB-gap white dwarfs from the SDSS

    NASA Astrophysics Data System (ADS)

    Hügelmeyer, S. D.; Dreizler, S.

    2009-06-01

    For a long time, no hydrogen-deficient white dwarfs have been known that have effective temperature between 30 kK and < 45 kK, i. e. exceeding those of DB white dwarfs and having lower ones than DO white dwarfs. Therefore, this temperature range was long known as the DB-gap. Only recently, the SDSS provided spectra of several candidate DB-gap stars. First analyses based on model spectra calculated under the assumption of local thermodynamic equilibrium (LTE) confirmed that these stars had 30 kK < Teff < 45 kK (Eisenstein et al. 2006). It has been shown for DO white dwarfs that the relaxation of LTE is necessary to account for non local effects in the atmosphere caused by the intense radiation field. Therefore, we calculated a non-LTE model grid and re-analysed the aforementioned set of SDSS spectra. Our results confirm the existence of DB-gap white dwarfs.

  18. The WIRED Survey. IV. New Dust Disks from the McCook & Sion White Dwarf Catalog

    NASA Technical Reports Server (NTRS)

    Hoard, D.W.; Debes, John H.; Wachter, Stefanie; Leisawitz, David T.; Cohen, Martin

    2013-01-01

    We have compiled photometric data from the Wide-field Infrared Survey Explorer All Sky Survey and other archival sources for the more than 2200 objects in the original McCook & Sion Catalog of Spectroscopically Identified White Dwarfs. We applied color-selection criteria to identify 28 targets whose infrared spectral energy distributions depart from the expectation for the white dwarf photosphere alone. Seven of these are previously known white dwarfs with circumstellar dust disks, five are known central stars of planetary nebulae, and six were excluded for being known binaries or having possible contamination of their infrared photometry. We fit white dwarf models to the spectral energy distributions of the remaining ten targets, and find seven new candidates with infrared excess suggesting the presence of a circumstellar dust disk. We compare the model dust disk properties for these new candidates with a comprehensive compilation of previously published parameters for known white dwarfs with dust disks. It is possible that the current census of white dwarfs with dust disks that produce an excess detectable at K-band and shorter wavelengths is close to complete for the entire sample of known WDs to the detection limits of existing near-IR all-sky surveys. The white dwarf dust disk candidates now being found using longer wavelength infrared data are drawn from a previously underrepresented region of parameter space, in which the dust disks are overall cooler, narrower in radial extent, and/or contain fewer emitting grains.

  19. The white dwarf mass-radius relation with Gaia, Hubble and FUSE

    NASA Astrophysics Data System (ADS)

    Joyce, Simon R. G.; Barstow, Martin A.; Casewell, Sarah L.; Holberg, Jay B.; Bond, Howard E.

    2018-04-01

    White dwarfs are becoming useful tools for many areas of astronomy. They can be used as accurate chronometers over Gyr timescales. They are also clues to the history of star formation in our galaxy. Many of these studies require accurate estimates of the mass of the white dwarf. The theoretical mass-radius relation is often invoked to provide these mass estimates. While the theoretical mass-radius relation is well developed, observational tests of this relation show a much larger scatter in the results than expected. High precision observational tests to confirm this relation are required. Gaia is providing distance measurements which will remove one of the main source of uncertainty affecting most previous observations. We combine Gaia distances with spectra from the Hubble and FUSE satelites to make precise tests of the white dwarf mass-radius relation.

  20. The Fate of Exploding White Dwarfs

    NASA Astrophysics Data System (ADS)

    Fisher, Robert

    2018-01-01

    Type Ia supernovae play an important role as standardizable candles for cosmology, providing one of the most important probes into the nature of dark energy. Yet, the nature of the stellar progenitors which give rise to Type Ia supernovae remains elusive. For decades, the leading model explaining Type Ia supernovae properties consisted of a white dwarf accreting to near the Chandrasekhar mass, in the single-degenerate channel. More recently, a variety of lines of evidence point instead towards merging binary white dwarfs, in the double-degenerate channel, as the progenitors of most Type Ia supernovae. In this talk, I will focus upon recent advances at the interface between observation and theory which will help crack the Type Ia progenitor problem. In particular, I will present new insights obtained from recent multidimensional numerical simulations of both the double-degenerate and single-degenerate channels which I have undertaken with my students and collaborators. I will discuss how new models and observations will help elucidate the long-standing mystery of Type supernovae.

  1. 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.

  2. 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.

  3. A large oxygen-dominated core from the seismic cartography of a pulsating white dwarf

    NASA Astrophysics Data System (ADS)

    Giammichele, N.; Charpinet, S.; Fontaine, G.; Brassard, P.; Green, E. M.; Van Grootel, V.; Bergeron, P.; Zong, W.; Dupret, M.-A.

    2018-02-01

    White-dwarf stars are the end product of stellar evolution for most stars in the Universe. Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution. Moreover, they are important chronometers for dating galactic stellar populations, and their mergers with other white dwarfs now appear to be responsible for producing the type Ia supernovae that are used as standard cosmological candles. However, the internal structure of white-dwarf stars—in particular their oxygen content and the stratification of their cores—is still poorly known, because of remaining uncertainties in the physics involved in stellar modelling codes. Here we report a measurement of the radial chemical stratification (of oxygen, carbon and helium) in the hydrogen-deficient white-dwarf star KIC08626021 (J192904.6+444708), independently of stellar-evolution calculations. We use archival data coupled with asteroseismic sounding techniques to determine the internal constitution of this star. We find that the oxygen content and extent of its core exceed the predictions of existing models of stellar evolution. The central homogeneous core has a mass of 0.45 solar masses, and is composed of about 86 per cent oxygen by mass. These values are respectively 40 per cent and 15 per cent greater than those expected from typical white-dwarf models. These findings challenge present theories of stellar evolution and their constitutive physics, and open up an avenue for calibrating white-dwarf cosmochronology.

  4. A large oxygen-dominated core from the seismic cartography of a pulsating white dwarf.

    PubMed

    Giammichele, N; Charpinet, S; Fontaine, G; Brassard, P; Green, E M; Van Grootel, V; Bergeron, P; Zong, W; Dupret, M-A

    2018-02-01

    White-dwarf stars are the end product of stellar evolution for most stars in the Universe. Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution. Moreover, they are important chronometers for dating galactic stellar populations, and their mergers with other white dwarfs now appear to be responsible for producing the type Ia supernovae that are used as standard cosmological candles. However, the internal structure of white-dwarf stars-in particular their oxygen content and the stratification of their cores-is still poorly known, because of remaining uncertainties in the physics involved in stellar modelling codes. Here we report a measurement of the radial chemical stratification (of oxygen, carbon and helium) in the hydrogen-deficient white-dwarf star KIC08626021 (J192904.6+444708), independently of stellar-evolution calculations. We use archival data coupled with asteroseismic sounding techniques to determine the internal constitution of this star. We find that the oxygen content and extent of its core exceed the predictions of existing models of stellar evolution. The central homogeneous core has a mass of 0.45 solar masses, and is composed of about 86 per cent oxygen by mass. These values are respectively 40 per cent and 15 per cent greater than those expected from typical white-dwarf models. These findings challenge present theories of stellar evolution and their constitutive physics, and open up an avenue for calibrating white-dwarf cosmochronology.

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

    NASA Astrophysics Data System (ADS)

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

    2017-02-01

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

  6. 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.

  7. Panel 1: A pulsating red giant star and a compact, hot white dwarf star orbit each other.

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Panel 1: A pulsating red giant star and a compact, hot white dwarf star orbit each other. Panel 2: The red giant sheds much of its outer layers in a stellar wind. The white dwarf helps concentrate the wind along a thin equatorial plane. The white dwarf accretes some of this escaping gas forming a disk around the itself. Panel 3: When enough gas accumulates on the white dwarf's surface it explodes as a nova outburst. Most of the hot gas forms a pair of expanding bubbles above and below the equatorial disk. Panel 4: A few thousand years after the bubbles expand into space, the white dwarf goes through another nova outburst and makes another pair of bubbles, which form a distinctive hourglass shape.

  8. Generalized uncertainty principle and the maximum mass of ideal white dwarfs

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Rashidi, Reza, E-mail: reza.rashidi@srttu.edu

    The effects of a generalized uncertainty principle on the structure of an ideal white dwarf star is investigated. The equation describing the equilibrium configuration of the star is a generalized form of the Lane–Emden equation. It is proved that the star always has a finite size. It is then argued that the maximum mass of such an ideal white dwarf tends to infinity, as opposed to the conventional case where it has a finite value.

  9. 70 DA White Dwarfs Identified in LAMOST Pilot Survey

    NASA Astrophysics Data System (ADS)

    Zhao, J. K.; Luo, A. L.; Oswalt, T. D.; Zhao, G.

    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 ~0.6 M ⊙, 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.

  10. WDEC: A Code for Modeling White Dwarf Structure and Pulsations

    NASA Astrophysics Data System (ADS)

    Bischoff-Kim, Agnès; Montgomery, Michael H.

    2018-05-01

    The White Dwarf Evolution Code (WDEC), written in Fortran, makes models of white dwarf stars. It is fast, versatile, and includes the latest physics. The code evolves hot (∼100,000 K) input models down to a chosen effective temperature by relaxing the models to be solutions of the equations of stellar structure. The code can also be used to obtain g-mode oscillation modes for the models. WDEC has a long history going back to the late 1960s. Over the years, it has been updated and re-packaged for modern computer architectures and has specifically been used in computationally intensive asteroseismic fitting. Generations of white dwarf astronomers and dozens of publications have made use of the WDEC, although the last true instrument paper is the original one, published in 1975. This paper discusses the history of the code, necessary to understand why it works the way it does, details the physics and features in the code today, and points the reader to where to find the code and a user guide.

  11. Distances to White Dwarf Stars from HIPPARCOS

    NASA Astrophysics Data System (ADS)

    Provencal, J.; Shipman, H.; Hoeg, E.; Thejll, P.

    1996-12-01

    We will present the results of a HIPPARCOS campaign to determine the distances to a number of white dwarf stars and we will discuss their implications. For bright stars, HIPPARCOS parallaxes have uncertainties that approach 1 milliarcsecond and thus they are considerably more accurate than earlier, ground-based parallaxes. Our most important finding is that the positions of important white dwarf stars in the mass-radius diagram, used to test our understanding of stellar degeneracy, have not changed appreciably. As a result the well known puzzles associated with 40 Eri B are still with us. The HIPPARCOS results indicate that the important binary V 471 Tau is a member of the Hyades cluster. The calibration star G 191-B2B is only an optical companion to the star G 191-B2A; these two objects are at different distances. The analysis of HIPPARCOS data has been supported by a grant from NASA.

  12. Unstable low-mass planetary systems as drivers of white dwarf pollution

    NASA Astrophysics Data System (ADS)

    Mustill, Alexander J.; Villaver, Eva; Veras, Dimitri; Gänsicke, Boris T.; Bonsor, Amy

    2018-05-01

    At least 25 {per cent} of white dwarfs show atmospheric pollution by metals, sometimes accompanied by detectable circumstellar dust/gas discs or (in the case of WD 1145+017) transiting disintegrating asteroids. Delivery of planetesimals to the white dwarf by orbiting planets is a leading candidate to explain these phenomena. Here, we study systems of planets and planetesimals undergoing planet-planet scattering triggered by the star's post-main-sequence mass loss, and test whether this can maintain high rates of delivery over the several Gyr that they are observed. We find that low-mass planets (Earth to Neptune mass) are efficient deliverers of material and can maintain the delivery for Gyr. Unstable low-mass planetary systems reproduce the observed delayed onset of significant accretion, as well as the slow decay in accretion rates at late times. Higher-mass planets are less efficient, and the delivery only lasts a relatively brief time before the planetesimal populations are cleared. The orbital inclinations of bodies as they cross the white dwarf's Roche limit are roughly isotropic, implying that significant collisional interactions of asteroids, debris streams and discs can be expected. If planet-planet scattering is indeed responsible for the pollution of white dwarfs, many such objects, and their main-sequence progenitors, can be expected to host (currently undetectable) super-Earth planets on orbits of several au and beyond.

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

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Bell, Keaton J.; Winget, D. E.; Montgomery, M. H.

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

  14. Ultra-high energy cosmic rays from white dwarf pulsars and the Hillas criterion

    NASA Astrophysics Data System (ADS)

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

    2017-06-01

    The origins of ultra-high-energy cosmic rays (E ≳ 1019 eV) are a mystery and still under debate in astroparticle physics. In recent years some efforts were made to understand their nature. In this contribution we consider the possibility of Some Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) beeing white dwarf pulsars, and show that these sources can achieve large electromagnetic potentials on their surface that accelerate particle almost at the speed of light, with energies E ~ 1020-21 eV. The sources SGRs/AXPs considered as highly magnetized white dwarfs are well described in the Hillas diagram, lying close to the AR Sorpii and AE Aquarii which are understood as white dwarf pulsars.

  15. Ultra-cool dwarfs viewed equator-on: surveying the best host stars for biosignature detection in transiting exoplanets

    NASA Astrophysics Data System (ADS)

    Miles-Paez, Paulo; Metchev, Stanimir; Burgasser, Adam; Apai, Daniel; Palle, Enric; Zapatero Osorio, Maria Rosa; Artigau, Etienne; Mace, Greg; Tannock, Megan; Triaud, Amaury

    2018-05-01

    There are about 150 known planets around M dwarfs, but only one system around an ultra-cool (>M7) dwarf: Trappist-1. Ultra-cool dwarfs are arguably the most promising hosts for atmospheric and biosignature detection in transiting planets because of the enhanced feature contrast in transit and eclipse spectroscopy. We propose a Spitzer survey to continuously monitor 15 of the brightest ultra-cool dwarfs over 3 days. To maximize the probability of detecting transiting planets, we have selected only targets seen close to equator-on. Spin-orbit alignment expectations dictate that the planetary systems around these ultra-cool dwarfs should also be oriented nearly edge-on. Any planet detections from this survey will immediately become top priority targets for JWST transit spectroscopy. No other telescope, present or within the foreseeable future, will be able to conduct a similarly sensitive and dedicated survey for characterizeable Earth analogs.

  16. COS Spectroscopy of White Dwarf Companions to Blue Stragglers

    NASA Astrophysics Data System (ADS)

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

    2017-01-01

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

  17. 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%)

  18. Formation Mechanisms for Helium White Dwarfs in Binaries

    NASA Astrophysics Data System (ADS)

    Sandquist, E. L.; Taam, R. E.; Burkert, A.

    1999-05-01

    We discuss the constraints that can be placed on formation mechanisms for helium degenerate stars in binary systems, as well as the orbital parameters of the progenitor binaries, by using observed systems and numerical simulations of common envelope evolution. For pre-cataclysmic variable stars having a helium white dwarf, common envelope simulations covering the range of observed companion masses indicate that the initial mass of the red giant (parent of the white dwarf) can be constrained by the final period of the system. The formation mechanisms for double helium degenerate systems are also restricted. Using energy arguments, we find that there are almost no parameter combinations for which such a system can be formed using two successive common envelope phases. Observed short-period systems appear to favor an Algol-like phase of stable mass transfer followed by a common envelope phase. However, theory predicts that the brighter component is also the most massive, which is not observed in at least one system. This may require that nuclear burning must have occurred on the white dwarf that formed first, but after its formation. Systems which instead go through a common envelope episode, followed by a phase of nonconservative mass transfer from secondary to primary, would tend to form double degenerates with low mass ratios, which have not been observed to date. Finally, we discuss a new mechanism for producing subdwarf B stars in binaries. This work was supported by NSF grants AST-9415423 and AST-9727875.

  19. 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.

  20. 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.

  1. 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.

  2. 70 DA WHITE DWARFS IDENTIFIED IN LAMOST PILOT SURVEY

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Zhao, J. K.; Luo, A. L.; Zhao, G.

    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 aremore » 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.« less

  3. How much hydrogen is there in a white dwarf?

    NASA Technical Reports Server (NTRS)

    Macdonald, James; Vennes, Stephane

    1991-01-01

    Stratified hydrogen/helium envelope models in diffusive equilibrium are calculated for a 0.6-solar-mass white dwarf for effective temperatures between 10,000 and 80,000 K in order to investigate the observational constraints placed on the total hydrogen mass. Convective mixing is included ab initio in the calculations, and synthetic spectra are used for comparing these models with observational materials. It is shown that evolutionary changes in the surface composition of white dwarfs cannot be explained by a model in which a small amount of hydrogen floats to the surface from initially being mixed in the outer parts of a helium envelope. It is pointed out that the shape of the hydrogen lines can be used for constraining theories of convective overshoot.

  4. 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.

  5. DA white dwarfs from the LSS-GAC survey DR1: the preliminary luminosity and mass functions and formation rate

    NASA Astrophysics Data System (ADS)

    Rebassa-Mansergas, A.; Liu, X.-W.; Cojocaru, R.; Yuan, H.-B.; Torres, S.; García-Berro, E.; Xiang, M.-X.; Huang, Y.; Koester, D.; Hou, Y.; Li, G.; Zhang, Y.

    2015-06-01

    Modern large-scale surveys have allowed the identification of large numbers of white dwarfs. However, these surveys are subject to complicated target selection algorithms, which make it almost impossible to quantify to what extent the observational biases affect the observed populations. The LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) Spectroscopic Survey of the Galactic anticentre (LSS-GAC) follows a well-defined set of criteria for selecting targets for observations. This advantage over previous surveys has been fully exploited here to identify a small yet well-characterized magnitude-limited sample of hydrogen-rich (DA) white dwarfs. We derive preliminary LSS-GAC DA white dwarf luminosity and mass functions. The space density and average formation rate of DA white dwarfs we derive are 0.83 ± 0.16 × 10-3 pc-3 and 5.42 ± 0.08 × 10-13 pc-3 yr-1, respectively. Additionally, using an existing Monte Carlo population synthesis code we simulate the population of single DA white dwarfs in the Galactic anticentre, under various assumptions. The synthetic populations are passed through the LSS-GAC selection criteria, taking into account all possible observational biases. This allows us to perform a meaningful comparison of the observed and simulated distributions. We find that the LSS-GAC set of criteria is highly efficient in selecting white dwarfs for spectroscopic observations (80-85 per cent) and that, overall, our simulations reproduce well the observed luminosity function. However, they fail at reproducing an excess of massive white dwarfs present in the observed mass function. A plausible explanation for this is that a sizable fraction of massive white dwarfs in the Galaxy are the product of white dwarf-white dwarf mergers.

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

    NASA Astrophysics Data System (ADS)

    Heber, U.

    2017-03-01

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

  7. An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate.

    PubMed

    Gilfanov, Marat; Bogdán, Akos

    2010-02-18

    There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10(7) years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of approximately 30-50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.

  8. The Merger Rate of Binary White Dwarfs in the Galactic Disk

    NASA Astrophysics Data System (ADS)

    Badenes, Carles; Maoz, Dan

    2012-04-01

    We use multi-epoch spectroscopy of ~4000 white dwarfs in the Sloan Digital Sky Survey to constrain the properties of the Galactic population of binary white dwarf systems and calculate their merger rate. With a Monte Carlo code, we model the distribution of ΔRVmax, the maximum radial velocity shift between exposures of the same star, as a function of the binary fraction within 0.05 AU, f bin, and the power-law index in the separation distribution at the end of the common-envelope phase, α. Although there is some degeneracy between f bin and α, the 15 high-ΔRVmax systems that we find constrain the combination of these parameters, which determines a white dwarf merger rate per unit stellar mass of 1.4+3.4 -1.0 × 10-13 yr-1 M -1 ⊙ (1σ limits). This is remarkably similar to the measured rate of Type Ia supernovae (SNe Ia) per unit stellar mass in Milky-Way-like Sbc galaxies. The rate of super-Chandrasekhar mergers is only 1.0+1.6 -0.6 × 10-14 yr-1 M -1 ⊙. We conclude that there are not enough close binary white dwarf systems to reproduce the observed SN Ia rate in the "classic" double degenerate super-Chandrasekhar scenario. On the other hand, if sub-Chandrasekhar mergers can lead to SNe Ia, as has been recently suggested by some studies, they could make a major contribution to the overall SN Ia rate. Although unlikely, we cannot rule out contamination of our sample by M-dwarf binaries or non-Gaussian errors. These issues will be clarified in the near future by completing the follow-up of all 15 high-ΔRVmax systems.

  9. LIMB-DARKENING COEFFICIENTS FOR ECLIPSING WHITE DWARFS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

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

    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 coefficientsmore » 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.« less

  10. Panchromatic Calibration of Astronomical Observations with State-of-the-Art White Dwarf Model Atmospheres

    NASA Astrophysics Data System (ADS)

    Rauch, T.

    2016-05-01

    Theoretical spectral energy distributions (SEDs) of white dwarfs provide a powerful tool for cross-calibration and sensitivity control of instruments from the far infrared to the X-ray energy range. Such SEDs can be calculated from fully metal-line blanketed NLTE model-atmospheres that are e.g. computed by the Tübingen NLTE Model-Atmosphere Package (TMAP) that has arrived at a high level of sophistication. TMAP was successfully employed for the reliable spectral analysis of many hot, compact post-AGB stars. High-quality stellar spectra obtained over a wide energy range establish a data base with a large number of spectral lines of many successive ions of different species. Their analysis allows to determine effective temperatures, surface gravities, and element abundances of individual (pre-)white dwarfs with very small error ranges. We present applications of TMAP SEDs for spectral analyses of hot, compact stars in the parameter range from (pre-) white dwarfs to neutron stars and demonstrate the improvement of flux calibration using white-dwarf SEDs that are e.g. available via registered services in the Virtual Observatory.

  11. FUSE Observations of Heavy Elements in the Photospheres of Cool DB White Dwarfs

    NASA Astrophysics Data System (ADS)

    Desharnais, S.; Wesemael, F.; Chayer, P.; Kruk, J. W.; Saffer, R. A.

    2008-01-01

    We present a comprehensive analysis of the far-ultraviolet spectra of five DB white dwarfs spanning the effective temperature range between 14,700 and 20,800 K. The FUSE line analysis shows that carbon features, previously observed in several hot DB stars at or above 22,000 K, are present in the two coolest (GD 408 and GD 378) and in the hottest (G270-124) target. The observed carbon abundances range from log N(C)/N(He) ~ - 6.9 to ~ - 8.8. In addition, four of the five objects display photospheric lines of silicon. Other elements such as oxygen, iron, and sulfur are also observed in some objects. The variations of the abundances of heavy elements as a function of effective temperature in DB stars are discussed in terms of a competition between a stellar wind, gravitational settling, accretion from interstellar (and circumstellar) matter, and convective dredge-up. FUSE is operated by the Johns Hopkins University under NASA contract NAS5-32985.

  12. Searching for new white dwarf pulsators for TESS observations at Konkoly Observatory

    NASA Astrophysics Data System (ADS)

    Bognár, Zs; Kalup, Cs; Sódor, Á.; Charpinet, S.; Hermes, J. J.

    2018-07-01

    We present the results of our survey searching for new white dwarf pulsators for observations by the TESS space telescope. We collected photometric time-series data on 14 white dwarf variable candidates at Konkoly Observatory, and found two new bright ZZ Ceti stars, namely EGGR 120 and WD 1310+583. We performed a Fourier analysis of the datasets. In the case of EGGR 120, which was observed on one night only, we found one significant frequency at 1332μHz with 2.3 mmag amplitude. We successfully observed WD 1310+583 on eight nights, and determined 17 significant frequencies in the whole dataset. Seven of them seem to be independent pulsation modes between 634 and 2740μHz, and we performed preliminary asteroseismic investigations of the star utilizing six of these periods. We also identified three new light variables on the fields of white dwarf candidates: an eclipsing binary, a candidate delta Scuti/beta Cephei and a candidate W UMa-type star.

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

    NASA Technical Reports Server (NTRS)

    Boehm-Vitense, Erika

    1993-01-01

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

  14. Low Magnetic Fields in White Dwarfs and their Direct Progenitors?

    NASA Astrophysics Data System (ADS)

    Jordan, S.; Bagnulo, S.; Landstreet, J.; Fossati, L.; Valyavin, G. G.; Monin, D.; Wade, G. A.; Werner, K.; O'Toole, S. J.

    2013-01-01

    We have carried out a re-analysis of polarimetric data of central stars of planetary nebulae, hot subdwarfs, and white dwarfs taken with FORS1 (FOcal Reducer and low dispersion Spectrograph) on the VLT (Very Large Telescope), and added a large number of new observations in order to increase the sample. A careful analysis of the observations using only one wavelength calibration for the polarimetrically analysed spectra and for all positions of the retarder plate of the spectrograph is crucial in order to avoid spurious signals. We find that the previous detections of magnetic fields in subdwarfs and central stars could not be confirmed while about 10% of the observed white dwarfs have magnetic fields at the kilogauss level.

  15. RCB stars from double degenerate white dwarf mergers

    NASA Astrophysics Data System (ADS)

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

    2018-01-01

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

  16. Long-term eclipse timing of white dwarf binaries: an observational hint of a magnetic mechanism at work

    NASA Astrophysics Data System (ADS)

    Bours, M. C. P.; Marsh, T. R.; Parsons, S. G.; Dhillon, V. S.; Ashley, R. P.; Bento, J. P.; Breedt, E.; Butterley, T.; Caceres, C.; Chote, P.; Copperwheat, C. M.; Hardy, L. K.; Hermes, J. J.; Irawati, P.; Kerry, P.; Kilkenny, D.; Littlefair, S. P.; McAllister, M. J.; Rattanasoon, S.; Sahman, D. I.; Vučković, M.; Wilson, R. W.

    2016-08-01

    We present a long-term programme for timing the eclipses of white dwarfs in close binaries to measure apparent and/or real variations in their orbital periods. Our programme includes 67 close binaries, both detached and semi-detached and with M-dwarfs, K-dwarfs, brown dwarfs or white dwarfs secondaries. In total, we have observed more than 650 white dwarf eclipses. We use this sample to search for orbital period variations and aim to identify the underlying cause of these variations. We find that the probability of observing orbital period variations increases significantly with the observational baseline. In particular, all binaries with baselines exceeding 10 yr, with secondaries of spectral type K2 - M5.5, show variations in the eclipse arrival times that in most cases amount to several minutes. In addition, among those with baselines shorter than 10 yr, binaries with late spectral type (>M6), brown dwarf or white dwarf secondaries appear to show no orbital period variations. This is in agreement with the so-called Applegate mechanism, which proposes that magnetic cycles in the secondary stars can drive variability in the binary orbits. We also present new eclipse times of NN Ser, which are still compatible with the previously published circumbinary planetary system model, although only with the addition of a quadratic term to the ephemeris. Finally, we conclude that we are limited by the relatively short observational baseline for many of the binaries in the eclipse timing programme, and therefore cannot yet draw robust conclusions about the cause of orbital period variations in evolved, white dwarf binaries.

  17. Infrared observations of white dwarfs and the implications for the accretion of dusty planetary material

    NASA Astrophysics Data System (ADS)

    Bonsor, Amy; Farihi, Jay; Wyatt, Mark C.; van Lieshout, Rik

    2017-06-01

    Infrared excesses around metal-polluted white dwarfs have been associated with the accretion of dusty planetary material. This work analyses the available infrared data for an unbiased sample of white dwarfs and demonstrates that no more than 3.3 per cent can have a wide, flat, opaque dust disc, extending to the Roche radius, with a temperature at the disc inner edge of Tin = 1400 K, the standard model for the observed excesses. This is in stark contrast to the incidence of pollution of about 30 per cent. We present four potential reasons for the absence of an infrared excess in polluted white dwarfs, depending on their stellar properties and inferred accretion rates: (I) their dust discs are opaque, but narrow, thus evading detection if more than 85 per cent of polluted white dwarfs have dust discs narrower than δr < 0.04r, (II) their dust discs have been fully consumed, which only works for the oldest white dwarfs with sinking time-scales longer than hundreds of years, (III) their dust is optically thin, which can supply low accretion rates of <107 gs-1 if dominated by (Poynting-Robertson) PR-drag, and higher accretion rates, if inwards transport of material is enhanced, e.g. due to the presence of gas, (IV) their accretion is supplied by a pure gas disc, which could result from the sublimation of optically thin dust for T* > 20 000 K. Future observations sensitive to faint infrared excesses or the presence of gas can test the scenarios presented here, thereby better constraining the nature of the material fuelling accretion in polluted white dwarfs.

  18. 3D hydrodynamic simulations of tidal disruption of terrestrial planets around white dwarfs

    NASA Astrophysics Data System (ADS)

    Liu, Shangfei; Zhang, Jinsu; Lin, Douglas N. C.

    2018-01-01

    Recent K2 mission spotted striking variability due to a group of minor bodies transiting white dwarf WD 1145+017 with periods ranging from 4.5 hours to 4.9 hours. One of the formation scenarios is that those transiting objects are the debris of a tidally disrupted minor planet. This scenario is consistent with fact that the white dwarf also hosts a dusty disk and displays strong metal atmospheric pollution. In this work, we perform state-of-the-art three-dimensional hydrodynamic simulations to study the consequences of tidal disruption of planets with various differentiated compositions by a white dwarf. We study the general outcomes of tidal disruption including partially disruption and total disruption. We also apply our results to the WD 1145+017 system to infer the physical and orbital properties of the progenitor.

  19. The binary white dwarf LHS 3236

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Harris, Hugh C.; Dahn, Conard C.; Canzian, Blaise

    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 binarymore » 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.« less

  20. Hydrodynamic Simulations of the Consequences of Accretion onto ONe White Dwarfs

    NASA Astrophysics Data System (ADS)

    Starrfield, Sumner; Bose, Maitrayee; Iliadis, Christian; Hix, William Raphael; Woodward, Charles E.; Wagner, Robert M.; José, Jordi; Hernanz, Margarita; Feng, Wanda

    2018-06-01

    Mass and luminosity variations of the white dwarf, combined with changes in the mass accretion rate and composition of the accreted material affect the evolution of the thermonuclear runaway (TNR) in classical and recurrent novae. Here we highlight continued investigations of these effects on accreting Oxygen-Neon (ONe) white dwarfs. We now use the results of the multi-dimensional studies of TNRs in white dwarfs, accreting only solar matter, which show that sufficient core material is dredged-up during the TNR to agree with the measurements of ejecta abundances in classical nova explosions. Therefore, we first accrete solar material and follow the evolution until a TNR is ongoing. We then switch the composition to a mixture with either 25% core material or 50% core material (plus accreted material) and follow the resulting evolution of the TNR through peak nuclear burning and decline. We use our 1D, Lagrangian, hydrodynamic code: NOVA. We will report on the results of these new simulations and compare the ejecta abundances to those measured in pre-solar grains that are thought to arise from classical nova explosions. We will also compare these results to our companion studies, done in a similar fashion, where we have followed the consequences of accretion onto Carbon-Oxygen white dwarfs. This work was supported in part by NASA under the Astrophysics Theory Program grant 14-ATP14-0007 and the U.S. DOE under Contract No. DE-FG02- 97ER41041. SS acknowledges partial support from NASA, NSF, and HST grants to ASU and WRH is supported by the U.S. Department of Energy, Office of Nuclear Physics.

  1. Studying white dwarf merger remnants with FLASH

    NASA Astrophysics Data System (ADS)

    Jenks, Malia

    2017-01-01

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

  2. A New Stellar Outburst Associated with the Magnetic Activities of the K-type Dwarf in a White Dwarf Binary

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Qian, S.-B.; Han, Z.-T.; Zhang, B.

    1SWASP J162117.36+441254.2 was originally classified as an EW-type binary with a period of 0.20785 days. However, it was detected to have undergone a stellar outburst on 2016 June 3. Although the system was later classified as a cataclysmic variable (CV) and the event was attributed as a dwarf nova outburst, the physical reason is still unknown. This binary has been monitored photometrically since 2016 April 19, and many light curves were obtained before, during, and after the outburst. Those light and color curves observed before the outburst indicate that the system is a special CV. The white dwarf is notmore » accreting material from the secondary and there are no accretion disks surrounding the white dwarf. By comparing the light curves obtained from 2016 April 19 to those from September 14, it was found that magnetic activity of the secondary is associated with the outburst. We show strong evidence that the L {sub 1} region on the secondary was heavily spotted before and after the outburst and thus quench the mass transfer, while the outburst is produced by a sudden mass accretion of the white dwarf. These results suggest that J162117 is a good astrophysical laboratory to study stellar magnetic activity and its influences on CV mass transfer and mass accretion.« less

  3. A New Stellar Outburst Associated with the Magnetic Activities of the K-type Dwarf in a White Dwarf Binary

    NASA Astrophysics Data System (ADS)

    Qian, S.-B.; Han, Z.-T.; Zhang, B.; Zejda, M.; Michel, R.; Zhu, L.-Y.; Zhao, E.-G.; Liao, W.-P.; Tian, X.-M.; Wang, Z.-H.

    2017-10-01

    1SWASP J162117.36+441254.2 was originally classified as an EW-type binary with a period of 0.20785 days. However, it was detected to have undergone a stellar outburst on 2016 June 3. Although the system was later classified as a cataclysmic variable (CV) and the event was attributed as a dwarf nova outburst, the physical reason is still unknown. This binary has been monitored photometrically since 2016 April 19, and many light curves were obtained before, during, and after the outburst. Those light and color curves observed before the outburst indicate that the system is a special CV. The white dwarf is not accreting material from the secondary and there are no accretion disks surrounding the white dwarf. By comparing the light curves obtained from 2016 April 19 to those from September 14, it was found that magnetic activity of the secondary is associated with the outburst. We show strong evidence that the L 1 region on the secondary was heavily spotted before and after the outburst and thus quench the mass transfer, while the outburst is produced by a sudden mass accretion of the white dwarf. These results suggest that J162117 is a good astrophysical laboratory to study stellar magnetic activity and its influences on CV mass transfer and mass accretion.

  4. Relativistic deflection of background starlight measures the mass of a nearby white dwarf star.

    PubMed

    Sahu, Kailash C; Anderson, Jay; Casertano, Stefano; Bond, Howard E; Bergeron, Pierre; Nelan, Edmund P; Pueyo, Laurent; Brown, Thomas M; Bellini, Andrea; Levay, Zoltan G; Sokol, Joshua; Dominik, Martin; Calamida, Annalisa; Kains, Noé; Livio, Mario

    2017-06-09

    Gravitational deflection of starlight around the Sun during the 1919 total solar eclipse provided measurements that confirmed Einstein's general theory of relativity. We have used the Hubble Space Telescope to measure the analogous process of astrometric microlensing caused by a nearby star, the white dwarf Stein 2051 B. As Stein 2051 B passed closely in front of a background star, the background star's position was deflected. Measurement of this deflection at multiple epochs allowed us to determine the mass of Stein 2051 B-the sixth-nearest white dwarf to the Sun-as 0.675 ± 0.051 solar masses. This mass determination provides confirmation of the physics of degenerate matter and lends support to white dwarf evolutionary theory. Copyright © 2017, American Association for the Advancement of Science.

  5. A young contracting white dwarf in the peculiar binary HD 49798/RX J0648.0-4418?

    NASA Astrophysics Data System (ADS)

    Popov, S. B.; Mereghetti, S.; Blinnikov, S. I.; Kuranov, A. G.; Yungelson, L. R.

    2018-02-01

    HD 49798/RX J0648.0-4418 is a peculiar X-ray binary with a hot subdwarf (sdO) mass donor. The nature of the accreting compact object is not known, but its spin period P = 13.2 s and \\dot{P} =-2.15 × 10^{-15} s s-1 proves that it can be only either a white dwarf or a neutron star. The spin-up has been very stable for more than 20 yr. We demonstrate that the continuous stable spin-up of the compact companion of HD 49798 can be best explained by contraction of a young white dwarf with an age ˜2 Myr. This allows us to interpret all the basic parameters of the system in the framework of an accreting white dwarf. We present examples of binary evolution, which result in such systems. If correct, this is the first direct evidence for a white dwarf contraction in early evolutionary stages.

  6. A Search for Metal Lines in the Spectra of DA White Dwarfs

    NASA Technical Reports Server (NTRS)

    Wegner, G. A.

    1986-01-01

    A theoretical analysis was carried out in order to interpret the ultraviolet spectra of DB white dwarfs obtained earlier with the International Ultraviolet Explorer (IUE) satellite. Here the results of the IUE ultraviolet spectroscopy combined with visual data and model atmospheres of DB white dwarfs are reported. In particular, a search for spectra lines due to the element carbon using the ultraviolet was made. In no case is there a positive detection of carbon and from these data, and upper limits for carbon by number relative to helium are derived in the range of C: He 10 to the minus 5 power to 10 to the minus 7 power for the 16 DB stars with ultraviolet spectra in the temperature range 11400 K T sub EFF less than 2300 K. The low carbon abundances found in the atmospheres of the DB stars agree well with the hypothesis that the atmospheric carbon observed in the cooler DQ members of the helium-rich white dwarf sequence is produced by a convective dredging mechanism.

  7. White Dwarf Asteroseismology and the 12C(α,γ)16O Rate

    NASA Astrophysics Data System (ADS)

    Metcalfe, Travis S.

    2003-04-01

    Due to a new global analysis method, it is now possible to measure the internal composition of pulsating white dwarf stars, even with relatively simple theoretical models. The precise internal mixture of carbon and oxygen is the largest single source of uncertainty in ages derived from white dwarf cosmochronometry, and it contains information about the rate of the astrophysically important, but experimentally uncertain, 12C(α,γ)16O nuclear reaction. Recent determinations of the internal composition and structure of two helium-atmosphere variable (DBV) white dwarf stars, GD 358 and CBS 114, initially led to conflicting implied rates for the 12C(α,γ)16O reaction. If both stars were formed through single-star evolution, then the initial analyses of their pulsation frequencies must have differed in some systematic way. I present improved fits to the two sets of pulsation data, resolving the tension between the initial results and leading to a value for the 12C(α,γ)16O reaction rate that is consistent with recent laboratory measurements.

  8. ON THE EFFECT OF EXPLOSIVE THERMONUCLEAR BURNING ON THE ACCRETED ENVELOPES OF WHITE DWARFS IN CATACLYSMIC VARIABLES

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Sion, Edward M.; Sparks, Warren, E-mail: edward.sion@villanova.edu, E-mail: warrensparks@comcast.net

    2014-11-20

    The detection of heavy elements at suprasolar abundances in the atmospheres of some accreting white dwarfs in cataclysmic variables (CVs), coupled with the high temperatures needed to produce these elements, requires explosive thermonuclear burning. The central temperatures of any formerly more massive secondary stars in CVs undergoing hydrostatic CNO burning are far too low to produce these elements. Evidence is presented that at least some CVs contain donor secondaries that have been contaminated by white dwarf remnant burning during the common envelope phase and are transferring this material back to the white dwarf. This scenario does not exclude the channelmore » in which formerly more massive donor stars underwent CNO processing in systems with thermal timescale mass transfer. Implications for the progenitors of CVs are discussed and a new scenario for the white dwarf's accretion-nova-outburst is given.« less

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

    NASA Technical Reports Server (NTRS)

    Peery, Benjamin F., Jr.

    1986-01-01

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

  10. Ultrahigh-energy cosmic rays from tidally-ignited white dwarfs

    NASA Astrophysics Data System (ADS)

    Alves Batista, Rafael; Silk, Joseph

    2017-11-01

    Ultrahigh-energy cosmic rays (UHECRs) can be accelerated by tidal disruption events of stars by black holes. We suggest a novel mechanism for UHECR acceleration wherein white dwarfs (WDs) are tidally compressed by intermediate-mass black holes (IMBHs), leading to their ignition and subsequent explosion as a supernova. Cosmic rays accelerated by the supernova may receive an energy boost when crossing the accretion-powered jet. The rate of encounters between WDs and IMBHs can be relatively high, as the number of IMBHs may be substantially augmented once account is taken of their likely presence in dwarf galaxies. Here we show that this kind of tidal disruption event naturally provides an intermediate composition for the observed UHECRs, and suggest that dwarf galaxies and globular clusters are suitable sites for particle acceleration to ultrahigh energies.

  11. 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

  12. Hydrodynamic Simulations of Classical Novae: Accretion onto CO White Dwarfs as SN Ia Progenitors

    NASA Astrophysics Data System (ADS)

    Starrfield, Sumner; Bose, Maitrayee; Iliadis, Christian; Hix, William R.; José, Jordi; Hernanz, Margarita

    2017-06-01

    We have continued our studies of accretion onto white dwarfs by following the evolution of thermonuclear runaways on Carbon Oxygen (CO) white dwarfs. We have varied the mass of the white dwarf and the composition of the accreted material but chosen to keep the mass accretion rate at 2 x 10^{-10} solar masses per year to obtain the largest amount of accreted material possible with rates near to those observed. We assume either 25% core material or 50% core material has been mixed into the accreting material prior to the explosion. We use our 1D, lagrangian, hydrodynamic code: NOVA. We will report on the results of these simulations and compare the ejecta abundances to those measured in pre-solar grains that are thought to arise from classical nova explosions. These results will also be compared to recent results with SHIVA (Jose and Hernanz). We find that in all cases and for all white dwarf masses that less mass is ejected than accreted and, therefore, the white dwarf is growing in mass as a result of the accretion and resulting explosion.This work was supported in part by NASA under the Astrophysics Theory Program grant 14-ATP14-0007 and the U.S. DOE under Contract No. DE-FG02- 97ER41041. SS acknowledges partial support from NASA, NSF, and HST grants to ASU and WRH is supported by the U.S. Department of Energy, Office of Nuclear Physics. The results reported herein benefitted from collaborations and/or information exchange within NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate.

  13. The Fate of Exoplanetary Systems and the Implications for White Dwarf Pollution

    NASA Astrophysics Data System (ADS)

    Veras, D.; Mustill, A. J.; Bonsor, A.; Wyatt, M. C.

    2013-09-01

    Mounting discoveries of extrasolar planets orbiting post-main-sequence stars motivate studies to understand the fate of these planets. Also, polluted white dwarfs (WDs) likely represent dynamically active systems at late times. Here, we perform full-lifetime simulations of one-, two- and three-planet systems from the endpoint of formation to several Gyr into the WD phase of the host star. We outline the physical and computational processes which must be considered for post-main-sequence planetary studies, and characterize the challenges in explaining the robust observational signatures of infrared excess in white dwarfs by appealing to late-stage planetary systems.

  14. A SUBSTELLAR COMPANION TO THE WHITE DWARF-RED DWARF ECLIPSING BINARY NN Ser

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Qian, S.-B.; Dai, Z.-B.; Liao, W.-P.

    2009-11-20

    NN Ser is a short-period (P = 3.12 hr) close binary containing a very hot white dwarf primary with a mass of 0.535 M{sub sun} and a fully convective secondary with a mass of 0.111 M{sub sun}. The changes in the orbital period of the eclipsing binary were analyzed based on our five newly determined eclipse times together with those compiled from the literature. A small-amplitude (0fd00031) cyclic period variation with a period of 7.56 years was discovered to be superimposed on a possible long-term decrease. The periodic change was plausibly explained as the light-travel time effect via the presencemore » of a tertiary companion. The mass of the tertiary companion is determined to be M{sub 3}sin i' = 0.0107(+-0.0017) M{sub sun} when a total mass of 0.646 M{sub sun} for NN Ser is adopted. For orbital inclinations i' >= 49.{sup 0}56, the mass of the tertiary component was calculated to be M {sub 3} <= 0.014 M{sub sun}; thus it would be an extrasolar planet. The third body is orbiting the white dwarf-red dwarf eclipsing binary at a distance shorter than 3.29 AU. Since the observed decrease rate of the orbital period is about two orders larger than that caused by gravitational radiation, it can be plausibly interpreted by magnetic braking of the fully convective component, which is driving this binary to evolve into a normal cataclysmic variable.« less

  15. Formation of Extremely Low-mass White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Sun, M.; Arras, P.

    2018-05-01

    Motivated by the discovery of several pulsating, extremely low-mass white dwarfs (ELM WDs, mass M ≲ 0.18 M ⊙) that likely have WD companions, this paper discusses binary formation models for these systems. ELM WDs are formed using angular momentum losses by magnetic braking. Evolutionary models are constructed using the Modules for Experiments in Stellar Astrophysics (MESA), with ELM WD progenitors in the range 1.0 ≲ M d/M ⊙ ≲ 1.5 and WD companions in the range 0.4 ≲ M a/M ⊙ ≲ 0.9. A prescription to reduce magnetic braking for thin surface convection zones is included. Upon the thinning of the evolved donor envelope, the donor star shrinks out of contact and mass transfer (MT) ceases, revealing the ELM WD. Systems with low masses have previously been suggested as possible AM CVNs. Systems with high masses, up to the limit M ≃ 0.18 M ⊙ at which shell flashes occur on the WD cooling track, tend to expand out to orbital periods P orb ≳ 15 hr. In between this range, ELM WDs may become pulsators both as pre-WDs and on the WD cooling track. Brickhill’s criterion for convective mode driving is used to estimate the location of the blue edge of the g-mode instability strip. In the appendix, we show that the formation of an ELM WD by unstable MT or a common-envelope event is unlikely. Stable Roche-lobe overflow with conservative MT produces only M ≳ 0.2 M ⊙.

  16. Nucleus-acoustic shock waves in white dwarfs

    NASA Astrophysics Data System (ADS)

    Jannat, S.; Mamun, A. A.

    2018-04-01

    The nucleus-acoustic shock waves (NASWs) propagating in a white dwarf plasma system, which contain non-relativistically or ultrarelativistically degenerate electrons, non-relativistically degenerate, viscous fluid of light nuclei, and immobile nuclei of heavy elements, have been theoretically investigated. We have used the reductive perturbation method, which is valid for small but finite-amplitude NASWs to derive the Burgers equation. The NASWs are, in fact, associated with the nucleus-acoustic (NA) waves in which the inertia is provided by the light nuclei, and restoring force is provided by the degenerate pressure of electrons. On the other hand, the stationary heavy nuclei participate only in maintaining the background charge neutrality condition at equilibrium. It is found that the viscous force acting in the fluid of light nuclei is a source of dissipation, and is responsible for the formation of NASWs. It is also observed that the basic features (polarity, amplitude, width, etc.) of the NASWs are significantly modified by the presence of heavy nuclei, and that NASWs are formed with either positive or negative potential depending on the values of the charge density of the heavy nuclei. The basic properties are also found to be significantly modified by the effects of ultrarelativistically degenerate electrons. The implications of our results in white dwarfs are briefly discussed.

  17. 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.

  18. Cool Dwarfs 1o-7

    NASA Astrophysics Data System (ADS)

    Ambruster, Carol W.

    Most of the cool dwarfs in the interesting age range 10^7-10^8 yr are too faint for IUE, yet such stars are critically important from the viewpoint of stellar evolution. Among stars of this age are the Pleiades K dwarfs, some of which appear to be on the main sequence, and some of which are still arriving there. Up until last year, only 2 stars in this age range had been observed by IUE, both recently: HD 36705 (AB Dor) and HD 17433. Three more stars were identified by the present investigators and observed with IUE during the past (11th) year: HD 129333, a single, nearby solar-type GOV star; HD 82558, a rapidly rotating, single, K2V star; and Ross 137B, the M dwarf common proper motion companion to AB Dor. We have since identified 5 more stars between 10^7 and 10^8 years old that are bright enough to be observed by IUE. They are physically associated, but distant, companions to main sequence O and B stars, identified in the survey of Lindroos (1986). Their ages are thus determined by the short main sequence lifetimes of the hot primaries. Rotational velocities are not yet known for our 5 proposed targets; we will be obtaining these and other data in the coming year. We therefore request time for basic IUE observations of these stars, an LWP-lo, LWP-hi and SWP-lo, for each star. This will ensure that crucial basic fluxes are in the IUE archives, should the satellite die in the coming year. Furthermore these data are immediately useful in filling the gap in the exhaustive study by Simon, Herbig and Boesgaard (1985) of the evolution of TR and chromospheric activity with age. More in-depth coverage will be proposed next year.

  19. The influence of H2O line blanketing on the spectra of cool dwarf stars

    NASA Technical Reports Server (NTRS)

    Allard, F.; Hauschildt, P. H.; Miller, S.; Tennyson, J.

    1994-01-01

    We present our initial results of model atmosphere calculations for cool M dwarfs using an opacity sampling method and a new list of H2O lines. We obtain significantly improved fits to the infrared spectrum of the M dwarf VB10 when compared to earlier models. H2O is by far the dominant opacity source in cool stars. To illustrate this, we show the Rosseland mean of the total extinction under various assumptions. Our calculations demonstrate the importance of a good treatment of the water opacities in cool stars and the improvements possible by using up-to-date data for the water line absorption.

  20. 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

  1. A burst from a thermonuclear runaway on an ONeMg white dwarf

    NASA Technical Reports Server (NTRS)

    Starrfield, S.; Politano, M.; Truran, J. W.; Sparks, W. M.

    1992-01-01

    Studies which examine the consequences of accretion, at rates of 10(exp -9) solar mass/yr and 10(exp -10) solar mass/yr, onto an ONeMg white dwarf with a mass of 1.35 solar masses are performed. In these studies, a Lagrangian, hydrodynamic, one-dimensional computer code was used. The code now includes a network with 89 nuclei up to Ca-40, elemental diffusion, new opacities, and new equation of state. The initial abundance distribution corresponded to a mixture that was enriched to either 25, 50, or 75 percent in products of carbon burning. The remaining material in each case is assumed to have a solar composition. The evolution of the thermonuclear runaway in the 1.35 solar mass white dwarf, with M = 10(exp -9) solar mass, produced peak temperatures in the shell source exceeding 300 million degrees. The sequence produced significant amounts of Na-22 from proton captures onto Ne-20 and significant amounts of Al-26 from proton captures on Mg-24. This sequence ejected 5.2 x 10(exp -6) solar mass moving with speeds from approximately 100 km/s to 2300 km/s. When the mass accretion rate was decreased to 10(exp -10) solar mass, the resulting thermonuclear runaway produced a shock that moved through the outer envelope of the white dwarf and raised the surface luminosity to L greater than 10(exp 7) solar luminosity and the effective temperature to values exceeding 10(exp 7) K. The interaction of the material expanding from off of the white dwarf with the accretion disk should produce a burst of gamma-rays.

  2. A reevaluation of the proposed spin-down of the white dwarf pulsar in AR Scorpii.

    NASA Astrophysics Data System (ADS)

    Potter, Stephen B.; Buckley, David A. H.

    2018-05-01

    We present high-speed optical photometric observations, spanning ˜2 years, of the recently-discovered white dwarf pulsar AR Scorpii. The amplitudes of the orbital, spin and beat modulations appear to be remarkably stable and repeatable over the time span of our observations. It has been suggested that the polarized and non-polarized emission from AR Scorpii is powered by the spin-down of the white dwarf. However, we find that our new data is inconsistent with the published spin-down ephemeris. Whilst our data is consistent with a constant spin period further observations over an extended time-base are required in order to ascertain the true spin-evolution of the white dwarf. This may have implications for the various models put forward to explain the energetics and evolution of AR Scorpii.

  3. A SUBTLE INFRARED EXCESS ASSOCIATED WITH A YOUNG WHITE DWARF IN THE EDINBURGH-CAPE BLUE OBJECT SURVEY

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Dennihy, E.; Dunlap, B. H.; Clemens, J. C.

    We report the discovery of a subtle infrared excess associated with the young white dwarf EC 05365–4749 at 3.35 and 4.6 μ m. Follow-up spectroscopic observations are consistent with a hydrogen atmosphere white dwarf of effective temperature 22,800 K and log [ g (cm s{sup −2})] = 8.19. High-resolution spectroscopy reveals atmospheric metal pollution with logarithmic abundances of [Mg/H] = −5.36 and [Ca/H] = −5.75, confirming the white dwarf is actively accreting from a metal-rich source with an intriguing abundance pattern. We find that the infrared excess is well modeled by a flat, opaque debris disk, though disk parameters aremore » not well constrained by the small number of infrared excess points. We further demonstrate that relaxing the assumption of a circular dusty debris disk to include elliptical disks expands the widths of acceptable disks, adding an alternative interpretation to the subtle infrared excesses commonly observed around young white dwarfs.« less

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

    NASA Astrophysics Data System (ADS)

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

    2011-01-01

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

  5. The coolest DA white dwarfs detected at soft X-ray wavelengths

    NASA Technical Reports Server (NTRS)

    Kidder, K. M.; Holberg, J. B.; Barstow, M. A.; Tweedy, R. W.; Wesemael, F.

    1992-01-01

    New soft X-ray/EUV photometric observations of the DA white dwarfs KPD 0631 + 1043 = WD 0631 + 107 and PG 1113 + 413 = WD 1113 + 413 are analyzed. Previously reported soft X-ray detections of three other DAs and the failure to detect a fourth DA in deep Exosat observations are investigated. New ground-based spectra are presented for all of the objects, with IUE Ly-alpha spectra for some. These data are used to constrain the effective temperatures and surface gravities. The improved estimates of these parameters are employed to refer a photospheric He abundance for the hotter objects and to elucidate an effective observational low-temperature threshold for the detection of pure hydrogen DA white dwarfs at soft X-ray wavelengths.

  6. A MODEL OF WHITE DWARF PULSAR AR SCORPII

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Geng, Jin-Jun; Huang, Yong-Feng; Zhang, Bing, E-mail: gengjinjun@gmail.com, E-mail: hyf@nju.edu.cn, E-mail: zhang@physics.unlv.edu

    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 accountmore » for the broadband (from radio to X-rays) spectral energy distribution of the system.« less

  7. Surface Inhomogeneities of the White Dwarf in the Binary EUVE J2013+400

    NASA Astrophysics Data System (ADS)

    Vennes, Stephane

    We propose to study the white dwarf in the binary EUVE J2013+400. The object is paired with a dMe star and new extreme ultraviolet (EUV) observations will offer critical insights into the properties of the white dwarf. The binary behaves, in every other aspects, like its siblings EUVE J0720-317 and EUVE J1016-053 and new EUV observations will help establish their class properties; in particular, EUV photometric variations in 0720-317 and 1016-053 over a period of 11 hours and 57 minutes, respectively, are indicative of surface abundance inhomogeneities coupled with the white dwarfs rotation period. These variations and their large photospheric helium abundance are best explained by a diffusion-accretion model in which time-variable accretion and possible coupling to magnetic poles contribute to abundance variations across the surface and possibly as a function of depth. EUV spectroscopy will also enable a study of the helium abundance as a function of depth and a detailed comparison with theoretical diffusion profile.

  8. The Ages of the Thin Disk, Thick Disk, and the Halo from Nearby White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kilic, Mukremin; Munn, Jeffrey A.; Harris, Hugh C.; von Hippel, Ted; Liebert, James W.; Williams, Kurtis A.; Jeffery, Elizabeth; DeGennaro, Steven

    2017-03-01

    We present a detailed analysis of the white dwarf luminosity functions derived from the local 40 pc sample and the deep proper motion catalog of Munn et al. Many previous studies have ignored the contribution of thick disk white dwarfs to the Galactic disk luminosity function, which results in an erroneous age measurement. We demonstrate that the ratio of thick/thin disk white dwarfs is roughly 20% in the local sample. Simultaneously fitting for both disk components, we derive ages of 6.8-7.0 Gyr for the thin disk and 8.7 ± 0.1 Gyr for the thick disk from the local 40 pc sample. Similarly, we derive ages of 7.4-8.2 Gyr for the thin disk and 9.5-9.9 Gyr for the thick disk from the deep proper motion catalog, which shows no evidence of a deviation from a constant star formation rate in the past 2.5 Gyr. We constrain the time difference between the onset of star formation in the thin disk and the thick disk to be {1.6}-0.4+0.3 Gyr. The faint end of the luminosity function for the halo white dwarfs is less constrained, resulting in an age estimate of {12.5}-3.4+1.4 Gyr for the Galactic inner halo. This is the first time that ages for all three major components of the Galaxy have been obtained from a sample of field white dwarfs that is large enough to contain significant numbers of disk and halo objects. The resultant ages agree reasonably well with the age estimates for the oldest open and globular clusters.

  9. HST eclipse mapping of dwarf nova OY Carinae in quiescence: An 'Fe II curtain' with Mach approx. = 6 velocity dispersion veils the white dwarf

    NASA Technical Reports Server (NTRS)

    Horne, Keith; Marsh, T. R.; Cheng, F. H.; Hubeny, Ivan; Lanz, Theirry

    1994-01-01

    Hubble Space Telescope (HST) observations of the eclipsing dwarf nova OY Car in its quiescent state are used to isolate the ultraviolet spectrum (1150-2500 A at 9.2 A Full Width at Half Maximum (FWHM) resolution) of the white dwarf, the accretion disk, and the bright spot. The white dwarf spectrum has a Stark-broadened photospheric L(alpha) absorption, but is veiled by a forest of blended Fe II features that we attribute to absorption by intervening disk material. A fit gives T(sub w) approx. = 16.5 x 10(exp 3) K for the white dwarf with a solar-abundance, log g = 8 model atmosphere, and T approx. = 10(exp 4) K, n(sub e) approx. = 10(exp 13)/cu cm, N(sub H) approx. = 10(exp 22) sq cm, and velocity dispersion delta V approx. = 60 km/s for the veil of homogeneous solar-abundance local thermodynamic equilibrium (LTE) gas. The veil parameters probably measure characteristic physical conditions in the quiescent accretion disk or its chromosphere. The large velocity dispersion is essential for a good fit; it lowers (chi square)/778 from 22 to 4. Keplerian shear can produce the velocity dispersion if the veiling gas is located at R approx. = 5 R(sub W) with (delta R)/R approx. = 0.3, but this model leaves an unobscured view to the upper hemisphere of the white dwarf, incompatible with absorptions that are up to 80% deep. The veiling gas may be in the upper atmosphere of the disk near its outer rim, but we then require supersonic (Mach approx. = 6) but sub-Keplerian (delta V/V(sub Kep) approx. = 0.07) velocity disturbances in this region to produce both the observed radial velocity dispersion and vertical motions sufficient to elevate the gas to z/R = cos i = 0.12. Such motions might be driven by the gas stream, since it may take several Kepler periods to reestablish the disk's vertical hydrostatic equilibrium. The temperature and column density of the gas we see as Fe II absorption in the ultraviolet are similar to what is required to produce the strong Balmer jump and

  10. NORMAL TYPE Ia SUPERNOVAE FROM VIOLENT MERGERS OF WHITE DWARF BINARIES

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Pakmor, R.; Kromer, M.; Taubenberger, S.

    2012-03-15

    One of the most important questions regarding the progenitor systems of Type Ia supernovae (SNe Ia) is whether mergers of two white dwarfs can lead to explosions that reproduce observations of normal events. Here we present a fully three-dimensional simulation of a violent merger of two carbon-oxygen white dwarfs with masses of 0.9 M{sub Sun} and 1.1 M{sub Sun} combining very high resolution and exact initial conditions. A well-tested combination of codes is used to study the system. We start with the dynamical inspiral phase and follow the subsequent thermonuclear explosion under the plausible assumption that a detonation forms inmore » the process of merging. We then perform detailed nucleosynthesis calculations and radiative transfer simulations to predict synthetic observables from the homologously expanding supernova ejecta. We find that synthetic color light curves of our merger, which produces about 0.62 M{sub Sun} of {sup 56}Ni, show good agreement with those observed for normal SNe Ia in all wave bands from U to K. Line velocities in synthetic spectra around maximum light also agree well with observations. We conclude that violent mergers of massive white dwarfs can closely resemble normal SNe Ia. Therefore, depending on the number of such massive systems available these mergers may contribute at least a small fraction to the observed population of normal SNe Ia.« less

  11. Detectable close-in planets around white dwarfs through late unpacking

    NASA Astrophysics Data System (ADS)

    Veras, Dimitri; Gänsicke, Boris T.

    2015-02-01

    Although 25-50 per cent of white dwarfs (WDs) display evidence for remnant planetary systems, their orbital architectures and overall sizes remain unknown. Vibrant close-in (≃1 R⊙) circumstellar activity is detected at WDs spanning many Gyr in age, suggestive of planets further away. Here we demonstrate how systems with 4 and 10 closely packed planets that remain stable and ordered on the main sequence can become unpacked when the star evolves into a WD and experience pervasive inward planetary incursions throughout WD cooling. Our full-lifetime simulations run for the age of the Universe and adopt main-sequence stellar masses of 1.5, 2.0 and 2.5 M⊙, which correspond to the mass range occupied by the progenitors of typical present-day WDs. These results provide (i) a natural way to generate an ever-changing dynamical architecture in post-main-sequence planetary systems, (ii) an avenue for planets to achieve temporary close-in orbits that are potentially detectable by transit photometry and (iii) a dynamical explanation for how residual asteroids might pollute particularly old WDs.

  12. The 2011 Outburst of Recurrent Nova T Pyx: X-Ray Observations Expose the White Dwarf Mass and Ejection Dynamics

    NASA Technical Reports Server (NTRS)

    Chomiuk, Laura; Nelson, Thomas; Mukai, Koji; Solokoski, J. L.; Rupen, Michael P.; Page, Kim L.; Osborne, Julian P.; Kuulkers, Erik; Mioduszewski, Amy J.; Roy, Nirupam; hide

    2014-01-01

    The recurrent nova T Pyx underwent its sixth historical outburst in 2011, and became the subject of an intensive multi-wavelength observational campaign.We analyze data from the Swift and Suzaku satellites to produce a detailed X-ray light curve augmented by epochs of spectral information. X-ray observations yield mostly non-detections in the first four months of outburst, but both a super-soft and hard X-ray component rise rapidly after Day 115. The super-soft X-ray component, attributable to the photosphere of the nuclear-burning white dwarf, is relatively cool (approximately 45 electron volts) and implies that the white dwarf in T Pyx is significantly below the Chandrasekhar mass (approximately 1 M). The late turn-on time of the super-soft component yields a large nova ejecta mass (approximately greater than 10(exp -5) solar mass), consistent with estimates at other wavelengths. The hard X-ray component is well fit by a approximately 1 kiloelectron volt thermal plasma, and is attributed to shocks internal to the 2011 nova ejecta. The presence of a strong oxygen line in this thermal plasma on Day 194 requires a significantly super-solar abundance of oxygen and implies that the ejecta are polluted by white dwarf material. The X-ray light curve can be explained by a dual-phase ejection, with a significant delay between the first and second ejection phases, and the second ejection finally released two months after outburst. A delayed ejection is consistent with optical and radio observations of T Pyx, but the physical mechanism producing such a delay remains a mystery.

  13. The 2011 outburst of recurrent nova T Pyx: X-ray observations expose the white dwarf mass and ejection dynamics

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Chomiuk, Laura; Nelson, Thomas; Mukai, Koji

    2014-06-20

    The recurrent nova T Pyx underwent its sixth historical outburst in 2011, and became the subject of an intensive multi-wavelength observational campaign. We analyze data from the Swift and Suzaku satellites to produce a detailed X-ray light curve augmented by epochs of spectral information. X-ray observations yield mostly non-detections in the first four months of outburst, but both a super-soft and hard X-ray component rise rapidly after Day 115. The super-soft X-ray component, attributable to the photosphere of the nuclear-burning white dwarf, is relatively cool (∼45 eV) and implies that the white dwarf in T Pyx is significantly below themore » Chandrasekhar mass (∼1 M {sub ☉}). The late turn-on time of the super-soft component yields a large nova ejecta mass (≳ 10{sup –5} M {sub ☉}), consistent with estimates at other wavelengths. The hard X-ray component is well fit by a ∼1 keV thermal plasma, and is attributed to shocks internal to the 2011 nova ejecta. The presence of a strong oxygen line in this thermal plasma on Day 194 requires a significantly super-solar abundance of oxygen and implies that the ejecta are polluted by white dwarf material. The X-ray light curve can be explained by a dual-phase ejection, with a significant delay between the first and second ejection phases, and the second ejection finally released two months after outburst. A delayed ejection is consistent with optical and radio observations of T Pyx, but the physical mechanism producing such a delay remains a mystery.« less

  14. 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.

  15. Angular momentum exchange in white dwarf binaries accreting through direct impact

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Sepinsky, J. F.; Kalogera, V., E-mail: jeremy.sepinsky@scranton.edu, E-mail: vicky@northwestern.edu

    We examine the exchange of angular momentum between the component spins and the orbit in semi-detached double white dwarf binaries undergoing mass transfer through direct impact of the transfer stream. We approximate the stream as a series of discrete massive particles ejected in the ballistic limit at the inner Lagrangian point of the donor toward the accretor. This work improves upon similar earlier studies in a number of ways. First, we self-consistently calculate the total angular momentum of the orbit at all times. This includes changes in the orbital angular momentum during the ballistic trajectory of the ejected mass, asmore » well as changes during the ejection/accretion due to the radial component of the particle's velocity. Second, we calculate the particle's ballistic trajectory for each system, which allows us to determine the precise position and velocity of the particle upon accretion. We can then include specific information about the radius of the accretor as well as the angle of impact. Finally, we ensure that the total angular momentum is conserved, which requires the donor star spin to vary self-consistently. With these improvements, we calculate the angular momentum change of the orbit and each binary component across the entire parameter space of direct impact double white dwarf binary systems. We find a significant decrease in the amount of angular momentum removed from the orbit during mass transfer, as well as cases where this process increases the angular momentum of the orbit at the expense of the spin angular momentum of the donor. We conclude that, unlike earlier claims in the literature, mass transfer through direct impact need not destabilize the binary and that the quantity and sign of the orbital angular momentum transfer depends on the binary properties, particularly the masses of the double white dwarf binary component stars. This stabilization may significantly impact the population synthesis calculations of the expected numbers

  16. Throwing Icebergs at White Dwarfs

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Stephan, Alexander P.; Naoz, Smadar; Zuckerman, B., E-mail: alexpstephan@astro.ucla.edu

    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,more » 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.« less

  17. Measurements of Physical Parameters of White Dwarfs: A Test of the Mass–Radius Relation

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Bédard, A.; Bergeron, P.; Fontaine, G., E-mail: bedard@astro.umontreal.ca, E-mail: bergeron@astro.umontreal.ca, E-mail: fontaine@astro.umontreal.ca

    We present a detailed spectroscopic and photometric analysis of 219 DA and DB white dwarfs for which trigonometric parallax measurements are available. Our aim is to compare the physical parameters derived from the spectroscopic and photometric techniques, and then to test the theoretical mass–radius relation for white dwarfs using these results. The agreement between spectroscopic and photometric parameters is found to be excellent, especially for effective temperatures, showing that our model atmospheres and fitting procedures provide an accurate, internally consistent analysis. The values of surface gravity and solid angle obtained, respectively, from spectroscopy and photometry, are combined with parallax measurementsmore » in various ways to study the validity of the mass–radius relation from an empirical point of view. After a thorough examination of our results, we find that 73% and 92% of the white dwarfs are consistent within 1 σ and 2 σ confidence levels, respectively, with the predictions of the mass–radius relation, thus providing strong support to the theory of stellar degeneracy. Our analysis also allows us to identify 15 stars that are better interpreted in terms of unresolved double degenerate binaries. Atmospheric parameters for both components in these binary systems are obtained using a novel approach. We further identify a few white dwarfs that are possibly composed of an iron core rather than a carbon/oxygen core, since they are consistent with Fe-core evolutionary models.« less

  18. Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star.

    PubMed

    Nugent, Peter E; Sullivan, Mark; Cenko, S Bradley; Thomas, Rollin C; Kasen, Daniel; Howell, D Andrew; Bersier, David; Bloom, Joshua S; Kulkarni, S R; Kandrashoff, Michael T; Filippenko, Alexei V; Silverman, Jeffrey M; Marcy, Geoffrey W; Howard, Andrew W; Isaacson, Howard T; Maguire, Kate; Suzuki, Nao; Tarlton, James E; Pan, Yen-Chen; Bildsten, Lars; Fulton, Benjamin J; Parrent, Jerod T; Sand, David; Podsiadlowski, Philipp; Bianco, Federica B; Dilday, Benjamin; Graham, Melissa L; Lyman, Joe; James, Phil; Kasliwal, Mansi M; Law, Nicholas M; Quimby, Robert M; Hook, Isobel M; Walker, Emma S; Mazzali, Paolo; Pian, Elena; Ofek, Eran O; Gal-Yam, Avishay; Poznanski, Dovi

    2011-12-14

    Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.

  19. Iron abundance in the hot DA white dwarfs Feige 24 and G191 B2B

    NASA Technical Reports Server (NTRS)

    Vennes, Stephane; Chayer, Pierre; Thorstensen, John R.; Bowyer, Stuart; Shipman, Harry L.

    1992-01-01

    Attention is given to model calculations of the far- and extreme-UV line spectra of highly ionized Fe species (Fe IV, Fe V, and Fe VI) for hot high-gravity H-rich stars. A spectral analysis of 31 hr of exposure of the DA white dwarf Feige 24 with IUE in the echelle mode reveals the presence of Fe with an abundance relative to H by number of (5-10) x 10 exp -6 with an uncertainty dominated by the determination of stellar parameters. An analysis of IUE data from the white dwarf G191 B2B results in a similar Fe abundance if this star shares similar atmospheric parameters (Teff, g) with Feige 24. Fe is thus the second most abundant photospheric element in hot DA white dwarfs.

  20. Evolution of double white dwarf binaries undergoing direct-impact accretion: Implications for gravitational wave astronomy

    NASA Astrophysics Data System (ADS)

    Kremer, Kyle; Breivik, Katelyn; Larson, Shane L.; Kalogera, Vassiliki

    2017-01-01

    For close double white dwarf binaries, the mass-transfer phenomenon known as direct-impact accretion (when the mass transfer stream impacts the accretor directly rather than forming a disc) may play a pivotal role in the long-term evolution of the systems. In this analysis, we explore the long-term evolution of white dwarf binaries accreting through direct-impact and explore implications of such systems to gravitational wave astronomy. We cover a broad range of parameter space which includes initial component masses and the strength of tidal coupling, and show that these systems, which lie firmly within the LISA frequency range, show strong negative chirps which can last as long as several million years. Detections of double white dwarf systems in the direct-impact phase by detectors such as LISA would provide astronomers with unique ways of probing the physics governing close compact object binaries.

  1. Exoplanet recycling in massive white-dwarf debris discs

    NASA Astrophysics Data System (ADS)

    van Lieshout, R.; Kral, Q.; Charnoz, S.; Wyatt, M. C.; Shannon, A.

    2018-05-01

    Several tens of white dwarfs are known to host circumstellar discs of dusty debris, thought to arise from the tidal disruption of rocky bodies originating in the star's remnant planetary system. This paper investigates the evolution of such discs if they are very massive, as may be the case if their progenitor was a terrestrial planet, moon, or dwarf planet. Assuming the discs are physically thin and flat, like Saturn's rings, their evolution is governed by Poynting-Robertson drag or viscous spreading, where the disc's effective viscosity is due to self-gravity wakes. For discs with masses ≳ 1026 g, located in the outer parts of the tidal disruption zone, viscous spreading dominates the evolution, and mass is transported both in- and outwards. When outwards-spreading material flows beyond the Roche limit, it coagulates into new (minor) planets in a process analogous to the ongoing formation of moonlets at the outer edge of Saturn's rings. The newly formed bodies migrate outwards by exchanging angular momentum with the disc and coalesce into larger objects through mutual collisions. Eventually, the disc's Roche-limit overflow recycles tens of percent of the original disc mass; most ends up in a single large body near 2:1 mean-motion resonance with the disc's outer edge. Hence, the recycling of a tidally disrupted super-Earth, for example, could yield an Earth-mass planet on a ˜10-h orbit, located in the habitable zone for 2-to-10-Gyr-old white dwarfs. The recycling process also creates a population of smaller bodies just outside the Roche limit, which may explain the minor planets recently postulated to orbit WD 1145+017.

  2. Importance of fingering convection for accreting white dwarfs in the framework of full evolutionary calculations: the case of the hydrogen-rich white dwarfs GD 133 and G 29-38

    NASA Astrophysics Data System (ADS)

    Wachlin, F. C.; Vauclair, G.; Vauclair, S.; Althaus, L. G.

    2017-05-01

    Context. A large fraction of white dwarfs show photospheric chemical composition that is polluted by heavy elements accreted from a debris disk. Such debris disks result from the tidal disruption of rocky planetesimals that have survived to whole stellar evolution from the main sequence to the final white dwarf stage. Determining the accretion rate of this material is an important step toward estimating the mass of the planetesimals and understanding the ultimate fate of the planetary systems. Aims: The accretion of heavy material with a mean molecular weight, μ, higher than the mean molecular weight of the white dwarf outer layers, induces a double-diffusive instability producing the fingering convection and an extra-mixing. As a result, the accreted material is diluted deep into the star. We explore the effect of this extra-mixing on the abundance evolution of Mg, O, Ca, Fe and Si in the cases of the two well-studied polluted DAZ white dwarfs: GD 133 and G 29-38. Methods: We performed numerical simulations of the accretion of material that has a chemical composition similar to the bulk Earth composition. We assumed a continuous and uniform accretion and considered a range of accretion rates from 104 g/s to 1010 g/s. Two cases are simulated, one using the standard mixing length theory (MLT) and one including the double-diffusive instability (fingering convection). Results: The double-diffusive instability develops on a very short timescale. The surface abundance rapidly reaches a stationary value while the depth of the zone mixed by the fingering convection increases. In the case of GD 133, the accretion rate needed to reproduce the observed abundances exceeds by more than two orders of magnitude the rate estimated by neglecting the fingering convection. In the case of G 29-38 the needed accretion rate is increased by approximately 1.7 dex. Conclusions: Our numerical simulations of the accretion of heavy elements on the hydrogen-rich white dwarf GD 133 and G 29

  3. 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.

  4. Effects of magnetic fields and slow rotation in white dwarfs

    NASA Astrophysics Data System (ADS)

    Terrero, D. Alvear; Paret, D. Manreza; Martínez, A. Pérez

    In this work we use Hartle’s formalism to study the effects of rotation in the structure of magnetized white dwarfs within the framework of general relativity. We describe the inner matter by means of an equation of state for electrons under the action of a constant magnetic field, which introduces an anisotropy in the pressures. Solutions correspond to typical densities of white dwarfs and values of magnetic field below 1013G considering perpendicular and parallel pressures independently, as if associated to two different equations of state. Rotation effects obtained account for an increase of the maximum mass for both magnetized and nonmagnetized stable configurations, up to about 1.5M⊙. Further effects studied include the deformation of the stars, which become oblate spheroids and the solutions for other quantities of interest, such as the moment of inertia, quadrupolar momentum and eccentricity. In all cases, rotation effects are dominant with respect to those of the magnetic field.

  5. Helium ignition in rotating magnetized CO white dwarfs leading to fast and faint rather than classical Type Ia supernovae

    NASA Astrophysics Data System (ADS)

    Neunteufel, P.; Yoon, S.-C.; Langer, N.

    2017-06-01

    Context. Based mostly on stellar models that do not include rotation, CO white dwarfs that accrete helium at rates of about 10-8M⊙/ yr have been put forward as candidate progenitors for a number of transient astrophysical phenomena, including Type Ia supernovae and the peculiar and fainter Type Iax supernovae. Aims: Here we study the impact of accretion-induced spin-up including the subsequent magnetic field generation, angular momentum transport, and viscous heating on the white dwarf evolution up to the point of helium ignition. Methods: We resolve the structure of the helium accreting white dwarf models with a one-dimensional Langrangian hydrodynamic code, modified to include rotational and magnetic effects, in 315 model sequences adopting different mass-transfer rates (10-8-10-7M⊙/ yr), and initial white dwarf masses (0.54-1.10 M⊙) and luminosities (0.01-1 L⊙). Results: We find magnetic angular momentum transport, which leads to quasi-solid-body rotation, profoundly impacts the evolution of the white dwarf models, and the helium ignition conditions. Our rotating lower mass (0.54 and 0.82 M⊙) models accrete up to 50% more mass up to ignition than the non-rotating case, while it is the opposite for our more massive models. Furthermore, we find that rotation leads to helium ignition densities that are up to ten times smaller, except for the lowest adopted initial white dwarf mass. Ignition densities on the order of 106 g/cm3 are only found for the lowest accretion rates and for large amounts of accreted helium (≳0.4M⊙). However, correspondingly massive donor stars would transfer mass at much higher rates. We therefore expect explosive He-shell burning to mostly occur as deflagrations and at Ṁ > 2 × 10-8M⊙/ yr, regardless of white dwarf mass. Conclusions: Our results imply that helium accretion onto CO white dwarfs at the considered rates is unlikely to lead to the explosion of the CO core or to classical Type Ia supernovae, but may instead

  6. Iron and magnesium in the white dwarf GD 40 - A test of diffusion theory

    NASA Technical Reports Server (NTRS)

    Shipman, H. L.; Greenstein, J. L.

    1983-01-01

    An outstanding problem in interpreting the properties of white-dwarf stars is related to the understanding of the chemical abundances in their atmospheres. The hydrogen-rich white-dwarf stars have monoelemental atmospheres, with small quantities of helium and no heavier elements observed in most, possibly all, stars with visible H lines. The helium-rich stars are more complex, and the existence of metallic lines in many of their spectra is now well confirmed. The DB star GD 40 (Gr 384) is the hottest He-rich white-dwarf star to show metal lines, apart from the extremely hot objects, such as HZ 21 and HD 149499B. The present investigation is concerned with IUE spectra which have been obtained of the GD 40. It is found that the near-UV is marked by strong Mg II lines along with broad blends of Fe II lines. An analysis is conducted of the abundances of Fe and Mg at the surface of GD 40 using a limited spectrum-synthesis technique with the measured equivalent widths of the best Fe II features.

  7. Temperature and Gravity Dependence of Trace Element Abundances in Hot DA White Dwarfs (94-EUVE-094)

    NASA Technical Reports Server (NTRS)

    Finley, David S.

    1998-01-01

    EUV spectroscopy has shown that DA white dwarfs hotter than about 45,000 K may contain trace heavy elements, while those hotter than about 50,000 K almost always have significant abundances of trace heavy elements. One of our continuing challenges is to identify and determine the abundances of these trace constituents, and then to relate the observed abundance patterns to the present conditions and previous evolutionary histories of the hot DA white dwarfs.

  8. Metal abundances in hot white dwarfs with signatures of a superionized wind

    NASA Astrophysics Data System (ADS)

    Werner, K.; Rauch, T.; Kruk, J. W.

    2018-01-01

    About a dozen hot white dwarfs with effective temperatures Teff = 65 000-120 000 K exhibit unusual absorption features in their optical spectra. These objects were tentatively identified as Rydberg lines of ultra-high excited metals in ionization stages v-x, indicating line formation in a dense environment with temperatures near 106 K. Since some features show blueward extensions, it was argued that they stem from a superionized wind. A unique assignment of the lines to particular elements is not possible, although they probably stem from C, N, O, and Ne. To further investigate this phenomenon, we analyzed the ultraviolet spectra available from only three stars of this group; that is, two helium-rich white dwarfs, HE 0504-2408 and HS 0713+3958 with spectral type DO, and a hydrogen-rich white dwarf, HS 2115+1148 with spectral type DAO. We identified light metals (C, N, O, Si, P, and S) with generally subsolar abundances and heavy elements from the iron group (Cr, Mn, Fe, Co, Ni) with solar or oversolar abundance. The abundance patterns are not unusual for hot WDs and can be interpreted as the result of gravitational settling and radiative levitation of elements. As to the origin of the ultra-high ionized metals lines, we discuss the possible presence of a multicomponent radiatively driven wind that is frictionally heated.

  9. Accreting CO material onto ONe white dwarfs towards accretion-induced collapse

    NASA Astrophysics Data System (ADS)

    Wu, Cheng-Yuan; Wang, Bo

    2018-03-01

    The final outcomes of accreting ONe white dwarfs (ONe WDs) have been studied for several decades, but there are still some issues that are not resolved. Recently, some studies suggested that the deflagration of oxygen would occur for accreting ONe WDs with Chandrasekhar masses. In this paper, we aim to investigate whether ONe WDs can experience accretion-induced collapse (AIC) or explosions when their masses approach the Chandrasekhar limit. Employing the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), we simulate the long-term evolution of ONe WDs with accreting CO material. The ONe WDs undergo weak multicycle carbon flashes during the mass-accretion process, leading to mass increase of the WDs. We found that different initial WD masses and mass-accretion rates influence the evolution of central density and temperature. However, the central temperature cannot reach the explosive oxygen ignition temperature due to neutrino cooling. This work implies that the final outcome of accreting ONe WDs is electron-capture induced collapse rather than thermonuclear explosion.

  10. Radio pulsar death lines to SGRs/AXPs and white dwarfs pulsars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Lobato, Ronaldo V.; Malheiro, M.; Coelho, J. G.

    Recently, an alternative model based on white dwarfs pulsars has been proposed to explain a class of pulsars known as Soft Gamma Repeaters (SGR) and Anomalus X-Ray Pulsars (AXP) [1], usually named as magnetars. In this model, the magnetized white dwarfs can have surface magnetic field B ∼ 10{sup 7} − 10{sup 10} G and rotate very fast with angular frequencies Ω ∼ 1 rad/s, allowing them to produce large electromagnetic (EM) potentials and generate electron-positron pairs. These EM potentials are comparable with the ones of neutron star pulsars with strong magnetic fields and even larger. In this study wemore » consider two possible processes associated with the particle acceleration, both of them are common used to explain radio emission in neutron star pulsars: in the first process the pair production happens near to the star polar caps, i.e. inside of the light cylinder where magnetic field lines are closed; in the second one the creation of pair happens in the outer magnetosphere, i.e. far away of the star surface where magnetic field lines are open [2]. The analysis of the possibility of radio emission were done for 23 SGRs/AXPs of the McGill Online Magnetar Catalog [3] that contains the current information available on these sources. The results of this work show that the model where the particles production occur in the outer magnetosphere emission “o2” is the process compatible with the astronomical observations of absence of radio emission for almost all SGRs/AXPs when these sources are understood as white dwarf pulsars. Our work is a first attempted to find an explanation for the puzzle why for almost all the SGRs/AXPs was expected radio emission, but it was observed in only four of them. These four sources, as it was suggested recently [4], seem to belong to an high magnetic field neutron star pulsar category, different from all the others SGRs/AXPs that our work indicate to belong to a new class of white dwarf pulsars, very fast and magnetized.« less

  11. The broadening of the calcium resonance line in a high-temperature helium atmosphere. [solar and white dwarf atmospheric studies

    NASA Technical Reports Server (NTRS)

    Driver, R. D.; Snider, J. L.

    1976-01-01

    The paper describes experimental measurements of the shape of the Ca I resonance line at 4227 A in a high-temperature helium atmosphere. A ballistic piston compressor was used to produce hot helium in the temperature range from 3000 to 7000 K and the number-density range from 2 to 4 by 10 to the 20th power per cu cm, which conditions approximate those postulated for the atmospheres of certain cool white-dwarf stars. The Boltzmann temperature of the calcium atoms was measured by the brightness-emissivity method, the absorption line shape was measured with a twelve-channel polychromator, and the helium kinetic temperature was calculated from the equation of an ideal gas. The observed deviation from thermodynamic equilibrium is illustrated. The value of the ratio of the damping constant to the helium number density at 5000 K is found to be 45 (+ or - 4) by 10 to the -22nd power A cu cm, or 4.7 (+ or - 0.4) by 10 to the -9th power rad/s cu cm. It is noted that no existing line-broadening calculation for the calcium-helium system is consistent with the present results. Recent studies of the 4227-A line in spectra of the sun and the white dwarf van Maanen 2 are discussed.

  12. An X-ray survey of hot white dwarf stars - Evidence for a m(He)/n(H) versus Teff correlation

    NASA Technical Reports Server (NTRS)

    Petre, R.; Shipman, H. L.; Canizares, C. R.

    1986-01-01

    Observations of 13 white dwarf and subdwarf stars using the Einstein Observatory High Resolution Image are reported. Included are stars of classes DA, DB, DAV, sDO, and sDB, with optically determined effective temperatures in the range 10,000-60,000 K. X-ray emission was detected from two of the 13: the very hot (55,000 K) DA1 star WD 2309 + 105 (= EG 233), with a count rate one-fifth that of HZ 43, and the relatively cool (26,000 K) DA3 star WD 1052 - 273 (=GD 125). The effective temperatures determined from ultraviolet and optical observations were used to place limits on the He content of the white dwarf photospheres, presuming that trace photospheric He is the missing opacity source which quenches the thermal X-rays in these stars. When presently obtained results were combined with those available from the literature evidence was found for a correlation between Teff and n(He)/n(H), in which HZ 43 is a conspicuous exception to the general trend. Both this correlation and the exceptional behavior of HZ 43 are qualitatively accounted for by a radiative acceleration model, in which the rate of upward movement of the He is a function of temperature and surface gravity

  13. Crystallization of the Pulsating White Dwarf Star, BPM 37093

    NASA Astrophysics Data System (ADS)

    Salois, Amee; Winget, D.

    2010-01-01

    BPM 37093 is unique among pulsating white dwarf stars because it is expected to have a highly crystallized interior. By understanding how this star is crystallizing, we gain a better understanding of extreme physics. Theoretical models of the evolution of white dwarf stars suggest that they crystallize from the inside out. The pulsations of the star, which we see as intensity variations, cannot penetrate this crystallized interior. Therefore, as the star crystallizes there is a smaller volume for the propagation of the pulsations and the pulsation periods are changed accordingly. We studied these changes in the periods of the pulsations of the star over ten weeks during the McDonald Observatory Research Experience for Undergraduates Program. By studying the changes in the pulsations periods of the star we can determine the mass fraction of the star that is crystallized. Comparing Fourier transforms of our observed light curves taken in 2004 and 2005 at CTIO with data taken in 1998 and 1999 by Kanaan et al. we hope to see the changes that have occurred in the star as well as determining a better approximation of the star's crystallized mass fraction.

  14. Three-Dimensional Simulations of the Convective Urca Process in Pre-Supernova White Dwarfs

    NASA Astrophysics Data System (ADS)

    Willcox, Donald E.; Townsley, Dean; Zingale, Michael; Calder, Alan

    2017-01-01

    A significant source of uncertainty in modeling the progenitor systems of Type Ia supernovae is the dynamics of the convective Urca process in which beta decay and electron capture reactions remove energy from and decrease the buoyancy of carbon-fueled convection in the progenitor white dwarf. The details of the Urca process during this simmering phase have long remained computationally intractable in three-dimensional simulations because of the very low convective velocities and the associated timestep constraints of compressible hydrodynamics methods. We report on recent work simulating the A=23 (Ne/Na) Urca process in convecting white dwarfs in three dimensions using the low-Mach hydrodynamics code MAESTRO. We simulate white dwarf models inspired by one-dimensional stellar evolution calculations at the stage when the outer edge of the convection zone driven by core carbon burning reaches the A=23 Urca shell. We compare our methods and results to those of previous work in one and two dimensions, discussing the implications of three dimensional turbulence. We also comment on the prospect of our results informing one-dimensional stellar evolution calculations and the Type Ia supernovae progenitor problem.This work was supported in part by the Department of Energy under grant DE-FG02-87ER40317.

  15. Dwarf mistletoe in red and white firs in California–23 to 28 years after inoculation

    Treesearch

    John R. Parmeter Jr.; Robert F. Scharpf

    1989-01-01

    Spread and buildup of dwarf mistletoe, Arceuthobium abietinum, was studied on inoculated white fir, Abies concolor, and red fir, A. magnifica, in northern California for 23 to 28 years. At the end of these studies (1986), and in the absence of overstory infection, 13 of 23 trees had dwarf mistletoe populations...

  16. A strange dwarf scenario for the formation of the peculiar double white dwarf binary SDSS J125733.63+542850.5

    NASA Astrophysics Data System (ADS)

    Jiang, Long; Chen, Wen-Cong; Li, Xiang-Dong

    2018-05-01

    The Hubble Space Telescope observation of the double white dwarf (WD) binary SDSS J125733.63+542850.5 reveals that the massive WD has a surface gravity log g1 ˜ 8.7 (which implies a mass of M1 ˜ 1.06 M⊙) and an effective temperature T1 ˜ 13 000 K, while the effective temperature of the low-mass WD (M2 < 0.24 M⊙) is T2 ˜ 6400K. Therefore, the massive and the low-mass WDs have a cooling age τ1 ˜ 1 Gyr and τ2 ≥ 5 Gyr, respectively. This is in contradiction with traditional binary evolution theory. In this paper, we propose a strange dwarf (SD) scenario to explain the formation of this double WD binary. We assume that the massive WD is a SD originating from a phase transition (PT) in a ˜1.1 M⊙ WD, which has experienced accretion and spin-down processes. Its high effective temperature could arise from the heating process during the PT. Our simulations suggest that the progenitor of SDSS J125733.63+542850.5 can be a binary system consisting of a 0.65 M⊙ WD and a 1.5 M⊙ main-sequence star in a 1.492 d orbit. Especially, the secondary star (i.e. the progenitor of the low-mass WD) is likely to have an ultra-low metallicity of Z = 0.0001.

  17. Accreting white dwarf models for type 1 supernovae. 1: Presupernova evolution and triggering mechanisms

    NASA Technical Reports Server (NTRS)

    Nomoto, K.

    1981-01-01

    As a plausible explosion model for a Type I supernova, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs. The relationship between the conditions in the binary system and the triggering mechanism for the supernova explosion is discussed, especially for the cases with relatively slow accretion rate. It is found that the growth of a helium zone on the carbon-oxygen core leads to a supernova explosion which is triggered either by the off-center helium detonation for slow and intermediate accretion rates or by the carbon deflagration for slow and rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion, since in this case the initial mass of the white dwarf is an important parameter for determining the mode of ignition. Finally, various modes of building up the helium zone on the white dwarf, namely, direct transfer of helium from the companion star and the various types and strength of the hydrogen shell flashes are discussed in some detail.

  18. 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.

  19. 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.

  20. Two Massive White Dwarfs from NGC 2323 and the Initial-Final Mass Relation for Progenitors of 4 to 6.5 M

    NASA Astrophysics Data System (ADS)

    Cummings, Jeffrey D.; Kalirai, Jason S.; Tremblay, P.-E.; Ramirez-Ruiz, Enrico

    2016-02-01

    We observed a sample of 10 white dwarf candidates in the rich open cluster NGC 2323 (M50) with the Keck Low-Resolution Imaging Spectrometer. The spectroscopy shows eight to be DA white dwarfs, with six of these having high signal-to-noise ratio appropriate for our analysis. Two of these white dwarfs are consistent with singly evolved cluster membership, and both are high mass ˜1.07 M⊙, and give equivalent progenitor masses of 4.69 M⊙. To supplement these new high-mass white dwarfs and analyze the initial-final mass relation (IFMR), we also looked at 30 white dwarfs from publicly available data that are mostly all high-mass (≳ 0.9 M⊙). These original published data exhibited significant scatter, and to test if this scatter is true or simply the result of systematics, we have uniformly analyzed the white dwarf spectra and have adopted thorough photometric techniques to derive uniform cluster parameters for their parent clusters. The resulting IFMR scatter is significantly reduced, arguing that mass-loss rates are not stochastic in nature and that within the ranges of metallicity and mass analyzed in this work mass loss is not highly sensitive to variations in metallicity. Lastly, when adopting cluster ages based on Y2 isochrones, the slope of the high-mass IFMR remains steep and consistent with that found from intermediate-mass white dwarfs, giving a linear IFMR from progenitor masses between 3 and 6.5 M⊙. In contrast, when adopting the slightly younger cluster ages based on PARSEC isochrones, the high-mass IFMR has a moderate turnover near an initial mass of 4 M⊙. Based on observations with the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, was made possible by the generous financial support of the W.M. Keck Foundation.

  1. Evidence for an oscillation of the magnetic axis of the white dwarf in the polar DP Leonis

    NASA Astrophysics Data System (ADS)

    Beuermann, K.; Dreizler, S.; Hessman, F. V.; Schwope, A. D.

    2014-02-01

    From 1979 to 2001, the magnetic axis of the white dwarf in the polar DP Leo slowly rotated by 50° in azimuth, possibly indicating a small asynchronism between the rotational and orbital periods of the magnetic white dwarf. Using the MONET/North telescope, we have obtained phase-resolved orbital light curves between 2009 and 2013, which show that this trend has not continued in recent years. Our data are consistent with the theoretically predicted oscillation of the magnetic axis of the white dwarf about an equilibrium orientation, which is defined by the competition between the accretion torque and the magnetostatic interaction of the primary and secondary star. Our data indicate an oscillation period of ~60 yr, an amplitude of about 25°, and an equilibrium orientation leading the connecting line of the two stars by about 7°.

  2. Detection of H-alpha emission in the hot white dwarf G191-B2B

    NASA Astrophysics Data System (ADS)

    Reid, Neill; Wegner, Gary

    1988-12-01

    High-resolution spectra of G191-B2B, the hottest known DA white dwarf were obtained which reveal emission in the core of the H-alpha line. The observations show little variation in the line profile over a period of four days, ruling out line-doubling in a close binary as an explanation. The observed emission cannot be due to a nearby red dwarf companion, while the absence of any spatially extended emission argues against either a planetary nebula remnant or local ionization of the interstellar medium. The determination of the systemic velocity, using the companion red dwarf G191-B2A, is 5 + or - 2 km/s and shows that both the H-alpha emission and the high-excitation species observed in the ultraviolet are redshifted by 19 + or - 3 km/s, suggesting a photospheric origin. The low redshift implies a mass of 0.45 solar mass for this hot white dwarf, although the uncertainties in the effective temperature and parallax permit masses in the range 0.29 to 0.60 solar mass.

  3. Three new cool brown dwarfs discovered with the wide-field infrared survey explorer (WISE) and an improved spectrum of the Y0 dwarf wise J041022.71+150248.4

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Cushing, Michael C.; Kirkpatrick, J. Davy; Gelino, Christopher R.

    2014-05-01

    As part of a larger search of Wide-field Infrared Survey Explorer (WISE) data for cool brown dwarfs with effective temperatures less than 1000 K, we present the discovery of three new cool brown dwarfs with spectral types later than T7. Using low-resolution, near-infrared spectra obtained with the NASA Infrared Telescope Facility and the Hubble Space Telescope, we derive spectral types of T9.5 for WISE J094305.98+360723.5, T8 for WISE J200050.19+362950.1, and Y0: for WISE J220905.73+271143.9. The identification of WISE J220905.73+271143.9 as a Y dwarf brings the total number of spectroscopically confirmed Y dwarfs to 17. In addition, we present an improvedmore » spectrum (i.e., higher signal-to-noise ratio) of the Y0 dwarf WISE J041022.71+150248.4 that confirms the Cushing et al. classification of Y0. Spectrophotometric distance estimates place all three new brown dwarfs at distances less than 12 pc, with WISE J200050.19+362950.1 lying at a distance of only 3.9-8.0 pc. Finally, we note that brown dwarfs like WISE J200050.19+362950.1 that lie in or near the Galactic plane offer an exciting opportunity to directly measure the mass of a brown dwarf via astrometric microlensing.« less

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

    NASA Technical Reports Server (NTRS)

    Boehm-Vitense, E.

    1981-01-01

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

  5. Accretion onto Carbon-Oxygen White Dwarfs as a possible mechanism for growth to the Chandrasekhar Limit

    NASA Astrophysics Data System (ADS)

    Starrfield, Sumner; Bose, Maitrayee; Iliadis, Christian; Hix, William R.; José, Jordi; Hernanz, Margarita

    2017-08-01

    We have continued our studies of accretion onto white dwarfs by following the evolution of thermonuclear runaways (TNRs) on Carbon Oxygen (CO) white dwarfs. We have varied the mass of the white dwarf and the composition of the accreted material. We use the results of the multi-dimensional studies of TNRs in white dwarfs, accreting only Solar matter, which show that sufficient core material is dredged-up by the TNR and then ejected by the explosion to agree with the observations of the ejecta abundances. We have also found that the initial 12C abundance is inversely proportional to the amount of material accreted prior to the TNR. Therefore, we first accrete Solar material and follow the evolution until a TNR occurs. Because the 12C abundance is significantly smaller then if we had initially mixed the accreting gas with the carbon-oxygen core, more matter takes part in the explosion than if we had begun the evolution with the mixed composition. We then instantaneously switch the composition to a mixture with either 25% core material or 50% core material (plus accreted material) and follow the resulting evolution of the TNR. We use our 1D, Lagrangian, hydrodynamic code: NOVA. We report on the results of these new simulations and compare the ejecta abundances to those measured in pre-solar grains that are thought to arise from classical nova explosions. These results will also be compared to recent results with SHIVA (Josè and Hernanz). We find that there are some white dwarf masses where significantly less mass is ejected than accreted during the Classical Nova event and, therefore, the white dwarf is growing in mass as a result of the accretion and in spite of the resulting explosion.This work was supported in part by NASA under the Astrophysics Theory Program grant 14-ATP14-0007 and the U.S. DOE under Contract No. DE-FG02- 97ER41041. SS acknowledges partial support from NASA, NSF, and HST grants to ASU and WRH is supported by the U.S. Department of Energy, Office

  6. A NEW TIMESCALE FOR PERIOD CHANGE IN THE PULSATING DA WHITE DWARF WD 0111+0018

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Hermes, J. J.; Montgomery, M. H.; Winget, D. E.

    2013-03-20

    We report the most rapid rate of period change measured to date for a pulsating DA (hydrogen atmosphere) white dwarf (WD), observed in the 292.9 s mode of WD 0111+0018. The observed period change, faster than 10{sup -12} s s{sup -1}, exceeds by more than two orders of magnitude the expected rate from cooling alone for this class of slow and simply evolving pulsating WDs. This result indicates the presence of an additional timescale for period evolution in these pulsating objects. We also measure the rates of period change of nonlinear combination frequencies and show that they share the evolutionarymore » characteristics of their parent modes, confirming that these combination frequencies are not independent modes but rather artifacts of some nonlinear distortion in the outer layers of the star.« less

  7. Precision asteroseismology of the pulsating white dwarf GD 1212 using a two-wheel-controlled Kepler spacecraft

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Hermes, J. J.; Charpinet, S.; Barclay, Thomas

    We present a preliminary analysis of the cool pulsating white dwarf (WD) GD 1212, enabled by more than 11.5 days of space-based photometry obtained during an engineering test of the two-reaction-wheel-controlled Kepler spacecraft. We detect at least 19 independent pulsation modes, ranging from 828.2-1220.8 s, and at least 17 nonlinear combination frequencies of those independent pulsations. Our longest uninterrupted light curve, 9.0 days in length, evidences coherent difference frequencies at periods inaccessible from the ground, up to 14.5 hr, the longest-period signals ever detected in a pulsating WD. These results mark some of the first science to come from amore » two-wheel-controlled Kepler spacecraft, proving the capability for unprecedented discoveries afforded by extending Kepler observations to the ecliptic.« less

  8. Astrophysical Implications of a New Dynamical Mass for the Nearby White Dwarf 40 Eridani B

    NASA Astrophysics Data System (ADS)

    Bond, Howard E.; Bergeron, P.; Bédard, A.

    2017-10-01

    The bright, nearby DA-type white dwarf (WD) 40 Eridani B is orbited by the M dwarf 40 Eri C, allowing determination of the WD’s mass. Until recently, however, the mass depended on orbital elements determined four decades ago, and that mass was so low that it created several astrophysical puzzles. Using new astrometric measurements, the binary-star group at the U.S. Naval Observatory has revised the dynamical mass upward, to 0.573 ± 0.018 M ⊙. In this paper, we use model-atmosphere analysis to update other parameters of the WD, including effective temperature, surface gravity, radius, and luminosity. We then compare these results with WD interior models. Within the observational uncertainties, theoretical cooling tracks for CO-core WDs of its measured mass are consistent with the position of 40 Eri B in the H-R diagram; equivalently, the theoretical mass-radius relation (MRR) is consistent with the star’s location in the mass-radius plane. This consistency is, however, achieved only if we assume a “thin” outer hydrogen layer, with q H = M H/M WD ≃ 10-10. We discuss other evidence that a significant fraction of DA WDs have such thin H layers, in spite of the expectation from canonical stellar-evolution theory of “thick” H layers with q H ≃ 10-4. The cooling age of 40 Eri B is ˜122 Myr, and its total age is ˜1.8 Gyr. We present the MRRs for 40 Eri B and three other nearby WDs in visual binaries with precise mass determinations, and show that the agreement of current theory with observations is excellent in all cases.

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

    Context. The discovery of pulsations in low-mass stars opens an opportunity to probe their interiors and determine their evolution by employing the tools of asteroseismology. Aims: We aim to analyse high-speed photometry of SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25 and discover brightness variabilities. In order to locate these stars in the Teff - log g diagram, we fit optical spectra (SDSS) with synthetic non-magnetic spectra derived from model atmospheres. Methods: To carry out this study, we used the photometric data we obtained for these stars with the 2.15 m telescope at CASLEO, Argentina. We analysed their light curves and applied the discrete Fourier transform (FT) to determine the pulsation frequencies. Finally, we compare both stars in the Teff - log g diagram, with two known pre-white dwarfs and seven pulsating pre-ELM white dwarf stars, δ Scuti, and SX Phe stars Results: We report the discovery of pulsations in SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25. We determine their effective temperature and surface gravity to be Teff = 7972 ± 200 K, log g = 4.25 ± 0.5 and Teff = 7925 ± 200 K, log g = 4.25 ± 0.5, respectively. With these parameters, these new pulsating low-mass stars can be identified with either ELM white dwarfs (with ~0.17 M⊙) or more massive SX Phe stars. We identified pulsation periods of 3278.7 and 1633.9 s for SDSS J145847.02+070754.46 and a pulsation period of 3367.1 s for SDSS J173001.94+070600.25. These two new objects, together with those of Maxted et al. (2013, 2014), indicate the possible existence of a new instability domain towards the late stages of evolution of low-mass white dwarf stars, although their identification with SX Phe stars cannot be discarded. Visiting Astronomer, Complejo Astronómico El Leoncito operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba, and San Juan.

  10. Numerical Simulations of Gaseous Disks Generated from Collisional Cascades at the Roche Limits of White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Kenyon, Scott J.; Bromley, Benjamin C.

    2017-11-01

    We consider the long-term evolution of gaseous disks fed by the vaporization of small particles produced in a collisional cascade inside the Roche limit of a 0.6 {M}⊙ white dwarf. Adding solids with radius {r}0 at a constant rate {\\dot{M}}0 into a narrow annulus leads to two distinct types of evolution. When {\\dot{M}}0≳ {\\dot{M}}0,{crit}≈ 3× {10}4 {({r}0/1{km})}3.92 {{g}} {{{s}}}-1, the cascade generates a fairly steady accretion disk where the mass transfer rate of gas onto the white dwarf is roughly {\\dot{M}}0 and the mass in gas is {M}g≈ 2.3× {10}22 ({\\dot{M}}0/{10}10 {{g}} {{{s}}}-1) (1500 {{K}}/{T}0) ({10}-3/α ) g, where T 0 is the temperature of the gas near the Roche limit and α is the dimensionless viscosity parameter. If {\\dot{M}}0≲ {\\dot{M}}0,{crit}, the system alternates between high states with large mass transfer rates and low states with negligible accretion. Although either mode of evolution adds significant amounts of metals to the white dwarf photosphere, none of our calculations yield a vertically thin ensemble of solids inside the Roche limit. X-ray observations can place limits on the mass transfer rate and test this model for metallic line white dwarfs.

  11. Discovery of an extremely weak magnetic field in the white dwarf LTT 16093 = WD 2047+372

    NASA Astrophysics Data System (ADS)

    Landstreet, J. D.; Bagnulo, S.; Martin, A.; Valyavin, G.

    2016-06-01

    Context. Magnetic fields have been detected in several hundred white dwarfs, with strengths ranging from a few kG to several hundred MG. Only a few of the known fields have a mean magnetic field modulus below about 1 MG. Aims: We are searching for new examples of magnetic white dwarfs with very weak fields, and trying to model the few known examples. Our search is intended to be sensitive enough to detect fields at the few kG level. Methods: We have been surveying bright white dwarfs for very weak fields using spectropolarimeters at the Canada-France-Hawaii telescope, the William Herschel Telescope (WHT), the European Southern Observatory, and the Russian Special Astrophysical Observatory. We discuss in some detail tests of the WHT spectropolarimeter ISIS using the known magnetic strong-field Ap star HD 215441 (Babcock's star) and the long-period Ap star HD 201601 (γ Equ). Results: We report the discovery of a field with a mean field modulus of about 57 kG in the white dwarf LTT 16093 = WD 2047+372. The field is clearly detected through the Zeeman splitting of Hα seen in two separate circularly polarised spectra from two different spectropolarimeters. Zeeman circular polarisation is also detected, but only barely above the 3σ level. Conclusions: The discovery of this field is significant because it is the third weakest field ever unambiguously discovered in a white dwarf, while still being large enough that we should be able to model the field structure in some detail with future observations. Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, and on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and

  12. 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.

  13. RE 1016-053 - A pre-cataclysmic binary, and the first extreme ultraviolet and X-ray detections of a DAO white dwarf

    NASA Technical Reports Server (NTRS)

    Tweedy, R. W.; Holberg, J. B.; Barstow, M. A.; Bergeron, P.; Grauer, A. D.; Liebert, James; Fleming, T. A.

    1993-01-01

    Photometric observations and analysis of the optical, UV, EUV, and X-ray spectra are presented for the EUV/X-ray source RE 1016-53. Multiwavelength observations of RE 1016-53 point out that it is a precataclysmic binary. Optical spectra exhibit the steep blue continuum and Balmer absorption typical of a hot white dwarf, but there are bright, narrow emission lines of H I, He I, and Ca II superimposed on this. The white dwarf component, with T (eff) = 55,800 +/- 1000 K and log g = 7.81 +/- 0.007, dominates the spectrum from the optical to the EUV/X-ray. An He II 4686 A absorption line suggests that the white dwarf is a hydrogen-helium (DAO) hybrid star. Four of the five precataclysmic binaries with white dwarfs with T(eff) greater than 40,000 K appear to be DAOs. A mass of 0.57 +/- 0.003 solar mass has been derived.

  14. A Multi-Survey Approach to White Dwarf Discovery

    DTIC Science & Technology

    2012-04-01

    LSPM-North), the Two Micron All Sky Survey ( 2MASS ), and the USNO-B1.0 catalog, we use a succession of methods to isolate white dwarf (WD) candidates...including SDSS, the Two Micron All Sky Survey ( 2MASS ; Skrutskie et al. 2006), USNO-B1.0 (Monet et al. 2003), and the proper motion survey of Lépine & Shara...Shara Proper Motion North Catalog (LSPM-North), the Two Micron All Sky Survey ( 2MASS ), and the USNO-B1.0 catalog, we use a succession of methods to

  15. The constitution of the atmospheric layers and the extreme ultraviolet spectrum of hot hydrogen-rich white dwarfs

    NASA Technical Reports Server (NTRS)

    Vennes, Stephane

    1992-01-01

    An analysis is presented of the atmospheric properties of hot, H-rich, DA white dwarfs that is based on optical, UV, and X-ray observations aimed at predicting detailed spectral properties of these stars in the range 80-800 A. The divergences between observations from a sample of 15 hot DA white dwarfs emitting in the EUV/soft X-ray range and pure H synthetic spectra calculated from a grid of model atmospheres characterized by Teff and g are examined. Seven out of 15 DA stars are found to consistently exhibit pure hydrogen atmospheres, the remaining seven stars showing inconsistency between FUV and EUV/soft X-ray data that can be explained by the presence of trace EUV/soft X-ray absorbers. Synthetic data are computed assuming two other possible chemical structures: photospheric traces of radiatively levitated heavy elements and a stratified hydrogen/helium distribution. Predictions about forthcoming medium-resolution observations of the EUV spectrum of selected hot H-rich white dwarfs are made.

  16. 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}⊙ =(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 .

  17. The historical record for Sirius - Evidence for a white-dwarf thermonuclear runaway?

    NASA Technical Reports Server (NTRS)

    Bruhweiler, Frederick C.; Kondo, Yoji; Sion, Edward M.

    1986-01-01

    Evidence was recently presented that in medieval times Sirius was a bright red star, rather than the present bluish-white star. Here, the results of attempts to detect possible planetary nebula ejecta toward Sirius using data obtained by the IUE are presented. Based on these results and in the light of recent advances in understanding white-dwarf evolution, it is proposed that Sirius B underwent a recent thermonuclear runaway event triggered by a diffusion-induced CN reaction.

  18. Studing the Post Merger Evolution of White Dwarf Mergers with FLASH

    NASA Astrophysics Data System (ADS)

    Jenks, Malia

    2017-06-01

    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 an equal mass model with FLASH. These simulations test if a similar mass merger can lead to an SN Ia.

  19. Characterizing Accreting Double White Dwarf Binaries with the Laser Interferometer Space Antenna and Gaia

    NASA Astrophysics Data System (ADS)

    Breivik, Katelyn; Kremer, Kyle; Bueno, Michael; Larson, Shane L.; Coughlin, Scott; Kalogera, Vassiliki

    2018-02-01

    We demonstrate a method to fully characterize mass-transferring double white dwarf (DWD) systems with a helium-rich (He) white dwarf (WD) donor based on the mass–radius (M–R) relationship for He WDs. Using a simulated Galactic population of DWDs, we show that donor and accretor masses can be inferred for up to ∼60 systems observed by both Laser Interferometer Space Antenna (LISA) and Gaia. Half of these systems will have mass constraints {{Δ }} {M}{{D}} ≲ 0.2 {M}ȯ and {{Δ }} {M}{{A}} ≲ 2.3 {M}ȯ . We also show how the orbital frequency evolution due to astrophysical processes and gravitational radiation can be decoupled from the total orbital frequency evolution for up to ∼50 of these systems.

  20. CONSTRAINING THE SPIN-DOWN TIMESCALE OF THE WHITE DWARF PROGENITORS OF TYPE Ia SUPERNOVAE

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Meng, Xiangcun; Podsiadlowski, Philipp, E-mail: xiangcunmeng@hotmail.com

    2013-12-01

    Justham and Di Stefano et al. proposed that the white dwarf progenitor of a Type Ia supernova (SN Ia) may have to spin down before it can explode. As the white dwarf spin-down timescale is not well known theoretically, here we try to constrain it empirically (within the framework of this spin-down model) for progenitor systems that contain a giant donor and for which circumbinary material has been detected after the explosion: we obtain an upper limit of a few 10{sup 7}yr. Based on the study of Di Stefano and Kilic, this means that it is too early to rulemore » out the existence of a surviving companion in SNR 0509–67.5.« less

  1. Relativistic tidal interaction of a white dwarf with a massive black hole

    NASA Technical Reports Server (NTRS)

    Frolov, V. P.; Khokhlov, A. M.; Novikov, I. D.; Pethick, C. J.

    1994-01-01

    We compute encounters of a realistic white dwarf model with a massive black hole in the regime where relativistic effects are important, using a three-dimensional, finite-difference, Eulerian, piecewise parabolic method (PPM) hydrodynamical code. Both disruptive and nondisruptive encounters are considered. We identify and discuss relativistic effects important for the problem: relativistic shift of the pericenter distance, time delay, relativistic precession, and the tensorial structure of the tidal forces. In the nondisruptive case, stripping of matter takes place. In the surface layers of the surviving core, complicated hydrodynamical phenomena are revealed. In both disruptive and nondispruptive encounters, material flows out in the form of two thin, S-shaped, supersonic jets. Our results provide realistic initial conditions for the subsequent investigation of the dynamics of the debris in the field of the black hole. We evaluate the critical conditions for complete disruption of the white dwarf, and compare our results with the corresponding results for nonrelativistic encounters.

  2. Detection of Double White Dwarf Binaries with Gaia, LSST and eLISA

    NASA Astrophysics Data System (ADS)

    Korol, V.; Rossi, E. M.; Groot, P. J.

    2017-03-01

    According to simulations around 108 double degenerate white dwarf binaries are expected to be present in the Milky Way. Due to their intrinsic faintness, the detection of these systems is a challenge, and the total number of detected sources so far amounts only to a few tens. This will change in the next two decades with the advent of Gaia, the LSST and eLISA. We present an estimation of how many compact DWDs with orbital periods less than a few hours we will be able to detect 1) through electromagnetic radiation with Gaia and LSST and 2) through gravitational wave radiation with eLISA. We find that the sample of simultaneous electromagnetic and gravitational waves detections is expected to be substantial, and will provide us a powerful tool for probing the white dwarf astrophysics and the structure of the Milky Way, letting us into the era of multi-messenger astronomy for these sources.

  3. Naming Disney's Dwarfs.

    ERIC Educational Resources Information Center

    Sidwell, Robert T.

    1980-01-01

    Discusses Disney's version of the folkloric dwarfs in his production of "Snow White" and weighs the Disney rendition of the dwarf figure against the corpus of traits and behaviors pertaining to dwarfs in traditional folklore. Concludes that Disney's dwarfs are "anthropologically true." (HOD)

  4. WD 0158-160, a new pulsating DB white dwarf

    NASA Astrophysics Data System (ADS)

    Kilkenny, D.

    2016-03-01

    The DB star, WD 0158-160 (=EC 01585-1600) is shown to be a variable white dwarf with a rich pulsation spectrum, dominated by a strong variation with a frequency near 1637 μHz (amplitude ˜0.024 mag, period 598 s), though at least 10 frequencies are detected between about 1285 and 5747 μHz (780-173 s) in a relatively small data set. At ˜14.5 mag, the star is one of the brightest known DBV stars.

  5. VizieR Online Data Catalog: White dwarf candidates using LAMOST DR3 (Gentile Fusillo+, 2015)

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

    We cross-matched all 65768 objects from the Gentile Fusillo et al. (2015, Cat. J/MNRAS/448/2260) catalogue with the list of the 4.6 million LAMOST spectra and retrieved 6101 spectra corresponding to 5173 unique objects. Thirty-five hundred of these have also received SDSS spectroscopic follow-up and 64 further objects had already been identified on the base of their LAMOST spectra as white dwarfs or white dwarf binaries by Zhang et al. (2013, Cat. J/AJ/146/34), Zhao et al. (2013AJ....145..169Z), Ren et al. (2014, Cat. J/A+A/570/A107) and Rebassa-Mansergas et al. (2015MNRAS.450..743R). (1 data file).

  6. Thermodynamic functions, freezing transition, and phase diagram of dense carbon-oxygen mixtures in white dwarfs

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Iyetomi, H.; Ogata, S.; Ichimaru, S.

    1989-07-01

    Equations of state for dense carbon-oxygen (C-O) binary-ionic mixtures (BIM's) appropriate to the interiors of white dwarfs are investigated through Monte Carlo simulations, by solution of relevant integral equations andvariational calculations in the density-functional formalism. It is thereby shown that the internal energies of the C-O BIM solids and fluids both obey precisely the linear mixing formulas. We then present an accurate calculation of the phase diagram associated with freezing transitions in such BIM materials, resulting in a novel prediction of an azeotropic diagram. Discontinuities of the mass density across the azeotropic phase boundaries areevaluated numerically for application to amore » study of white-dwarf evolution.« less

  7. Astrophysical Implications of a New Dynamical Mass for the Nearby White Dwarf 40 Eridani B

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Bond, Howard E.; Bergeron, P.; Bédard, A., E-mail: heb11@psu.edu

    The bright, nearby DA-type white dwarf (WD) 40 Eridani B is orbited by the M dwarf 40 Eri C, allowing determination of the WD’s mass. Until recently, however, the mass depended on orbital elements determined four decades ago, and that mass was so low that it created several astrophysical puzzles. Using new astrometric measurements, the binary-star group at the U.S. Naval Observatory has revised the dynamical mass upward, to 0.573 ± 0.018 M {sub ☉}. In this paper, we use model-atmosphere analysis to update other parameters of the WD, including effective temperature, surface gravity, radius, and luminosity. We then comparemore » these results with WD interior models. Within the observational uncertainties, theoretical cooling tracks for CO-core WDs of its measured mass are consistent with the position of 40 Eri B in the H-R diagram; equivalently, the theoretical mass–radius relation (MRR) is consistent with the star’s location in the mass–radius plane. This consistency is, however, achieved only if we assume a “thin” outer hydrogen layer, with q {sub H} = M {sub H}/ M {sub WD} ≃ 10{sup −10}. We discuss other evidence that a significant fraction of DA WDs have such thin H layers, in spite of the expectation from canonical stellar-evolution theory of “thick” H layers with q {sub H} ≃ 10{sup −4}. The cooling age of 40 Eri B is ∼122 Myr, and its total age is ∼1.8 Gyr. We present the MRRs for 40 Eri B and three other nearby WDs in visual binaries with precise mass determinations, and show that the agreement of current theory with observations is excellent in all cases.« less

  8. Low-mass White Dwarfs with Hydrogen Envelopes as a Missing Link in the Tidal Disruption Menu

    NASA Astrophysics Data System (ADS)

    Law-Smith, Jamie; MacLeod, Morgan; Guillochon, James; Macias, Phillip; Ramirez-Ruiz, Enrico

    2017-06-01

    We construct a menu of objects that can give rise to bright flares when disrupted by massive black holes (BHs), ranging from planets to evolved stars. Through their tidal disruption, main sequence and evolved stars can effectively probe the existence of otherwise quiescent supermassive BHs, and white dwarfs can probe intermediate mass BHs. Many low-mass white dwarfs possess extended hydrogen envelopes, which allow for the production of prompt flares in disruptive encounters with moderately massive BHs of 105-{10}7 {M}⊙ —masses that may constitute the majority of massive BHs by number. These objects are a missing link in two ways: (1) for probing moderately massive BHs and (2) for understanding the hydrodynamics of the disruption of objects with tenuous envelopes. A flare arising from the tidal disruption of a 0.17 {M}⊙ white dwarf by a {10}5 {M}⊙ {BH} reaches a maximum between 0.6 and 11 days, with a peak fallback rate that is usually super-Eddington and results in a flare that is likely brighter than a typical tidal disruption event. Encounters stripping only the envelope can provide hydrogen-only fallback, while encounters disrupting the core evolve from H- to He-rich fallback. While most tidal disruption candidates observed thus far are consistent with the disruptions of main sequence stars, the rapid timescales of nuclear transients such as Dougie and PTF10iya are naturally explained by the disruption of low-mass white dwarfs. As the number of observed flares continues to increase, the menu presented here will be essential for characterizing nuclear BHs and their environments through tidal disruptions.

  9. THE DROP DURING LESS THAN 300 DAYS OF A DUSTY WHITE DWARF'S INFRARED LUMINOSITY

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Xu, S.; Jura, M., E-mail: sxu@astro.ucla.edu, E-mail: jura@astro.ucla.edu

    2014-09-10

    We report Spitzer/Infrared Array Camera photometry of WD J0959–0200, a white dwarf that displays excess infrared radiation from a disk, likely produced by a tidally disrupted planetesimal. We find that in 2010, the fluxes in both 3.6 μm and 4.5 μm decreased by ∼35% in less than 300 days. The drop in the infrared luminosity is likely due to an increase of the inner disk radius from one of two scenarios: (1) a recent planetesimal impact; (2) instability in the circumstellar disk. The current situation is tantalizing; high-sensitivity, high-cadence infrared studies will be a new tool to study the interplay between a diskmore » and its host white dwarf star.« less

  10. THE CIRCUMSTELLAR ENVIRONMENT OF R CORONAE BOREALIS: WHITE DWARF MERGER OR FINAL-HELIUM-SHELL FLASH?

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Clayton, Geoffrey C.; Andrews, J. E.; Sugerman, Ben E. K.

    2011-12-10

    In 2007, R Coronae Borealis (R CrB) went into a historically deep and long decline. In this state, the dust acts like a natural coronagraph at visible wavelengths, allowing faint nebulosity around the star to be seen. Imaging has been obtained from 0.5 to 500 {mu}m with Gemini/GMOS, Hubble Space Telescope/WFPC2, Spitzer/MIPS, and Herschel/SPIRE. Several of the structures around R CrB are cometary globules caused by wind from the star streaming past dense blobs. The estimated dust mass of the knots is consistent with their being responsible for the R CrB declines if they form along the line of sightmore » to the star. In addition, there is a large diffuse shell extending up to 4 pc away from the star containing cool 25 K dust that is detected all the way out to 500 {mu}m. The spectral energy distribution of R CrB can be well fitted by a 150 AU disk surrounded by a very large diffuse envelope which corresponds to the size of the observed nebulosity. The total masses of the disk and envelope are 10{sup -4} and 2 M{sub Sun }, respectively, assuming a gas-to-dust ratio of 100. The evidence pointing toward a white dwarf merger or a final-helium-shell flash origin for R CrB is contradictory. The shell and the cometary knots are consistent with a fossil planetary nebula. Along with the fact that R CrB shows significant lithium in its atmosphere, this supports the final-helium-shell flash. However, the relatively high inferred mass of R CrB and its high fluorine abundance support a white dwarf merger.« less

  11. OPTICAL IDENTIFICATION OF He WHITE DWARFS ORBITING FOUR MILLISECOND PULSARS IN THE GLOBULAR CLUSTER 47 TUCANAE

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Cadelano, M.; Pallanca, C.; Ferraro, F. R.

    2015-10-10

    We used ultra-deep UV observations obtained with the Hubble Space Telescope to search for optical companions to binary millisecond pulsars (MSPs) in the globular cluster 47 Tucanae. We identified four new counterparts (to MSPs 47TucQ, 47TucS, 47TucT, and 47TucY) and confirmed those already known (to MSPs 47TucU and 47TucW). In the color–magnitude diagram, the detected companions are located in a region between the main sequence and the CO white dwarf (WD) cooling sequences, consistent with the cooling tracks of He WDs with masses between 0.15 M{sub ⊙} and 0.20 M{sub ⊙}. For each identified companion, mass, cooling age, temperature, andmore » pulsar mass (as a function of the inclination angle) have been derived and discussed. For 47TucU we also found that the past accretion history likely proceeded at a sub-Eddington rate. The companion to the redback 47TucW is confirmed to be a non-degenerate star, with properties particularly similar to those observed for black widow systems. Two stars have been identified within the 2σ astrometric uncertainty from the radio positions of 47TucH and 47TucI, but the available data prevent us from firmly assessing whether they are the true companions of these two MSPs.« less

  12. HOT WHITE DWARF SHINES IN YOUNG STAR CLUSTER

    NASA Technical Reports Server (NTRS)

    2002-01-01

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

  13. Periodic Variations in the O - C Diagrams of Five Pulsation Frequencies of the DB White Dwarf EC 20058-5234

    NASA Astrophysics Data System (ADS)

    Dalessio, J.; Sullivan, D. J.; Provencal, J. L.; Shipman, H. L.; Sullivan, T.; Kilkenny, D.; Fraga, L.; Sefako, R.

    2013-03-01

    Variations in the pulsation arrival time of five independent pulsation frequencies of the DB white dwarf EC 20058-5234 individually imitate the effects of reflex motion induced by a planet or companion but are inconsistent when considered in unison. The pulsation frequencies vary periodically in a 12.9 year cycle and undergo secular changes that are inconsistent with simple neutrino plus photon-cooling models. The magnitude of the periodic and secular variations increases with the period of the pulsations, possibly hinting that the corresponding physical mechanism is located near the surface of the star. The phase of the periodic variations appears coupled to the sign of the secular variations. The standards for pulsation-timing-based detection of planetary companions around pulsating white dwarfs, and possibly other variables such as subdwarf B stars, should be re-evaluated. The physical mechanism responsible for this surprising result may involve a redistribution of angular momentum or a magnetic cycle. Additionally, variations in a supposed combination frequency are shown to match the sum of the variations of the parent frequencies to remarkable precision, an expected but unprecedented confirmation of theoretical predictions. Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the US National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).

  14. 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.

  15. Constraints on the age and evolution of the Galaxy from the white dwarf luminosity function

    NASA Technical Reports Server (NTRS)

    Wood, M. A.

    1992-01-01

    The white dwarf disk luminosity function is explored using observational results of Liebert et al. (1988, 1989) as a template for comparison, and the cooling curves of Wood (1990, 1991) as the input basis functions for the integration. The star formation rate over the history of the Galaxy is found to be constant to within an order of magnitude, and the disk age lies in the range 6-13.5 Gyr, where roughly 40 percent of the uncertainty is due to the observational uncertainties. Using the best current estimates as inputs to the integration, the disk ages range from 7.5 to 11 Gyr, i.e., they are substantially younger than most estimates for the halo globular clusters but in reasonable agreement with those for the disk globular clusters and open clusters. The ages of these differing populations, taken together, are consistent with the pressure-supported collapse models of early spiral Galactic evolution.

  16. All things White Dwarf: The State of Stellar Forensics at the University of Texas and Sandia National Laboratories

    NASA Astrophysics Data System (ADS)

    Winget, Donald

    2011-10-01

    Astronomy has always been considered an observational science, in contrast with other experimental sciences like physics, chemistry, biology, and geology. This is because it has not been possible to perform experiments on the objects we observe. This situation has changed in a way that is transformational. We are now able to make macroscopic bits of star stuff in the lab: plasmas created under conditions that are the same as the plasmas in stars. Although laboratory astrophysics has long been an important part of astronomical research, what has changed is the ability to produce large enough chunks of a star that we can make measurements and perform experiments. In this way, astronomy joins her sister sciences in becoming an experimental science as well as an observational one. I will describe how this came about, the technology behind it, and the results of recent laboratory experiments. Most importantly, we will discuss how this will change our understanding of the universe and its contents. This work will shed new light on our recent discoveries involving McDonald Observatory: planets around white dwarf stars, massive carbon/oxygen variable white dwarf stars, and white dwarf-white dwarf binaries -- including one detached double eclipsing system with an orbital period of 12 minutes. We should measure the rate of change of the orbital period in this system within a year and we expect it to be the highest S/N source of gravitational radiation, easily detectable with LISA or similar approaches.

  17. On the absence of young white dwarf companions to five technetium stars

    NASA Technical Reports Server (NTRS)

    Smith, Verne V.; Lambert, David L.

    1987-01-01

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

  18. Kepler Beyond Planets: Finding Exploding Stars (Type Ia Supernova from a White Dwarf Merger)

    NASA Image and Video Library

    2018-03-26

    This frame from an animation shows the merger of two white dwarfs. A white dwarf is an extremely dense remnant of a star that can no longer burn nuclear fuel at its core. This is another way that a "type Ia" supernova occurs. Stellar explosions forge and distribute materials that make up the world in which we live, and also hold clues to how fast the universe is expanding. By understanding supernovae, scientists can unlock mysteries that are key to what we are made of and the fate of our universe. But to get the full picture, scientists must observe supernovae from a variety of perspectives, especially in the first moments of the explosion. That's really difficult -- there's no telling when or where a supernova might happen next. An animation is available at https://photojournal.jpl.nasa.gov/catalog/PIA22353

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

    NASA Astrophysics Data System (ADS)

    Salaris, Maurizio; Cassisi, Santi

    2018-04-01

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

  20. 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.

  1. Heavy Metals Resisting Gravity in White Dwarfs?

    NASA Technical Reports Server (NTRS)

    Rauch, Thomas; Gamrath, Sebastien; Quinet, Pascal; Hoyer, Denny; Werner, Klaus; Kruk, Jeffrey Walter

    2017-01-01

    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 thermo-dynamic equilibrium stellar-atmosphere models provided that reliable atomic data is available. Such analyses of Zn (atomic number Z equals 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 (decimal exponent)). 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.

  2. TOWARD A NETWORK OF FAINT DA WHITE DWARFS AS HIGH-PRECISION SPECTROPHOTOMETRIC STANDARDS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Narayan, G.; Matheson, T.; Saha, A.

    We present the initial results from a program aimed at establishing a network of hot DA white dwarfs to serve as spectrophotometric standards for present and future wide-field surveys. These stars span the equatorial zone and are faint enough to be conveniently observed throughout the year with large-aperture telescopes. The spectra of these white dwarfs are analyzed in order to generate a non-local-thermodynamic-equilibrium model atmosphere normalized to Hubble Space Telescope colors, including adjustments for wavelength-dependent interstellar extinction. Once established, this standard star network will serve ground-based observatories in both hemispheres as well as space-based instrumentation from the UV to themore » near IR. We demonstrate the effectiveness of this concept and show how two different approaches to the problem using somewhat different assumptions produce equivalent results. We discuss the lessons learned and the resulting corrective actions applied to our program.« less

  3. Rapid Evolution of the Gaseous Exoplanetary Debris around the White Dwarf Star HE 1349–2305

    NASA Astrophysics Data System (ADS)

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

    2018-02-01

    Observations of heavy metal pollution in white dwarf stars indicate that metal-rich planetesimals are frequently scattered into star-grazing orbits, tidally disrupted, and accreted onto the white dwarf surface, offering direct insight into the dynamical evolution of post-main-sequence exoplanetary systems. Emission lines from the gaseous debris in the accretion disks of some of these systems show variations on timescales of decades, and have been interpreted as the general relativistic precession of a recently formed, elliptical disk. Here we present a comprehensive spectroscopic monitoring campaign of the calcium infrared triplet emission in one system, HE 1349–2305, which shows morphological emission profile variations suggestive of a precessing, asymmetric intensity pattern. The emission profiles are shown to vary on a timescale of one to two years, which is an order of magnitude shorter than what has been observed in other similar systems. We demonstrate that this timescale is likely incompatible with general relativistic precession, and consider alternative explanations for the rapid evolution, including the propagation of density waves within the gaseous debris. We conclude with recommendations for follow-up observations, and discuss how the rapid evolution of the gaseous debris in HE 1349–2305 could be leveraged to test theories of exoplanetary debris disk evolution around white dwarf stars.

  4. A hydrodynamic study of a slow nova outburst. [computerized simulation of thermonuclear runaway in white dwarf envelope

    NASA Technical Reports Server (NTRS)

    Sparks, W. M.; Starrfield, S.; Truran, J. W.

    1978-01-01

    The paper reports use of a Lagrangian implicit hydrodynamics computer code incorporating a full nuclear-reaction network to follow a thermonuclear runaway in the hydrogen-rich envelope of a 1.25 solar-mass white dwarf. In this evolutionary sequence the envelope was assumed to be of normal (solar) composition and the resulting outburst closely resembles that of the slow nova HR Del. In contrast, previous CNO-enhanced models resemble fast nova outbursts. The slow-nova model ejects material by radiation pressure when the high luminosity of the rekindled hydrogen shell source exceeds the local Eddington luminosity of the outer layers. This is in contrast to the fast nova outburst where ejection is caused by the decay of the beta(+)-unstable nuclei. Nevertheless, radiation pressure probably plays a major role in ejecting material from the fast nova remnants. Therefore, the sequence from slow to fast novae can be interpreted as a sequence of white dwarfs with increasing amounts of enhanced CNO nuclei in their hydrogen envelopes, although other parameters such as the white-dwarf mass and accretion rate probably contribute to the observed variation between novae.

  5. K2 Ultracool Dwarfs Survey. III. White Light Flares Are Ubiquitous in M6-L0 Dwarfs

    NASA Astrophysics Data System (ADS)

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

    2018-05-01

    We report the white light flare rates for 10 ultracool dwarfs using Kepler K2 short-cadence data. Among our sample stars, two have spectral type M6, three are M7, three are M8, and two are L0. Most of our targets are old low-mass stars. We identify a total of 283 flares in all of the stars in our sample, with Kepler energies in the range log E Kp ∼ (29–33.5) erg. Using the maximum-likelihood method of line fitting, we find that the flare frequency distribution (FFD) for each star in our sample follows a power law with slope ‑α in the range ‑(1.3–2.0). We find that cooler objects tend to have shallower slopes. For some of our targets, the FFD follows either a broken power law, or a power law with an exponential cutoff. For the L0 dwarf 2MASS J12321827-0951502, we find a very shallow slope (‑α = ‑1.3) in the Kepler energy range (0.82–130) × 1030 erg: this L0 dwarf has flare rates which are comparable to those of high-energy flares in stars of earlier spectral types. In addition, we report photometry of two superflares: one on the L0 dwarf 2MASS J12321827-0951502 and another on the M7 dwarf 2MASS J08352366+1029318. In the case of 2MASS J12321827-0951502, we report a flare brightening by a factor of ∼144 relative to the quiescent photospheric level. Likewise, for 2MASS J08352366+1029318, we report a flare brightening by a factor of ∼60 relative to the quiescent photospheric level. These two superflares have bolometric (ultraviolet/optical/infrared) energies 3.6 × 1033 erg and 8.9 × 1033 erg respectively, while the full width half maximum timescales are very short, ∼2 min. We find that the M8 star TRAPPIST-1 is more active than the M8.5 dwarf 2M03264453+1919309, but less active than another M8 dwarf (2M12215066-0843197).

  6. 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

  7. Soft gamma-ray repeaters and anomalous X-ray pulsars as highly magnetized white dwarfs

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Mukhopadhyay, Banibrata; Rao, A.R., E-mail: bm@physics.iisc.ernet.in, E-mail: arrao@tifr.res.in

    We explore the possibility that soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are powered by highly magnetized white dwarfs (B-WDs). We take a sample of SGRs and AXPs and provide the possible parameter space in mass, radius, and surface magnetic field based on their observed properties (period and its derivative) and the assumption that these sources obey the mass-radius relation derived for the B-WDs. The radius and magnetic field of B-WDs are adequate to explain energies in SGRs/AXPs as the rotationally powered energy. In addition, B-WDs also adequately explain the perplexing radio transient GCRT J1745-3009 as a whitemore » dwarf pulsar. Note that the radius and magnetic fields of B-WDs are neither extreme (unlike of highly magnetized neutron stars) nor ordinary (unlike of magnetized white dwarfs, yet following the Chandrasekhar's mass-radius relation (C-WDs)). In order to explain SGRs/AXPs, while the highly magnetized neutron stars require an extra, observationally not well established yet, source of energy, the C-WDs predict large ultra-violet luminosity which is observationally constrained from a strict upper limit. Finally, we provide a set of basic differences between the magnetar and B-WD hypotheses for SGRs/AXPs.« less

  8. Limits on the dependence of the fine-structure constant on gravitational potential from white-dwarf spectra.

    PubMed

    Berengut, J C; Flambaum, V V; Ong, A; Webb, J K; Barrow, John D; Barstow, M A; Preval, S P; Holberg, J B

    2013-07-05

    We propose a new probe of the dependence of the fine-structure constant α on a strong gravitational field using metal lines in the spectra of white-dwarf stars. Comparison of laboratory spectra with far-UV astronomical spectra from the white-dwarf star G191-B2B recorded by the Hubble Space Telescope Imaging Spectrograph gives limits of Δα/α=(4.2±1.6)×10(-5) and (-6.1±5.8)×10(-5) from FeV and NiV spectra, respectively, at a dimensionless gravitational potential relative to Earth of Δφ≈5×10(-5). With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to 2 orders of magnitude.

  9. Limits on the Dependence of the Fine-Structure Constant on Gravitational Potential from White-Dwarf Spectra

    NASA Astrophysics Data System (ADS)

    Berengut, J. C.; Flambaum, V. V.; Ong, A.; Webb, J. K.; Barrow, John D.; Barstow, M. A.; Preval, S. P.; Holberg, J. B.

    2013-07-01

    We propose a new probe of the dependence of the fine-structure constant α on a strong gravitational field using metal lines in the spectra of white-dwarf stars. Comparison of laboratory spectra with far-UV astronomical spectra from the white-dwarf star G191-B2B recorded by the Hubble Space Telescope Imaging Spectrograph gives limits of Δα/α=(4.2±1.6)×10-5 and (-6.1±5.8)×10-5 from FeV and NiV spectra, respectively, at a dimensionless gravitational potential relative to Earth of Δϕ≈5×10-5. With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to 2 orders of magnitude.

  10. 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.

  11. An extrasolar extreme-ultraviolet object. II - The nature of HZ 43. [hot white dwarf star

    NASA Technical Reports Server (NTRS)

    Margon, B.; Liebert, J.; Lampton, M.; Spinrad, H.; Bowyer, S.; Gatewood, G.

    1976-01-01

    A variety of data are presented concerning the spectrum, distance, temperature, and evolutionary state of the hot white dwarf HZ 43, the first extrasolar object to be detected in the EUV band. The data include spectrophotometry of the star and its red dwarf companion (HZ 43B), a trigonometric parallax for the star, its tangential velocity, and results of soft X-ray and EUV observations. The main conclusions are that: (1) the spectrum of HZ 43A is that of a hot DAwk star, (2) HZ 43B is a dM3.5e star, (3) the distance of the system is about 65 pc, (4) the tangential velocity is not atypical of white dwarfs, and (5) the stellar energy distribution of HZ 43A is well fitted by a black body with an effective temperature of approximately 110,000 K. Evolutionary implications of the existence of an object as hot as HZ 43A are briefly considered, and it is suggested that the progenitors of hot DA stars must include objects hotter than spectral type sdB, with logical possibilities being nuclei of planetary nebulae and sdO stars.

  12. Energy Levels and Spectral Lines of Li Atoms in White Dwarf Strength Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Zhao, L. B.

    2018-04-01

    A theoretical approach based on B-splines has been developed to calculate atomic structures and discrete spectra of Li atoms in a strong magnetic field typical of magnetic white dwarf stars. Energy levels are presented for 20 electronic states with the symmetries 20+, 20‑, 2(‑1)+, 2(‑1)‑, and 2(‑2)+. The magnetic field strengths involved range from 0 to 2350 MG. The wavelengths and oscillator strengths for the electric dipole transitions relevant to these magnetized atomic states are reported. The current results are compared to the limited theoretical data in the literature. A good agreement has been found for the lower energy levels, but a significant discrepancy is clearly visible for the higher energy levels. The existing discrepancies of the wavelengths and oscillator strengths are also discussed. Our investigation shows that the spectrum data of magnetized Li atoms previously published are obviously far from meeting requirements of analyzing discrete atomic spectra of magnetic white dwarfs with lithium atmospheres.

  13. VizieR Online Data Catalog: Spectroscopically identified white dwarfs (McCook+, 2014)

    NASA Astrophysics Data System (ADS)

    McCook, G. P.; Sion, E. M.

    2016-10-01

    This is an update of the Villanova catalog published in the ApJS paper, corresponding to the Web version of the catalog in Apr 2014 (see http://www.astronomy.villanova.edu/WDCatalog/index.html). The introduction to the 1999 catalog is in the file "preface.tex". This updated version lists 14294 unique white dwarfs for a total of 27975 rows (multiple observations). (5 data files).

  14. Surprising Rapid Collapse of Sirius B from Red Giant to White Dwarf Through Mass Transfer to Sirius a

    NASA Astrophysics Data System (ADS)

    Yousef, Shahinaz; Ali, Ola

    2013-03-01

    Sirius was observed in antiquity as a red star. In his famous astronomy textbook the Almagest written 140 AD, Ptolemy described the star Sirius as fiery red. He curiously depicted it as one of six red-colored stars. The other five are class M and K stars, such as Arcturus and Betelgeuse. Apparent confirmation in ancient Greek and Roman sources are found and Sirius was also reported red in Europe about 1400 years ago. Sirius must have changed to a white dwarf in the night of Ascension. The star chapter in the Quran started with "by the star as it collapsed (1) your companion have not gone astray nor being misled (2), and in verse 49 which is the rotation period of the companion Sirius B around Sirius A, it is said" He is the Lord of Sirius (49). If Sirius actually was red what could have caused it to change into the brilliant bluish-white star we see today? What the naked eye perceives as a single star is actually a binary star system, consisting of a white main sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The red color indicates that the star seen then was a red giant. It looks that what they have seen in antiquity was Sirius B which was then a red giant and it collapsed to form a white dwarf. Since there is no evidence of a planetary nebula, then the red Sirius paradox can be solved in terms of stellar evolution with mass transfer. Sirius B was the most massive star which evolved to a red giant and filled the Roche lobe. Mass transfer to Sirius A occurred through the Lagrangian point. Sirius A then became more massive while Sirius B lost mass and shrank. Sirius B then collapsed abruptly into a white dwarf. In the case of Algol, Ptolmy observed it as white star but it was red at the time of El sufi. At present it is white. The rate of mass transfer from Sirius B to Sirius A, and from Algol B to A is estimated from observational data of colour change from red to bullish white to be 0

  15. Three new barium dwarfs with white dwarf companions: BD+68°1027, RE J0702+129 and BD+80°670

    NASA Astrophysics Data System (ADS)

    Kong, X. M.; Bharat Kumar, Y.; Zhao, G.; Zhao, J. K.; Fang, X. S.; Shi, J. R.; Wang, L.; Zhang, J. B.; Yan, H. L.

    2018-02-01

    We report three new barium (Ba) dwarfs lying in Sirius-like systems. They provide direct evidence that Ba dwarfs are companions to white dwarfs (WDs). Atmospheric parameters, stellar masses and the chemical abundances of 25 elements, including light, α, Fe-peak and s-process elements, are derived from high-resolution and high S/N spectra. The enhancement of s-process elements with [s/Fe] ratios between 0.4 and 0.6 confirm them as mild barium stars. The estimated metallicities (-0.31, -0.06 and 0.13) of BD+68°1027, RE J0702+129 and BD+80°670 are in the range of known Ba dwarfs and giants. As expected, the observed indices [hs/ls], [s/Fe] and [C/Fe] show an anti-correlation with metallicity. Asymptotic giant branch (AGB) progenitor masses are estimated for the WD companions of RE J0702+129 (1.47 M⊙) and BD+80°670 (3.59 M⊙). These confirm the predicted range of progenitor AGB masses (1.5-4 M⊙) for unseen WDs around Ba dwarfs. The surface abundances of s-process elements in RE J0702+129 and BD+80°670 are compared with AGB models and they are in close agreement, within the predicted accretion efficiencies and pollution factors for Ba stars. These results support that the origin of s-process overabundances in Ba dwarfs is similar to those of Ba giants via the McClure hypothesis in which Ba stars accumulate s-process elements through mass transfer from their host companions during the AGB phase.

  16. International Ultraviolet Explorer observations of the white dwarf nucleus of the very old, diffuse planetary nebula, IW-2

    NASA Technical Reports Server (NTRS)

    Bruhweiler, F. C.; Feibelman, Walter A.

    1993-01-01

    UV low-dispersion spectra of the central star of the faint planetary nebula, IW-2, were obtained with the IUE. The apparent large diameter of the very diffuse nebula, about half that of the moon, as seen on the Palomar Sky Survey plates by Ishida and Weinberger (1987), indicates this object to be potentially quite evolved, and nearby. The IUE spectra clearly reveal a hot stellar continuum extending over the entire wavelength range of the short-wavelength prime camera (1200-2000 A). This object with V = 17.7 +/- 0.4 is definitely one of the faintest stars ever successfully observed with the IUE. Comparisons of the IUE observed fluxes with those from white dwarf model atmospheres suggest extinction near E(B - V) = 0.45 for a white dwarf of T(eff) roughly 100,000 K. Constraints from estimates of the nebular emission measure and observed visual magnitude also argue for a white dwarf of T(eff) roughly 100,000 K at a distance of 300 to 350 pc. The nucleus of IW-2 is one of the most evolved stars to be identified with a planetary nebula.

  17. The White-Dwarf Mass-Radius Relation from 40 Eridani B and Other Nearby Visual Binaries

    NASA Astrophysics Data System (ADS)

    Bond, Howard E.; Bergeron, P.; Bedard, A.

    2018-01-01

    The bright, nearby DA-type white dwarf (WD) 40 Eridani B is orbited by the M dwarf 40 Eri C, allowing determination of the WD's mass. Until recently, however, the mass depended on orbital elements determined four decades ago, and that mass was so low that it created several astrophysical puzzles. Using new astrometric measurements, the binary-star group at the U.S. Naval Observatory has revised the dynamical mass upward, to 0.573 ± 0.018 M⊙. We have used model-atmosphere analysis to update other parameters of the WD, including effective temperature, surface gravity, radius, and luminosity. We then comparethese results with WD interior models.Within the observational uncertainties, theoretical cooling tracks for CO-core WDs of its measured mass are consistent with the position of 40 Eri B in the H-R diagram; equivalently, the theoretical mass-radius relation (MRR) is consistent with the star's location in the mass-radius plane. This consistency is, however, achieved only if we assume a "thin'' outer hydrogen layer, with qH = MH/MWD ∼ 10–10.We discuss other evidence that a significant fraction of DA WDs have such thin H layers, in spite of expectation from canonical stellar-evolution theory of "thick'' H layers with qH ∼ 10–4 . The cooling age of 40 Eri B is ~122 Myr, and its total age is ~1.8 Gyr. We present the MRRs for 40 Eri B and three other nearby WDs in visual binaries with precise mass determinations, and show that the agreement of current theory with observation is excellent in all cases.However, astrophysical puzzles remain. The eccentricity of the BC orbit has remained high (0.43), even though the progenitor of B ought to have interacted tidally with C when it was an AGB star. This puzzle exists also for the Sirius and Procyon systems. If thin hydrogen layers are common among WDs, the mass scale will need to be shifted downwards by a few hundredths of a solar mass.

  18. The Impact of Accurate Distances on UV Spectroscopy of White Dwarfs and Cataclysmic Variables

    DTIC Science & Technology

    2009-01-01

    evolution. Four instability strips in the HR diagram are associated with planetary nebulae nuclei (PNN) and white dwarfs (WDs). The rst instability...strip occurs during the high luminosity planetary nebula phase. The second is during the pre- WD stars of the PG 1159 spectral type, which are direct

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

    NASA Astrophysics Data System (ADS)

    Hermes, J. J.

    2013-08-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2018-06-01

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

  1. The movie about the magnetism in isolated white dwarfs

    NASA Astrophysics Data System (ADS)

    Sliwinski, M. S.; Krzyczkowska, L. I.

    2004-10-01

    We present the movie about magnetism in the isolated white dwarfs. We tried to create a brand new form of presentation of astronomical knowledge. This one was made to show it to the scientists, so we assumed some level of knowledge that was needed to understand the movie. However it still can be a way to arouse interest of non-astronomers (even children) in astronomical research. This could also be the best way to teach young people or help them to understand astronomy and astrophysics that they have to learn at school. Below we report the plot of the movie adding, in some parts, comments that will follow.

  2. High-Time-Resolution Photometry of the White Dwarf Pulsar AR Scorpii

    NASA Astrophysics Data System (ADS)

    Stiller, Robert A.; Littlefield, Colin; Garnavich, Peter

    2018-01-01

    The cataclysmic variable AR Sco was recently discovered to be the first-ever white dwarf pulsar by Marsh et al. (2016) and Buckley et al. (2017). AR Sco has a 3.56-hour orbital period, a beat period of 1.97 minutes, and a spin period of 1.95 minutes. The flux varies by up to a factor of four during the beat period. It is believed that there is little to no accretion because of the weak X-ray emissions from the system. The white dwarf pulsar is believed to be spin-powered and is in an ejector state (Beskrovnaya et. al 2017) which is further evidence of little to no accretion. 24 hours of high-time-resolution photometry was taken using the 0.8 meter Sarah L. Krizmanich Telescope at the University of Notre Dame. We used our own observations and previous observations to calculate a new spin down timescale. In our data, AR Sco is brightest at an orbital phase of approximately 0.4 which suggests that if the orbital modulation is a reflection effect, the inner hemisphere in not uniformly irradiated. We establish that the amplitude and waveform of the beat pulse changes as function of orbital phase and that this can be attributed to the beat and spin pulses constructively and destructively interfering with one another.

  3. Dynamo generation of magnetic field in the white dwarf GD 358

    NASA Technical Reports Server (NTRS)

    Markiel, J. Andrew; Thomas, John H.; Van Horn, H. M.

    1994-01-01

    On the basis of Whole Earth Telescope observations of the g-mode oscillation spectrum of the white dwarf GD 358, Winget et al. find evidence for significant differential rotation and for a time-varying magnetic field concentrated in the surface layers of this star. Here we argue on theoretical grounds that this magnetic field is produced by an alpha omega dynamo operating in the lower part of a surface convection zone in GD 358. Our argument is based on numerical solutions of the nonlinear, local dynamo equations of Robinson & Durney, with specific parameters based on our detailed models of white-dwarf convective envelopes, and universal constants determined by a calibration with the the Sun's dynamo. The calculations suggest a dynamo cycle period of about 6 years for the fundamental mode, and periods as short as 1 year for the higher-order modes that are expected to dominate in view of the large dynamo number we estimate for GD 358. These dynamo periods are consistent with the changes in the magnetic field of GD 358 over the span of 1 month inferred by Winget et. al. from their observations. Our calculations also suggest a peak dynamo magnetic field strength at the base of the surface convection zone of about 1800 G, which is consistent with the field strength inferred from the observations.

  4. Topics in solid-state astrophysics: Magnetized neutron star crusts and multicomponent crusts/white dwarfs

    NASA Astrophysics Data System (ADS)

    Engstrom, Tyler A.

    Two research endeavors are described in this dissertation; both undertake problems in solid-state astrophysics, which is a branch of solid-state physics concerning the extreme conditions found within white dwarfs and the solid crusts of neutron stars. As much of our knowledge about these compact objects comes from observation of astrophysical phenomena, Chapter 1 is devoted to the phenomena, and how they can be exploited as material property probes. Several of the most interesting phenomena involve the enormous magnetic fields (B ≥ 1012 gauss) harbored by many neutron stars, and the interaction between these fields and the charged particles within the solid crust. Accordingly, Chapter 2 reviews some theory of strongly-magnetized electrons, which both sets the stage for Chapter 3, and (hopefully) serves as a useful reference for future research. Let it now be made clear that this dissertation focuses exclusively on the "outer crusts," of neutron stars, where no free neutrons are present (rho < 4x1011 g/cc), and the similarly-composed interiors of white dwarfs, which have central densities ˜ 107 g/cc. For the most part we specialize to even lower densities. In Chapter 3, static and dynamic properties of low density (rho ≥ 106 g/cc) outer envelopes of neutron stars are calculated within the nonlinear magnetic Thomas-Fermi model, assuming degenerate electrons. A novel domain decomposition enables proper description of lattice symmetry and may be seen as a prototype for the general class of problems involving nonlinear charge screening of periodic, quasi-low-dimensionality structures, e.g. liquid crystals. We describe a scalable implementation of the method using Hypre. Over the density range considered, the effective shear modulus appears to be a factor of ≈ 20 larger than in the linearlyscreened Coulomb crystal model, which could have implications for observables related to astroseismology as well as low temperature phonon-mediated thermal conductivity. Other

  5. A Spectroscopic Search for White Dwarf Companions to 101 Nearby M Dwarfs

    NASA Astrophysics Data System (ADS)

    Bar, Ira; Vreeswijk, Paul; Gal-Yam, Avishay; Ofek, Eran O.; Nelemans, Gijs

    2017-11-01

    Recent studies of the stellar population in the solar neighborhood (<20 pc) suggest that there are undetected white dwarfs (WDs) in multiple systems with main-sequence companions. Detecting these hidden stars and obtaining a more complete census of nearby WDs is important for our understanding of stellar and galactic evolution, as well as the study of explosive phenomena. In an attempt to uncover these hidden WDs, we present intermediate resolution spectroscopy over the wavelength range of 3000-25000 Å of 101 nearby M dwarfs (dMs), observed with the Very Large Telescope X-Shooter spectrograph. For each star we search for a hot component superimposed on the dM spectrum. X-Shooter has excellent blue sensitivity and thus can reveal a faint hot WD despite the brightness of its red companion. Visual examination shows no clear evidence of a WD in any of the spectra. We place upper limits on the effective temperatures of WDs that may still be hiding by fitting dM templates to the spectra and modeling the WD spectra. On average our survey is sensitive to WDs hotter than about 5300 K. This suggests that the frequency of WD companions of {T}{eff}≳ 5300 {{K}} with separation of the order of ≲50 au among the local dM population is <3% at the 95% confidence level. The reduced spectra are made available via the WISeREP3 repository. Based on observations collected in service mode using the Very Large Telescope (VLT) under program IDs 095_D-0949(A) and 096_D-0963(A).

  6. WIRED for EC: New White Dwarfs with WISE Infrared Excesses and New Classification Schemes from the Edinburgh–Cape Blue Object Survey

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Dennihy, E.; Clemens, J. C.; Dunlap, B. H.

    We present a simple method for identifying candidate white dwarf systems with dusty exoplanetary debris based on a single temperature blackbody model fit to the infrared excess. We apply this technique to a sample of Southern Hemisphere white dwarfs from the recently completed Edinburgh–Cape Blue Object Survey and identify four new promising dusty debris disk candidates. We demonstrate the efficacy of our selection method by recovering three of the four Spitzer confirmed dusty debris disk systems in our sample. Further investigation using archival high-resolution imaging shows that Spitzer data of the unrecovered fourth object is likely contaminated by a line-of-sightmore » object that either led to a misclassification as a dusty disk in the literature or is confounding our method. Finally, in our diagnostic plot, we show that dusty white dwarfs, which also host gaseous debris, lie along a boundary of our dusty debris disk region, providing clues to the origin and evolution of these especially interesting systems.« less

  7. Effect of cooling of cooked white rice on resistant starch content and glycemic response.

    PubMed

    Sonia, Steffi; Witjaksono, Fiastuti; Ridwan, Rahmawati

    2015-01-01

    Cooling of cooked starch is known to cause starch retrogradation which increases resistant starch content. This study aimed to determine the effect of cooling of cooked white rice on resistant starch content and glycemic response in healthy subjects. Resistant starch contents were analyzed on freshly cooked white rice (control rice), cooked white rice cooled for 10 hours at room temperature (test rice I), and cooked white rice cooled for 24 hours at 4°C then reheated (test rice II). The results showed that resistant starch contents in control rice, test rice I, and test rice II were 0.64 g/100 g, 1.30 g/100 g, and 1.65 g/100 g, respectively. Test rice II had higher resistant starch content than test rice I, hence used in the clinical study along with control rice to characterize glycemic response in 15 healthy adults. The clinical study was a randomized, single-blind crossover study. In the clinical study, test rice II significantly lowered glycemic response compared with control rice (125±50.1 vs 152±48.3 mmol.min/L, respectively; p=0.047). In conclusion, cooling of cooked white rice increased resistant starch content. Cooked white rice cooled for 24 hours at 4°C then reheated lowered glycemic response compared with freshly cooked white rice.

  8. Sub-percent Photometry: Faint DA White Dwarf Spectrophotometric Standards for Astrophysical Observatories

    NASA Astrophysics Data System (ADS)

    Narayan, Gautham; Axelrod, Tim; Calamida, Annalisa; Saha, Abhijit; Matheson, Thomas; Olszewski, Edward; Holberg, Jay; Holberg, Jay; Bohlin, Ralph; Stubbs, Christopher W.; Rest, Armin; Deustua, Susana; Sabbi, Elena; MacKenty, John W.; Points, Sean D.; Hubeny, Ivan

    2018-01-01

    We have established a network of faint (16.5 < V < 19) hot DA white dwarfs as spectrophotometric standards for present and future wide-field observatories. Our standards are accessible from both hemispheres and suitable for ground and space-based covering the UV to the near IR. The network is tied directly to the most precise astrophysical reference presently available - the CALSPEC standards - through a multi-cycle program imaging using the Wide-Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). We have developed two independent analyses to forward model all the observed photometry and ground-based spectroscopy and infer a spectral energy distribution for each source using a non-local-thermodynamic-equilibrium (NLTE) DA white dwarf atmosphere extincted by interstellar dust. The models are in excellent agreement with each other, and agree with the observations to better than 0.01 mag in all passbands, and better than 0.005 mag in the optical. The high-precision of these faint sources, tied directly to the most accurate flux standards presently available, make our network of standards ideally suited for any experiments that have very stringent requirements on absolute flux calibration, such as studies of dark energy using the Large Synoptic Survey Telescope (LSST) and the Wide-Field Infrared Survey Telescope (WFIRST).

  9. VizieR Online Data Catalog: Census of nearby white dwarfs from SUPERBLINK (Limoges+, 2015)

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

    In Paper I (Limoges+, 2013, J/AJ/145/136), we compiled a list of 1341 white dwarf candidates within 40pc of the Sun -but likely extending to 55pc given the uncertainties- and reported spectroscopic observations for 422 objects from this target list, including 192 new white dwarf identifications. The spectroscopic observing log of our earlier survey is presented in Table 2 of Paper I. Since then, additional optical spectra have been obtained with the Steward Observatory 2.3m telescope and the NOAO Mayall 4 and 2.1m telescopes during 7 different observing campaigns between 2011 January and 2013 October. A few of the brightest candidates were also observed in spectroscopy with the 1.6m Mont-Megantic Observatory, while ~60hr with the Gemini North and South 8m telescopes were used to observe our faintest candidates (V~17-18). The adopted configurations allow a spectral coverage of λ~3200-5300Å and ~3800-6700Å, at an intermediate resolution of ~6Å FWHM. (4 data files).

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

    DTIC Science & Technology

    2008-05-20

    Anderson,4 Patrick B . Hall,5 Jeffrey A. Munn,1 James Liebert,6 Gillian R. Knapp,7 D. Bizyaev,8 E. Malanushenko,8 V. Malanushenko,8 K . Pan,8 Donald P...ADDITIONAL ULTRACOOL WHITE DWARFS FOUND IN THE SLOAN DIGITAL SKY SURVEY Hugh C. Harris,1 Evalyn Gates,2 Geza Gyuk,2,3 Mark Subbarao ,2,3 Scott F...effective temperature of roughly 4000 K , the density of gas in the photosphere increases to a point where models of the atmosphere must include effects not

  11. Accreting Double White Dwarf Binaries: Implications for LISA

    NASA Astrophysics Data System (ADS)

    Kremer, Kyle; Breivik, Katelyn; Larson, Shane L.; Kalogera, Vassiliki

    2017-09-01

    We explore the long-term evolution of mass-transferring white dwarf (WD) binaries undergoing both direct-impact and disk accretion and explore implications of such systems to gravitational-wave (GW) astronomy. We cover a broad range of initial component masses and show that these systems, the majority of which lie within the Laser Interferometer Space Antenna (LISA) sensitivity range, exhibit prominent negative orbital frequency evolution (chirp) for a significant fraction of their lifetimes. Using a galactic population synthesis, we predict ˜2700 of these systems will be observable with a negative chirp of 0.1 yr-2 by a space-based GW detector like LISA. We also show that detections of mass-transferring double WD systems by LISA may provide astronomers with unique ways of probing the physics governing close compact object binaries.

  12. VizieR Online Data Catalog: White dwarfs in SDSS Stripe 82 (Hernandez Santisteban+, 2018)

    NASA Astrophysics Data System (ADS)

    Hernandez Santisteban, J. V.; Knigge, C.; Pretorius, M. L.; Sullivan, M.; Warner, B.

    2017-08-01

    We present the photometrically selected white dwarfs in SDSS Stripe 82, used to find total eclipses or dropouts in the SDSS and PTF surveys. The Table presents the location as measured by SDSS as well as average photometric values of each WD. This data corresponds to the Table B1 in the paper. (1 data file).

  13. Direct imaging and new technologies to search for substellar companions around MGs cool dwarfs

    NASA Astrophysics Data System (ADS)

    Gálvez-Ortiz, M. C.; Clarke, J. R. A.; Pinfield, D. J.; Folkes, S. L.; Jenkins, J. S.; García Pérez, A. E.; Burningham, B.; Day-Jones, A. C.; Jones, H. R. A.

    2011-07-01

    We describe here our project based in a search for sub-stellar companions (brown dwarfs and exo-planets) around young ultra-cool dwarfs (UCDs) and characterise their properties. We will use current and future technology (high contrast imaging, high-precision Doppler determinations) from the ground and space (VLT, ELT and JWST), to find companions to young objects. Members of young moving groups (MGs) have clear advantages in this field. We compiled a catalogue of young UCD objects and studied their membership to five known young moving groups: Local Association (Pleiades moving group, 20-150 Myr), Ursa Mayor group (Sirius supercluster, 300 Myr), Hyades supercluster (600 Myr), IC 2391 supercluster (35 Myr) and Castor moving group (200 Myr). To assess them as members we used different kinematic and spectroscopic criteria.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

  15. The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems

    NASA Astrophysics Data System (ADS)

    Silvestri, Nicole M.; Hawley, Suzanne L.; Oswalt, Terry D.

    2005-05-01

    We investigate the relationship between age and chromospheric activity for 139 M dwarf stars in wide binary systems with white dwarf companions. The age of each system is determined from the cooling age of its white dwarf component. The current limit for activity-age relations found for M dwarfs in open clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for the exploration of this relationship at ages older than 4 Gyr. The general trend of stars remaining active for a longer time at a later spectral type is confirmed. However, our larger sample and greater age range reveal additional complexity in assigning age based on activity alone. We find that M dwarfs in wide binaries older than 4 Gyr depart from the loglinear relation for clusters and are found to have activity at magnitudes, colors, and masses that are brighter, bluer, and more massive than predicted by the cluster relation. In addition to our activity-age results, we present the measured radial velocities and complete space motions for 161 white dwarf stars in wide binaries. Based on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium; the Cerro Tololo Inter-American Observatory 4.0 m telescope, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation (NSF) as part of the National Optical Astronomy Observatory (NOAO), which also operates Kitt Peak National Observatory in Tucson, Arizona; and the SARA Observatory 0.9 m telescope at Kitt Peak, which is owned and operated by the Southeastern Association for Research in Astronomy (http://www.saraobservatory.org).

  16. Interstellar absorption of the extreme ultraviolet flux from two hot white dwarfs

    NASA Technical Reports Server (NTRS)

    Cash, W.; Bowyer, S.; Lampton, M.

    1979-01-01

    Photometric upper limits on the 300 A flux from the hot white dwarfs Feige 24 and G191-B2B are presented. The limits, which were obtained with a rocket-borne extreme ultraviolet imaging telescope, are interpreted as lower limits on the density of the intervening interstellar matter. The limits are used to investigate the state of interstellar gas within 100 pc. A local clumpiness factor, which is of value in planning future extreme ultraviolet observations, is derived.

  17. Accretion onto CO White Dwarfs using MESA

    NASA Astrophysics Data System (ADS)

    Feng, Wanda; Starrfield, Sumner

    2018-06-01

    The nature of type Ia Supernovae (SNe Ia) progenitor systems and their underlying mechanism are not well understood. There are two competing progenitor scenarios: the single-degenerate scenario wherein a white dwarf (WD) star accretes material from a companion star, reaching the Chandrasekhar mass limit; and, the double-degenerate scenario wherein two WDs merge. In this study, we investigate the single-degenerate scenario by accretion onto carbon-oxygen (CO) WDs using the Modules for Experiments in Stellar Astrophysics (MESA). We vary the WD mass, composition of the accreting material, and accretion rate in our models. Mixing between the accreted material and the WD core is informed by multidimensional studies that suggest occurance after thermonuclear runaway (TNR) ensues. We compare the accretion of solar composition material onto CO WDs with the accretion of mixed solar and core material after TNR. As many of our models eject less material than accreted, our study supports that accretion onto CO WDs is a feasible channel for SNe I progenitors.

  18. DOES A DIFFERENTIATED, CARBONATE-RICH, ROCKY OBJECT POLLUTE THE WHITE DWARF SDSS J104341.53+085558.2?

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Melis, Carl; Dufour, P., E-mail: cmelis@ucsd.edu

    We present spectroscopic observations of the dust- and gas-enshrouded, polluted, single white dwarf star SDSS J104341.53+085558.2 (hereafter SDSS J1043+0855). Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet spectra combined with deep Keck HIRES optical spectroscopy reveal the elements C, O, Mg, Al, Si, P, S, Ca, Fe, and Ni and enable useful limits for Sc, Ti, V, Cr, and Mn in the photosphere of SDSS J1043+0855. From this suite of elements we determine that the parent body being accreted by SDSS J1043+0855 is similar to the silicate Moon or the outer layers of Earth in that it is rocky and iron-poor.more » Combining this with comparison to other heavily polluted white dwarf stars, we are able to identify the material being accreted by SDSS J1043+0855 as likely to have come from the outermost layers of a differentiated object. Furthermore, we present evidence that some polluted white dwarfs (including SDSS J1043+0855) allow us to examine the structure of differentiated extrasolar rocky bodies. Enhanced levels of carbon in the body polluting SDSS J1043+0855 relative to the Earth–Moon system can be explained with a model where a significant amount of the accreted rocky minerals took the form of carbonates; specifically, through this model the accreted material could be up to 9% calcium-carbonate by mass.« less

  19. Crystal Chemistry of Three-component White Dwarfs and Neutron Star Crusts: Phase Stability, Phase Stratification, and Physical Properties

    NASA Astrophysics Data System (ADS)

    Engstrom, T. A.; Yoder, N. C.; Crespi, V. H.

    2016-02-01

    A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are “bred” by a genetic algorithm and optimized at constant pressure under the assumption of linear response (Thomas-Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the T = 0 bulk phase diagrams, five of which are complicated multinary structures not previously predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low-density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase-layering diagram and mass-radius-composition dependence, both of which are reported for He-C-O and C-O-Ne white dwarfs. Finite thickness interfacial phases (“interphases”) show up at the boundaries between single-component body-centered cubic (bcc) crystalline regions, some of which have lower lattice symmetry than cubic. A second application—quasi-static settling of heavy nuclei in white dwarfs—builds on our equilibrium phase-layering method. Tests of this nonequilibrium method reveal extra phases that play the role of transient host phases for the settling species.

  20. Activity in Very Cool Stars: Magnetic Dissipation in Late M and L Dwarf Atmospheres

    NASA Astrophysics Data System (ADS)

    Mohanty, Subhanjoy; Basri, Gibor; Shu, Frank; Allard, France; Chabrier, Gilles

    2002-05-01

    Recent observations show that chromospheric Hα activity in late M and L dwarfs is much lower than in the earlier M types. This is particularly surprising given that the late M and L dwarfs are comparatively very rapid rotators: in the early M dwarfs, rapid rotation is associated with high activity levels. One possibility is that the drop-off in activity in the late M and L dwarfs is a result of very high electrical resistivities in their dense, cool, and predominantly neutral atmospheres.We calculate the magnetic field diffusivity in the atmospheres of objects with Teff in the range 3000-1500 K (mid M to late L) using the atmospheric structure models of Allard and Hauschildt. We find that the combination of very low ionization fraction and high density in these atmospheres results in very large resistivities and thus efficient field diffusion. While both ambipolar diffusion and Ohmic decay of currents due to ion-electron collisions occur, the primary diffusion effects are due to current decay through collisions of charged particles with neutrals. Moreover, the latter resistivity is a strong function of both effective temperature and optical depth, increasing rapidly as either Teff or optical depth decreases. This has two implications: (1) Any magnetic field present is increasingly decoupled from atmospheric fluid motions as one moves from mid M to L. In the late M and L dwarfs, atmospheric motions cannot lead to equilibrium field configurations very different from potential ones. That is, the magnitude of magnetic stresses generated by atmospheric motions is very small in these objects. We quantify this effect by a simple Reynolds number calculation. (2) Even if magnetic stresses are easily produced by fluid motions in the hot interior (where the coupling between field and matter is good), their propagation up through the atmosphere will be increasingly hampered by the growing atmospheric resistivity as one moves from mid M to late L. Thus both the generation and

  1. Accreting Double White Dwarf Binaries: Implications for LISA

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Kremer, Kyle; Breivik, Katelyn; Larson, Shane L.

    We explore the long-term evolution of mass-transferring white dwarf (WD) binaries undergoing both direct-impact and disk accretion and explore implications of such systems to gravitational-wave (GW) astronomy. We cover a broad range of initial component masses and show that these systems, the majority of which lie within the Laser Interferometer Space Antenna ( LISA ) sensitivity range, exhibit prominent negative orbital frequency evolution (chirp) for a significant fraction of their lifetimes. Using a galactic population synthesis, we predict ∼2700 of these systems will be observable with a negative chirp of 0.1 yr{sup −2} by a space-based GW detector like LISAmore » . We also show that detections of mass-transferring double WD systems by LISA may provide astronomers with unique ways of probing the physics governing close compact object binaries.« less

  2. The conductive propagation of nuclear flames. I - Degenerate C + O and O + Ne + Mg white dwarfs

    NASA Technical Reports Server (NTRS)

    Timmes, F. X.; Woosley, S. E.

    1992-01-01

    The paper determines the physical properties - speed, width, and density structure - of conductive burning fronts in degenerate carbon-oxygen (C + O) and oxygen-neon-magnesium (O + Ne + Mg) compositions for a grid of initial densities and compositions. The dependence of the physical properties of the flame on the assumed values of nuclear reaction rates, the nuclear reaction network employed, the thermal conductivity, and the choice of coordinate system are investigated. The occurrence of accretion-induced collapse of a white dwarf is found to be critically dependent on the velocity of the nuclear conductive burning front and the growth rate of hydrodynamic instabilities. Treating the expanding area of the turbulent burning region as a fractal whose tile size is identical to the minimum unstable Rayleigh-Taylor wavelength, it is found, for all reasonable values of the fractal dimension, that for initial C + O or O + Ne + Mg densities above about 9 x 10 exp 9 g/cu cm the white dwarf should collapse to a neutron star.

  3. X-ray reflection from cold white dwarfs in magnetic cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Hayashi, Takayuki; Kitaguchi, Takao; Ishida, Manabu

    2018-02-01

    We model X-ray reflection from white dwarfs (WDs) in magnetic cataclysmic variables (mCVs) using a Monte Carlo simulation. A point source with a power-law spectrum or a realistic post-shock accretion column (PSAC) source irradiates a cool and spherical WD. The PSAC source emits thermal spectra of various temperatures stratified along the column according to the PSAC model. In the point-source simulation, we confirm the following: a source harder and nearer to the WD enhances the reflection; higher iron abundance enhances the equivalent widths (EWs) of fluorescent iron Kα1, 2 lines and their Compton shoulder, and increases the cut-off energy of a Compton hump; significant reflection appears from an area that is more than 90° apart from the position right under the point X-ray source because of the WD curvature. The PSAC simulation reveals the following: a more massive WD basically enhances the intensities of the fluorescent iron Kα1, 2 lines and the Compton hump, except for some specific accretion rate, because the more massive WD makes a hotter PSAC from which higher-energy X-rays are preferentially emitted; a larger specific accretion rate monotonically enhances the reflection because it makes a hotter and shorter PSAC; the intrinsic thermal component hardens by occultation of the cool base of the PSAC by the WD. We quantitatively estimate the influences of the parameters on the EWs and the Compton hump with both types of source. We also calculate X-ray modulation profiles brought about by the WD spin. These depend on the angles of the spin axis from the line of sight and from the PSAC, and on whether the two PSACs can be seen. The reflection spectral model and the modulation model involve the fluorescent lines and the Compton hump and can directly be compared to the data, which allows us to estimate these geometrical parameters with unprecedented accuracy.

  4. A Semi-analytic Criterion for the Spontaneous Initiation of Carbon Detonations in White Dwarfs

    NASA Astrophysics Data System (ADS)

    Garg, Uma; Chang, Philip

    2017-02-01

    Despite over 40 years of active research, the nature of the white dwarf progenitors of SNe Ia remains unclear. However, in the last decade, various progenitor scenarios have highlighted the need for detonations to be the primary mechanism by which these white dwarfs are consumed, but it is unclear how these detonations are triggered. In this paper we study how detonations are spontaneously initiated due to temperature inhomogeneities, e.g., hotspots, in burning nuclear fuel in a simplified physical scenario. Following the earlier work by Zel’Dovich, we describe the physics of detonation initiation in terms of the comparison between the spontaneous wave speed and the Chapman-Jouguet speed. We develop an analytic expression for the spontaneous wave speed and utilize it to determine a semi-analytic criterion for the minimum size of a hotspot with a linear temperature gradient between a peak and base temperature for which detonations in burning carbon-oxygen material can occur. Our results suggest that spontaneous detonations may easily form under a diverse range of conditions, likely allowing a number of progenitor scenarios to initiate detonations that burn up the star.

  5. A Semi-analytic Criterion for the Spontaneous Initiation of Carbon Detonations in White Dwarfs

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Garg, Uma; Chang, Philip, E-mail: umagarg@uwm.edu, E-mail: chang65@uwm.edu

    Despite over 40 years of active research, the nature of the white dwarf progenitors of SNe Ia remains unclear. However, in the last decade, various progenitor scenarios have highlighted the need for detonations to be the primary mechanism by which these white dwarfs are consumed, but it is unclear how these detonations are triggered. In this paper we study how detonations are spontaneously initiated due to temperature inhomogeneities, e.g., hotspots, in burning nuclear fuel in a simplified physical scenario. Following the earlier work by Zel’Dovich, we describe the physics of detonation initiation in terms of the comparison between the spontaneousmore » wave speed and the Chapman–Jouguet speed. We develop an analytic expression for the spontaneous wave speed and utilize it to determine a semi-analytic criterion for the minimum size of a hotspot with a linear temperature gradient between a peak and base temperature for which detonations in burning carbon–oxygen material can occur. Our results suggest that spontaneous detonations may easily form under a diverse range of conditions, likely allowing a number of progenitor scenarios to initiate detonations that burn up the star.« less

  6. Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with mass approximately 0.9M[symbol: see text].

    PubMed

    Pakmor, Rüdiger; Kromer, Markus; Röpke, Friedrich K; Sim, Stuart A; Ruiter, Ashley J; Hillebrandt, Wolfgang

    2010-01-07

    Type Ia supernovae are thought to result from thermonuclear explosions of carbon-oxygen white dwarf stars. Existing models generally explain the observed properties, with the exception of the sub-luminous 1991bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but hitherto simulations have failed to produce an explosion. Here we report a simulation of the merger of two equal-mass white dwarfs that leads to a sub-luminous explosion, although at the expense of requiring a single common-envelope phase, and component masses of approximately 0.9M[symbol: see text]. The light curve is too broad, but the synthesized spectra, red colour and low expansion velocities are all close to what is observed for sub-luminous 1991bg-like events. Although the mass ratios can be slightly less than one and still produce a sub-luminous event, the masses have to be in the range 0.83M[symbol: see text] to 0.9M[symbol: see text].

  7. Asteroseismology of ZZ Ceti stars with full evolutionary white dwarf models. II. The impact of AGB thermal pulses on the asteroseismic inferences of ZZ Ceti stars

    NASA Astrophysics Data System (ADS)

    De Gerónimo, F. C.; Althaus, L. G.; Córsico, A. H.; Romero, A. D.; Kepler, S. O.

    2018-05-01

    Context. The thermally pulsing phase on the asymptotic giant branch (TP-AGB) is the last nuclear burning phase experienced by most low- and intermediate-mass stars. During this phase, the outer chemical stratification above the C/O core of the emerging white dwarf (WD) is built up. The chemical structure resulting from progenitor evolution strongly impacts the whole pulsation spectrum exhibited by ZZ Ceti stars, which are pulsating C/O core white dwarfs located on a narrow instability strip at Teff 12 000 K. Several physical processes occurring during progenitor evolution strongly affect the chemical structure of these stars; those found during the TP-AGB phase are the most relevant for the pulsational properties of ZZ Ceti stars. Aims: We present a study of the impact of the chemical structure built up during the TP-AGB evolution on the stellar parameters inferred from asteroseismological fits of ZZ Ceti stars. Methods: Our analysis is based on a set of carbon-oxygen core white dwarf models with masses from 0.534 to 0.6463 M⊙ derived from full evolutionary computations from the ZAMS to the ZZ Ceti domain. We computed evolutionary sequences that experience different number of thermal pulses (TP). Results: We find that the occurrence or not of thermal pulses during AGB evolution implies an average deviation in the asteroseimological effective temperature of ZZ Ceti stars of at most 8% and on the order of ≲5% in the stellar mass. For the mass of the hydrogen envelope, however, we find deviations up to 2 orders of magnitude in the case of cool ZZ Ceti stars. Hot and intermediate temperature ZZ Ceti stars show no differences in the hydrogen envelope mass in most cases. Conclusions: Our results show that, in general, the impact of the occurrence or not of thermal pulses in the progenitor stars is not negligible and must be taken into account in asteroseismological studies of ZZ Ceti stars.

  8. CRYSTAL CHEMISTRY OF THREE-COMPONENT WHITE DWARFS AND NEUTRON STAR CRUSTS: PHASE STABILITY, PHASE STRATIFICATION, AND PHYSICAL PROPERTIES

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Engstrom, T. A.; Yoder, N. C.; Crespi, V. H., E-mail: tae146@psu.edu, E-mail: ncy5007@psu.edu, E-mail: vhc2@psu.edu

    A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are “bred” by a genetic algorithm and optimized at constant pressure under the assumption of linear response (Thomas–Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the T = 0 bulk phase diagrams, five of which are complicated multinary structures not previously predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferencesmore » of systems with pairwise Yukawa interactions, including and extending to the regime of low-density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase-layering diagram and mass–radius-composition dependence, both of which are reported for He–C–O and C–O–Ne white dwarfs. Finite thickness interfacial phases (“interphases”) show up at the boundaries between single-component body-centered cubic (bcc) crystalline regions, some of which have lower lattice symmetry than cubic. A second application—quasi-static settling of heavy nuclei in white dwarfs—builds on our equilibrium phase-layering method. Tests of this nonequilibrium method reveal extra phases that play the role of transient host phases for the settling species.« less

  9. The white dwarfs within 25 pc of the Sun: Kinematics and spectroscopic subtypes

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Sion, Edward M.; McCook, George P.; Wasatonic, Richard

    2014-06-01

    We present the fractional distribution of spectroscopic subtypes, range and distribution of surface temperatures, and kinematical properties of the white dwarfs (WDs) within 25 pc of the Sun. There is no convincing evidence of halo WDs in the total 25 pc sample of 224 WDs. There is also little to suggest the presence of genuine thick disk subcomponent members within 25 pc. It appears that the entire 25 pc sample likely belongs to the thin disk. We also find no significant kinematic differences with respect to spectroscopic subtypes. The total DA to non-DA ratio of the 25 pc sample ismore » 1.8, a manifestation of deepening envelope convection, which transforms DA stars with sufficiently thin H surface layers into non-DAs. We compare this ratio with the results of other studies. We find that at least 11% of the WDs within 25 pc of the Sun (the DAZ and DZ stars) have photospheric metals that likely originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, then it suggests the possibility that a similar percentage have planets, asteroid-like bodies, or debris disks orbiting them. Our volume-limited sample reveals a pileup of DC WDs at the well-known cutoff in DQ WDs at T {sub eff} ∼ 6000 K. Mindful of small number statistics, we speculate on its possible evolutionary significance. We find that the incidence of magnetic WDs in the 25 pc sample is at least 8% in our volume-limited sample, dominated by cool WDs. We derive approximate formation rates of DB and DQ degenerates and present a preliminary test of the evolutionary scenario that all cooling DB stars become DQ WDs via helium convective dredge-up with the diffusion tail of carbon extending upward from their cores.« less

  10. The Tübingen Model-Atom Database: A Revised Aluminum Model Atom and its Application for the Spectral Analysis of White Dwarfs

    NASA Astrophysics Data System (ADS)

    Löbling, L.

    2017-03-01

    Aluminum (Al) nucleosynthesis takes place during the asymptotic-giant-branch (AGB) phase of stellar evolution. Al abundance determinations in hot white dwarf stars provide constraints to understand this process. Precise abundance measurements require advanced non-local thermodynamic stellar-atmosphere models and reliable atomic data. In the framework of the German Astrophysical Virtual Observatory (GAVO), the Tübingen Model-Atom Database (TMAD) contains ready-to- use model atoms for elements from hydrogen to barium. A revised, elaborated Al model atom has recently been added. We present preliminary stellar-atmosphere models and emergent Al line spectra for the hot white dwarfs G191-B2B and RE 0503-289.

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

    PubMed

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

    2010-06-11

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

  12. Monitoring and modelling of white dwarfs with extremely weak magnetic fields. WD 2047+372 and WD 2359-434

    NASA Astrophysics Data System (ADS)

    Landstreet, J. D.; Bagnulo, S.; Valyavin, G.; Valeev, A. F.

    2017-11-01

    Magnetic fields are detected in a few percent of white dwarfs. The number of such magnetic white dwarfs known is now some hundreds. Fields range in strength from a few kG to several hundred MG. Almost all the known magnetic white dwarfs have a mean field modulus ≥1 MG. We are trying to fill a major gap in observational knowledge at the low field limit (≤200 kG) using circular spectro-polarimetry. In this paper we report the discovery and monitoring of strong, periodic magnetic variability in two previously discovered "super-weak field" magnetic white dwarfs, WD 2047+372 and WD 2359-434. WD 2047+372 has a mean longitudinal field that reverses between about -12 and + 15 kG, with a period of 0.243 d, while its mean field modulus appears nearly constant at 60 kG. The observations can be interpreted in terms of a dipolar field tilted with respect to the stellar rotation axis. WD 2359-434 always shows a weak positive longitudinal field with values between about 0 and + 12 kG, varying only weakly with stellar rotation, while the mean field modulus varies between about 50 and 100 kG. The rotation period is found to be 0.112 d using the variable shape of the Hα line core, consistent with available photometry. The field of this star appears to be much more complex than a dipole, and is probably not axisymmetric. Available photometry shows that WD 2359-434 is a light variable with an amplitude of only 0.005 mag; our own photometry shows that if WD 2047+372 is photometrically variable, the amplitude is below about 0.01 mag. These are the first models for magnetic white dwarfs with fields below about 100 kG based on magnetic measurements through the full stellar rotation. They reveal two very different magnetic surface configurations, and that, contrary to simple ohmic decay theory, WD 2359-434 has a much more complex surface field than the much younger WD 2047+372. Based, in part, on observations collected at the European Organisation for Astronomical Research in the

  13. Brown Dwarf Weather (Artist's Concept)

    NASA Image and Video Library

    2017-06-06

    This artist's concept shows what the weather might look like on cool star-like bodies known as brown dwarfs. These giant balls of gas start out life like stars, but lack the mass to sustain nuclear fusion at their cores, and instead, fade and cool with time. Observations from NASA's Spitzer Space Telescope suggest that most brown dwarfs are roiling with one or more planet-size storms akin to Jupiter's "Great Red Spot." https://photojournal.jpl.nasa.gov/catalog/PIA21475

  14. 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).

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

    PubMed

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

    2010-05-20

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

  16. Discovery of the Closest Hot Subdwarf Binary with White Dwarf Companion

    NASA Astrophysics Data System (ADS)

    Geier, S.; Marsh, T. R.; Dunlap, B. H.; Barlow, B. N.; Schaffenroth, V.; Ziegerer, E.; Heber, U.; Kupfer, T.; Maxted, P. F. L.; Miszalski, B.; Shporer, A.; Telting, J. H.; Ostensen, R. H.; O'Toole, S. J.; Gänsicke, B. T.; Napiwotzki, R.

    2013-01-01

    We report the discovery of an extremely close, eclipsing binary system. A white dwarf is orbited by a core He-burning compact hot subdwarf star with a period as short as ≃ 0.04987 d making this system the most compact hot subdwarf binary discovered so far. The subdwarf will start to transfer helium-rich material on short timescales of less than 50 Myr. The ignition of He-burning at the surface may trigger carbon-burning in the core although the WD is less massive than the Chandrasekhar limit (> 0.74 M⊙) making this binary a possible progenitor candidate for a supernova type Ia event.

  17. Dark-ages Reionization and Galaxy Formation Simulation - XIV. Gas accretion, cooling, and star formation in dwarf galaxies at high redshift

    NASA Astrophysics Data System (ADS)

    Qin, Yuxiang; Duffy, Alan R.; Mutch, Simon J.; Poole, Gregory B.; Geil, Paul M.; Mesinger, Andrei; Wyithe, J. Stuart B.

    2018-06-01

    We study dwarf galaxy formation at high redshift (z ≥ 5) using a suite of high-resolution, cosmological hydrodynamic simulations and a semi-analytic model (SAM). We focus on gas accretion, cooling, and star formation in this work by isolating the relevant process from reionization and supernova feedback, which will be further discussed in a companion paper. We apply the SAM to halo merger trees constructed from a collisionless N-body simulation sharing identical initial conditions to the hydrodynamic suite, and calibrate the free parameters against the stellar mass function predicted by the hydrodynamic simulations at z = 5. By making comparisons of the star formation history and gas components calculated by the two modelling techniques, we find that semi-analytic prescriptions that are commonly adopted in the literature of low-redshift galaxy formation do not accurately represent dwarf galaxy properties in the hydrodynamic simulation at earlier times. We propose three modifications to SAMs that will provide more accurate high-redshift simulations. These include (1) the halo mass and baryon fraction which are overestimated by collisionless N-body simulations; (2) the star formation efficiency which follows a different cosmic evolutionary path from the hydrodynamic simulation; and (3) the cooling rate which is not well defined for dwarf galaxies at high redshift. Accurate semi-analytic modelling of dwarf galaxy formation informed by detailed hydrodynamical modelling will facilitate reliable semi-analytic predictions over the large volumes needed for the study of reionization.

  18. Population buildup and vertical spread of dwarf mistletoe on young red and white firs in California

    Treesearch

    Robert F. Scharpf; John R. Parmeter Jr.

    1976-01-01

    Rate of population buildup of dwarf mistletoe, Arceuthobium abietinum Engelm. ex Munz., was slow in most small red firs and white firs 12 to 15 years after inoculation with the parasite. Where population buildup did occur, it remained clustered in the lower portions of tree crowns near inoculation sites. Maximum distance of vertical spread was 16...

  19. The mass spectrum of the white dwarfs in cataclysmic binaries - Supplementary computations

    NASA Astrophysics Data System (ADS)

    Ritter, H.; Ozkan, M. T.

    1986-10-01

    Numerical computations supplementing those of Ritter and Burkert (1986) for the study of observational selection in favor of massive white dwarfs among cataclysmic binaries (CBs) are presented. In particular the contribution of CBs beyond the period minimum and the influence of the inclination and of limb-darkening of the accretion disk are investigated. It is found that: (1) the main conclusions of Ritter and Burkert remain unchanged; (2) neither the inclusion of CBs containing a black-dwarf secondary nor the variation of the inclination or limb-darkening change the selection significantly; (3) in a magnitude-limited sample, about 22 percent of the ultra-short-period CBs contain a degenerate secondary; (4) the mean inclination of the accretion disk in a magnitude-limited sample is always close to 60 deg; (5) the fraction of eclipsing systems is of order 10-15 percent at m(v) = 10.0; (6) the mean inclination and the fraction of eclipsing systems increases with the limiting magnitude; and (7) the intrinsic space density of CBs is n(CB) approximately .0001-.0002/cu pc.

  20. Component masses of young, wide, non-magnetic white dwarf binaries in the Sloan Digital Sky Survey Data Release 7

    NASA Astrophysics Data System (ADS)

    Baxter, R. B.; Dobbie, P. D.; Parker, Q. A.; Casewell, S. L.; Lodieu, N.; Burleigh, M. R.; Lawrie, K. A.; Külebi, B.; Koester, D.; Holland, B. R.

    2014-06-01

    We present a spectroscopic component analysis of 18 candidate young, wide, non-magnetic, double-degenerate binaries identified from a search of the Sloan Digital Sky Survey Data Release 7 (DR7). All but two pairings are likely to be physical systems. We show SDSS J084952.47+471247.7 + SDSS J084952.87+471249.4 to be a wide DA + DB binary, only the second identified to date. Combining our measurements for the components of 16 new binaries with results for three similar, previously known systems within the DR7, we have constructed a mass distribution for the largest sample to date (38) of white dwarfs in young, wide, non-magnetic, double-degenerate pairings. This is broadly similar in form to that of the isolated field population with a substantial peak around M ˜ 0.6 M⊙. We identify an excess of ultramassive white dwarfs and attribute this to the primordial separation distribution of their progenitor systems peaking at relatively larger values and the greater expansion of their binary orbits during the final stages of stellar evolution. We exploit this mass distribution to probe the origins of unusual types of degenerates, confirming a mild preference for the progenitor systems of high-field-magnetic white dwarfs, at least within these binaries, to be associated with early-type stars. Additionally, we consider the 19 systems in the context of the stellar initial mass-final mass relation. None appear to be strongly discordant with current understanding of this relationship.

  1. Two new hot white dwarfs in a region of exceptionally low hi density

    NASA Technical Reports Server (NTRS)

    Barstow, M. A.; Wesemael, F.; Holberg, J. B.; Werner, K.; Buckley, D. A. H.; Stobie, R. S.; Fontaine, G.; Rosen, S. R.; Demers, S.; Lamontagne, R.

    1993-01-01

    We report the discovery of two hot white dwarfs which have the lowest line-of-sight neutral hydrogen column densities yet measured. The stars were found independently by the ROSAT EUV, Montreal-Cambridge-Tololo, and Edinburgh-Cape surveys. Follow-up observations made using the Voyager 2 ultraviolet spectrometer reveal strong continua shortward of the 912A Lyman limit from which we deduce that the neutral hydrogen column densities are 1.3 x 10(exp 17) and 2.0 x 10(exp 17) atoms/sq cm.

  2. Cool carbon stars in the halo and in dwarf galaxies: Hα, colours, and variability

    NASA Astrophysics Data System (ADS)

    Mauron, N.; Gigoyan, K. S.; Berlioz-Arthaud, P.; Klotz, A.

    2014-02-01

    The population of cool carbon (C) stars located far from the galactic plane is probably made of debris of small galaxies such as the Sagittarius dwarf spheroidal galaxy (Sgr), which are disrupted by the gravitational field of the Galaxy. We aim to know this population better through spectroscopy, 2MASS photometric colours, and variability data. When possible, we compared the halo results to C star populations in the Fornax dwarf spheroidal galaxy, Sgr, and the solar neighbourhood. We first present a few new discoveries of C stars in the halo and in Fornax. The number of spectra of halo C stars is now 125. Forty percent show Hα in emission. The narrow location in the JHK diagram of the halo C stars is found to differ from that of similar C stars in the above galaxies. The light curves of the Catalina and LINEAR variability databases were exploited to derive the pulsation periods of 66 halo C stars. A few supplementary periods were obtained with the TAROT telescopes. We confirm that the period distribution of the halo strongly resembles that of Fornax, and we found that it is very different from the C stars in the solar neighbourhood. There is a larger proportion of short-period Mira/SRa variables in the halo than in Sgr, but the survey for C stars in this dwarf galaxy is not complete, and the study of their variability needs to be continued to investigate the link between Sgr and the cool halo C stars. Based on observations made with the NTT and 3.6 m telescope at the European Southern Observatory (La Silla, Chile; programs 084.D-0302 and 070.D-0203), with the TAROT telescopes at La Silla and at Observatoire de la Côte d'Azur (France), and on the exploitation of the Catalina Sky Survey and the LINEAR variability databases.Appendix A is available in electronic form at http://www.aanda.org

  3. Metal Accretion onto White Dwarfs. II. A Better Approach Based on Time-Dependent Calculations in Static Models

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Dufour, P.; Chayer, P.; Dupuis, J.; Brassard, P.

    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. Inferences on the process based on diffusion timescale arguments make the implicit assumption that the concentration gradient of a given metal at the base of the convection zone is negligible. This assumption is, in fact, not rigorously valid, but it allows the decoupling of the surface abundance from the evolving distribution of a given metal in deeper layers. A better approach is a full time-dependent calculation of the evolution of the abundance profile of an accreting-diffusing element. We used the same approach as that developed by Dupuis et al. to model accretion episodes involving many more elements than those considered by these authors. Our calculations incorporate the improvements to diffusion physics mentioned in Paper I. The basic assumption in the Dupuis et al. approach is that the accreted metals are trace elements, i.e, that they have no effects on the background (DA or non-DA) stellar structure. This allows us to consider an arbitrary number of accreting elements.

  4. Down the Tubes: Vetting the Apparent Water-rich Parent Body being Accreted by the White Dwarf GD 16

    NASA Astrophysics Data System (ADS)

    Melis, Carl

    2015-10-01

    How water is distributed in a planetary system critically affects the formation, evolution, and habitability of its constituent rocky bodies. White dwarf stars provide a unique method to probe the prevalence of water-rich rocky bodies outside of our Solar system and where they preferentially reside in a planetary system. However, as evidenced by the case of GD 362, some parent bodies that at first glance might appear to be water-rich can actually be quite water-scarce. At this time there are only a small number of plausibly water-rich rocky bodies that are being actively accreted by their host white dwarf star. Given such a sample size it is crucial to characterize each one in sufficient detail to remove interlopers like GD 362 that might otherwise affect future statistical analyses. In this proposal we seek to vet GD 16, a water-rich candidate yet to be observed with HST-COS that is the brightest remaining such target in the UV.

  5. The double helium-white dwarf channel for the formation of AM CVn binaries

    NASA Astrophysics Data System (ADS)

    Zhang, Xian-Fei; Liu, Jin-Zhong; Jeffery, C. Simon; Hall, Philip D.; Bi, Shao-Lan

    2018-01-01

    Most close double helium white dwarfs will merge within a Hubble time due to orbital decay by gravitational wave radiation. However, a significant fraction with low mass ratios will survive for a long time as a consequence of stable mass transfer. Such stable mass transfer between two helium white dwarfs (HeWDs) provides one channel for the production of AM CVn binary stars. In previous calculations of double HeWD progenitors, the accreting HeWD was treated as a point mass. We have computed the evolution of 16 double HeWD models in order to investigate the consequences of treating the evolution of both components in detail. We find that the boundary between binaries having stable and unstable mass transfer is slightly modified by this approach. By comparing with observed periods and mass ratios, we redetermine masses of eight known AM CVn stars by our double HeWDs channel, i.e. HM Cnc, AM CVn, V406 Hya, J0926, J1240, GP Com, Gaia14aae and V396 Hya.We propose that central spikes in the triple-peaked emission spectra of J1240, GP Com and V396 Hya and the surface abundance ratios of N/C/O in GP Com can be explained by the stable double HeWD channel. The mass estimates derived from our calculations are used to discuss the predicted gravitational wave signal in the context of the Laser Interferometer Space Antenna (LISA) project.

  6. KOI-256's Magnetic Activity Under the Influence of the White Dwarf

    NASA Astrophysics Data System (ADS)

    Yoldaş, Ezgi; Dal, Hasan Ali

    2017-11-01

    We present the findings about chromospheric activity nature of KOI-256 obtained from the Kepler Mission data. First, it was found that there are some sinusoidal variations out-of-eclipses due to cool spot activity. The sinusoidal variations modelled by the spotmodel program indicate that the active component has two different active regions. Their longitudinal variation revealed that one of them has a migration period of 3.95 yrs, while the other has a migration period of 8.37 yrs. Second, 225 flares were detected from the short cadence data in total. The parameters, such as increase (T r) and decay (T d) times, total flare time (T t), equivalent durations (P), were calculated for each flare. The distribution of equivalent durations versus total flare times in logarithmic scale is modelled to find flare activity level. The Plateau value known as the saturation level of the active component was calculated to be 2.3121 ± 0.0964 s, and the Half-life value, which is required flare total time to reach the saturation, was computed to be 2233.6 s. In addition, the frequency of N 1, which is the number of flares per an hour in the system, was found to be 0.05087 h-1, while the flare frequency N 2 that the flare-equivalent duration emitting per an hour was found to be 0.00051. Contrary to the spot activity, it has been found that the flares are in tends to appear at specific phases due to the white dwarf component.

  7. NLTE Line Blanketed Model Atmospheres for Hot, Metal-rich White Dwarfs

    NASA Astrophysics Data System (ADS)

    Hubeny, I.; Lanz, T.

    1993-05-01

    Recent observations of some hot DA white dwarfs (Feige 24, G191 B2B - Sion et al. 1992, Ap.J. 391, L29; Vennes et al. 1992, Ap.J. 392, L27) and subsequent analyses have demonstrated that their atmospheres are contaminated with heavy metal species with appreciable abundances. So far, modeling was mostly limited to calculating synthetic spectra (usually in LTE), based on previously calculated model atmospheres that were constructed assuming a simplified chemical composition. However, in reality the metal lines may also significantly influence the temperature structure, and consequently the ionization balance of some important species. Since the effective temperature is rather high (55000 to 60000 K), the NLTE effects may be important despite the high gravity. Therefore, in order to settle the question on metal abundances of hot DA white dwarfs, we need to calculate NLTE fully line blanketed model atmospheres. Using our previously developed hybrid complete linearization/accelerated lambda iteration method, we have calculated a set of NLTE models including H, He, C, N, O, and Fe, with some 13000 lines of Fe IV, Fe V and Fe VI taken into account explicitly in model construction. The NLTE departure coefficients for all levels of Fe IV, Fe V, and Fe VI are then employed in the spectrum synthesis program SYNSPEC. In the first part of this study, we present a theoretical comparison between models computed with adding more and more opacity sources, and show how the temperature structure and synthetic spectra develop. We also discuss possible errors arising from using an inconsistent spectrum synthesis (the metals considered only in the spectrum synthesis, not in the model construction). In the second part, we compare the computed UV spectra with with available observations of Feige 24 and G191 B2B, and deduce limits for the iron abundance of these stars.

  8. Ultraviolet Spectra of Two Magnetic White Dwarfs and Ultraviolet Spectra of Subluminous Objects Found in the Kiso Schmidt Survey and Ultraviolet Absorptions in the Spectra of DA White Dwarfds

    NASA Technical Reports Server (NTRS)

    Wegner, Gary A.

    1988-01-01

    Research under NASA Grant NAG5-287 has carried out a number of projects in conjunction with the International Ultraviolet Explorer (IUE) satellite. These include: (1) studies of the UV spectra of DA white dwarfs which show quasi-molecular bands of H2 and H2(+); (2) the peculiar star HR6560; (3) the UV spectra of two magnetic white dwarfs that also show the quasi-molecular features; (4) investigations of the UV spectra of subluminous stars, primarily identified from visual wavelength spectroscopy in the Kiso survey of UV excess stars, some of which show interesting metal lines in their UV spectra; and (5) completion of studies of UV spectra of DB stars. The main result of this research has been to further knowledge of the structure and compositions of subluminous stars which helps cast light on their formation and evolution.

  9. A dearth of small particles in the transiting material around the white dwarf WD 1145+017

    NASA Astrophysics Data System (ADS)

    Xu, S.; Rappaport, S.; van Lieshout, R.; Vanderburg, A.; Gary, B.; Hallakoun, N.; Ivanov, V. D.; Wyatt, M. C.; DeVore, J.; Bayliss, D.; Bento, J.; Bieryla, A.; Cameron, A.; Cann, J. M.; Croll, B.; Collins, K. A.; Dalba, P. A.; Debes, J.; Doyle, D.; Dufour, P.; Ely, J.; Espinoza, N.; Joner, M. D.; Jura, M.; Kaye, T.; McClain, J. L.; Muirhead, P.; Palle, E.; Panka, P. A.; Provencal, J.; Randall, S.; Rodriguez, J. E.; Scarborough, J.; Sefako, R.; Shporer, A.; Strickland, W.; Zhou, G.; Zuckerman, B.

    2018-03-01

    White dwarf WD 1145+017 is orbited by several clouds of dust, possibly emanating from actively disintegrating bodies. These dust clouds reveal themselves through deep, broad, and evolving transits in the star's light curve. Here, we report two epochs of multiwavelength photometric observations of WD 1145+017, including several filters in the optical, Ks and 4.5 μm bands in 2016 and 2017. The observed transit depths are different at these wavelengths. However, after correcting for excess dust emission at Ks and 4.5 μm, we find the transit depths for the white dwarf itself are the same at all wavelengths, at least to within the observational uncertainties of ˜5-10 per cent. From this surprising result, and under the assumption of low optical depth dust clouds, we conclude that there is a deficit of small particles (with radii s ≲ 1.5 μm) in the transiting material. We propose a model wherein only large particles can survive the high equilibrium temperature environment corresponding to 4.5 h orbital periods around WD 1145+017, while small particles sublimate rapidly. In addition, we evaluate dust models that are permitted by our measurements of infrared emission.

  10. Assessing the dependency of the fine structure constant on gravity using hot DA white dwarfs

    NASA Astrophysics Data System (ADS)

    Barstow, Martin

    2016-10-01

    Variation of fundamental constants is a common theme of many theories of quantum gravity and Grand Unification. Using spectra obtained with the Hubble Space Telescope, it has been shown by Berengut et al. (2013), and Bagdonaite et al. (2014), that it is possible to place strong constraints on gravitational variations of the fine structure constant (alpha), and the proton to electron mass ratio (mu) in white dwarf stars.As part of the UV initiative, we propose to observe four hot DA white dwarf stars using STIS with the E140H grating, totalling 12 orbits. These four stars have been chosen so as to have a wide range of masses, allowing a full exploration of the compactness parameter space (M/R). We will measure several absorption features of Fe V and Ni V, and extract any potential variation in alpha in a manner similar to Berengut et al. (2013).This proposal will be a significant advance in the effort to detect gravitational variations in alpha. A confirmed detection of alpha variation would have extensive consequences for fundamental physics, cosmology, and would also signal the breakdown of Einstein's Equivalence principle, and hence, general relativity. Furthermore, a null detection would also allow strong limits to be placed on any potential alpha variation in a strong gravitational field.

  11. Formation of Millisecond Pulsars with Heavy White Dwarf Companions: Extreme Mass Transfer on Subthermal Timescales.

    PubMed

    Tauris; van Den Heuvel EP; Savonije

    2000-02-20

    We have performed detailed numerical calculations of the nonconservative evolution of close X-ray binary systems with intermediate-mass (2.0-6.0 M middle dot in circle) donor stars and a 1.3 M middle dot in circle accreting neutron star. We calculated the thermal response of the donor star to mass loss in order to determine its stability and follow the evolution of the mass transfer. Under the assumption of the "isotropic reemission model," we demonstrate that in many cases it is possible for the binary to prevent a spiral-in and survive a highly super-Eddington mass transfer phase (1white dwarfs and relatively short orbital periods (3-50 days). However, we conclude that to produce a binary pulsar with a O-Ne-Mg white dwarf or Porb approximately 1 day (e.g., PSR B0655+64) the above scenario does not work, and a spiral-in phase is still considered the most plausible scenario for the formation of such a system.

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

    PubMed

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

    2006-08-03

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

  13. SGR 0418+5729, Swift J1822.3-1606, and 1E 2259+586 as massive, fast-rotating, highly magnetized white dwarfs

    NASA Astrophysics Data System (ADS)

    Boshkayev, K.; Izzo, L.; Rueda Hernandez, J. A.; Ruffini, R.

    2013-07-01

    Aims: We describe the so-called low magnetic field magnetars, SGR 0418+5729, Swift J1822.3-1606, and the AXP prototype 1E 2259+586 as massive, fast-rotating, highly magnetized white dwarfs. Methods: We give bounds for the mass, radius, moment of inertia, and magnetic field for these sources by requesting the stability of realistic, general relativistic, uniformly rotating white dwarfs. We also present the theoretical expectation of the infrared, optical, and ultraviolet emission of these objects and show their consistency with the current available observational data. Results: We improve the theoretical prediction of the lower limit of the spindown rate of SGR 0418+5729; for a white dwarf close to its maximum stable mass we obtain the very stringent interval for the spindown rate of 4.1 × 10-16 < Ṗ < 6 × 10-15, where the upper value is the known observational limit. A lower limit has been also set for Swift J1822.3-1606, whose spindown rate is not yet fully confirmed. Our model provides for the source Ṗ ≥ 2.13 × 10-15 if the star is close to its maximum stable mass. We give in addition the frequencies at which absorption features could be present in the spectrum of these sources as the result of the scattering of photons with the quantized electrons by the surface magnetic field.

  14. New Y and T Dwarfs from WISE Identified by Methane Imaging

    NASA Astrophysics Data System (ADS)

    Tinney, C. G.; Kirkpatrick, J. Davy; Faherty, Jacqueline K.; Mace, Gregory N.; Cushing, Mike; Gelino, Christopher R.; Burgasser, Adam J.; Sheppard, Scott S.; Wright, Edward L.

    2018-06-01

    We identify new Y- and T-type brown dwarfs from the WISE All Sky data release using images obtained in filters that divide the traditional near-infrared H and J bands into two halves—specifically {CH}}4{{s}} and CH4l in the H and J2, and J3 in the J. This proves to be very effective at identifying cool brown dwarfs via the detection of their methane absorption, as well as providing preliminary classification using methane colors and WISE -to-near-infrared colors. New and updated calibrations between T/Y spectral types and {CH}}4{{s}}–CH4l J3–W2, and {CH}}4{{s}}–W2 colors are derived, producing classification estimates good to a few spectral sub-types. We present photometry for a large sample of T and Y dwarfs in these filters, together with spectroscopy for 23 new ultra-cool dwarfs—2 Y dwarfs and 21 T dwarfs. We identify a further 8 new cool brown dwarfs, which we have high confidence are T dwarfs based on their methane photometry. We find that, for objects observed on a 4 m class telescope at J-band magnitudes of ∼20 or brighter, {CH}}4{{s}}–CH4l is the more powerful color for detecting objects and then estimating spectral types. Due to the lower sky background in the J-band, the J3 and J2 bands are more useful for identifying fainter cool dwarfs at J ≳ 22. The J3–J2 color is poor at estimating spectral types. But fortunately, once J3–J2 confirms that an object is a cool dwarf, the J3–W2 color is very effective at estimating approximate spectral types.

  15. The rotationally modulated Zeeman spectrum at nearly 10 to the 9th Gauss of the white dwarf PG 1031 + 234

    NASA Technical Reports Server (NTRS)

    Latter, William B.; Schmidt, Gary D.; Green, Richard F.

    1987-01-01

    Detailed analyses are performed of high-quality, phase-resolved CCD spectroscopy of the absorption-line spectrum throughout its rotation period of the new white dwarf PG 1031 + 234. The spectral variations are discussed and compared with new theoretical calculations of the behavior of hydrogen in strong magnetic fields. This analysis is then extended through a modeling procedure which produces a synthetic magnetically distorted spectrum for a star of arbitrary field strength and structure. The results confirm that PG 1031 + 234 possesses the strongest field yet detected on a white dwarf, with regions on the surface spanning the range of about 200 to nearly 1000 MG. The spectroscopic data reflect a field pattern containing a slightly offset global component of polar field strength of about 500 MG together with a localized magnetic 'spot' whose central field approaches 2000 MG.

  16. On the possibility of detecting weak magnetic fields in variable white dwarfs

    NASA Technical Reports Server (NTRS)

    Jones, Philip W.; Hansen, Carl J.; Pesnell, W. Dean; Kawaler, Steven D.

    1989-01-01

    It is suggested that 'weak' magnetic fields of strengths less than 10 to the 6th G may be detectable in some variable white dwarfs. Weak fields can cause subtle changes in the Fourier power spectra of these stars in the form of 'splitting' in frequency of otherwise degenerate signals. Present-day observational and analysis techniques are capable of detecting these changes. It is suggested suggested, by listing some well-studied candidate stars, that perhaps the magnetic signature of splitting has already been observed in at least one object and that the difficult task of intensive measurements of weak fields should now be undertaken of those candidates.

  17. Limits on a gravitational field dependence of the proton-electron mass ratio from H2 in white dwarf stars.

    PubMed

    Bagdonaite, J; Salumbides, E J; Preval, S P; Barstow, M A; Barrow, J D; Murphy, M T; Ubachs, W

    2014-09-19

    Spectra of molecular hydrogen (H2) are employed to search for a possible proton-to-electron mass ratio (μ) dependence on gravity. The Lyman transitions of H2, observed with the Hubble Space Telescope towards white dwarf stars that underwent a gravitational collapse, are compared to accurate laboratory spectra taking into account the high temperature conditions (T∼13 000  K) of their photospheres. We derive sensitivity coefficients Ki which define how the individual H2 transitions shift due to μ dependence. The spectrum of white dwarf star GD133 yields a Δμ/μ constraint of (-2.7±4.7stat±0.2syst)×10(-5) for a local environment of a gravitational potential ϕ∼10(4) ϕEarth, while that of G29-38 yields Δμ/μ=(-5.8±3.8stat±0.3syst)×10(-5) for a potential of 2×10(4) ϕEarth.

  18. Hot DA white dwarf model atmosphere calculations: including improved Ni PI cross-sections

    NASA Astrophysics Data System (ADS)

    Preval, S. P.; Barstow, M. A.; Badnell, N. R.; Hubeny, I.; Holberg, J. B.

    2017-02-01

    To calculate realistic models of objects with Ni in their atmospheres, accurate atomic data for the relevant ionization stages need to be included in model atmosphere calculations. In the context of white dwarf stars, we investigate the effect of changing the Ni IV-VI bound-bound and bound-free atomic data on model atmosphere calculations. Models including photoionization cross-section (PICS) calculated with AUTOSTRUCTURE show significant flux attenuation of up to ˜80 per cent shortward of 180 Å in the extreme ultraviolet (EUV) region compared to a model using hydrogenic PICS. Comparatively, models including a larger set of Ni transitions left the EUV, UV, and optical continua unaffected. We use models calculated with permutations of these atomic data to test for potential changes to measured metal abundances of the hot DA white dwarf G191-B2B. Models including AUTOSTRUCTURE PICS were found to change the abundances of N and O by as much as ˜22 per cent compared to models using hydrogenic PICS, but heavier species were relatively unaffected. Models including AUTOSTRUCTURE PICS caused the abundances of N/O IV and V to diverge. This is because the increased opacity in the AUTOSTRUCTURE PICS model causes these charge states to form higher in the atmosphere, more so for N/O V. Models using an extended line list caused significant changes to the Ni IV-V abundances. While both PICS and an extended line list cause changes in both synthetic spectra and measured abundances, the biggest changes are caused by using AUTOSTRUCTURE PICS for Ni.

  19. Outbursts by low-mass white dwarfs in symbiotic variables

    NASA Technical Reports Server (NTRS)

    Sion, Edward M.; Ready, Christian J.

    1992-01-01

    The high-resolution IUE spectra of the symbiotic variables BF Cygni and EG Andromedae are studied in order to describe the P Cygni-like features of these objects. The 10 high-dispersion IUE spectra are examined for orbital phase-dependent variations in the C IV resonance doublet in terms of velocity and/or structure. One image is found to have a strong He-II absorption feature that coincides in velocity with the C-IV absorption component in P Cygni. The absorbing material for both lines is related to outflow and P Cygni self-absorption near the hot component. The P Cygni profiles do not appear to be related to a red-giant wind nor an expanding circumbinary shell in the in both BF Cyg and EG And. Quasi-static evolutionary model calculations demonstrate an unexpected outburst behavior in response to the assumed accretion. These data are shown to be important for the study of symbiotic systems that contain low-mass white dwarfs.

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

    NASA Technical Reports Server (NTRS)

    Keeley, D. A.

    1979-01-01

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

  1. Discovery of White Dwarfs in the Globular Clusters M13 and M22 Using HST ACS Photometric Data

    NASA Astrophysics Data System (ADS)

    Cho, Dong-Hwan; Yoon, Tae Seog; Lee, Sang-Gak; Sung, Hyun-Il

    2015-12-01

    A search for hot and bright white dwarfs (WDs) in the Milky Way globular clusters M13 (NGC 6205) and M22 (NGC 6656) is carried out using the deep and homogeneous VI photometric catalog of Anderson et al. and and Sarajedini et al., based on data taken with the ACS/WFC aboard the Hubble Space Telescope (HST). V versus V-I color-magnitude diagrams (CMDs) of M13 and M22 are constructed and numerous spurious detections are rejected according to their photometric quality parameters qfit(V) and qfit(I). In the case of M13, further radial restriction is applied to reject central stars with higher photometric errors due to central crowding. From each resultant V versus V-I CMD, sixteen and thirteen WD candidates are identified in M13 and M22, respectively. They are identified as stellar objects in the accompanying ACS/WFC images and are found to be randomly distributed across the central regions of M13 and M22. Their positions in the CMDs are in the bright part of the DA WD cooling sequences indicating that they are true WDs. In order to confirm their nature, follow-up spectroscopic observations are needed.

  2. Radio Observations as a Tool to Investigate Shocks and Asymmetries in Accreting White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Weston, Jennifer Helen Seng; E-Nova Project

    2017-01-01

    In this dissertation, I use radio observations with the Karl G. Jansky Very Large Array (VLA) to reveal that colliding flows within the ejecta from nova explosions can lead to shocks that accelerate particles and produce radio synchrotron emission. In both novae V1723 Aql and V5589 Sgr, radio emission within the first one to two months deviated strongly from the classic thermal model for radio emission from novae. Three years of radio observations of V1723 Aql show that multiple outflows from the system collided to create non-thermal shocks with a brightness temperature of >106 K. After these shocks faded, the radio light curve became roughly consistent with an expanding thermal shell. However, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months. In the case of nova V5589 Sgr, I show that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTx > 33 keV) X-rays. These findings have important implications for understanding how normal novae generate GeV gamma-rays.Additionally, I present VLA observations of the symbiotic star CH Cyg and two small surveys of symbiotic binaries. Radio observations of CH Cyg tie the ejection of a collimated jet to a change of state in the accretion disk, strengthening the link between bipolar outflows from accreting white dwarfs and other types of accreting compact objects. Next, I use a survey of eleven accretion-driven symbiotic binaries to determine that the radio brightness of a symbiotic system could potentially be used as an indicator of whether it is powered predominantly by shell burning on the surface of the white dwarf or by accretion. This survey also produces the first radio detections of seven of the target systems. In the second survey of seventeen symbiotic binaries, I spatially resolve extended radio emission in several systems for the first time. The results from these surveys provide some support for the

  3. A BAYESIAN APPROACH TO DERIVING AGES OF INDIVIDUAL FIELD WHITE DWARFS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    O'Malley, Erin M.; Von Hippel, Ted; Van Dyk, David A., E-mail: ted.vonhippel@erau.edu, E-mail: dvandyke@imperial.ac.uk

    2013-09-20

    We apply a self-consistent and robust Bayesian statistical approach to determine the ages, distances, and zero-age main sequence (ZAMS) masses of 28 field DA white dwarfs (WDs) with ages of approximately 4-8 Gyr. Our technique requires only quality optical and near-infrared photometry to derive ages with <15% uncertainties, generally with little sensitivity to our choice of modern initial-final mass relation. We find that age, distance, and ZAMS mass are correlated in a manner that is too complex to be captured by traditional error propagation techniques. We further find that the posterior distributions of age are often asymmetric, indicating that themore » standard approach to deriving WD ages can yield misleading results.« less

  4. AN EMPIRICAL CALIBRATION TO ESTIMATE COOL DWARF FUNDAMENTAL PARAMETERS FROM H-BAND SPECTRA

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Newton, Elisabeth R.; Charbonneau, David; Irwin, Jonathan

    Interferometric radius measurements provide a direct probe of the fundamental parameters of M dwarfs. However, interferometry is within reach for only a limited sample of nearby, bright stars. We use interferometrically measured radii, bolometric luminosities, and effective temperatures to develop new empirical calibrations based on low-resolution, near-infrared spectra. We find that H-band Mg and Al spectral features are good tracers of stellar properties, and derive functions that relate effective temperature, radius, and log luminosity to these features. The standard deviations in the residuals of our best fits are, respectively, 73 K, 0.027 R {sub ☉}, and 0.049 dex (an 11% error on luminosity).more » Our calibrations are valid from mid K to mid M dwarf stars, roughly corresponding to temperatures between 3100 and 4800 K. We apply our H-band relationships to M dwarfs targeted by the MEarth transiting planet survey and to the cool Kepler Objects of Interest (KOIs). We present spectral measurements and estimated stellar parameters for these stars. Parallaxes are also available for many of the MEarth targets, allowing us to independently validate our calibrations by demonstrating a clear relationship between our inferred parameters and the stars' absolute K magnitudes. We identify objects with magnitudes that are too bright for their inferred luminosities as candidate multiple systems. We also use our estimated luminosities to address the applicability of near-infrared metallicity calibrations to mid and late M dwarfs. The temperatures we infer for the KOIs agree remarkably well with those from the literature; however, our stellar radii are systematically larger than those presented in previous works that derive radii from model isochrones. This results in a mean planet radius that is 15% larger than one would infer using the stellar properties from recent catalogs. Our results confirm the derived parameters from previous in-depth studies of KOIs 961

  5. DETECTION OF WHITE DWARF COMPANIONS TO BLUE STRAGGLERS IN THE OPEN CLUSTER NGC 188: DIRECT EVIDENCE FOR RECENT MASS TRANSFER

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Gosnell, Natalie M.; Mathieu, Robert D.; Geller, Aaron M.

    2014-03-01

    Several possible formation pathways for blue straggler stars have been developed recently, but no one pathway has yet been observationally confirmed for a specific blue straggler. Here we report the first findings from a Hubble Space Telescope Advanced Camera for Surveys/Solar Blind Channel far-UV photometric program to search for white dwarf companions to blue straggler stars. We find three hot and young white dwarf companions to blue straggler stars in the 7 Gyr open cluster NGC 188, indicating that mass transfer in these systems ended less than 300 Myr ago. These companions are direct and secure observational evidence that these blue straggler starsmore » were formed through mass transfer in binary stars. Their existence in a well-studied cluster environment allows for observational constraints of both the current binary system and the progenitor binary system, mapping the entire mass transfer history.« less

  6. IUE observations of a hot DAO white dwarf: Implications for diffusion theory and photospheric stratification

    NASA Technical Reports Server (NTRS)

    Holberg, J. B.; Sion, E. M.; Liebert, J.; Vauclair, Gerard

    1988-01-01

    Observations of the DAO white dwarf PG1210+533, including the first high dispersion spectrum of a hybrid H-He object of this nature were obtained by IUE. In contrast with hot DAs in the 50,000 K temperature range, PG1210+533 shows no narrow interstellar-like metal lines, in spite of an optically observed He/H abundance of 0.1. This lack of metal makes accretion from the ISM an unlikely source for the He in the PG1210+533 photosphere. A significant discovery in the high dispersion spectrum is the existence of a sharp, non-LTE like, core seen in the He II 1640 line. Such features are detected in DO white dwarfs. A small aperture SWP low dispersion observation reveals the Lyman alpha profile of PG1210+533 to be surprisingly weak and narrow. Fits of this profile using pure H models yielded a T(eff) = 56,000 K. Fits of the Balmer H gamma profile however, yield T(eff) = 42,300 K and log g = 8.5 + or - 0.5 for the same models. It is unlikely that homogeneously mixed H-He atmospheres can resolve the inconsistency between the Lyman alpha and H gamma features in this star. Stratified models involving thin H photospheres may be necessary to explain these results.

  7. Non-LTE line-blanketed model atmospheres of hot stars. 2: Hot, metal-rich white dwarfs

    NASA Technical Reports Server (NTRS)

    Lanz, T.; Hubeny, I.

    1995-01-01

    We present several model atmospheres for a typical hot metal-rich DA white dwarf, T(sub eff) = 60,000 K, log g = 7.5. We consider pure hydrogen models, as well as models with various abundances of two typical 'trace' elements-carbon and iron. We calculte a number of Local Thermodynamic Equilibrium (LTE) and non-LTE models, taking into account the effect of numerous lines of these elements on the atmospheric structure. We demostrate that while the non-LTE effects are notvery significant for pure hydrogen models, except for describing correctly the central emission in H-alpha they are essential for predicting correctly the ionization balance of metals, such as carbon and iron. Previously reported discrepancies in LTE abundances determinations using C III and C IV lines are easily explained by non-LTE effects. We show that if the iron abundance is larger than 10(exp -5), the iron line opacity has to be considered not only for the spectrum synthesis, but also in the model construction itself. For such metal abundances, non-LTE metal line-blanketed models are needed for detailed abundance studies of hot, metal-rich white dwarfs. We also discuss the predicted Extreme Ultraviolet (EUV) spectrum and show that it is very sensitive to metal abundances, as well as to non-LTE effects.

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  9. The separation distribution and merger rate of double white dwarfs: improved constraints

    NASA Astrophysics Data System (ADS)

    Maoz, Dan; Hallakoun, Na'ama; Badenes, Carles

    2018-05-01

    We obtain new and precise information on the double white dwarf (DWD) population and on its gravitational-wave-driven merger rate by combining the constraints on the DWD population from two previous studies on radial velocity variation. One of the studies is based on a sample of white dwarfs (WDs) from the Sloan Digital Sky Survey (SDSS, which with its low spectral resolution probes systems at separations a < 0.05 au) and the other is based on the ESO-VLT Supernova-Ia Progenitor surveY (SPY, which with its high spectral resolution is sensitive to a < 4 au). From a joint likelihood analysis, the DWD fraction among WDs is fbin = 0.095 ± 0.020 (1σ, random) +0.010 (systematic) in the separation range ≲4 au. The index of a power-law distribution of initial WD separations (at the start of solely gravitational-wave-driven binary evolution), N(a)da ∝ aαda, is α = -1.30 ± 0.15 (1σ) +0.05 (systematic). The Galactic WD merger rate per WD is Rmerge = (9.7 ± 1.1) × 10-12 yr-1. Integrated over the Galaxy lifetime, this implies that 8.5-11 per cent of all WDs ever formed have merged with another WD. If most DWD mergers end as more-massive WDs, then some 10 per cent of WDs are DWD-merger products, consistent with the observed fraction of WDs in a `high-mass bump' in the WD mass function. The DWD merger rate is 4.5-7 times the Milky Way's specific Type Ia supernova (SN Ia) rate. If most SN Ia explosions stem from the mergers of some DWDs (say, those with massive-enough binary components) then ˜15 per cent of all WD mergers must lead to a SN Ia.

  10. An asteroseismic constraint on the mass of the axion from the period drift of the pulsating DA white dwarf star L19-2

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Córsico, Alejandro H.; Althaus, Leandro G.; Bertolami, Marcelo M. Miller

    We employ an asteroseismic model of L19-2, a relatively massive ( M {sub *} ∼ 0.75 M {sub ⊙}) and hot ( T {sub eff} ∼ 12 100 K) pulsating DA (H-rich atmosphere) white dwarf star (DAV or ZZ Ceti variable), and use the observed values of the temporal rates of period change of its dominant pulsation modes (Π ∼ 113 s and Π ∼ 192 s), to derive a new constraint on the mass of the axion, the hypothetical non-barionic particle considered as a possible component of the dark matter of the Universe. If the asteroseismic model employed ismore » an accurate representation of L19-2, then our results indicate hints of extra cooling in this star, compatible with emission of axions of mass m {sub a} cos{sup 2}β ∼< 25 meV or an axion-electron coupling constant of g {sub ae} ∼< 7 × 10{sup −13}.« less

  11. Collecting Brown Dwarfs in the Night Sky

    NASA Image and Video Library

    2010-11-09

    The green dot in the middle of this image might look like an emerald amidst glittering diamonds, but is a dim star belonging to a class called brown dwarfs; it is the first ultra-cool brown dwarf discovered by NASA Wide-field Infrared Survey Explorer.

  12. A NICMOS direct imaging search for giant planets around the seven single white dwarfs in the Hyades

    NASA Astrophysics Data System (ADS)

    Zinnecker, Hans

    2003-07-01

    We propose to use the NIC1 camera on HST to search for massive giant planets around the known seven single white dwarfs in the nearby Hyades cluster at sub-arcsec separations. At an age of 625 Myr, the white dwarfs had protogenitor masses of about 3 solar masses, and massive gaseous giant planets should have formed in the massive circumstellar disks around these ex Herbig A0 stars, probably at orbital separations similar or slightly larger than that of Jupiter {5 AU} in our own solar system. Such planets would have survived the post-Main Sequence mass loss of the parent star, and would have migrated outward adiabatically by a factor 4.5, equal to the ratio of initial to final stellar mass {3Mo/0.66Mo}, due to conservation of orbital angular momentum during the mass loss {AGB and PN} phase. Thus the orbital separation NOW would be 4.5 x 5 AU = 22.5 AU, which at the distance of the Hyades {45 pc} corresponds to 0.50 arcsec. Simulations with TinyTim then show that giant planets at this separation with masses in the range 6-12 Jupiter masses and apparent J and H magnitudes in the range 20.5-23.3 mag {from Baraffe or Burrows models} can be spatially resolved around the Hyades white dwarfs. Their J and H brightnesses are known to be 15 +/- 0.5 mag, implying a median star-planet brightness ratio of 1000:1 {7.5 mag}. This combination of dynamic range and orbital separation is observable with NICMOS, by subtracting images taken at two roll angles. Therefore, the proposed near-IR diffraction-limited observations in the F110W and F160W filters promise to resolve giant planets around low-mass stars for the first time. If successful, the observations would also prove that giant planets do form around early-type stars more massive than the Sun.

  13. Evidence for a cool wind from the K2 dwarf in the detached binary V471 Tauri

    NASA Technical Reports Server (NTRS)

    Mullan, D. J.; Sion, E. M.; Bruhweiler, F. C.; Carpenter, K. G.

    1989-01-01

    Evidence for mass loss from the K2 dwarf in V471 Tauri is found in the form of discrete absorption features in lines of various elements (Mg, Fe, Cr, Mn) and ionization stages (Mg I, Mg II, Fe I, Fe II). Resonant Mg II absorption indicates a mass loss rate of at least 10 to the -11th solar masses per year. The wind appears to be cool (no more than a few times 10,000 K).

  14. The OmegaWhite Survey for short-period variable stars - IV. Discovery of the warm DQ white dwarf OW J175358.85-310728.9

    NASA Astrophysics Data System (ADS)

    Macfarlane, S. A.; Woudt, P. A.; Dufour, P.; Ramsay, G.; Groot, P. J.; Toma, R.; Warner, B.; Paterson, K.; Kupfer, T.; van Roestel, J.; Berdnikov, L.; Dagne, T.; Hardy, F.

    2017-09-01

    We present the discovery and follow-up observations of the second known variable warm DQ white dwarf OW J175358.85-310728.9 (OW J1753-3107). OW J1753-3107 is the brightest of any of the currently known warm or hot DQ and was discovered in the OmegaWhite Survey as exhibiting optical variations on a period of 35.5452 (2) min, with no evidence for other periods in its light curves. This period has remained constant over the last 2 yr and a single-period sinusoidal model provides a good fit for all follow-up light curves. The spectrum consists of a very blue continuum with strong absorption lines of neutral and ionized carbon, a broad He I λ4471 line and possibly weaker hydrogen lines. The C I lines are Zeeman split, and indicate the presence of a strong magnetic field. Using spectral Paschen-Back model descriptions, we determine that OW J1753-3107 exhibits the following physical parameters: Teff = 15 430 K, log (g) = 9.0, log (N(C)/N(He)) = -1.2 and the mean magnetic field strength is Bz =2.1 MG. This relatively low temperature and carbon abundance (compared to the expected properties of hot DQs) is similar to that seen in the other warm DQ SDSS J1036+6522. Although OW J1753-3107 appears to be a twin of SDSS J1036+6522, it exhibits a modulation on a period slightly longer than the dominant period in SDSS J1036+6522 and has a higher carbon abundance. The source of variations is uncertain, but they are believed to originate from the rotation of the magnetic white dwarf.

  15. Spectral and Timing Investigations of Dwarf Novae Selected in Hard X-Rays

    NASA Technical Reports Server (NTRS)

    Thorstensen, John; Remillard, Ronald A.

    2000-01-01

    There are 9 dwarf novae (DN) among the 43 cataclysmic variables (accreting white dwarfs in close binary systems) that were detected during the HEAO-1 all-sky X-ray survey (1977-1979). On the other hand, there are roughly one hundred dwarf novae that are closer and/or optically brighter and yet they were not detected as hard X-ray sources. Two of the HEAO-1 DN show evidence for X-ray pulsations that imply strong magnetic fields on the white dwarf surface, and magnetic CVs are known to be strong X-ray sources. However, substantial flux in hard X-rays may be caused by non-magnetic effects, such as an optically thin boundary layer near a massive white dwarf. We proposed RXTE observations to measure plasma temperatures and to search for X-ray pulsations. The observations would distinguish whether these DN belong to one of (rare) magnetic subclasses. For those that do not show pulsations, the observations support efforts to define empirical relations between X-ray temperature, the accretion rate, and the mass of the white dwarf. The latter is determined via optical studies of the dynamics of the binary constituents.

  16. Post-main-sequence Evolution of Icy Minor Planets. II. Water Retention and White Dwarf Pollution around Massive Progenitor Stars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Malamud, Uri; Perets, Hagai B., E-mail: uri.mal@tx.technion.ac.il, E-mail: hperets@physics.technion.ac.il

    Most studies suggest that the pollution of white dwarf (WD) atmospheres arises from the accretion of minor planets, but the exact properties of polluting material, and in particular the evidence for water in some cases, are not yet understood. Here we study the water retention of small icy bodies in exo-solar planetary systems, as their respective host stars evolve through and off the main sequence and eventually become WDs. We explore, for the first time, a wide range of star masses and metallicities. We find that the mass of the WD progenitor star is of crucial importance for the retentionmore » of water, while its metallicity is relatively unimportant. We predict that minor planets around lower-mass WD progenitors would generally retain more water and would do so at closer distances from the WD than compared with high-mass progenitors. The dependence of water retention on progenitor mass and other parameters has direct implications for the origin of observed WD pollution, and we discuss how our results and predictions might be tested in the future as more observations of WDs with long cooling ages become available.« less

  17. The white dwarf binary pathways survey - II. Radial velocities of 1453 FGK stars with white dwarf companions from LAMOST DR 4

    NASA Astrophysics Data System (ADS)

    Rebassa-Mansergas, A.; Ren, J. J.; Irawati, P.; García-Berro, E.; Parsons, S. G.; Schreiber, M. R.; Gänsicke, B. T.; Rodríguez-Gil, P.; Liu, X.; Manser, C.; Nevado, S. P.; Jiménez-Ibarra, F.; Costero, R.; Echevarría, J.; Michel, R.; Zorotovic, M.; Hollands, M.; Han, Z.; Luo, A.; Villaver, E.; Kong, X.

    2017-12-01

    We present the second paper of a series of publications aiming at obtaining a better understanding regarding the nature of type Ia supernovae (SN Ia) progenitors by studying a large sample of detached F, G and K main-sequence stars in close orbits with white dwarf companions (i.e. WD+FGK binaries). We employ the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) data release 4 spectroscopic data base together with Galaxy Evolution Explorer (GALEX) ultraviolet fluxes to identify 1549 WD+FGK binary candidates (1057 of which are new), thus doubling the number of known sources. We measure the radial velocities of 1453 of these binaries from the available LAMOST spectra and/or from spectra obtained by us at a wide variety of different telescopes around the globe. The analysis of the radial velocity data allows us to identify 24 systems displaying more than 3σ radial velocity variation that we classify as close binaries. We also discuss the fraction of close binaries among WD+FGK systems, which we find to be ∼10 per cent, and demonstrate that high-resolution spectroscopy is required to efficiently identify double-degenerate SN Ia progenitor candidates.

  18. The White Dwarf Mass and the Accretion Rate of Recurrent Novae: An X-ray Perspective

    NASA Technical Reports Server (NTRS)

    Mukai, Koji; Sokoloski, Jennifer L.; Nelson, Thomas; Luna, Gerardo J. M.

    2011-01-01

    We present recent results of quiescent X-ray observations of recurrent novae (RNe) and related objects. Several RNe are luminous hard X-ray sources in quiescence, consistent with accretion onto a near Chandrasekhar mass white dwarf. Detection of similar hard X-ray emissions in old novae and other cataclysmic variables may lead to identification of additional RN candidates. On the other hand, other RNe are found to be comparatively hard X-ray faint. We present several scenarios that may explain this dichotomy, which should be explored further.

  19. Search for trans-iron elements in hot, helium-rich white dwarfs with the HST Cosmic Origins Spectrograph

    NASA Astrophysics Data System (ADS)

    Hoyer, D.; Rauch, T.; Werner, K.; Kruk, J. W.

    2018-04-01

    The metal abundances in the atmospheres of hot white dwarfs (WDs) entering the cooling sequence are determined by the preceding Asymptotic Giant Branch (AGB) evolutionary phase and, subsequently, by the onset of gravitational settling and radiative levitation. In this paper, we investigate three hot He-rich WDs, which are believed to result from a late He-shell flash. During such a flash, the He-rich intershell matter is dredged up and dominates the surface chemistry. Hence, in contrast to the usual H-rich WDs, their spectra allow direct access to s-process element abundances in the intershell that were synthesized during the AGB stage. In order to look for trans-iron group elements (atomic number Z > 29), we performed a non-local thermodynamic equilibrium model atmosphere analysis of new ultraviolet spectra taken with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. One of our program stars is of PG 1159 spectral type; this star, PG 1707+427, has effective temperature Teff = 85 000 K, and surface gravity logg = 7.5. The two other stars are DO white dwarfs: WD 0111+002 has Teff = 58 000 K and log g = 7.7, and PG 0109+111 has Teff = 70 000 K and log g = 8.0. These stars trace the onset of element diffusion during early WD evolution. While zinc is the only trans-iron element we could detect in the PG 1159 star, both DOs exhibit lines from Zn, Ga, Ge, Se; one additionally exhibits lines from Sr, Sn, Te, and I and the other from As. Generally, the trans-iron elements are very abundant in the DOs, meaning that radiative levitation must be acting. Most extreme is the almost six orders of magnitude oversolar abundance of tellurium in PG 0109+111. In terms of mass fraction, it is the most abundant metal in the atmosphere. The two DOs join the hitherto unique hot DO RE 0503-289, in which 14 trans-iron elements had even been identified. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which

  20. The periodicities in the infrared excess of G29-38 - An oscillating brown dwarf?

    NASA Technical Reports Server (NTRS)

    Marley, Mark S.; Lunine, Jonathan I.; Hubbard, William B.

    1990-01-01

    The oscillatory behavior of brown dwarfs has been investigated. The observed periodicities in the infrared excess of the white dwarf Giclas 29-38 are consistent with low-degree, intermediate radial order p-mode oscillations of a brown dwarf companion to the white dwarf. These oscillation modes have the correct frequencies, act on observable layers of the atmosphere, and may be excited to sufficient amplitudes to explain the observations.

  1. Follow-Up Observations of the Second and Third Known Pulsating Hot DQ White Dwarfs

    NASA Astrophysics Data System (ADS)

    Dufour, P.; Green, E. M.; Fontaine, G.; Brassard, P.; Francoeur, M.; Latour, M.

    2009-09-01

    We present follow-up time-series photometric observations that confirm and extend the results of the significant discovery made by Barlow et al. that the Hot DQ white dwarfs SDSS J220029.08 - 074121.5 and SDSS J234843.30 - 094245.3 are luminosity variable. These are the second and third known members of a new class of pulsating white dwarfs, after the prototype SDSS J142625.71+575218.3. We find that the light curve of SDSS J220029.08 - 074121.5 is dominated by an oscillation at 654.397 ± 0.056 s, and that the light pulse folded on that period is highly nonlinear due to the presence of the first and second harmonic of the main pulsation. We also present evidence for the possible detection of two additional pulsation modes with low amplitudes and periods of 577.576 ± 0.226 s and 254.732 ± 0.048 s in that star. Likewise, we find that the light curve of SDSS J234843.30 - 094245.3 is dominated by a pulsation with a period of 1044.168 ± 0.012 s, but with no sign of harmonic components. A new oscillation, with a low amplitude and a period of 416.919 ± 0.004 s, is also probably detected in that second star. We argue, on the basis of the very different folded pulse shapes, that SDSS J220029.08 - 074121.5 is likely magnetic, while SDSS J234843.30 - 094245.3 is probably not.

  2. The accretion and spreading of matter on white dwarfs

    NASA Astrophysics Data System (ADS)

    Fisker, Jacob Lund; Balsara, Dinshaw S.; Burger, Tom

    2006-10-01

    For a slowly rotating non-magnetized white dwarf the accretion disk extends all the way to the star. At the interface between the accretion disk and the star, the matter moves through a boundary layer (BL) and then spreads toward the poles as new matter continuously piles up behind it. We have solved the 3d compressible Navier-Stokes equations on an axisymmetric grid to determine the structure of this BL for different accretion rates (states). The high states show a spreading BL which sets off a gravity wave in the surface matter. The accretion flow moves supersonically over the cusp making it susceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz instabilities. This BL is optically thick and extends more than 30° to either side of the disk plane after 3/4 of a Keplerian rotation period (tK = 19 s). The low states also show a spreading BL, but here the accretion flow does not set off gravity waves and it is optically thin.

  3. Numerical simulations of the convective flame in white dwarfs

    NASA Technical Reports Server (NTRS)

    Livne, Eli

    1993-01-01

    A first step toward better understanding of the mechanism driving convective flames in exploding white dwarfs is presented. The propagation of the convective flame is examined using a two-dimensional implicit hydrodynamical code. The large scales of the instability are captured by the grid while the scales that are smaller than the grid resolution are approximated by a mixing-length approximation. It is found that largescale perturbations (of order of the pressure scale height) do grow significantly during the expansion, leading to a very nonspherical burning front. The combustion rate is strongly enhanced (compared to the unperturbed case) during the first second, but later the expansion of the star suppresses the flame speed, leading to only partial incineration of the nuclear fuel. Our results imply that large-scale perturbations by themselves are not enough to explain the mechanism by which convective flames are driven, and a study of the whole spectrum of relevant perturbations is needed. The implications of these preliminary results on future simulations, in the context of current models for Type Ia supernovae, are discussed.

  4. HS 2231+2441: an HW Vir system composed of a low-mass white dwarf and a brown dwarf★

    NASA Astrophysics Data System (ADS)

    Almeida, L. A.; Damineli, A.; Rodrigues, C. V.; Pereira, M. G.; Jablonski, F.

    2017-12-01

    HW Vir systems are rare evolved eclipsing binaries composed of a hot compact star and a low-mass main sequence star in a close orbit. These systems provide a direct way to measure the fundamental properties, e.g. masses and radii, of their components, hence they are crucial in studying the formation of subdwarf B stars and low-mass white dwarfs, the common-envelope phase and the pre-phase of cataclysmic variables. Here, we present a detailed study of HS 2231+2441, an HW Vir type system, by analysing BVRCIC photometry and phase-resolved optical spectroscopy. The spectra of this system, which are dominated by the primary component features, were fitted using non-local thermodynamic equilibrium models providing an effective temperature Teff = 28 500 ± 500 K, surface gravity log g = 5.40 ± 0.05 cm s-2 and helium abundance log (n(He)/n(H)) = -2.52 ± 0.07. The geometrical orbit and physical parameters were derived by simultaneously modelling the photometric and spectroscopic data using the Wilson-Devinney code. We derive two possible solutions for HS 2231+2441 that provide the component masses: M1 = 0.19 M⊙ and M2 = 0.036 M⊙ or M1 = 0.288 M⊙ and M2 = 0.046 M⊙. Considering the possible evolutionary channels for forming a compact hot star, the primary of HS 2231+2441 probably evolved through the red-giant branch scenario and does not have a helium-burning core, which is consistent with a low-mass white dwarf. Both solutions are consistent with a brown dwarf as the secondary.

  5. Post-main-sequence Evolution of Icy Minor Planets. III. Water Retention in Dwarf Planets and Exomoons and Implications for White Dwarf Pollution

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Malamud, Uri; Perets, Hagai B., E-mail: uri.mal@tx.technion.ac.il, E-mail: hperets@physics.technion.ac.il

    Studies suggest that the pollution of white dwarf (WD) atmospheres arises from the accretion of minor planets, but the exact properties of polluting material, and in particular the evidence for water in some cases are not yet understood. Several previous works studied the possibility of water surviving inside minor planets around evolving stars. However, they all focused on small, comet-sized to moonlet-sized minor planets, when the inferred mass inside the convection zones of He-dominated WDs could actually be compatible with much more massive minor planets. Here we explore for the first time, the water retention inside exoplanetary dwarf planets, ormore » moderate-sized moons, with radii of the order of hundreds of kilometers. This paper concludes a series of papers that has now covered nearly the entire potential mass range of minor planets, in addition to the full mass range of their host stars. We find that water retention is (a) affected by the mass of the WD progenitor, and (b) it is on average at least 5%, irrespective of the assumed initial water composition, if it came from a single accretion event of an icy dwarf planet or moon. The latter prediction strengthens the possibility of habitability in WD planetary systems, and it may also be used in order to distinguish between pollution originating from multiple small accretion events and singular large accretion events. To conclude our work, we provide a code that calculates ice and water retention by interpolation and may be freely used as a service to the community.« less

  6. The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star.

    PubMed

    Howell, D Andrew; Sullivan, Mark; Nugent, Peter E; Ellis, Richard S; Conley, Alexander J; Le Borgne, Damien; Carlberg, Raymond G; Guy, Julien; Balam, David; Basa, Stephane; Fouchez, Dominique; Hook, Isobel M; Hsiao, Eric Y; Neill, James D; Pain, Reynald; Perrett, Kathryn M; Pritchet, Christopher J

    2006-09-21

    The accelerating expansion of the Universe, and the need for dark energy, were inferred from observations of type Ia supernovae. There is a consensus that type Ia supernovae are thermonuclear explosions that destroy carbon-oxygen white dwarf stars that have accreted matter from a companion star, although the nature of this companion remains uncertain. These supernovae are thought to be reliable distance indicators because they have a standard amount of fuel and a uniform trigger: they are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass of 1.4 solar masses (M(o)). Here we show that the high-redshift supernova SNLS-03D3bb has an exceptionally high luminosity and low kinetic energy that both imply a super-Chandrasekhar-mass progenitor. Super-Chandrasekhar-mass supernovae should occur preferentially in a young stellar population, so this may provide an explanation for the observed trend that overluminous type Ia supernovae occur only in 'young' environments. As this supernova does not obey the relations that allow type Ia supernovae to be calibrated as standard candles, and as no counterparts have been found at low redshift, future cosmology studies will have to consider possible contamination from such events.

  7. New nearby white dwarfs from Gaia DR1 TGAS and UCAC5/URAT

    NASA Astrophysics Data System (ADS)

    Scholz, R.-D.; Meusinger, H.; Jahreiß, H.

    2018-05-01

    Aims: Using an accurate Tycho-Gaia Astrometric Solution (TGAS) 25 pc sample that is nearly complete for GK stars and selecting common proper motion (CPM) candidates from the 5th United States Naval Observatory CCD Astrograph Catalog (UCAC5), we search for new white dwarf (WD) companions around nearby stars with relatively small proper motions. Methods: To investigate known CPM systems in TGAS and to select CPM candidates in TGAS+UCAC5, we took into account the expected effect of orbital motion on the proper motion and proper motion catalogue errors. Colour-magnitude diagrams (CMDs) MJ /J - Ks and MG /G - J were used to verify CPM candidates from UCAC5. Assuming their common distance with a given TGAS star, we searched for candidates that occupied similar regions in the CMDs as the few known nearby WDs (four in TGAS) and WD companions (three in TGAS+UCAC5). The CPM candidates with colours and absolute magnitudes corresponding neither to the main sequence nor to the WD sequence were considered as doubtful or subdwarf candidates. Results: With a minimum proper motion of 60 mas yr-1, we selected three WD companion candidates; two of which are also confirmed by their significant parallaxes measured in URAT data, whereas the third may also be a chance alignment of a distant halo star with a nearby TGAS star that has an angular separation of about 465 arcsec. One additional nearby WD candidate was found from its URAT parallax and GJKs photometry. With HD 166435 B orbiting a well-known G1 star at ≈24.6 pc with a projected physical separation of ≈700 AU, we discovered one of the hottest WDs, classified by us as DA2.0 ± 0.2, in the solar neighbourhood. We also found TYC 3980-1081-1 B, a strong cool WD companion candidate around a recently identified new solar neighbour with a TGAS parallax corresponding to a distance of ≈8.3 pc and our photometric classification as ≈M2 dwarf. This raises the question of whether previous assumptions on the completeness of the WD

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  9. High Transparency of Photosphere Plasma for Electromagnetic Waves Polarized Across Strong Magnetic Field on White Dwarfs

    NASA Astrophysics Data System (ADS)

    Koryagin, S. A.

    2015-06-01

    We showed that, in the photosphere of a white dwarf with strong magnetic field, the collisional absorption significantly decreases at the frequencies below the electron cyclotron frequency for the electromagnetic waves linearly polarized across the magnetic field lines (for the so-called extraordinary waves in a magnetized plasma). As a result, the extraordinary waves can escape from the deeper and hotter photosphere layers than the ordinary waves and, in this way, can determine the high linear polarization at the infrared and optical continuum.

  10. The ultracool-field dwarf luminosity-function and space density from the Canada-France Brown Dwarf Survey

    NASA Astrophysics Data System (ADS)

    Reylé, C.; Delorme, P.; Willott, C. J.; Albert, L.; Delfosse, X.; Forveille, T.; Artigau, E.; Malo, L.; Hill, G. J.; Doyon, R.

    2010-11-01

    Context. Thanks to recent and ongoing large scale surveys, hundreds of brown dwarfs have been discovered in the last decade. The Canada-France Brown Dwarf Survey is a wide-field survey for cool brown dwarfs conducted with the MegaCam camera on the Canada-France-Hawaii Telescope. Aims: Our objectives are to find ultracool brown dwarfs and to constrain the field brown-dwarf luminosity function and the mass function from a large and homogeneous sample of L and T dwarfs. Methods: We identify candidates in CFHT/MegaCam i' and z' images and follow them up with pointed near infrared (NIR) imaging on several telescopes. Halfway through our survey we found ~50 T dwarfs and ~170 L or ultra cool M dwarfs drawn from a larger sample of 1400 candidates with typical ultracool dwarfs i'-z' colours, found in 780 square degrees. Results: We have currently completed the NIR follow-up on a large part of the survey for all candidates from mid-L dwarfs down to the latest T dwarfs known with utracool dwarfs' colours. This allows us to draw on a complete and well defined sample of 102 ultracool dwarfs to investigate the luminosity function and space density of field dwarfs. Conclusions: We found the density of late L5 to T0 dwarfs to be 2.0+0.8-0.7 × 10-3 objects pc-3, the density of T0.5 to T5.5 dwarfs to be 1.4+0.3-0.2 × 10-3 objects pc-3, and the density of T6 to T8 dwarfs to be 5.3+3.1-2.2 × 10-3 objects pc-3. We found that these results agree better with a flat substellar mass function. Three latest dwarfs at the boundary between T and Y dwarfs give the high density 8.3+9.0-5.1 × 10-3 objects pc-3. Although the uncertainties are very large this suggests that many brown dwarfs should be found in this late spectral type range, as expected from the cooling of brown dwarfs, whatever their mass, down to very low temperature. Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT), which is operated by

  11. Detection of lithium in the cool halo dwarfs Groombridge 1830 and HD 134439: Implications for internal stellar mixing and cosmology

    NASA Technical Reports Server (NTRS)

    Deliyannis, Constantine P.; Ryan, Sean G.; Beers, Timothy C.; Thorburn, Julie A.

    1994-01-01

    Lithium abundances in halo stars, when interpreted correctly, hold the key to uncovering the primordial Li abundance Li(sub p). However, whereas standard stellar evolutionary models imply consistency in standard big bang nucleosynthesis (BBN), models with rotationally induced mixing imply a higher Li(sub p), possibly implying an inconsistency in standard BBN. We report here Li detections in two cool halo dwarfs, Gmb 1830 and HD 134439. These are the coolest and lowest Li detections in halo dwarfs to date, and are consistent with the metallicity dependence of Li depletion in published models. If the recent report of a beryllium deficiency in Gmb 1830 represents a real Be depletion, then the rotational models would be favored. We propose tests to reduce critical uncertainties.

  12. White Dwarf WD-1145+17 "Zombie Star" Consumes Asteroid

    NASA Astrophysics Data System (ADS)

    Kaye, Thomas G.; Gary, Bruce L.; Rappaport, Saul A.; Foote, Jerry, Benni, Paul

    2016-05-01

    It has long been suspected that white dwarfs accrete asteroid debris as evidenced by heavy metals in many white dwarf spectra. WD1145 was initially detected in Kepler data as an exoplanet candidate with a repeating 1.3% dip over the course of the Jul-Sep 2014 observing season. Follow-up ground based observations were conducted with professional telescopes during March through May of 2015, and these showed that the Kepler dip must likely consist of deeper and shorter dips which come and go with an uncertain pattern. It was hypothesized that the observations were due to an asteroid in a 4.5 hour orbit. In anticipation of its return to nighttime visibility, major observatories scheduled time starting in 2016 Feb. A pro/am collaboration was formed in late 2015 for amateur observations prior to the 2016 Feb professional observations in order to determine an ephemeris for fade activity for the purpose of scheduling relatively short observations with professional telescopes. The amateur observations began in 2015 Nov, sooner than requested, and they showed that the fade activity level had exploded, becoming 20 times the level measured by Kepler. As many as 13 different fades per 4.5-hour orbit were measured, and these varied in depth from night to night. The amateur project turned into a full assault on the star with as many as 4 amateur telescopes observing on the same night. Continuous monitoring mysteriously showed that the clouds drifted in phase with respect to the dominant period i.e., they have a shorter period than measured by Kepler; this would imply that the orbiting dust clouds were located inside the orbit of the parent planetesimal. The best model indicated that the parent planetesimal was releasing fragments from inside its Hill sphere at the L1 Lagrange point, causing them to fall into an inner orbit. New astrophysics was described for the first time when the team used the diameter of the planetesimal orbit, and the diameter of the drift fragment orbit, to

  13. On the nature of the dwarf carbon star G77-61

    NASA Technical Reports Server (NTRS)

    Dearborn, D. S. P.; Liebert, J.; Aaronson, M.; Dahn, C. C.; Harrington, R.

    1986-01-01

    In the present study of astrometric, photometric, and spectrophotometric data for the low luminosity carbon star G77-61, radial velocity variations are detected which have a binary period of 245 days. The unseen companion is probably a cool white dwarf of much higher mass than the visible object. The most straightforward evolutionary hypothesis is that this star has an extremely metal-poor composition, and that it accreted a small amount of carbon-rich material when the now-unseen primary was at maximum radius. This may have inverted the C/O abundance of the secondary without achieving common envelope evolution and a sorter period.

  14. Discovery of a Detached, Eclipsing 40 Minute Period Double White Dwarf Binary and a Friend: Implications for He+CO White Dwarf Mergers

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Brown, Warren R.; Kilic, Mukremin; Kosakowski, Alekzander

    We report the discovery of two detached double white dwarf (WD) binaries, SDSS J082239.546+304857.19 and SDSS J104336.275+055149.90, with orbital periods of 40 and 46 minutes, respectively. The 40 minute system is eclipsing; it is composed of a 0.30 M {sub ⊙} and a 0.52 M {sub ⊙} WD. The 46 minute system is a likely LISA verification binary. The short 20 ± 2 Myr and ∼34 Myr gravitational-wave merger times of the two binaries imply that many more such systems have formed and merged over the age of the Milky Way. We update the estimated Milky Way He+CO WD binarymore » merger rate and affirm our previously published result: He+CO WD binaries merge at a rate at least 40 times greater than the formation rate of stable mass-transfer AM CVn binaries, and so the majority must have unstable mass-transfer. The implication is that spin–orbit coupling in He+CO WD mergers is weak, or perhaps nova-like outbursts drive He+CO WDs into merger, as proposed by Shen.« less

  15. The Star, the Dwarf and the Planet

    NASA Astrophysics Data System (ADS)

    2006-10-01

    properties place it in the category of cool T-type brown dwarfs. ESO PR Photo 38b/06 ESO PR Photo 39b/06 The Relative Position of the Companion to HD 3651 "Due to their faintness even in the infrared, these cool T dwarfs are very difficult to find", said Mugrauer. "Only two other brown dwarfs with similar brightness are presently known. Their study will provide important insights into the atmospheric properties of cool sub-stellar objects." More than 170 stars are currently known to host exoplanets. In some cases, these stars were also found to have one or several stellar companions, showing that planet formation can also take place in a dynamically more complex environment than our own Solar System where planet formation occurred around an isolated single star. In 2001, Mugrauer and his colleagues started an observational programme to find out whether exoplanet host stars are single or married. In this programme, known exoplanet host stars are systematically imaged at two different epochs, at least several months apart. True companions can be distinguished from coincidental background objects as only they move together with the stars over time. With this effective search strategy several new companions of exoplanet host stars have been detected. Most of the detected companions are low-mass stars in the same evolutionary state as the Sun. In two cases, however, the astronomers found the companions to be white dwarfs, that is, stars at the end of their life. These intriguing systems bear evidence that planets can even survive the troubled last moments in the life of a nearby star. The planet host star HD 3651 is thus surrounded by two sub-stellar objects. The planet, HD 3651b, is very close, while the newly found brown dwarf companion revolves around the star 1500 times farther away than the planet. This system is the first imaged example that planets and brown dwarfs can form around the same star.

  16. Mass-accreting white dwarfs and type Ia supernovae

    NASA Astrophysics Data System (ADS)

    Wang, Bo

    2018-05-01

    Type Ia supernovae (SNe Ia) play a prominent role in understanding the evolution of the Universe. They are thought to be thermonuclear explosions of mass-accreting carbon-oxygen white dwarfs (CO WDs) in binaries, although the mass donors of the accreting WDs are still not well determined. In this article, I review recent studies on mass-accreting WDs, including H- and He-accreting WDs. I also review currently most studied progenitor models of SNe Ia, i.e., the single-degenerate model (including the WD+MS channel, the WD+RG channel and the WD+He star channel), the double-degenerate model (including the violent merger scenario) and the sub-Chandrasekhar mass model. Recent progress on these progenitor models is discussed, including the initial parameter space for producing SNe Ia, the binary evolutionary paths to SNe Ia, the progenitor candidates for SNe Ia, the possible surviving companion stars of SNe Ia, some observational constraints, etc. Some other potential progenitor models of SNe Ia are also summarized, including the hybrid CONe WD model, the core-degenerate model, the double WD collision model, the spin-up/spin-down model and the model of WDs near black holes. To date, it seems that two or more progenitor models are needed to explain the observed diversity among SNe Ia.

  17. Using Close White Dwarf + M Dwarf Stellar Pairs to Constrain the Flare Rates in Close Stellar Binaries

    NASA Astrophysics Data System (ADS)

    Morgan, Dylan P.; West, Andrew A.; Becker, Andrew C.

    2016-05-01

    We present a study of the statistical flare rates of M dwarfs (dMs) with close white dwarf (WD) companions (WD+dM; typical separations <1 au). Our previous analysis demonstrated that dMs with close WD companions are more magnetically active than their field counterparts. One likely implication of having a close binary companion is increased stellar rotation through disk-disruption, tidal effects, and/or angular momentum exchange; increased stellar rotation has long been associated with an increase in stellar activity. Previous studies show a strong correlation between dMs that are magnetically active (showing Hα in emission) and the frequency of stellar flare rates. We examine the difference between the flare rates observed in close WD+dM binary systems and field dMs. Our sample consists of a subset of 181 close WD+dM pairs from Morgan et al. observed in the Sloan Digital Sky Survey Stripe 82, where we obtain multi-epoch observations in the Sloan ugriz-bands. We find an increase in the overall flaring fraction in the close WD+dM pairs (0.09 ± 0.03%) compared to the field dMs (0.0108 ± 0.0007%) and a lower flaring fraction for active WD+dMs (0.05 ± 0.03%) compared to active dMs (0.28 ± 0.05%). We discuss how our results constrain both the single and binary dM flare rates. Our results also constrain dM multiplicity, our knowledge of the Galactic transient background, and may be important for the habitability of attending planets around dMs with close companions.

  18. Clouds and hazes in exoplanets and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Morley, Caroline Victoria

    The formation of clouds significantly alters the spectra of cool substellar atmospheres from terrestrial planets to brown dwarfs. In cool planets like Earth and Jupiter, volatile species like water and ammonia condense to form ice clouds. In hot planets and brown dwarfs, iron and silicates instead condense, forming dusty clouds. Irradiated methane-rich planets may have substantial hydrocarbon hazes. During my dissertation, I have studied the impact of clouds and hazes in a variety of substellar objects. First, I present results for cool brown dwarfs including clouds previously neglected in model atmospheres. Model spectra that include sulfide and salt clouds can match the spectra of T dwarf atmospheres; water ice clouds will alter the spectra of the newest and coldest brown dwarfs, the Y dwarfs. These sulfide/salt and ice clouds potentially drive spectroscopic variability in these cool objects, and this variability should be distinguishable from variability caused by hot spots. Next, I present results for small, cool exoplanets between the size of Earth and Neptune. They likely have sulfide and salt clouds and also have photochemical hazes caused by stellar irradiation. Vast resources have been dedicated to characterizing the handful of super Earths and Neptunes accessible to current telescopes, yet of the planets smaller than Neptune studied to date, all have radii in the near-infrared consistent with being constant in wavelength, likely showing that these small planets are consistently enshrouded in thick hazes and clouds. For the super Earth GJ 1214b, very thick, lofted clouds of salts or sulfides in high metallicity (1000x solar) atmospheres create featureless transmission spectra in the near-infrared. Photochemical hazes also create featureless transmission spectra at lower metallicities. For the Neptune-sized GJ 436b, its thermal emission and transmission spectra combine indicate a high metallicity atmosphere, potentially heated by tides and affected by

  19. Photospheric, circumstellar, and interstellar features of HE, C, N. O, and Si in the HST spectra of four hot white dwarf stars

    NASA Technical Reports Server (NTRS)

    Shipman, Harry L.; Provencal, Judi; Roby, Scott W.; Barstow, Martin; Bond, Howard; Bruhweiler, Fred; Finley, David; Fontaine, Gilles; Holberg, Jay; Nousek, John

    1995-01-01

    This paper reports on the observations of four hot white dwarf stars with the spectrographs on the Hubble Space Telescope (HST). The higher resolving power and higher signal/noise, in comparison with IUE, reveals a very rich phenomomenology, including photospheric features from heavy elements, circumstellar features, and the first direct detection of accretion onto the white dwarf component of a binary system. Specific results include the following: Our observations of the ultrahot degenerate H1504+65 confirm that it has a photosphere which is depleted in both H and He, and reveals features of C IV and O VI. The spectrum fits previously published models extremely well. The intermediate-temperature DO star PG 1034+001 has an ultraviolet spectrum showing complex profiles of the well-known resonance doublets of C IV, N v, and Si IV. The O V 1371 line shows a clear separation into a photospheric and a circumstellar component, and it is likely that the same two components can explain the other lines as well. The cooler DA star GD 394 has an extensive system of heavy-element features, but their radial velocity is such that it is highly unlikely that they are formed in the stellar photosphere. Time-resolved spectra of the accreting white dwarf in the V 471 Tau binary system are briefly presented here; they do show the presence of C IV, Si IV, and He II. However, the C IV and He II lines are in emission, rather than in aborption as had been expected.

  20. A Deep Proper Motion Catalog Within the Sloan Digital Sky Survey Footprint. II. The White Dwarf Luminosity Function

    NASA Astrophysics Data System (ADS)

    Munn, Jeffrey A.; Harris, Hugh C.; von Hippel, Ted; Kilic, Mukremin; Liebert, James W.; Williams, Kurtis A.; DeGennaro, Steven; Jeffery, Elizabeth; Dame, Kyra; Gianninas, A.; Brown, Warren R.

    2017-01-01

    A catalog of 8472 white dwarf (WD) candidates is presented, selected using reduced proper motions from the deep proper motion catalog of Munn et al. Candidates are selected in the magnitude range 16< r< 21.5 over 980 square degrees, and 16< r< 21.3 over an additional 1276 square degrees, within the Sloan Digital Sky Survey (SDSS) imaging footprint. Distances, bolometric luminosities, and atmospheric compositions are derived by fitting SDSS ugriz photometry to pure hydrogen and helium model atmospheres (assuming surface gravities {log} {\\text{}}g=8). The disk white dwarf luminosity function (WDLF) is constructed using a sample of 2839 stars with 5.5< {M}{bol}< 17, with statistically significant numbers of stars cooler than the turnover in the luminosity function. The WDLF for the halo is also constructed, using a sample of 135 halo WDs with 5< {M}{bol}< 16. We find space densities of disk and halo WDs in the solar neighborhood of 5.5+/- 0.1× {10}-3 {{pc}}-3 and 3.5+/- 0.7× {10}-5 {{pc}}-3, respectively. We resolve the bump in the disk WDLF due to the onset of fully convective envelopes in WDs, and see indications of it in the halo WDLF as well.