Seismic test of the mass-radius relationship of hydrogen-atmospheric white dwarf stars

We report the discovery of 6576 new spectroscopically confirmed white dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 12. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found one central star of a planetary nebula, one ultra-compact helium binary (AM CVn), one oxygen line dominated white dwarf, 15 hot DO/PG1159s, 12 new cataclysmic variables, 36 magnetic white dwarf stars, 54 DQs, 115 helium dominated white dwarfs, 148 white dwarf+main sequence star binaries, 236 metal polluted white dwarfs, 300 continuum spectra DCs, 230 hot subdwarfs, 2936 new hydrogen dominated white dwarf stars, and 2675 cool hydrogen dominated subdwarf stars. We calculate the mass distribution of all 5883 DAs with S/N>15 in DR12, including the ones in DR7 and DR10, with an average S/N=26, corrected to the 3D convection scale, and also the distribution after correcting for the observed volume, using 1/Vmax.

view Abstract Citations (14) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Structure and Thermal Evolution of Strange Dwarf Stars Benvenuto, O. G. ; Althaus, L. G. Abstract We study the structure and evolution of strange dwarf stars: stellar objects composed of a compact core made up of strange matter surrounded by a normal matter envelope, which at its bottom has a density lower than or equal to that of the neutron drip. We restrict our analysis to the case of low- central pressure objects that have a mass-radius relation very similar to the corresponding to white dwarf stars. We show by means of a simplified analysis that strange dwarfs resembling white dwarfs correspond to a very narrow range of central pressures. The almost discontinuous behavior of these structures with respect to changes of the central pressure is studied by means of a polytropic-like analysis, which shows that the envelope of all these objects is well described by the Lane-Emden equation with n = 3 but with boundary conditions different from the ordinary ones. In contrast to earlier expectations, we show that strange dwarf stars are stable only if the density at the bottom of the normal matter envelope is lower than that of the neutron drip. We have computed the evolution of strange dwarf stars of 0.4, 0.55, and 0.8 Msun in the range of luminosities usually attributed to white dwarf stars. Because of the lack of computations of the previous evolution for such objects, two types of chemical composition were assumed: carbon-oxygen up to a density ρ of ρ = 109 g cm-3 (type A models) and up to ρ = 107 g cm-3 (type B models), respectively. For higher densities, we assumed nuclear statistical equilibrium. We show the central and maximum temperature, neutrino emission, crystallization profile, ages, and the luminosity function versus the stellar luminosity for each type of model and each stellar mass. We found that if the density at the base of the normal matter envelope is slightly lower than that of neutron drip, these objects have a luminosity function observationally indistinguishable from the corresponding luminosity function in white dwarf stars. This is independent of the chemical composition of the normal matter, high-density layers. Thus, the observational data on the cooling of white dwarfs are not in contradiction with the strange matter hypothesis. However, strange dwarfs should behave very differently from white dwarfs in mass-exchanging close binary systems. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177158 Bibcode: 1996ApJ...462..364B Keywords: DENSE MATTER; ELEMENTARY PARTICLES; STARS: INTERIORS; STARS: WHITE DWARFS full text sources ADS |

Models predict that the well-studied mass–radius relation of white dwarf stars depends on the temperature of the star, with hotter white dwarfs having larger masses at a given radius than cooler stars. In this paper, we use a catalog of 26,041 DA white dwarfs observed in Sloan Digital Sky Survey Data Releases 1–19. We measure the radial velocity, effective temperature, surface gravity, and radius for each object. By binning this catalog in radius or surface gravity, we average out the random motion component of the radial velocities for nearby white dwarfs to isolate the gravitational redshifts for these objects and use them to directly measure the mass–radius relation. For gravitational redshifts measured from binning in either radius or surface gravity, we find strong evidence for a temperature-dependent mass–radius relation, with warmer white dwarfs consistently having greater gravitational redshifts than cool objects at a fixed radius or surface gravity. For warm white dwarfs, we find that their mean radius is larger and mean surface gravity is smaller than those of cool white dwarfs at 5.2σ and 6.0σ significance, respectively. Selecting white dwarfs with similar radii or surface gravities, the significance of the difference in mean gravitational redshifts between the warm and cool samples is >6.1σ and >3.6σ for measurements binned in radius and surface gravity, respectively, in the direction predicted by theory. This is an improvement over previous implicit detections, and our technique can be expanded to precisely test the white dwarf mass–radius relation with future surveys.

Super-massive white dwarf (WD) stars in the mass range 2.4 - 2.8 solar masses are believed to be the progenitors of “super-luminous” Type Ia supernovae according to a hypothesis proposed by some researchers. They theorize such a higher mass of the WD due to the presence of a very strong magnetic field inside it. We revisit their first work on magnetic WDs (MWDs) and present our theoretical results that are very different from theirs. The main reason for this difference is in the use of the equation of state (EoS) to make stellar models of MWDs. An electron gas in a magnetic field is Landau quantized and hence, the resulting EoS becomes non-polytropic. By constructing models of MWDs using such an EoS, we highlight that a strong magnetic field inside a WD would make the star super-massive. We have found that our stellar models do indeed fall in the mass range given above. Moreover, we are also able to address an observational finding that the mean mass of MWDs are almost double that of non-magnetic WDs. Magnetic field changes the momentum-space of the electrons which in turn changes their density of states (DOS), and that in turn changes the EoS of matter inside the star. By correlating the magnetic DOS with the non-polytropic EoS, we were also able to find a physical reason behind our theoretical result of super-massive WDs with strong magnetic fields. In order to construct these models, we have considered different equations of state with at most three Landau levels occupied and have plotted our results as mass-radius relations for a particular chosen value of maximum Fermi energy. Our results also show that a multiple Landau-level system of electrons leads to such an EoS that gives multiple branches in the mass-radius relations, and that the super-massive MWDs are obtained when the Landau-level occupancy is limited to just one level. Finally, our theoretical results can be explained solely on the basis of quantum and statistical mechanics that warrant no assumptions regarding stars.

White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with 3σ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and 311 cataclysmic variables. We obtain Teff, log g, and mass for hydrogen atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs), hot subdwarfs (sdBs and sdOs), and estimate photometric Teff for white dwarf stars with continuum spectra (DCs). We find 15 793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below $T_\mathrm{eff}\simeq 10\, 000$ K; most are likely low-mass metal-poor main-sequence stars, but some could be the result of interacting binary evolution.

view Abstract Citations (47) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The collapse of rotating stellar cores - Equilibria between white dwarf and neutron star densities Tohline, J. E. Abstract A simple analytical model is developed through which properties of a stellar core at the endpoint of its collapse can be estimated without performing a detailed collapse calculation. The model permits the range of physical conditions for which dynamical collapse to nuclear density will be prevented to be specified. The basic physical relations that allow the endpoint of collapse to be predicted are first outlined. The core is assumed to remain spherical during its collapse, even though it is rotating. A more accurate model of the endpoint of collapse is then obtained by allowing the core to acquire oblate spheroidal equilibrium structures. These analytical descriptions of the endpoint of core collapse require that collapse proceed along a fixed adiabat. The analysis is also performed without this assumption. An attempt is made to mimic the pressure-density conditions encountered during a hot core collapse, and final equilibrium conditions in the core are predicted for all possible initial core rotation rates. Publication: The Astrophysical Journal Pub Date: October 1984 DOI: 10.1086/162550 Bibcode: 1984ApJ...285..721T Keywords: Gravitational Collapse; Neutron Stars; Stellar Cores; Stellar Evolution; Stellar Rotation; White Dwarf Stars; Computational Astrophysics; Density Distribution; Equilibrium Equations; Stellar Models; Stellar Structure; Astrophysics full text sources ADS |

view Abstract Citations (19) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Nature of the X-Ray Pulsar near Lynds 1457 Patterson, J. ; Halpern, J. P. Abstract The rotation of a magnetized white dwarf or neutron star accidentally located within or behind the Lynds 1457 molecular cloud, is considered as a model for the hard X-ray source H0253 + 193. It is calculated that all models of this type have a low a priori probability because the positions are so remarkably coincident and because the objects involved are not sufficiently common. A neutron star accreting in a close binary is rejected because its likelihood is extremely small and because the observed position, luminosity, and spectrum of the object are all quite atypical for X-ray binaries. Two other possibilities are considered: (1) an isolated white dwarf or neutron star drifting accidentally into the cloud and accreting sufficient gas to power the X-ray source and (2) a white dwarf accreting in a close binary. Publication: The Astrophysical Journal Pub Date: September 1990 DOI: 10.1086/169180 Bibcode: 1990ApJ...361..173P Keywords: Astronomical Models; Molecular Clouds; Pulsars; X Ray Binaries; Cataclysmic Variables; Magnetic Stars; Neutron Stars; Stellar Rotation; White Dwarf Stars; Astrophysics; NEBULAE: INDIVIDUAL ALPHANUMERIC: L1457 (LYNDS 1457); PULSARS; X-RAYS: BINARIES full text sources ADS | data products SIMBAD (12)

view Abstract Citations (26) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS IUE spectrophotometry of the DA4 primary in the short-period white dwarf red dwarf spectroscopic binary Case 1. Sion, E. M. ; Wesemael, F. ; Guinan, E. F. Abstract 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. Publication: The Astrophysical Journal Pub Date: April 1984 DOI: 10.1086/161944 Bibcode: 1984ApJ...279..758S Keywords: Binary Stars; Red Dwarf Stars; Satellite Observation; Stellar Spectrophotometry; Ultraviolet Spectra; White Dwarf Stars; Angular Momentum; Iue; Spectral Energy Distribution; Stellar Evolution; Stellar Spectra; Stellar Temperature; Ultraviolet Astronomy; Astrophysics full text sources ADS | data products SIMBAD (18) MAST (1) INES (1)

White dwarf stars exceeding the Chandrasekhar mass limit

We present a study of the dependence of the mass-radius relation for DA white dwarf stars on the hydrogen envelope mass and the impact on the value of log g, and thus the determination of the stellar mass. We employ a set of full evolutionary carbon-oxygen core white dwarf sequences with white dwarf mass between 0.493 and 1.05 Msun. Computations of the pre-white dwarf evolution uncovers an intrinsic dependence of the maximum mass of the hydrogen envelope with stellar mass, i.e., it decreases when the total mass increases. We find that a reduction of the hydrogen envelope mass can lead to a reduction in the radius of the model of up to ~12%. This translates directly into an increase in log g for a fixed stellar mass, that can reach up to 0.11 dex, mainly overestimating the determinations of stellar mass from atmospheric parameters. Finally, we find a good agreement between the results from the theoretical mass-radius relation and observations from white dwarfs in binary systems. In particular, we find a thin hydrogen mass of MH ~ 2 10^-8 Msun, for 40 Eridani B, in agreement with previous determinations. For Sirius B, the spectroscopic mass is 4.3% lower than the dynamical mass. However, the values of mass and radius from gravitational redshift observations are compatible with the theoretical mass-radius relation for a thick hydrogen envelope of MH = 2 10^-6 Msun.

White dwarfs (WDs) are the stellar core remnants of low mass stars. They are\ntypically divided into three main composition groups: Oxygen Neon (ONe), Carbon\nOxygen (CO) and Helium (He) WDs. The evolution of binary systems can\nsignificantly change the evolution of the binary stellar components. In\nparticular, striping the envelope of an evolved star can give rise to a core\nremnant, which can later evolve into a WD with significantly different\ncomposition. Here we focus on the formation and evolution of hybrid HeCO WDs.\nWe follow the formation and stellar evolution of such WDs for a range of\ninitial conditions and provide their detailed structure, mass-radius relation\nand luminosity temperature evolution. We find that both low mass WDs (< 0.45M ,\ntypically thought to be He WDs) and intermediate-mass WDs (0.45 < MWD < 0.7,\ntypically thought to be CO WDs) could in fact be hybrid HeCO WDs, with 5-25 (75\n-95)% of their mass in He (CO). We use population synthesis calculations to\ninfer the birth rate and properties of such WDs. We find that hybrid HeCO WD\ncomprise the majority of young (< 2Gyr) WDs in binaries, but are more rare\namong older WDs in binaries. The high frequency and large He content of such\nWDs could have an important role in WD WD mergers, and may give rise to sub\nChandrasekhar thermonuclear supernova explosions.\n

In quantum gravity phenomenology, the effect of the generalized uncertainty principle (GUP) on white dwarf structure has been given much attention in recent literature. However, these studies assume a zero temperature equation of state (EoS), excluding young white dwarfs whose initial temperatures are substantially high. To that cause, this paper calculates the Chandrasekhar EoS and resulting mass-radius relations of finite temperature white dwarfs modified by the quadratic GUP, an approach that extends Heisenberg’s uncertainty principle by a quadratic term in momenta. The EoS was first approximated by treating the quadratic GUP parameter as perturbative, causing the EoS to exhibit expected thermal deviations at low pressures, and conflicting behaviors at high pressures, depending on the order of approximation. We then proceeded with a full numerical simulation of the modified EoS, and showed that in general, finite temperatures cause the EoS at low pressures to soften, while the quadratic GUP stiffens the EoS at high pressures. This modified EoS was then applied to the Tolman–Oppenheimer–Volkoff equations and its classical approximation to obtain the modified mass-radius relations for general relativistic and Newtonian white dwarfs. The relations for both cases were found to exhibit the expected thermal deviations at small masses, where low-mass white dwarfs are shifted to the high-mass regime at large radii, while high-mass white dwarfs acquire larger masses, beyond the Chandrasekhar limit. Additionally, we find that for sufficiently large values of the GUP parameter and temperature, we obtain mass-radius relations that are completely removed from the ideal case, as high-mass deviations due to GUP and low-mass deviations due to temperature are no longer mutually exclusive.

We explore the luminosity L of magnetized white dwarfs and its effect on the mass–radius relation. We self-consistently obtain the interface between the electron degenerate-gas dominated inner core and the outer ideal gas surface layer or envelope by incorporating both the components of gas throughout the model white dwarf. This is obtained by solving the set of magnetostatic equilibrium, photon diffusion, and mass conservation equations in the Newtonian framework, for different sets of luminosity and magnetic field. We appropriately use magnetic opacity, instead of Kramer’s opacity, wherever required. We show that the Chandrasekhar limit is retained, even at high luminosity up to about $10^{-2}\, L_\odot$ but without magnetic field, if the temperature is set constant inside the interface. However, there is an increased mass for large-radius white dwarfs, an effect of photon diffusion. Nevertheless, in the presence of strong magnetic fields, with central strength of about 1014 G, super-Chandrasekhar white dwarfs, with masses of about $1.9\, {\rm M}_{\odot }$, are obtained even when the temperature inside the interface is kept constant. Most interestingly, small-radius magnetic white dwarfs remain super-Chandrasekhar even if their luminosity decreases to as low as about $10^{-20}\, L_{\odot }$. However, their large-radius counterparts in the same mass–radius relation merge with Chandrasekhar’s result at low L. Hence, we argue for the possibility of highly magnetized, low luminous super-Chandrasekhar mass white dwarfs that, owing to their faintness, can be practically hidden.

Measuring the mass–radius relation of individual white dwarfs is an empirically challenging task that has been performed for only a few dozen stars. We measure the white dwarf mass–radius relation using the gravitational redshifts and radii of 135 white dwarfs in wide binaries with main-sequence companions. We obtain the radial velocities of these systems using the main-sequence companion, and subtract these Doppler redshifts from the white dwarfs’ apparent motions, isolating their gravitational redshifts. We use Gaia data to calculate the surface temperatures and radii of these white dwarfs, thereby deriving an empirical gravitational redshift–radius relation. This work demonstrates the utility of low-resolution Galactic surveys to measure the white dwarf equation of state. Our results are consistent with theoretical models, and represent the largest sample of individual white dwarf gravitational redshift measurements to date.

With space missions such as Kepler, TESS, and Gaia, we have a wealth of data on pulsating white dwarfs that can be leveraged in white dwarf asteroseismology. We address the question of the proportion of white dwarfs with thin hydrogen layers versus those with thick hydrogen layers. We also provide a mass–radius relation for carbon–oxygen-core, hydrogen-atmosphere white dwarfs. Such a relationship can be used in conjunction with magnitudes and distance measurements to constrain the mass and effective temperature of the white dwarfs. We select nine hydrogen-atmosphere pulsating white dwarfs for their rich pulsation spectra. From such pulsation spectra, we can derive the asymptotic period spacing, which in turn allows us to determine the thickness of the hydrogen and helium envelope of the models, without having to perform period-by-period fitting. We find that the majority of the white dwarfs have thicker hydrogen layers and we determine an upper limit of M r = 1–10−2.2 for the location of the base of the helium layer, in accordance with stellar evolution models. We confirm a finding from earlier studies that used a mass–radius relation and Gaia data to determine the effective temperatures of white dwarfs. The Gaia data systematically point to white dwarfs of lower effective temperature than indicated by the spectroscopy. Our results also support the hypothesis that white dwarfs with thicker hydrogen layers are more common than those with thinner layers.