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White dwarf

About: White dwarf is a research topic. Over the lifetime, 15004 publications have been published within this topic receiving 430597 citations. The topic is also known as: degenerate dwarf.


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Journal ArticleDOI
TL;DR: In this article, the authors presented time-dependent multi-wavelength radiative transfer calculations for models with minimum helium shell mass and derived synthetic observables for both the optical and γ-ray spectral regions.
Abstract: In the double-detonation scenario for Type Ia supernovae, it is suggested that a detonation initiates in a shell of helium-rich material accreted from a companion star by a sub-Chandrasekhar-mass white dwarf. This shell detonation drives a shock front into the carbon-oxygen white dwarf that triggers a secondary detonation in the core. The core detonation results in a complete disruption of the white dwarf. Earlier studies concluded that this scenario has difficulties in accounting for the observed properties of Type Ia supernovae since the explosion ejecta are surrounded by the products of explosive helium burning in the shell. Recently, however, it was proposed that detonations might be possible for much less massive helium shells than previously assumed (Bildsten et al.). Moreover, it was shown that even detonations of these minimum helium shell masses robustly trigger detonations of the carbon-oxygen core (Fink et al.). Therefore, it is possible that the impact of the helium layer on observables is less than previously thought. Here, we present time-dependent multi-wavelength radiative transfer calculations for models with minimum helium shell mass and derive synthetic observables for both the optical and γ-ray spectral regions. These differ strongly from those found in earlier simulations of sub-Chandrasekhar-mass explosions in which more massive helium shells were considered. Our models predict light curves that cover both the range of brightnesses and the rise and decline times of observed Type Ia supernovae. However, their colors and spectra do not match the observations. In particular, their B – V colors are generally too red. We show that this discrepancy is mainly due to the composition of the burning products of the helium shell of the Fink et al. models which contain significant amounts of titanium and chromium. Using a toy model, we also show that the burning products of the helium shell depend crucially on its initial composition. This leads us to conclude that good agreement between sub-Chandrasekhar-mass explosions and observed Type Ia supernovae may still be feasible but further study of the shell properties is required.

267 citations

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the stabilizing effect of a dissipative torque between the accretor and the binary orbit on the survival rate of white dwarfs in cataclysmic variable stars.
Abstract: Three periodically variable stars have recently been discovered (V407 Vul, P=9.5 min; ES Cet, P=10.3 min; RX J0806.3+1527, P=5.3 min) with properties that suggest that their photometric periods are also their orbital periods, making them the most compact binary stars known. If true, this might indicate that close, detached, double white dwarfs are able to survive the onset of mass transfer caused by gravitational wave radiation and emerge as the semi-detached, hydrogen-deficient stars known as the AM CVn stars. The accreting white dwarfs in such systems are large compared to the orbital separations. This has two effects. First, it makes it likely that the mass-transfer stream can hit the accretor directly. Secondly, it causes a loss of angular momentum from the orbit which can destabilize the mass transfer unless the angular momentum lost to the accretor can be transferred back to the orbit. The effect of the destabilization is to reduce the number of systems which survive mass transfer by as much as one hundredfold. In this paper we analyse this destabilization and the stabilizing effect of a dissipative torque between the accretor and the binary orbit. We obtain analytical criteria for the stability of both disc-fed and direct impact accretion, and we carry out numerical integrations to assess the importance of secondary effects, the chief one being that otherwise stable systems can exceed the Eddington accretion rate. We show that to have any effect upon survival rates, the synchronizing torque must act on a time-scale of the order of 1000 yr or less. If synchronization torques are this strong, then they will play a significant role in the spin rates of white dwarfs in cataclysmic variable stars as well.

266 citations

Journal ArticleDOI
TL;DR: In this article, the authors derived general-relativistic equations of structure under the assumptions of hydrostatic and thermal equilibrium, spherical symmetry, no rotation and no magnetic field, and derived analytic expressions for the stellar structure in various interior regions.
Abstract: Stars with massive envelopes (M/sub env/approximately-greater-than1 M/sub sun/) and degenerate neutron cores (M/sub core/approx.1 M/sub sun/, R/sub core/approx.10 km) are analyzed theoretically: General-relativistic equations of structure are derived under the assumptions of hydrostatic and thermal equilibrium, spherical symmetry, no rotation and no magnetic field. Numerical models are constructed, and analytic expressions are derived for the stellar structure in various interior regions. It is argued that all nonrotating, equilibrium models probably resemble qualitatively those constructed in this paper. Brief discussions are given of the stability and evolution of the models, and of prospects for identifying such stars observationally.Viewed externally, our models are extreme M supergiants (Lapprox.3 x 10/sup 4/ to 1.3 x 10/sup 5/ L/sub sun/, T/sub photosphere/approx.2600 to 3100 K, R/sub photosphere/approx. =1000 R/sub sun/). The large, diffuse envelope of each model is separated from its compact core by a thin (approx.40 meter) energy-generation layer called the ''halo.'' The envelope convects from the outer edge of the halo all the way out to the photosphere. Matter contracts from the envelope through the halo and into the core at a rate of approx.1 x 10/sup -8/ M/sub sun/ yr/sup -1/. The contracting matter releases its gravitational energy and burns its hydrogenmore » and helium while passing through the halo. When the envelope mass exceeds approx.10 M/sub sun/, the hydrogen-burning shell occurs at the halo-envelope interface, and the products of hot (Tapprox. =1 x 10/sup 9/ K) nonequilibrium hydrogen burning are convected directly from the burning shell out to the photosphere, where they should be observable.« less

266 citations

Journal ArticleDOI
TL;DR: In this article, a comprehensive spectroscopic survey of DA white dwarfs hotter than 25,000 K is presented, with the majority of the spectra covering the wavelength range of 3500-7500 A, allowing the detection of a number of cool companions.
Abstract: Results are presented from a comprehensive spectroscopic survey of DA white dwarfs hotter than ~25,000 K. The observations consisted of CCD spectra with signal-to-noise ratios of ~100 in the blue, with ~5 A resolution. The majority of the spectra covered the wavelength range of 3500-7500 A, allowing the detection of a number of cool companions. To date, spectra have been obtained and analyzed for 174 DA stars. The spectra were analyzed using our model atmospheres, which are described in detail here, resulting in temperatures and gravities with average internal errors of 1% and 0.04 dex, respectively. Comparisons with previously published results showed that temperature determinations for Teff 1.1 M☉. The ultramassive DA's comprise a much larger fraction of the total than was the case for cooler samples. Calculations based on white dwarf evolutionary models showed that a higher proportion of massive white dwarfs is expected to be found in samples with Teff 30,000 K as a result of differential cooling effects. Within the range Teff > 40,000 K, the EUV-selected subsample did have proportionately more massive stars than the optically selected subsample. However, a detailed comparison showed that EUV and optical surveys were equally capable of detecting relatively nearby massive white dwarfs. On the other hand, interstellar EUV absorption eliminated from the EUV sample many of the more distant stars that were detectable optically. Therefore, the apparent excess of massive DA white dwarfs in the EUV sample is largely due to a relative deficit of stars with more typical masses. Results are presented for individual stars, including a number of subdwarf identifications and reports of detections of cool companions. Properties of some of the more interesting binaries are discussed. We also report on some stars in the sample that evidently have varying He abundances. WD 0612+177 (G104-27) had been observed, at one point, to have photospheric He I; we confirm that He has remained absent since then. WD 0718-316 (RE 0720-314) is in a post-common-envelope binary and has photospheric He II, the abundance of which appears to vary by more than an order of magnitude over timescales of months.

266 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023367
2022667
2021495
2020557
2019548
2018515