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.

Double-detonation sub-Chandrasekhar supernovae: can minimum helium shell masses detonate the core?

Abstract: The explosion of sub-Chandrasekhar mass white dwarfs via the double detonation scenario is a potential explanation for type Ia supernovae. In this scenario, a surface detonation in a helium layer initiates a detonation in the underlying carbon/oxygen core leading to an explosion. For a given core mass, a lower bound has been determined on the mass of the helium shell required for dynamical burning during a helium flash, which is a necessary prerequisite for detonation. For a range of core and corresponding minimum helium shell masses, we investigate whether an assumed surface helium detonation is capable of triggering a subsequent detonation in the core even for this limiting case. We carried out hydrodynamic simulations on a co-expanding Eulerian grid in two dimensions assuming rotational symmetry. The detonations are propagated using the level-set approach and a simplified scheme for nuclear reactions that has been calibrated with a large nuclear network. The same network is used to determine detailed nucleosynthetic abundances in a post-processing step. Based on approximate detonation initiation criteria in the literature, we find that secondary core detonations are triggered for all of the simulated models, ranging in core mass from 0.810 up to 1.385 M_solar with corresponding shell masses from 0.126 down to 0.0035 M_solar. This implies that, as soon as a detonation triggers in a helium shell covering a carbon/oxygen white dwarf, a subsequent core detonation is virtually inevitable.

White Dwarfs and Neutron Stars

Abstract: This chapter on the physics of compact objects begins with a section on white dwarfs. It will be shown that the famous Chandrasekhar equation is just the relativistic Thomas–Fermi equation. For white dwarfs the Thomas–Fermi approximation is ideally justified. For neutron stars the general relativistic stellar structure equations are needed. In view of the fact that we do not have an accurate quantitative understanding of neutron stars, we begin with a brief qualitative overview of the interior of neutron stars and discuss some simplified models. Then we turn to the equation of state at high densities. Since the densities in the central regions of neutron stars can be almost an order of magnitude higher than the nuclear density, we need a reliable equation of state at super-nuclear densities. This is a very difficult problem and large uncertainties remain. In spite of this, it is possible to establish reliable, rather tight upper limits of the largest possible mass of a non-rotating neutron star. This plays a decisive role in the observational identification of black holes, to be discussed in Chap. 7. Additional sections are devoted to rotating neutron stars, the cooling of neutron stars, and to neutron stars in binaries.

Population synthesis for double white dwarfs. I. Close detached systems.

Abstract: We model the population of double white dwarfs in the Galaxy and find a better agreement with observations compared to earlier studies, due to two modifications. The first is the treatment of the first phase of unstable mass transfer and the second the modelling of the cooling of the white dwarfs. A satisfactory agreement with observations of the local sample of white dwarfs is achieved if we assume that the initial binary fraction is ∼ 50% and that the lowest mass white dwarfs () cool faster than the most recently published cooling models predict. With this model we find a Galactic birth rate of close double white dwarfs of 0.05 yr-1 , a birth rate of AM CVn systems of 0.005 yr-1 , a merger rate of pairs with a combined mass exceeding the Chandrasekhar limit (which may be progenitors of SNe Ia) of 0.003 yr-1 and a formation rate of planetary nebulae of 1 yr-1 . We estimate the total number of double white dwarfs in the Galaxy as 2.5 108 . In an observable sample with a limiting magnitude we predict the presence of ∼ 855 white dwarfs of which ∼ 220 are close pairs. Of these 10 are double CO white dwarfs of which one has a combined mass exceeding the Chandrasekhar limit and will merge within a Hubble time.

A comprehensive spectroscopic analysis of db white dwarfs

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

Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe

Abstract: Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system1, 2, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models1, 3: double-degenerate (involving two white dwarfs in a close binary system2, 4) and single-degenerate models5, 6. In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy7. Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10–100 times fainter than previous limits on other type Ia supernova progenitor systems8, 9, 10, allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf.