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.

Evolution of 3-9 M☉ Stars for Z = 0.001-0.03 and Metallicity Effects on Type Ia Supernovae

Abstract: Recent observations have revealed that Type Ia supernovae (SNe Ia) are not perfect standard candles; they show variations in their absolute magnitudes, light-curve shapes, and spectra. The C/O ratio in the SNe Ia progenitors (C-O white dwarfs) may be related to this variation. In this work, we systematically investigate the effects of stellar mass (M) and metallicity (Z) on the C/O ratio and its distribution in the C-O white dwarfs by calculating stellar evolution from the main sequence through the end of the second dredge-up for M=3-9 M? and Z=0.001-0.03. We find that the total carbon mass fraction just before SN Ia explosion varies in the range 0.36-0.5. We also calculate the metallicity dependence of the main-sequence mass range of the SN Ia progenitor white dwarfs. Our results show that the maximum main-sequence mass to form C-O white dwarfs decreases significantly toward lower metallicity, and the number of SN Ia progenitors may be underestimated if metallicity effect is neglected. We discuss the implications of these results on the variation of SNe Ia, determination of cosmological parameters, luminosity function of white dwarfs, and galactic chemical evolution.

Inward Propagation of Nuclear-burning Shells in Merging C-O and He White Dwarfs

Abstract: We have investigated the consequences of merging double white dwarf systems by calculating evolutionary models of accreting white dwarfs. We have considered two cases: a massive C-O white dwarf of ~1 M☉ accreting C-O mixture and a low-mass white dwarf with an initial mass of 0.4 M☉ accreting matter composed mostly of helium. The accretion rate of the C-O white dwarf is assumed to be 1 × 10-5 M☉ yr-1. After carbon burning is ignited at Mr ~ 1.04 M☉, the flame propagates inward because of heat conduction. By inserting enough grid points to resolve the structure of the flame, we have obtained almost steady burning in most phases of evolution, but we have found a new phenomenon that the strength of flame sometimes oscillates because of a thermal instability in an early phase of the evolution. In calculating evolutionary models, we have occasionally employed a steady-burning approximation, in which the propagation speed of flame is given a priori. We have considered two extreme cases for the interior abundance of the massive white dwarf: XC = 0.5 and XC = 0.2. For XC = 0.5, the flame becomes very weak at Mr ~ 0.4 M☉ and the inward propagation stalls there. But a few thousand years later, the flame is reactivated because of contraction and propagates to the center. For XC = 0.2, the flame propagates smoothly and reaches the center in ~1000 yr. In both cases the C-O mixture has been burned into an O-Ne-Mg mixture without causing an explosive phenomenon. For helium-accreting low-mass white dwarfs, we have considered accretion rates of 1 × 10-7 and 1 × 10-6 M☉ yr-1. After a fraction of M☉ is accreted to the white dwarf, helium is ignited in the outer part and a shell flash occurs. Such a shell flash diminishes when about 10% of helium in the convective shell is burned into carbon and oxygen. The next shell flash occurs at a shell interior to the previously flashed shell. After less than 30 shell flashes, the helium ignition occurs at the center and steady burning begins. Thus, the merging produces a helium star that burns helium at the center. The mechanism of the propagation of the burning shell in this case is compressional heating during interpulse phases, in contrast to the case of the carbon-burning flame where the conduction drives the inward propagation of the flame. We infer that such a low-mass double white dwarf system could be a progenitor of the AM CVn stars.

Deflagration-to-Detonation Transition in Thermonuclear Supernovae

Abstract: We derive the criteria for deflagration to detonation transition (DDT) in a Type Ia supernova. The theory is based on two major assumptions: (1) detonation is triggered via the Zeldovich gradient mechanism inside a region of mixed fuel and products, and (2) the mixed region is produced by a turbulent mixing of fuel and products either inside an active deflagration front or during the global expansion and subsequent contraction of an exploding white dwarf. We determine the critical size of the mixed region required to initiate a detonation in a degenerate carbon-oxygen mixture. This critical length is much larger than the width of the reaction front of a Chapman-Jouguet detonation. However, at densities greater than 5 × 106 g cm-3, it is much smaller than the size of a white dwarf. We derive the critical turbulent intensity required to create the mixed region inside an active deflagration front in which a detonation can form. We conclude that the density ρtr at which a detonation can form in a carbon-oxygen white dwarf is low, less than 2-5 × 107 g cm-3 but greater than 5 × 106 g cm-3.

Constraining asymmetric dark matter through observations of compact stars

Abstract: We put constraints on asymmetric dark matter candidates with spin-dependent interactions based on the simple existence of white dwarfs and neutron stars in globular clusters. For a wide range of the parameters (WIMP mass and WIMP-nucleon cross section), WIMPs can be trapped in progenitors in large numbers and once the original star collapses to a white dwarf or a neutron star, these WIMPs might self-gravitate and eventually collapse forming a mini-black hole that eventually destroys the star. We impose constraints competitive to direct dark matter search experiments, for WIMPs with masses down to the TeV scale.