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.

Accretion torques in X-ray pulsars

Abstract: An analysis of the accretion process in an X-ray pulsar, whereby angular momentum is transferred to the star and its rotation period is changed, is presented, and an expression for the fractional rate of change of the pulse period in terms of X-ray luminosity and other star parameters is derived. It is shown that observed characteristic spin-up time scales for seven X-ray pulsars strongly support the view that in every source (1) the pulse period reflects the rotation period of a compact object, (2) the accretion is mediated by a disk surrounding the compact object and rotating in the same sense, and (3) the compact object is a neutron star rather than a white dwarf.

Evolution towards and beyond accretion-induced collapse of massive white dwarfs and formation of millisecond pulsars

Abstract: Context. Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs), formed via type Ib/c core-collapse supernovae (SNe), which have been spun up to high rotation rates via accretion from a companion star in a low-mass X-ray binary (LMXB). In an alternative formation channel, NSs are produced via the accretion-induced collapse (AIC) of a massive white dwarf (WD) in a close binary. Aims. Here we investigate binary evolution leading to AIC and examine if NSs formed in this way can subsequently be recycled to form MSPs and, if so, how they can observationally be distinguished from pulsars formed via the standard core-collapse SN channel in terms of their masses, spins, orbital periods and space velocities. Methods. Numerical calculations with a detailed stellar evolution code were used for the first time to study the combined pre- and post-AIC evolution of close binaries. We investigated the mass transfer onto a massive WD (treated as a point mass) in 240 systems with three different types of non-degenerate donor stars: main-sequence stars, red giants, and helium stars. When the WD is able to accrete sufficient mass (depending on the mass-transfer rate and the duration of the accretion phase) we assumed it collapses to form a NS and we studied the dynamical effects of this implosion on the binary orbit. Subsequently, we followed the mass-transfer epoch which resumes once the donor star refills its Roche lobe and calculated the continued LMXB evolution until the end. Results. We show that recycled pulsars may form via AIC from all three types of progenitor systems investigated and find that the final properties of the resulting MSPs are, in general, remarkably similar to those of MSPs formed via the standard core-collapse SN channel. However, as a consequence of the fine-tuned mass-transfer rate necessary to make the WD grow in mass, the resultant MSPs created via the AIC channel preferentially form in certain orbital period intervals. In addition, their predicted small space velocities can also be used to identify them observationally. The production time of NSs formed via AIC can exceed 10 Gyr which can therefore explain the existence of relatively young NSs in globular clusters. Our calculations are also applicable to progenitor binaries of SNe Ia under certain conditions.

LINKING TYPE Ia SUPERNOVA PROGENITORS AND THEIR RESULTING EXPLOSIONS

Abstract: Comparing the ejecta velocities at maximum brightness and narrow circumstellar/interstellar Na D absorption line profiles of a sample of 23 Type Ia supernovae (SNe Ia), we determine that the properties of SN Ia progenitor systems and explosions are intimately connected. As demonstrated by Sternberg et al., half of all SNe Ia with detectable Na D absorption at the host-galaxy redshift in high-resolution spectroscopy have Na D line profiles with significant blueshifted absorption relative to the strongest absorption component, which indicates that a large fraction of SN Ia progenitor systems have strong outflows. In this study, we find that SNe Ia with blueshifted circumstellar/interstellar absorption systematically have higher ejecta velocities and redder colors at maximum brightness relative to the rest of the SN Ia population. This result is robust at a 98.9%-99.8% confidence level, providing the first link between the progenitor systems and properties of the explosion. This finding is further evidence that the outflow scenario is the correct interpretation of the blueshifted Na D absorption, adding additional confirmation that some SNe Ia are produced from a single-degenerate progenitor channel. An additional implication is that either SN Ia progenitor systems have highly asymmetric outflows that are also aligned with the SN explosion or SNe Ia come from a variety of progenitor systems where SNe Ia from systems with strong outflows tend to have more kinetic energy per unit mass than those from systems with weak or no outflows.

The rate of cooling of the pulsating white dwarf star G117−B15A: a new asteroseismological inference of the axion mass

Abstract: We employ a state-of-the-art asteroseismological model of G117−B15A, the archetypeof the H-rich atmosphere (DA) white dwarf pulsators (also known as DAV or ZZ Cetivariables), and use the most recently measured value of the rate of period changefor the dominant mode of this pulsating star to derive a new constraint on the massof axion, the still conjectural non-barionic particle considered as candidate for darkmatter of the Universe. Assuming that G117−B15A is truly represented by our as-teroseismological model, and in particular, that the period of the dominant mode isassociated to a pulsation g-mode trapped in the H envelope, we find strong indicationsof the existence of extra cooling in this star, compatible with emission of axions ofmass m a cos 2 β =17.4 +2.3−2.7 meV.Key words: elementary particles – stars: oscillations – stars: individual: ZZ Cetistars – stars: white dwarfs 1 INTRODUCTION AND CONTEXTAxions are hypothetical weakly interacting particles whoseexistence was proposed about 35 years ago as a solutionto the strong charge-parity problem in quantum chromo-dynamics (Peccei & Quinn 1977; Weinberg 1978; Wilczek1978). They are well-motivated candidates for dark mat-ter of the Universe, and their contribution depends on theirmass (Raffelt 2007), a quantity that is not given by the the-ory that predicts their existence. There are two types ofaxion models: the KVSZ model (Kim 1979; Shifman et al.1980), where the axions couple with photons and hadrons,and the DFSZ model (Dine et al. 1981; Zhimitskii 1980),where they also couple to charged leptons like electrons. Inthis paper, we are interested in DFSZ axions, those thatinteract with electrons. The coupling strength of DFSZ ax-ions to electrons is defined through a dimensionless couplingconstant, g

An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey

Abstract: An initial assessment is made of white dwarf and hot subdwarf stars observed in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees), observed much like the full survey will be, 269 white dwarfs (WDs) and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and five hot subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and DB (helium) types. In addition, in the full survey to date, a number of WDs have been found with uncommon spectral types. Among these are blue DQ stars displaying lines of atomic carbon; red DQ stars showing molecular bands of C2 with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na, and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry alone allows identification of stars hotter than 12,000 K, and the density of these stars for 15 < g < 20 is found to be ~2.2 deg-2 at Galactic latitudes of 29°–62°. Spectra are obtained for roughly half of these hot stars. The spectra show that for 15 < g < 17, 40% of hot stars are WDs, and the fraction of WDs rises to ~90% at g = 20. The remainder are hot sdB and sdO stars.