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.

Evolutionary and pulsational properties of white dwarf stars

Abstract: White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications in different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, such as our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow these stars to be used as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos and to study problems related to the variation of fundamental constants. These potential applications of white dwarfs have led to renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers, and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with a unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows one to measure stellar masses with unprecedented precision and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.

On variations in the peak luminosity of type ia supernovae

Abstract: We explore the idea that the observed variations in the peak luminosities of Type Ia supernovae (SNe Ia) originate in part from a scatter in metallicity of the main-sequence stars that become white dwarfs. Previous numerical studies have not self-consistently explored metallicities greater than solar. One-dimensional Chandrasekhar mass models of SNe Ia produce most of their 56Ni in a burn to nuclear statistical equilibrium between the mass shells 0.2 and 0.8 M?, for which the electron-to-nucleon ratio Ye is constant during the burn. We show analytically that under these conditions, charge and mass conservation constrain the mass of 56Ni produced to depend linearly on the original metallicity of the white dwarf progenitor. Detailed postprocessing of W7-like models confirms this linear dependence. The effect that we have identified is most evident at metallicities larger than solar and is in agreement with previous self-consistent calculations over the metallicity range common to both calculations. The observed scatter in the metallicity (-3 Z?) of the solar neighborhood is enough to induce a 25% variation in the mass of 56Ni ejected by SNe Ia. This is sufficient to vary the peak V-band brightness by |?MV| ? 0.2. This scatter in metallicity is present out to the limiting redshifts of current observations (z 1). Sedimentation of 22Ne can possibly amplify the variation in 56Ni mass to 50%. Further numerical studies can determine if other metallicity-induced effects, such as a change in the mass of the 56Ni-producing region, offset or enhance the variation that we identify.

A Tidally-Disrupted Asteroid Around the White Dwarf G29-38

Abstract: The infrared excess around the white dwarf G29-38 can be explained by emission from an opaque flat ring of dust with an inner radius 0.14 of the radius of the Sun and an outer radius approximately equal to the Sun's. This ring lies within the Roche region of the white dwarf where an asteroid could have been tidally destroyed, producing a system reminiscent of Saturn's rings. Accretion onto the white dwarf from this circumstellar dust can explain the observed calcium abundance in the atmosphere of G29-38. Either as a bombardment by a series of asteroids or because of one large disruption, the total amount of matter accreted onto the white dwarf may have been comparable to the total mass of asteroids in the Solar System, or, equivalently, about 1% of the mass in the asteroid belt around the main sequence star zeta Lep.

Ultraviolet Radiation from Evolved Stellar Populations -- I. Models

Abstract: This series of papers comprises a systematic exploration of the hypothesis that the far ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the Zero Age Horizontal Branch through to a point late in post-HB evolution or a point on the white dwarf cooling track. We use the term `Extreme Horizontal Branch' (EHB) to refer to HB sequences of constant mass that do not reach the thermally-pulsing stage on the AGB. These models evolve after core helium exhaustion

Luminous supersoft X-ray sources

Abstract: We discuss possible evolution channels that lead to the formation of luminous supersoft X-ray sources, subclasses of which may be progenitors of type Ia supernovae. We carry out full evolution calculations from the zero-age main sequence to the supersoft source. A novel feature of our calculations is the inclusion of thermohaline mixing after mass transfer during binary evolution. The main effect of this is to produce secondaries of non-solar composition. Candidate initial progenitors are intermediate-mass donors of about 7 Mwith companions in the range 1.5−3.0 M� . We concentrate on early case-C evolution, which means that the primary fills its Roche lobe when it ascends the Early Asymptotic Giant Branch while its core is highly evolved and massive enough to form a CO white dwarf. A crucial role, established by observations in this part of HR diagram, is played by mass loss in winds and we treat winds with a new approach. Since common-envelope evolution (CE) is generally invoked to explain the formation of close binaries with one or two degenerate components, we assume that the progenitors undergo severe mass and angular momentum loss through such a phase. We further study how the configurations of the post- CE systems, composed of a massive white dwarf and a 1.5−3.0 Mcompanion, depend on the parameters of CE-evolution and mass-loss rates in various phases of evolution. Under these general assumptions a new path for the formation of SSSs is found which differs from that of the, usually assumed, solar composition donors. Our results may explain supersoft systems with enhanced helium abundances such as U Sco and very luminous extragalactic supersoft sources such as CAL 83 in the LMC and possibly the CHANDRA source (N1) in M 81.