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.

Fermi Establishes Classical Novae as a Distinct Class of Gamma-ray Sources

Abstract: A classical nova results from runaway thermonuclear explosions on the surface of a white dwarf that accretes matter from a low-mass main-sequence stellar companion. In 2012 and 2013, three novae were detected in γ rays and stood in contrast to the first γ-ray-detected nova V407 Cygni 2010, which belongs to a rare class of symbiotic binary systems. Despite likely differences in the compositions and masses of their white dwarf progenitors, the three classical novae are similarly characterized as soft-spectrum transient γ-ray sources detected over 2- to 3-week durations. The γ-ray detections point to unexpected high-energy particle acceleration processes linked to the mass ejection from thermonuclear explosions in an unanticipated class of Galactic γ-ray sources.

Evidence for water in the rocky debris of a disrupted extrasolar minor planet.

Abstract: The existence of water in extrasolar planetary systems is of great interest because it constrains the potential for habitable planets and life. We have identified a circumstellar disk that resulted from the destruction of a water-rich and rocky extrasolar minor planet. The parent body formed and evolved around a star somewhat more massive than the Sun, and the debris now closely orbits the white dwarf remnant of the star. The stellar atmosphere is polluted with metals accreted from the disk, including oxygen in excess of that expected for oxide minerals, indicating that the parent body was originally composed of 26% water by mass. This finding demonstrates that water-bearing planetesimals exist around A- and F-type stars that end their lives as white dwarfs.

X and UV radiation from accreting magnetic degenerate dwarfs.

Abstract: We have carried out detailed numerical calculations of high-harmonic cyclotron emission from a hot plasma, and from these calculations we have developed at self-consistent, quantitative model of X-ray and UV emission from accreting magnetic degenerate dwards. We find that such stars are typically strong UV sources, and emit only at small fraction of their luminosity in soft and hard X-rays. The qualitative features of the X and UV spectra suffice to determine the mass, magnetic field strength, and luminosity of the source with relative confidence. Our results show that the soft and hard X-ray emission observed from AM Her, AN UMa, and 3A 0211--227 are typical of these objects.

The Progenitors of Type Ia Supernovae

Abstract: Type Ia supernovae (SNe Ia) occur in both old, passive galaxies and active, star-forming galaxies. This fact, coupled with the strong dependence of SN Ia rate on star formation rate, suggests that SNe Ia form from stars with a wide range of ages. Here we show that the rate of SN Ia explosions is about 1% of the stellar death rate, independent of star formation history. The dependence of SN Ia rate on star formation rate implies a delay time distribution proportional to t−0.5 ± 0.2. The single-degenerate channel for SNe Ia can be made to match the observed SN Ia rate-SFR relation, but only if white dwarfs are converted to SNe Ia with a uniform efficiency of ~1%, independent of mass. Since low-mass progenitors are expected to have lower conversion efficiencies than high-mass progenitors, we conclude that some other progenitor scenario must be invoked to explain some, or perhaps all, SNe Ia.

Semi-empirical white dwarf initial-final mass relationships: a thorough analysis of systematic uncertainties due to stellar evolution models

Abstract: Using the most recent results about white dwarfs (WDs) in ten open clusters, we revisit semiempirical estimates of the initial-final mass relation (IFMR) in star clusters, with emphasis on the use of stellar evolution models. We discuss the influence of these models on each step of the derivation. One intention of our work is to use consistent sets of calculations both for the isochrones and the WD cooling tracks. The second one is to derive the range of systematic errors arising from stellar evolution theory. This is achieved by using different sources for the stellar models and by varying physical assumptions and input data. We find that systematic errors, including the determination of the cluster age, are dominating the initial mass values, while observational uncertainties influence the final mass primarily. After having determined the systematic errors, the initial-final mass relation allows us finally to draw conclusions about the physics of the stellar models, in particular about convective overshooting.