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.

Cataclysmic Variable Primary Effective Temperatures: Constraints on Binary Angular Momentum Loss

Abstract: We review the most decisive currently available measurements of the surface effective temperatures, Teff, of white dwarf (WD) primaries in cataclysmic variables (CVs) during accretion quiescence, and use these as a diagnostic for their time averaged accretion rate, . Using time-dependent calculations of the WD envelope, we investigate the sensitivity of the quiescent Teff to long term variations in the accretion rate. We find that the quiescent Teff provides one of the best available tests of predictions for the angular momentum loss and resultant mass transfer rates which govern the evolution of CVs. While gravitational radiation is sufficient to explain the of strongly magnetic CVs at all Porb, faster angular momentum loss is required by the temperatures of dwarf nova primaries (non-magnetic systems). This provides evidence that a normal stellar magnetic field structure near the secondary is essential for the enhanced braking mechanism to work, supporting the well-known stellar wind braking hypothesis. The contrast in is most prominent for orbital periods Porb > 3 hours, above the period gap, but a modest enhancement is also present at shorter Porb. The averaging time which reflects is as much as 10^5 years for low- systems and as little as 10^3 years for high- systems. We discuss the security of conclusions drawn about the CV population in light of these time scales and our necessarily incomplete sample of systems. Measurements for non-magnetic systems above the period gap fall below predictions from traditional stellar wind braking prescriptions, but above more recent predictions with somewhat weaker angular momentum loss. We also discuss the apparently high Teff's found in the VY Scl stars. (abridged)

The binarity of the local white dwarf population

Abstract: White dwarfs (WDs) are powerful tools to study the evolutionary history of stars and binaries in the Galaxy. But do we understand their multiplicity from a theoretical point of view? This can be tested by a comparison with the sample of WDs within 20 pc, which is minimally affected by selection biases. From the literature, we compile the available information of the local WD sample with a particular emphasis on their multiplicity, and compare this to synthetic models of WD formation in single stars and binaries. As part of our population synthesis approach, we also study the effect of different assumptions concerning the star formation history, binary evolution, and the initial distributions of binary parameters. We find that the observed space densities of single and binary WDs are well reproduced by the models. The space densities of the most common WD systems (single WDs and unresolved WD-MS binaries) are consistent within a factor two with the observed value. We find a discrepancy only for the space density of resolved double WDs. We exclude that observational selection effects, fast stellar winds, or dynamical interactions with other objects in the Milky Way explain this discrepancy. We find that either the initial mass ratio distribution in the Solar neighbourhood is biased towards low mass-ratios, or more than ten resolved DWDs have been missed observationally in the 20 pc sample. Furthermore, we show that the low binary fraction of WD systems (~25%) compared to Solartype MS-MS binaries (~50%) is consistent with theory, and is mainly caused by mergers in binary systems, and to a lesser degree by WDs hiding in the glare of their companion stars. Lastly, Gaia will dramatically increase the size of the volume-limited WD sample, detecting the coolest and oldest WDs out to 50 pc. We provide a detailed estimate of the number of single and binary WDs in the Gaia sample.

Rapidly Decaying Supernova 2010X: A Candidate ".Ia" Explosion

Abstract: We present the discovery, photometric and spectroscopic follow-up observations of SN 2010X (PTF 10bhp). This supernova decays exponentially with tau_d=5 days, and rivals the current recordholder in speed, SN 2002bj. SN 2010X peaks at M_r=-17mag and has mean velocities of 10,000 km/s. Our light curve modeling suggests a radioactivity powered event and an ejecta mass of 0.16 Msun. If powered by Nickel, we show that the Nickel mass must be very small (0.02 Msun) and that the supernova quickly becomes optically thin to gamma-rays. Our spectral modeling suggests that SN 2010X and SN 2002bj have similar chemical compositions and that one of Aluminum or Helium is present. If Aluminum is present, we speculate that this may be an accretion induced collapse of an O-Ne-Mg white dwarf. If Helium is present, all observables of SN 2010X are consistent with being a thermonuclear Helium shell detonation on a white dwarf, a ".Ia" explosion. With the 1-day dynamic-cadence experiment on the Palomar Transient Factory, we expect to annually discover a few such events.

Magnetic White Dwarfs from the SDSS. The First Data Release

Abstract: Beyond its goals related to the extragalactic universe, the Sloan Digital Sky Survey (SDSS) is an effective tool for identifying stellar objects with unusual spectral energy distributions. Here we report on the 53 new magnetic white dwarfs discovered during the first two years of the survey, including 38 whose data are made public in the 1500 square-degree First Data Release. Discoveries span the magnitude range 16.3 3 MG and g > 15. The new objects nearly double the total number of known magnetic white dwarfs, and include examples with polar field strengths B > 500 MG as well as several with exotic atmospheric compositions. The improved sample statistics and uniformity indicate that the distribution of magnetic white dwarfs has a broad peak in the range ~5-30 MG and a tail extending to nearly 10^9 G. Degenerates with polar fields B > 50 MG are consistent with being descendents of magnetic Ap/Bp main-sequence stars, but low- and moderate-field magnetic white dwarfs appear to imply another origin. Yet-undetected magnetic F-type stars with convective envelopes that destroy the ordered underlying field are attractive candidates.

Is FIRST J102347.6+003841 Really a Cataclysmic Binary?

Abstract: The radio source FIRST J102347.6+003841 was presented as the first radio-selected cataclysmic variable star. In the discovery paper, Bond et al. (2002) show a spectrum consistent with a magnetic AM Her–type system, or polar, featuring strong Balmer lines, He I and He II emission lines, and a light curve with rapid, irregular flickering. In contrast, Woudt, Warner, and Pretorius found a smoothly varying light curve with a period near 4.75 hr and one minimum per orbit, indicating that the state of the system had changed dramatically. We present time-resolved spectra showing a superficially normal, mid-G type photosphere, with no detectable emission lines. The absorption-line radial velocity varies sinusoidally, with semiamplitude 268 ± 4 km s-1, on the orbital period, which is refined to 0.198094(2) days. At this orbital period, the secondary's spectral type is atypically early, suggesting an unusual evolutionary history. We also obtained photometry around the orbit in B, V, and I. The light curve resembles that observed by Woudt, Warner, and Pretorius, and the colors are modulated in a manner consistent with a heating effect. A simple illumination model matches the observations strikingly well, with a Roche lobe–filling secondary near Teff = 5650 K being illuminated by a primary with an isotropic luminosity of ~2 L⊙. The modest amplitude of the observed modulation constrains the orbital inclination i ~ 55° or less, unless the gravity darkening is artificially reduced. Combining the low i with the secondary's velocity amplitude gives a primary star mass above the Chandrasekhar limit when conventional gravity darkening is assumed. We consider the robustness of this conclusion and examine the possibility that the compact object in this system is not a white dwarf, in which case this is not actually a cataclysmic variable. On close examination, FIRST J102347.6+003841 defies easy classification.