Results 1 - 10 of 3600
Results 1 - 10 of 3600. Search took: 0.025 seconds
|Sort by: date | relevance|
[en] We report the discovery of a bright (V ∼ 13.7), extremely low mass white dwarf in a close double degenerate system. We originally selected GALEX J171708.5+675712 for spectroscopic follow-up among a group of white dwarf candidates in an ultraviolet-optical reduced proper-motion diagram. The new white dwarf has a mass of 0.18 Msun and is the primary component of a close double degenerate system (P = 0.246137 days, K1 = 288 km s-1) comprising a fainter white dwarf secondary with M2 ∼ 0.9 Msun. Light curves phased with the orbital ephemeris show evidence of relativistic beaming and weaker ellipsoidal variations. The light curves also reveal secondary eclipses (depth ∼8 mmag) while the primary eclipses appear partially compensated by the secondary gravitational deflection and are below detection limits. Photospheric abundance measurements show a nearly solar composition of Si, Ca, and Fe (0.1-1 sun), while the normal kinematics suggest a relatively recent formation history. Close binary evolutionary scenarios suggest that extremely low mass white dwarfs form via a common-envelope phase and possible Roche lobe overflow.
[en] This study was designed to investigate the apparent deficiency, compared to theoretical predictions, of cool degenerate stars. Two approaches to the problem were employed: a spectroscopic survey designed to identify red degenerates, and a model atmospheres study of the spectroscopic and photometric differences between red dwarfs and red degenerate stars. On computed atmospheric models for white dwarfs at the temperatures under investigation. Line profiles obtained from these models indicate that degenerate stars with T/sub e/ approximately 60000K and depleted surface metals would be extremely difficult to identify spectroscopically. Their hydrogen and calcium line profiles would strongly resemble those of classical sub-dwarfs. Three apparently degenerate stars whose spectral features match our predictions have been identified. These results indicate that the existence of the previously postulated deficiency of red degenerate stars is uncertain
[en] There are two recognized subsets of cataclysmic binaries that contain white dwarfs: the AM Her stars, whose white dwarf components are sufficiently magnetized to ensure synchronous rotation with the orbital period, and the DQ Her stars, which do not maintain synchronism and probably have been spun up by mass accretion. A pilot survey has shown that for all objects but V533 Her the amplitude of the periodic component of the circular polarization is not greater than 0.10-0.15% in the blue band. In the case of V533 Her approximate 0.5% sinusoidal changes in a circular polarization have been detected on two nights. This might mean that in DQ Her stars the dilution by an unpolarized background is very large, and furthermore, that this dilution is variable. (Auth.)
[en] To date the search for habitable Earth-like planets has primarily focused on nuclear burning stars. I propose that this search should be expanded to cool white dwarf stars that have expended their nuclear fuel. I define the continuously habitable zone of white dwarfs and show that it extends from ∼0.005 to 0.02 AU for white dwarfs with masses from 0.4 to 0.9 Msun, temperatures less than ∼104 K, and habitable durations of at least 3 Gyr. As they are similar in size to Earth, white dwarfs may be deeply eclipsed by terrestrial planets that orbit edge-on, which can easily be detected with ground-based telescopes. If planets can migrate inward or reform near white dwarfs, I show that a global robotic telescope network could carry out a transit survey of nearby white dwarfs placing interesting constraints on the presence of habitable Earths. If planets were detected, I show that the survey would favor detection of planets similar to Earth: similar size, temperature, and rotation period, and host star temperatures similar to the Sun. The Large Synoptic Survey Telescope could place even tighter constraints on the frequency of habitable Earths around white dwarfs. The confirmation and characterization of these planets might be carried out with large ground and space telescopes.
[en] Evidence is building that remnants of solar systems might orbit a large percentage of white dwarfs, as the polluted atmospheres of DAZ and DBZ white dwarfs indicate the very recent accretion of metal-rich material. Some of these polluted white dwarfs are found to have large mid-infrared excesses from close-in debris disks that are thought to be reservoirs for the metal accretion. These systems are coined DAZd white dwarfs. Here we investigate the claims of Bonnet-Bidaud and Pantin that Sirius B, the nearest white dwarf to the Sun, might have an infrared excess from a dusty debris disk. Sirius B's companion, Sirius A, is commonly observed as a mid-infrared photometric standard in the Southern hemisphere. We combine several years of Gemini/T-ReCS photometric standard observations to produce deep mid-infrared imaging in five ∼10 μm filters (broad N + four narrow band), which reveal the presence of Sirius B. Our photometry is consistent with the expected photospheric emission such that we constrain any mid-infrared excess to ∼<10% of the photosphere. Thus, we conclude that Sirius B does not have a large dusty disk, as seen in DAZd white dwarfs.