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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.


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Journal ArticleDOI
TL;DR: In this article, an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations is presented, and it is shown that effective temperatures derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models.
Abstract: We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz, and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the first data release of the Sloan Digital Sky Survey. Our analysis confirms that effective temperatures derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly the mean mass of the trigonometric parallax sample, = 0.62 Mo, is significantly lower than that obtained from pure helium models, = 0.73 Mo, and more consistent with the spectroscopic mean mass of DB stars, = 0.59 Mo, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well defined sequence in a carbon abundance versus effective temperature diagram; below Teff~10,000 K, carbon pollution decreases monotonically with decreasing effective temperature. Improved evolutionary models including diffusion and connecting to the PG 1159 phase are used to infer a typical value for the thickness of the helium layer M_He/M_* between 10^{-3} and 10^{-2}, compatible with the predictions of post-AGB models.

127 citations

Journal ArticleDOI
TL;DR: The Evryscope as discussed by the authors is an under-construction 780 MPix telescope which is capable of detecting transiting exoplanets around every solar-type star brighter than mV = 12, providing at least few-millimagnitude photometric precision in longterm light curves.
Abstract: Low-cost mass-produced sensors and optics have recently made it feasible to build telescope arrays which observe the entire accessible sky simultaneously. In this article, we discuss the scientific motivation for these telescopes, including exoplanets, stellar variability, and extragalactic transients. To provide a concrete example we detail the goals and expectations for the Evryscope, an under-construction 780 MPix telescope which covers 8660 sq. deg. in each 2-minute exposure; each night, 18,400 sq. deg. will be continuously observed for an average of ≈6 hr. Despite its small 61 mm aperture, the system's large field of view provides an etendue which is ~10% of LSST. The Evryscope, which places 27 separate individual telescopes into a common mount which tracks the entire accessible sky with only one moving part, will return 1%-precision, many-year-length, high-cadence light curves for every accessible star brighter than ~16th magnitude. The camera readout times are short enough to provide near-continuous observing, with a 97% survey time efficiency. The array telescope will be capable of detecting transiting exoplanets around every solar-type star brighter than mV = 12, providing at least few-millimagnitude photometric precision in long-term light curves. It will be capable of searching for transiting giant planets around the brightest and most nearby stars, where the planets are much easier to characterize; it will also search for small planets nearby M-dwarfs, for planetary occultations of white dwarfs, and will perform comprehensive nearby microlensing and eclipse-timing searches for exoplanets inaccessible to other planet-finding methods. The Evryscope will also provide comprehensive monitoring of outbursting young stars, white dwarf activity, and stellar activity of all types, along with finding a large sample of very-long-period M-dwarf eclipsing binaries. When relatively rare transients events occur, such as gamma-ray bursts (GRBs), nearby supernovae, or even gravitational wave detections from the Advanced LIGO/Virgo network, the array will return minute-by-minute light curves without needing pointing toward the event as it occurs. By coadding images, the system will reach V ~ 19 in 1-hr integrations, enabling the monitoring of faint objects. Finally, by recording all data, the Evryscope will be able to provide pre-event imaging at 2-minute cadence for bright transients and variable objects, enabling the first high-cadence searches for optical variability before, during and after all-sky events.

126 citations

Journal ArticleDOI
TL;DR: A theoretical light curve for the 1999 outburst of U Scorpii is presented in order to obtain various physical parameters of the recurrent nova, which are exactly the same as those predicted in a new progenitor model of Type Ia supernovae.
Abstract: A theoretical light curve for the 1999 outburst of U Scorpii is presented in order to obtain various physical parameters of the recurrent nova. Our U Sco model consists of a very massive white dwarf (WD) with an accretion disk and a lobe-filling, slightly evolved, main-sequence star (MS). The model includes a reflection effect by the companion and the accretion disk together with a shadowing effect on the companion by the accretion disk. The early visual light curve (with a linear phase of t ~ 1-15 days after maximum) is well reproduced by a thermonuclear runaway model on a very massive WD close to the Chandrasekhar limit (MWD = 1.37 ± 0.01 M⊙), in which optically thick winds blowing from the WD play a key role in determining the nova duration. The ensuing plateau phase (t ~ 15-30 days) is also reproduced by the combination of a slightly irradiated MS and a fully irradiated flaring-up disk with a radius ~1.4 times the Roche lobe size. The cooling phase (t ~ 30-40 days) is consistent with a low-hydrogen content of X ≈ 0.05 of the envelope for the 1.37 M⊙ WD. The best-fit parameters are the WD mass of MWD ~ 1.37 M⊙, the companion mass of MMS ~ 1.5 M⊙ (0.8-2.0 M⊙ is acceptable), the inclination angle of the orbit (i ~ 80°), and the flaring-up edge, the vertical height of which is ~0.30 times the accretion disk radius. The duration of the strong wind phase (t ~ 0-17 days) is very consistent with the BeppoSAX supersoft X-ray detection at t ~ 19-20 days because supersoft X-rays are self-absorbed by the massive wind. The envelope mass at the peak is estimated to be ~3 × 10-6 M⊙, which is indicates an average mass accretion rate of ~2.5 × 10-7 M⊙ yr-1 during the quiescent phase between 1987 and 1999. These quantities are exactly the same as those predicted in a new progenitor model of Type Ia supernovae.

126 citations

Journal ArticleDOI
TL;DR: In this article, the authors used a large nuclear reaction network to determine the abundances of neutron-rich nuclei in the mass range 12-90 for realistic models of the explosion.
Abstract: Accreting carbon-oxygen white dwarfs approaching the Chandrasekhar mass may provide a substantial fraction of Type Ia supernovae. The hydrodynamics of nuclear burning in these models remains uncertain, but all current models are characterized by an initial period of slow, nearly laminar flame propagation at a well-known conductive speed. For cold white dwarfs and slow accretion, the density at the center of the white dwarf at ignition may be quite high, extending perhaps beyond the highest value allowed before accretion-induced collapse occurs, ~9 × 109 g cm-3. The nucleosynthesis that occurs in stars slightly below this critical value, specifically ρc = 2-8 × 109 g cm-3, is explored here using a large nuclear reaction network that allows the resulting abundances of neutron-rich nuclei in the mass range 12-90 to be determined accurately for realistic models of the explosion. It is found that these explosions are responsible for producing the solar abundances of 48Ca,50Ti,54Cr, and 70Zn, with appreciable contributions to 58Fe,64Ni,66Zn,76Ge,82Se, and the gamma-astronomy candidate,60Fe. Provided a prompt detonation does not occur, these results are insensitive to the physics of flame propagation after the first few hundredths of a solar mass has burned. They are, however, mildly sensitive to the ignition density and uncertain weak interaction rates below Ye ≈ 0.42. Since these nuclei, especially 48Ca, cannot be produced anywhere else in nature, the results show that Chandrasekhar mass explosions must occasionally occur (an event rate about 2% of the observed Type Ia supernova rate is estimated for these higher density explosions); however, most Type Ia supernovae must ignite at an appreciably lower density, 2 × 109 g cm-3. Implications for gamma-ray astronomy and meteorite anomalies are briefly discussed.

126 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023367
2022667
2021495
2020557
2019548
2018515