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.

Observations of Doppler Boosting in Kepler Lightcurves

Abstract: Among the initial results from Kepler were two striking lightcurves, for KOI 74 and KOI 81, in which the relative depths of the primary and secondary eclipses showed that the more compact, less luminous object was hotter than its stellar host. That result became particularly intriguing because a substellar mass had been derived for the secondary in KOI 74, which would make the high temperature challenging to explain; in KOI 81, the mass range for the companion was also reported to be consistent with a substellar object. We re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs. We also find that the photometric data for KOI 74 show a modulation in brightness as the more luminous star orbits, due to Doppler boosting. The magnitude of the effect is sufficiently large that we can use it to infer a radial velocity amplitude accurate to 1 km/s. As far as we are aware, this is the first time a radial-velocity curve has been measured photometrically. Combining our velocity amplitude with the inclination and primary mass derived from the eclipses and primary spectral type, we infer a secondary mass of 0.22+/-0.03 Msun. We use our estimates to consider the likely evolutionary paths and mass-transfer episodes of these binary systems.

Binaries Like to Be Twins: Implications for Doubly Degenerate Binaries, the Type Ia Supernova Rate, and Other Interacting Binaries

Abstract: A recently published sample of 21 detached eclipsing binaries in the Small Magellanic Cloud provides a valuable test of the binary mass function for massive stars. We show that 50% of detached binaries have companions with very similar masses, q = M2/M1 > 0.87, where M1 and M2 denote the masses of the two binary components, M1 ≥ M2. A Salpeter relative mass function for the secondary is very strongly excluded, and the data are consistent with a flat mass function containing 55% of the systems and a "twin" population with q > 0.95 containing the remainder. We survey the existing literature on binary mass ratios and conclude that a significant twin population (of order 20%-25%) exists in binaries that are likely to interact across a broad range of stellar masses and metallicity. Interactions involving twins have distinctly different properties from those involving stars of unequal mass; the secondaries will tend to be evolved, and common-envelope evolution is qualitatively different. The implications of such a population for both binary interactions and star formation are substantial, and we present some examples. We argue that twin systems may provide a natural stellar population to explain the recently proposed prompt channel for Type Ia supernovae, and the presence of a twin population dramatically reduces the maximum inferred merger rate between neutron stars (NSs) and black holes relative to the NS-NS merger rate. Twins may also be important for understanding the tendency of white dwarf and NS binaries to be nearly equal in mass, and inclusion of twins in population studies will boost the blue straggler production rate.

White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8

Abstract: We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere white dwarfs (DAVs; a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The space-based photometry reveals pulsation properties previously inaccessible to ground-based observations. We observe a sharp dichotomy in oscillation mode line widths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode line widths, more reminiscent of a damped harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also permits extensive mode identification: we identify the spherical degree of 87 out of 201 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4 m-class telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra, we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low- and intermediate-mass stars. We find that 0.51–0.73 M ⊙ white dwarfs, which evolved from 1.7–3.0 M ⊙ ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Finally, we announce an online repository for our Kepler data and follow-up spectroscopy, which we collect at http://k2wd.org.

Carbon Deficiency in Externally Polluted White Dwarfs: Evidence for Accretion of Asteroids

Abstract: Existing determinations show that n(C)/n(Fe) is more than a factor of 10 below solar in the atmospheres of three white dwarfs that appear to be externally polluted. These results are not easily explained if the stars have accreted interstellar matter, and we reinterpret these measurements as evidence that these stars have accreted asteroids with a chondritic composition.

An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi.

Abstract: In February this year the recurrent nova RS Ophiuchi (RS Oph) burst into life. Every 20 years or so the white dwarf component of this binary accumulates sufficient material from its red giant companion to power a thermonuclear explosion that we see as an increase in magnitude from a very dim 12.5 to magnitude 5. Two groups report observations of the recent outburst. Satellite X-ray observations by Sokoloski et al. reveal an initial phase in which the blast wave expanded freely. Within two days the outbound wave started to slow, suggesting that there was much less debris than had been expected from such an event. O'Brien et al. trained the largest terrestrial radio telescope arrays on RS Oph and were able to directly image a shock wave in a nova explosion for the first time, 14 days after its initial discovery. The structures revealed show an evolution to a remnant similar to that of a type II supernova — but evolving over months rather than millennia. Nova outbursts1 take place in binary star systems comprising a white dwarf and either a low-mass Sun-like star or, as in the case of the recurrent nova RS Ophiuchi2, a red giant. Although the cause of these outbursts is known to be thermonuclear explosion of matter transferred from the companion onto the surface of the white dwarf3, models of the previous (1985) outburst of RS Ophiuchi failed to adequately fit the X-ray evolution4 and there was controversy over a single-epoch high-resolution radio image, which suggested that the remnant was bipolar5,6 rather than spherical as modelled. Here we report the detection of spatially resolved structure in RS Ophiuchi from two weeks after its 12 February 2006 outburst. We track an expanding shock wave as it sweeps through the red giant wind, producing a remnant similar to that of a type II supernova but evolving over months rather than millennia7. As in supernova remnants, the radio emission is non-thermal (synchrotron emission), but asymmetries and multiple emission components clearly demonstrate that contrary to the assumptions of spherical symmetry in models of the 1985 explosion, the ejection is jet-like, collimated by the central binary whose orientation on the sky can be determined from these observations.