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, T eff, 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 T eff to long-term variations in the accretion rate. We find that the quiescent T eff 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 completely sufficient to explain the of strongly magnetic CVs at all P orb, faster angular momentum loss is required to explain the temperatures of dwarf nova primaries (nonmagnetic systems). This provides evidence that a normal stellar magnetic field structure near the secondary, providing for wind launching and attachment, is essential for the enhanced braking mechanism to work, directly supporting the well-known stellar wind braking hypothesis. The contrast in is most prominent for orbital periods P orb > 3 h, above the so-called period gap, where differs by orders of magnitude, but a modest enhancement is also present at shorter P orb. The averaging time which reflects depends on itself, being as much as 105 years for low- systems and as little as 103 years for high- systems. We discuss in some detail the security of conclusions drawn about the CV population in light of these time scales and our necessarily incomplete sample of systems, finding that, due to the time necessary for the quiescent T eff to adjust, the consistency of measurements between different systems places significant constraints on possible long-timescale variation in . Measurements for nonmagnetic 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 T eff's found in the VY Scl stars, showing that these most likely indicate in this subclass even larger than predicted by stellar wind braking.

Dynamos in Asymptotic-Giant-Branch Stars As the Origin of Magnetic Fields Shaping Planetary Nebulale

Abstract: Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage$^{1}$. A shock forms near the boundary between the winds, which creates a relatively dense shell that provides the characteristic appearance of a planetary nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape$^{2}$. While a magnetic field could launch and collimate a bipolar outflow, the origin of such a field has hitherto been unclear, as previous work suggested that a field could not be generated${^3}$. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, in a dynamo at the interface between a rapidly rotating core and the more slowly rotating envelope of the star. The field is strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly$^{4}$ slow rotation of most white dwarf stars.

NO STRIPPED HYDROGEN IN THE NEBULAR SPECTRA OF NEARBY TYPE Ia SUPERNOVA 2011fe

Abstract: A generic prediction of the single-degenerate model for Type Ia supernovae (SNe Ia) is that a significant amount of material will be stripped from the donor star (~0.5 M ☉ for a giant donor and ~0.15 M ☉ for a main-sequence donor) by the supernova ejecta. This material, excited by gamma-rays from radioactive decay, would then produce relatively narrow (1000 km s–1) emission features observable once the supernova enters the nebular phase. Such emission has never been detected, which already provides strong constraints on Type Ia progenitor models. In this Letter, we report the deepest limit yet on the presence of Hα emission originating from the stripped hydrogen in the nebular spectrum of an SN Ia obtained using a high signal-to-noise spectrum of the nearby normal SN Ia 2011fe 274 days after B-band maximum light with the Large Binocular Telescope's Multi-Object Double Spectrograph. We put a conservative upper limit on the Hα flux of 3.14 × 10–17 erg s–1 cm–2, which corresponds to a luminosity of 1.57 × 1035 erg s–1. By scaling models from the literature, our flux limit translates into an upper limit of 0.001 M ☉ of stripped material. This is an order of magnitude stronger than previous limits. SN 2011fe was a typical SN Ia, special only in its proximity, and we argue that lack of hydrogen emission in its nebular spectrum adds yet another strong constraint on the single-degenerate class of models for SNe Ia.

Physics of relativistic objects in compact binaries: from birth to coalescence

Abstract: This book provides a comprehensive, authoritative and timely review of the astrophysical approach to the investigation of gravity theories. Particular attention is paid to strong-field tests of general relativity and alternative theories of gravity, performed using collapsed objects (neutron stars, black holes and white dwarfs) in relativistic binaries as laboratories. The book starts with an introduction which gives the background linking experimental gravity in cosmic laboratories to astrophysics and fundamental physics. Subsequent chapters cover observational and theoretical aspects of the following topics: from binaries as test-beds of gravity theories to binary pulsars as cosmic laboratories; from binary star evolution to the formation of relativistic binaries; from short gamma-ray bursts to low mass X-ray binaries; from stellar-mass black hole binaries to coalescing super-massive black holes in galaxy mergers.

Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low-Mass White Dwarfs* **

Abstract: We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ +0.4) open clusters in our Galaxy and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster using Keck/LRIS spectra suggests that most of these stars are undermassive, M = 0.43 ± 0.06 M☉, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791's evolved stars must have lost enough mass on the red giant branch to avoid the flash and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium-core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen-core stars, and therefore the corrected white dwarf cooling age is in fact 7 Gyr, consistent with the well-measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high-metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.