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.

EVLA OBSERVATIONS CONSTRAIN THE ENVIRONMENT AND PROGENITOR SYSTEM OF Type Ia SUPERNOVA 2011fe

Abstract: We report unique Expanded Very Large Array observations of SN 2011fe representing the most sensitive radio study of a Type Ia supernova to date. Our data place direct constraints on the density of the surrounding medium at radii ~1015-1016 cm, implying an upper limit on the mass loss rate from the progenitor system of (assuming a wind speed of 100 km s–1) or expansion into a uniform medium with density n CSM 6 cm–3. Drawing from the observed properties of non-conservative mass transfer among accreting white dwarfs, we use these limits on the density of the immediate environs to exclude a phase space of possible progenitor systems for SN 2011fe. We rule out a symbiotic progenitor system and also a system characterized by high accretion rate onto the white dwarf that is expected to give rise to optically thick accretion winds. Assuming that a small fraction, 1%, of the mass accreted is lost from the progenitor system, we also eliminate much of the potential progenitor parameter space for white dwarfs hosting recurrent novae or undergoing stable nuclear burning. Therefore, we rule out much of the parameter space associated with popular single degenerate progenitor models for SN 2011fe, leaving a limited phase space largely inhabited by some double degenerate systems, as well as exotic single degenerates with a sufficient time delay between mass accretion and SN explosion.

XLVIII. The density of white dwarf stars

Abstract: Summary The density of the white dwarfs stars is reconsidered from the point of view of the theory of the polytropic gas spheres, and gives for the mean density of a white dwarf (under ideal conditions) the formula

Evidence for Asphericity in a Subluminous Type Ia Supernova: Spectropolarimetry of SN 1999by

Abstract: We present polarization spectra near maximum light for the strongly subluminous Type Ia supernova SN 1999by that show that the supernova is intrinsically polarized. SN 1999by has an observed, overall level of polarization of ≈0.3%-0.8%, a rise of the polarization P redward of 6500 A, and a change in polarization across the Si II λ6150 feature of about 0.4%. The presentation of the polarization at different wavelengths in the Q-U plane is shown to be a powerful tool to determine the overall geometry and the interstellar component. The distribution of points with wavelength using this empirical Q-U plane method reveals that SN 1999by has a well-defined axis of symmetry and suggests an interstellar polarization (ISP) vector with PISP = 0.3% and position angle Θ = 150° with an error circle in the Q-U plane of radius about 0.1%. Synthetic non-LTE spectra for axisymmetric configurations based on delayed-detonation models have been computed assuming ellipsoidal geometry. The input ejecta structure and composition are based on a Chandrasekhar mass delayed-detonation model. The parameters of the explosion are chosen to reproduce the time evolution of IR spectra of SN 1999by without further adjustments. Spherical models are then mapped onto ellipsoidal geometries and the axis ratio, viewing angle, and ISP adjusted to provide the best agreement with the polarization spectra. Both flux and polarization spectra can be reasonably well reproduced by models with an asphericity of ≈20% observed equator-on. The general properties of the polarization can be understood as a consequence of the structure of subluminous models. Best fits are obtained for the theoretical models with PISP = 0.25% and Θ = 140°, consistent with the empirical method. We discuss our results for this subluminous Type Ia in the context of "normally bright" Type Ia supernovae. For normally bright Type Ia, the photosphere is near the inner iron-rich layers at maximum light and the ubiquitous iron lines give a rapid variation to the model polarization spectra. In subluminous models, the photosphere near maximum is in the silicon layers with fewer lines and a smoother overall polarization spectrum, as observed for SN 1999by. Though data are sparse, the low upper limits for polarization determined for many normal events in contrast to the high polarization in SN 1999by may suggest a relation between the asymmetry we observed and the mechanism that produces a subluminous Type Ia. Among various mechanisms, rapid rotation of the progenitor white dwarf and/or an explosion during a binary white dwarf merger process are likely candidates to explain the asphericity in SN 1999by.

Discovery of a High-Latitude Accreting Millisecond Pulsar in an Ultracompact Binary

Abstract: We have identified the third known accretion-powered millisecond pulsar, XTE J0929-314, with the Rossi X-Ray Timing Explorer. The source is a faint, high Galactic latitude X-ray transient (d 5 kpc) that was in outburst during 2002 April-June. The 185 Hz (5.4 ms) pulsation had a fractional rms amplitude of 3%-7% and was generally broad and sinusoidal, although occasionally double-peaked. The hard X-ray pulses arrived up to 770 ?s earlier than the soft X-ray pulses. The pulsar was spinning down at an average rate of = (-9.2 ? 0.4) ? 10-14 Hz s-1; the spin-down torque may arise from magnetic coupling to the accretion disk, a magnetohydrodynamic wind, or gravitational radiation from the rapidly spinning pulsar. The pulsations were modulated by a 43.6 minute ultracompact binary orbit, yielding the smallest measured mass function (2.7 ? 10-7 M?) of any stellar binary. The binary parameters imply an 0.01 M? white dwarf donor and a moderately high inclination. We note that all three known accreting millisecond pulsars are X-ray transients in very close binaries with extremely low mass transfer rates. This is an important clue to the physics governing whether or not persistent millisecond pulsations are detected in low-mass X-ray binaries.

A Theory of Hydrogen Shell Flashes on Accreting White Dwarfs - Part Two - the Stable Shell Burning and the Recurrence Period of Shell Flashes

Abstract: By means of analytical solutions of the envelope, thermal properties of hydrogen shell burning on accreting white dwarfs are studied and a general picture for their progress is presented which is described by two parameters, the accretion rate and the mass of the white dwarf. On a white dwarf, the thermal behavior of gas in the burning shell depends on the configuration of the envelope, which gives birth to two distinct types of stable configurations in thermal equilibrium, a high and a low state. In the high state, the nuclear shell burning makes up for the energy loss from the surface. There exists the lower limit to the envelope mass for this state. The nuclear burning rate lies in a narrow range of about a factor of 2.5, irrespective of the mass of the white dwarf, while the range itself varies greatly with the latter. In the low state, the nuclear burning is extinct, and yet the compressional heating by accreted gas balances with the cooling through the diffusion of heat. Therefore, the structure depends on the accretion rate. Thermal instability of nuclear burning sets the upper limit to the envelope mass of this state.