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.

Distances and absolute magnitudes of dwarf novae: murmurs of period bounce

Abstract: We redetermine the relationship between absolute magnitude and orbital period for dwarf novae, based on 46 stars with good distance estimates. This improves upon Warner's previous relation, building upon today's improved estimates of distance and binary inclination, and greater wavelength coverage. Together with other distance and dynamical constraints, this calibration is then applied to a set of ∼300 known or likely dwarf novae of short orbital period, to study the dependence of quiescent M υ , time-averaged M υ , mass ratio q and white-dwarf temperature T WD , on P or b. These distributions show that stars become much fainter as they approach the minimum P orb , and appear to show evolutionary tracks as the secondary is whittled down by mass-loss. Stars on the lower branch have the expected properties of 'period bouncers' - with a feeble secondary, faint accretion light, cool white dwarf and long recurrence time between eruptions. Period bounce seems to occur at a mass of 0.058 ± 0.008 M o . Stars on the lower branch may also have higher velocities and heights above the Galactic plane, consistent with a greater age. Some are very nearby, despite strong selection effects discriminating against the discovery of these faint binaries accreting at very low rates. Period bouncers appear to be very common, and probably would dominate a complete census of cataclysmic variables.

The Outermost Ejecta of Type Ia Supernovae

Abstract: The properties of the highest velocity ejecta of normal Type Ia supernovae (SNe Ia) are studied via models of very early optical spectra of six SNe. At epochs earlier than 1 week before maximum, SNe with a rapidly evolving Si II λ6355 line velocity (HVG) have a larger photospheric velocity than SNe with a slowly evolving Si II λ6355 line velocity (LVG). Since the two groups have comparable luminosities, the temperature at the photosphere is higher in LVG SNe. This explains the different overall spectral appearance of HVG and LVG SNe. However, the variation of the Ca II and Si II absorptions at the highest velocities (v 20,000 km s−1) suggests that additional factors, such as asphericity or different abundances in the progenitor white dwarf, affect the outermost layers. The C II λ6578 line is marginally detected in three LVG SNe, suggesting that LVGs undergo less intense burning. The carbon mass fraction is small, only less than 0.01 near the photosphere, so that he mass of unburned C is only 0.01 M☉. Radioactive 56Ni and stable Fe are detected in both LVG and HVG SNe. Different Fe-group abundances in the outer layers may be one of the reasons for spectral diversity among SNe Ia at the earliest times. The diversity among SNe Ia at the earliest phases could also indicate an intrinsic dispersion in the LC width-luminosity relation.

The imprint of a symbiotic binary progenitor on the properties of Kepler's supernova remnant

Abstract: We present a model for the type Ia supernova remnant (SNR) of SN 1604, also known as Kepler’s SNR. We find that its main features can be explained by a progenitor model of a symbiotic binary consisting of a white dwarf and an AGB donor star with an initial mass of 4−5 M� . The slow, nitrogen-rich wind emanating from the donor star has partially been accreted by the white dwarf, but has also created a circumstellar bubble. On the basis of observational evidence, we assume that the system moves with a velocity of 250 km s −1 . Owing to the spatial velocity, the interaction between the wind and the interstellar medium has resulted in the formation of a bow shock, which can explain the presence of a one-sided, nitrogen-rich shell. We present two-dimensional hydrodynamical simulations of both the shell formation and the SNR evolution. The SNR simulations show good agreement with the observed kinematic and morphological properties of Kepler’s SNR. In particular, the model reproduces the observed expansion parameters (m = V/(R/t)) of m ≈ 0.35 in the north and m ≈ 0.6 in the south of Kepler’s SNR. We discuss the variations among our hydrodynamical simulations in light of the observations, and show that part of the blast wave may have completely traversed through the one-sided shell. The simulations suggest a distance to Kepler’s SNR of 6 kpc, or otherwise imply that SN 1604 was a sub-energetic type Ia explosion. Finally, we discuss the possible implications of our model for type Ia supernovae and their remnants in general.

On the thermonuclear runaway in type ia supernovae: how to run away?

Abstract: Type Ia supernovae (SNe Ia) are thought to be thermonuclear explosions of massive white dwarfs (WDs). We present the first study of multidimensional effects during the final hours prior to the thermonuclear runaway that leads to the explosion. The calculations utilize an implicit, two-dimensional hydrodynamic code. Mixing and the ignition process are studied in detail. We find that the initial chemical structure of the WD is changed, but the material is not fully homogenized. In particular, the exploding WD sustains a central region with a low C/O ratio. This implies that the explosive nuclear burning will begin in a partially carbon-depleted environment. The thermonuclear runaway happens in a well-defined region close to the center. It is induced by compressional heat when matter is brought inward by convective flows. We find no evidence for multiple spot or strong off-center ignition. Convective velocities in the WD are on the order of 100 km s-1, which is well above the effective burning speeds in SNe Ia previously expected right after the runaway. In our calculations, the ignition occurs near the center. Then, for ≈ 0.5-1 s, the speed of the burning front will neither be determined by the laminar speed nor the Rayleigh-Taylor instabilities but by convective flows produced prior to the runaway. The consequences are discussed for our understanding of the detailed physics of the flame propagation, the deflagration to detonation transition, and the nucleosynthesis in the central layers. Our results strongly suggest the preconditioning of the progenitor as a key factor for our understanding of the diversity in SNe Ia.