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.

Diffusion in white dwarfs - New results and comparative study

Abstract: On presente de nouvelles estimations des echelles de temps de diffusion des elements lourds dans les naines blanches

Collisions of White Dwarfs as a New Progenitor Channel for Type Ia Supernovae

Abstract: We present the results of a systematic numerical study of an alternative progenitor scenario to produce Type Ia supernova explosions, which is not restricted to the ignition of a CO white dwarf (WD) near the Chandrasekhar mass. In this scenario, a shock-triggered thermonuclear explosion ensues from the collision of two WDs. Consistent modeling of the gas dynamics together with nuclear reactions using both a smoothed particle and a grid-based hydrodynamics code are performed to study the viability of this alternative progenitor channel. We find that shock-triggered ignition and the synthesis of Ni are in fact a natural outcome for moderately massive WD pairs colliding close to head-on. We use a multi-dimensional radiative transfer code to calculate the emergent broadband light curves and spectral time series of these events. The synthetic spectra and light curves compare well to those of normal Type Ia supernovae over a similar B-band decline rate and are broadly consistent with the Phillips relation, although a mild dependence on viewing angle is observed due to the asymmetry of the ejected debris. While event rates within galactic centers and globular clusters are found to be much too low to explain the bulk of the Type Ia supernovae population, they may be frequent enough to make as much as a one percent contribution to the overall rate. Although these rate estimates are still subject to substantial uncertainties, they do suggest that dense stellar systems should provide upcoming supernova surveys with hundreds of such collision-induced thermonuclear explosions per year.

US 708 – an unbound hyper-velocity subluminous O star

Abstract: We report the discovery of an unbound hyper-velocity star, US 708, in the Milky Way halo, with a heliocentric radial velocity of +708 ± 15 km s -1 . A quantitative NLTE model atmosphere analysis of optical spectra obtained with LRIS at the Keck I telescope shows that US 708 is an extremely helium-rich (N He /N H = 10) subluminous O type star with T eff = 44 500 K, logy = 5.23 at a distance of 19 kpc. Its Galactic rest frame velocity is at least 751 km s -1 , much higher than the local Galactic escape velocity indicating that the star is unbound to the Galaxy. It has been suggested that such hyper-velocity stars can be formed by the tidal disruption of a binary through interaction with the super-massive black hole (SMBH) at the Galactic centre (GC). Numerical kinemalical experiments are carried out to reconstruct the path from the GC. US 708 needs about 32 Myrs to travel from the GC to its present position, less than its evolutionary lifetime. Its predicted proper motion μ α cos δ = -2.3 mas yr -1 and μ δ = -2.4 mas yr -1 should be measurable by future space missions. We conjecture that US 708 is formed by the merger of two helium white dwarfs in a close binary induced by the interaction with the SMBH in the GC and then escaped.

A faint discrete source origin for the highly ionized iron emission from the Galactic Centre region.

Abstract: The origin of the X-ray emission for the central region of our Galaxy has remained a mystery1,2,3,4. In particular, the relative spectral contributions of the diffuse emission and discrete sources, which are critical to understanding the high-energy phenomena in this environment, have been unclear because of the lack of sufficient spatial resolution. Here we report the results of a large-scale imaging survey of the Galactic Centre that resolves these components. We find that the Kα emission from iron that has been highly ionized (so that it has only two electrons left), which has previously been attributed to the diffuse component1, actually arises mainly from discrete sources. This suggests that the presence of a large amount of hot gas (T ≈ 108 K) is no longer required to explain the iron line emission. The spectra of the discrete sources indicate the presence of numerous accreting white dwarfs, neutron stars, and/or black holes in the region. The diffuse emission dominates over the contribution from the faint point sources, and is shown to be associated globally with interstellar features that have been observed at radio and mid-infrared wavelengths, suggesting that it is the product of recent massive star formation.