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.

Type ia supernovae from very long delayed explosion of core−wd merger

Abstract: We study the spinning down time scale of rapidly rotating white dwarfs (WDs) in the frame of the core-degenerate (CD) scenario for type Ia supernovae (SNe Ia). In the CD scenario the Chandrasekhar or super-Chandrasekhar mass WD is formed at the termination of the common envelope phase or during the planetary nebula phase, from a merger of a WD companion with the hot core of a massive asymptotic giant branch star. In the CD scenario the rapidly rotating WD is formed shortly after the stellar formation episode, and the delay from stellar formation to explosion is basically determined by the spin-down time of the rapidly rotating merger remnant. We find that gravitational radiation is inefficient in spinning downWDs, while the magnetodipole radiation torque can lead to delay times that are required to explain SNe Ia. To explain the delay-time-distribution of SNe Ia the merger remnants distribution should be dN/dlog(B sinδ) ≈ constant, for 10 6 G . Bsinδ . 10 8 G where B is the dipole magnetic value and δ the angle between the magnetic dipole axis and rotation axis. 1. INTODUCTION

A single-degenerate channel for the progenitors of Type Ia supernovae with different metallicities

Abstract: A single-degenerate channel for the progenitors of Type Ia supernovae (SNe Ia) is currently accepted, in which a carbon-oxygen white dwarf (CO WD) accretes hydrogen-rich material from its companion, increases its mass to the Chandrasekhar mass limit and then explodes as a SN Ia. Incorporating the prescription of Hachisu et al. for the accretion efficiency into Eggleton's stellar evolution code, and assuming that the prescription is valid for all metallicities, we performed binary stellar evolution calculations for more than 25 000 close WD binaries with metallicities Z = 0.06, 0.05, 0.04, 0.03, 0.02, 0.01, 0.004, 0.001, 0.0003 and 0.0001. For our calculations, the companions are assumed to be unevolved or slightly evolved stars (WD + MS). As a result, the initial parameter spaces for SNe Ia at various Z are presented in the orbital period-secondary mass (log P(i), M(2)(i)) plane. Our study shows that both the initial mass of the secondary and the initial orbital period increase with metallicity. Thus, the minimum mass of the CO WD for SNe Ia decreases with metallicity Z. The difference in the minimum mass may be as large as 0.24M(circle dot) for different Z. Adopting the results above, we studied the birth rate of SNe Ia for various Z via a binary population synthesis approach. If a single starburst is assumed, SNe Ia occur systemically earlier and the peak value of the birth rate is larger for a high Z. The Galactic birth rate from the WD + MS channel is lower than ( but comparable to) that inferred from observations. Our study indicates that supernovae like SN2002ic will not occur in extremely low-metallicity environments, if the delayed dynamical-instability model is appropriate.

Strong neutrino cooling by cycles of electron capture and β − decay in neutron star crusts

Abstract: Cycles of electron capture and β− decay involving neutron-rich nuclei at a typical depth of about 150 metres are found to cool the outer crust of a neutron star by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust; this mechanism has been studied in other astrophysical environments, but has not hitherto been considered in neutron stars. It has been suggested that the heated crust of a neutron star — its outermost kilometre — influences observable phenomena at shallower depths. Hendrik Schatz et al. have now identified a novel cooling process taking place at the relatively shallow depth of 150 metres in the crust of a neutron star, in which nuclei continuously decay and re-form through cycles of electron-capture and β−-decay, emitting neutrinos in the process. This 'Urca' mechanism has been seen in other bodies such as white dwarfs, but has not been previously linked to neutron stars. This thermal decoupling implies that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The temperature in the crust of an accreting neutron star, which comprises its outermost kilometre, is set by heating from nuclear reactions at large densities1,2,3,4, neutrino cooling5,6 and heat transport from the interior7,8,9,10,11. The heated crust has been thought to affect observable phenomena at shallower depths, such as thermonuclear bursts in the accreted envelope10,11. Here we report that cycles of electron capture and its inverse, β− decay, involving neutron-rich nuclei at a typical depth of about 150 metres, cool the outer neutron star crust by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust. This ‘Urca’ mechanism12 has been studied in the context of white dwarfs13 and type Ia supernovae14,15, but hitherto was not considered in neutron stars, because previous models1,2 computed the crust reactions using a zero-temperature approximation and assumed that only a single nuclear species was present at any given depth. The thermal decoupling means that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The unexpectedly short recurrence times, of the order of years, observed for very energetic thermonuclear superbursts16 are therefore not an indicator of a hot crust, but may point instead to an unknown local heating mechanism near the neutron star surface.

DA white dwarfs in Sloan Digital Sky Survey Data Release 7 and a search for infrared excess emission

Abstract: We present a method which uses colour–colour cuts on the Sloan Digital Sky Survey (SDSS) photometry to select white dwarfs with hydrogen-rich (DA) atmospheres without the recourse to spectroscopy. This method results in a sample of DA white dwarfs that is 95 per cent complete at an efficiency of returning a true DA white dwarf of 62 per cent. The approach was applied to SDSS Data Release 7 for objects with and without SDSS spectroscopy. This led to 4636 spectroscopicially confirmed DA white dwarfs with g≤ 19; a ∼70 per cent increase compared to Eisenstein et al.’s 2006 sample. Including the photometric-only objects, we estimate a factor of 3 increase in DA white dwarfs. We find that the SDSS spectroscopic follow-up is 44 per cent complete for DA white dwarfs with Teff≳ 8000 K. We further cross-correlated the SDSS sample with Data Release 8 of the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Large Area Survey. The spectral energy distributions (SED) of both subsets, with and without SDSS spectroscopy, were fitted with white dwarf models to determine the fraction of DA white dwarfs with low-mass stellar companions or dusty debris discs via the detection of excess near-infrared emission. From the spectroscopic sample we find that 2.0 per cent of white dwarfs have an excess consistent with a brown dwarf type companion, with a firm lower limit of 0.8 per cent. From the white dwarfs with photometry only, we find that 1.8 per cent are candidates for having brown dwarf companions. Similarly, both samples show that ∼1 per cent of white dwarfs are candidates for having a dusty debris disc.

On the consequences of low-mass white dwarf mergers

Abstract: The theory of binary star evolution suggests that about 10 percent of all main-sequence binary systems should evolve into a close pair of light white dwarfs which merge within a Hubble time. This paper explores the consequences of such mergers on the assumption that a merger can be approximated by a mass-transfer event which occurs on a time scale shorter than that given by the Eddington accretion limit. The evolution of He + He mergers and of CO + He and of hybrid + He mergers are discussed. The birthrate of helium degenerate pairs which merge in less than a Hubble time is estimated, and the space density of low-luminosity merger products currently present in the Galaxy is predicted. It is shown that the evolutionary tracks of models of simulated mergers pass through the region in the H-R diagram occupied by subdwarfs, but that the predicted space density of merger products exceeds by over a factor of three the space density of subdwarf estimated form the known sample of such stars. 61 refs.