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.

On the interpretation and implications of nova abundances: An abundance of riches or an overabundance of enrichments

Abstract: We reexamine the question of the frequency of occurrence of oxygen-neon-magnesium (ONeMg) degenerate dwarfs in classical nova systems, in light of recent observations which have been interpreted as suggesting that 'neon novae' can be associated with relatively low mass white dwarfs. Determinations of heavy-element concentrations in nova ejecta are reviewed, and possible interpretations of their origin are examined. We conclude that, of the 18 classical novae for which detailed abundance analyses are availble, only two (or possibly three) seem unambiguously to demand the presence of an underlying ONeMg white dwarf: V693 CrA 1981, V1370 Aql 1982, and possibly QU Vul 1984. Three other novae which exhibit significant neon enrichments, relative to their total heavy-element concentrations, are RR Pic 1925, V977, Sco 1989, and LMC 1990 No. 1. This result is entirely consistent with present frequency estimates, and our interpretation of the lower levels of enrichment in other systems explains, in a natural way, the existence of relatively low mass white dwarfs in some of the 'neon' novae.

Shock Breakout from Type Ia Supernova

Abstract: The mode of explosive burning in Type Ia supernovae (SNe Ia) remains an outstanding problem. It is generally thought to begin as a subsonic deflagration, but this may transition into a supersonic detonation (the delayed detonation transition, DDT). We argue that this transition leads to a breakout shock, which would provide the first unambiguous evidence that DDTs occur. Its main features are a hard X-ray flash (~20 keV) lasting ~10–2 s with a total radiated energy of ~1040 erg, followed by a cooling tail. This creates a distinct feature in the visual light curve, which is separate from the nickel decay. This cooling tail has a maximum absolute visual magnitude of MV ≈ –9 to –10 at ≈1 day, which depends most sensitively on the white dwarf radius at the time of the DDT. As the thermal diffusion wave moves in, the composition of these surface layers may be imprinted as spectral features, which would help to discern between SN Ia progenitor models. Since this feature should accompany every SNe Ia, future deep surveys (e.g., m = 24) will see it out to a distance of ≈80 Mpc, giving a maximum rate of ~60 yr-1. Archival data sets can also be used to study the early rise dictated by the shock heating (at ≈20 days before maximum B-band light). A similar and slightly brighter event may also accompany core bounce during the accretion-induced collapse to a neutron star, but with a lower occurrence rate.

Supersoft X-Ray Light Curve of RS Ophiuchi (2006)

Abstract: One of the candidates for Type Ia supernova progenitors, the recurrent nova RS Ophiuchi, underwent its sixth recorded outburst in 2006 February, and for the first time a complete light curve of supersoft X-rays has been obtained. It shows the much earlier emergence and longer duration of a supersoft X-ray phase than expected before. These characteristics can be naturally understood when a significant amount of helium layer piles up beneath the hydrogen-burning zone during the outburst, suggesting that the white dwarf (WD) is effectively growing in mass. We have estimated the WD mass in RS Oph to be 1.35 ± 0.01 M☉ and the growth rate of the WD mass to be at an average rate of ~1 × 10-7 M☉ yr-1. The white dwarf will probably reach the critical mass for Type Ia explosion if the present accretion continues further for a few to several times 105 yr.

Population synthesis of triple systems in the context of mergers of carbon–oxygen white dwarfs

Abstract: Hierarchical triple systems are common among field stars yet their long-term evolution is poorly understood theoretically. In such systems Kozai cycles can be induced in the inner binary system during which the inner orbit eccentricity and the inclination between both binary orbits vary periodically. These cycles, combined with tidal friction and gravitational wave emission, can significantly affect the inner binary evolution. To investigate these effects quantitatively we perform a population synthesis study of triple systems and focus on evolutionary paths that lead to mergers of carbon-oxygen (CO) white dwarfs (WDs), which constitute an important candidate progenitor channel for type Ia supernovae (SNe Ia). We approach this problem by Monte Carlo sampling from observation-based distributions of systems within the primary mass range 1.0 - 6.5 M_Sun and inner orbit semi-major axes a_1 and eccentricities e_1 satisfying a_1 (1-e_1^2) > 12 AU, i.e. non-interacting in the absence of a tertiary component. We evolve these systems by means of a newly developed algorithm that couples secular triple dynamics with an existing binary population synthesis code. We find that the tertiary significantly alters the inner binary evolution in about 24% of all sampled systems. In particular, we find several channels leading to CO WD mergers. Amongst these is a novel channel in which a collision occurs in wide inner binary systems as a result of extremely high eccentricities induced by Kozai cycles. With assumptions on which CO WD mergers lead to a SN Ia explosion we estimate the SNe Ia delay time distribution resulting from triples and compare to a binary population synthesis study and to observations. Although we find that our triple rate is low, we have determined a lower limit of the triple-induced SNe Ia rate and further study is needed that includes triples with initially tighter inner orbits.