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.

Have the Elusive Progenitors of Type Ia Supernovae Been Discovered

Abstract: The recent detection of Hα emission in the Type Ia supernova SN 2002ic could be taken to mean that the elusive progenitor systems of Type Ia supernovae have finally been identified. At first glance, the observation appears to support a single-degenerate scenario, in which the white dwarf accretes from a normal companion. In this Letter we show that the opposite may be true, and the observations may support the merger of two white dwarfs as the cause for Type Ia supernovae.

Dynamics and Interactions of Binaries and Neutron Stars in Globular Clusters

Abstract: We model the dynamics of test binaries in isotropic, multi-mass models of galactic globular clusters. The evolution of binary orbits through the cluster potentials is modeled, including second order diffusion terms, and probabilities for close encounters with field stars are calculated. We carry out Monte Carlo simulations of the effects of the binary--single star encounters on the binary population and distribution in the cluster, and estimate the collision rate for different stellar populations in globular clusters with different structural parameters. Assuming a Salpeter IMF, for low concentration clusters the core encounter rate is dominated by turnoff mass main--sequence stars and medium mass white dwarfs. For high concentration, high density clusters the encounter probabilities are increasingly dominated by neutron stars and heavy white dwarfs. Hence we predict a smaller ratio of blue stragglers and cataclysmic variables to pulsars in high concentration clusters. The total number of millisecond pulsars, and the ratio of single to binary pulsars, is broadly consistent with the observed population, suggesting the binary--single star encounters contribute significantly to the pulsar formation rate in globular clusters, for the whole range of globular cluster types. The number of millisecond pulsars and the ratio of pulsars in different globular clusters is best explained by a total binary fraction comparable to that of the galaxy, and a modest number of primordial neutron stars in the globular clusters.

Evolution of the stars

Abstract: Modern theories on the evolution of stars are extended to the phases of helium burning, carbon burning, and later burnings as far as possible, and the theoretical results are compared wth the Hertzsprung-Russell diagrams of star clusters. The topics discussed include nuclear generation and energy loss by neutrinos, quasi-static equilibrium, envelope,core, and surface solutions, stellar models with homogeneous chemical composition, hydrogen burning phase, helium burning phase, advanced phases of nuclear burning, iinal phase toward white dwarfs, and pre-mainsequence contricting phase. (C.E.S.)

Where, oh where has the r-process gone?

Abstract: We present a review of the possible sources for r-process nuclei. It is known that there is as yet no self-consistent mechanism to provide abundant neutrons for a robust r-process in the neutrino-driven winds from nascent neutron stars. We consider that the heavy r-nuclei with mass numbers A>130 (Ba and above) cannot be produced in the neutrino-driven winds. Nonetheless, the r-process and the neutrino-driven winds may be directly or indirectly related by some unknown additional mechanism, which, for example, could provide ejecta with very short dynamic timescales of <0.004 s. This undetermined mechanism must supply a neutron source within the same general stellar sites that undergo core collapse to produce the neutron star. Observational data on low-metallicity stars in the Galactic halo show that sites producing the heavy r-nuclei do not produce Fe or any other elements between N and Ge. Insofar as a forming neutron star is key to producing the heavy r-nuclei, then the only possible sources are supernovae resulting from collapse of O-Ne-Mg cores or accretion-induced collapse of white dwarfs, neither of which produce the elements of the Fe group or those of intermediate mass (above C and N). Using a template star with high enrichments of heavy r-nuclei and another with low enrichments we develop a two-component model based on the abundances of Eu (from sources for heavy r-nuclei) and Fe (from Fe core-collapse supernovae). This model gives very good quantitative predictions for the abundances of all the other elements in those metal-poor stars with [Fe/H]<-1.5 for which the Eu and Fe abundances are known. (Abridged)

Hard X-rays from accretion disk boundary layers

Abstract: ACCRETION disks1,2 are found in many astrophysical objects, ranging from newly formed stars and mass-transferring binary systems to quasars and other active galactic nuclei. An important feature of accretion disks is the boundary layer—the interface between the disk and the accreting objects—where up to half the accretion luminosity may be liberated. The lack of a satisfactory description of the flow and thermal structure of this layer has long been a handicap when modelling disk spectra. Here we report numerical solutions of a model of thin accretion disks around a central white dwarf which includes a self-consistent description of the boundary layer. We find two distinct kinds of solution depending on the mass accretion rate Ṁ. At high rates, we find optically thick boundary layers whose radial width and peak temperature decrease with decreasing Ṁ, but when the accretion rate falls below a critical value, the boundary layer becomes optically thin, and the width and temperature increase dramatically. Our results provide an explanation for the hard X-rays observed3 in cataclysmic variables, particularly at low Ṁ. It should be possible to extend our analysis to other accretion-disk systems.