Showing papers on "White dwarf published in 1969"

Magnetic Dipole Radiation from Pulsars

Abstract: RADIO astronomers1–3 have recently discovered pulsars in Vela and near the Crab nebula which seem to be associated with supernova remnants and have repetition periods (0.0892 s and 0.0331 s) considerably shorter than those previously found. The very wide range (60-fold) of periods at present observed is difficult to understand if pulsars are in fact pulsating objects of a single class, because in general (period) ∝ (density)−1/2 and an extremely wide range of density would be required. Of the remaining clock mechanisms proposed, rotation4,5 seems most consistent with the observations. The extremely short periods exclude white dwarfs and make the suggestion of neutron stars more likely.

A calculation of a white dwarf supernova

Abstract: A hydrodynamic calculation of a 1.42M⊙ white dwarf supernova is described. Instability and collapse are initiated by electron capture and the subsequent nuclear detonation of C12 at high densities is sufficient to disperse all the material of the star with an average velocity of 7000 km/sec

Observation of a Rapidly Pulsating Radio Source (Reprinted from Nature, February 24, 1968)

Abstract: Unusual signals from pulsating radio sources have been recorded at the Mullard Radio Astronomy Observatory. The radiation seems to come from local objects within the galaxy, and may be associated with oscillations of white dwarf or neutron stars.

Volcano Theory of Pulsars

Abstract: IT has recently become clear1 that at least some white dwarfs are likely to have convective envelopes Neutron star models2 are generally similar to white dwarf models, so it is possible that neutron stars will also have superadiabatic temperature gradients in their outer layers Convection in the ordinary sense cannot occur in neutron star envelopes, however, because these envelopes are solid3 How will heat be transported through a solid mantle of a neutron star which is convectively unstable? It seems reasonable to expect that one or more weak points will develop through which liquid material from the core will pour out to the surface In other words, energy will be transported by volcanoes

Treatment of Pulsating White Dwarfs including General Relativistic Effects.

Abstract: In this paper, pulsating white dwarfs are treated via general relativity. Numerical integration of Einstein's equations was used to find equilibrium white dwarfs models and the fundamental periods of small oscillations about these equilibrium models. In these calculations account was taken of coulomb, Thomas-Fermi, and exchange interactions as well as ion zero point energies. It is shown that general relativity makes not just a quantitative difference in the results but a qualitative differences; pure C12 models which are stable in Newtonian mechanics can be unstable against collapse (at a central density of 3×1010 g/cm3) when general relativity is taken into account. The collapsing model may become a neutron star or may continue towards the Schwarzschild radius. More realistic white dwarf models with carbon burning products at the center, also were studied. For these models, the density at which the star becomes unstable against collapse due to electron capture (3×109 g/cm3) was found to be lower than the density at which general relativistic instability occurs.

Ultrashort-period stellar oscillations. III. Power-spectrum analysis of photometric observations of white dwarfs.

Abstract: Power spectrum analysis of photometric observations of white dwarf stars for measuring variability of stellar luminosity

Mass Loss from Pulsating Neutron Stars

Abstract: DESPITE the trend of recent evidence, both observational and theoretical1, that radial pulsations may not be the source of variation in pulsars, the pulsational hypothesis has still not been disproved In particular, the expected pulsational damping times of degenerate stars may be sufficiently long (for small oscillations) that observable variations could occur over a period of millions of years (J M Cohen and L C Rosen, private communication), in spite of some earlier theoretical evidence to the contrary2 One of the chief difficulties with the pulsational hypothesis, however, is that the calculated fundamental periods are longer than 15 s for white dwarfs3,4 and shorter than 1 ms for most neutron-star models5, whereas most pulsars have periods lying between these extremes Overtone pulsations in white dwarfs have been suggested as a mechanism3, but this possibility seems rather unlikely, and neither it nor the suggestion that pulsations are occurring in the non-degenerate envelope6 or atmosphere7 of a white dwarf can explain the 33 ms variations of the Crab Nebula pulsar in Taurus In this article, a new possibility involving radial pulsations of a neutron star which is undergoing mass loss will be pointed out, with a view to interpreting the observed period8,9 and secular increase of period10,11 of the Crab Nebula pulsar

Pulsars: White Dwarf Atmospheric Pulsations

Abstract: RECENT discussions of the pulsed radio sources discovered by Hewish et al.1 have demonstrated the theoretical difficulties associated with these objects. The observed periods suggest that atmospheric pulsations of white dwarfs is the mechanism. I consider here a model of a thin, adiabatic atmosphere with a constant lapse rate. Only the non-degenerate portion of the star is considered as participating in the oscillation.

Mass Loss from Stars: A Review

Abstract: For many decades astronomers have realized that some stars lose mass in the catastrophic events that produce supernovae, ordinary novae, and planetary nebulae. Only recently have we learned that less conspicuous mass-loss processes occur throughout much of the lifetimes of most normal stars. My purpose in this Introduction is to summarize briefly the kinds of evidence we have for this conclusion; also what can be said about the properties of the various kinds of flow, and about their significance relative to stellar evolution.

White-Dwarf Production in Binary Systems of Large Separation

Abstract: Numerical calculations are carried out for the evolution of a binary system with a primary of 5 M⊙ and a secondary of 2 M⊙, revolving round another in a circular orbit of 300 M⊙. After finishing central helium burning, the primary starts to transfer mass to its companion. After the mass loss, the star of originally 5 M⊙ has become a star of 1 M⊙. This star has a carbon-oxygen core which is a well-developed white dwarf, and a very extended hydrogen shell. The final system has a distance of 815 M⊙; the period is 2.3 years.

Superdense matter in stars

Abstract: After the nuclear evolution of stellar cores has ended stars rapidly approach their final superdense state. The regime of central densities encompassed in such objets extends from 105 g cm-3 in the lighter white dwarts, to 1015 g cm-3 and more, in canonical neutron stars, or stars indefinitely collapsing toward their Schwarzschild surface. Such dense matter generally consists of a highly degenerate (relativistic) electron sea in which are imbedded conventional nuclei for white dwarfs, and mainly neutrons (or hyperons) with a small proton and electron component in the center of typical neutron stars. The very high momentum of the degenerate electrons so greatly reduces their dielectric polarizability that protons, neutrons and nuclei move in an essentially uniform background sea. Such matter has calculable properties for very dense stars : some white dwarts will be crystalline. The heavier (smaller) ones will have a T3 Debye heat capacity and will consequently cool rapidly as is suggested by observations. The outer regions of a neutron star will also be crystalline since the melting temperature exceeds 3 x 108 °K. Below the solid mantle the neutrons and protons form quantum superfluids with a BCS type gap of order one MeV. The gap is isotropic balow nuclear densities and probably anisotropic above. In the center where fhe density can exceed that within a nucleus the pressure is no longer negligible next to the total relativistic energy density and the nature of such matter is very uncertain. Some fundamental aspects of this regime are discussed.

Photometric Effects for Highly Distorted White Dwarf Secondaries in Close Binary Systems

Abstract: As a by-product of an analysis of the light curves of the early-type close binary systems (Rucinski 1969a), similar computations for the highly distorted white dwarf hypothetical secondaries of certain peculiar systems were made. The computations were carried out numerically by integrating the monochromatic fluxes emerging from the atmosphere over the visible surface of the star. The effects of eclipses were not taken into account; the reflection effect was also excluded at this step of analysis. A slightly different model atmosphere was used at each point of the star’s surface depending on the local effective temperature (with the assumption of von Zeipel proportionality T_e ∼ g 1/4). The shape, variations of the effective gravitaty g, and the cosine of the angle between the local normal to the surface and the direction to the observer, μ, were described using the first order perturbation theory for close binary systems (Chandrasekhar 1933). In that theory the Legendre polynomial P2 gives the ellipticity of the star; the next P3 and P4 polynomials describe respectively the non-symmetric and symmetric deviations from ellipticity.

The Nearest Stars (L)

Abstract: Adjoined to this text is the raw material: a list of some 40 stars with the greatest known parallaxes, for which some properties have been measured. Of these well-known stars we shall compute several other properties and then survey the whole collection. Each pair of students works on 5 stars; all results are finally combined.