Showing papers on "White dwarf published in 1980"

Presupernova models and supernovae

Abstract: The present status of theories of presupernova stellar evolution and the triggering mechanisms of supernova explosions are reviewed The validity of the single-star approximation for stellar core evolution is considered, and the central density and temperature of the stellar core are discussed Attention is then given to the results of numerical models of supernova explosions by carbon deflagration of an intermediate mass star, resulting in the total disruption of the star; the photodissociation of iron nuclei in a massive star, resulting in neutron star or black hole formation; and stellar core collapse triggered by electron capture in stars of mass ranging between those of the intermediate mass and massive stars, resulting in neutron star formation despite oxygen deflagration Helium and carbon combustion and detonation in accreting white dwarfs and the gravitational collapse triggered by electron-pair creation in supermassive stars are also discussed, and problems requiring future investigation are indicated

The surface chemistry of stars. I - Diffusion of heavy ions in white dwarf envelopes. II - Fractionated accretion of interstellar matter

Abstract: We have examined the diffusion separation of heavy elements in the envelopes of white dwarfs. Even when there is a deep convective zone, gravitational sedimentation is an efficient process. The convective mixing enhances the settling process. The surface abundances are determined by a balance between accreting interstellar matter and this diffusive separation. The abundances of trace elements are extremely low, and bear no relation to the primordial abundances.

White Dwarf Stars

Abstract: The violated rule to which Professor Eddington referred was the mass­ luminosity relation for dwarf stars. The "strange objects" had the exceed­ ingly low luminosities of the faint red dwarfs, yet their colors were quite bluish. Thus, they came to be called white dwarfs. The high surface emissivities required very small radii-of order 102 Ro-compared with known stars. Yet, at least the companion star to Sirius was known from the orbital solution to have a mass of � 1 Mo. This implied interior densities and surface gravities orders of magnitude higher than for dwarf stars. The physical state of such superdense matter could not be understood until the quantum-statistical theory of the electron gas was worked out by E. Fermi and P. Dirac in the mid-1920's. R. H. Fowler then showed that the enhanced pressure of a degenerate electron gas could support an object of stellar mass against its own self-gravitation at precisely the radii of white dwarfs. Chandrasekhar (1939) modeled the basic interior structure and determined the basic mass-radius-density-composition relationships. Most import­ antly, Chandrasekhar established a critical mass (� 1.44 M 0) above which stable degenerate configurations cannot exist. In the wake of their successes, theoreticians called the configurations degenerate stars or degenerate dwarfs, though observers still used white dwarfs. The stars are certainly not main-sequence dwarfs, and most are too cool to properly be called white. However, the term degenerate stars encompasses neutron stars as well. Since all of the above terms are in widespread use, it is futile to argue which of them is the biggest misnomer ; we shall use them interchangeably in this discussion.

Helium runaways in white dwarfs

Abstract: The long term evolution of an accreting carbon white dwarf was studied from the onset of accretion to the ignition of helium. The variations in the details of the helium shell flash examined with respect to variations in mass accretion rate. For intermediate rates the helium flash is potentially explosive whereas for high rates the shell flash is relatively weak. The results are discussed in the context of the long term evolution of novae.

Soft X-ray pulsations from SS Cygni

Abstract: Pulsed soft X-rays (01 to 05 keV) with a period of 9 sec and a pulsed fraction that varies between 0 and 100% were detected from the dwarf nova SS Cygni at the peak of an optical outburst This detection confirms for the first time the supposed high energy origin of optical pulsations seen in erupting dwarf novae The pulse shape is remarkably sinusoidal for such a large-amplitude oscillation The X-ray pulsation observed in this outburst is not coherent, in contrast to previous claims for the related optical oscillations

Determination of the upper mass limit for stars producing white-dwarf remnants

Abstract: We have searched ultraviolet and red plates of four open clusters (NGC 2168, 2287, 2422, and 6633) for faint blue objects which might be white dwarf members of the clusters. The most massive stars in these clusters range from 3 to 6 M/sub sun/. We find a definite concentration of faint blue objects in the clusters. This fact, plus initial photoelectric photometry, provides strong support for the identification of many of these objects as cluster white dwarfs. By modeling the expected number of possible white dwarfs in each cluster, we are able to put some limits on m/sub w/, the upper stellar mass limit for formation of white dwarfs. Our data require that some stars of at least 5 M/sub sun/ have evolved into white dwarfs and give a most probable value of 7 M/sub sun/ for m/sub w/.

Comments on the evolution and origin of cataclysmic binaries

Abstract: Some aspects of the observational data on cataclysmic binaries are discussed and some possible correlations between type of behaviour and binary period are noted. A gap between 2 and 3 hours in the histogram of binary periods is estimated to be real. A numerical procedure for following the evolution of Roche-lobe-fiUing stars using simplified equations is described. This procedure Is applied to white/red dwarf binaries for a variety of initial conditions, and of mass loss and angular momentum loss mechanisms. The results of these calculations, in which we ignore the short timescale behaviour of the systems, are classified into four modes of evolution: normal, nuclear evolution dominated, angular momentum loss dominated and hydrodynamical. The results are discussed in connection with cataclysmic binaries. The clustering in period below 2 hours is Interpreted in terms of evolution following the hydrodynamical mode, and it is suggested that such systems contain low mass white dwarfs as well as low mass secondaries. These may be the most common type of cataclysmic binary. A possible explanation of the clustering of classical novae systems to binary periods of 3 to 5 hours is mentioned, and evolutionary scenarios for cataclysmic binaries are outlined. We suggest, following Ritter and Webbink, that the short period systems (≲ 2 hrs) arise mainly from late Case B mass transfer in the original binary (original primary mass 1.5 to 3M ⊙ ) and the longer period systems arise mainly from Case C mass transfer. Full text to be published in Monthly Notices of the Royal Astronomical Society.

Thermal and electrical conductivity in white dwarfs and neutron stars

Abstract: Simple relativistic expressions are derived for the electron contribution to the thermal and electrical conductivites kappa, sigma in the degenerate core of a white dwarf and the degenerate layers of a neutron-star envelope (in which the density rho<4 x 10/sup 11/ g/cm/sup 3/). At temperatures T above the crystallization temperature T/sub M/ of the ions (the crystal melting point), the main factor governing kappa and sigma will be the scattering of electrons by ions; for T

The Surface Chemistry of Stars - Part Two - Fractionated Accretion of Interstellar Matter

Abstract: The rate of accretion of the interstellar medium onto a star is discussed. It is shown that this is a complex, fractionated process. The relative abundances of heavy elements accreting onto the star are very different from the abundances in the ambient medium. These accretion rates are important in determining the abundances on the surfaces of white dwarf stars.

The collapse of carbon-oxygen white dwarfs.

Abstract: Carbon-oxygen white dwarfs formed in close binary systems may become unstable by mass accretion. Recent results concerning carbon-oxygen separation at the freezing point during the phase of cooling may have very important consequences for the problem of neutron star formation. The central, high-density regions of the star are then made of pure oxygen, the carbon being rejected to lower-density layers. When the star is compressed, carbon ignition can only happen after neutronization of the central (oxygen) regions. We show that, in this case, the chances of collapse to a neutron star are independent from the rate of mass accretion, in contrast with previous studies. A likely mechanism for neutron star formation emerges from this picture.

4U 1626-67 and the character of highly compact binary X-ray sources

Abstract: We present the results of new observations of the 7.7 s X-ray pulsar 4U 1626--67 with SAS 3. We confirm the presence of quasi-periodic oscillations in the X-ray intensity and demonstrate that these oscillations have a preferred time scale of approx.1 x 10/sup 3/ s but are not strictly periodic. We also place stringent new upper limits on orbital motion of the X-ray star for orbital periods between 10 s and 7 hr. This evidence lends further support to a model for this source composed of a neutron star that is accreting matter from a low-mass main-sequence dwarf or degenerate dwarf companion in a highly compact binary system. If this picture is correct, it is likely that the time scale of the quasi-periodic oscillations is directly related to the orbital period. We discuss other properties of this model, which may be applicable to other galactic X-ray sources. In particular, we analyze the temporal evolution of X-ray luminosity, L/sub x/, from such a system under the assumption that the mass transfer is driven by the decay of the orbit due to gravitational radiation, and we show that high luminosities (L/sub x/> or approx. =10/sup 36/ ergs s/sup -1/) may well bemore » attained during some phases of the binary evolution.« less

White dwarf models for type I supernovae and quiet supernovae, and presupernova evolution

Abstract: Supernova mechanisms in accreting white dwarfs (WDs) are presented, i.e., the carbon deflagration as a plausible mechanism for producing Type I supernovae and electron captures to form quiet supernovae leaving neutron stars. These outcomes depend on accretion rate of helium, initial mass and composition of the WD. The various types of hydrogen shell-burning in the presupernova stage are also discussed.

Acoustic fluxes in white dwarfs

Abstract: Estimates of the acoustic flux generated in white dwarf convection zones are given. Wide ranges of mass, effective temperature, and convective efficiency have been considered. Homogeneous hydrogen- and helium-rich convective envelope models were computed as well as inhomogeneous models representing DA white dwarfs with thin layers of hydrogen surrounding helium-dominated regions. The calculations of the transition zone that separates the hydrogen- and helium-rich layers of these models is extended to take into account the fact that both hydrogen and helium are non-trace species. Only very thin layers of hydrogen are allowed if an underlyign helium convection zone is to produce acoustic noise in these inhomogeneous models. It is suggested that such layers are unstable against mixing. Using the input acoustic flux of the homogeneous envelopes, the soft X-ray luminosities of acoustically heated white dwarf coronae are estimated according to Hearn's coronal model. If such coronae exist, they could be detectable by present day techniques.

Cygnus X-2 - Neutron star or degenerate dwarf?

Abstract: Some conflicting models have been proposed for Cyg X-2: a degenerate dwarf model which predicts a distance of 250 + or 50 pc; and a neutron star model which implies a distance of about 8000 pc. Based on a reddening study, it is found that the distance to Cyg X-2 is greater than 1100 pc, which rules strongly against the degenerate dwarf model. This conclusion is based on observations of the 2200 A feature in the spectrum of Cyg X-2 made with the International Ultraviolet Explorer (IUE), and UBV and spectroscopic observations of 38 field stars. For the reddening of Cyg X-2 values of E(B-V) = 0.40 + or - 0.07 (1 sigma) are found and are consistent with the reddening to infinity in that direction inferred from radio data. Consequently, Cyg X-2 may be located in the halo at about 8 kpc as proposed in 1979 by Cowley, Crampton, and Hutchings.

Masses and radii of white-dwarf stars. IV. The two-color diagram

Abstract: Newly calculated model atmospheres, including both the effects of Lyman and Balmer line blanketing and the effects of convection, are used to determine the surface gravities of white-dwarf stars by the use of the separation of stars with different surface gravities in the Stroemgren and multichannel two-color diagrams. The mean surface gravity of 40 stars in the appropriate temperature range with multichannel colors is log g=8.04 +- 0.25, and the mean value for 35 stars with Stroemgren photomety is log g=7.86 +- 0.25. Comparison of all available investigations of white-dwarf masses shows that they are internally consistent with the possible exception of masses derived from gravitational redshifts, where a slight, marginally significant discrepancy may exist. The weighted mean of all mass determination is 0.75 M/sub sun/, allowing for selection effects. The 1 sigma range of white-dwarf masses extends from 0.5 M/sub sun/ to 1.0 M/sub sun/; we do not find evidence for a smaller mass range.

Accretion disks in cataclysmic variables. I - The eclipse-related phase shifts in DQ Herculis and UX Ursae Majoris

Abstract: A model is described for a cataclysmic-variable binary system in which a large accretion disk reflects radiation from a progradely rotating source on or near the white dwarf sufrace, thus producing rapid oscillations in the optical light. Numerical calculations of the reflection process are presented which simulate the behavior of the oscillations as the system goes through exlipse.

Radial accretion of H-rich material onto a He white dwarf

Abstract: The nova outburst is modeled by spherically accreting H-rich material onto a 1 solar mass He white dwarf at a rate of 10 to the -8th solar mass/yr. The star accretes for 5848 years, when the nuclear reactions run away near the base of the accreted envelope. The nuclear-energy generation rate rises to 4.6(8) solar luminosities, and the envelope expands in response to it. However, nova-like mass ejection does not occur because the envelope is of insufficient mass, the base of the envelope is only mildly electron degenerate, and there is no enrichment of the CNO abundance. To overcome these limiting conditions, it is suggested that the H-rich material be accreted either more slowly than 10 to the -10th solar mass/yr or with angular momentum. Outbursts resulting in the former case should be similar to the nova models computed by Starrfield, Truran, and Sparks. Outbursts in the latter case should be strengthened, and novae might result because the H-rich material will be mixed into the surface layers of the white dwarf, as first suggested by Kippenhahn and Thomas (1978).

The system AM Her = 4U 1814 + 50

Abstract: The binary system AM Herculis = 4U 1814 + 50 gives the first well ascertained example of an X-ray emitting magnetic white dwarf. The orbital period (3.1h) is apparent from X-ray to IR frequencies and in linear and circular polarization. Since the time of the identification of the X-ray source the system has been extensively studied. The observations (which range from 1 MeV to 20 μm) are reviewed and compared with the present theory of X-ray emitting white dwarfs.

Gravitational radiation and the evolution of cataclysmic binaries

Abstract: Results are presented for several evolutionary sequences of close binary systems which resemble dwarf novae. The systems, containing white dwarfs and main-sequence companions, have masses less than 3.4 M/sub sun/ and periods less than 17 hours. The loss of angular momentum by gravitational radiation is included in the calculations. It is found that a system can be captured as a result of evolution dominated by gravitational radiation losses when the primary encounters its Roche lobe while near the main sequence. In such a case, the system evolves to even shorter periods, with the nuclear time scale of the primary increasing so that the latter is unable to exhaust hydrogen.