Showing papers on "White dwarf published in 1979"

X and UV radiation from accreting magnetic degenerate dwarfs.

Abstract: We have carried out detailed numerical calculations of high-harmonic cyclotron emission from a hot plasma, and from these calculations we have developed at self-consistent, quantitative model of X-ray and UV emission from accreting magnetic degenerate dwards. We find that such stars are typically strong UV sources, and emit only at small fraction of their luminosity in soft and hard X-rays. The qualitative features of the X and UV spectra suffice to determine the mass, magnetic field strength, and luminosity of the source with relative confidence. Our results show that the soft and hard X-ray emission observed from AM Her, AN UMa, and 3A 0211--227 are typical of these objects.

Rapid oscillations in cataclysmic variables. III. An oblique rotator in AE aquarii

Abstract: A rapid, strictly periodic oscillation has been discovered in the light curve of the novalike variable AE Aquarii. The fundamental period is 33.076737 s, with comparable power at the first harmonic. The amplitude averages 0.2--0.3% but can exceed 1% in flares. Pulse timings around the binary orbit prove that the periodicity arises in the white dwarf, and lead to an accurate measurement of the projected orbital velocity. The velocity curve and other constraints lead to a mass determination for the component stars :0.74 +- 0.06 M/sub sun/ for the late-type star and 0.94 +- 0.10 M/sub sun/ for the white dwarf. Estimates are also given for the system dimensions, luminosity, distance, and mass transfer rate.Quasi-periodic oscillations are also detected in flares, and have periods near the coherent periods of 16.5 and 33 s. Their characteristics suggest an origin in gaseous blobs produced by instabilities near the inner edge of the accretion disk.A model is presented in which the strict periodicity arises from the rotation of an accreting, magnetized white dwarf, with a surface field of 10/sup 6/--10/sup 7/ gauss. Future spectroscopic, polarimetric, and X-ray observations should provide critical tests for predictions of the model.

Diffusion time scales in white dwarfs

Abstract: The time scales of a few representative elements diffusion in hydrogen- and helium-rich white-dwarf envelopes are presented. The dependences on depth, the stellar mass, and effective temperature are investigated. It is shown that diffusion processes are sufficiently rapid that the observed monoelemental character of white-dwarf spectra can be explained. Even deep convective mixing, present in the surface layers of cooler white dwarfs, cannot bring back to the photosphere heavy elements that have diffused downward. The present calculations lead to estimates of diffusion time scales that are so short that no trace element should be seen in the spectra of white dwarfs. Mechanisms such as accretion competing with diffusion may have to be invoked to explain the small but measurable abundances of heavy elements in the spectra of these objects. For the vast majority of white dwarfs these conclusions are not affected by our having neglected radiative forces or by inaccuracies in the transport coefficients. However, improvements in the estimates of transport coefficients in dense plasmas are needed.

Hydrogen shell flashes in massive accreting white dwarfs

Abstract: The long-term evolution of massive (M>1.1 M/sub sun/) accreting white dwarfs in close binary systems is discussed in the context of several astrophysical problems. The evolution of massive white dwarfs was followed with model calculations. The models have /sup 12/C--/sup 16/O cores and envelopes with composition X=0.7, Z=0.03. The accreted hydrogen-rich matter was assumed to have the same composition as the stellar envelope. A 1.2 M/sub sun/ white dwarf accreting at the rate 1.03 x 10/sup -7/ M/sub sun/ yr/sup -1/ gives rise to repetitive hydrogen shell flashes with an interflash period of 17 years. A 1.2 M/sub sun/ white dwarf accreting at the rate 1.03 x 10/sup -8/ M/sub sun/ yr/sup -1/ has an interflash period of approximately 630 years. A 1.3 M/sub sun/ white dwarf accreting at the rate 2.71 x 10/sup -7/ M/sub sun/ yr/sup -1/ undergoes stable hydrogen burning in a steady state with accretion.The possible relationship of our models to understanding the underlying variability of the blue component of symbiotic variables is discussed. The application of our models to X-ray sources containing massive degenerate dwarfs undergoing rapid accretion is considered.

Synchronous rotation in magnetic X-ray binaries.

Abstract: AM Herculis is thought to be a binary stellar system that contains an accreting magnetic degenerate dwarf whose rotation is synchronous with the orbital period. This synchronism is remarkable, particularly because of the small moment of inertia of a degenerate dwarf and the large specific angular momentum of the accreted matter. This paper demonstrates that ohmic dissipation from the magnetic interaction of the stars is capable of bringing about exact synchronism, provided that some other process has brought the rotation period of the degenerate dwarf to the same order of magnitude as the orbital period. It is also shown that magnetostatic interaction in the synchronous state leads to oscillatory drifts in phase about exact synchronism with periods of approximately 1-10 yr. These phase drifts could manifest themselves in long-term periodic variability in the X-ray or optical properties of the source. Accretion torques could excite such oscillatory motions but need not disrupt synchronism once it has been established.

Mass Loss and Stellar Evolution

Abstract: The available observational evidence about stellar winds and mass loss is briefly reviewed, and its implications for stellar evolution are discussed. Particular emphasis is placed on post-main sequence evolution of low-mass and intermediate-mass stars, including the final evolution from red giant to the white dwarf stage. For massive stars, as well as for the implications of stellar mass loss for supernovae and supernova remnants, the reader is referred to other recent review papers.

Quasi-periodic luminosity variations in dwarf novae.

Abstract: We have identified quasi-periodic oscillations in the light curves of five dwarf novae--U Gem, SS Cyg, RU Peg, KT Per, and VW Hyi-- and in the light curve of the quasi-periodic X-ray source Sco X-1. The mean periods of the quasi-periodic oscillations range from 32 s in SS Cyg to 147 s in KT Per and 165 s in Sco X-l. Their amplitudes are typically 0.005--0.0l mag. The properties of the quasi-periodic oscillations are represented well by a second-order autoregressive process. Use of this representation shows that the length of time over which the quasi-periodic oscillations maintain coherence is very short, typically 3--5 cycles of the oscillations. Thus the quasi-periodic oscillations can be distinguished from the short-period coherent oscillations in dwarf novae, which are usually interpreted as white dwarf pulsations, because t the periods of the quasi-periodic oscillations are 3--4 times longer and their coherence time is much shorter. The quasi-periodic oscillations occur in dwarf novae only during their eruptions and occur in Sco X-l only when the system is bright. The presence of the oscillations does not depend on the subclass to which a dwarf nova belongs or on the morphology of the individual eruptions. We argue that theirmore » short periods, their short coherence times, and their presence in Sco X-l require that the quasi-periodic oscillations be produced by the accretion disk, and not by the stars or by the boundary between the a accretion disk and its central star.« less

PG1159-035: a new, hot, non-DA pulsating degenerate

Abstract: The star was observed with a computer-controlled high-speed photometer. The light curve showed nearly sinusoidal variations with a period of about 8 minutes and a peak-to-peak amplitude of about 0.02 magnitude. Power spectra of the light curves were calculated. They showed peaks corresponding to periods of 460 and 539 seconds. The effective temperature of the star was found to exceed 5 x 10/sup 4/ K; lack of circular polarization indicated that the star does not have a large surface magnetic field. The surface gravity is indeterminate, but probably lower than log g = 8. It is suggested that this star is pulsating in some manner; it is likely to change rapidly. 3 figures. (RWR)

Ultraviolet observations of AM Herculis with IUE

Abstract: Ultraviolet observations of the binary X-ray source AM Herculis from the IUE satellite show strong emission lines of ions from O I to N V, probably originating in photoionized gas, and a continuum which is partially eclipsed in phase with the X-ray eclipse. The emission lines have broad (about 600 km/s) and sharp (about 80 km/s) components at different velocities, as has been seen in optical lines. The continuum is interpreted as two components, a blackbody (blackbody kT of 25-30 eV) which accounts for the X-ray emission below 0.5 keV and the eclipsed part of the UV continuum, and a component which is never eclipsed, whose spectrum is roughly proportional to the inverse of frequency. Strong ultraviolet emission due to optically thick cyclotron emission, which has been predicted theoretically, is not observed. This presents a severe difficulty for the theory of accretion onto the magnetic pole of a white dwarf.

The physics of white dwarfs

Abstract: White dwarf stars, so called because of the color of the first few to be discovered, occupy a key position in astrophysical theory. Together with neutron stars and black holes, they are the terminal points of stellar evolution. Their properties thus provide clues to the physical processes that take place during the rapid and often spectacular evolutionary stages near the ends of stellar lifetimes. In addition, white dwarfs provide astrophysical “laboratories” for “measuring” the physical properties of matter under extreme conditions. These extend from conditions like those in laser‐produced plasmas to those typical of the solid crusts of neutron stars. White‐dwarf stars also occur as components of cataclysmic binary systems—novae, dwarf novae and related objects—and knowledge of the properties of white dwarfs is essential to the development of satisfactory theoretical models for these systems.

Stellar coronae—Evidence for their existence from X- and UV observations

Abstract: Stellar coronae were among the first predicted X-ray sources. Because of their relatively low X-ray luminosities, however, they have been discovered only during the last few years. In the present paper the current state of stellar coronal X- and UV observations has been reviewed, including some preliminary observational results from the HEAO-1 and IUE satellites, but still without any result from the recently launched X-ray satellite HEAO-2. Late 1978 about two dozens of stellar soft X-ray sources have been detected, e.g., normal stars like the Sun (e.g., α Cen), very active stars (RS CVn systems), and possibly a corona around an intermediately hot white dwarf (Sirius B). The observational results of various objects have been discussed and compared with X-ray luminosity predictions based on minimum-flux coronal models.

A spectrophotometric parallax for U Geminorum

Abstract: Spectrophotometry of U Geminorum from lambda3200 to lambda10 900 shows an upturn in flux for wavelengths longward of lambda6000. The energy distribution can be interpreted as the sum of fluxes from an M4.5 or M5 star and a source giving a flat continuum. The presence of an M star is confirmed by a higher-resolution spectrum in the red and near infrared and by the work of Stauffer, Spinrad, and Thorstensen. The M companion is less dense than a main-sequence star of the same color index. The absolute photometry allows a distance modulus of 4.4 +- 0.4 mag to be derived for U Geminorum. The absolute visual magnitude of the quiescent system at orbital phase 0.08 is M/sub V/=10.3 +- 0.8.

X-ray observations of H1908+050 /=SS 433/

Abstract: The X-ray source H1908+050 (=4U 1908+05=A1909+04) was observed for three 6 day periods in 1977 and 1978 with the HEAO A-2 experiment. Because of the positional error box and variability of the source, the unusual emission-line object and variable radio source SS 433 has been suggested as the potical counterpart. The X-ray luminosity of the source varied by a factor of approx.2 on a time scale of 6 months, and the spectrum of the object is consistent with either a power law of photon index GAMMA of 2.1 or with 14.3 keV thermal bremsstrahlung emission with a approx.575 eV equivalent-width iron line. These X-ray characteristics argue against the source being extragalactic, but do not uniquely identify the type of source. The measurements are consistent with emission from a white dwarf with approx.10/sup 8/ gauss magnetic field, but are also similar to the X-ray emission sometines seen from Cir X-1. A search has been made for X-ray emission from similar radio sources, but no new X-ray sources were detected. A previously known source, A1850+00, is a possible counterpart for one of these radio sources.