Showing papers on "White dwarf published in 1981"

Light curve types, absolute magnitudes, and physical properties of galactic novae.

Abstract: For 31 classical galactic novae and four recurrent novae distances, absolute magnitudes and extinction data are derived by different methods. A new absolute magnitude - t3 time relation is established. Two groups of novae are found: very fast and fast novae with smooth light curves and absolute magnitudes between -8 and -11, and fast, slow, and very slow novae with structured light curves and absolute magnitudes between -6 and -7. Group I exhibits quasi-instantaneous mass loss at a luminosity far above the Eddington limit. Group II exhibits continued radiation at the Eddington limit from a bloated white dwarf. A classification scheme for nova light curves which is useful in the context of the new luminosity calibration is described and applied to 95 novae.

Thermal instability in accretion flows onto degenerate stars

Abstract: We report the discovery of a new kind of thermal instability in accretion flows onto stellar atmospheres. The instability results in periodic variations in the height of the standoff shock which forms above the stellar surface. The hard X-ray temperature and luminosity vary with this period, as does any other radiation produced in the hot gas between the shock and the surface. Conditions for the appearance of this instability are most favorable in accreting white dwarf stars, such as cataclysmic variables and AM Her stars. We suggest that a search for such variability be carried out in AM Her stars in both the hard X-ray and UN-optical bands. The resulting period, which is of order 1--100 s for AM Her stars, permits the determination of the accretion rate per unit area at the stellar surface.

Stellar coronae in the Hyades - A soft X-ray survey with the Einstein Observatory

Abstract: An X-ray survey of the central region of the Hyades cluster demonstrates that soft X-ray emission is a common property of the stars in the cluster. Half of the 85 stars surveyed are detected above a sensitivity threshold of 10 to the 28.5th ergs/s at the Hyades distance of 45 pc. The high incidence of X-ray emission and range of observed X-ray luminosities indicate that stellar coronas produce the observed X-ray emission, with a typical X-ray luminosity for solar-type Hyades of 10 to the 29th ergs/s. The use of coronal scaling laws is found to yield reasonable values of maximum coronal temperatures and the fraction of stellar surface covered for the Hyades coronas.

Atmospheres for hot, high-gravity stars. II. Pure helium models

Abstract: We present an extensive grid of pure helium stellar model atmosphere calculations for hot, high-gravity stars. These models are intended to help elucidate the nature and characteristics of the progenitors of the DB white dwarfs. The surface gravity in the model grid ranges from log g = 6.0 (1.0) 9.0, and the effective temperature ranges from T/sub e/ = 25,000 K at log g = 6.0 and 8.0 (T/sub e/ = 40,000 K at log g = 7.0 and 9.0) up to temperatures close to the Eddington limit at each gravity. All our models assume hydrostatic equilibrium, steady-state statistical equilibrium, pure helium composition, and plane-parallel geometry. Most of our models are unblanketed and in LTE, and include convective energy transport. In addition to these, smaller preliminary grids of NLTE models (assuming detailed radiative balance in the helium lines) and of LTE, helium-line--blanketed models (including convective energy transport) were computed to assess the importance of these effects.

The X-ray spectrum of AM Herculis from 0.1 to 150 keV

Abstract: No significant flux at 100 keV was detected in the observations by the HEAO 1 satellite (March and April of 1978) and in several OSO 7 observations The spectrum above 2 keV can be fitted by a composite thermal bremsstrahlung model that includes an approximation to the albedo expected from the white dwarf The bremsstrahlung kT sub e from this model (309 + or - 45 keV) implies a white dwarf mass in excess of 06 solar mass An emission feature at 65 + or - 015 keV and equivalent width of 08 + or - 01 keV is confirmed; it is thought that this might be due to fluorescence from the white dwarf by the bremsstrahlung from a small thin shocked region It is noted that the continuum could also have been steepened at high energy in scattering in the accretion column, but the line photons cannot have gone through the same optical depths

Coordinated X-ray, ultraviolet and optical observations of AM Herculis, U Geminorum, and SS Cygni

Abstract: Simultaneous and quasi-simultaneous optical, UV and X-ray observations of the accreting degenerate dwarf stars AM Herculis, U Geminorum and SS Cygni are reported. The observations were obtained in March 1979, by instruments on board the Einstein Observatory and the IUE satellite, and optical properties were monitored on the ground. AM Her was found to be in a high state at the time of the observations, while SS Cyg and U Gem appeared to be in the optical low state. The presence of a strong UV excess is found in SS Cyg, U Gem and AM Her, which most likely originates from the boundary of the accretion disk in U Gem. The observed excess, with a blackbody component greater than 10 eV, is noted to be inconsistent with standard accretion disk and column models. It is suggested that nuclear burning at the surface of the white dwarf may be responsible for the excess UV flux in the three systems, with differences in spectral distributions resulting from different levels of magnetic field intensity and accretion rate.

The interstellar medium and the highly ionized species observed in the spectrum of the nearby white dwarf G191-B2B

Abstract: High-resolution spectra of the nearby (48 pc) white dwarf G191-B2B, obtained with the International Ultraviolet Explorer, reveal sharp resonance lines of N V, C IV, and Si IV. The origin of these features is most likely linked to the white dwarf, possibly being formed in an expanding halo around the star. Interstellar lines of C II, N I, Mg II, Si II, and Fe II are also seen in the spectrum. Analysis of these features indicates an average neutral hydrogen number density of 0.064 for this line of sight. In combination with the recent EUV and soft X-ray results, this is interpreted to mean that the interstellar medium in the most immediate solar vicinity is of the normal density n approximately equal to 0.1/cu cm of lower ionization, while just beyond it, at least in some directions, is a hot lower density plasma. These results are apparently in conflict with the model of the interstellar medium by McKee and Ostriker (1977) in its present form.

Rapid oscillations in cataclysmic variables. V - H2252-035, a single-sideband X-ray and optical pulsar

Abstract: Photometric and spectroscopic observations of the optical counterpart of the X-ray source H2252-035 reveal the existence of stable periodicities of 3.59 hr and 14.31 min. Both radial velocity and photometric variations occur at the longer period, suggesting that it is the true orbital period of the underlying cataclysmic binary. The shorter period appears as a 10% modulation in the light curve, superposed on the 3.59 hr variation. The frequency is the lower orbital sideband of the X-ray pulsation frequency, suggesting that the optical pulses arise from reprocessing in the atmosphere of the secondary. Unlike the majority of X-ray pulsars, H2252-035 appears to contain a white dwarf rather than a neutron star.

Accreting white dwarf models for type 1 supernovae. 1: Presupernova evolution and triggering mechanisms

Abstract: As a plausible explosion model for a Type I supernova, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs. The relationship between the conditions in the binary system and the triggering mechanism for the supernova explosion is discussed, especially for the cases with relatively slow accretion rate. It is found that the growth of a helium zone on the carbon-oxygen core leads to a supernova explosion which is triggered either by the off-center helium detonation for slow and intermediate accretion rates or by the carbon deflagration for slow and rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion, since in this case the initial mass of the white dwarf is an important parameter for determining the mode of ignition. Finally, various modes of building up the helium zone on the white dwarf, namely, direct transfer of helium from the companion star and the various types and strength of the hydrogen shell flashes are discussed in some detail.

Identification of 4U 1849-31 with V1223 Sagittarii and discovery of optical pulsations

Abstract: The X-ray source 4U 1849-31 is identified with the 13 mag star V1223 Sgr, on the basis of a precise HEAO 1 modulation collimator X-ray position uniquely selected by the Ariel-5 sky survey location. It is shown by optical spectroscopy and photometry that V1223 Sgr is a cataclysmic variable, displaying a photometric period of 13.2 min that is interpreted as being due to the rotation of a degenerate dwarf. The similarity of this system to H2252-035 is noted.

The origin of supernovae

Abstract: The observations and theory of stellar evolution and supernova explosions give an increasingly refined picture of the origin of supernovae. Stars evolving singly or in wide binaries probably leave stable white dwarf remnants if their mass is or approximately=70 M(.) are rare, but may have played an important role in galactic nucleosynthesis by exploding via the pair formation process. Recent calculations have given support to the notion that radioactive decay of 56Ni to 56Co to 56Fe determines the exponential light curve and the late-time spectra of Type I supernovae (SN I). The ejection of 0.5-1 M(.) of Ni seems to be required to account for the observations.

IUE observations of eight dwarf novae - A study of the outburst cycle from 0.12 to 3.5 microns

Abstract: The dwarf novae are a class of close binary systems with a G-M star transferring mass to a white dwarf. The transferred material accumulates in a disk and accretes onto the white dwarf. Dwarf novae are characterized by outbursts on time scales of weeks to months possibly triggered by instabilities in the disk. To test the steady state models and the behavior of the disk at various stages of the outburst cycle, IUE observations were conducted for eight dwarf novae. Selected UV continuum points were joined to nonsimultaneous optical and IR spectrophotometry and broad-band photography to yield flux distributions over a large wavelength range. These flux distributions are discussed in terms of available accretion disk models.

X-ray observations of symbiotic stars

Abstract: Observations are reported of 19 symbiotic stars made with the imaging proportional counter of the Einstein Observatory. Three of the objects (HM Sge, V 1016 Cyg and RR Tel) were detected as soft X-ray sources. All three have shown slow-nova eruptions in the past 40 years. The data are interpreted as support of a model for slow novae involving thermonuclear events on white dwarfs which accrete from M giant companions. Symbiotic stars in their steady state, not being detected X-ray sources, are presumed to be powered by the accretion process alone.

On intermediate-mass single stars and accreting white dwarfs as sources of neutron-rich isotopes.

Abstract: During the most luminous portion of the asymptotic giant-branch phase, models of intermediate-mass stars first become carbon stars and then produce s-process isotopes in the solar-system distribution. Recent observations of the optically most luminous carbon stars in the Magellanic Clouds introduce the possibility that real intermediate-mass stars lose their hydrogen-rich envelopes during the asymptotic giant-branch phase before they have made s-process isotopes both in large quantities and in the solar system distribution. This encourages a search for alternate sources of these isotopes. A promising site for the production of some neutron-rich isotopes isthe convective helium-carbon region that appears in accreting white dwarfs during helium shell flashes. For appropriate accretion rates, overlap of matter in successive convective zones may lead to an exponential distribution of exposures. Further, because of a small entropy barrier between the convective shell and the hydrogen-rich envelope, protons enter the shell and provide a source of neutrons that, for appropriate accretion rates, is repetitive in strength and either dominates or is complementary to the /sup 22/Ne(..cap alpha..,n)/sup 25/Mg source. This permits an estimate of the distribution of neutron-rich isotopes that is formed after many flashes. The distribution, in most instances, tends to be weighted more toward heaviermore » elements than is the case when /sup 22/Ne(..cap alpha.., n)/sup 25/Mg is the sole source of neutrons. Hence, accreting white dwarfs cannot be major contributors to the enrichment of the interstellar medium in most s-process isotopes. Considerable effort should be devoted toward demonstrating whether or not the bolometrically most lumious asymptotic giant branch stars in local systems obey M/sub BOL/ /sup min/ -6.5, then either the source of most Galactic s-process isotopes is as yet unknown, or the rate of the /sup 22/Ne(..cap alpha.., n)/sup 25/Mg reaction has been considerably underestimated.« less

The ultraviolet spectrum of the twin-degenerate interacting binary G61-29

Abstract: A low-resolution IUE spectrum (1200 A-1900 A) of the twin-degenerate white-dwarf binary star G61-29 is presented. A comparison is made to similar IUE spectra of the old novae DQ Her and V603 Aql. G61-29 shows N V 1240 A and He II 1640 A in emission. Unlike the old novae, the C IV 1550 A doublet is not present. It is proposed that the mass-losing He white dwarf (with a mass of 0.02 solar mass) is the H-exhausted, He-rich core of an old star in which the original CNO nuclei have been converted to N-14 by the CNO-cycles.

The amazing X-ray light curve of 2A 0311-227

Abstract: The X-ray light curve of the AM Herculis-type binary 2A 0311-227 is presented. A smooth modulation of the X-ray flux at the orbital period is obtained, along with an eclipse feature that is total for 0.1-0.8 keV photons yet not visible for photons above about 3 keV. This strong and smooth dependence on energy suggests an origin in the obscuration of the X-ray emitting region by a column of accreting gas. Furthermore, very strong quasi-periodic pulsations with a period of about 6 min are found in the soft X-ray light curve. This periodicity is also present in the optical and hard X-ray light curves, and in the circular polarization data. It seems likely that the periodicity arises from a modulation of the mass accretion rate onto the white dwarf, and possible that it originates in pulsations of a disk-magnetosphere boundary.

The ultraviolet spectrum of the magnetic degenerate star GD229

Abstract: The ultraviolet spectrum obtained with the IUE satellite of the magnetic degenerate white dwarf star GD 229 is presented. The ultraviolet energy distribution measured by the short- and long-wavelength cameras covering the range 1170-3200 A, when combined with optical data, indicates a best fit blackbody temperature of 16,000 K, which is significantly lower than previous estimates. A broad absorption trough in the 2000-3000 A region is observed which may be produced by opacity in the fundamental cyclotron frequency for surface magnetic fields from 3.2 to 5 x 10 to the 8th gauss. The absence of a strong Lyman-alpha sigma + component suggests that the primary atmospheric constituent is not hydrogen, with various absorption features probably attributable to Zeeman components of neutral helium, or possibly Mg II, C II and other metal lines. The superposition of many strong Zeeman features is also an alternative interpretation of the absorption in the 2000-3000 A interval.

An X-ray survey of nine historical novae

Abstract: The Einstein Observatory imaging proportional counter was used to search for X-ray emission from nine nearby historical novae. Six of the novae were detected with estimated X-ray intensities between .1 to 4 keV of 10 to the -13th power to 10 to the -11th power ergs/sq cm-s, comparable to the intensities of previously detected cataclysmic variables. The X-ray intensity of one of the novae, V603 Aql, varies over times of several hundred seconds. The data suggest a correlation between the decay rate of the historical outburst and the current X-ray luminosity. Alternatively, the X-ray luminosity may be related to the inclination of the binary system.