Showing papers on "White dwarf published in 1989"

Theory of accretion disks

Abstract: Conference Photograph.- Preface.- List of Participants.- Welcoming Address.- Observational Evidence for Accretion Disks in Galactic Nuclei.- The Low-Frequency Spectra of Accretion Disks in Active Galactic Nuclei: The Dark Side of the Big Blue Bump.- Observational Constraints on Viscosity in the AGN Accretion Discs.- Accretion Disk Magnetohydrodynamics and the Origin of Jets.- Formation of Relativistic MHD Jets in the Magnetosphere of Accretion Disks.- Nonsteady MHD Jets from Magnetized Accretion Disks - Sweeping-Magnetic-Twist Mechanism -.- Statistical Modelling of IR/UV Spectra in AGN.- Formation and Evolution of the Solar Nebula.- Angular Momentum Transport in Protostellar Disks.- Protostellar Discs.- Formation of Viscous Protostellar Accretion Disks.- Formation and Structure of Protostellar Accretion Disks.- Merging White Dwarfs, Disk Formation and Type I Super-Novae.- Variability of Active Galactic Nuclei and Galactic QPO Sources: A Diagnosis.- Nonlinear Pulsation in the Transonic Region of Geometrically Thin Accretion Disks.- One-Armed Oscillations Of Disks and Their Applications.- Thermal and Tidal Instabilities in Accretion Disks of Dwarf Novae.- Hydrodynamic Simulation of Accretion Disks in Cataclysmic Variables.- Simulations of Accretion Flow in Close Binary Stars.- Viscous Evolution of Accretion Discs in the Quiescence of Dwarf Novae.- Black Hole Accretion Disc Instability and Soft X-Ray Transients.- Non-Axisymmetric Shear Instabilities in Thick Accretion Disks.- Instabilities of Accretion Flows Caused by the Interaction of Internal Gravity Modes.- Simulations of Three-Dimensional Slender Tori.- Accretion Disks in Low Mass X-Ray Binaries.- Reflected Accretion Disk Emission Lines in Cataclysmic Variables.- The Structure of the Boundary Layer of Accretion Disks in Cataclysmic Binaries.- The Dynamics of Twisted Accretion Discs.- A Free Accretion Disk in Sn 1987A?.- The Spectra of Relativistic Accretion Disks.- The Compact Object in SS433: Neutron Star or Black Hole?.- Physics of Accretion by Spiral Shock Waves.- Spiral Shocks in Accretion Disks: A Preliminary Numerical Study.- Mass Transfer by Tidally Induced Spiral Shocks in an Accretion Disc.- Accretion Disks and the Link Between an AGN and its Host Galaxy.- Large-Scale Accretion Flows in AGN.- The Disk Accretion of a Tidally Disrupted Star onto a Massive Black Hole.- Line Radiation from Stationary Accretion Disks.- Continuum Spectra of Accretion Discs.- Influence of Radiative Transfer on the Vertical Structure of Accretion Disks.- Dusty Disks and the Infrared Emission from AGN.

Recent progress in understanding the eruptions of classical novae

Abstract: Dramatic progress has occurred in the last two decades in understanding the physical processes and events leading up to, and transpiring during the eruption of a classical nova. The mechanism whereby a white dwarf accreting hydrogen-rich matter from a low-mass main-sequence companion produces a nova eruption has been understood since 1970. The mass-transferring binary stellar configuration leads inexorably to thermonuclear runaways detected at distances of megaparsecs. Summarized here are the efforts of many researchers in understanding the physical processes which generate nova eruptions; the effects upon nova eruptions of different binary-system parameters (e.g., chemical composition or mass of the white dwarf, different mass accretion rates); the possible metamorphosis from dwarf to classical novae and back again; and observational diagnostics of novae, including x ray and gamma ray emission, and the characteristics and distributions of novae in globular clusters and in extragalactic systems. While the thermonuclear-runaway model remains the successful cornerstone of nova simulation, it is now clear that a wide variety of physical processes, and three-dimensional hydrodynamic simulations, will be needed to explain the rich spectrum of behavior observed in erupting novae.

Three-dimensional hydrodynamical simulations of stellar collisions. II. White dwarfs

Abstract: Three-dimensional numerical simulations are presented for collisions between white dwarfs, using a smooth-particle hydrodynamics code with 5000 particles. The code allows for radiation and degenerate pressure and uses a reduced nuclear network which models the large release of nuclear energy. Two different collision models are considered over a range of impact parameters: between two 0.06 solar-mass C-O white dwarfs and between 0.9 solar-mass and 0.7 solar-mass C-O white dwarfs. In nearly head-on collisions, a very substantial fraction of the mass is lost as a result of a large release of nuclear energy. In grazing collisions, the fraction of mass lost is close to that produced in collisions between main-sequence stars. The quantity of processed elements ejected into the ISM by these collisions does not significantly affect the chemical evolution of the Galaxy. 24 refs.

A study of the white dwarf luminosity function

Abstract: The dependence of white dwarf luminosity functions on the assumed age of the Galactic disk is examined, taking into account the relationship between white dwarf mass and progenitor mass, the progenitor lifetime, the mass dependence of cooling curves, and possible variations in the birthrate function in the past. At low luminosities, the resultant distribution in number versus luminosity depends very strongly not only on the shape of the cooling curves for different masses, but also on the spectrum of white dwarf masses that contribute to the distribution. For an age of about 9 Gyr, it is suggested that at luminosities only a quarter of a magnitude fainter than the dimmest white dwarfs yet found, the space density of white dwarfs is larger by a factor of 10 than the space density which the Winget et al. (1987) luminosity function suggests. 43 refs.

Model stars with degenerate dwarf cores and helium-burning shells: a stationary-burning approximation

Abstract: The characteristics of model stars consisting of a degenerate dwarf core and an envelope which is burning a nuclear fuel or fuels in its interior are explored. The models are relevant to stars which are accreting matter from a companion, to single stars in late stages of evolution, to stripped noninteracting remnants of binary star evolution, and to merging and merged degenerate dwarfs. For any given mass and choice of nuclear fuels, a sequence of models is constructed which differ with respect to the mass of the degenerate core and the envelope characteristics. Each sequence has at least three distinct branches: a degenerate dwarf branch along which envelope mass increases with decreasing luminosity, a plateau branch characterized by a very small envelope mass and by a nearly constant luminosity which reaches the maximum achievable value for the sequence, and an asymptotic giant branch which is at the lowest temperatures achievable and along which envelope mass decreases with increasing luminosity. 78 refs.

Subdwarf studies. I. UBVRI photometry of NLTT stars

Abstract: UBVRI photometry is presented for a sample of 1656 southern stars, including 1211 that were previously unmeasured, drown from the NLTT proper-motion catalog. The catalog is shown to be a rich source of subdwarfs. The normalized ultraviolet excess delta (U - B)0.6, photometric parallax, and interstellar reddening are calculated for each star when possible. Photometric parallaxes are compared with trigonometric parallaxes from the literature. It is found that the former do not have systematic errors greater than about 25 percent. In agreement with other studies, the bluest subdwarfs are found at B - V = 0.35. The selection of the program stars on the basis of large reduced proper motions restricted subgiant contamination of the sample to about 5 percent and increased the discovery fraction of halo stars relative to disk stars. The claim is made here that the sample can be used to investigate the abundance distribution of the halo. The sample includes stars with ultraviolet excesses characteristic of disk abundances but with velocities up to 150 km/s. These are believed to be stars that, quite expectedly, reside in the high-velocity tail of the disk velocity distribution. 63 refs.

The optical/infrared counterpart(s) of IRAS 18333-2357

Abstract: Observations of the potential optical counterparts of the unusual source IRAS 18333-2357 show that this source is associated with an extraordinary planetary nebula system in the galactic globular cluster M22. Three distinct optical objects were found within 2" of the IRAS 18333-2357 position as determined by precisely locating the 20 μm infrared source. One object is a red star with m_v ≈ 14.7 mag, which appears to be an unrelated background field star that is possibly significantly reddened beyond the line-of-sight reddening to M22. The second stellar object is a very blue star with m_v ≈ 14.3 mag located about 1~3 south of the red star. Absorption lines of He 11 and possibly H are present in 4000-5000 A spectra of the stellar pair, similar to spectra of planetary nebula nuclei. The third member of this optical triple is an extended emission line nebulosity approximately 10" x 7" in size, centered about 1" east and south of the red star. The ionized gas in this nebulosity is extraordinarily oxygen-rich and neon-rich relative to both hydrogen and helium compared to the atmospheres of M22 red giants and is substantially oxygen-rich and neon-rich relative to hydrogen in comparison with typical planetary nebulae. This nebulosity is almost certainly in M22. The blue star is also very likely to be a member of M22, the source of ionizing photons for the nebulosity and probably the luminosity source for IRAS 18333-2357. We suggest that the dust responsible for the strong infrared emission of IRAS 18333-2357 is physically associated with the M22 nebulosity. In this case the total nebular mass, comprised of 3-10 x 10^(-4) M_☉ of ionized gas plus > 6 x 10^(-4) M_☉ of silicate or carbon-based grains, is possibly dominated by the dust component. The 0, Mg, Si abundances in the case of silicate grains, or carbon in the case of carbon-based grains, may be enhanced relative to hydrogen by at least a factor of 1000 compared to solar abundances. The relative abundances and mass of the M22 nebula are very unusual among known planetary nebulae. It is speculated that this system in M22 may be the result of the interaction within a close binary system containing at least one 0-Ne white dwarf component, or perhaps related to planetary nebulae like A30 and A78.

On the disrupted magnetic braking model for the period gap of cataclysmic variables

Abstract: The disrupted magnetic braking theory for the period gap of cataclysmic variable systems is used to study the binary evolution of low-mass main-sequence-like stars with white dwarf companions. The model is able to reproduce the observed location and width of the gap provided that the average mass transfer rates above the upper edge of the gap are greater than about 1.9 x 10 to the -9th solar masses/yr. For the case of angular momentum loss by magnetic braking, the slope of the mass transfer rate with respect to orbital period is shown to range from 3.4 to 3.7. For the evolutionary sequences considered, the He-3 abundance at the surface of the secondary exceeds 0.0015 after the complete mixing phase, resulting in modifications in the nuclear burning development of nova explosions. 31 refs.

The Fastest Evolving White Dwarfs

Abstract: The evolution of white dwarfs (WDs) at their lowest luminosities is investigated by computing a reference track with solar metal and helium abundances down to the beginning of WD evolution. The main characteristics of the cooling tracks are described, including the onset of crystallization and its completion, and the differentiation in the relation T(c) - T(eff) is shown for the tracks. It is shown why the evolutionary times do not shorten abruptly at a given luminosity as a result of Debye cooling. The structure of the coolest models is shown to consist of dense atmospheres, with photospheres lying at the boundary of pressure ionization. A study of the resulting luminosity functions (LFs) shows that fast cooling never occurs, and that the LF in the crucial region log L/L(solar) between -4 and -6 is either flat or slowly decreasing. Comparisons with the observed LFs explains well the peak or flattening of the LF at log L/L(solar) = -3 or less but fails to reproduce the drop at log L/L(solar) = -4.5. 48 refs.

Eclipse line profiles in cataclysmic variables - Evidence for absence of accretion disks

Abstract: Emission-line profiles of several cataclysmic variables have been observed through eclipse, and they do not show strong evidence for the rotational signature that is characteristic of an accretion disk. The lines appear to be formed in material that has some rotation with respect to the white dwarf, but that also has a substantial velocity directed radially with respect to the degenerate dwarf. The data are suggestive that the emission lines of many novae and novalike variables may not originate in disks, but in rotating polar accretion columns, and that the existence of a strong magnetic field may be an important parameter for the nova phenomenon. 31 refs.

Stellar wind during helium nova outburst

Abstract: The mass ejection process during helium shell flashes on a 13 solar masses white dwarf which accretes pure helium matter is examined Full cycles of helium shell flashes are followed by two different kinds of approaches: one is a time-dependent hydrostatic calculation, and the other is the construction of the sequence consisting of static models and steady state models with wind mass loss It is found that a stellar wind occurs if the shell flash is strong enough, ie, the mass accretion rate is lower than 17 x 10 to the -6th solar masses/year The mass accumulation ratio, ie, the ratio of the processed matter which remains after a shell flash to the initial envelope mass, has been obtained This ratio depends strongly on the mass accretion rate If the binary size is small enough, further mass loss occurs due to the outer critical Roche lobe overflow This mass loss seriously affects the existing scenarios on the neutron star formation induced by the accretion 31 refs

The effect of the white dwarf magnetic field on dwarf nova outbursts

Abstract: Si la naine blanche dans une variable cataclysmique possede un champ magnetique intense, la region interne du disque d'accretion est brisee. Cet effet est calcule sur la forme des sursauts de novae naines dans des modeles d'instabilites du disque et dans des modeles d'instabilite de transfert de masse

A soft X-ray survey of hot white dwarfs with Exosat

Abstract: Broadband multicolor soft X-ray photometry of 34 hot white dwarfs is presented, obtained with the Low Energy Imaging Telescopes on Exosat. By combining the soft X-ray fluxes with measurements of the stellar parameters in the UV and optical bands, photospheric He abundances for 16 of the 21 hot DA white dwarfs are derived. Supplemented by five stars previously observed with Einstein, a total of 21 helium abundances is compiled which span a range between He/H less than 10 to the minus 5 and He/H = 1/100, for effective temperatures (Te) between 25,000 and 65,000 K. For the cooler DAs in the sample (Te less than 45,000 K) evidence is found that accretion from the interstellar medium is the dominant mechanism determining the photospheric He content. For the six stars at higher Te, no compelling evidence for a positive correlation between Te and He/H is found. 43 refs.

Determination of the atmospheric parameters of the binary DA white dwarf L870-2 (EG 11)

Abstract: An analysis of the close detached binary DA white dwarf system L870-2 is presented, with the aim of determining atmospheric parameters for both components of the system. Medium-resolution spectrophotometry of the high Balmer lines, Stroemgren photometry, and the observed luminosity excess are used, together with the observed mass ratio, to constrain the effective temperature and surface gravity of each star. Values obtained included T(1) = 7470 + or - 500 K, log g(1) = 7.80 + or - 0.10, T(2) = 6920 + or - 500 K, log g(2) = 7.89 + or - 0.10. The corresponding masses are 0.47 + or - 0.05 M solar mass and 0.52 + or - 0.05 solar mass for an assumed helium core composition. 15 refs.

A spectroscopic, photometric, and X-ray study of the DQ Herculis system 1H0542-407

Abstract: Spectroscopic and photometric observations of the DQ Herculis binary 1H0542-407 are reported, and models are derived based on these and earlier observations. The system consists of a high-mass, about 1.3 solar mass white dwarf with a spin period of 1911 sec in a 5.7-h orbital period with about a 0.6 solar mass M0 dwarf secondary. The system is viewed at low inclination, so that only one accreting pole is observed. An accretion disk is disrupted by the white dwarf's magnetic field at a distance of about 20,000 km. The gas subsequently accretes along the field lines in the manner of an auroral curtain or arc. A strong shock is produced just above the surface, with the hot gas cooling via bremsstrahlung emission, producing a luminosity of (5-10) x 10 to the 34th ergs/sec. It is shown that the magnetospheric gating model is inapplicable to the star unless the magnetic field strength is inordinately high. 84 refs.

The nebular phase of Nova GQ Muscae 1983 - Evolution of the ionization of the optical spectrum

Abstract: Spectroscopic observations of Nova GQ Muscae 1983 over a period of 4 years during its nebular stage reveal a steady increase in the ionization of the ejected envelope. The forbidden Fe x 6374 A coronal line first appears in the spectrum more than 2 years after maximum, and by the end of the fourth year has become the strongest line in the spectrum. It is shown that the high ionization is caused by photoionization from a hot radiation source with T greater than about 400,000 K whose temperature increases with time. The observations can be explained in the context of the constant bolometric luminosity phase caused by hydrostatic hydrogen burning on the white dwarf remnant. They appear to be an important observational verification of this phase in the nova which is predicted by the thermonuclear model of the classical nova outburst. 18 refs.

Type IA Supernovae in Elliptical and Spiral Galaxies: Possible Differences in Photometric Homogeneity

Abstract: It is shown that β, the initial postmaximum rate of SN brightness decline (in the B band) defined by Pskovskii, may have a smaller dispersion among SNe Ia in elliptical galaxies than in all other types of galaxies. Contamination of the sample by SNe Ib is unlikely to be the primary cause of this difference. Although the number of objects is very small, it is also possible that the velocity of SN Ia ejecta in elliptical galaxies is lower than in spiral galaxies. If correct, these observations provide the first direct evidence for physical differences among SNe Ia in different environments; reddening variations due to gas and dust are unlikely to produce most of the observed dispersion in β among spirals. One obvious possibility is that the SNe Ia in spiral galaxies come from intermediate-mass stars and that differences in the metallicities, accretion rates, or other properties account for the observations. A more extreme, improbable explanation is that not all SNe Ia in spiral galaxies result from carbon deflagrations of carbon-oxygen white dwarfs.

Modeling the classical nova outburst. I - Exploring the physics of a new mechanism

Abstract: Model calculations were performed to describe a mechanism that produces classical nova outbursts on white dwarfs of 1 solar mass or less and for accretion rates of 4 x 10 to the -10th solar mass/yr or greater, i.e., the parameters corresponding to observed data of nova systems. Calculations point to four factors that can induce nuclear runaways of sufficient strength to eject about 0.0001 solar mass at speeds of several hundred to a few thousand km per second, as is observed in classical novae. These are (1) the effects of storage of angular momentum in the star's envelope during the accretion phase; (2) the reduction of centrifugal forces in the star's outer layers during the early nuclear runaway phase, through the inward transport of angular momentum; (3) the inward movement of the zone of peak nuclear burning through the convectively induced shear instability during the runaway phase; and (4) the mixing of original CO stellar matter and H-rich matter, also through the convectively induced shear instability.

Neutron stars and white dwarfs in galactic halos

Abstract: The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

Angular momentum of accreting white dwarfs - Implications for millisecond pulsar formation

Abstract: The evolution of the angular momentum and angular velocity of uniformly rotating magnetized white dwarfs (WDs) accreting at a constant rate from a thin disk is examined. Depending on the initial mass of the WD, its field strength, and mass accretion rate, the end result is one of the following cases: (1) the central density of the WD exceeds the critical density for collapse and a rapidly rotating neutron star is formed directly; (2) the WD collapses first to a fizzler and later becomes a neutron star; or (3) the WD attains the maximum angular momentum allowed for its mass (in order to accrete any more mass and collapse the WD must lose angular momentum, but the mechanism by which it might do this is not clear). Unless case (3) can produce collapse, the number of systems that lead to low-field short-period neutron stars, similar to the known msec pulsars, is small. 22 refs.

A Compact Planetary Nebula around the Hot White Dwarf EGB 6/PG 0950+139

Abstract: The remarkable central star (0950 + 139), a very hot DA/DAO white dwarf, of the planetary nebula EGB 6 is described. Follow-up observations relevant to the analyses of both the nebula and the stellar photosphere are presented. Three kinds of scenarios are discussed to account for the existence of this peculiar nebula, but none appears very promising. The first consideration is that the nebula was ejected from the white dwarf as a discret event. This hypothesis is heavily constrained by the nebular size, density, and expansion rate; by the low luminosity and radius of the star; and by the absence of evidence for variation in density-sensitive forbidden lines from 1978 to 1987. No plausible mechanism can cause the observed amount of mass to be lost directly from a white dwarf in a steady or sporadic wind, at outflow velocities orders of magnitude below the escape velocity. Final consideration is given to the possibility that the gas is lost from a close companion star, but there is no evidence that this is a close binary system.

Radio emission from stellar flares

Abstract: An overview is given of the observations of stellar radio ‘flares’, defined as radio emission which is both variable in time and created by explosive releases of magnetic energy The main sources of such flares are late-type Main-Sequence stars, classic close binaries, X-ray binaries, and pre-Main-Sequence stars

The luminosity function of white dwarfs in the local disk and halo

Abstract: The luminosity function (LF) and total space density of white dwarfs in the solar neighborhood contain important information about the star formation history of the stellar population, and provide an independent method of measuring its age. The first empirical estimates of the LF for degenerate stars were those of Weidemann (1967), Kovetz and Shaviv (1976) and Sion and Liebert (1977). The follow-up investigations made possible by the huge Luyten Palomar proper motion surveys, however, added many more faint white dwarfs to the known sample. While the number of known cool white dwarfs grew to nearly one hundred, these did not include any that were much fainter intrinsically than the coolest degenerates found from the early Luyten, van Biesbroeck and Eggen-Greenstein lists.

An emission-line model for AM Herculis systems

Abstract: The optical spectra of the AM Herculis binaries are characterized by extremely complex emission lines whose profiles can be resolved into at least three components which are formed in different regions of the accretion stream leading from the companion star toward the magnetic white dwarf. A theoretical model is presented for the radial velocity and velocity dispersion of the broad emission line component assuming that it originates mainly in the gas which is diverted out of the orbital plane and funneled onto the white dwarf surface along magnetic field lines. The model is used to locate the line-forming region in three AM Her variables: E1405-451, CW 1103+254, and EXO 033319-2554.2, using as constraints the radial velocity and velocity dispersion data. The analyses of these systems show that the material is threaded by the magnetic field in a very azimuthally extended coupling region located 0.5-0.75 of the way between the white dwarf and the inner Lagrange point. 36 refs.

Accretion onto Hot White Dwarfs in Relation to Symbiotic Novae

Abstract: Numerical calculations are used to study the hydrodynamic evolution of a hot white dwarf with 1 solar mass accreting hydrogen-rich matter at rates between 10 to the -8th and 10 to the -6th solar masses/yr. It is found that for accretion at a rate of about 10 to the -8th solar masses/yr, nova-type outbursts of long duration occur at intervals of about 1500 yr. About half of the accreted envelope is ejected during these outbursts. At a rate of about 10 to the -7th solar masses/yr, the star alternates between comparable periods at a high plateau luminosity and giant dimensions and periods at a low luminosity and white dwarf dimension. At 10 to the -6th solar masses/yr, equilibrium is achieved with a typical red giant luminosity supported by steady hydrogen burning. It is concluded that symbiotic novae are more likely to occur in detached systems involving wind accretors. Thus, the contribution of symbiotic stars to the frequency of type I supernovae is severely constrained. 39 refs.

Offset magnetic dipoles in white dwarfs

Abstract: Various authors have reported observations of the flux and circular polarization for the four stars KPD 0253 + 5052, PG 1658 + 441, PG 1533 - 057 and K 813 - 14. On the basis of the observational data, the stars were classified as magnetic white dwarfs. To place constraints on the magnetic field strengths and geometries of these stars, the relevant authors qualitatively compared the data with available theory and, in two cases, used a model of optically thin hydrogen threaded by a magnetic field. In this paper, a more detailed model for magnetic white dwarfs is used to assess the results previously obtained for these four stars. For one star (KPD 0253 + 5052), clear evidence is found for deviation from a centered dipole field structure. The observational data for the other stars are not uniquely explained by centered dipole models. The observations can be interpreted, however, through the additional use of dipole field geometries offset from the stellar center. 15 refs.

The coalescence of white dwarfs and type I supernovae

Abstract: In the context of the white dwarf coalescence model for type la supernovae, we compute post-coalescence configurations involving a thick disk, rotating around a central white dwarf (the original primary), having the same total mass, angular momentum and energy as the initial system. We show that carbon ignition in rather low density material (105 − 106, g.cm−3) can be triggered during the merging process itself or later, by dissipation due to turbulence in the disk. The evolution of the object following carbon ignition is very uncertain.

How much boundary layer heating occurs in an accreting prenova white dwarf

Abstract: Understanding boundary layer heating is crucial in determining the thermal structure of the accreted envelope of a prenova white dwarf. The matched asymptotic expansion method was used to solve consistently for the structure of accretion disks transferring matter onto rotating white dwarfs. The fraction of accretion energy transported into prenova white dwarf envelopes was calculated. These results should be used by modelers of nova eruptions; they will produce significantly lower degeneracies and weaker explosions than expected until now. Detailed models of accretion disks and boundary layers can also be used to calculate the amount of white dwarf heating during a dwarf nova outburst. In general, such models can serve as input to model atmosphere codes to predict more realistic spectra of disk-accreting objects. 29 refs.

Evidence for a High-Mass White Dwarf in Nova V1500 Cygni 1975

Abstract: Time-resolved spectroscopy of V1500 Cyg are presented from 1981, when the nebular lines from the nova ejecta had faded sufficiently to permit reliably radial velocity measurements on emission lines from the binary remnant. The spectra possess a typical AM Herculis signature. The wavelength region 3800-5100 A contains a number of strong lines, in particular the Balmer series and He II 4686 A. All lines show periodic velocity changes with the same period as the photometric variations. On the first night the lines are relatively weak and narrow, are in phase with the photometric cycle, and suffer extreme modulation; two months later they are strong and broad, are roughly 90 deg out of phase with the photometric cycle, and have relatively less variation in their line fluxes. These features support the suggestion that the remnant is an AM Her system. The observed radial velocity of the secondary is 193 + or - 9 km/s. Various constraints suggest that the magnetic white dwarf in this system has a mass over 0.9 solar.