Showing papers on "White dwarf published in 1985"

On the evolution of close binaries with components of initial mass between 3 solar masses and 12 solar masses

Abstract: The evolution of components of close binary systems of intermediate mass (component masses in the range 3--12 M/sub sun/) and of Population I composition (X = 0.7, Z = 0.02) is studied numerically. We assume that a model star first fills its Roche lobe after it has developed a helium core and becomes a giant and that mass loss from the model continues until model radius decreases below a predetermined limit. How much of the matter that is lost from the Roche-lobe--filling model is transferred to its companion and how much is lost from the system are left undetermined. In the mass loss/exchange process, a model of initial mass in the 3--10 M/sub sun/ range is transformed into a compact ''helium'' star (which, nevertheless, retains some hydrogen-rich matter in its envelope). In our experiments, models of initial mass 3, 4, 5 6.95, and 9.85 M/sub sun/ produce remnants of mass 0.378, 0.523, 0.765, 1.11, and 1.95 M/sub sun/, respectively. The lifetime of the core helium-burning phase of each remnant ranges from 200% (.0378 M/sub sun/ remnant) to 20% (1.95 M/sub sun/ remnant) of the main-sequence lifetime of its progenitor, and the possibility arises that the primordial secondary, if it ismore » initially massive enough or if it has accreted sufficient matter during the first mass-transfer episode, will expand and transfer matter back to the primary remnant while this remnant is still relatively compact. We show that, then, most of the matter outside the helium core of the secondary will be ejected from the system.« less

Carbon ignition in a rapidly accreting degenerate dwarf - A clue to the nature of the merging process in close binaries.

Abstract: Recent studies have suggested that the merging of two degenerate dwarfs composed of carbon and oxygen and of total mass larger than the Chandrasekhar limit occurs at a frequency comparable to that of Type I supernovae. The rate at which mass is transferred in the merging process is at present unknown, except that it must be less than some appropriate Eddington limit. It is found that, unless mass transfer occurs at a rate less than one-fifth of the Eddington limit for an isolated dwarf, carbon is ignited off-center, and ignition does not lead immediately to a deflagration. If carbon continues to burn quiescently until it is exhausted throughout the star, a neon-oxygen-magnesium white dwarf will be formed. If a Type I supernova is to follow from merging white dwarfs, a thick disk must be formed as an intermediate stage in the merging process, with transfer from the disk onto the central degenerate dwarf occurring at a rate sufficiently less than Eddington that a deflagration induced by carbon burning occurs. Thus, the outcome of the merging of two massive carbon-oxygen degenerate dwarfs is not trivially a Type I supernova explosion. 43 references.

X-ray emission from cataclysmic variables with accretion disks. I - Hard X-rays. II - EUV/soft X-ray radiation

Abstract: Theoretical models explaining the hard-X-ray, soft-X-ray, and EUV emission of accretion-disk cataclysmic variables in terms of the disk boundary layer (DBL) are developed on the basis of a survey of the published observational data. The data are compared with model predictions in graphs for systems with high or low (greater than or less than 10-Pg/s) accretion rates. Good agreement is obtained both at low accretion rates, where an optically thin rarefied hot (Te = 10 to the 8th K) DBL radiates most of its energy as hard X-rays, and at high accretion rates, where an optically thick 100,000-K DBL radiates most of its energy in the EUV and as soft X-rays. Detailed analysis of the old nova V603 Aql suggests that previous models predicting more detections of soft-X-ray/EUV emissions from thick-DBL objects (Ferland et al., 1982) used inappropriate dwarf masses, interstellar column densities, or classical-nova space densities.

Cataclysmic variables and low-mass X-ray binaries

Abstract: There is little doubt that the cataclysmic variables are closely related to the low-mass X-ray binaries, in which the accelerating star is usually, or perhaps always, a neutron star. The binary periods are roughly the same, the acceleration rates are similar, both have low-mass companion stars, and both show lots of variability. Despite all these similarities, however, research has proceeded more or less independently for each of the two classes of accreting binaries, and has focused on different aspects of the systems. Papers include major review articles on the observational properties and evolution of low-mass X-ray binaries, recent observational and theoretical work on cataclysmic variables containing a magnetic white dwarf (the ''AM Her'' and ''DQ Her'' stars), and theories of dwarf novae outbursts.

Recurrent novae as a consequence of the accretion of solar material onto a 1.38 M-white dwarf

Abstract: We have computed three evolutionary sequences which treat the accretion of hydrogen-rich material onto 1.38 M/sub sun/ white dwarfs. In each of these sequences the accreting matter had only a solar composition of the CNO nuclei (Z = 0.015). In the first sequence we utilized an accretion rate of 1.7 x 10/sup hyphen8/ M/sub sun/ yr/sup hyphen1/ onto a white dwarf with an initial luminosity of 0.1 L/sub sun/ . It took this sequence '33 yr to reach the peak of the thermonuclear runaway which resulted in an outburst that ejected 3x10/sup hyphen8/ M/sub sun/ . of material moving at speeds up to 2900 kn s/sup hyphen1/. The light curve, the time to outburst, and the amount of mass ejected during the evolution are in excellent agreement with the observed outburst of Nova U Sco 1979. However, only 6% of the accreted envelope was ejected during the outburst. The remaining material quickly burned to helium ('2 yr) and settled back onto the white dwarf. The second study involved an accretion rate of 1.7x10/sup hyphen9/ M/sub sun/ yr/sup hyphen1/ onto a white dwarf with an initial luminosity of 10/sup hyphen2/ L/sub sun/ . It took nearly 1600 yr to reach themore » burst phase of the evolution, and by this time the dwarf had accreted '3x10/sup hyphen6/ M/sub sun/ . Peak temperature in the shell source reached 3.5x10/sup 8/ K, about 1.3x10/sup 8/ K higher than was found for model 1. This sequence ejected 3x10/sup hyphen7/ M/sub sun/, only 13% of the accreted envelope, moving at low velocities. For both of these evolutionary sequences, we find that as a result of the accretion of matter onto a massive white dwarf, the mass of the white dwarf grows toward the Chandrasekhar limit. If our study is a realistic representation of the evolution of U Sco, then this star is well on its way to becoming an SN I.« less

The effects of diffusion due to gravity and due to composition gradients on the rate of hydrogen burning in a cooling degenerate dwarf. I - The case of a thick helium buffer layer

Abstract: The evolution of an isolated carbon-oxygen degenerate dwarf which has a surface layer initially containing hydrogen and helium and is of mass 0.6 M/sub sun/ is followed from the planetary nebula nucleus stage at a luminosity of 5 x 10TL/sub sun/ until luminosity drops to 10 VL/sub sun/. Wind mass loss and accretion from the interstellar medium are neglected. Shortly after the model reaches a maximum surface temperature of about 1.5 x 10V K, CN cycle reactions cease abruptly to contribute dominantly to surface luminosity, which drops to below 100 L/sub sun/. However, hydrogen-burning reactions continue to take place and, at intermediate times in the cooling history, can even contribute energy to surface luminosity comparably with the contribution of energy by cooling ions. Diffusion induced by the gravitational force and by composition gradients affects the distributions of element abundances below the stellar surface in such a way that the rates of burning by both pp-initiated reactions and the SC+p-initiated reactions are altered from the situation when diffusion is neglected. The chemical diffusion of hydrogen inward causes the SC UN reactions to contribute energy for a somewhat longer time than is the case when diffusion is neglected. The gravitational settling ofmore » all heavy elements out of subphotospheric layers causes these elements to effectively disappear from surface layers before the surface temperature is reduced much below 80,000 K and the concomitant drop in opacity in surface layers accelerates the rate at which the interior cools and decreases the rate at which hydrogen burns by the pp and subsequent reactions relative to the situation when diffusion is neglected.« less

Hubble's constant and exploding carbon-oxygen white dwarf models for Type I supernovae

Abstract: The immediate progenitor of a Type I supernova (SN I) is thought to be a mass-accreting carbon–oxygen (C–O) white dwarf in a binary system When the mass of the white dwarf approaches the Chandrasekhar mass (14 M⊙) the C–O nuclear fuel ignites, part of the star is incinerated to radioactive 56Ni, and the thermonuclear energy completely disrupts the star The optical luminosity results from the trapping and thermalization of the γ rays and positrons emitted by the decay of 56Ni through 56Co to stable 56Fe The amount of 56Ni synthesized, MNi, and the corresponding peak luminosity, Lmax, can be used with the observed Hubble diagram for SN I to determine the value of Hubble's constant, H0 We argue here that if this model is correct, MNi is in the range 04–14 M⊙, the best estimate being 06 M⊙, and that H0 is in the range 39–73 km s−1 Mpc−1 with a best estimate of 59 km s−1 Mpc−1 This line of reasoning does not require knowledge of the temperature of the supernova and, therefore, is not subject to the uncertainties associated with attempts to determine supernova luminosities and distances by the Baade method1 It relies on the physical correctness of the model, which is subject to independent tests

Accreting white dwarf models for type I supernovae. IV: The optical spectrum of a carbon-deflagration supernova

Abstract: Theoretical optical spectra for a carbon-deflagration model of a Type I supernova are computed and compared to spectra of supernova 1981b in NGC 4536 observed at the time of maximum light and 17 days later. If the radial stratification of the composition in the outer layers of the supernova model is assumed to be removed by mixing, the theoretical and observed spectra are in good agreement; the discrepancies could be due to the approximations in the synthetic-spectrum calculations. Spectra for the carbon deflagration model in its unmixed form appear to be inconsistent with observation.

X-ray emission from cataclysmic variables with accretion disks. II. EUV/soft X-ray radiation.

Abstract: About half of the gravitational luminosity released by gas accreting onto a white dwarf through a disk should emerge from the star/disk boundary layer. For the accretion rates present in many cataclysmic variables, theory predicts that this luminosity should be in the form of an optically thick EUV/soft X-ray component, with T/sub e/roughly-equal(1-3) x 10/sup 5/ K. We compare the theoretical predictions with presently available soft X-ray observations and find satisfactory agreement. Previous doubts on this point were based on inappropriate choices for several critical parameters: white dwarf mass, interstellar column density, and the space density of classical novae. We also attempt to constrain the boundary layer radiation by comparing observed and predicted strengths of the He II lambda1640 and lambda4686 emission lines, assuming that these are produced by photoionization in the upper layers of the disk. The results support the simple optically thick model for high-M systems, but may require complicated X-ray spectra in low-M systems.

Spectrophotometric and model-atmosphere analyses of the hot DO and DAO white dwarfs from the Palomar-Green survey

Abstract: Recent optical and spectrophotometric observations of all currently known DAO and DO hot white dwarf candidates from the Palomar-Green Survey are presented. The optical observations were carried out using the Kitt Peak National Observatory Intensified Image Dissector Scanner (IIDS) system, the Multiple Mirror Telescope (MMT), and two photon-counting Reticon systems. Ultraviolet spectrophotometry was obtained using the International Ultraviolet Explorer (IUE) short-wavelength and long-wavelength cameras. The following spectroscopic subgroups were identified in the observations: the cool DO stars with spectra showing H I features together with He II + H blends; the hot DO stars, with spectra exhibiting weak He II (+H) lines; and hydrogen-rich DAO stars with broad H lines and sharp H II lines at 4686 A. A fourth subgroup of stars resembling PG 1159 with He at He II lines at 4686, and a C, N ion blend in the optical spectrum were also found. The spectroscopic data were analyzed in terms of a grid of hot, high-gravity LTE model atmospheres composed of a mixture of hydrogen and helium, and atmospheric parameters are deduced for each class of objects. On the basis of the model atmosphere analysis, the local space density of the DO stars is estimated to be 1.9-3.0 x 10 to the -7th per cubic pc, depending on the scale height.

On the Number of Comets Around Other Single Stars

Abstract: It is shown how to obtain interesting limits on the number of comets in orbit around other single stars in clouds similar to the one present in our own solar system. The number of comets around white dwarf stars is constrained directly, and this in turn constrains the presence of comets around main-sequence stars. If a white dwarf possesses a cloud of comets similar to the Oort cloud, it will occassionally accrete a comet. The accretion of a comet may add enough heavy elements to the atmosphere of the white dwarf to produce a recognizable spectral signature; it is suggested that the calcium reported in the white dwarf G 74-7 is due to recent comet accretion. This signature will persist until sedimentation has removed the heavy elements from the atmosphere; the mean time between accretion events is comparable to the sedimentation time for cool DA stars, so this signature should be frequently observed if cometary systems are common. The possibility that cometary systems greater than or approximately equal to 3 times more massive than the Oort cloud are present around most white dwarf stars are excluded and how this limit can be improved is indicated. It is shown that a star keeps most of its comets during the vigorous mass loss phase prior to the appearance of the white dwarf star. Thus, the constraints obtained here may be applied to the main-sequence progenitors of the white dwarfs.

Kinematics and internal dynamics of planetary nebulae in the small Magellanic Cloud

Abstract: Kinematical and dynamical data for 44 SMC planetary nebulae (PNs) are presented at a velocity resolution of 12 km/s.The PNs form an unstructured spheroidal population apparently associated with the Bar with centroid at alpha 00 h 49 m 39 s, delta -73 deg 30 min (1950) and with a mean radial velocity of -17 km/s.Their velocity dispersion implies a mass of 900 million solar masses within a radius of 3 kpc of the centroid.These PNs, which form a population of age greater than about 100 million years, appear to be loosely associated with the lower velocity component of the H I, suggesting that the high-velocity H I component is dynamically young.A remarkably good correlation between the expansion velocity and the excitation class is found, which may show that the mass of the white dwarf central star is the major variable influencing the expansion velocity of PNs. 39 references.

CNO abundances resulting from diffusion in accreting nova progenitors

Abstract: The configurations of potential nova progenitors at the onset of a thermonuclear runaway (TNR) are obtained by evolving a series of white dwarf (WD) models through the quasi-static accretion phase preceding the TNR. Special attention is paid to the effect of diffusion on composition profiles. The initial parameters of the accretion phase are the WD mass, the WD luminosity, and the accretion rate. They lead to a wide range of combinations of accreted mass and heavy element content of the envelope Z(env). It is found that the WD mass is the most important parameter in determining the accreted mass. A lower limit exists for the mass of a WD capable of producing a nova-like outburst which decreases with decreasing WD mass. There is a strong correlation between the accretion rate and Z(env), higher Z(env) values resulting from lower accretion rates. Changes in WD luminosity have no dramatic effect on the outcome of the accretion phase.

Evolution of the pulsation properties of hot pre-white dwarf stars.

Abstract: Resolution des equations d'oscillations adiabatiques, lineaires non radiales, pour des modeles de pre-naine blanche

A measurement of secular evolution in the pre-white dwarf star PG 1159-035.

Abstract: Analyse de 96 heures de photometrie a haute vitesse de la variable pulsante PG 1159-035, obtenues de 1979 a 1984

Ultraviolet spectral evolution and heavy element abundances in Nova Austrinae 1981

Abstract: Nova CrA 1981, a moderately fast galactic nova, was systematically observed over a period of seven months with the IUE satellite. The initial evolution of the UV spectrum was similar to that of previously observed novae; however, several months after maximum, weak emission due to very high ionizating forbidden lines of Na, Mg, Al, and Si appeared, eventually decreasing in strength over the next two months. The spectral development of the lines is consistent with the photoionization of an expanding shell; a high-temperature 'coronal' origin is ruled out. Abundances deduced from the spectra show that most of the observed heavy elements in the ejected material are enhanced with respect to helium, with neon the most abundant of these. In addition, there are substantial deviations of the Na/Mg/Al/Si abundance ratios from solar values. This is interpreted as evidence for the mixing of core material with the accreted envelope on a massive O-Ne-Mg (as opposed to a C-O) white dwarf. Furthermore, the proportion of core material in the ejecta implies that the mass of the white dwarf decreased as a consequence of the outburst.

The decline and fall of classical novae

Abstract: We address questions concerning the evolution of classical nova systems through outburst and the time scale for the return to their preoutburst state. Observations indicate that evolution on a purely nuclear burning time scale is not sufficient. We call attention to the sequence of events involving common envelope evolution which naturally occurs following thermonuclear runaways in the accreted hydrogen shells on the white dwarf components of these close binary systems. We estimate that a significant fraction of the envelope may be lost as a result of dynamical friction during a relatively short-lived common envelope phase which is experienced by most classical nova systems. The residual hydrogen-rich envelope matter may be consumed by nuclear burning or wind-driven mass loss or both on time scales compatible with observations of nova systems. As mass is consumed, the photospheric radius decreases and consequently the photospheric temperature rises. The associated hardening of the radiation from novae causes them progressively to become UV, EUV, and ultimately soft X-ray sources. Estimates of the fluxes during these late stages are provided and shown to be compatible with their being detected with EXOSAT or equivalent X-ray facilities. Such detection would provide strong confirmation of theoretical models for the classicalmore » novae. We also point out that infrared emission from X-ray heated grains may be detectable during the soft X-ray phase.« less

The AM Herculis Magnetic Variables

Abstract: The observational properties of the ten known AM Her systems are reviewed. A multitude of components of continuum and line radiation from these objects are outlined. We discuss the important physical processes involved in the accretion flow and funnel shock, with emphasis on the properties of the emission line regions. Other topics include the properties of the white dwarf primaries and M dwarf secondaries, the maintenance of synchronous rotation, and the implications of “dumpings” in the known orbital periods (80–115 minutes) and magnetic field strengths (20-35 megagauss).

Planetary nebulae and their central stars.

Abstract: La discussion concerne les nebuleuses planetaires et leurs proprietes puis les caracteristiques des etoiles qui leur sont associees. On examine enfin les questions liees a l'evolution des etoiles et des nebuleuses

Observable quantities of nonradial pulsations in the presence of slow rotation

Abstract: On presente les spectres de puissance theoriques des variations lumineuses et de vitesse radiale d'une etoile pulsante non radialement, en incluant les effets de rotation. Application au cas d'une naine blanche a hydrogene et d'une etoile B de la sequence principale

TT Arietis: the low state

Abstract: A comprehensive photometric and spectroscopic study of the low state of the novalike variable TT Ari has been made with observations spanning the UV to IR spectral regions. The roughly 5 mag drop in the system's luminosity from the high state indicates that the mass-transfer rate decreased by more than two orders of magnitude. The drastic reduction in the luminosity of the accretion disk between the high and low states enabled the white dwarf component to be directly observed for the first time. The broad absorption profiles at various lines and the UV energy distribution are best fitted by a hot white dwarf. A lower limit of 200 pc for the distance to TT Ari is derived, and the behavior of TT Ari is compared to that of another novalike variable, MV Lyr. The long-term photometric behavior of TT Ari makes it a typical member of the VY Scl subclass of novalike variables.

Identification of the 1400 and 1600 A features observed in the ultraviolet spectra of DA white dwarfs

Abstract: The 600 A absorption feature in the spectra of cool DA white dwarf stars has been identified as a resonance broadening of Ly-alpha due to the hydrogen quasi molecule. Likewise, the 1400 A absorption feature in the spectra of cool and moderately warm DA white dwarf stars appears to be due to a Ly-alpha satellite line arising from the hydrogen ion quasi molecule. The strength of both features is gravity sensitive and therefore promises to be an excellent indicator of surface gravity.

Evolution of Cataclysmic Binaries

Abstract: A standard scenario for the evolutionary origin of cataclysmic binaries is presented. In this scenario the initial periods of the ancestor binaries was between a few months and a few years, and the initial mass ratio was very different from unity. The period was reduced, and a lot of mass and angular momentum was lost during the common envelope phase of evolution. If this scenario is correct then there should be a lot of short period binaries made of pairs of white dwarfs, which were produced by the systems with initial mass ratios close to unity. About 10% of all “single” white dwarfs could be short period binaries, or could have passed through such a phase sometime in the past. Such binaries should evolve due to angular momentum losses caused by gravitational radiation and could produce R CrB stars, AM CVn cataclysmic binaries, DB white dwarfs, and Type I Supernovae. Inconsistencies in the standard evolutionary scenario are discussed. It is possible that a binary nature of a star is the necessary condition for a formation of a high surface brightness planetary nebula.

Emission lines in the magnetic white dwarf GD 356

Abstract: GD 356 est l'unique naine blanche montrant des triplets de Zeeman resolus pour les raies d'emission Hα et Hβ

An analysis of nonradial pulsations of the central star of the planetary nebula K1-16.

Abstract: We have performed a linear, nonradial, nonadiabatic pulsation stability analysis of stellar models with effective temperatures of 1.50 x 10/sup 5/ K, 1.55 x 10/sup 5/ K, 1.60 x 10/sup 5/ K, and 1.65 x 10/sup 5/ K, and luminosities characteristic of stars evolving from the AGB to the white dwarf cooling sequence. Our purpose is to find unstable pulsation modes with periods as long as those observed in the central star of the planetary nebula, K1-16. These models have masses of 0.6 M/sub sun/ and homogeneous compositions of half-carbon and half-oxygen by mass. All these stellar models exhibit unstable nonradial modes with periods ranging from 1000 to 4000 s. The longest periods were found in the model with T/sub e/ = 1.55 x 10/sup 5/ K, L = 2000 L/sub sun/. The pulsation driving mechanism is the cyclical ionization of carbon and oxygen at temperatures of (2--5) x 10/sup 6/ K. This analysis demonstrates that K1-16 is probably a member of the PG 1159-035 class of sdO variable stars. This star is, therefore, an evolutionary link from the central stars of the OVI planetary nebulae to the hottest non-Da white dwarfs as proposed by Sion, Liebert, and Starrfield. Itmore » must also be evolving very rapidly.« less

Time-dependent accretion onto magnetic white dwarfs: Effects of cyclotron emission

Abstract: Previous accretion models have shown that if the cooling is due to optically thin bremsstrahlung, the flow is unstable. The shock height oscillates with a period approx.1 s, which corresponds to the cooling time for parameters characteristic of the AM Her binaries. Here we show that the effects of cyclotron cooling stabilize the flow if the magnetic field is large enough. For a 0.5 M/sub sun/ white dwarf accreting 1 g cm S s , the flow is stable if B is more than or approximately equal to 2 x 10X gauss. We also show that the bremsstrahlung X-ray emission and the optical cyclotron emission are 180 out of phase when oscillations are present. The relevance of these calculations to the observed optical oscillations, with periods of approx.1 s, in two AM Her stars is discussed.