Showing papers on "White dwarf published in 1988"

The common envelope phase in the evolution of binary stars

Abstract: The common envelope phase in the evolution of binary systems is examined. Three parameters are identified which characterize the efficiency of energy deposition, the importance of three-dimensional effects, and the efficiency of spin-up of the envelope. It is demonstrated that the efficiency of deposition of orbital energy into envelope ejection can be quite low. It is found that significant spin-up of the envelope can be expected to occur in relatively early stages, when the spiralling-in occurs inside evolved supergiant envelopes. In normal giants spin-up can occur only in the final stages of the spiralling-in process. The results of a simplified three-dimensional numerical calculation of the common envelope phase are presented, and the implications of the results for the formation of planetary nebulae with binary nuclei, double white dwarf systems, and FK Com stars are discussed. 52 references.

A low-temperature companion to a white dwarf star

Abstract: An infrared object located about 120 AU from the white dwarf GD165 has been discovered. With the exception of the possible brown dwarf companion to Giclas 29-38 reported last year, the companion to GD165 is the coolest (2100 K) dwarf star ever reported and, according to some theoretical models, it should be a substellar brown dwarf with a mass between 0.06 and 0.08 solar mass. These results, together with newly discovered low-mass stellar companions to white dwarfs, change the investigation of very low-mass stars from the study of a few chance objects to that of a statistical distribution. In particular, it appears that very low-mass stars and perhaps even brown dwarfs could be quite common in the Galaxy.

The Luminosity Function of White Dwarfs

Abstract: Trigonometric parallaxes, optical colors, and spectrophotometry are used to derive an empirical luminosity function for cool white dwarfs using the 1/V(max) method. To facilitate comparison with theoretical cooling curves, relations for cool white dwarfs are estimated for T(eff) versus M(V) and for M(V) versus M(bol). The results show that a downturn occurs in the distribution of cool degenerate stars near log luminosity equals about -4.4. The indicated local space density of observed degenerate dwarfs is 0.003 stars/pc exp 3, which corresponds to about 1 percent of the dynamical mass density in the solar neighborhood. 107 references.

Tidal enhancement by a binary companion of stellar winds from cool giants

Abstract: Certaines binaires RS CVn ont une inversion de masse avant l'ecoulement de leur lobe de Roche. On postule que la composante secondaire a l'origine moins massive a augmente par maree le taux de perte de masse par vent stellaire de la primaire

On the outburst recurrence time for the accretion disk limit cycle mechanism in dwarf novae

Abstract: An attempt is made here to elucidate the dwarf nova outburst time scale in terms of basic physics, and to place limits on the model by using the observed recurrence times. A simple analytical model is developed to describe the limit cycle process, and an expression is obtained for the interval between eruptions. It is shown that the smallness of the amount of mass accreted onto the central white dwarf during eruption can be explained in terms of the propagation of heating and cooling fronts. The dependency of the fraction of the disk mass accreted on the model parameters is derived, and the result is used to construct a semianalytic expression for the recurrence time.

Discovery of a Close Detached Binary DA White Dwarf System

Abstract: It has been found that the cool white dwarf L870-2 (EG 11, WD 0135-052) is a double-lined spectroscopic binary composed of a detached pair of DA white dwarfs. A time series of echelle spectra centered on the H-alpha absorption lines was obtained in November 1987 with the MMT. This data is combined with spectra from September 1987 to determine the orbital and stellar parameters of the system. 42 references.

Quasiperiodicity in cataclysmic variable stars caused by solar-type magnetic cycles

Abstract: Cyclical variations of orbital periods, quiescent magnitudes and outburst intervals in the activity of cataclysmic variable binary stars are inter-related and are ascribed to variations in radii of the secondaries, caused by solar-type (sunspot) magnetic cycles. In the nova remnant DQ Herculis the observed variations in orbital period and quiescent magnitude are consistent with this mechanism. But accretion onto the white dwarf, from an accretion disk acquired from its companion, cannot explain the observed variation of the 71-second oscillations.

A model for the 1987 outburst of the recurrent nova U Scorpii

Abstract: The paper presents simulations of the outburst of U Sco by accreting material, with a solar abundance of the CNO nuclei, at a rate of 1.1 x 10 to the -6th solar mass/yr onto a 1.35-solar mass white dwarf. This evolutionary sequence takes about 2.6 yr to reach runaway conditions and ultimately eject 4 x 10 to the -7th solar mass moving with speeds exceeding about 400 km/s. It is found that ejection occurred by radiation pressure rather than by explosive CNO burning. 22 references.

Properties of high-density binary mixtures and the age of the Universe from white dwarf stars

Abstract: The luminosity of white dwarf stars can be attributed to the cooling process of their degenerate cores. The simple relationship existing between their luminosity and their age, together with the lack of white dwarfs fainter than log (L/L⊙)≈−4.5, provides a method of measuring the age of the disk and consequently that of the Universe1,2. Winget et al.2 have derived an age of the galactic disk of 9.3 Gyr and have then found that the Universe is young: 10.3 Gyr. These values depend on the assumption that completely ionized carbon and oxygen (the most abundant elements in white dwarf interiors) are miscible in solid phase. It is possible, however, that completely ionized carbon and oxygen separate during the process of crystallization3,4. The consequences of this behaviour on the evolution of mass-accreting carbon–oxygen white dwarfs in stellar binary systems5–7 and on the cooling process of white dwarfs8,9 have been already described. Here, we attempt to show that a galactic disk'age of 15 Gyr cannot be excluded by the white dwarf observations if carbon anil oxygen are immiscible in solid phase.

V1500 Cygni - Discovery of a magnetic nova

Abstract: Polarimetric observations of V1500 Cygni, the remnant of Nova Cygni 1975, reveal periodic variations of optical circular polarization with semi-amplitudes of about + or - 1.5 pct. Measured over a six-month baseline, the period of variation, 0.137154 + or - 0.000004 days, is 1.8 percent shorter than the stable photometric period, 0.1396129 days, obtained by Patterson (1979), and refined by Kaluzny and Semeniuk (1987). The circular polarization is interpreted as high-harmonic optical cyclotron emission from an accreting, magnetic white dwarf primary. However, the dominant system light and photometric variability are due to the heated companion star. In this picture, the prenova system was a synchronized AM Herculis-type magnetic variable. During the outburst and subsequent dense-wind phase, coupling between the white dwarf and the expanded envelope and the interaction of the orbiting secondary star were responsible for the asynchronism of the current system and the remarkable photometric variations observed in 1975-1976. Other unusual characteristics of Nova Cygni 1975 are also explained by this model.

Birth of millisecond pulsars in globular clusters

Abstract: It is argued here that accretion-induced collapse of white dwarfs in binaries can form millisecond pulsars directly without requiring a precursor low-mass X-ray binary stage. Ablation of the precollapse binary companion by the millisecond pulsar's radiation field, a process invoked to explain some of the characteristics of the recently discovered eclipsing millisecond pulsar, can then yield isolated neutron stars witout requiring an additional stellar encounter.

The Presence of Helium in Hot DA White Dwarfs: The Role of Radiative Levitation and the Case for Stratified Atmospheres

Abstract: The role of helium radiative levitation in the envelopes of hot DA white dwarfs is examined. Explicit time-dependent calculations of helium diffusion in the presence of radiative support carried out in evolving DA models show that the helium distribution in the outermost layers of the models relaxes rapidly to a configuration corresponding to diffusive equilibrium. Observations of these regions show only equilbrium helium abundances and are totally decoupled from the evolving internal helium profile. It is shown that the helium abundances supported by radiative forces in the atmospheric layers of these stars are too small to account for the observations. It is suggested that a mechanism other than radiative levitation must be used to compete against gravitational settling in these stars. A number of mechanisms are discussed and it is concluded that an equilibrium between ordinary diffusion driven by a composition gradient and settling may be the best possibility. The implications of this model are discussed in detail. 66 references.

What does an erupting nova do to its red dwarf companion

Abstract: During nova eruptions and for decades afterward, the red dwards in cataclysmic binaries are irradiated with hundreds of times more luminosity than they themselves produce. Simulations of the time-dependent irradiation of three red dwarf models (0.25, 0.50, and 0.75 solar mass) are presented. The mass transfer rates forced by irradiation after nova eruption are found to be enhanced by two orders of magnitude because of the irradiation. The time scale for irradiation to become unimportant is that of the white dwarf cooling time scale, a few centuries. These two results support the hibernation scenario of novae, which suggests that novae remain bright for a few centuries after eruption because of irradiation-induced mass transfer. After irradiation decreases mass transfer slows, and some very old novae may then become extremely faint. 26 references.

Gamma-ray spectra and energy deposition for type Ia supernovae

Abstract: The standard model for SNIa involves the partial incineration of a carbon-oxygen white dwarf Confirmation or rejection of the model awaits observation of the large gamma-ray fluxes expected when the supernova ejecta become sufficiently transparent that a large fraction of the gamma rays escape with little or no interaction This paper presents results of Monte Carlo calculations of the gamma-ray spectra for a grid of models that spans the parameter space associated with the radioactive decay model of SNIa 32 references

Formation of Xe-HL-enriched diamond grains in stellar environments

Abstract: Recently Lewis et al.1 have identified the C δstructure, previously found in C1, C2 and.C3 carbonaceous chrondrites, as diamond powder (that is, grains typically 50 A in diameter). The Cδ is enriched (compared to the atmosphere of the Sun) in heavy and light isotopes of Xe (Xe-HL) and therefore assumed to be of pre-solar origin. In laboratory experiments Roy2 has described how small layers of diamonds can form at low pressure and at temperatures normally attributed to the grain-forming layers of cool stellar atmospheres (∼1,000–2,000 K). Lewis et al.1 therefore suggest following the original idea of Clayton3, that the diamonds found in meteorites are formed in the upper atmosphere of red giant stars. Here it is shown that the diamonds must have formed in the type of red giants called carbon stars, and that Xe-HL and diamonds cannot have formed in the same star, but must be produced one in each component of a close binary system. The diamonds are produced in the smaller of the components, near the end of its evolution, the Xe-HL is produced when the compact remnant (a white dwarf) of the bigger component has accreted so much mass from the carbon star that it explodes as a supernova. This is consistent with the normal evolution of type I supernovae (SNI), and it is demonstrated that there were appreciably more carbon stars participating in SNI formation at the time of Solar System formation than today. Finally, the thermodynamic conditions are found under which the diamonds are produced, and it is explained why Xe-HL is found in diamonds but not in silicon carbide, and why s-process Xe is expected to be found in silicon carbide (Cβ).

Kinematical Tests of White Dwarf Formation Channels and Evolution

Abstract: Published observational data on the effective temperature, radius, mass, and kinematics of 420 DA stars, 53 DB stars, 88 DC stars, 13 DH/DP stars, 26 DQ stars, and 24 DZ stars from the catalog of McCook and Sion (1987) are compiled in extensive tables and analyzed statistically. The results are presented graphically and discussed in detail. Major findings reported include identification of the magnetic degenerate stars as a kinematically distinct group of young disk objects, higher-than-average mean motions and velocity dispersions for carbon-band DQ stars, and no kinematic differences between DA and DB stars of similar color and magnitude. From the general kinematic similarity of DA stars and their non-DA counterparts it is inferred that they probably did not evolve from more massive progenitors. 47 references.

An optical spectrophotometric survey of abundances in Magellanic Cloud Planetary Nebulae

Abstract: Optical spectroscopic data for 71 Planetary Nebulae (PN) in the Large and Small Magellanic Clouds have been analysed. The line fluxes have been used to determine nebular temperatures, densities, and the abundances of He, N, O, Ne, and Ar, relative to H. In our sample there are 12 nebulae with N/O≽0.5, resembling Peimbert's Type I PN; six low-excitation (LE) objects [1≼I(5007)/I(Hβ)≼4]; and four very-low-excitation (VLE) nebulae [I(Hβ)>I(5007)], similar to the Galactic VLE class. Mean abundances have been calculated for the nebulae not in these special groups. After correction for collisional excitation contributions to the nebular He I lines, PN in the SMC and LMC yield mass fractions of Y=0.249±0.025 and Y=0.258±0.015, respectively. Compared with PN in our own Galaxy, the abundances of Ne and Ar, which are the elements in our sample least affected by nucleosynthesis, are lower by 0.6 and 0.35 dex for the SMC and LMC respectively. The oxygen and neon abundances in the Magellanic Cloud PN are the same as those previously found for H II regions in the LMC and SMC, but the nitrogen in PN is enhanced by 0.9 and 1.0 dex in each galaxy, respectively. This is found to be consistent with the processing of all of the original carbon to nitrogen by the CN cycle, operating in the progenitor stars at the time of the first dredge-up. This process seems to have operated much more efficiently in the metal-poor Magellanic Clouds than in the Milky Way, in agreement with theoretical predictions. Five Wolf–Rayet central stars are detected in the sample (two SMC, three LMC). The frequency of occurrence of these helium-rich central stars in low- and medium-excitation PN (15 per cent) is very similar to that of helium-rich white dwarfs in the solar neighbourhood, suggesting that events at the end of the AGB phase may be reponsible for the observed fractions of helium- and hydrogen-rich white dwarf stars.

Searches for magnetic fields in white dwarfs since Babcock

Abstract: Discoveries of magnetic fields in 26 white dwarfs since 1970 do not negate the basic inference from the work of Babcock - that most of the over 1000 known white dwarfs do not have strong magnetic fields. The origin of the strong fields in the minority remains an unsolved problem, although evidence is growing that the magnetic degenerates are more massive than the average white dwarf. This paper is one of a series of invited reviews celebrating the centenary of the Astronomical Society of the Pacific, see also 004.165.

Basic elements of the theory of accretion

Abstract: The paper reviews what we consider to be the essential aspects of the theory of accretion. A general overview is given in Section I. The treatments of accretion of cold gas onto a moving star by Hoyle and Lyttleton and of spherical adiabatic accretion by Bondi are summarized in Section II. The formation of spectra in accretion flows onto neutron stars and white dwarfs is described in Section III, with brief mention of the effects of a magnetic field anchored to the star. Spherical accretion onto black holes is treated in Section IV; in particular we underline the differences between classes of models leading to different efficiencies and spectra. We stress the role of a magnetic field entangled in the infalling plasma and the importance of the Comptonization process. The effects of radiation pressure in spherical accretion are then discussed. Thin accretion disks in the standard α approximation are introduced in Section V. The stability for thermal and viscous processes are examined and the bivalued region of the {SIGMA}, Mdot^ plot is discussed in view of a possible limit cycle behavior. Thick disks and their stability are briefly treated in Section VI. Section VII offers threads between theory and observations mentioning the astrophysical systems where accretion plays a major role (cataclysmic variables, X-ray binaries, Active Galactic Nuclei) pointing out successes and weaknesses of the present models.

Optical detection and characterization of the eclipsing pulsar's companion

Abstract: The recently discovered millisecond pulsar PSR1957 + 20 is eclipsed for ∼50 min out of each 9.16-h orbit1; several authors suggested1–4 that spin-down radiation from the pulsar is evaporating material from the surface of the ˜0.02Mṁ companion. Here we report observations intended to find the optical counterpart of the pulsar and to study luminosity variations with orbital phase. We show that the candidate optical counterpart5 is actually two stars, one the true counterpart, the other an unrelated background star. The luminosity of the system varies by at least a factor of five, with a maximum of magnitude 20.3 when the pulsar hides the companion. The phase of the variations implies that the light comes mostly from the companion star, and suggests that it is tidally locked, with the bright side constantly illuminated by the pulsar. The companion has a low colour temperature, ∼5,500 K, which with its magnitude and the pulsar's dispersion-measure distance indicates a radius of 0.15R⊙, about the size of a 0.02M⊙ hydrogen white dwarf.

A Centered Dipole Model for the High Field Magnetic White Dwarf GRW +70 degrees 8247

Abstract: Construction de modeles dipolaires centres pour cette naine blanche magnetique a fort champ, en utilisant les calculs de Zeeman sur l'hydrogene des groupes de Tubingen et de l'Universite de Louisiane

The Formation and Evolution of Symbiotic Stars

Abstract: The evolutionary origins of symbiotic stars containing (i) disk-accreting main sequence stars, (ii) wind-fed, shell-burning white dwarfs, and (iii) disk-accreting neutron stars are described. Of particular interest are those white dwarf systems which have orbital periods too short to have escaped tidal mass transfer prior to becoming symbiotics. We show here that, under suitable circumstances, low-mass, long period binaries may undergo quasi-conservative mass transfer, rather than evolving through common envelope evolution to the cataclysmic variable state, thus accounting for the existence of these systems. Approximate expressions are given for the lifetimes, and relative efficiencies (mass accreted/mass of donor) for different modes of interaction among symbiotic binary systems.

The formation of Algols without catastrophes

Abstract: Detailed models of binary systems where ordinary stellar wind mass loss is enhanced by a binary companion are presented. The present Z-Her-type model is used to interpret the evolution of Algol-like and related binaries. One consequence of the present theory is that the mass of the white dwarf resulting from the primary is rather low, and is not dependent on the initial period. 36 references.

The 1985 May superoutburst of the dwarf nova OY Carinae. II - IUE and Exosat observations

Abstract: Ultraviolet (IUE) and X-ray (Exosat) observations of the eclipsing dwarf nova OY Carinae during the superoutburst of May 1985 are presented. From the lack of X-ray eclipse and UV behavior, it is deduced that the X-ray flux originates in an optically thin corona comparable in size to the Roche lobe, and not directly from the white dwarf or boundary layer. The asymmetric UV line emission originates partly in the accretion disk and partly in a wind. There is a strong modulation of the UV continuum flux that is thought to be caused by extended vertical disk structure shadowing the inner regions.

The Final Stages of Evolution of Cold, Mass-accreting White Dwarfs

Abstract: The evolution of solid C + O white dwarf models upon mass accretion is calculated up to the point of either explosive thermonuclear ignition or gravitational collapse. It is shown that both explosions and quiet collapses to a neutron star are possible for each of two different phase diagrams for high-density C + O mixtures. The ranges of initial masses and temperatures and of accretion rates leading to the different outcomes are determined. Problems concerning the chemical composition of the accreted matter and the effects of tidal dissipation are discussed. 68 references.

Existence and Nature of Dark Matter in the Universe

Abstract: Ann. Rev. Astron. Astrophys. 1987. 25: 425-72 Copyright © 1987 by Annual Reviews Inc. All rights reserved EXISTENCE AND NATURE OF DARK MATTER IN THE UNIVERSE Virginia T rimble Astronomy Program, University of Maryland, College Park, Maryland 20742, and Department of Physics, University of California, Irvine, California 92717 1. HISTORICAL INTRODUCTION AND THE SCOPE OF THE PROBLEM The first detection of nonluminous matter from its gravitational effects occurred in 1844, when Friedrich Wilhelm Bessel announced that several decades of positional measurements of Sirius and Procyon implied that each was in orbit with an invisible companion of mass comparable to its own. The companions ceased to be invisible in 1862, when Alvan G. Clark turned his newly-ground 18%” objective toward Sirius and resolved the 10q‘ of the photons from the system emitted by the white dwarf Sirius B. Studies of astrometric and single-line spectroscopic binaries are the modern descendants of Bessel’s work. A couple of generations later, data implying nonluminous matter on two very different scales surfaced almost simultaneously. First, Oort (498, 499) analyzed numbers and velocities of stars near the Sun and concluded that visible stars fell shy by 30-50% of adding up to the amount of gravitating matter implied by the velocities. Then, in 1933, Zwicky (777) concluded that the velocity dispersions in rich clusters of galaxies required 10 to 100 times more mass to keep them bound than could be accounted for by the luminous galaxies themselves. The former result was taken much more seriously than the latter by contemporary and succeeding astronomers (being dignified by the name “the Oort limit”), which is perhaps more a statement about the personalities of Oort and Zwicky than about anything else. 425 0066-4146/87/0915—0425$02.00 © Annual Reviews Inc. - Provided by the NASA Astrophysics Data System

The 1987 Outburst of the Recurrent Nova U Sco

Abstract: Spectral observations obtained soon after the 1987 brightening of U Sco support a thermonuclear runaway model for outbursts of this object. Spectra later in the decline are, however, more characteristic of a hot accretion disc. These observations are reconciled in a model where the low-mass high-velocity shell ejected from the surface of the white dwarf collides with the accretion disc causing it to brighten.