Showing papers on "White dwarf published in 1991"
TL;DR: In this paper, a reexamination is conducted as to whether or not AIC occurs for the new models of O + Ne + Mg white dwarfs and solid C + O white- dwarfs that can ignite nuclear burning at significantly lower central densities than in the previous models.
Abstract: Recent discovery of an unexpectedly large number of low-mass binary pulsars (LMBPs) in globular clusters has instigated active discussions on the evolutionary origin of binary pulsars. Prompted by the possibility that at least some of LMBPs originate from accretion-induced collapse (AIC) of white dwarfs, a reexamination is conducted as to whether or not AIC occurs for the new models of O + Ne + Mg white dwarfs and solid C + O white dwarfs that can ignite explosive nuclear burning at significantly lower central densities than in the previous models. Even with low critical densities, AIC is still much more likely than explosion for both types of white dwarfs. Possible regions for AIC are presented in a diagram of mass accretion rate vs initial mass of the white dwarfs.
455 citations
University of Texas at Austin1, Alfred P. Sloan Foundation2, University of Arkansas at Little Rock3, Ames Research Center4, University of Colorado Boulder5, Université de Montréal6, Australian National University7, Indian Space Research Organisation8, University of Cape Town9, University of Toulouse10, PSL Research University11, University of Leicester12, Universidade Federal do Rio Grande do Sul13, Tennessee State University14, Iowa State University15
TL;DR: In this article, the power spectrum of a pulsating prewhite dwarf star (DPV) PG 1159 - 035 was resolved into 125 individual frequencies; 101 of them were identified with specific quantized pulsation modes, and the rest were completely consistent with such modal assignment.
Abstract: Results are reported from 264.1 hr of nearly continuous time-series photometry on the pulsating prewhite dwarf star (DPV) PG 1159 - 035. The power spectrum of the data set is completely resolved into 125 individual frequencies; 101 of them are identified with specific quantized pulsation modes, and the rest are completely consistent with such modal assignment. It is argued that the luminosity variations are certainly the result of g-mode pulsations. Although the amplitudes of some of the peaks exhibit significant variations on the time scales of a year or so, the underlying frequency structure of the pulsations is stable over much longer intervals. The existing linear theory is invoked to determine, or strongly constrain, many of the fundamental physical parameters describing this star. Its mass is found to be 0.586 solar mass, is rotation period 1.38 days, its magnetic field less than 6000 G, its pulsation and rotation axes to be aligned, and its outer layers to be compositionally stratified.
234 citations
TL;DR: The physical properties of dense plasmas that are necessary to construct theoretical models of such degenerate stars include the equation of state, transport properties, and nuclear reaction rates as discussed by the authors.
Abstract: Degenerate bodies composed primarily of dense hydrogen and helium plasmas range from giant planets to the so far hypothetical brown dwarfs. More massive objects begin their lives as nondegenerate stars and may end as white dwarfs, composed primarily of carbon and oxygen, or as neutron stars, with solid crusts of iron or heavier elements and cores of neutron matter. The physical properties of dense plasmas that are necessary to construct theoretical models of such degenerate stars include the equation of state, transport properties, and nuclear reaction rates.
161 citations
TL;DR: In this paper, a general account of the observed global properties of single stars of low, intermediate, and high mass, together with their theoretical Hertzsprung-Russell diagram evolution, is given to the observed properties of various evolved close binaries.
Abstract: After presenting a general account of the observed global properties of single stars of low, intermediate, and high mass, together with their theoretical Hertzsprung-Russell diagram evolution, attention is given to the observed properties of various evolved close binaries and to an assessment of the value of comparisons between observation and crude theory in characterizing the physics of mass transfer within interacting binary systems Detailed consideration is then undertaken of such topics as stellar evolution in globular clusters, interior star changes due to nucleosynthesis and mixing, asymptotic giant branch stars of intermediate mass, the response of white dwarfs in binary systems to mass accretion, and scenarios for binary star evolution tending toward close white dwarf pairs
158 citations
TL;DR: In this paper, it was shown that the mass transfer rate of a carbon-oxygen white dwarf is approximately 3 × 10 -8 M ○. yr -1, remarkably insensitive to the mass ratio of the components.
Abstract: Scenarios for close binary star evolution suggest that there is a finite probability for the formation of systems in which a carbon-oxygen white dwarf is accreting helium from a nondegenerate companion that is burning helium in its core. If it is gravitational wave radiation alone which drives mass transfer, the mass-transfer rate in such systems is approximately 3 × 10 -8 M ○. yr -1 , remarkably insensitive to the mass ratio of the components.
152 citations
TL;DR: In this article, a steady-state thin accretion disk is modeled around a uniformly rotating unmagnetized star in order to investigate whether disk accretion can continue as the accreted angular momentum spins the central star to near breakup.
Abstract: A steady-state thin accretion disk is modeled around a uniformly rotating unmagnetized star in order to investigate whether disk accretion can continue as the accreted angular momentum spins the central star to near breakup. A mapping between the specific angular momentum (SAM) added to the star and the stellar rotation rate (SRR) is obtained. When SRR is somewhat less than the breakup rotation rate of the star, a class of solutions is found where the angular velocity of the disk attains a maximum close to the star and then decreases rapidly in a boundary layer to match SRR. If SRR is near breakup, a second class of solutions is found where the disk angular velocity has no maximum but increases monotonically all the way down to the stellar surface. SAM decreases very rapidly with increasing SRR and even takes on fairly large negative values. The spin-up of an accreting star slows down and eventually stops at a rotation rate near breakup. Beyond this point, the star can continue to accrete any amount of matter without actually breaking up. 18 refs.
148 citations
Journal Article•
TL;DR: In this article, a delayed detonation model for type Ia supernova explosions is considered and the results of the gas-dynamical computations are compared with both detonation and deflagration models and with observations.
Abstract: A new delayed detonation model for type Ia supernova explosions is considered According to the model a detonation of a massive carbon-oxygen white dwarf occurs after an episode of slow burning (deflagration) near the white dwarf centre The numerical scheme used in the simulations is described and the results of the gasdynamical computations are presented The results are compared with both detonation and deflagration models and with observations
138 citations
Book•
01 Jan 1991
TL;DR: In this paper, theoretical and observational views of star formation, spectroscopic constraints on the evolution of massive stars, very low mass stars and brown dwarfs, asteroseismology, globular clusters as tests of stellar evolution, observational tests, and mass loss from cool evolved giant stars.
Abstract: The present conference discusses theoretical and observational views of star formation, spectroscopic constraints on the evolution of massive stars, very low mass stars and brown dwarfs, asteroseismology, globular clusters as tests of stellar evolution, observational tests of stellar evolution, and mass loss from cool evolved giant stars. Also discussed are white dwarfs and hot subdwarfs, neutron stars and black holes, supernovae from single stars, close binaries with evolved components, accretion disks in interacting binaries, supernovae in binary systems, stellar evolution and galactic chemical evolution, and interacting binaries containing compact components.
110 citations
TL;DR: In this article, a new generation of model atmospheres and synthetic appropriate to the study of cool DA white dwarfs is presented, with a standard complete linearization technique modified to take into account the energy transport by convection.
Abstract: A new generation of model atmospheres and synthetic appropriate to the study of cool DA white dwarfs is presented. The thermodynamic structure of the model atmospheres is calculated in LTE, with a standard complete linearization technique modified to take into account the energy transport by convection. The emergent fluxes are calculated, providing an improved theoretical framework for the computation of detailed atomic level populations which permits a careful analysis of the gravity- and helium-sensitive high Balmer lines. A comparison with results based on older, less satisfactory, formalisms reveals that this improved treatment leads to significant differences not only in the predicted line profiles, but also in the absolute continuum flux. These new synthetic spectra are used to study some of the systematic properties of the atmospheres of cool DA white dwarfs. 61 refs.
84 citations
TL;DR: The fundamental issue of the region of period formaton in a degenerate star is examined in this article, with special attention given to the treatment of the Brunt-Vaisala frequency.
Abstract: The fundamental issue of the region of period formaton in a degenerate star is examined, with special attention given to the treatment of the Brunt-Vaisala frequency. It is shown that, in order to obtain reliable numerical results in degenerate stellar models, the Brunt-Vaisala frequency must be appropriately transformed, because it is defined in terms of a difference between two numbers which become nearly equal in highly degenerate matter, causing serious numerical problems and systematic errors. An alternative expression is derived, which is valid for multicomponent nonideal partially degenerate and partially ionized plasmas such as those encountered in white dwarf envelopes. This expression is used to compute the period structure of the same white dwarf considered by Pesnell (1987). It is shown that the implicit numerical differencing used in the Lagrangian pulsation code of Pesnell leads to very serious difficulties when used with models of degenerate stars.
TL;DR: In this paper, high-speed multicolor light curves were derived from high-quality mean light curves that cleanly separate the white dwarf and accretion disk eclipses, but the bright spot is weak or absent during their observations, but a 0.3 mag orbital modulation centered on mid-eclipse is seen in the U-band.
Abstract: Optical eclipses are used to investigate the structure of the dwarf nova HT Cas. We present high-speed multicolor light curves covering four eclipses at UBR in 1982 September and 12 eclipses at UBVR in 1983 November-December. From the latter we derive high-quality mean light curves that cleanly separate the white dwarf and accretion disk eclipses. The bright spot is weak or absent during our observations, but a 0.3 mag orbital modulation, centered on mid-eclipse, is seen in the U-band. We define a purely photometric model of the system using contact timings of the white dwarf from the data presented here and measurements of the bright spot eclipses in other data (published by Patterson in 1981).
TL;DR: In this paper, it is suggested that the physics of the explosive ignition of ONeMg cores may be a key to such phenomena as SN Ia and SN Ib/c outbursts, or perhaps to a new type of objects.
Abstract: Models are used to explore the outcome of explosive ignition of ONeMg cores, which result from the evolution of stars in a mass range between 8 and 10 solar masses. Arguments are presented showing that the thermonuclear supernova production or the 'iron' dwarf formation by milder outbursts are possibile outcomes of the explosive ignition of ONeMg cores. It is suggested that the physics of the explosive ignition of ONeMg cores may well be a key to such phenomena as SN Ia and SN Ib/c outbursts, or perhaps to a new type of objects. 21 refs.
01 Sep 1991
TL;DR: In this article, it was shown that the white dwarfs in these systems accrete essentially the specific angular momentum of the secondary star, averaged over time scales ∼10 4 yr.
Abstract: A number of intermediate polars (IPs) are observed to have spin periods very close to 10% of the orbital period. We show that if they spin in equilibrium, the white dwarfs in these systems accrete essentially the specific angular momentum of the secondary star, averaged over time scales ∼10 4 yr. Thus either no accretion disk forms or this is only marginally possible, and there is very little reduction of specific angular momentum during the accretion process. The relation between the spin and orbital periods is difficult to reconcile with accretion predominantly through disks around white dwarfs spinning at the equilibrium rate, but does not preclude their presence in observed systems
10 Sep 1991
TL;DR: In this article, the time rate of change for the main pulsation period of the 13,000 K DA white dwarf G117 - B15A has been detected using the Whole Earth Telescope (WET).
Abstract: The time rate of change for the main pulsation period of the 13,000 K DA white dwarf G117 - B15A has been detected using the Whole Earth Telescope (WET). The observed rate of period change, P(dot) = (12.0 + or - 3.5) x 10 to the -15th s/s, is somewhat larger than the published theoretical calculations of the rate of period change due to cooling, based on carbon core white dwarf models. Other effects that could contribute to the observed rate of period change are discussed.
TL;DR: In this article, a 3D smoothed particle hydrodynamics code is used to simulate collisions between a 0.8 solar mass red giant and impactors of mass 0.4 and 0.6 solar mass.
Abstract: A 3D smoothed particle hydrodynamics code is used to simulate collisions between a 0.8 solar mass red giant and impactors of mass 0.4 and 0.6 solar mass. The red giant is modeled as a massive core surrounded by a gaseous envelope, while the impactor, assumed to be a white dwarf or a main-sequence star, is treated as a single particle. Encounters at various impact parameters and with relative velocities pertinent to global clusters are studied. It is found that, if the impactor passes within twice the radius of the red giant, mass loss occurs from the red giant envelope and both objects are left bound in elliptical orbits. Calculations suggest that if the impactor has an initial distance at the closest approach of approximately 1.6 R(RG) or less, the impactor will spiral into the red giant envelope on subsequent periastron passages. For bound systems with a greater separation, the orbit may circularize with a radius approximately double that of the initial counter. 27 refs.
TL;DR: In this paper, a 0.6-solar-mass white dwarf was investigated for convective mixing and the shape of the hydrogen lines can be used for constraining theories of convective overshoot.
Abstract: Stratified hydrogen/helium envelope models in diffusive equilibrium are calculated for a 0.6-solar-mass white dwarf for effective temperatures between 10,000 and 80,000 K in order to investigate the observational constraints placed on the total hydrogen mass. Convective mixing is included ab initio in the calculations, and synthetic spectra are used for comparing these models with observational materials. It is shown that evolutionary changes in the surface composition of white dwarfs cannot be explained by a model in which a small amount of hydrogen floats to the surface from initially being mixed in the outer parts of a helium envelope. It is pointed out that the shape of the hydrogen lines can be used for constraining theories of convective overshoot.
TL;DR: In this paper, a detailed model of white dwarf mass distribution in newly formed cataclysmic binaries produced by close binary evolution was used to derive the distributions for such systems' subsequent nova outbursts, in view of selection effects due to the dependences of envelope ignition mass, companion mass, and outburst luminosity on the masses of white dwarfs in nova systems.
Abstract: A detailed model of white dwarf mass distribution in newly formed cataclysmic binaries produced by close binary evolution is presently used to derive the distributions for such systems' subsequent nova outbursts, in view of selection effects due to the dependences of envelope ignition mass, companion mass, and outburst luminosity on the masses of white dwarfs in nova systems. As much as 25-27 percent of observed nova outbursts should occur on O-Ne-Mg white dwarfs, with the rest occurring almost entirely on C-O white dwarfs. Some evidence of a bimodal character is noted in the empirical mass distribution for white dwarfs, as anticipated by the models. 45 refs.
TL;DR: In this paper, the degenerate structures obtained in these computations have been successively accreted at various He accretion rates in order to better define the fate of the accreting dwarf versus its mass and accretion rate for a fixed degeneracy level.
Abstract: He star models in the mass range 0.4-1.0 solar mass have been evolved until the red giant phase or, depending on their mass, until crystallization on the white-dwarf cooling sequence. Some of the degenerate structures obtained in these computations have been successively accreted at various He accretion rates in order to better define the fate of the accreting dwarf versus its mass and accretion rate for a fixed degeneracy level of the accreting dwarf. He stars have been further induced to transfer mass to a degenerate companion through Roche lobe overflow, in conditions of large gravitational wave radiation by the system. CO dwarfs in binary systems with He stars are found to experience a thermal behavior whose effects are such to locate the structure on the verge of obtaining a strong SN-like explosive event. 22 refs.
TL;DR: In this article, it is argued that the spin-modulated components of the X-ray, optical-continuum and line emission originate in magnetically controlled "accretion curtains" above the white dwarf's magnetic poles.
Abstract: Observations of AO Psc are presented which suggest that it has a magnetic field much weaker than the AM Her stars, that it possesses an accretion disc extending inwards to approximately 8 white-dwaft radii and outwards to fill the Roche lobe, and that it accretes on to narrow arcs around the magnetic poles of the white dwarf with areas less-than-or-similar-to 0.01 of the white-dwaft surface. We exclude the alternative of an intermediate polar having a field similar to the AM Her stars, not possessing a disc, and accreting primarily through ballistic 'blobs' which strike the white dwarf over approximately 0.5 of its surface.It is argued that the spin-modulated components of the X-ray, optical-continuum and line emission originate in magnetically controlled 'accretion curtains' above the white dwarf's magnetic poles. The modulations are all caused by the variation in our view of the optically thick accretion curtain with spin phase. When the upper pole points away from the observer, we see maximum area of the accretion curtain and hence maximum flux; the material is flowing on the pole from the far side of the disc, producing blueshifted emission lines. Occultation due to the upper pole disappearing over the limb does not contribute significantly to the modulation of the X-ray light curve, implying that any disappearance is compensated for by the simultaneous appearance of the lower pole. Hence, we must see similar X-ray fluxes from both poles.Intensity dips in both the X-ray light curve and the optical emission lines are caused by material thrown out of the orbital plane, perhaps by the impact of the accretion stream with the disc, in a manner analogous to the dipping activity of many low-mass X-ray binaries. This implies a moderately high inclination of approximately 60-degrees.The different amplitudes of the optical spin-pulses from one star to another can be explained by different locations of the emission region within the accretion curtain, coupled with the effect of inclination. The spin-pulse amplitude is greatest when the system is viewed edge-on.
TL;DR: In this article, a simple model was proposed to predict the merger rate of binary degenerates, based on the observations presented on the observed luminosity function of white dwarfs, on assumptions about the distribution of initial binary periods, and on estimates of the fraction of all white dwarf pairs whose combined mass exceeds the Chandrasekhar limit.
Abstract: A search for variable radial velocities in 25 hot white dwarfs yielded no binaries with orbital periods between 3 and 10 hr. The fraction of all white dwarfs with spectral classes DA1 to DA5 that are binary with periods in this range is less than or equal to 0.12, with 90 percent confidence. A simple model to predict the merger rate of binary degenerates, based on the observations presented on the observed luminosity function of white dwarfs, on assumptions about the distribution of initial binary periods, and on estimates of the fraction of all white dwarf pairs whose combined mass exceeds the Chandrasekhar limit. The upper limits so derived are less than the observed rate of Galactic Type Ia supernovae. 24 refs.
TL;DR: In this article, rotational ephemerides for four isolated magnetic white dwarfs are derived from more than a decade of polarimetric monitoring, and the stars span the period range 99 min-17.9 days.
Abstract: New rotational ephemerides for four isolated magnetic white dwarfs are derived from more than a decade of polarimetric monitoring. The stars span the period range 99 min-17.9 days and effectively double the number of white dwarfs with measured rotation periods. In addition, five stars whose polarization is constant on time scales up to at least 10 years may have very long rotation periods (P greater than about 100 yr). The lack of very short period white dwarfs attests to the coupling of angular momentum into the giant envelope or interstellar medium during late stages of evolution, but thus far there is no evidence that this process involves magnetic braking. With few exceptions, the disk-averaged fields of magnetic degenerates are dominated by dipolar patterns (as opposed to higher order multipoles or spots), and cover a broad range of obliquity to the rotation axis. 30 refs.
TL;DR: In this article, a preliminary investigation of the asteroseismological properties of chemically stratified evolutionary DA and DB white dwarf models is reported, where the period and kinetic energy distributions for nonradial g-modes of spherical harmonic indices l = 1-3 are computed in the adiabatic approximation.
Abstract: A preliminary investigation of the asteroseismological properties of chemically stratified evolutionary DA and DB white dwarf models is reported. The period and kinetic energy distributions for nonradial g-modes of spherical harmonic indices l = 1-3 are computed in the adiabatic approximation, and the effects of varying the total stellar masss and the surface layer masses on the pulsation properties are studied using an extensive grid of models. Significant resonant mode trapping due to chemical stratification is found. Modes trapped in the outer layers have much lower kinetic energies; these minima also show up as minima in the period spacing between modes of consecutive radial overtone k. Mode trapping occurs at the same or nearly the same value of k for different l-values. Thus, l-values of trapped modes may be identified on the basis of their period ratios. It is shown that observational identification of these period ratios can be used to constrain the mass of the star and its surface layer. 68 refs.
TL;DR: In this paper, high-speed photometric observations of the eclipse light curves of the AM Herculis binary UZ For (EXO 033319-25542) are presented.
Abstract: CCD high-speed photometric observations of the eclipse light curves of the AM Herculis binary UZ For (EXO 033319-25542) are presented Circular polarization observations at lower time resolution, covering the whole binary cycle are presented The eclipse light curves are clearly resolved into two components which can be identified with the white dwarf photosphere, and the accretion spot on its surface This provides a particularly direct confirmation of the standard model of AM Herculis systems The eclipse light curve is modeled in order to set constraints on the geometrical parameters of the binary system, and the location of the accretion spot The duration of the ingress and egress of the white dwarf eclipse constrains the radius and hence mass of the white dwarf It is found that the white dwarf mass is between 061 and 079 solar mass The accretion spot is found to have a radius of about 012 times the radius of the white dwarfs
TL;DR: In this paper, a set of evolutionary tracks of large-metallicity low-mass stars, covering the advanced evolutionary phases from the zero-age horizontal branch to the final white dwarf cooling phase, at the onset of crystallization, is presented.
Abstract: A set of evolutionary tracks of large-metallicity low-mass stars, covering the advanced evolutionary phases from the zero-age horizontal branch to the final white dwarf cooling phase, at the onset of crystallization, is presented. These star models are intended to be representative of the hot component of the stellar population of elliptical galaxies and are suitable for population synthesis purposes. Physical parameters of the evolved stars have been selected to search for models leaving the asymptotic giant branch phase before the onset of the thermal pulses and evolving to the blue side of the H-R diagram according to an evolutionary scheme earlier defined as post-early AGB evolution (Brocato et al., 1990, ApJ)
TL;DR: In this article, the polarization properties of extended accretion shocks on the surface of a magnetic white dwarf have been investigated, and it has been shown conclusively that the null detection of circular polarization in most IPs imply fields of less than 5 MG.
Abstract: Detailed calculations are presented of the polarization properties of extended accretion shocks on the surface of a magnetic white dwarf, where allowance is made both for field spread and for the change in shock height as a function of specific accretion rate. These results are used to show conclusively that the null detection of circular polarization in most IPs imply fields of less than 5 MG. The X-ray properties of MCVs depends critically on the fractional area of the white-dwarf surface over which accretion occurs, and on the type of accretion (smooth or clumpy). In the known IPs, accretion occurs via a disk. The accretion flow is smooth and a strong shock forms making them a powerful source of hard X-rays. There is a new class of MCV distinct from the IPs, where the white dwarf is asynchronous and accretes without a disk in which the accretion is clumpy and the radiation is mainly in the EUV region. A significant fraction of the IPs above the period gap evolve into such systems below the gap and not into AM Hers, as some workers have previously proposed. Any IP-AM Her transitions occur mainly above the period gap at P(O) greater than 4 hr.
TL;DR: The magnetic cataclysmic variable stars (polars, intermediate polars and DQ Her stars) are grouped about three lines in the orbital period-spin period diagram.
Abstract: The magnetic cataclysmic variable stars (polars, intermediate polars and DQ Her stars) are grouped about three lines in the orbital period-spin period diagram. This segregation is shown to be the consequence of competition between braking and accretion torques when combined with the effects of cyclical variations in rate of mass transfer.
TL;DR: In this paper, two alternative explanations for the existence and orbital parameters of the claimed planet-mass object were presented, in which the pulsar forms from the coalescence of two white dwarfs, and the planet condenses from a massive disk of material left behind.
Abstract: BAILES et al.1 have reported the discovery of a planet-mass object in a six-month orbit around the radio pulsar PS R1829–10. The parameters of the orbit, in particular its circularity, make it unlikely that this planet existed before the supernova explosion that presumably created the pulsar, and somehow survived1. Here we present two alternative explanations for the existence and orbital parameters of the claimed planet-mass object. In the first, the pulsar forms from the coalescence of two white dwarfs, and the planet condenses from a massive disk of material left behind. In the second, a neutron star collides with and cannibalizes the central star of a solar-type planetary system. The orbits of the inner planets are made elliptical by this collision, but then recircularized by drag forces due to the extended but short-lived envelope of the disrupted central star.
TL;DR: In this paper, an update of determinations of the CV white dwarf effective-temperature, T(eff), together with an initial exploration of the possible implications and constraints on the CV lifetimes and evolution based on the ensemble of white dwarf T (eff) values as a function of orbital period is presented.
Abstract: This paper presents an update of determinations of the CV white dwarf effective-temperature, T(eff), together with an initial exploration of the possible implications and constraints on the CV lifetimes and evolution based on the ensemble of white dwarf T(eff) values as a function of orbital period. The CV dwarf luminosities are derived by using the T(eff) data and adopting the masses of individual CV white dwarfs determined by Webbink (1990). The present ensemble of empirically determined white dwarf effective temperatures reveals a distribution centered near 16,000 K, implying a mean lower limit total cooling lifetime of 5 x 10 to the 8th yr for the majority of CV degenerates. The two coolest CV degenerates, VV Puppis and St LMi, were found among the strongly magnetic AM Her CVs.