Showing papers on "White dwarf published in 1992"

Millisecond pulsars with extremely strong magnetic fields as a cosmological source of γ-ray bursts

Abstract: THE spatial and luminosity distribution of γ-ray bursts as observed by the BATSE instrument on the Compton Gamma Ray Observatory1,2 provides support for the revival of the idea3,4 that the burst sources are at cosmological distances5. I present here a new model for γ-ray bursts at cosmological distances, based on the formation of rapidly rotating neutron stars with surface magnetic fields of the order of 1015. Such objects could form by the gravitational collapse of accreting white dwarfs with anomalously high magnetic fields in binaries, as in magnetic cataclysmic binaries. Once formed, such rapidly rotating and strongly magnetized neutron stars would lose their rotational kinetic energy catastrophically, on a timescale of seconds or less: rotation of the magnetic field creates a strong electric field, and hence an electron–positron plasma, which I show to be optically thick and in quasi-thermodynamic equilibrium. This plasma flows away from the neutron star at relativistic speeds, and X-ray and γ-ray emission at the photosphere of this relativistic wind may then reproduce the observational characteristics of a γ-ray burst.

A spectroscopic determination of the mass distribution of DA white dwarfs

Abstract: A mass distribution for a sample of 129 DA white dwarfs is presented, based on fitting hydrogen line profiles to the predictions of stellar atmosphere models. The atmospheric parameters determined from spectroscopy are shown to be of improved accuracy over those obtained with other techniques. The spectroscopic masses are in good agreement with those inferred from gravitational redshift measurements

The peculiar type Ia SN 1991T : detonation of a white dwarf ?

Abstract: SN 1991T was a peculiar object whose premaximum optical spectrum did not resemble that of any known supernova; it appears to have been dominated by lines of iron-group elements. Near maximum brightness, however, lines of intermediate-mass elements slowly appeared, and the spectrum began to resemble that of Type Ia supernovae (SNs Ia). With time, the spectral similarity to classical SNs Ia grew progressively stronger. Two months after the explosion, the spectrum was once again dominated by iron-group elements and appeared almost identical to that of typical SNs Ia

The conductive propagation of nuclear flames. I. Degenerate C+O and O+ Ne + Mg white dwarfs

Abstract: The paper determines the physical properties - speed, width, and density structure - of conductive burning fronts in degenerate carbon-oxygen (C + O) and oxygen-neon-magnesium (O + Ne + Mg) compositions for a grid of initial densities and compositions. The dependence of the physical properties of the flame on the assumed values of nuclear reaction rates, the nuclear reaction network employed, the thermal conductivity, and the choice of coordinate system are investigated. The occurrence of accretion-induced collapse of a white dwarf is found to be critically dependent on the velocity of the nuclear conductive burning front and the growth rate of hydrodynamic instabilities. Treating the expanding area of the turbulent burning region as a fractal whose tile size is identical to the minimum unstable Rayleigh-Taylor wavelength, it is found, for all reasonable values of the fractal dimension, that for initial C + O or O + Ne + Mg densities above about 9 x 10 exp 9 g/cu cm the white dwarf should collapse to a neutron star.

Constraints on the age and evolution of the Galaxy from the white dwarf luminosity function

Abstract: The white dwarf disk luminosity function is explored using observational results of Liebert et al. (1988, 1989) as a template for comparison, and the cooling curves of Wood (1990, 1991) as the input basis functions for the integration. The star formation rate over the history of the Galaxy is found to be constant to within an order of magnitude, and the disk age lies in the range 6-13.5 Gyr, where roughly 40 percent of the uncertainty is due to the observational uncertainties. Using the best current estimates as inputs to the integration, the disk ages range from 7.5 to 11 Gyr, i.e., they are substantially younger than most estimates for the halo globular clusters but in reasonable agreement with those for the disk globular clusters and open clusters. The ages of these differing populations, taken together, are consistent with the pressure-supported collapse models of early spiral Galactic evolution.

The collapse of white dwarfs to neutron stars

Abstract: The observable consequences of an accreting white dwarf collapsing directly to a neutron star are considered. The outcome depends critically upon the nature of the wind that is driven by neutrino absorption in the surface layers as the dwarf collapses. Unlike previous calculations which either ignored mass loss or employed inadequate zoning to resolve it, a characteristic mass-loss rate of about 0.005 solar mass/s and an energy input of 5 x 10 exp 50 ergs/s is found. Such a large mass-loss rate almost completely obscures any prompt electromagnetic display and certainly rules out the production by this model of gamma-ray bursts situated at cosmological distances. The occurrence of such collapses with the Milky Way Galaxy might, however, be detected and limited by their nucleosynthesis and gamma-ray line emission. To avoid the overproduction of rare neutron-rich isotopes heavier than iron, such events must be very infrequent, probably happening no more than once every thousand years.

Discovery of a sub-megagauss magnetic white dwarf through spectropolarimetry

Abstract: A circular polarization survey for weakly magnetized (about 10 kG-1 MG) white dwarfs has been initiated using a newly developed CCD imaging/spectropolarimeter. Among the first two-dozen stars studied, WD 0637 + 477 revealed strong polarization reversals across both H-alpha and H-beta, indicating a mean longitudinal ('effective') field of Be = +349 +/- 19 kG. Corresponding estimates of the mean surface field and dipolar field values are Bs = 950 +/- 100 kG and Bp = 1.2 +/- 0.2 MG. WD 0637 + 477 portrays the weakest field yet found on a magnetic white dwarf by a factor of 2. Given the small survey sample observed to date, statistics on the incidence of magnetic fields in the range 100 kG-1 MG permit a distribution which either increases smoothly with decreasing field strength, or is bimodal, wherein a small percentage of stars possess fields above about 1 MG and the vast majority are essentially nonmagnetized.

Magnetic Fields of Degenerate Stars

Abstract: Magnetic fields are ubiquitous in the universe and were known over three decades ago to range from � 1 J.LG in interstellar space up to about 30,000 G in nondegenerate magnetic stars (Parker 1 979). The possible existence of even stronger magnetic fields in white dwarfs was suggested earlier by Blackett (1 947), who proposed that the magnetic moment, J.L = BR3, of a star or planet, with surface field B and radius R, is proportional to its angular momentum. He hence argued that if the angular momentum of a star is conserved during its evolution and collapse, white dwarfs with strong magnetic fields (�1 MG) may be formed since they have smaller radii than their main-sequence progenitors. However, Ginzburg ( 1 964) and W oltjer ( 1 964) proposed instead that the magnetic flux (�BR2) of a star is conserved during its evolution and collapse, so that strong magnetic fields would be generated in degenerate stars. Hence, a main-sequence star with R � 10 11 cm and B � 1 0-1 000 G would collapse to form a neutron star with R � 106 cm and B � 10 1 1_1013 G, or a white dwarf with R � 1 09 cm and B � 105_1 07 G. Shortly thereafter pulsars were discovered ser­ endipitously by Hewish et al ( 1 968) and identified as rotating neutron stars (Gold 1 968). Their magnetic fields were soon deduced to be � 10 1 1_10 1 3 G (see e.g. reviews by Ruderman 1 972, Michel 1 982, Taylor & Stine bring 1 986), apparently consistent with field amplification by flux conservation. These developments were followed by the identification of several X-ray sources as binaries containing accreting magnetized neutron stars (Schreier

The carbon-to-oxygen ratio in halo dwarfs

Abstract: We have used four C I lines at 9100 A and the O I triplet at 7770 A to determine C and O abundances in a sample of 34 field halo dwarfs, which span the metallicity range −3.0 ≤ [Fe/H] ≤ −0.8. The C I line enable us to obtain, for the first time, C abundances for these metal-deficient stars that are based on atomic, rather than molecular, lines. Additional observations of Fe I and Fe II lines at 4500 and 5200 A have been used to help determine atmosphere parameters of the stars

Axion cooling of white dwarfs

Abstract: The cooling rate of the white dwarf star G117-B15A has been recently measured using asteroseismological techniques It has been found that it is higher than predicted from current theoretical models As the modifications introduced into the standard models to accomodate such a result are not satisfactory, we propose to interpret this phenomenon in terms of axion cooling We show in this paper that in order to account for the properties of G117-B15A, the mass of the axions should be 0008 eV, a quantity that is compatible with all the other astrophysical bounds

Dwarf nova outbursts – the ultraviolet delay and the effect of a weakly magnetized white dwarf

Abstract: We investigate the effect on the properties of the outbursts of a dwarf nova (in the disc instability model) of the central white dwarf having a weak (∼10 4 G) magnetic field. We find that in quiescence the inner regions of the disc are evacuated by the presence of the field. This implies that at the beginning of the outburst there is a delay before the hotter inner disc regions are filled and radiating, and thus the optical flux can rise 12 h or so before the ultraviolet flux.

Adiabatic properties of pulsating DA white dwarfs. II: Mode trapping in compositionally stratified models

Abstract: As part of a series of investigations aimed at understanding the adiabatic gravity-mode period structure of models of pulsating DA white dwarfs, we examine qualitatively and quantitatively the phenomenon of mode trapping caused by compositional layering in these stars. This is motivated by the real possibility that the pulsation modes detected in ZZ Ceti stars are actually trapped in the outer hydrogen layer. We first discuss the various manifestations of compositional layering on the pulsational properties of a representative ZZ Ceti star model

Are symbiotic stars the precursors of type Ia supernovae

Abstract: Arguments have been presented recently which favor a membership in the bulge/thick-disk population for symbiotic stars together with a shorter distance scale than previously accepted. Observational evidence from eclipsing symbiotic novae suggests that the large-amplitude thermonuclear outbursts of the white dwarf components are not accompanied by mass loss. On these premises we reanalyze the question of the frequency of symbiotics in the Galaxy, notice that their discovery probability so far has been grossly overestimated, and derive under conservative assumptions a total number for them of ∼3x10 5 , some 100 times more than currently reported

Modeling the Iron-dominated Spectra of the Type IA Supernova SN 1991T at Premaximum

Abstract: SN 1991T in NGC 4527, a galaxy of the Virgo Cluster, has been an unusual Type Ia supernova, being at least one magnitude brighter at maximum than other SN Ia's in Virgo Unlike SN 1990N, another recent Type Ia SN observed also at premaximum, this supernova has not shown in this phase lines of Si II or Ca II To study the composition of the ejecta an approximate NLTE treatment of excitation and ionization based on the Sobolev escape probability concept has been developed and incorporated in the Monte Carlo transfer code of Lucy

A study of metal abundance patterns in cool white dwarfs. I: Time-dependent calculations of gravitational settling

Abstract: As a first step toward understanding the presence of traces of metals in a minority of cool white dwarfs, the settling of Mg, Si, Ca, and Fe in models of these stars is investigated in detail. Powerful numerical tools based on finite-element techniques are used to study the evolution of the abundance distribution of a given metal over 25 orders of magnitude

Late Detonation Models for the Type IA Supernovae SN 1991T and SN 1990N

Abstract: We present hydrodynamical models of exploding white dwarfs to account for the newly observed premaximum features of Type Ia supernovae 1990N and 1991T in a unified manner. In these models, a carbon deflagration, producing a central Fe/Co/Ni core and an intermediate Si/S/Ca layer, is later transformed into a detonation in the outermost layer. Depending on the transition density, the nucleosynthesis in such late detonations results in the formation of Fe-dominated surface layers or Si/Fe-rich surface layers, which are qualitatively consistent with the inferred composition of SN 1991T and 1990N, respectively

Companions to white dwarfs : very low-mass stars and the brown dwarf candidate GD 165B

Abstract: We estimate the initial mass function (IMF) for star formation at the bottom of the main sequence by measuring the number of low-luminosity companions to white dwarfs. A histogram of the number of companions versus luminosity indicates that the IMF is flat or increasing with decreasing stellar mass down to, at least, 0.1 M ⊙ .

Adiabatic Properties of Pulsating DA White Dwarfs. IV. an Extensive Survey of the Period Structure of Evolutionary Models

Abstract: The results of a detailed adiabatic survey of the pulsation properties of evolutionary models of pulsating DA white dwarfs are presented. Pulsation periods, kinetic energies, and first-order rotation coefficients were calculated for all g-modes with l ≤ 3 in the period window 0-1000 s. The survey was carried out with the help of a new, high-performance adiabatic pulsation code based on the Galerkin finite-element method of weighted residuals. A very large volume of parameter space was explored in order to obtain a complete picture of the fundamental asteroseismological properties of DA white dwarfs

Possible models for the Type Ia supernova 1990N

Abstract: Possible interpretations of new spectral and photometric features of Type Ia supernova 1990N with carbon deflagration/detonation models are presented. To explain the presence of Si and Ca at high expansion velocities in SN 1990N, we suggest four possible models, among which is a carbon detonation in a white dwarf with a mass ∼1 M ⊙ . The slow rise of the light curve of SN 1990N can be reproduced by both the deflagration and detonation models if the optical opacity is somewhat larger than that previously adopted

The Hubble Constant from Nickel Radioactivity in Type IA Supernovae

Abstract: On the assumptions that Type Ia supernovae are thermonuclear disruptions of carbon-oxygen white dwarfs near the Chandrasekhar mass, that their light curves are powered by the radioactive decay of 56 Ni and 56 Co, and that scatter in their peak luminosities can be disregarded, the ejected nickel mass, the peak luminosity, and the value of the Hubble constant are estimated. The nickel mass is constrained by the direct spectroscopic detection of elements of intermediate mass and by the explosion kinetic energy as inferred from the spectroscopic velocities. The peak luminosity follows from the nickel mass and the rise time to maximum light. The luminosity, combined with the observational Hubble diagram for Type Ia supernovae, corresponds to a Hubble constant of 61 ± 10 km/(s × Mpc)

Two Ultramassive White Dwarfs Found among Candidates for Magnetic Fields

Abstract: The enhanced and broad Balmer lines evident in spectrophotometry of the white dwarfs PG 1658+441 and PG 0136+251 were previously interpreted in terms of comparatively weak surface magnetic fields, in the range 1-3 MG. New CCD spectrophotometry and circular spectropolarimetry confirm a magnetic field of mean surface strength B s =2.3±0.2 MG on the former star, but provide only marginal evidence for a much weaker longitudinal component of B e =−75±35 kG (1 σ) on the latter object

Accreting white dwarf models for CAL 83, CAL 87 and other ultrasoft X-ray sources in the LMC

Abstract: The study demonstrates that the ultrasoft X-ray emission observed in the three strong LMC X-ray sources CAL 83, CAL 87, and RXJ 0527.8-6954 can be explained by steady nuclear burning of hydrogen accreted onto white dwarfs with masses in the range of 0.7 to 1.2 solar mass. The observed optical and X-ray characteristics of the binary systems CAL 83 and CAL 87 are shown to be consistent with such a model. In both systems the companions are main-sequence stars with masses in the range of 1.5 to 2 solar masses. They are transferring mass unstably on a thermal time scale by Roche-lobe overflow, at rates between 1.0 and 4.0 × 10 exp -7 solar mass/yr. It is argued that the stellar wind emanating from the heated star interacts with that from the disk to generate the He II 4686 emission line with a radial velocity amplitude much lower than the actual radial velocity amplitude of the white dwarf, thus yielding an apparently excessively large mass estimate of this compact star. It is suggested that CAL 83 and CAL 87 are the white dwarf analogs of X-ray binaries like Her X-1.

Gamma-ray bursts and cosmic rays from accretion-induced collapse

Abstract: It is shown that the birth of naked or nearly naked neutron stars in accretion-induced collapse or in the bare collapse of white dwarfs can produce cosmological gamma-ray bursts and can provide the required injection rate of cosmic rays into the interstellar space. It is estimated that most of the e(+)e(-) pairs annihilate in flight on a short time scale in the vicinity of the neutron star. It is shown that the gamma-ray bursts, the 0.511 MeV Galactic annihilation radiation, and the cosmic rays exclude the possibility that the large uncertainties in the Galactic pulsar birthrate and the Galactic SN II explosion rate would allow a significant contribution to the pulsar birthrate from naked or nearly naked neutron star formation. The upper bound on the Galactic birthrate of naked or nearly naked neutron stars of less than 1 in 1000 yr makes it very unlikely that a neutrino burst unaccompanied by optical emission from the birth of a naked or nearly naked neutron star will be detected in the near future by underground neutrino telescopes.

Diffusion and Mixing in Accreting White Dwarfs

Abstract: Numerical experiments have been conducted to determine the degree of enhancement of CNO elements in the envelope of a 1 M ⊙ carbon-oxygen white dwarf accreting hydrogen-rich material at rates of 10 −10 , 10 −9 , and 10 −8 M ⊙ yr −1 . Three initial configurations have been adopted : (1) no initial surface helium layer, 10 9 yr of cooling prior to start of accretion; (2) no initial surface helium layer, a steady state interior thermal structure that is expected after many thermonuclear outbursts; and (3) an initial layer of 10 −3 M ⊙ of helium, 10 9 yr of cooling, with diffusion, before the start of accretion at the rate 10 −10 M ⊙ yr −1

Irradiation of the secondary star in cataclysmic variables

Abstract: It is shown how the relative strength of the Na I doublet over the surface of the secondary star can be calculated from radial velocity curves obtained using the Na I absorption feature around 8190 A. The results for 11 dwarf novae and one magnetic cataclysmic binary system are presented. For the five dwarf nova secondary stars that showed significant heating, it is found that the surface distribution is far more asymmetrical than would be expected from irradiation by the white dwarf and disc hotspot

Type I Supernovae and Accretion-induced Collapses from Cataclysmic Variables?

Abstract: We examine the possibility that the white dwarfs in cataclysmic variables might grow in mass to the Chandrasekhar limit. We demonstrate that the masses of the white dwarfs in classical nova systems generally decrease as a result of nova explosions. We present observational and theoretical evidence in support of the view that the masses of the white dwarfs in recurrent nova systems increase. The critical parameters for which a mass increase is obtained are approximately M WD >1.2 M ⊙ and M>10 −8 M ⊙ yr −1

On the nova-like eruptions of symbiotic binaries

Abstract: Three popular explanations for the nova-like eruptions observed in symbiotic binary stars are discussed and model predictions compared with recent observations. Most outbursts occur when unstable hydrogen shell burning causes a hot white dwarf to expand to a radius of 1-100 R ⊙ . This model predicts a long-duration, constant-luminosity phase following visual maximum, and observations of the symbiotic novae and several steady-burning sources confirm this feature of thermonuclear runaway calculations. At least two symbiotics erupt when the accretion rate from a circumstellar disc onto a solar-type central star increases by 1-2 orders of magnitude

White Dwarfs in the Halo of the Hyades Cluster: The Case of the Missing White Dwarfs

Abstract: On subtracting the cooling age of the dimmest white dwarf in the Hyades cluster (3×10 8 yr) from the age of the cluster (7×10 8 yr), we estimate the mass of the progenitor of this white dwarf to be 3.0 M ○ ., compared with the mass 2.25 M ○ . of a main-sequence star near the turnoff point in the cluster. Adopting a conventional mass function and normalizing to the 24 brightest main-sequence stars in the Hyades, we infer that there should be at least 21 white dwarfs dimmer than the 7 confirmed white dwarf cluster members which presumably derive from stars of initial mass in the range 2.5-3 M ○ .

A Multiwavelength Study of Nova QU Vulpeculae 1984

Abstract: We have combined ground-based optical and satellite ultraviolet spectrophotometric data to study the ejected shell of Nova Vulpecula 1984 No. 2 (QU Vul). Substantial uncertainties concerning the nature of the ionizing continuum, and the role played by density inhomogeneities, now exist. Because of these uncertainties, the chemical composition is deduced from emission-line pairs chosen to minimize the effects of variations in temperature, density, and ionization. We find that the chemical composition of the ejected shell show enhancements of several elements relative to hydrogen, namely, He, N, O, Ne, Mg, Al, and Si, as is expected for an outburst occurring on an ONeMg white dwarf. It is also shown that QU Vul ejected a large amount of aluminum into the interstellar medium, and it is estimated that only a small fraction of slow novae of this type can account for the observed abundance of aluminum in the Galaxy.

X-ray detection of Nova Herculis 1991 five days after optical outburst

Abstract: CLASSICAL nova outbursts are thought to occur in binary systems in which a white dwarf accretes material from a main-sequence dwarf. The outburst is due to thermonuclear runaway in the accreted material, and results in the ejection of about 10−5–10−4 solar masses of material at velocities of up to several thousand kilometres per second (ref. 1). Previous X-ray observations of classical novae in the early stages of outburst have resulted only in upper limits to the X-ray flux2–4. Here we report a positive detection by the Rosat satellite of X-ray emission from Nova Herculis 1991 five days after its optical discovery. Standard nova models predict X-ray emission to arise directly from nuclear burning on the surface of the white dwarf, and suggest that X-rays should not be seen until later in the outburst5. We argue that the emission from Nova Her 1991 comes from hot, shocked circuin-stellar material, which may be the ejected material itself or preexisting circumstellar matter. In either case, however, the required density of material is higher than models of nova binary systems would suggest.