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Showing papers on "White dwarf published in 1994"



Journal ArticleDOI
TL;DR: In this paper, the properties of the DQ Herculis stars are reviewed and compared to those of the AM Hercis stars, which are characterized by strong X-ray emission, high-excitation spectra, and very stable optical and Xray pulsations.
Abstract: We review the properties of the DQ Herculis stars: cataclysmic variables containing an accreting, magnetic, rapidly rotating white dwarf. These stars are characterized by strong X-ray emission, high-excitation spectra, and very stable optical and X-ray pulsations in their light curves. There is considerable resemblance to their more famous cousins, the AM Herculis stars, but the latter class is additionally characterized by spin-orbit synchronism and the presence of strong circular polarization. We list eighteen stars passing muster as certain or very likely DQ Her stars. The rotational periods range from 33 s to 2.0 hr. Additional periods can result when the rotating searchlight illuminates other structures in the binary. A single hypothesis explains most of the observed properties: magnetically channeled accretion within a truncated disk. Some accretion flow still seems to proceed directly to the magnetosphere, however. The white dwarfs' magnetic moments are in the range 10(sup 32) - 10(sup 34) G cc, slightly weaker than in AM Her stars but with some probable overlap. The more important reason why DQ Hers have broken synchronism is probably their greater accretion rate and orbital separation. The observed L(sub x)/L(sub V) values are surprisingly low for a radially accreting white dwarf, suggesting that most of the accretion energy is not radiated in a strong shock above the magnetic pole. The fluxes can be more satisfactorily explained if most of the radial infall energy manages to bypass the shock and deposit itse lf directly in the white dwarf photosphere, where it should emerge as extreme ultraviolet (EUV) radiation. This also provides an adequate source of ionizing photons to power the high-excitation optical and UV emission lines. This is probably the DQ Her analog to the famous 'soft X-ray excess' in AM Her stars. However, unlike the AM Her case, this radiation has not been directly observed, so the analogy must not (yet) be embraced too firmly. There is some conventional wisdom today which segregates the short-period from the long-period DQ Her stars. But the observational grounds for this distinction are slim, except in one respect: X-ray emission from short-period systems appears to be weaker and softer. This must be due to the shallower depth of the potential well, and/or the greater difficulty the fast rotators have in enforcing radial accretion flow.

453 citations


Journal ArticleDOI
TL;DR: In this paper, high signal-to-noise ratio optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet-excess survey is analyzed with a new grid of model atmospheres and synthetic spectra.
Abstract: High signal-to-noise ratio optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet-excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a detailed analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the subluminous B stars in the theoroetical Hertzsprung-Russell (H-R) diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-burning cores of approximately 0.5 solar mass overlain by very thin layers of hydrogen (approximately less than 0.02 solar mass). Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into white dwarfs of lower than average mass.

326 citations


Journal ArticleDOI
TL;DR: Most of the known pulsars are single and located in the disk of the Galaxy as mentioned in this paper, and there is circumstantial evidence that the pulsars in this majority are created in supernova (SN) explosions, by the collapse of the cores of massive massively massive stars.
Abstract: Most of the ~600 known pulsars are single and located in the disk of our Galaxy. There is circumstantial evidence that the pulsars in this majority are created in supernova (SN) explosions, by the collapse of the cores of massive stars (initial mass M_i ≳ M_(cr) ≃ 8 M_⊙). One is created roughly every 100 y in the Galaxy.

273 citations


Journal ArticleDOI
TL;DR: The theory of polytropic gas spheres in conjunction with the equation of state of arelativislically degenerate electrongas leads to a unique value for the mass of a star built on this model.
Abstract: The theory of thepolytropic gas spheres in conjunction with the equation of state of arelativislically degenerate electrongas leads to aunique value for the mass of a star built on this model. This mass (=0.9I⊙) is interpreted as representing the upper limit to the mass of an ideal white dwarf.

248 citations


Journal ArticleDOI
TL;DR: In this article, the authors report on the analysis of 154 hours of early continuous high-speed photometry on the pulsating DB white dwarf (DBV) GD 358, obtained during the Whole Earth Telescope (WET) run of 1990 May.
Abstract: We report on the analysis of 154 hours of early continuous high-speed photometry on the pulsating DB white dwarf (DBV) GD 358, obtained during the Whole Earth Telescope (WET) run of 1990 May. The power spectrum of the light curve is dominated by power in the range from 1000 to 2400 microHz with more than 180 significant peaks in the total spectrum. We identify all of the triplet frequencies as degree l = 1, and from the details of their spacings we derive the total stellar mass as 0.61 + or - 0.03 solar mass, the mass of the outer helium envelope as 2.0 + or - 1.0 x 10(exp -6) M(sub *), the absolute luminosity as 0.050 + or - 0.012 solar luminosity and the distance as 42 + or - 3 pc. We find strong evidence for differential rotation in the radial direction -- the outer envelope is rotating at least 1.8 times faster than the core -- and we detect the presence of a weak magnetic field with a strength of 1300 + or - 300 G. We also find a significant power at the sums and differences of the dominant frequencies, indicating nonlinear processes are significant, but they have a richness and complexity that rules out resonant mode coupling as a major cause.

191 citations



Journal ArticleDOI
TL;DR: In this paper, a Monte Carlo simulation of the formation and evolution of supernova supernovae is presented, and it is shown that a sufficient subset of the initial binary systems evolve to become systems with the requisite properties, so that they can account for the population of supersoft sources that is inferred from observations.
Abstract: Luminous supersoft X-ray sources, with characteristic luminosities of approximately 10(exp 38) ergs/s and temperatures, kT, of approximately 35 eV, have been established as a new and distinct class of X-ray source through recent Roentgen Satellite (ROSAT) observations. Several possible physical models have been proposed for these sources. One promising scenario (van den Heuvel et al. 1992) involves mass transfer, which is unstable on a thermal timescale, from a main-sequence or subgiant donor star onto the surface of a white dwarf. For a narrow range of accretion rates, steady nuclear burning of the accreted matter can take place. This process can provide the high luminosities and the correct range of temperatures observed in the supersoft sources. However, given the limited range of mass transfer rates that are consistent with this phenomenon, it is far from obvious that a sufficient population of such systems exists in galaxies such as our own, M31, and the Magellanic Clouds, in order to account for the large number of supersoft sources which can be inferred from present observations. This work addresses the population question in detail, through a Monte Carlo simulation of the formation and evolution of such systems, which starts with zero-age primordial binaries. In order to evolve into close binary systems which contain a white dwarf component and a companion transferring mass at a rate within the requisite narrow range, a binary system must undergo a specific progression of evolutionary steps. We find that a sufficient subset of our initial binaries evolve to become systems with the requisite properties, so that they can account for the population of supersoft sources that is inferred from observations. In particular, we find that there should be more than 1000 systems in the Galaxy today with properties that very closely match those of the observed supersoft sources. From our models, we find expected luminosities, white dwarf effective temperatures, and orbital periods in the ranges of 10(exp 37) - 10(exp 38) ergs/s, (1-5) x 10(exp 5) K, and 8 hr-1.4 days, respectively. The masses of the white dwarf and donor star are expected to lie in the range of 0.7-1.05 solar mass and 1.3-2.7 solar mass. Finally, we discuss the role that supersoft X-ray sources may play as the progenitors of Type Ia supernovae, and estimate the rate of production via this channel.

150 citations


Journal ArticleDOI
TL;DR: In this article, the authors reexamine the question of the frequency of occurrence of oxygen-neon-magnesium (ONeMg) degenerate dwarfs in classical nova systems, in light of recent observations which have been interpreted as suggesting that 'neon novae' can be associated with relatively low mass white dwarfs.
Abstract: We reexamine the question of the frequency of occurrence of oxygen-neon-magnesium (ONeMg) degenerate dwarfs in classical nova systems, in light of recent observations which have been interpreted as suggesting that 'neon novae' can be associated with relatively low mass white dwarfs. Determinations of heavy-element concentrations in nova ejecta are reviewed, and possible interpretations of their origin are examined. We conclude that, of the 18 classical novae for which detailed abundance analyses are availble, only two (or possibly three) seem unambiguously to demand the presence of an underlying ONeMg white dwarf: V693 CrA 1981, V1370 Aql 1982, and possibly QU Vul 1984. Three other novae which exhibit significant neon enrichments, relative to their total heavy-element concentrations, are RR Pic 1925, V977, Sco 1989, and LMC 1990 No. 1. This result is entirely consistent with present frequency estimates, and our interpretation of the lower levels of enrichment in other systems explains, in a natural way, the existence of relatively low mass white dwarfs in some of the 'neon' novae.

117 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reported the discovery of a rapid spindown of the 33-s rotation period of the white dwarf in AE Aqr at a steady rate of 5.64×10 −14 s s −1 over a baseline of at least 14.5 yr.
Abstract: We report the discovery of a rapid spindown of the 33-s rotation period of the white dwarf in AE Aqr at a steady rate of 5.64×10 −14 s s −1 over a baseline of at least 14.5 yr. The newly derived orbital period by Welsh, Horne & Gomer is confirmed, and it is shown that the secondary star's absorption-line radial velocities provide the correct phasing for the 33-s oscillations if they originate at the white dwarf. The amplitude of the pulse-timing delays, a wd sin i=2.04 s, yields a white dwarf mass of between 0.9 and 1.0 M ○. . The spindown power of the white dwarf is −IΩΩ˙=6×10 33 I 50 erg s −1 , which exceeds the accretion luminosity of AE Aqr by a factor of ∼120

115 citations


Book ChapterDOI
01 Jan 1994
TL;DR: In this article, it was shown that the subgiant components in Algol systems were originally the more massive components of these systems and that the outer layers of the expanding (sub)giant came under the gravitational influence of the companion, causing the latter to increase its mass at the expense of the sub(giant) companion.
Abstract: The ideas of how binary systems evolve with mass exchange have been largely inspired by the surprising characteristics of Algol-type ecUpsing binary systems^. Such systems consist of an unevolved main-sequence star, in the case of Algol: a B 8 V star of 3.7 MQ, together with a less massive subgiant (i.e. more evolved) companion star, in the case of Algol: of spectral type G 8 III (0.8 M©). This situation, with the more evolved star having the smaller mass of the two, is just opposite to what one would expect on evolutionary grounds, as stars of larger mass are expected to live shorter and thus at any time to be in a more advanced stage of evolution than stars of smaller mass. This is what is called the "Algol-paradox". Crawford [50] was the first one to realize that this paradoxical situation can be explained if one asstmaes that largescale mass transfer can take place during the evolution of a binary system: he hypothesized that the subgiant components in Algol systems were originally the more massive components of these systems. As the more massive star evolved faster than its less massive companion it was the first to have evolved away from the main-sequence towards the giant branch. The presence of the close companion, however, prevented such an evolution: when the outer layers of the expanding (sub)giant came under the gravitational influence of the companion, they were captured by that star, causing the latter to increase its mass at the expense of the sub(giant) companion. The (sub)giant transferred so much of its mass that it was finally able to restabilize its interior structure. At that moment it had become the less massive of the two stars. The first -very courageousattempt to carry out a real calculation of this type of evolution with mass transfer was by Morton [174]. He demonstrated the correctness of Crawford's conjecture that mass tranfer, once it begins, becomes unstable and continues until the (sub)giant has become the less massive star of the two. In his calculations, however, he still assumed that the orbital period of the system does not change during the mass transfer. He just kept it fixed.

Journal ArticleDOI
TL;DR: In this article, the speeds of flame fronts that propagate inward into degenerate and semidegenerate cores of carbon and oxygen (CO) and neon-oxy (NeOMg) white dwarfs are determined.
Abstract: We determine the speeds, and many other physical properties, of flame fronts that propagate inward into degenerate and semidegenerate cores of carbon and oxygen (CO) and neon and oxygen (NeOMg) white dwarfs when such flames are bounded on their exterior by a convective region. Combustion in such fronts, per se, is incomplete, with only a small part of the initial mass function burned. A condition of balanced power is set up in the star where the rate of energy emitted as neutrinos from the convective region equals the power available from the unburned fuel that crosses the burning front. The propagation of the burning front itself is in turn limited by the temperature at the base of the convective shell, while cannot greatly exceed the adiabatic value. Solving for consistency between these two conditions gives a unique speed for the flame. Typical values for CO white dwarfs are a few hundredths of a centimeter per second. Flames in NeOMg mixtures are slower. Tables are presented in a form that can easily be implemented in stellar evolution codes and yield the rate at which the convective shell advances into the interior. Combining these velocities with the local equations for stellar structure, we find a minimum density for each gravitational potential below with the local equations for stellar structure, we find a minimum density for each gravitational potential below which the flame cannot propagate, and must die. Although detailed stellar models will have to be constructed to reslove some issues conclusively, our results that a CO white dwarf inginted at its edge will not burn carbon all the way to its center unless the mass of the white dwarf exceeds 0.8 solar mass. On the other hand, it is difficult to ignite carbon burning by compression alone anywhere in a white dwarf whose mass does not exceed 1.0 solar mass. Thus, compressionally ignited shell carbon burning in an accerting CO dwarf almost certainly propagates all the way to the center of the star. Implications for neutron star formation, and Type Ia supernova models, are briefly discussed. These are also applicable to massive stars in the about 10-12 solar mass range which ignite neon burning off center.

Journal ArticleDOI
TL;DR: In this paper, the authors used the HST observations of the eclipsing dwarf nova OY Car in its quiescent state to isolate the ultraviolet spectrum of the white dwarf, the accretion disk, and the bright spot.
Abstract: Hubble Space Telescope (HST) observations of the eclipsing dwarf nova OY Car in its quiescent state are used to isolate the ultraviolet spectrum (1150-2500 A at 9.2 A Full Width at Half Maximum (FWHM) resolution) of the white dwarf, the accretion disk, and the bright spot. The white dwarf spectrum has a Stark-broadened photospheric L(alpha) absorption, but is veiled by a forest of blended Fe II features that we attribute to absorption by intervening disk material. A fit gives T(sub w) approx. = 16.5 x 10(exp 3) K for the white dwarf with a solar-abundance, log g = 8 model atmosphere, and T approx. = 10(exp 4) K, n(sub e) approx. = 10(exp 13)/cu cm, N(sub H) approx. = 10(exp 22) sq cm, and velocity dispersion delta V approx. = 60 km/s for the veil of homogeneous solar-abundance local thermodynamic equilibrium (LTE) gas. The veil parameters probably measure characteristic physical conditions in the quiescent accretion disk or its chromosphere. The large velocity dispersion is essential for a good fit; it lowers (chi square)/778 from 22 to 4. Keplerian shear can produce the velocity dispersion if the veiling gas is located at R approx. = 5 R(sub W) with (delta R)/R approx. = 0.3, but this model leaves an unobscured view to the upper hemisphere of the white dwarf, incompatible with absorptions that are up to 80% deep. The veiling gas may be in the upper atmosphere of the disk near its outer rim, but we then require supersonic (Mach approx. = 6) but sub-Keplerian (delta V/V(sub Kep) approx. = 0.07) velocity disturbances in this region to produce both the observed radial velocity dispersion and vertical motions sufficient to elevate the gas to z/R = cos i = 0.12. Such motions might be driven by the gas stream, since it may take several Kepler periods to reestablish the disk's vertical hydrostatic equilibrium. The temperature and column density of the gas we see as Fe II absorption in the ultraviolet are similar to what is required to produce the strong Balmer jump and line emissions seen in optical spectra of OY Car and similar quiescent dwarf novae. The outer accretion disk is detected at mid-eclipse with a spectrum that rises from 0.05 to 0.3 mJy between 2000 and 2500 A, consistent with combinations of cool blackbodies, blended Fe II emission lines, and Balmer continuum emission. The total disk flux density is 0.5 mJy at 2500 A, and this shallow disk eclipse implies a roughly flat surface brightness distribution. The bright spot, somewhat bluer than the disk, has a flux density rising from 0.05 to 0.15 mJy between 1600 and 2500 A. The C IV emission line has a broad shallow eclipse, but the radial velocity variations observed during the eclipse do not clearly distinguish between a disk or wind origin. The only possible indications of boundary layer emission are fast UV flares that appear to arise from near the central object -- not from the bright spot.

Journal ArticleDOI
TL;DR: In this article, the spectral properties of Type II supernova 1993J have been analyzed for the first 14 months of its existence, revealing its transition to the nebular phase, consistent with the progenitor having lost a majority of its hydrogen envelope prior to exploding.
Abstract: We present optical spectra of the bright, peculiar Type II supernova 1993J in M81 spanning the first 14 months of its existence, revealing its transition to the nebular phase. Unlike the case in normal Type II supernovae, during the first 2-10 months the H-alpha emission line gradually becomes less prominent relative to other features such as (O I) lambda lambda 6300, 6364 and (Ca II) lambda lambda 7291, 7324, as we had predicted based on early-time (tau less than or approximately equal to 2 months) spectra. The nebular spectrum resembles those of the Type Ib/Ic supernovae 1985F and 1987M, although weak H-alpha emission is easily visible even at late times in SN 1993J. At tau = 8 months a close similarity is found with the spectrum of SN 1987K, the only other Type II supernova known to have undergone such a metamorphosis. The emission lines are considerably broader than those of normal Type II supernovae at comparable phases, consistent with the progenitor having lost a majority of its hydrogen envelope prior to exploding. Consequently, there is now little doubt that Type Ib, and probably Type Ic, supernovae result from core collapse in stripped, massive stars; models of the chemical evolution of galaxies in which these subtypes are ascribed to exploding white dwarfs must be appropriately modified. Although all of the emission lines in spectra of SN 1993J fade roughly exponentially for a considerable time, the fading of H-alpha begins to slow down at tau approximately = 8 months, and in the interval tau = 10-14 months its flux is constant, or even slightly rising in the wings of the line. This behavior, together with the box-like shape and great breadth (full width at half maximum (FWHM) approximately = 17 000 km/s) of the line profile, suggests that the H-alpha emission is being produced by the high-velocity outer layer of hydrogen ejecta interacting with circumstellar gas released by the progenitor prior to its explosion. A similar phenomenon has previously been seen at later phases in several Type II supernovae, most notably SN 1980K. Bumps (FWHM approximately = 1000 km/s, amplitude approximately = 20%) in the H-alpha profile are probably indicative of Rayleigh-Taylor instabilities in the cool gas behind the reverse shock. A very narrow component (unresolved, FWHM less than or approximately equal to 200 km/s) of H-alpha at the symmetric velocity of SN 1993J may instead be produced by a superposed H II region, or perhaps by recombination in a large circumstellar shell or ring that was ionized during the first few hours after outburst. In the near future the spectrum of SN 1993J should become increasingly dominated by broad H-alpha emission.


Journal ArticleDOI
TL;DR: In this article, the authors examined the energy balance in AM Her stars with a view to quantifying the incidence of the'soft X-ray excess' due to the deposition of energy directly into the white dwarf photosphere by the accretion stream.
Abstract: We present the results of ROSAT pointed observations of the AM Her stars AN UMa and MR Ser. AN UMa was in a high-brightness state. We examine its soft X-ray light curve, and fit its spectrum with a two-component model consisting of an absorbed soft blackbody and a harder thermal bremsstrahlung distribution. MR Ser was found to be in a low-brightness state, and had a spectrum which was well fitted with a single-component, thermal bremsstrahlung model.The question of the energy balance in AM Her stars is reviewed with a view to quantifying the incidence of the 'soft X-ray excess'. The excess has been explained as due to the deposition of energy directly into the white dwarf photosphere by the accretion stream, supplementing the energy of photons from the hot accretion column which irradiate the photosphere. We have calculated the energy balance in AN UMa and 16 other AM Her stars observed with ROSAT. We find that, although the majority of AM Her stars exhibit a soft X-ray excess, a number of objects are consistent with a model which includes radiative heating only. We examine possible factors which could determine the value of the soft X-ray excess. We find a correlation of soft X-ray excess with magnetic field strength or, alternatively, the radius at which material couples on to the magnetic field lines.

Journal ArticleDOI
TL;DR: In this paper, the results of high-time resolution UV spectroscopy and simultaneous high-speed UBVR photometry of AE Aqr were obtained with the Faint Object Spectrograph aboard the Hubble Space Telescope (HST), and the photometry was carried out with the 82 sec telescope at McDonald Observatory.
Abstract: We present the results of high time resolution UV spectroscopy and simultaneous high-speed UBVR photometry of AE Aqr. The UV spectra were obtained with the Faint Object Spectrograph aboard the Hubble Space Telescope (HST), and the photometry was carried out with the 82 sec telescope at McDonald Observatory. Our study focuses on the coherent 33 sceond oscillations, whose amplitude is found to be very large in the UV (40% of the mean quiescent level). The mean pulse profile has two broad unequal peaks spaced by half an oscillation cycle. The pulse profiles in the UV and optical bands appear quite similar in shape, with no discernible shifts. The orbital delay curve of the UV pulses establishes the white dwarf as their origin. The (UV+optical) spectrum of the pulsations is well described by a white dwarf atmosphere model with a temperature of about 26,000 K. We find no oscillations in the UV emission-line fluxes, nor in their velocities, down to a limit of 800 km/s. Based on the properties of the UV and optical pulsations we suggest that they originate in the X-ray heated magnetic polar caps of the white dwarf. Under this assumption we produce maximum entropy maps of the brightness distribution of the white dwarf surface. Using this model we are able to reproduce the observed mean pulse profile and interpret fluctuations in the oscillation amplitude as small fluctuations in the accretion rate. We find that the amplitudes and profiles of the pulses are not strongly affectd by the large aperiodic flares exhibited by the system. This suggests that the large flares are not related to the process of depositing material onto the white dwarf and argues against models that place their origin at the white dwarf magnetosphere.

Journal ArticleDOI
TL;DR: In this article, the emission-line profiles of cataclysmic variable stars, seen at different orbital phases, are transformed into velocity space images, which makes many of the complex line profile changes easier to interpret.
Abstract: Doppler emission-line tomography is a technique similar to medical tomography. In this atlas the emission-line profiles of cataclysmic variable stars, seen at different orbital phases, are transformed into velocity space images. This transformation makes many of the complex line profile changes easier to interpret. The emission contributions of the disk and the s-wave are clearly separated in these images, and any emission from the stream and the secondary star can often be identified. In this atlas, Doppler tomograms of Hbeta, He I lambda 4471, and He II lambda 4686 emission lines of 18 cataclysmic variable stars are presented. The Doppler images provide insights into the individual systems and a better technique for measuring and radial velocity amplitude of the white dwarf.



Journal ArticleDOI
TL;DR: In this paper, the authors used CCDs to determine the extinction of a number of recent novae based upon emission-line ratios, which are generally observable using CCD.
Abstract: Interstellar reddening is determined for a number of recent novae based upon emission-line ratios which are generally observable using CCDs. Large values of extinction are found for most systems, possibly indicative of an intrinsic component of reddening in postoutburst novae. The unusual characteristics of the (O I) lines in novae, which are strong and optically thick, require a large population of very dense globules which are the likely sites of dust formation. These pyroclasts must be ejected from the white dwarf. The total mass of the neutral gas in the globules in some of the objects is substantially larger than the masses normally derived for the ionized ejecta of novae. The distribution of radial velocities of Galactic novae in the Tololo sample, although uncertain, shows an asymmetry in having predominantly negative values. Either high internal absorption in the expanding ejecta skews the emission lines to bluer wavelengths, or most of the novae are moving out from the center of the Galaxy.

Book ChapterDOI
TL;DR: In this paper, the influence of the larger cooling times of white dwarfs, obtained when crytallization effects are included, on the luminosity function has been calculated, taking into account the complete spectrum of white dwarf masses and their main sequence pre-cooling times.
Abstract: The influence of the larger cooling times of white dwarfs, obtained when crytallization effects are included, on the luminosity function has been calculated, taking into account the complete spectrum of white dwarf masses and their main sequence pre-cooling times. When the effect of 22Ne is included, different initial metallicities of the progenitor star are considered, according to a simplified chemical evolutionary model of the galaxy. Galactic disk ages larger by 2 or more Gyr than those obtained by models without separation are obtained, in better agreement with the globular cluster ages.

Journal ArticleDOI
TL;DR: In this paper, the cooling evolutionary sequences of hot (T eff > 12×10 3 K) white dwarf stars were analyzed and the best fit to the observations yields the mass M=0.7 M ○.
Abstract: We present calculations of the cooling evolutionary sequences of hot (T eff >12×10 3 K) white dwarf stars. The input physics is discussed in some detail, with special emphasis on the thermal conductivity and plasmon neutrino emission assumed. We suggest our own approximation for the latter. We find that the calculated effective temperature distribution of white dwarfs is rather sensitive to the assumed mass. The best fit to the observations yields the mass M=0.7 M ○. . With the inclusion of non-standard physics, we confirm the upper limit on the neutrino magnetic moment to be μ ν <10 −11 in units of the Bohr magneton. Taking into account the bremsstrahlung of axions, we find a constraint on the axion-electron coupling (the axion fine-structure constant) of α a <5×10 −26




Journal ArticleDOI
TL;DR: In this article, a pair of consecutive far-ultraviolet Goddard High-Resolution Spectrograph (GHRS) exposures of the Si IV region of the dwarf nova U Geminorum in early quiescence was obtained, 8 days after its return to optical quyingcence when the underlying white dwarf dominates the ultraviolet light of the system.
Abstract: We have obtained a pair of consecutive far-ultraviolet Goddard High-Resolution Spectrograph (GHRS) exposures of the Si IV region of the dwarf nova U Geminorum in early quiescence, 8 days after its return to optical quiescence when the underlying white dwarf dominates the ultraviolet light of the system. Our GHRS observation revealed a fully resolved line profile for the resonance doublet of Si IV. If it is associated with the white dwarf photosphere, then our best synthetic fits are consistent with T (sub eff) = 35,000 K-38,000 K, log g = 8, a rotational velocity of 50 to 100 km per sec, with a modestly enhanced silicon abundance (1.3-2.3 times solar) and our results suggest that at least in U Gem and perhaps in other similar dwarf novae, the missing boundary layer cannot be explained by rapid rotation of the white dwarf. However, the gamma-velocity of the system remains uncertain. If the gamma-velocity is 43 km per sec (Friend et al. 1990), then a gravitational redshift of approximately 50-60 km per sec is implied for the white dwarf. If the gamma-velocity is 84 km per sec (Wade 1981), then a gravitational redshift of only 10-30 km per sec is indicated, which may imply that either the white dwarf has a low (0.5-0.6 of Solar Mass) mass or an extended atmosphere (corona) due to the outburst heating 8 days earlier. The implications of our line-fitting results for the structure and temperature of boundary layers in cataclysmic variables are discussed.


Journal ArticleDOI
TL;DR: The Hopkins Ultraviolet Telescope was used to obtain a spectrum of the novalike variable IX Vel (CPD -48 deg 1577) in the wavelength range 830-1860 A as mentioned in this paper, which revealed a rich absorption-line and continuum spectrum that peaks near 1050 A at a flux of 1.6 x 10(exp -11) ergs/sq cm/s/A.
Abstract: The Hopkins Ultraviolet Telescope, an experiment flown on the Space Shuttle as part of the Astro-1 mission, was used to obtain a spectrum of the novalike variable IX Vel (= CPD -48 deg 1577) in the wavelength range 830-1860 A. The observation revealed a rich absorption-line and continuum spectrum that peaks near 1050 A at a flux of 1.6 x 10(exp -11) ergs/sq cm/s/A. In the sub-Lyman-alpha region, some of the more prominent absorption lines are S VI lambda lambda-933, 945, C III lambda-977, Lyman-beta, O VI lambda lambda-1032, 1038, P V lambda lambda-1118, 1128, and C III lambda-1176. No emission was detected below the Lyman limit. The overall continuum shape of IX Vel in the FUV can be approximated using models of an optically thick accretion disk in which the integrated spectrum has been constructed by summing model stellar atmospheres or proper disk model spectra. However, if the distance to IX Vel is approximately 95 pc, standard disk models without reddening cannot simultaneously reproduce the color and flux in the UV. While interstellar reddening can reconcile this difference, the amount of reddening appears inconsistent with the absence of a 2200 A bump in the spectrum and the very low H I column density measured along the line of sight. Improved fits to the data can be obtained by modifying the accretion disk stucture within three white dwarf radii. None of the models reproduces the profiles of the Li- and Na-like ions, which are observed as strong but relatively narrow absorption lines, and which are almost surely due to a wind above the disk.

Journal ArticleDOI
TL;DR: In this article, the relativistic shift of the pericenter distance, time delay, and precession of the tensorial structure of the tidal forces are considered in both disruptive and non-disruption cases of a white dwarf model with a massive black hole.
Abstract: We compute encounters of a realistic white dwarf model with a massive black hole in the regime where relativistic effects are important, using a three-dimensional, finite-difference, Eulerian, piecewise parabolic method (PPM) hydrodynamical code. Both disruptive and nondisruptive encounters are considered. We identify and discuss relativistic effects important for the problem: relativistic shift of the pericenter distance, time delay, relativistic precession, and the tensorial structure of the tidal forces. In the nondisruptive case, stripping of matter takes place. In the surface layers of the surviving core, complicated hydrodynamical phenomena are revealed. In both disruptive and nondispruptive encounters, material flows out in the form of two thin, S-shaped, supersonic jets. Our results provide realistic initial conditions for the subsequent investigation of the dynamics of the debris in the field of the black hole. We evaluate the critical conditions for complete disruption of the white dwarf, and compare our results with the corresponding results for nonrelativistic encounters.