Showing papers on "White dwarf published in 1983"

Highlights of astronomy

Abstract: Invited Discourses.- Pulsars and their Genesis.- Dark Matter in the Universe.- Venus.- Joint Discussions.- 1. Reference Frames.- Uses and Required Characteristics of Reference Frames for Galactic Astronomy.- Reference Frames for Minor Planets, Comets and Satellites.- Reference Frames and the Extragalactic Distance Scale.- Requirements for Earth Rotation Parameters.- Ephemerides and Celestial Mechanics.- Status Report on the Work on the FK5.- J2000.0.- Classical Absolute/Differential Programs.- The Background to the MERIT/COTES Recommendations on the Terrestrial and Celestial Reference Systems.- HIPPARCOS and Celestial Reference Frame.- The Use of the Hubble Space Telescope for Global Reference Frame Work.- Proper Motions Referred to Faint Galaxies.- Reference Frame Studies at JPL/ Caltech.- On the Use of Natural References.- Relativistic Reference Frames in Astrometry.- Comparison of the Optical and Radio Reference Frames.- The Promise of Optical/IR Interferometry and Space Astrometry.- Reference Frame/Coordinate System in General Relativity.- Discussions.- 2. Long-Period Eclipsing Binary Stars and Related Objects.- Photometry of the Recent Eclipse of Epsilon Aurigae.- A Spectroscopic View of Epsilon Aurigae.- Interpretation of the Data of Epsilon Aurigae.- Observation and Interpretation of Zeta Aurigae Stars.- Related Binaries, Including Symbiotic Stars.- Mass Transfer and Evolution in Long-Period Binary Systems.- Wind Accretion and Interaction in Long Period Binary Systems.- Summary and Critique of Joint Discussion II.- Extraction of the Shell Spectrum of Epsilon Aurigae.- Dynamics of the Epsilon Aurigae Ring Model.- Interim Discussion of the Orbit of ? Sagittae.- Evidence for the Roche Lobe Overflow in VV Cephei.- Infrared Michelson Interferometry of Zeta Aurigae Type Supergiants.- Very Long Period Supergiant Semidetached and Contact Systems.- 3. Solar and Stellar Nonradial Oscillations.- Progress and Problems in the Study of the Pulsating White Dwarf Stars.- The PG1159 Variables.- Non-Radial Oscillations in ? Scuti Stars and Rapidly Oscillating Ap Stars.- Beta Cephei Variables.- Nonradial and Radial Oscillations Observed in Non-Emission Line OB Dwarfs and Giants.- Non-Radial Pulsation in Be Stars.- Radial and Non-Radial Pulsations in Wolf-Rayet Stars and in Supergiants.- Solar and Solar-Like Oscillations: Theory.- 4. Radio Astronomy and Cosmology.- Cosmic Microwave Background Spectrum Measurements.- The Angular Distribution of the Cosmic Background Radiation.- Ovro Results on the Sunyaev-Zel'dovich Effect, 1983-1985.- The Microwave Background from Cambridge.- Searches for Primordial Pancakes.- Intergalactic Neutral Hydrogen.- The Nature of Faint Radio Sources.- Is the Upturn in the Source Counts Caused by Primeval Radio Galaxies ?.- A New Deep VLA Radio Survey at 6 cm.- Dependence of Linear Sizes and Spectral Indices of Extended Radio Galaxies on Redshift and Radio Luminosity.- Abundances of the Very Light Elements (D, 3He, 4He and 7Li) and Primordial Nucleosynthesis.- 5. Stellar Activity: Rotation and Magnetic Fields.- Stellar Dynamo Characteristics.- Stellar Activity Cycles.- On the Rotation-Activity Connection.- Convection as a Regulator of Dynamos.- The Measurement of Stellar Photospheric Magnetic Fields.- The Dichotomy Between CO Absorptions and Ca II Emissions in the Sun and Stars: An Indirect Diagnostic for Gas Disturbed by Magnetic Fields ?.- Starspots and Plages.- A Study of Three RS CVn-type Eclipsing Binaries.- Coronal Magnetic Fields.- Properties of Stellar Magnetic Fields in Close Binaries Deduced from Non-thermal Radio Observations.- Stellar X-ray and Radio Activities and Coronal Magnetic Field.- On Active Stars, Coronal Loops, Magnetic Braking and All That.- 6. Evolution in Young Populations in Galaxies.- The Concept of Young Populations.- Evolution of High Mass Stars.- The Dynamical Evolution of Young Clussters and Associations.- The Initial Mass Function in Young Star Clusters.- Discussion.- Structure and Evolution of Giant Molecular Clouds.- Molecular Clouds in M51 and in the Galaxy.- The Molecular Cloud Content of Spiral and Dwarf Galaxies.- Discussion.- Star Formation, Giant HII Regions and Spiral Structure.- Young Stars and Star Formation in Normal Irregular Galaxies.- Discussion.- Chemical Evolution of Galaxies.- Star Formation Bursts in Galaxies.- Discussion.- Summary.- 7. Supernovae.- Centenary of S Andromedae (SN 1885a).- Problems of Observing Nearby Galaxies Visually.- Some Possible Identification Between Chinese Guest Stars and Supernova Remnants.- Supernovae in Flocculent and Grand Design Galaxies.- Recent Optical Observations of Supernovae.- Supernovae and Stellar Mass Loss.- Models for the Early and Late Spectra of Supernovae.- NL TE-Effects in Supernovae Type II Photospheres.- Models of Type II Supernova Explosions.- Effects of Angular Momentum on Supernova Explosion and Stability of Rapidly Rotating Stellar Cores.- Gravitational Radiation by a Collapsing Rotating Stellar Core.- Phase Transitions of Superdense Matter and Supernova Explosion.- X-ray Emission from Supernova Remnants.- Supernovae and Cosmology.- VL BI Observations of the Compact Components in M82.- Radio Supernova Candidates in the M82 Starburst.- Joint Commission Meetings.- Hipparcos.- The ESA Space Astrometry Mission: Overview and Status.- Activities of the Input Catalogue Consortium.- Preparation of the Mission: Earth-based Photometry.- Preparation of the Mission: Earth-based Astrometry.- Activities of the Data Reduction Consortia.- Tycho: Photometry & Astrometry for more than 500 000 Stars.- The Tycho Input Catalogue.- Link with Extragalactic Reference Frames.- Coronal Activity and Interplanetary Disturbances.- Coronal Activity at Radio Wavelengths.- Coronal Response to Energy Release During Solar Flares.- Correlated Observations of Impulsive UV and Hard X-ray Bursts from the Solar Maximum Mission.- Coronal Mass Ejections.- The Sources of Major Heliospheric Disturbances.- The Response of Microwave Emission to the Development of Active Regions.- Coronal Holes and Flare Related Phenomena.- Coronal Structures Observed at Meter Wavelengths.- The Nancay Multifrequency Radioheliograph.- An Energy Storage Mechanism for a Solar Flare by Shearing the Magnetic Field.- Merit + Cotes.- Joint Summary Report of the IAU/IUGG Working Groups on the Rotation of the Earth and the Terrestrial Reference System.- Report of Joint Meeting Held on 1985 November 22 to consider the MERIT/COTES Recommendations for a New International Earth-rotation Service.- Synthetic Photometry.- Principles and Scope of Synthetic Photometry.- The Possibilities of Synthetic Photometry.- Passbands and Photometric Systems.- Observed Stellar Energy Distributions for Synthetic Photometry.- Theoretical Stellar Energy Distributions.- Synthetic Photometry and the Calibration of the Hubble Space Telescope.- Standard Stars.- The Microfiche of Standard Stars.- Energy Distribution, Photometry and Physical Characteristics of the Sun and Solar Analogs.- On the Absolute Energy Distributions of the Sun, of the "Solar Analogs" 16 Cyg B, 16 Cyg A, VB 64 and of Vega.- Note.- Additional Contributions.- New Radio Telescopes.- A Reconsideration of the Galactic Constants.- Author Index.

Late stages of stellar evolution. II: Mass loss and the transition of asymptotic giant branch stars into hot remnants

Abstract: We have computed the transition of asymptotic branch giants of 0.8 and 1.0 M/sub sun/ into remnants of 0.565, 0.553, and 0.546 M/sub sun/ by means of mass loss which removed the stellar envelopes in about 10/sup 3/ yr. The remnants were then evolved across the H-R diagram to the white dwarf configuration and their relevance for central stars of planetary nebulae studied. From the temporal evolution of these models we determined lower limits for mass and luminosity of central stars of approx.0.55 M/sub sun/ and approx.2500 L/sub sun/.

On the evolution of those nuclei of planetary nebulae that experiencea final helium shell flash.

Abstract: We suggest that some of the central stars of planetary nebulae experience a final thermal pulse having achieved a white dwarf configuration and begun their descent along a cooling white dwarf sequence of nearly constant radius. A concrete theoretical calculation demonstrates that, during such a pulse, most of the hydrogen remaining in the star at pulse onset is incorporated into the helium-burning convective shell and completely burned, and that, following the pulse, the star swells briefly to red giant dimensions. The model then proceeds to burn helium on a long time scale, retracing in the H-R diagram approximately the same path that it followed while burning hydrogen during the initial excitation of the nebula, which has by now expanded considerably in extent. We identify as being in the postpulse, quiescent helium burning phase the central stars of the planetary nebulae d Abell 78, and the central stars of a group of related high-excitation objects. These nebulae all have the large radii often found in conjunction with central stars of low luminosity that are thought to be cooling along the white dwarf sequence; however, they have the high luminosities that are characteristic of much smaller nebulae whose nuclei are thought tomore » be proceeding for the first time through the planetary nucleus regime in the (log L, log T/sub e/)-plane.« less

Evolutionary status of bright, low-mass x-ray sources/sup 1/

Abstract: A model of bright, low-mass X-ray binaries is proposed which features a lower giant-branch star losing mass on a nuclear time scale to an accreting compact companion. Simple numerical models show that mass transfer rates > or =10/sup -9/ M/sub sun/ yr/sup -1/ are sustained at very nearly a constant rate until the envelope of the donor star is exhausted. The model predicts orbital periods in the range 1/sup d/-200/sup d/ and X-ray to optical luminosity ratios L/sub x//L/sub opt/roughly-equal200-1000 for these sources. It accounts in a natural way for the large fraction of the total galactic bulge luminosity emitted by a few bright (> or =10/sup 37/ ergs s/sup -1/) sources. It also accords very well with the observed X-ray and optical properties of the halo source Cyg X-2 and also with those of 2S 0921-63, provided this latter system contains a massive accreting white dwarf rather than a neutron star. Problems of the prior evolution of low-mass X-ray sources are also briefly delineated.

The Optical Counterparts of Compact Galactic X-Ray Sources

Abstract: Ninety-six optical identifications of X-ray sources presumed to be compact objects are presented and discussed. X-ray and optical classifications of the sources are considered, and the several types of systems are discussed. These include neutron star binaries with massive and low-mass stellar components, neutron stars in supernova remnants, cataclysmic variables, and 'isolated' hot white dwarfs. The information that can be derived from optical observations of neutron-star systems with low-mass optical companions is emphasized.

The binary nature of the barium stars. II. Velocities, binary frequency, and preliminary orbits.

Abstract: A series of radial velocity observations has been obtained over the last 3 years of a sample of 20 Ba II stars. Seventeen of these (85%) are found to show long-term velocity variations ranging in amplitude from a few to tens of km s/sup -1/. Thus it seems likely that all Ba II stars are binary systems. An additional Ba II star in the open cluster NGC 2420 is also found to have a variable velocity. Orbits for seven Ba II stars indicate low-mass companions, compatible with their being white dwarfs. Separations are large, about 2 AU on average. The peculiar abundances are not explained, but the fact remains that multiplicity must bear a causal relationship to these peculiarities.

A one-zone model for shell flashes on accreting compact stars

Abstract: A one-zone model is developed for analysis of properties of nuclear shell flashes on accreting degenerate dwarfs and neutron stars. The model provides a description of a steady-state nuclear burning and a linear stability analysis with a small number of algebraic equations. Time evolution of the accreted layer is described with two first order ordinary differential equations: one for the heat balance, the second for the mass balance. A very small computing power is required for the analysis. This makes the model attractive for pilot studies, for a simple analysis of many properties of compact stars accreting nuclear fuel, and for teaching purposes. When the accretion rate is either very low or very high, then column density increases with the accretion rate and the models with a steady state nuclear burning are stable. For intermediate rates, the surface mass density decreases with increasing accretion rate and the models are thermally unstable. Near the transition from stability to instability the eigenvalues of the problem are always complex. The models are stable for any value of the accretion rate when either the heat flux from the core exceeds some critical value, or the accreted matter is rich in hydrogen but has nomore » metals, so that nuclear burning may proceed through the proton-proton chain only. Large-amplitude shell flashes develop for all unstable models provided that heat flux from the core is below a certain value. The time interval between the flashes decreases with increasing surface gravity, accretion rate and heat flux from the core. The shortest periods for accreting degenerate dwarfs are just 1 month for hydrogen-rich matter and 1 year for helium-rich matter. The shortest interflash period for a neutron star accreting helium is only 10 s. The values of these periods may be incorrect by a factor of 2 or so, because of simplifications inherent in the one-zone model.« less

White dwarfs and neutron stars in globular cluster X-ray sources

Abstract: We predict here that globular clusters contain at least as many cataclysmic variables, which contain white dwarfs, as bright (Lx>1036 erg s−1) X-ray sources, which contain neutron stars. Globular clusters contain many more white dwarfs than neutron stars, but the capture mechanisms for the formation of binaries are more efficient for neutron stars. We point out here the consequences of the frequent formation of temporarily bound triple systems (resonance scattering). These are: an extra enhancement of neutron star capture with respect to white dwarf capture, a considerable probability of actual collisions between stars, and the presence of cataclysmic variables in the outer regions of globular clusters.

Mixing by shear instabilities in differentially rotating inhomogeneous stars with application to accreting white dwarf models for novae

Abstract: The problem of how shear instabilities redistribute matter and angular momentum accreted by a star from a disk is considered Necessary conditions for stability of the star to nonaxisymmetric perturbations are derived by use of the short wavelength approximation By considering growth rates, it is shown that freshly accreted material rapidly takes up a quasi-spherical distribution due to dynamical instabilities However, mixing inward toward the stellar interior occurs on a thermal time scale or longer

Evolutionary history of the 6.1-ms pulsar

Abstract: Boriakoff et al.1 have reported the discovery of a 6.1-ms radio pulsar in a binary with an orbital period of 120 days, a1 sin i = 0.99 × 1012 cm, and e ≈ 0. This is the fourth binary radio pulsar, and the third to have a nearly circular orbit2. The origin of such systems has been discussed frequently3,4 and I now report that evolutionary history of the new millisecond pulsar is perhaps the simplest of them all. The rapid rotation of the pulsar, small mass function, circular orbit, and long binary period suggest that following the formation of a neutron star there had to be a phase of mass transfer from the low-mass secondary onto the neutron star primary. That mass transfer circularized the orbit and spun up the primary, and it required the secondary to be a red giant at that time. The present binary period of 120 days implies that the secondary has a mass of 0.3 M⊙. The observed mass function of 0.0027M⊙ implies that the primary must be less massive than 3.1M⊙, not an interesting mass limit for a neutron star. The secondary is probably a white dwarf at present. However, if the mass transfer terminated < 106 yr ago then the secondary may be still burning hydrogen at the rate of about 300L⊙, and it may be optically detectable.

Is the 6-ms binary pulsar the remnant of a bright galactic X-ray source?

Abstract: It is argued here that the recently discovered 6-ms radio pulsar (ref. 1) with orbital period Pb = 120≠4 days, a sin i = (0.92≠0.08) ×1012 cm and e ≈ 0) is the spun up remnant of a formerly heavily accreting (M ≈ 10−8M⊙ yr−1) X-ray source, belonging to a new type of low-mass X-ray binaries. This new type of X-ray binary consists of a critical lobe filling (lower) branch giant star of ≈ 1M⊙ that spills mass to its neutron star companion. Systems of this kind were recently introduced2 to explain the existence of the very bright (Lx ≈ 1038 erg s−1) X-ray sources located in the bulge of M31 and in the bulge of our Galaxy. Such systems will end their lives as wide binary systems (Pb ≳ 100 days), consisting of a≈0.3M⊙ helium white dwarf (the remnant core of the giant) in orbit about a rapidly spinning neutron star. For a suitable combination of magnetic field strength and accretion rate onto the neutron star, it will appear as a millisecond radio pulsar once the giant has lost its envelope and the heavy mass transfer ceases abruptly.

Radio emission from AM Herculis - The quiescent component and an outburst

Abstract: The VLA has been used to search for radio emission from the AM Her-type binaries VV Pup, EF Eri, PG 1550 + 191, CW 1103 + 354, and AN UMa, at 4.9 GHz. A remarkable 10-min outburst was detected from AM Her at 4.9 GHz, which was about 20 times more intense than the quiescent emission and was essentially 100 percent circularly polarized. It is suggested that the quiescent emission of AM Her can be accounted for by 500-keV electrons trapped in the magnetosphere of the white dwarf, provided that the electron energy spectrum is quite hard and that the spectral hardness or number density of energetic electrons increases with radius, while the outburst is probably due to an electron-cyclotron maser operating near the surface of the red dwarf companion. The implied existence of a 1000-gauss localized magnetic field and a corona on the red dwarf has consequences for mass transfer, field line interactions, and variable activity.

Do all barium stars have a white dwarf companion

Abstract: International Ultraviolet Explorer short-wavelength, low-dispersion spectra were analyzed for four barium, two mild barium, and one R-type carbon star in order to test the hypothesis that the barium and related giants are produced by mass transfer from a companion now present as a white dwarf. An earlier tentative identification of a white dwarf companion to the mild barium star Zeta Cyg is confirmed. For the other stars, no ultraviolet excess attributable to a white dwarf is seen. Limits are set on the bolometric magnitude and age of a possible white dwarf companion. Since the barium stars do not have obvious progenitors among main-sequence and subgiant stars, mass transfer must be presumed to occur when the mass-gaining star is already on the giant branch. This restriction, and the white dwarf's minimum age, which is greater than 8 x 10 to the 8th yr, determined for several stars, effectively eliminates the hypothesis that mass transfer from an asymptotic giant branch star creates a barium star. Speculations are presented on alternative methods of producing a barium star in a binary system.

X-ray evidence for white dwarf binaries in globular clusters

Abstract: Data from an X ray survey of globular clusters by means of instrumentation on the Einstein Observatory indicate the existence of a distinct class of low-luminosity (L) sources. L was measured in the 10 to the 32nd to 10 to the 34th ergs/sec range when viewing point-like objects. The L-value is three orders of magnitude less than that of neutron star binaries, and is consistent with projections for binaries with 0.5 solar mass white dwarfs. They were located outside the cluster cores, which was taken as an indication of low mass. More than three sources at the lowest luminosity bound were observed in two clusters, suggesting a steep rise in the luminosity function, which agreed with observational data on galactic plane cataclysmic variables.

Upper Limit on the Frequency of Pulsars

Abstract: Neutron stars for which the ratio T/Vertical BarWVertical Bar of kinetic to gravitational potential energy exceeds approx.0.08 are likely to be unstable and to spin down by gravitational-wave--driven oscillations. A class of pulsars with nearly identical frequencies may thus result from the collapse of accreting, rapidly rotating white dwarfs. It is likely that the fast pulsar, PSR 1937+214, rotates more slowly than this, but the question remains open partly because of uncertainty in the equation of state.

The outbursts of symbiotic novae

Abstract: We discuss possible conditions under which thermonuclear burning episodes in the hydrogen-rich envelopes of accreting white dwarfs give rise to outbursts similar in nature to those observed in the symbiotic stars AG Peg, RT Ser, RR Tel, AS 239, V1016 Cyg, V1329 Cyg, and HM Sge. In principle, thermonuclear runaways involving low-luminosity white dwarfs accreting matter at low rates produce configurations that evolve into A--F supergiants at maximum visual light and which resemble the outbursts of RR Tel, RT Ser, and AG peg. Very weak, nondegenerage hydrogen shell flashes on white dwarfs accreting matter at high rates (M> or approx. =10/sup -8/ M/sub sun/ yr/sup -1/) do not produce cool supergiants at maximum, and may explain the outbursts in V1016 Cyg, V1329 Cyg, and HM Sge. The low accretion rates demanded for systems developing strong hydrogen shell flashes on low-luminsoity white dwarfs are not compatible with observations of ''normal'' quiescent symbiotic stars. The extremely slow outbursts of symbiotic novae appear to be typical of accreting white dwarfs in wide binaries, which suggests that the outbursts of classical novae may be accelerated by the interaction of the expanding white dwarf envelope with its close binary companion.

The Intermediate Polars

Abstract: Until 1976, cataclysmic variable star research proceeded with few requirements for the inclusion of magnetic fields in theoretical models. Although models for low-mass X-ray binaries stressed the importance of magnetic fields (Lamb et al. 1973) and there was an increasing number of known magnetic single white dwarfs (Angel 1977), and a magnetised white dwarf had been one of the models proposed to explain the rapid oscillations in DQ Her (Herbst et al. 1974, Katz 1975), there was no anticipation of the more general role that magnetic fields now seem destined to play. The two major reviews of the time (Robinson 1976, Warner 1976) scarcely considered the presence of magnetic fields.

Synchronization of magnetic stars in binary systems.

Abstract: Asynchronous rotation of magnetic stars in close binary systems drives substantial field-aligned electrical currents between the magnetic star and its companion. The resulting magnetohydrodynamic torque is able to account for the heretofore unexplained synchronous rotation of the strongly magnetic degenerate dwarf component in systems like AM Her, VV Pup, AN UMa, and EF Eri as well as the magnetic A type component in systems like HD 98088 and 41 Tauri. The electric fields produced by even a small asynchronism are large and may accelerate some electrons to high energies, producing radio emission. The total energy dissipation rate in systems with degenerate dwarf spin periods as short as 1 minute may reach 10 to the 33rd ergs/s. Total luminosities of this order may be a characteristic feature of such systems.

X-ray emission and spin-up evolution of the 6.1-ms pulsar

Abstract: An upper bound is calculated for the X ray luminosity of the binary pulsar PSR1953+29 and discussed in terms of effects on theretical models for the pulsar's formation and spin-up. The upper limit was obtained by summing the detected counts measured by the Einstein Observatory within a circle of radius 150 arcsec and subtracting local background fluxes. A flux upper bound of less than 4 x 10 to the 32 erg/sec, a neutron star radius of 15 km, a temperature less than 1,000,000 K, and a pulsar age of at least 3000 year are obtained. The neutron star could be much older than 10,000,000 yr, the spin-down rate is up to 10 to the minus 17 str/sec, and formation occurred by accretion when the companion was in a red giant phase. The pulsar has a mass of about 0.3 solar mass, travels an orbit of about 10 to the 13 cm, and evolved from a white dwarf near its Chandrasekhar limit. It is suggested that the magnetic surface field of the pulsar has stabilized near 1 billion G.

Collapse and explosion of white dwarfs. I - Precollapse evolution

Abstract: The long-term evolution of accreting carbon-oxygen white dwarfs in close binary systems is considered. Depending on the time of onset of mass accretion (for a given stellar mass), thermonuclear ignition happens when the star's center is either in the fluid or in the solid phase. In the last case, burning propagation should be slow, and previous carbon-oxygen separation is likely. We show that by considering different degrees of chemical separation (associated with different cooling times) diverse outcomes are possible: total collapse for maximum separation and off-center ignitions for partial chemical differentiation. The off-center ignitions might provide a mechanism for Type I supernova outbursts, and, by implying the explosive burning of varying amounts of the carbon-oxygen mixture, they might also explain the ''fast'' to ''slow'' SN I range.

White dwarfs and the interstellar medium

Abstract: Radiation emanating from hot (T greater than 40,000 K) white dwarfs can create large volumes of ionized material containing substantial column densities of highly ionized species, in particular Si IV and C IV. The ions N V and O VI can also be produced by hot, hydrogen-rich white dwarfs. These ionization spheres may be detectable around the nearby dwarfs. The relatively high space motions of these stars coupled with long recombination times in the interstellar medium suggest that a white dwarf leaves a region of ionized material - a fossil Stroemgren trail - that marks its progress through the galaxy. White dwarfs create a patchy substrate of ionized gas in the galactic plane and lead to extended ionized regions out of the plane. The spatial frequency of hot white dwarfs indicates that they contribute a radiative energy comparable to that provided by nondegenerate stars and by supernovae and capable of affecting the ionization balance of the interstellar medium.

Mass loss, levitation, accretion, and the sharp-lined features in hot white dwarfs

Abstract: A study has been conducted of eight white dwarfs, including seven DA and one He-rich types. The study is based on high-resolution observations conducted with the aid of the International Ultraviolet Explorer. Four of the dwarfs show features related to heavy elements which are not interstellar in origin. It is tentatively suggested that, at least in the hottest low-gravity DA white dwarfs, the observed narrow-lined features are formed in expanding halos or winds associated with the white dwarfs. Theoretically, stable white dwarf halos should actually be coronae with temperatures in excess of 1,000,000 K. However, the observed narrow-lined features do not suggest such high temperatures. The observed radial velocities suggest weak stellar winds in two hot white dwarfs, namely, G191-B2B and 2111+49. It is tentatively proposed that radiative levitation can explain the appearance of the observed metallic lines in the hot DA white dwarfs.

White dwarfs and the interstellar medium

Abstract: Radiation emanating from hot (T greater than 40,000 K) white dwarfs can create large volumes of ionized material containing substantial column densities of highly ionized species, in particular Si IV and C IV. The ions N V and O VI can also be produced by hot, hydrogen-rich white dwarfs. These ionization spheres may be detectable around the nearby dwarfs. The relatively high space motions of these stars coupled with long recombination times in the interstellar medium suggest that a white dwarf leaves a region of ionized material - a fossil Stroemgren trail - that marks its progress through the galaxy. White dwarfs create a patchy substrate of ionized gas in the galactic plane and lead to extended ionized regions out of the plane. The spatial frequency of hot white dwarfs indicates that they contribute a radiative energy comparable to that provided by nondegenerate stars and by supernovae and capable of affecting the ionization balance of the interstellar medium.