Peter Thejll

Hot White Dwarfs in the Extreme Ultraviolet Explorer Survey. II. Mass Distribution, Space Density, and Population Age

We present new effective temperature and surface gravity determinations for a sample of 90 hot white dwarfs detected in the Extreme Ultraviolet Explorer (EUVE) all-sky survey. The measurements, based on spectroscopy of the Balmer line series obtained at the Michigan-Dartmouth-MIT Observatory, Mount Stromlo Observatory, Lick Observatory, and Cerro Tololo Inter-American Observatory, are used to constrain the space density as well as the population age and mass distribution of a sample of 110 EUV-selected DA white dwarfs in the solar neighborhood. We find a mass spectrum narrowly peaked over 0.56 M☉, indicative of a C-O core with a thin hydrogen layer, and a significant population of 10 ultramassive (M ≥ 1.1 M☉) white dwarfs; we also find that all objects fall between effective temperatures of ≈ 25,000 and ≈ 75,000 K and that most are younger than 30 Myr. Using Woods evolutionary models we determine a DA white dwarf birthrate in the solar neighborhood of (0.7-1.0) × 10-12 pc-3 yr-1. Although most objects are on normal C-O cooling tracks, we suggest that a few low-mass white dwarfs and the population of ultramassive white dwarfs may follow different paths with, respectively, He, and, possibly, O-Ne-Mg cores.

AGES OF GLOBULAR CLUSTERS : A NEW APPROACH

A B ST R A C T W ehaveapplied a new m ethod to analyzethehorizontalbranch (HB)m orphology in relationtothedistributionofstarsneartheredgiantbranch(RGB)tipfortheglobular clustersM 22,M 5,M 68,M 107,M 72,M 92,M 3 and 47 Tuc.Thisnew m ethod perm its determ ination ofthe cluster ages to greater accuracy than conventionalisochrone �tting.Using them ethod in conjunction with ournew high-quality photom etricdata

A critical analysis of the ultraviolet temperature scale of the helium-dominated DB and DBV white dwarfs

The temperatures of the hottest of the DB and of some of the variable DBV white dwarfs have been reanalyzed, using a new helium-rich model atmosphere grid and archival IUE data. The grid covers variations in the effective temperature, surface gravity, and trace hydrogen abundance. Significantly lower temperatures than those found by earlier investigations are found. This has influence on the issue of which mixing-length theory is the best one for treatment of convection in DB white dwarfs. The prototype of the DBV stars, GD 358, is given particular attention in view of the recently reported HE II 1640-A line. The sensitivity of the new results to input physics is analyzed. It is found that uncertainties in Stark broadening parameters, convective efficiency, and equation of state and the assumptions of a fixed gravity (log g = 8) and a negligible abundance of hydrogen can at most give an error in the effective temperature of 400 K for a typical DB star. The uncertainty on the absolute calibration of the IUE cameras and the effect of small interstellar reddening dominate the error analysis. 46 refs.

Abundances of Trace Heavy Elements in Hot DA White Dwarfs

We have examined the IUE high-dispersion spectra of 25 DA white dwarfs in order to establish the nature of the relationship between the EUV/soft X-ray opacities discovered by observations from the EXOSAT and Einstein satellites and the heavy element traces often apparent in the IUE ultraviolet spectra of these stars. The study of the ionization balance of silicon and carbon atoms as well as the wavelength shift of the lines relative to the photosphere give important clues to the physical nature of the line absorption. Some spectra suggest the existence of a relatively close and relatively dense shell of ionized material of uncertain composition surrounding at least 6 and maybe as many as 13 of the white dwarfs. The heavy elements may be photospheric only in the case of Feige 24. The inferred EUV/soft X-ray opacity may be provided by heavy elements in some cases and by helium beneath a thin layer of hydrogen in some other cases.

Stellar evolution with mass loss–comparison of numerical and semi-analytical computations

We present here results of stellar evolution calculations that include the latest advances in radiative opacities and neutrino cooling, and discuss on the basis of these models how the internal stellar structure responds to mass-loss from the stellar surface. This problem has particular importance for the development of semi-analytical algorithms for effi cient calculation of synthetic stellar populations with realistic (and hence complex) mass-loss scenarios. We therefore compare our numerical results with test calculations based on a semi-analytical stellar evolution method developed by us. Although small, but important, differences between results from the two methods are revealed, the evolutionary tracks in the HR-diagram predicted by the two approaches are almost identical.

An atmospheric analysis of the carbon-rich white dwarf G35 - 26

The first detailed atmospheric analysis of the DQ white dwarf G35 - 26 is presented. The analysis is based on published spectra and on helium-rich, homogeneous, blanketed LTE models containing carbon, hydrogen, and small amounts of N, O, and Ca, and on the assumption that G35 - 26 is a single star. The observed spectra are fitted against the model spectra, and T(eff), log(g), and abundances of H, He, C are determined along with upper limits on N, O, and Ca. The mass and zero temperature Hamada-Salpeter (ZTHS) radius, distance, parallax, and tangential velocity are derived. The mass, 1.2-1.33 solar, is the highest yet observed for a single white dwarf if the star is on the ZTHS relation. O + Ne + Mg white dwarf in this mass range are predicted by theory and G35 - 26 may be the first such object discovered. Given an ZTHS radius, the distance is between 33 and 69 pc. 36 refs.

The binary Feige 24 - The mass, radius, and gravitational redshift of the DA white dwarf

Observations are reported which refine the binary ephemeris of the Feige 24 system, which contains a peculiar hot DA white dwarf and an M dwarf with an atmosphere illuminated by extreme ultraviolet radiation from the white dwarf. With the new ephemeris and a set of IUE high-dispersion spectra, showing phase-dependent redshifted C IV, N V, and Si IV resonance lines, the orbital velocity, and hence the mass (0.54 + or {minus} 0.20 solar masses), and the gravitational redshift of the white dwarf (14.1 + or {minus} 5.2 km/s) are determined independently. It is shown that the measured Einstein redshift is consistent with an estimated radius for the white dwarf obtained from a model atmosphere solid angle and a parallax measurement. This radius is twice the Hamada-Salpeter radius for the given mass and offers a prospect to investigate the presence of a massive hydrogen envelope in that white dwarf star. 27 refs.

The ultraviolet radiation in metal-rich elliptical galaxies

By use of a new method that allows for an analysis of stellar evolution with a minimum of model dependency, we discuss the conditions for progenitors of UV-bright asymptotic giant branch (AGB) manque stars to form in galaxies of various metallicities. UV-bright manque stars in the so-called slow blue phase have been suggested as the explanation for the observed excess of UV flux shortward of ≃3000 A in metal-rich elliptical galaxies relative to metal-poor systems. We find that in systems where the ratio of helium enrichment relative to metallicity enrichment, R=ΔY/ΔZ, is close to the solar value, the possible UV-bright manque stars will give rise to UV radiation which is almost independent of metallicity

Temperature, radius, and rotational velocity of Sirius B

The authors have derived the interrelationship between radius, rotational velocity, and effective temperature for Sirius B. They find that (1) the radii, 0.76 to 0.81 in hundredths of solar radii, predicted by Teff between 26,000K and 28,000K and a mass of 1.05 Msun; places Sirius B somewhat above the Hamada-Salpeter mass-radius relation, by about 1 or 2 σ, (2) the rotational velocity of Sirius B is predicted to lie between 0 and 600 km s-1 (using 2 σ limits) and that only radii which place a white dwarf above the mass-radius line are allowed, and (3) in order to further limit the rotational velocity and the radius of Sirius B, better determinations of Teff and magnitude are needed.

Modeling Extreme Ultraviolet Spectra of White Dwarfs with High-Metallicity LTE Models

The presence of heavy elements in the hot hydrogen-and helium-rich atmospheres of DA and DO white dwarfs is evident in high-dispersion, far-ultraviolet (FUV) spectroscopy (see Holberg 1995; Werner, Dreizier, & Wolff 1995). The measured abundances are sometimes greatly in excess of the solar composition, and sometimes much below; the abundance pattern in young degenerate stars remains unexplained (Chayer, Fontaine, & Wesemael 1995). Theory shows that surface abundances rapidly reach diffusive equilibrium, and large departures from model predictions indicate that mass-loss or accretion may determine the atmospheric composition of hot white dwarf stars. Vennes et al. (1996a) also suggest that silicon overabundances observed in several hot DA white dwarfs may result from accretion of grains or larger particles.