A. Beauchamp

Spectroscopic Studies of DB White Dwarfs: The Instability Strip of the Pulsating DB (V777 Herculis) Stars

We have secured optical spectra for the eight currently known variable DB, or V777 Her, stars. With the help of a new generation of synthetic spectra, spectroscopic effective temperatures are derived for these objects, as well as for 15 other DB or DBA stars above 20,000 K. We find that the location of the boundaries of the instability strip is sensitive to the atmospheric hydrogen abundance assumed for DB stars: the strip covers the range 22,400-27,800 K if atmospheres of pure helium are used and the range 21,800-24,700 K if undetectable traces of hydrogen are allowed for in the DB models. These determinations provide independent constraints for current seismological analyses of the V777 Her stars. More sensitive searches for weak hydrogen features in hot DB stars should help decide between the two temperature scales.

Spectroscopic Studies of DB White Dwarfs: Improved Stark Profiles for Optical Transitions of Neutral Helium

New Stark profiles for 21 He I lines and their forbidden components have been calculated for electron densities ranging from 1014 to 6 × 1017 cm-3, and for temperatures ranging from 10,000 K to 40,000 K. The profiles include quasi-static broadening by ions as well as electron broadening—the latter treated within the impact approximation near the core, and within the one-perturber approximation in the far wings. The calculated profiles also allow for line dissolution. These new profiles are the cornerstone of a recent reanalysis of the optical spectrum of a large sample of DB white dwarfs. Our calculations provide a very good match to the observed profiles of all the lines, provided that Stark broadening dominates other broadening mechanisms.

LB 8827: A Variable, Magnetic DBA White Dwarf

We present accurate CCD spectropolarimetry and spectra that show that the hot DB star LB 8827 (PG 0853+164) is a weakly magnetic, variable DBA white dwarf. We confirm Putneys detection of circular dichroism in the He I lines, but on three occasions we measure a weaker field strength and opposite sign of circular polarization, Be 100 kG. The Balmer line strengths appear to vary with time, indicative of changes in the apparent H/He abundance ratio or irregularities over the stellar surface. Fits to the observed energy distribution and to the optical He I line spectrum with model atmospheres yield effective temperatures in the 21,200-27,700 K range, with lower values favored. This suggests that LB 8827 falls within or near the instability strip of the V777 Herculis stars. High time resolution photometric observations presented here rule out white-light brightness variations with amplitudes larger than 1.2 mmag in the period window 20 s to 2 hr. The possibility that the magnetic field might tend to suppress pulsations is discussed.

Analysis of Optical and Ultraviolet Spectroscopy of the DAZ White Dwarf G74-7

White dwarfs with spectral lines of more than one chemical element afford us the possibility of studying the competition between different physical mechanisms in determining the chemical composition of the stellar envelope. Among cool white dwarfs of the DA, or hydrogen-line variety, only the prototype DAZ star G74-7 (Lacombe et al. 1983) and the ZZ Ceti star G29-38 (Koester & Provencal 1997) are known to show metal lines. Because of the presence of the Ca II H & K lines in the optical, G74-7 (V ~ 14.5; T eff = 7200 K) was observed with the HST Faint Object Spectrograph by the white dwarf consortium of Shipman et al. (1995). The 1700–3200 A spectrum of this object revealed lines of Mg I, Mg II, Fe I, and Fe II. By providing additional relative abundance ratios, these lines can contribute in a useful way to our understanding of the diffusion process in white dwarfs.

White Dwarfs in M67

We present optical spectra taken of two white dwarfs in the field of the old open cluster M67. One is of type DA, the other of type DB. The DA is hot (68,000 K) and of standard mass (0.57 M⊙), while the DB is much cooler (17,000 K) and less massive (0.49 M⊙), athough the results for the DB are less certain. Should they prove to be members of the cluster, their existence shows that dwarf stars of near solar mass do beget both hydrogen and helium atmosphere white dwarfs.

Atmospheres with Inhomogeneous Surface Abundances and Their Relevance to the Dab White Dwarf GD 323

Koester (1989) has described the DAB star GD 323 as the only clear case of a white dwarf where homogeneous models fail, and stratified models succeed, in providing a fairly consistent fit to the spectral properties of the star. While this is an eminently reasonable physical model to explain the peculiarities of GD 323, we note that i) the fractional hydrogen layer mass required to fit GD 323 is extremely thin by any standard (log [ΔM H /M ⋆] ≡ log qH = -17.06, if M ~ 0.6 M⊙; Koester 1991), and that such thin layers could conceivably be destroyed by mixing with the underlying active helium convection zone; and ii) two recently-discovered DAB stars, with effective temperatures comparable to that of GD 323, might not — at first glance — be amenable to fits with stratified models (Wesemael et al. 1993). Accordingly, it seems appropriate to pursue the consideration of alternatives to a stratified atmospheric structure to explain both the line spectrum and energy distribution of at least some DAB stars.

Defining the Red Edge of the DB Gap with PG 0112+104

Stars around the DB gap, which separates the coolest DO stars from the hottest DB white dwarfs, are likely to hold important clues for our knowledge of the spectral evolution of white dwarf stars and deserve sustained attention. In particular PG 0112+104, the hottest DB star, may well be a critical object, emerging from a DA→DB transition and defining the red edge of the gap. We report here some preliminary results of a reanalysis of this pivotal object, which includes all data currently available: archival low-dispersion IUE spectra and high-dispersion GHRS data around La, which we combine here with our own FOS observations and optical spectrophotometry in the blue and around Hα.

Equilibrium Abundances of Heavy Elements Supported by Radiative Levitation in the Atmospheres of Hot DA White Dwarfs

We present revised estimates of the equilibrium abundances of heavy elements supported by radiative levitation in the atmospheres of hot DA white dwarfs. We emphasize, in particular, the role of trace pollutants that may be present in the background plasma, an effect which has been heretofore neglected. We take advantage of the availability of a table of detailed monochromatic opacities calculated for a plasma made of H containing small amounts of C, N, O, and Fe to illustrate how the equilibrium abundances of levitating elements react to the flux redistribution caused by the addition of these small traces of opaque material. We also consider two other improvements: a more sophisticated treatment of the momentum redistribution process and ion experiences following a photoexcitation, and use of an upgraded value for the line profile width associated with pressure broadening.

The Hot DAB Degenerates in the MCT Survey

The DAB white dwarfs represent a class of hybrid white dwarfs where weak neutral helium lines are superposed onto the classical hydrogen-line spectrum of DA stars. Such stars are rare: until recently the only known DAB star was the prototype GD 323, discovered independently by Oke, Weidemann, & Koester (1984) and Liebert et al. (1984). More recently, a second DAB star, G104-27, was discussed by Holberg, Kidder, & Wesemael (1990), but the exact nature of this object still remains unclear as the weak He I lines reported earlier have not been detected in the subsequent observations of Kidder et al. (1992).