R. A. Saffer

Discovery and Asteroseismological Analysis of the Pulsating sdB Star PG 0014+067*

We report the discovery of low-amplitude, short-period, multiperiodic luminosity variations in the hot B subdwarf PG 0014+067. This star was selected as a potential target in the course of our ongoing survey to search for pulsators of the EC 14026 type. Our model atmosphere analysis of the time-averaged Multiple Mirror Telescope (MMT) optical spectrum of PG 0014+067 indicates that this star has Teff = 33,550 ± 380 K and log g = 5.77 ± 0.10, which places it right in the middle of the theoretical EC 14026 instability region in the log g-Teff plane. A standard analysis of our Canada-France-Hawaii Telescope (CFHT) light curve reveals the presence of at least 13 distinct harmonic oscillations with periods in the range 80-170 s. Fine structure (closely spaced frequency doublets) is observed in three of these oscillations, and five high-frequency peaks due to nonlinear cross frequency superpositions of the basic oscillations are also possibly seen in the Fourier spectrum. The largest oscillation has an amplitude 0.22% of the mean brightness of the star, making PG 0014+067 the EC 14026 star with the smallest intrinsic amplitudes so far. On the basis of the 13 observed periods, we carry out a detailed asteroseismological analysis of the data starting with an extensive search in parameter space for a model that could account for the observations. To make this search efficient, objective, and reliable, we use a newly developed period matching technique based on an optimization algorithm. This search leads to a model that can account remarkably well for the 13 observed periods in the light curve of PG 0014+067. A detailed comparison of the theoretical period spectrum of this optimal model with the distribution of the 13 observed periods leads to the realization that 10 other pulsations, with lower amplitudes than the threshold value used in our standard analysis, are probably present in the light curve of PG 0014+067. Altogether, we tentatively identify 23 distinct pulsation modes in our target star (counting the frequency doublets referred to above as single modes). These are all low-order acoustic modes with adjacent values of k and with l = 0, 1, 2, and 3. They define a band of unstable periods, in close agreement with nonadiabatic pulsation theory. Furthermore, the average relative dispersion between the 23 observed periods and the periods of the corresponding 23 theoretical modes of the optimal model is only 0.8%, a remarkable achievement by asteroseismological standards. On the basis of our analysis, we infer that the global structural parameters of PG 0014+067 are log g = 5.780 ± 0.008, Teff = 34,500K ± 2690 K, M*/M☉ = 0.490 ± 0.019, log(Menv/M*) = -4.31 ± 0.22, and R/R☉ = 0.149 ± 0.004. If we combine these estimates of the surface gravity, total mass, and radius with our value of the spectroscopic temperature (which is more accurately evaluated than its asteroseismological counterpart, in direct contrast to the surface gravity), we also find that PG 0014+067 has a luminosity L/L☉ = 25.5 ± 2.5, has an absolute visual magnitude MV = 4.48 ± 0.12, and is located at a distance d = 1925 ± 195 pc (using V = 15.9 ± 0.1). If we interpret the fine structure (frequency doublets) observed in three of the 23 pulsations in terms of rotational splitting, we further find that PG 0014+067 rotates with a period of 29.2 ± 0.9 hr and has a maximum rotational broadening velocity of V sin i 6.2 ± 0.4 km s-1.

A Comparative Study of the Mass Distribution of Extreme-Ultraviolet-selected White Dwarfs

We present new determinations of effective temperature, surface gravity, and masses for a sample of 46 hot DA white dwarfs selected from the Extreme Ultraviolet Explorer (EUVE) and ROSAT Wide Field Camera bright source lists in the course of a near-infrared survey for low-mass companions. Our analysis, based on hydrogen non-LTE model atmospheres, provides a map of LTE correction vectors, which allow a thorough comparison with previous LTE studies. We find that previous studies underestimate both the systematic errors and the observational scatter in the determination of white dwarf parameters obtained via fits to model atmospheres. The structure of very hot or low-mass white dwarfs depends sensitively on their history. To compute white dwarf masses, we thus use theoretical mass-radius relations that take into account the complete evolution from the main sequence. We find a peak mass of our white dwarf sample of 0.59 M☉, in agreement with the results of previous analyses. However, we do not confirm a trend of peak mass with temperature reported in two previous analyses. Analogous to other EUV-selected samples, we note a lack of low-mass white dwarfs and a large fraction of massive white dwarfs. Only one white dwarf is likely to have a helium core. While the lack of helium white dwarfs in our sample can be easily understood from their high cooling rate, and therefore low detection probability in our temperature range, this is not enough to explain the large fraction of massive white dwarfs. This feature very likely results from a decreased relative sample volume for low-mass white dwarfs caused by interstellar absorption in EUV-selected samples.

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.

Close Binary White Dwarf Systems: Numerous New Detections and Their Interpretation

We describe radial velocity observations of a large sample of apparently single white dwarfs (WDs), obtained in a long-term effort to discover close double-degenerate (DD) pairs, which might comprise viable Type Ia supernova (SN Ia) progenitors. We augment the WD sample with a previously observed sample of apparently single subdwarf B (sdB) stars, which are believed to evolve directly to the WD cooling sequence after the cessation of core helium burning. We have identified 18 new radial velocity variables, including five confirmed sdB + WD short-period pairs. Our observations are in general agreement with the predictions of the theory of binary star evolution. We describe a numerical method to evaluate the detection efficiency of the survey and estimate the number of binary systems not detected because of the effects of varying orbital inclination, orbital phase at the epoch of the first observation, and the actual temporal sampling of each object in the sample. Follow-up observations are in progress to solve for the orbital parameters of the candidate velocity variables.

The First Direct Measurement of the Mass of a Blue Straggler in the Core of a Globular Cluster: BSS 19 in 47 Tucanae

Hot, young blue straggler stars are observed by the Hubble Space Telescope to exist in the cores of very old globular clusters. Theory maintains that these stars must be more massive than hydrogen-burning cluster members that are as old as the clusters themselves. We have, for the first time, verified this theoretical claim by directly measuring the mass of the blue straggler BSS 19 in the core of 47 Tucanae. The derived mass, M=1.7±0.4 M☉, is nearly twice that of the oldest primordial hydrogen-burning cluster star. BSS 19 is found to be rotating rapidly, with a derived projected rotational velocity of vsini=155±55 km s -->−1. This large value for the rotational velocity tends to support a model for the formation of BSS 19 in which the blue straggler was formed from a binary system via a noncollisional mechanism (e.g., the coalescence of a contact binary).

Detection of p-Mode Pulsations and Possible Ellipsoidal Luminosity Variations in the Hot Subdwarf B Star KPD 1930+2752

We report the discovery of multiperiodic luminosity variations in the hot B subdwarf KPD 1930+2752. This star was selected as a potential target in the course of our ongoing survey to search for pulsators of the EC 14026 type. Our model atmosphere analysis of the time-averaged optical spectrum of KPD 1930+2752 indicates that this star has Teff 33,280 K and log g 5.61, which places it well within the theoretical EC 14026 instability strip. At least 44 harmonic oscillations are seen in the light curve, with periods in the range 145-332 s, and amplitudes in the range 0.064%-0.451% of the mean brightness of the star. In addition, the light curve is dominated by a nearly sinusoidal variation with a period of ~4108.9 s and amplitude of ~1.39%. This latter variation is unique among the known EC 14026 stars. We argue that this relatively slow luminosity variation is likely due to the ellipsoidal deformation of the sdB star in a close binary system containing a faint invisible companion (possibly a white dwarf). Using a new period-matching technique based on a genetic algorithm, we demonstrate that the dense observed period spectrum in the 145-332 s interval is compatible with a theoretical low-degree p-mode spectrum that is rotationally split in a star rotating with a period of ~8217.8 s, the value expected from the ellipsoidal effect invoked to explain the observed long-period variation. This interpretation awaits the test of time-resolved spectroscopy. If confirmed, the potential of KPD 1930+2752 as a laboratory for EC 14026 seismology will become immense.

A Large-Scale Spectroscopic Survey of Early-Type Stars at High Galactic Latitudes*

We present new model atmosphere analyses of optical spectroscopy of a large sample of B-type stellar candidates. Of a total of 298 objects, the largest sample of its kind to date, 205 were drawn from the Palomar Green Survey of high Galactic latitude ultraviolet-excess stellar objects and comprise a complete magnitude-limited sample. Effective temperatures, surface gravities, and helium abundances for the hot subdwarf (high-gravity) component of the sample are derived from a detailed line profile analysis of the hydrogen and helium absorption lines in intermediate-resolution (3-5 A FWHM) optical spectra. A separate analysis of the lower gravity component is made using a newly calculated grid of synthetic spectra. Additional estimates of the effective temperatures are made from wide- and intermediate-band photometry taken from the literature. We are currently undertaking two follow-up programs. (1) Detailed abundance analyses of high-resolution echelle spectra of the lower gravity component of the survey using modern model atmosphere and synthetic spectrum techniques will differentiate between massive Population I main-sequence B stars and low-mass, lower luminosity Population II blue horizontal branch stars and post-asymptotic giant branch stars. (2) The derived atmospheric parameters for the higher gravity component, the field extended horizontal branch stars, will be combined with radial velocity measurements to determine their spatial and kinematic distributions, which will distinguish between competing evolutionary scenarios for this hot, evolved stellar population.

FUSE Observations of Heavy Elements in the Photospheres of Cool DB White Dwarfs

We present a comprehensive analysis of the far-ultraviolet spectra offive DB white dwarfs spanning the effective temperature range between 14,700 and 20,800 K. The FUSE line analysis shows that carbon features, previously observed in several hot DB stars at or above 22,000 K, are present in the two coolest (GD 408 and GD 378) and in the hottest (G270� 124) target. The observed carbon abundances range from log N(C)/N(He) �� 6: 9t o �� 8:8. In addition, fourofthefiveobjectsdisplayphotosphericlinesof silicon. Otherelementssuchasoxygen,iron,andsulfur are also observed in some objects. The variations of the abundances of heavy elements as a function of effective temperature in DB stars are discussed in terms of a competition between a stellar wind, gravitational settling, accretion from interstellar (and circumstellar) matter, and convective dredge-up. Subject headingg ultraviolet: stars — white dwarfs

Discovery of p-Mode Instabilities in the Hot Subdwarf B Star PG 1047+003

We report the discovery of multiperiodic luminosity variations in the hot B subdwarf PG 1047+003. At least five periodicities are seen in the light curve, from 104.2 s to 161.9 s, but others are also present at lower amplitudes in that interval. The largest oscillation has an amplitude of 9.2 millimag in white light and a period of 142.2 s. With atmospheric parameters Teff ~ 34,370 K and log g ~ 5.7 for PG 1047+003, these variations are identified with low-order radial and nonradial (p and f) pulsation modes. The similarity of periods and derived stellar parameters indicate that PG 1047+003 is a genuine member of the EC 14026 class, the latest and newest family of pulsators in the field of asteroseismology. However, it shows no evidence of a binary companion, implying that the mechanism for driving pulsations is internal to the star. We also report on the current status of our ongoing survey to search for additional sdB pulsators.

Early-Type Stars in the Galactic Halo from the Palomar-Green Survey. I. A Sample of Evolved, Low-Mass Stars

We present high-resolution spectroscopic observations of early-type stars drawn from a complete sample based on low-resolution spectroscopy of targets from the Palomar-Green Survey by Green, Schmidt, & Liebert. Qualitatively, the metal-line spectra are sharp and are therefore indicative of extremely low projected rotational velocities. Hence the objects are characterized as members of an old, evolved population (for example, blue horizontal branch or post-asymptotic giant branch). By careful choice of Population I, Galactic disk B stars, we have computed differential abundances between the targets and their main-sequence analogs. The CNO abundances from model-atmosphere analyses suggest the presence of nucleosynthesis dredge-up products in the stellar photospheres. With one exception, the stars all have [Fe/H] abundances consistent with their progenitor objects being metal deficient. Some conclusions are drawn as to the previous evolution (red giant branch, horizontal branch, or asymptotic giant branch) of the stars.