Thick to Thin: The Evolutionary Connection Between PG 1159 Stars and the Thin Helium-Enveloped Pulsating White Dwarf GD 358

Seismological observations with the Whole Earth Telescope (WET) allow the determination of the subsurface compositional structure of white dwarf stars. The hot DO PG 1159 has a helium surface layer with a mass of 0.001 Msun, while the cooler DB white dwarf GD 358 has a much thinner surface helium layer of 10^-6 Msun. These results imply that either there is no evolutionary relation between these two stars, or that there is an unknown mass loss mechanism. To investigate possible evolutionary links between these objects, we computed evolutionary sequences of white dwarf models including time-dependent diffusion. Our initial model is based on the PG~1159 pulsational data, and has a surface composition of 30% helium, 35% carbon, and 35% oxygen. Below this is a thin transition zone where the helium fraction falls to zero. As expected, diffusion caused a separation of the elements; a thickening surface layer of nearly pure helium overlays a deepening transition zone where the composition returns to the original surface composition. When the model reached the temperature range of GD~358 and the pulsating DB white dwarfs, this pure helium surface layer was 3x10^-6 stellar masses deep. The resulting evolved model is very similar to the model used by Bradley and Winget (1994) to match the pulsation observations of GD 358. The pulsation periods of this model also show a good fit to the WET observations. These results demonstrate the plausibility of a direct evolutionary path from PG 1159 stars to the much cooler DB white dwarfs by inclusion of time-dependent diffusion. A problem still remains in that our models have no hydrogen, and thus must retain their DB nature while their surface tempeture drops from 45,000K to 30,000K. Since there are no known DB stars in this range, we plan to address this problem in future calculations.Comment: LaTeX, 10 pages, using AAS Macros. 2 PostScript figures. Accepted for publication in The Astrophysical Journal Letters

New nonadiabatic pulsation computations on full PG1159 evolutionary models: the theoretical GW Vir instability strip revisited

We reexamine the theoretical instability domain of pulsating PG1159 stars (GW Vir variables). We performed an extensive g-mode stability analysis on PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo for which the complete evolutionary stages of their progenitors from the ZAMS, through the thermally pulsing AGB and born-again phases to the domain of the PG1159 stars have been considered. We found that pulsations in PG1159 stars are excited by the kappa-mechanism due to partial ionization of carbon and oxygen, and that no composition gradients are needed between the surface layers and the driving region, much in agreement with previous studies. We show, for the first time, the existence of a red edge of the instability strip at high luminosities. We found that all of the GW Vir stars lay within our theoretical instability strip. Our results suggest a qualitative good agreement between the observed and the predicted ranges of unstable periods of individual stars. Finally, we found that generally the seismic masses (derived from the period spacing) of GW Vir stars are somewhat different from the masses suggested by evolutionary tracks coupled with spectroscopy. Improvements in the evolution during the thermally pulsing AGB phase and/or during the core helium burning stage and early AGB could help to alleviate the persisting discrepancies.Comment: 10 pages, 8 figures. To be published in Astronomy and Astrophysic

On the systematics of asteroseismological mass determinations of PG1159 stars

We analyze systematics in the asteroseismological mass determination methods in pulsating PG 1159 stars. We compare the seismic masses resulting from the comparison of the observed mean period spacings with the usually adopted asymptotic period spacings, and the average of the computed period spacings. Computations are based on full PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo that take into account the complete evolution of progenitor stars. We conclude that asteroseismology is a precise and powerful technique that determines the masses to a high internal accuracy, but it depends on the adopted mass determination method. In particular, we find that in the case of pulsating PG 1159 stars characterized by short pulsation periods, like PG 2131+066 and PG 0122+200, the employment of the asymptotic period spacings overestimates the stellar mass by about 0.06 Mo as compared with inferences from the average of the period spacings. In this case, the discrepancy between asteroseismological and spectroscopical masses is markedly reduced when use is made of the mean period spacing instead of the asymptotic period spacing.Comment: 7 pages, 4 figures, 1 table. To be published in Astronomy and Astrophysic

Limits on the Halo White Dwarf Component of Baryonic Dark Matter from the {\em Hubble Deep Field}

The MACHO collaboration lensing event statistics suggest that a significant fraction of the dark galactic halo can be comprised of baryonic matter in the form of white dwarf stars with masses between 0.1 and 1.0 \Msun . Such a halo white dwarf population, in order to have escaped detection by those who observe the white dwarf luminosity function of the disk, must have formed from an old population. The observations indicate that the number of halo white dwarfs per cubic parsec per unit bolometric magnitude is less than $10^{-5}$ at $10^{-4.5}$\Lsun; the number must rise significantly at lower luminosities to provide the needed baryonic halo mass. Such white dwarfs may easily escape detection in most current and earlier surveys. Though it is limited in angular extent, the {\em Hubble Deep Field} (HDF) probes a sufficient volume of the galactic halo to provide interesting limits on the number of halo white dwarf stars, and on the fraction of the halo mass that they can make up. If the HDF field can be probed for stars down to $V=29.8$ then the MACHO result suggests that there could be up to 12 faint halo white dwarfs visible in the HDF. Finding (or not finding) these stars in turn places interesting constraints on star formation immediately following the formation of the galaxy.Comment: 10 pages, AASTEX, 1 table, no figures, accepted for publication in Ap.J. Letter

The asteroseismological potential of the pulsating DB white dwarf stars CBS 114 and PG 1456+103

We have acquired 65 h of single-site time-resolved CCD photometry of the pulsating DB white dwarf star CBS 114 and 62 h of two-site high-speed CCD photometry of another DBV, PG 1456+103. The pulsation spectrum of PG 1456+103 is complicated and variable on time scales of about one week and could only partly be deciphered with our measurements. The modes of CBS 114 are more stable in time and we were able to arrive at a frequency solution somewhat affected by aliasing, but still satisfactory, involving seven independent modes and two combination frequencies. These frequencies also explain the discovery data of the star, taken 13 years earlier. We find a mean period spacing of 37.1 +/- 0.7 s significant at the 98% level between the independent modes of CBS 114 and argue that they are due to nonradial g-mode pulsations of spherical degree l=1. We performed a global search for asteroseismological models of CBS 114 using a genetic algorithm, and we examined the susceptibility of the results to the uncertainties of the observational frequency determinations and mode identifications (we could not provide m values). The families of possible solutions are identified correctly even without knowledge of m. Our optimal model suggests Teff = 21,000 K and M_* = 0.730 M_sun as well as log(M_He/M_*) = -6.66, X_O = 0.61. This measurement of the central oxygen mass fraction implies a rate for the ^12C(alpha,gamma)^16O nuclear reaction near S_300=180 keV b, consistent with laboratory measurements.Comment: 10 pages, 10 embedded figures, 3 embedded tables. Accepted for publication in MNRA