Search

We present a spectroscopic analysis of white dwarfs found in the Kiso survey. Spectroscopic observations at high signal-to-noise ratio have been obtained for all DA and DB stars in the Kiso Schmidt ultraviolet excess survey (KUV stars). These observations led to the reclassification of several KUV objects, including the discovery of three unresolved DA+DB double degenerate binaries. The atmospheric parameters (Teff and log g) are obtained from detailed model atmosphere fits to optical spectroscopic data. The mass distribution of our sample is characterized by a mean value of 0.606 Msun and a dispersion of 0.135 Msun for DA stars, and 0.758 Msun and a dispersion of 0.192 Msun for DB stars. Absolute visual magnitudes obtained from our spectroscopic fits allow us to derive an improved luminosity function for the DA and DB stars identified in the Kiso survey. Our luminosity function is found to be significantly different from earlier estimates based on empirical photometric calibrations of Mv for the same sample. The results for the DA stars now appear entirely consistent with those obtained for the PG survey using the same spectroscopic approach. The space density for DA stars with Mv, 44 pages, 14 figures, accepted for publication in ApJ

A new type of oscillation, the so-called atmospheric oscillations resulting from the response of stellar atmospheres to propagating acoustic waves, must be present in white dwarfs with chromospheric activity. Since these oscillations exist only in atmospheres with an outward temperature increase, they can be used as a new indicator of chromospheric activity in white dwarfs. Theoretical predictions of periods and amplitudes of the oscillations are given for selected DA and DB stars. The best candidates for detection of these oscillations are also identified.

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.

Theoretical models of DB white dwarfs are unstable against radial pulsation at effective temperatures near 20,000-30,000 K. Many high-overtone modes are unstable, with periods ranging from 12 s down to the acoustic cutoff period of approximately 0.1 s. The blue edge for radial instability lies at slightly higher effective temperatures than for nonradial pulsations, with the temperature of the blue edge dependent on the assumed efficiency of convection. Models with increased convective efficiency have radial blue edges that are increasingly closer to the nonradial blue edge; in all models the instability persists into the nonradial instability strip. Radial pulsations therefore may exist in the hottest DB stars that lie below the DB gap; the greatest chance for detection would be observations in the ultraviolet. These models also explain why searches for radial pulsations in DA white dwarfs have failed: the efficient convection needed to explain the blue edge for nonradial DA pulsation means that the radial instability strip is 1000 K cooler than found in previous investigations. The multiperiodic nature of the expected pulsations can be used to advantage to identify very low amplitude modes using the uniform spacing of the modes in frequency. This frequency spacing is a direct indicator of the mass of the star.

The problems of nonradiative heating of outer atmospheric layers and p-mode oscillations in white dwarfs caused by acoustic waves generated in convective zones are discussed. These effects have been studied by calculating the cutoff periods for adiabatic and isothermal waves propagating in atmospheres of DA and DB stars with Teff greater than or equal 20,000 K and log g = 6-9. The obtained cutoff periods are approximately bounded by 0.01 and 40 sec for high- and low-gravity white dwarfs, respectively. Expected amplitudes of p-mode oscillations corresponding to trapped acoustic waves with small angular wave numbers are estimated, indicating that the amplitudes could be observed as Doppler shifts of spectral lines which might be detectable if adequate spectral resolution were available. The luminosity variations corresponding to these amplitudes are unlikely to be observable when all damping processes are accounted for. Results also indicate that the present theory of convection predicts some irregularities in the behavior of physical parameters.

Low-dispersion radial velocities of 53 white dwarfs have been measured on Palomar spectrograms. Table 1 contains the type, velocity, space-motion components, photometrically deduced temperature and radius, for each star. Table 4 contains 39 additional radial velocities of very low weight. A few members of wide binary systems and 6 white dwarfs in the Hyades provide direct measures of the Einstein gravitational redshift, with a mean value of +51 km/sec. Omitting the very-high-velocity star LP9-231, there are 37 DA stars, with a mean K-term (expansion velocity) of +65.6 km/sec. If the Hyades stars are omitted, the mean K term is +62.5 km/sec. A number of white dwarfs are members of the high-velocity population. Systematic wavelength shifts of He i lines in DB stars make their velocities more negative than those of DA stars; similar negative shifts may exist for metallic lines. The temperature scale is obtained from colors and, combined with luminosities, gives radii. The broad distribution of radii and redshifts is shown in Figure 2, and median values are derived. The median radial velocity for 37 DA stars is +58 km/sec, and the median radius 0.0107 R_⊙; the redshift and radius give a mass of 0.98 M_⊙. However, this value is almost certainly too high, if we expect accordance with the theoretical mass-radius relation. The theoretical M-R relation of a zero-temperature degenerate star predicts a redshift, for given mass, for various compositions. Two corrections could bring the theoretically expected redshifts into agreement with the observations. Either a systematic change in luminosity, ΔM_v of +0.25 mag, or a reciprocal temperature change of Δθ = —0.03, reduces the median radius to 0.0093 R_⊙. The mass derived from the redshift is then 0.86 M_⊙. These values are in accordance with the Hamada-Salpeter mass-radius relation, if the composition in the interior is pure helium. A carbon or magnesium interior also gives a radius not too different from the colorimetric radius. An iron core gives a mass of 0.73 M_⊙, but a radius of 0.008 R_⊙, sufficiently smaller to require substantial changes in the temperature scale. The mass now derived from the radial velocities is higher than that previously found from radii only and closer to the Chandrasekhar limit.