THE TEMPERATURE SCALE AND MASS-DISTRIBUTION OF HOT DA WHITE-DWARFS

Results are presented from a comprehensive spectroscopic survey of DA white dwarfs hotter than similar to 25,000 K. The observations consist ed of CCD spectra with signal-to-noise ratios of similar to 100 in the blue, with similar to 5 Angstrom resolution. The majority of the spec tra covered the wavelength range of 3500-7500 Angstrom, allowing the d etection of a number of cool companions. To date, spectra have been ob tained and analyzed for 174 DA stars. The spectra were analyzed using our model atmospheres, which are described in detail here, resulting i n temperatures and gravities with average internal errors of 1% and 0. 04 dex, respectively. Comparisons with previously published results sh owed that temperature determinations for T-eff < 30,000 K were general ly consistent at the 1% or better level, while spectroscopic gravity d eterminations based on the latest models were consistent at the 0.02 d ex level. Wood's evolutionary sequences were used to derive masses for the observed stars with average internal errors of 0.02 M.. The mass distribution for this hot sample peaked at 0.570 M., consistent with p revious spectroscopic results for cooler samples. One salient feature of this hot sample is the presence of a significant number of massive DA stars with T-eff < 50,000 K, many with M > 1.1 M.. The ultramassive DA's comprise a much larger fraction of the total than was the case f or cooler samples. Calculations based on white dwarf evolutionary mode ls showed that a higher proportion of massive white dwarfs is expected to be found in samples with T-eff greater than or similar to 30,000 K as a result of differential cooling effects. Within the range T-eff > 40,000 K, the EUV-selected subsample did have proportionately more ma ssive stars than the optically selected subsample. However, a detailed comparison showed that EUV and optical surveys were equally capable o f detecting relatively nearby massive white dwarfs. On the other hand, interstellar EUV absorption eliminated from the EUV sample many of th e more distant stars that were detectable optically. Therefore, the ap parent excess of massive DA white dwarfs in the EUV sample is largely due to a relative deficit of stars with more typical masses. Results a re presented for individual stars, including a number of subdwarf iden tifications and reports of detections of cool companions. Properties o f some of the more interesting binaries are discussed. We also report on some stars in the sample that evidently have varying He abundances. WD 0612+177 (G104-27) had been observed, at one point, to have photos pheric He I; we confirm that He has remained absent since then. WD 071 8-316 (RE 0720-314) is in a post-common-envelope binary and has photos pheric He II, the abundance of which appears to vary by more than an o rder of magnitude over timescales of months.