Spectra of Variable White Dwarfs

Extract

Spectra of seven ZZ Ceti DA variables have been obtained with the multichannel spectrophotometer, several also with 7 Å resolution with the SIT digital spectrophotometer, one with the IUE. The colors fit hydrogen model atmospheres well; the combined IUE and MCSP energy distribution of G29-38 show no peculiarity from 10,000 to 2000 Å. Hydrogen model atmospheres yield <log T> = 4.073±0.008; the dispersion is σ = ±0.02, quite near the total observational and model-fitting error. Using an estimate of the intrinsic 2σ as a half-width the instability strip for hydrogen degenerates lies at 11,800±600 K (for log g = 8). This applies, of course, only to the northern ZZ Ceti stars observed. Another measure of how normal these variables appear is the equivalent width of Balmer lines, which are normal for that T, and are at the line-strength maximum (estimated for log g = 8.2). Possibly the mean Hα is slightly strong; since the Balmer jump is also at maximum strength, a slightly anomalous low boundary temperature might be present. This is also suggested by marginally more positive U-B color for their G-R (or larger log g). I have counts from MCSP data giving the frequency of 162 DA effective temperatures. The ZZ Ceti stars represent 7 of 27 stars in the bin ±0.05 wide in log T centered on 4.05. They are a substantial fraction, but not necessarily all, stars at that temperature. An attempt to observe Ha emission with the Boksenberg IPCS at the Palomar coudé failed for lack of photons.