An EUV-selected sample of H-rich white dwarfs

Approximately 120 H-rich white dwarfs were detected in the EUV during the survey by the ROSAT Wide Field Camera. Around 90 of these are sufficiently bright for follow-up spectroscopy and photometry and have been included in programmes of observations using the 2.5m and 1.0m telescopes of the Isacc Newton Group in the northern hemisphere together with the 1.9m and 1.0m South African Astronomical Observatory telescopes in the south. The results of our studies of these objects as a group are included in two papers covering their optical and EUV properties ( Marsh et al, 1997a, MNRAS vol 286, pg 286, pg 369-383, and Marsh et all, 1997b, MNRAS , vol 287, pg 705-712, respectively).

Optical observations provide three essential pieces of information for each star - the effective temperature, surface gravity (measured from the Balmer line profiles) and visual magnitude. Using theoretical white dwarf evolutionary models, theses measurements then yield estimates of white dwarf mass and radius. Comparing the ROSAT sample with the cooler optically selected groups reveals a statistically significant excess (approximately 3 times the number expected) of hot, massive H-rich stars. As the optical and EUV samples do not cover the same range of white dwarf temperatures, the high mass excess might arise from differences in the cooling rates of `normal' (about 0.6 solar mass) and massive (more than 1.0 solar mass) stars. Consequently, this feature is not necessarily the result of selection on the basis of EUV flux and might also be present in an optically selected sample covering an appropriate temperature range. Nevertheless, the newly discovered high mass objects remain an interesting group, since they cannot be formed from single star evolution, and may represent a population of coalesced binary white dwarfs.

Distribution of surface gravity for all 89 stars in the EUV-selected sample, and all 129 stars in that of Bergeron, Saffer & Liebert (ApJ, 394 pp228, 1992). The BSL data are scaled by the ratio of the sample sizes forease of comparison.

The X-ray and EUV photometry can only be analysed once the temperature and surface gravity of each object are known. In keeping with earlier studies of a smaler group of stars, we find that most objects with temperatures below 40,000K have nearly pure H atmospheres, while the majority of hotter stars contain significant quantities of heavier elements. However, with the enlarged sample we are able to fill in much more detail. In contrast to earlier work we find a significant number of objects with pure H atmospheres above 40,000K indicating that the EUV sample is biased to objects with lower opacity and higher mass, as expected. Nevertheless, all the stars hotter than 55,000K contain significant quantities of heavy elements. A steep rise in opacity can be seen at 54,000K, which we interpret as arising from the appearance of significant quantities of radiatively supported Fe and other Fe-group elements.

Normalized emergent EUV/X-ray fluxes as a function of temperature for the S1, S2 and PSPC ROSAT survey bands. Lines correspond to predicted fluxes for a pure-H atmosphere and HI column of 5 x 10 18 cm -2 for each filter/instrument combination. Arrows depict upper limits.