Temperatures of the hottest white dwarfs

Earlier work has shown that the temperature inferred for the hottest ( T ~ 50,000 K) white dwarfs is very sensitive to the modelling approach, particularly that used to analyse the Balmer line profiles. For example, a significantly lower temperature is obtained if the models include appropriate abundances of elements heavier than H or He, compared to a pure H model. The most extreme effect is in the very hottest white dwarfs; an illustration is provided by WD2218+706 for which the best current value of surface temperature is nearly 20,000 K cooler (at 57,400K) than the previous estimates (Barstow et al. 2001 MNRAS, vol 325, p1149). As an old CSPN, WD2218+706 is an enigmatic object. The HeII 1640Å line is visible in the STIS spectrum which represents the first detection of trace He in an apparently isolated DA white dwarf. The helium feature is shown below.

However, the surface gravity of the star, and the resulting estimated mass of 0.41 solar masses, shows that the star lies on the boundary of normal post-AGB tracks and binary evolution routes (see figure below). Hence, it may provide an important observational constraint on the lower mass limit for post-AGB evolution, if it is indeed an isolated object. A search for a possible companion, to confirm or deny this status, is now important.

T eff versus log g diagram showing the location of WD2218+706 (marked as DeHt5) compared to single star (with M >= 0.524 Msolar) and binary evolution predictions (with M =< 0.414 Msolar).