5. DO white dwarfs
Hot helium-rich white dwarfs that show He II lines are classified DO. They are divided according to the presence or absence of He I lines into a cool DO and a hot DO region (Wesemael et al. 1985). The cool boundary of the DO sequence is defined by the so-called DB gap (28000 K K) in which no helium-rich white dwarfs are known. For the cooler DO stars with detectable He I lines the ionization equilibrium of He I /He II provides a valuable temperature indicator. However, the He I lines are invisible in the hottest DO white dwarfs and and g must be derived from a fit of the He II lines alone, similar to the case of the DA white dwarfs.
An exploratory calculation by Napiwotzki (1995b) resulted in strong NLTE deviations for a model of a cool DO white dwarf (55000 K). Werner (1996b) presented a LTE/NLTE comparison of two DO models with K and K focussed on effects on the He II lines coinciding to Balmer lines. Here we calculated a representative grid of model atmospheres covering the range from K up to 120000 K ( and ), which is representative for the whole DO class. The result is displayed in Fig. 7. Line profiles of representative models are shown in Fig. 8 (see also Dreizler & Werner 1996). At first glance the temperature dependence of the NLTE deviations is astonishing: strong effects are present around K. They become smaller for higher temperatures. The reason for this is the high sensitivity of the ionization equilibrium of helium at K. While at higher temperatures the ionization stage He III is highly dominant the equilibrium is just flipping from He III to He II in this cooler region. In addition the flux maximum for a 60000 K DO star is close to the He II 228 Å ionization edge causing an overionization of helium.
From Fig. 7 it can be seen that moderate to strong NLTE deviations are present for important lines of DO white dwarfs for virtually the complete temperature range covered by this class. Especially if one wants to use the full potential of the He I /He II equilibrium for the determination it is mandatory to use NLTE model atmospheres.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998