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Astron. Astrophys. 349, 553-572 (1999)

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3. Computation of non-LTE model atmospheres

A grid of non-LTE model atmospheres suitable for the analysis of B-type stars was generated using the code TLUSTY (Hubeny 1988). This grid covered an appropriate range in effective temperature (10 000 K [FORMULA] [FORMULA] [FORMULA] 35 000 K) and included logarithmic gravities from [FORMULA] = 4.5 down to near the Eddington stability limit. Models were calculated within the above parameter space, at increments of 2 500 K and 0.25 dex in effective temperature and logarithmic gravity respectively, and for two helium fractions, viz. y = 0.09 (solar) and y = 0.20, where y=N[He]/N[H+He].

The models, which contained only hydrogen and helium, assumed a plane-parallel geometry with both hydrostatic and radiative equilibrium and allow for departures from LTE. In all cases the hydrogen atom consisted of 8 non-LTE levels and the neutral helium atom consisted of 14 non-LTE levels. He I levels having their principal quantum number n = 6, 7, 8 were treated as hydrogenic, levels n = 3, 4, 5 were split into sublevels due to electron spin, but ignored angular momentum, and levels n = 1, 2 were treated fully. Models having effective temperatures greater than 20 000 K also included a He II ion which consisted of 14 non-LTE levels.

The models omit a number of physical effects. The most important is likely to be the lack of metal line-blanketing, but also neglected is the effect of wind-blanketing, whereby increased line opacities in the stellar envelope produces a back-warming effect and a change in the temperature structure through the photosphere. Furthermore, the assumption of a plane-parallel geometry may be of limited validity for the low gravity objects considered here. The consequences of these omissions will be discussed below.

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© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999
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