NLTE Line-blanketed Model Atmosphere Analysis \\ of the hot Subdwarf BD +75\deg~325

Previous abstract Next abstract

Session 80 -- Stellar Activity II: Early Type Stars, Normal Stars
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

[80.05] NLTE Line-blanketed Model Atmosphere Analysis \\ of the hot Subdwarf BD +75\deg~325

T. Lanz, I. Hubeny (USRA/GSFC), S. R. Heap (NASA/GSFC)

With the Goddard High Resolution Spectrograph aboard HST, we have recorded a high-resolution, high signal-to-noise ratio, far ultraviolet spectra of the hot subdwarf, BD +75\deg~325. Because of the low apparent rotational velocity ($V \sin\,i~<~$20~km/s) of the star, the high quality of the spectra and our newly developed NLTE line-blanketed model atmospheres, we have an unprecedented opportunity to derive accurate stellar parameters.

With our hybrid CL/ALI method, we have constructed a small grid of NLTE line-blanketed model atmospheres for hot subdwarfs. These model atmospheres include consistently about 200 NLTE levels (or superlevels) of H, He, C, N, O, Si, Fe and Ni, and all transitions between them. Individual atomic levels of iron and nickel are merged into a relatively small number of superlevels. The cross-sections of the transitions between superlevels are represented by NLTE Opacity Distribution Functions. This grid of NLTE model atmospheres is used for a thorough analysis of \mbox{BD +75\deg~325}, which then provides in addition a ``real'' check of our methodology.

Our results show that BD +75\deg~325 is a He enriched star, whose surface exhibit CNO-cycle products (N-rich and C-deficient). The very rich Fe~{\sc v} and Fe~{\sc vi} line spectra can only be reproduced by a significant overabundance of iron (about 5 to 10 times solar). With our metal line blanketed NLTE model atmospheres, we are now able to reproduce the deep line cores in the ultraviolet spectrum assuming a microturbulence of 5-10 km/s. Moreover, the line blanketed models are necessary to match the H~{\sc i}/He~{\sc ii} line cores in the visual spectrum (models including only H and He produce too shallow lines). Finally, we have found a weak asymmetry of the N~{\sc v} resonance line profiles, which can be attributed to a weak wind with \.{M}~=~9$\pm 2\times 10^{-11} M_\odot /{\rm yr}$.

Wednesday program listing