The helium abundance in thirty-three main sequence B stars.
Abstract
The main purpose of this paper is to determine the importance of non LTE effects in neutral helium line formation in main sequence B stars. Helium abundances are derived assuming LTE for thirty-three main sequence B stars ranging in spectral type from B8 to BO. The large number of stars analysed reveals definite trends in the behaviour of He 1 lines which are attributable to deviations from LTE. The following behaviour was observed. (a) The derived abundance and the scatter in the derived abundance are smaller for the higher members of a transition series than for the lower members. (b) For most lines the derived abundances increase with increase in effective temperature of the star. (c) The calibration of Vega has a significant effect on the derived abundance for the cool stars but has a negligible effect for the hot stars. Th addition is was found for certain lines (e.g. 5048, 4713) in the early B stars that the observed equivalent width increases with decreasing surface gravity of the star contrary to the predictions of LTE. Also the line profiles of a number of He 1 lines (e.g. 6678, 5876) have observed cores much deeper than those predicted by LTE theory. The non LTE calculations of Johnson and Poland (1969) and Auer and Mihalas (1971) improve the agreement between theory and observation but considerable discrepancies still remain. Auer and Mihalas (1971) have shown that there are pronounced departures from LTE in He I lines in 0 stars. We believe that these departures from LTE begin in the late B stars and become progressively more important as the effective temperature of the star increases. By selecting those lines which we consider to be not appreciably affected by deviations from LTE a mean abundance of N (He)/N (H) = 0.092+0.025 is obtained for main sequence B stars which is in good agreement with other abundance determinations. Key words: B stars - helium abundances - line profiles - non LTE
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- 1972
- Bibcode:
- 1972A&A....19..167O