Photospheric absorption lines in the UV spectra of O and B type stars.

Abstract

Measurements of the equivalent widths of ultraviolet photo spheric lines of He II λ1640, C III λ1247, N III λλ1748, 1752, O IV λ1339, Si II λ1265, Si III λ1299, Si III λ1417, Si IV λλ1394, 1403 and Fe IV λ1765 in O3 to B5 stars are presented, using high-resolution IUE spectra of 300 supergiant, giant and dwarf stars.

All the ions exhibit a well-defined dependence on effective temperature (or spectral type). The Si III λ1299 and Si II λ1265 lines are sensitive diagnostics of the effective temperature in O and B stars, respectively. The C III/O IV photospheric line ratio is dependent on the luminosity class in 0 stars. The data suggest some evidence for a decrease in the C III/N III and O IV/N III ratios for O supergiants compared to corresponding main sequence stars. This result may be consistent with recent suggestions that O supergiants are enriched in helium and exhibit CNO processed material. Only a limited comparison is currently possible between the observed UV photospheric line behaviour and model predictions.