Ultraviolet photometry from the Orbiting Astronomical Observatory. XXIII - The resonance lines of triply ionized carbon and silicon in the spectra of hot stars

Abstract

Quantitative measurements of the absorption strength of the Si IV (1400 A) and C IV (1550 A) resonanIVe doublets have been made from OAO-2 far-ultraviolet spectra of 118 O- and B00-type stars for both doublets we note a large increase in the absorption equivalent width with luminosity classification, the increase amounting to a factor of 2 to a factor of 2 to 3 between dwarfs and supergiants. In the case of the Si IV line for stars cooler than spectral type BO.5 the observed luminoisity effect is roughly consistent with the predictions of static NLTE models. However, for the static O stars the static models predict a decrease in equivalent width with increasing luminosity. The differences between theory and observation are associated with the effects of mass outflow.The observations include a number of B stars with peculiar visual spectra.exhibits. The various peculiar stars observed appear to have normal Si IV and C IV line strengths.For a group of rapidly rotating stars the C IV line appears marginally stronger than expected on the basis of MK classification, while Si IV appears normal. In the case of the Be shell star, ..gamma.. Cas, we have noted a factor of 2 increase inmore » the strength of Si iv over a time scale of months. (AIP)« less