THE THRESHOLD FOR STELLAR WINDS IN HOT MAIN-SEQUENCE STARS

摘要

The profiles of ultraviolet resonance lines of C Ⅳ were surveyed in a sample of 29 cluster and assocation members in the spectral type range O9-B2 Ⅲ-Ⅴ, together with a few field stars of interest. The temperatures and gravities of the stars were taken from the model atmosphere analysis by Grigsby, Morrison, & Anderson (1992), and the luminosities were estimated on the basis of cluster and association distances from the recent literature. A parameter P_w, was defined in order to describe the degree of asymmetry of the C Ⅳ profile. This parameter, together with total C Ⅳ equivalent width, was found to be well correlated with stellar luminosity and temperature. A few anomalous stars were noted: T Sco, HD 66665, HD 13621, and the ON stars HD12323 and HD 201345. The results suggest a sudden onset of observable mass loss at T_(eff)= 27,500 ± 500 K, log (L/solar luminosity) = 4.4 ± 0.1, in agreement with the previous study by Prinja (1989). At T_(eff)= 28,000 K and log g = 4, our non-LTE model atmospheres show an enhancement in the ground-state population of C~(+3) in their topmost layer, which could be responsible for initiation of the winds via radiation pressure on the C~(+3) ions, or for the onset of visibility of C~(+3) ions in the wind because of an increase in the optical depth in the C Ⅳ lines in the outermost layers.