Wind-line Variability and Intrinsic Errors in Observational Mass-loss Rates

Abstract

UV wind-line variability in OB stars appears to be universal. In order to quantify this variation and to estimate its effect on a mass-loss rate determined from a single observation, we use the International Ultraviolet Explorer archive to identify nonpeculiar OB stars with well-developed but unsaturated Si iv λ1400 doublets and at least 10 independent observations. This resulted in 1699 spectra of 25 stars. A simple model was used to translate the observed profile variations into optical depth variations and, hence, variations in measured mass-loss rates. These variations quantify the intrinsic error inherent in any single mass-loss rate derived from a single observation. The derived rates have an overall 1σ variation of about 22%, but this appears to differ with T eff, being as small at 8% for the hottest stars and up to 45% for the cooler ones. Furthermore, any single determination can differ from the mean by a factor of 2 or more. Our results also imply that mass-loss rates determined from nonsimultaneous observations (such as UV and ground-based data) need not agree. In addition, we use our results to examine the nature of the structures responsible for the variability. Our findings suggest that the optical depth variations result from optically very thick structures occulting more or less of the line of sight to the stellar disk. Further, the smaller optical depth variations in the hottest stars suggest that the structures responsible for the variations are disrupted in their more powerful winds.