The Discordance of Mass‐Loss Estimates for Galactic O‐Type Stars

We have determined accurate values of the product of the mass-loss rate andthe ion fraction of P^{4+}, Mdot q(P^{4+}), for a sample of 40 Galactic O-typestars by fitting stellar-wind profiles to observations of the P V resonancedoublet obtained with FUSE, ORFEUS/BEFS, and Copernicus. When P^{4+} is thedominant ion in the wind, Mdot q(P^{4+}) approximates the mass-loss rate towithin a factor of 2. Theory predicts that P^{4+} is the dominant ion in thewinds of O7-O9.7 stars, though an empirical estimator suggests that the rangefrom O4-O7 may be more appropriate. However, we find that the mass-loss ratesobtained from P V wind profiles are systematically smaller than those obtainedfrom fits to Halpha emission profiles or radio free-free emission by medianfactors of about 130 (if P^{4+} is dominant between O7 and O9.7) or about 20(if P^{4+} is dominant between O4 and O7). These discordant measurements can bereconciled if the winds of O stars in the relevant temperature range arestrongly clumped on small spatial scales. We use a simplified two-componentmodel to investigate the volume filling factors of the denser regions. Thisclumping implies that mass-loss rates determined from "density squared"diagnostics have been systematically over-estimated by factors of 10 or more,at least for a subset of O stars. Reductions in the mass-loss rates of thissize have important implications for the evolution of massive stars andquantitative estimates of the feedback that hot-star winds provide to theirinterstellar environments.Comment: 26 pages, 4 figures; accepted for publication in Ap