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

We have determined accurate values of the product of the mass-loss rate and the ion fraction of P +4 , q (P +4 ), for a sample of 40 Galactic O-type stars by fitting stellar wind profiles to observations of the P V resonance doublet obtained with FUSE , ORFEUS BEFS, and Copernicus . When P +4 is the dominant ion in the wind [i.e., 0.5 ≲ q (P +4 ) ≤ 1], q (P +4 ) approximates the mass-loss rate to within a factor of ≲2. Theory predicts that P +4 is the dominant ion in the winds of O7-O9.7 stars, although an empirical estimator suggests that the range O4-O7 may be more appropriate. However, we find that the mass-loss rates obtained from P V wind profiles are systematically smaller than those obtained from fits to Hα emission profiles or radio free-free emission by median factors of ~130 (if P +4 is dominant between O7 and O9.7) or ~20 (if P +4 is dominant between O4 and O7). These discordant measurements can be reconciled if the winds of O stars in the relevant temperature range are strongly clumped on small spatial scales. We use a simplified two-component model to investigate the volume filling factors of the denser regions. This clumping implies that mass-loss rates determined from "ρ 2 " diagnostics have been systematically overestimated by factors of 10 or more, at least for a subset of O stars. Reductions in the mass-loss rates of this size have important implications for the evolution of massive stars and quantitative estimates of the feedback that hot-star winds provide to their interstellar environments.