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, Mq(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 less than or similar to q(P+4) <= 1],. Mq(P+4) approximates the mass-loss rate to within a factor of less than or similar to 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 H alpha emission profiles or radio free-free emission by median factors of similar to 130 (if P+4 is dominant between O7 and O9.7) or similar to 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 rho(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.