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

A. W. Fullerton (1,2), D. L. Massa (3), and R. K. Prinja (4)

(1) Dept. of Physics & Astronomy, University of Victoria
(2) Dept. of Physics & Astronomy, Johns Hopkins University
(3) SGT Inc., NASA's Goddard Space Flight Center
(4) Dept. of Physics & Astronomy, University College London

We have determined accurate values of the product of the mass-loss rate
and the ion fraction of P^{4+}, Mdot 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, Mdot 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, though an empirical estimator
suggests that the range from 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 Halpha emission profiles or radio
free-free emission by median factors 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 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 "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 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.

Reference: ApJ, in press
Status: Manuscript has been accepted