AAS 205th Meeting, 9-13 January 2005
Session 53 Hot Stars, Atmospheres and Winds
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

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[53.04] The Discordance of Mass-Loss Estimates for O-Type Stars

A. W. Fullerton (UVic/JHU), D. L. Massa (SGT, Inc.), R. K. Prinja (UCL)

Although the UV resonance lines of ionized metals are the most sensitive indicators of the stellar winds of O-type stars, the wind lines accessible to IUE or HST do not provide reliable estimates of the mass-loss rate. The utility of these diagnostics is compromised because they are due to (a) dominant ions of abundant elements (e.g., C3+), and are generally saturated; or (b) trace ions (e.g., Si3+), whose abundances depend sensitively on local conditions in the wind, including the presence of X-rays. Consequently, mass-loss rates are typically derived from observations of emission in H\alpha or the radio continuum. However, determinations based on these ``density-squared" diagnostics are quite sensitive to the presence of inhomogeneities, and will over-estimate the true mass flux if the wind is clumped.

To circumvent these problems, we have used new FUSE and archival Copernicus and Orfeus observations of the P~{\sc v} resonance doublet to estimate mass-loss rates for a standard sample of Galactic O-type stars. Since P4+ is expected to be the dominant ion in the winds of mid-O stars, its ion fraction is known a priori; and, since P is much less abundant than C or N, its resonance lines are not saturated and provide reliable measurements of the ionic column density. The mass-loss rates derived from P~{\sc v} wind profiles are typically 1--2 orders of magnitude smaller than those obtained from H\alpha or radio observations. We interpret this discrepancy as an indication that the winds of O-type stars are strongly clumped.

The NASA-CNES-CSA FUSE mission is operated for NASA by the Johns Hopkins University. Financial support for this work was provided by NASA FUSE Guest Investigator program.

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