AAS 207th Meeting, 8-12 January 2006
Session 182 Stellar Winds, Circumstellar Matter, and Activity
Poster, Thursday, 9:20am-4:00pm, January 12, 2006, Exhibit Hall

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[182.23] Lower Mass Loss Rates in O Stars

J.-C. Bouret (Laboratoire Astrophysique de Marseille), T. Lanz (U. Maryland), D. J. Hillier (U. Pittsburgh)

We have analyzed the far-UV and optical spectra of 5 Galactic O4 supergiants with the NLTE wind code CMFGEN. Stellar and wind parameters, and the surface composition of the stars have been derived. The surface composition for all 5 stars is typical of an evolved O supergiant (nitrogen-rich, carbon and oxygen-poor), and reflects the appearance of CNO processed material at the stellar surface. For all stars only highly-clumped wind models match key lines of different species in the far-UV and optical spectrum, in particular the \ion{P}{5} resonance doublet, \ion{N}{4}1718, and H\alpha. Compared to homogeneous wind models, models with clumping exhibit increased recombination in the clumps and provide a better description of the wind ionization structure. We also achieve a good fit to \ion{He}{2}4686 (especially the electron scattering wings) with clumping. The sharp transition from absorption to emission in the profile of UV wind lines indicate that clumping starts deep in the wind, just above the sonic point. The most crucial consequence of wind clumping is that the mass loss rates of O stars need to be revised downward significantly, by a factor of 3 and more. Similarly, we also recently derived lower mass loss rates for a few O-type dwarfs. Our study therefore calls for a fundamental revision in our understanding of mass loss and of O-type star stellar winds.

Support from the NASA Astrophysics Data Program (grant NNG04GC81G) is gratefully acknowledged.

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