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A&A 476, 1331-1340 (2007)
DOI: 10.1051/0004-6361:20066377
Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates
L. M. Oskinova, W.-R. Hamann, and A. FeldmeierLehrstuhl Astrophysik der Universität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany
e-mail: lida@astro.physik.uni-potsdam.de
(Received 11 September 2006 / Accepted 18 September 2007)
Abstract
Context.The mass-loss rate is a key parameter of massive stars. Adequate
stellar atmosphere models are required for spectral analyses and
mass-loss determinations. Present models can only account for the
inhomogeneity of stellar winds in the approximation of small-scale
structures that are optically thin. Compared to previous homogeneous
models, this treatment of "microclumping" has led to reducing
empirical mass-loss rates by factors of two to three. Further
reductions are presently discussed in the literature, with far-reaching
consequences e.g. for stellar evolution and stellar yields.
Aims.Stellar wind clumps can be optically thick in spectral lines.
We investigate how this "macroclumping" influences the
radiative transfer and the emergent line spectra and discuss its
impact on empirical mass-loss rates.
Methods.The Potsdam Wolf-Rayet (PoWR) model atmosphere code is generalized in
the "formal integral" to account for clumps that are not necessarily
optically thin. The stellar wind is characterized by the filling
factor of the dense clumps and by their average separation. An
effective opacity is obtained by adopting a statistical distribution of
clumps and applied in the radiative transfer.
Results.Optically thick clumps reduce the effective opacity. This has a
pronounced effect on the emergent spectrum. Our modeling for the
O-type supergiant 
Puppis reveals that the optically thin
H
line is not affected by wind porosity, but that the P V resonance doublet becomes significantly weaker when macroclumping
is taken into account. The reported discrepancies between
resonance-line and recombination-line diagnostics can be resolved
entirely with the macroclumping modeling without downward revision of
the mass-loss rate. In the case of Wolf-Rayet stars, we demonstrate
for two representative models that stronger lines are typically
reduced by a factor of two in intensity, while weak lines remain
unchanged by porosity effects.
Conclusions.Mass-loss rates inferred from optically thin emission, such
as the H line in O stars, are not influenced by macroclumping.
The strength of optically thick lines, however, is reduced because of
the porosity effects. Therefore, neglecting the porosity in stellar
wind modeling can lead to underestimating empirical mass-loss rates.
Key words: stars: mass-loss -- stars: winds, outflows -- stars: atmospheres -- stars: early-type -- stars: individual:
Puppis
© ESO 2007