Macroclumping as solution of the discrepancy between H-alpha and P v mass loss diagnostics for O-type stars

B. Surlan(1,2), W.-R. Hamann(3), A. Aret(4), J. Kubat(1), L. M. Oskinova(3), A. F. Torres(5,6)

1-Astronomicky ustav, Akademie ved Ceske Republiky, CZ-251 65 Ondrejov, Czech Republic
2-Matematicki Institut SANU, Kneza Mihaila 36, 11001 Beograd, Republic of Serbia
3-Institut fur Physik und Astronomie, Universitat Potsdam, Karl-Liebknecht-Strase 24/25, 14476 Potsdam-Golm, Germany
4-Tartu Observatory, 61602, Toravere, Tartumaa, Estonia
5-Departamento de Espectroscopia, Facultad de Ciencias Astronomicas y Geofisicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, La Plata, B1900FWA, Buenos Aires, Argentina
6-Instituto de Astrofisica de La Plata (CCT La Plata - CONICET, UNLP), Paseo del Bosque S/N, La Plata, B1900FWA, Buenos Aires, Argentina

Recent studies of O-type stars demonstrated that discrepant mass-loss rates are obtained when different diagnostic methods are employed - fitting the unsaturated UV resonance lines (e.g. P v) gives drastically lower values than obtained from the H-alpha emission. Wind inhomogeneity
(so-called ``clumping'') may be the main cause for this discrepancy. In a previous paper, we have presented 3-D Monte-Carlo calculations for the formation of scattering lines in a clumped stellar wind. In the present paper we select five O-type supergiants (from O4 to O7) and test whether the reported discrepancies can be resolved this way. In the first step, the analyses start with simulating the observed spectra with Potsdam Wolf-Rayet (PoWR) non-LTE model atmospheres. The mass-loss rates are adjusted to fit best to the observed H-alpha emission lines. For the unsaturated UV resonance lines (i.e. P v) we then apply our 3-D Monte-Carlo code, which can account for wind clumps of any optical depths (``macroclumping''), a non-void inter-clump medium, and a velocity dispersion inside the clumps. The ionization stratifications and underlying photospheric spectra are adopted from the PoWR models. From fitting the observed resonance line profiles, the properties of the wind clumps are constrained. Our results show that with the mass-loss rates that fit H-alpha (and other Balmer and He II lines), the UV resonance lines (especially the unsaturated doublet of P v) can also be reproduced without problem when macroclumping is taken into account. There is no need to artificially reduce the mass-loss rates, nor to assume a sub-solar phosphorus abundance or an extremely high clumping factor, contrary to what was claimed by other authors. These consistent mass-loss rates are lower by a factor of 1.3 to 2.6, compared to the mass-loss rate recipe from Vink et al. Macroclumping resolves the previously reported discrepancy between H-alpha and P v mass-loss diagnostics.

Reference: Accepted for publication in A&A (in press)
Status: Manuscript has been accepted