A&A 420, 1087-1106 (2004)
Puzzling wind properties of young massive stars in SMC-N81F. Martins1, 2, D. Schaerer2, 1, D. J. Hillier3 and M. Heydari-Malayeri4
Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées,
14 Av. E. Belin, 31400 Toulouse, France
Observatoire de Genève, 51 Chemin des Maillettes, 1290 Sauverny,
Department of Physics and Astronomy, University of Pittsburgh, 3941
O'Hara Street, Pittsburgh, PA 15260, USA
LERMA, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014
(Recieved 14 October 2003 / Accepted 16 March 2004)
We present a quantitative study of massive stars in the High
Excitation Blob N81, a compact star forming region in the SMC.
The stellar content
was resolved by HST, and STIS was used to obtain medium
The qualitative analysis of the stellar properties presented in
Heydari-Malayeri et al. (2002a) is
non-LTE spherically extended atmosphere models including
line-blanketing computed with the code CMFGEN (Hillier & Miller
1998), and the wind properties are investigated. The main results
are the following:
The SMC-N81 components are young (~0-4 Myrs) O stars with effective temperatures compatible with
medium to late
subtypes and with luminosities lower than those of average
Galactic O dwarfs, rendering them possible ZAMS candidates.
The winds are extremely weak: with values of the order of 10
the mass loss rates are lower than observed so
far for Galactic dwarfs. Only the recent study of SMC stars by Bouret
et al. (2003) show the same trend. The modified wind momenta
) are also 1 to 2 orders of
magnitude lower than observed for Galactic stars. Both the mass loss
rates and the modified wind momenta are lower than the predictions of
the most recent hydrodynamical models.
The accuracy of the UV based mass loss rate determination, relying
in particular on the predicted ionisation fractions, are carefully
examined. We find that
could be underestimated by a factor
of up to 10. Even in this unlikely case, the above conclusions
remain valid qualitatively
The reasons for such weak winds are
investigated with special emphasis on the modified wind momenta:
There may be a break-down of the wind momentum-luminosity
relation (WLR) for dwarf stars at low luminosity (log
). However, reasons for such a
breakdown remain unknown.
The slope of the WLR may be steeper at low metallicity. This is
predicted by the radiation driven wind theory, but the current
hydrodynamical simulations do not show any change of the slope at SMC metallicity. Moreover, there are indications that some
have wind momenta similar to those of the SMC stars.
Decoupling may take place in the atmosphere of the SMC-N81 stars, leading
to multicomponent winds. However, various tests
indicate that this is not likely to be the case.
The origin of the weakness of the wind observed in the SMC-N81
stars remains unknown.
We suggest that this weakness may be linked with the youth of
these stars and represents possibly the onset of stellar winds in
recently formed massive stars.
stars: winds, outflows --
stars: atmospheres --
stars: mass-loss --
stars: early-type --
fundamental parameters --
ISM: HII region
Offprint request: F. Martins, email@example.comSIMBAD Objects
© ESO 2004