2004A&A...420.1087M

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 resolution spectra. The qualitative analysis of the stellar properties presented in Heydari-Malayeri et al. (2002A&A...381..951H) is extended using non-LTE spherically extended atmosphere models including line-blanketing computed with the code CMFGEN (Hillier & Miller, 1998ApJ...496..407H), and the wind properties are investigated. The main results are the following: . The SMC-N81 components are young (∼0-4Myrs) 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^–8/10^–9M_☉/yr the mass loss rates are lower than observed so far for Galactic dwarfs. Only the recent study of SMC stars by Bouret et al. (2003ApJ...595.1182B) show the same trend. The modified wind momenta ({dot}(M)v_∞sqrt(R)) 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 {dot}(M) 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: (o) There may be a break-down of the wind momentum-luminosity relation (WLR) for dwarf stars at low luminosity (log L/L_☉≲5.5). However, reasons for such a breakdown remain unknown. (o) 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 Galactic objects have wind momenta similar to those of the SMC stars. (o) 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.