Abstract. We study the stellar and wind properties of a sample of Galactic O dwarfs to track the conditions under which weak winds (i.e. mass loss rates lower than ∼10 −8 M yr −1 ) appear. The sample is composed of low and high luminosity dwarfs including Vz stars and stars known to display qualitatively weak winds. Atmosphere models including non-LTE treatment, spherical expansion and line blanketing are computed with the code CMFGEN (Hillier & Miller 1998, ApJ, 496, 407). Both UV and Hα lines are used to derive wind properties while optical H and He lines give the stellar parameters. We find that the stars of our sample are usually 1 to 4 Myr old. Mass loss rates of all stars are found to be lower than expected from the hydrodynamical predictions of Vink et al. (2001, A&A, 369, 574). For stars with log L L > ∼ 5.2, the reduction is by less than a factor 5 and is mainly due to the inclusion of clumping in the models. For stars with log L L < ∼ 5.2 the reduction can be as high as a factor 100. The inclusion of X-ray emission (possibly due to magnetic mechanisms) in models with low density is crucial to derive accurate mass loss rates from UV lines, while it is found to be unimportant for high density winds. The modified wind momentum -luminosity relation shows a significant change of slope around this transition luminosity. Terminal velocities of low luminosity stars are also found to be low. Both mass loss rates and terminal velocities of low L stars are consistent with a reduced line force parameter α. However, the physical reason for such a reduction is still not clear although the finding of weak winds in Galactic stars excludes the role of a reduced metallicity. There may be a link between an early evolutionary state and a weak wind, but this has to be confirmed by further studies of Vz stars. X-rays, through the change in the ionisation structure they imply, may be at the origin of a reduction of the radiative acceleration, leading to lower mass loss rates. A better understanding of the origin of X-rays is of crucial importance for the study of the physics of weak winds.

Abstract. 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) is extended using 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 −8 /10 −9 M yr −1 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 (Ṁ v ∞ √ 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 thatṀ 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 L/L < ∼ 5.5). 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 Galactic objects 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.

Abstract. We present a study of the LMC compact H ii region N 11A using Hubble Space Telescope imaging observations which resolve N 11A and reveal its unknown nebular and stellar features. The presence of a sharp ionization front extending over more than 4 (1 pc) and fine structure filaments as well as larger loops indicate an environment typical of massive star formation regions, in agreement with high [O iii]/Hβ line ratios. N 11A is a young region, as deduced from its morphology, reddening, and especially high local concentration of dust, as indicated by the Balmer decrement map. Our observations also reveal a cluster of stars lying towards the central part of N 11A. Five of the stars are packed in an area less than 2 (0.5 pc), with the most luminous one being a mid O type star. N 11A appears to be the most evolved compact H ii region in the Magellanic Clouds so far studied.