Quantitative Spectroscopy of O Stars at Low Metallicity: O Dwarfs in NGC 346^*

We present the results of a detailed analysis of the properties of dwarf O-type stars in a metal-poor environment. High-resolution, high-quality ultraviolet and optical spectra of six O-type stars in the H II region NGC 346 have been obtained from a spectroscopic survey of O stars in the SMC. Stellar parameters and chemical abundances have been determined using non-LTE (NLTE) line-blanketed photospheric models calculated with TLUSTY. Additionally, we have modeled the spectra with the NLTE line-blanketed wind code CMFGEN to derive wind parameters. Stellar parameters, chemical abundances, and in particular iron abundances obtained with the two NLTE codes compare quite favorably. This consistency demonstrates that basic photospheric parameters of main-sequence O stars can be reliably determined using NLTE static model atmospheres. With the two NLTE codes, we need to introduce a microturbulent velocity to match the observed spectra. Our results hint at a decrease of the required microturbulent velocity from a value close to the sonic velocity in early O stars to a low value in late O stars. As in several recent studies of Galactic, LMC, and SMC stars, we derive effective temperatures lower than predicted from the widely used relation between spectral type and T_eff, resulting in lower stellar luminosities and lower ionizing fluxes. From evolutionary tracks in the H-R diagram, we find the age 3 × 10⁶ yr for NGC 346. A majority of the stars in our sample reveal CNO cycle-processed material at their surface during the main-sequence stage, thus indicating fast stellar rotation and/or very efficient mixing processes. We obtain an overall metallicity Z = 0.2 Z_☉, in good agreement with other recent analyses of SMC stars. We study the dependence of the mass-loss rate on the stellar metallicity and find a satisfactory agreement with recent theoretical predictions for the three most luminous stars of the sample. The wind momentum-luminosity relation for our sample stars derived for these stars agrees with previous studies. However, the three other stars of our sample reveal very weak signatures of mass loss. We obtain mass-loss rates that are significantly lower than 10^-8 M_☉ yr^-1, below the predictions of radiative line-driven wind theory by an order of magnitude or more. Furthermore, evidence of clumping in the wind of main-sequence O stars is provided by O V λ1371. As in previous studies of O star winds, we are unable to reproduce this line with homogeneous-wind models, but we have achieved very good fits with clumped models. Clumped-wind models systematically yield lower mass-loss rates than theoretical predictions.