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 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 and 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 in order 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 .
Similarly to 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 HR diagram , we find an age of 3 10 ^ { 6 } years for NGC 346 .
A majority of the stars in our sample reveal CNO-cycle processed material at their surface during the main-sequence stage , indicating thus fast stellar rotation and/or very efficient mixing processes .
We obtain an overall metallicity , Z = 0.2 Z _ { \odot } , in good agreement with other recent analyses of SMC stars .
We study the dependence of the mass loss rate with the stellar metallicity and find a satisfactory agreement with recent theoretical predictions for three most luminous stars of the sample .
The wind-momentum luminosity relation for our sample stars derived for these stars agree 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 _ { \odot } yr ^ { -1 } , which is 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 of O stars is provided by O v { \lambda } 1371 .
Like 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 .