SIMBAD references

We investigate the stellar and wind properties of a sample of late-type O dwarfs. Previous analyses of such stars have found very low mass-loss rates; rates much lower than predicted by theory (the weak wind problem). Far-UV to optical spectra of five Galactic O stars were analyzed: HD 216898 (O9IV/O8.5V), HD 326329 (O9V), HD 66788 (O8V/O9V), ζ Oph (O9.5Vnn), and HD 216532 (O8.5V((n))). We used a grid of TLUSTY models to obtain effective temperatures, gravities, rotational velocities, and to identify wind lines. Wind parameters for each object were obtained using expanding atmosphere models calculated with the CMFGEN code. The spectra of our sample have primarily a photospheric origin. A weak wind signature is seen in CIV λλ1548, 1551, from which mass-loss rates consistent with previous CMFGEN results for O8-O9V stars were derived (∼10^–10-10^–9M_☉/yr). A discrepancy of roughly two orders of magnitude is found between these mass-loss rates and the values predicted by theory ({dot}(M)_Vink), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log L_*/L_☉≲5.2. We have estimated the carbon abundance for the stars of our sample and concluded that its value cannot be reduced to sufficiently small values to solve the weak wind problem. Upper limits on {dot}(M) were established for all objects using lines of different ions: P v λλ1118, 1128, CIII λ1176, NV λλ1239, 1243, SiIV λλ1394, 1403, and NIV λ1718. All the values obtained are in disagreement with theoretical predictions, bringing support to the reality of weak winds. Together with CIV λλ1548, 1551, the use of NV λλ1239, 1243 results in the lowest mass-loss rates: the upper limits indicate that {dot}(M) must be less than about -1.0dex {dot}(M)_Vink. Upper mass-loss rate limits obtained for other transitions are also low: they indicate that {dot}(M) must be less than about (-0.5±0.2)dex {dot}(M)_Vink. We studied the behavior of the Hα line with different mass-loss rates. For two stars, only models with very low {dot}(M)'s provide the best fit to the UV and optical spectra. We also explored ways to fit the observed spectra with the theoretical mass-loss rates. By using large amounts of X-rays, we could reduce the predicted wind emission to the observed levels. However, unrealistic X-ray luminosities had to be used (logL_X/L_Bol>-3.5). The validity of the models used in our analyses is discussed.