Analysis of Galactic late-type O dwarfs: more constraints on the weak
  wind problem
  We have investigated the stellar and wind properties of a sample of five
late-type O dwarfs in order to address the weak wind problem. A grid of TLUSTY
models was used to obtain the stellar parameters, and the wind parameters were
determined by using the CMFGEN code. We found that the spectra have mainly a
photospheric origin. A weak wind signature is seen in CIV 1549, from where
mass-loss rates consistent with previous CMFGEN results regarding O8-9V stars
were obtained. A discrepancy of roughly 2 orders of magnitude is found between
these mass-loss rates and the values predicted by theory (Mdot(Vink)),
confirming a breakdown or a steepening of the modified wind momentum-luminosity
relation at log L/Lsun < 5.2. We have estimated the carbon abundance for the
stars of our sample and concluded that its uncertainty cannot cause the weak
wind problem. Upper limits on Mdot were established for all objects using lines
of different ions, namely, PV 1118,28, CIII 1176, NV 1239,43, Si IV 1394,03,
and NIV 1718. All the values obtained are also in disagreement with theoretical
predictions, bringing support to the reality of weak winds. Together with CIV
1549, the use of NV 1239,43 results in the lowest mass-loss rates: the upper
limits indicate that Mdot must be less than about -1.0 dex Mdot(Vink).
Regarding the other transitions, the upper limits still point to low rates:
Mdot must be less than about $(-0.5 \pm 0.2)$ dex Mdot(Vink). We have studied
the behavior of the Halpha line with different mass-loss rates. We have also
explored ways to fit the observed spectra with Mdot(Vink). By using large
amounts of X-rays, we verified that few wind emissions take place, as in weak
winds. However, unrealistic X-rays luminosities had to be used (log Lx/Lbol >
-3.5) (abridged).
