On the effective temperature scale of O stars

We rediscuss the temperature of O dwarfs based on new non-LTE line blanketed atmosphere models including stellar winds computed with the CMFGEN code of Hillier & Miller (\cite{hm98}). Compared to the latest calibration of Vacca et al. (\cite{vacca}), the inclusion of line blanketing leads to lower effective temperatures, typically by ~ 4000 to 1500 K for O3 to O9.5 dwarf stars. The dependence of the T_eff-scale on stellar and model parameters - such as mass loss, microturbulence, and metallicity - is explored, and model predictions are compared to optical observations of O stars. Even for an SMC metallicity we find a non-negligible effect of line blanketing on the T_eff-scale. The temperature reduction implies downward revisions of luminosities by ~ 0.1 dex and Lyman continuum fluxes Q₀ by approximately 40% for dwarfs of a given spectral type.