A new calibration of stellar parameters of Galactic O stars Fabrice Martins, Daniel Schaerer, and D. John Hillier present new calibrations of stellar parameters of O stars at solar metallicity, taking into account non-LTE, wind, and line-blanketing effects. Gravities and absolute visual magnitudes are derived from recent spectroscopic analyses. Two types of effective temperature scales are derived: one from a compilation of recent spectroscopic studies of a sample of massive stars ("observational scale") and the other from direct interpolations on a grid of non-LTE spherically extended line-blanketed models computed with the code CMFGEN ("theoretical scale"). These T_eff scales are used to calibrate other parameters (bolometric correction, luminosity, radius, spectroscopic mass, and ionising fluxes) as a function of spectral type and luminosity class. Compared to the earlier calibrations of Vacca et al. (1996), the main results are: - Luminosities are reduced by 0.20 to 0.35 dex for dwarfs, by ~0.25 for all giants, and by 0.25 to 0.35 dex for supergiants. The reduction is essentially the same for both T_eff scales. It is independent of spectral type for giants and supergiants and is slightly larger for late type than for early type dwarfs. - Lyman continuum fluxes are reduced. Theoretical values for the hydrogen ionising photon fluxes for dwarfs are 0.20 to 0.80 dex lower than those of Vacca et al. (1996), the difference being larger at late spectral types. For giants, the reduction is of 0.25 to 0.55 dex, while for supergiants it is of 0.30 to 0.55 dex. Using the observational T_eff scale leads to smaller reductions at late spectral types. The present results represent a significant improvement over previous calibrations, given the detailed treatment of non-LTE line-blanketing in the expanding atmospheres of massive stars. The main ingredients in massive star atmospheres are non-LTE treatment, spherical expansion due to stellar wind, and line-blanketing. The effects of line-blanketing on atmosphere models lead to quantitative modifications of the stellar and wind properties of massive stars. The most studied effect is the reduction of the effective temperature scale. The effective temperature scale is fundamental since T_eff cannot be derived from optical photometry. Several T_eff scales have been proposed in the past, with the most recent being that of Vacca et al. (1996). The main approaches to derive a temperature scale as a function of spectral type and luminosity class are the determination of average T_eff's from an observed sample of O stars or the determination of T_eff's from theA new calibration of stellar parameters of Galactic O stars Fabrice Martins, Daniel Schaerer, and D. John Hillier present new calibrations of stellar parameters of O stars at solar metallicity, taking into account non-LTE, wind, and line-blanketing effects. Gravities and absolute visual magnitudes are derived from recent spectroscopic analyses. Two types of effective temperature scales are derived: one from a compilation of recent spectroscopic studies of a sample of massive stars ("observational scale") and the other from direct interpolations on a grid of non-LTE spherically extended line-blanketed models computed with the code CMFGEN ("theoretical scale"). These T_eff scales are used to calibrate other parameters (bolometric correction, luminosity, radius, spectroscopic mass, and ionising fluxes) as a function of spectral type and luminosity class. Compared to the earlier calibrations of Vacca et al. (1996), the main results are: - Luminosities are reduced by 0.20 to 0.35 dex for dwarfs, by ~0.25 for all giants, and by 0.25 to 0.35 dex for supergiants. The reduction is essentially the same for both T_eff scales. It is independent of spectral type for giants and supergiants and is slightly larger for late type than for early type dwarfs. - Lyman continuum fluxes are reduced. Theoretical values for the hydrogen ionising photon fluxes for dwarfs are 0.20 to 0.80 dex lower than those of Vacca et al. (1996), the difference being larger at late spectral types. For giants, the reduction is of 0.25 to 0.55 dex, while for supergiants it is of 0.30 to 0.55 dex. Using the observational T_eff scale leads to smaller reductions at late spectral types. The present results represent a significant improvement over previous calibrations, given the detailed treatment of non-LTE line-blanketing in the expanding atmospheres of massive stars. The main ingredients in massive star atmospheres are non-LTE treatment, spherical expansion due to stellar wind, and line-blanketing. The effects of line-blanketing on atmosphere models lead to quantitative modifications of the stellar and wind properties of massive stars. The most studied effect is the reduction of the effective temperature scale. The effective temperature scale is fundamental since T_eff cannot be derived from optical photometry. Several T_eff scales have been proposed in the past, with the most recent being that of Vacca et al. (1996). The main approaches to derive a temperature scale as a function of spectral type and luminosity class are the determination of average T_eff's from an observed sample of O stars or the determination of T_eff's from the