First results from the Goddard High-Resolution Spectrograph: Evidence for photospheric microturbulence in early O stars: Are surface gravities systematically underestimated?

GHRS spectra of two very hot stars (BD +75°325 and Melnick 42) provide evidence for the presence of microturbulence in their photospheres. In attempting to reproduce the observed spectra, we have built theoretical models in which the microturbulence is allowed to modify not only the Doppler line widths (classical "spectroscopic" microturbulence), but also the turbulent pressure (thus mimicking a "physical" turbulence). We find that a corresponding modification of the temperature-pressure stratification influences the hydrogen and helium line profiles to the extent that the surface gravities of early O stars determined without considering microturbulence are too low by 0.1-0.15 dex. Thus, including microturbulence would reduce, or resolve completely, a long-standing discrepancy between evolutionary and spectroscopic stellar masses.