Toward resolving the ''mass discrepancy'' in O-type stars

We show that metal line blanketing has an important effect on the atmospheres of hot stars, and we argue that the omission of metal line blanketing in previous non-LTE model atmospheres of O-type stars has led to underestimates of surface gravity and stellar mass. In addition to wind emission, metal line blanketing contributes to the solution of the long-standing discrepancy between spectroscopic and evolutionary masses for O-type stars. To support our argument, we calculated a series of non-LTE stellar atmospheres for O and Of-type stars. We compared the predicted profiles of hydrogen and helium lines produced by (1) a static plane-parallel H-He model, (2) a metal line-blanketed static model, and (3) an extended expanding model atmosphere. We find that simple H-He models produce stronger lines than do our metal line-blanketed models. Consequently, they lead to underestimated gravities. Wind emission is more effective in filling in the wings of H or He lines in the case of large mass-loss rates typical of extreme Of stars. These findings apply equally to young, massive O-type stars and to highly evolved stars, i.e., central stars of planetary nebulae. The lower gravities derived from H-He models yield underestimated masses for young O-type stars, and overestimated masses for highly evolved stars.