What are the mass-loss rates of O stars?

Consideration is given to the mass-loss rates of 28 luminous galactic OB stars derived from thermal radio emission and from H-alpha recombination radiation. The results of previous UV line-profile analyses are combined with the radio and H-alpha data for a precise and consistent determination of stellar mass-loss rates. Radio and H-alpha rates are found to agree within the observational errors. It is inferred that significant clumping in the wind is unlikely due to the very different radius of origin of radio and H-alpha radiation within the wind. Comparison of the empirical mass-loss rates with values derived theoretically from the theory of radiatively driven winds demonstrates that theoretical mass-loss rates are lower by an average of 0.29 dex. Theoretical terminal velocities are higher by 40 percent. The empirical momentum flux are also shown to be higher by 0.17 dex than the prediction of the theory. The theoretical momentum fluxes are independent of the stellar masses. Therefore, the momentum discrepancy cannot be explained by systematic errors in the adopted stellar masses.