The theory of radiatively driven stellar winds. II - The line acceleration

Abstract

Numerical values of the radiation pressure on spectral lines are presented for the envelopes of stars with type O--G, and with any luminosity, metallicity, or velocity structure. The line acceleration was calculated using a tabulation of atomic lines that is complete for the elements H--Zn. The line acceleration is remarkably constant over the temperature range 50,000> or =T/sub eff/> or =10,000 K, but drops off sharply for cooler stars. The anomalous ionization observed in OB stars decreases the acceleration by up to a factor of 2 from radiative equilibrium values. More than one-half of the acceleration comes from lines whose frequencies overlap with those of neighboring lines at some point in the wind. Line blanketing of the continuum flux by the wind becomes significant for mass loss rates exceeding approx.10/sup -6/ M/sub sun/ yr/sup -1/. The predicted and observed mass loss rates are in complete agreement for the OB stars, and we conclude that radiation pressure is the dominant mechanism driving the winds from these stars. The mass loss rates of Wolf-Rayet stars are not explained by these models, even accounting for the effects of chemical enrichment on the line acceleration. Radiation-driven mass loss is still significant in F andmore » G supergiants, but the predicted rates are smaller than for OBA supergiants of comparable luminosity. The predicted mass loss rate scales nearly linearly with metallicity, with obvious consequences for stellar systems formed in regions of very high, or very low, metallicity. Subject headings: stars: early-type: stars: mass loss: stars: supergiants: stars: winds: stars: Wolf-Rayet« less