The theory of line driven stellar winds

Abstract

We describe the theory of line driven winds. First we explain the concept and give some simple estimates. Next we derive expressions for the radiative acceleration due to lines. The equation of motion for a line driven wind from a star in the point source limit is solved. This gives simple analytical expressions for the predicted mass loss rates and the terminal velocities. The equations show that M ∼ L ^1.6_* and v _∞ ∼ v _esc ∼ v _esc, in agreement with the observations. The theory is refined by a correction for the finite size of the star. We also discuss the stability of line driven wind and show that these winds are intrinsically unstable. This explains the observed super-ionization and the X-rays from line driven winds. We compare the predictions with observations and show that the theory is very successful in explaining the overall features of the winds of hot stars.