Escape probability treatment of doublet resonance lines in expanding stellar winds

When stellar winds have expansion velocities greater than the velocity separation between doublet line transitions, the line source function of the component with the longer wavelength depends on nonlocal values of the other component's source function. By using the Sobolev approximation and escape probability methods, the nonlocal radiative coupling for the case of spherically symmetric outflow with a monotonically increasing velocity can be solved explicitly and efficiently. This case is directly applicable to the doublet P Cygni type profiles observed in the ultraviolet spectra of O and B stars and leads to an improved determination of the line strengths. The nonlocal coupling also modifies the radiative forces that act on the mass outflow. Examples of how the nonlocal radiative coupling changes the line source functions, the line profiles, and the radiative forces are presented