An atlas of theoretical P Cygni profiles.

An atlas of theoretical P Cygni-type line profiles is presented. The profiles are calculated by assuming resonance scattering and using the Sobolev approximation. The expanding envelope is characterized by two functions: the optical depth and the velocity law. The velocity law, the process of line formation, and the accuracy of the theoretical line profiles are discussed. The effect of collisional excitation is discussed and found not to be important in most UV resonance lines, except in a star with a very high rate of mass loss and a small wind velocity. A simple procedure to correct for the presence of underlying photospheric profiles is described, and it is shown that the simple addition of the calculated P Cygni profile and photospheric profile can give very wrong results. A fairly simple procedure for calculating the shape of partly overlapping doublet lines is also described. The accuracy of the information that can be derived from a comparison between observed and theoretical P Cygni profiles is discussed critically.