Spectral line formation in YY ORI envelopes : a multi-level hydrogen atom.

A numerical method for incorporating a many-level atom into the generalized Sobolev theory is presented. The basic, statistical equilibrium equations and Sobolev-type equations for the line radiation fields are reviewed, and the method of solution is set forth, including the treatment of rate equations, calculation of the coupling integrals, integration of flux profiles, and application to Balmer lines of spherical protostars. The results are compared with observations of YY Orionis stars, and it is found that the many-level ansatz is successful in several ways, particularly in the reduction in model envelope density. The detailed shape of the line profiles is not reproduced, however, and the model profiles are too uniform and in particular cannot explain the observed differences between the hydrogen alpha line and higher Balmer profiles.