ON HYDROGEN LINE FORMATION IN THE ATMOSPHERES OF PULSATING GIANTS

An analysis of the effects of radial pulsation and shock propagation on the first three hydrogen lines formed in the atmosphere of a 13.3-d W Vir model has been performed using a method allowing the solution of the transfer equations for an arbitrary velocity distribution. A three-level atom is incorporated into the comoving transfer theory, and a non-LTE self-consistent solution is obtained by the equivalent two-level-atom formalism. The calculated H-alpha profiles and spectroscopic velocity curve are in semi-quantitative agreement with observations of the Population II Cepheids. The emission component, formed behind the shock front, is purely thermal. Its strength and frequency position reflect, respectively, the height of the post-shock temperature peak and the post-shock gas velocity. Maximum equivalent width of H-alpha was about 1 angstrom. The absorption components may originate, mostly due to scattering, either in the pre-shock or post-shock regions, or in both, and this causes a discontinuity on the empirical velocity curve when DELTA-tau-l of the pre-shock matter becomes < 10. It is found that strong absorption line velocities are very close to 17/24 times the radial velocities and hence may be used for the radial displacement measurements.