Shock waves and Hα profiles in the hydrodynamical model for RR Lyrae

The shock phenomena and Hα formation in the atmosphere of RR Lyrae are investigated by means of numerical simulations. The full amplitude hydrodynamical model is generated adopting |M=0.578 M_, L=62 L_, T e=7175 K| and population II abundance. The Hα profiles are obtained by solution of the non-LTE line transfer problem for the multilevel hydrogen atom. A close quantitative agreement between the predicted and observed profiles is found, confirming the adequacy of the model used. In the course of pulsations the model produces an extended atmosphere of about 16 static scaleheights, in which two shocks are successively generated during one period. These results generally confirm the earlier conclusions of Hill. It is suggested that the bump of the light curve is connected with the early shock generation, the mechanism for which is discussed. The full development of the main shock occurs very high in the atmosphere. This can explain both the lack of doubling of the weak metallic lines, formed at a relatively large depth, and the absence of prominent emission in Hα due to large deviations from LTE near the shock front. Strong emission in the Lyman lines is predicted, reaching maximum intensity near the light maximum.