Bump, hump and shock waves in the RR Lyrae stars : X Ari and RR Lyr.

High spectral resolution (0.2 and 0.7 A) observations of the H-alpha profile in RR Lyrae stars X Ari and RR Lyr are presented. The observations have been carried out during bump and hump phases. A weak emission is presented during the bump (phi about 0.7) while during the hump (phi about 0.9), another stronger emission is observed. Only the emission associated with the hump is followed by a line absorption doubling. Both emissions are interpreted as the consequence of the propagation of two independent shock waves. The 'main' (phi about 0.9) shock would be produced by the opacity mechanism giving the pulsation. The first or 'early' shock (phi about 0.7), associated with the bump, would be the consequence of the colliding of the upper atmospheric layers with deeper ones during the infall phase. This is quite consistent with the nonlinear hydrodynamic model of Hill (1972). The physical origin of the bump in RR Lyrae stars would therefore be different than that in Classical Cepheids (resonance mechanism).