1996A&A...308..481C

For the first time, the observation of the line doubling phenomenon over two metallic absorption lines of a RR Lyrae star (RR Lyr) is reported. A resolving power of 42,000 and a time resolution close to 1% of the pulsating period were necessary. Although a smaller exposure time would be required to completely resolve this phenomenon over the whole spectrum, we interpret it as the consequence of a "two-step" Schwarzschild's mechanism. Thus a strong shock wave propagating throughout the photosphere around phase 0.93 would be at the origin of the observed line doubling. Because the shock undergoes a sudden acceleration phase, a "jump" appears in the line profile evolution. The shock is first receding and then almost stationary. Our qualitative interpretation needs a theoretical confirmation with the help of a nonlinear nonadiabatic pulsational model with an extended atmosphere taking into account the presence of shock waves. Nevertheless it appears from our observations that the dynamics of the atmosphere of RR Lyr is important just above the photosphere and that strong shock exists at this level because the doubling is already present in FeI lines.