Hypersonic shock wave evidence in RR Lyrae stars First detection of neutral line disappearance in S Arae

Context. We carried out a new high resolution spectroscopic analysis of S Arae, one of the largest amplitude RR Lyrae star, to constrain the dynamical structure of the outermost layers of RR Lyrae stars. Aims. New high precision spectroscopic observations of S Arae were obtained using UVES at VLT-UT2 and FEROS at ESO-1.52 telescopes, during the rise of the SArae light curve, over an entire pulsation period and covering a large wavelength range, from 376 to 1000 nm. Methods. We used the first line moment method to measure the radial velocity and to establish the radial velocity curves. The atmosphere stratification was done by the line identification method derived from the Kurucz atmosphere models. Results. For the first time, we show the existence of hypersonic shock waves in the photosphere of RR Lyrae stars. Our spectroscopic results reveal a new mechanism: the neutral line disappearance phenomenon. All neutral metallic absorption lines, over the whole spectrum, undergo a total line disappearance during the hump, while the ionized lines show line doubling connected by the two-step Schwarzschild's mechanism. Thus, a hypersonic shock wave propagating through the photosphere around the maximum light phase results in the line disappearance. The shock energy is high enough to ionize the atoms, forming the layer crossed by the shock wave. The shock at first recedes, then becomes stationary and finally advances with a very high Mach number crossing the high atmosphere of the star. We stress the need for additional theoretical efforts to come to a better understanding of hypersonic shock wave and dynamic pulsational motion relationship.