1998A&A...335..255C

From theoretical models, it is now well known that the pulsation motion of atmospheric layers located above the photosphere have not the form of a standing wave especially in large amplitude pulsating stars such as RR Lyrae stars. In this paper, we report for the first time a weak but clear Van Hoof effect between some metallic lines in the atmosphere of the brightest RR Lyrae star of the sky: RR Lyrae. This shows that the propagation time of the pulsation wave through the atmosphere is now measurable with high resolution spectrographs. This provides a power tool to test nonlinear nonadiabatic pulsating models. We also confirm, but with a better accuracy, the Van Hoof effect with hydrogen lines which are formed higher in the atmosphere. Thus our observations put into evidence that large amplitude nonlinear motions of the atmospheric layers already exist at the photospheric level i.e., in the deep atmosphere.