VAN-HOOF EFFECT BETWEEN METALLIC LINES IN RR LYRAE

From theoretical models, it is now well known that the pulsation motio n of atmospheric layers located above the photosphere have not the for m of a standing wave especially in large amplitude pulsating stars suc h as RR Lyrae stars. In this paper, we report for the first time a wea k but clear Van Hoof effect between some metallic lines in the atmosph ere of the brightest RR Lyrae star of the sky: RR Lyrae. This shows th at the propagation time of the pulsation wave through the atmosphere i s now measurable with high resolution spectrographs. This provides a p ower tool to test nonlinear nonadiabatic pulsating models. We also con firm, but with a better accuracy, the Van Hoof effect with hydrogen li nes which an formed higher in the atmosphere. Thus our observations pu t into evidence that large amplitude nonlinear motions of the atmosphe ric layers already exist at the photospheric level i.e., in the deep a tmosphere.