Atmospheric dynamics and shock waves in RR Lyr

Abstract

Context. Although spectroscopic observations of RR Lyrae stars have now been carried out for almost a century, it is only recently that it has been identified that the hydrogen line exhibits three successive emissions in each pulsation cycle. Aims. The purpose of this study is to clarify the physical origin of these three emissions and their connection with atmospheric dynamics and the influence of Blazhko modulation on their intensity. Methods. We used 2437 high-resolution spectra over a total of 81 nights made with the spectrograph ELODIE (Haute Provence observatory, France) during years 1994-1997 completed with 2015 run from Oukaïmeden observatory (Morocco). A detailed analysis of lines profile variations over the whole pulsation cycle is performed. Results. From the value of the blueshift of the main H𝛼 emission, the velocity of the hypersonic shock front was estimated between 100 and 150 ±10 km s −1 (Mach number between 10 and 15). It has been established that the shock velocity increases from the minimum Blazhko to its maximum, and after that, gradually decreases to the Blazhko minimum to start growing again. This observational result is consistent with the shock model proposed in 2013 to explain the Blazhko effect. The intensity of the H𝛼 emission increases with the shock velocity up to a maximum value around 137 km s −1 and then decreases as the shock velocity increases further. This effect would be the consequence of the more and more important ionization of the atoms in the radiative shock wake. The second (blueshifted) H𝛼 emission, is the consequence of an approximately constant supersonic compression (Mach number between 2 and 3) of the upper atmosphere falling on the photospheric layers, during 3 to 16% of the pulsation period. Finally, the third H𝛼 emission (P-Cygni profile) would be the consequence of the expansion of the high atmosphere induced by the shock wave during its final weakening.