Dynamical structure of the pulsating atmosphere of RR Lyrae
I. A typical pulsation cycle
Observatoire de Haute-Provence – CNRS/PYTHEAS/Université d’Aix-Marseille,
2 Observatoire du Val de l’Arc, route de Peynier, 13530 Trets, France
3 Observatoire de Chelles, 23 avenue Hénin, 77500 Chelles, France
4 IRAP, Université de Toulouse, CNRS, UPS, CNES, 57 avenue d’Azereix, 65000 Tarbes, France
5 Observatoire OAV, 13 rue du Moulin, 34290 Alignan-du-Vent, France
6 Observatoire des Tourterelles, 5 impasse Tourterelles, 34140 Méze, France
7 Observatoire de Fontcaude, 19 avenue du hameau du Golf, 34990 Juvignac, France
8 Observatoire de Haute-Provence, 04870 Saint-Michel l’Observatoire, France
Accepted: 16 January 2019
Context. RRab stars are large amplitude pulsating stars in which the pulsation wave is a progressive wave. Consequently, strong shocks, stratification effects, and phase lag may exist between the variations associated with line profiles formed in different parts of the atmosphere, including the shock wake. The pulsation is associated with a large extension of the expanding atmosphere, and strong infalling motions are expected.
Aims. The objective of this study is to provide a general overview of the dynamical structure of the atmosphere occurring over a typical pulsation cycle.
Methods. We report new high-resolution observations with high time resolution of Hα and sodium lines in the brightest RR Lyrae star of the sky: RR Lyr (HD 182989). A detailed analysis of line profile variations over the whole pulsation cycle is performed to understand the dynamical structure of the atmosphere.
Results. The main shock wave appears when it exits from the photosphere at φ ≃ 0.89, i.e., when the main Hα emission is observed. Whereas the acceleration phase of the shock is not observed, a significant deceleration of the shock front velocity is clearly present. The radiative stage of the shock wave is short: 4% of the pulsation period (0.892 < φ < 0.929). A Mach number M > 10 is required to get such a radiative shock. The sodium layer reaches its maximum expansion well before that of Hα (Δφ = 0.135). Thus, a rarefaction wave is induced between the Hα and sodium layers. A strong atmospheric compression occurring around φ = 0.36, which produces the third Hα emission, takes place in the highest part of the atmosphere. The region located lower in the atmosphere where the sodium line is formed is not involved. The amplification of gas turbulence seems mainly due to strong shock waves propagating in the atmosphere rather than to the global compression of the atmosphere caused by the pulsation. It has not yet been clearly established whether the microturbulence velocity increases or decreases with height in the atmosphere. Furthermore, it seems very probable that an interstellar component is visible within the sodium profile.
Key words: stars: variables: RR Lyrae / shock waves / stars: atmospheres / line: profiles / hydrodynamics
The Groupe de Recherche sur RR Lyrae (GRRR) is an association of professionals and amateur astronomers leading high-resolution spectroscopic and photometric monitoring of complex phenomena such as the RR Lyrae Blazhko effect.
© D. Gillet et al. 2019
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.