Abstract
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.
Highlights
The kinematics and dynamics of the outer layers of pulsating stars, a fortiori those undergoing Blazhko effect (Blazhko 1907), is usually not known in detail
HJD0B, we developed an innovative method, which will be described in a forthcoming paper (Gillet et al, in prep.), based on the equivalent width (EW) and main shock velocities driven by our many years of experience in performing spectral observations
The objective of this paper was to determine from spectral observations the main dynamic phenomena occurring in the atmosphere of RR Lyr during a typical pulsation cycle
Summary
The kinematics and dynamics of the outer layers of pulsating stars, a fortiori those undergoing Blazhko effect (Blazhko 1907), is usually not known in detail. Stellingwerf (2013) developed an adiabatic three-dimensional (3D) pulsation model for RR Lyrae stars that he applied to RR Lyr itself It considers the outer boundary condition and the interaction between pulsation and turbulence, but this model does not include radiative effects such as radiation and ionization. A work dedicated to RR Lyr was done by Fokin & Gillet (1997) They built a series of nonlinear, nonadiabatic models including atmosphere and shock wave propagation. They used a purely radiative approach because convection is thought to strongly affect the helium ionization zone of RR Lyr according to its effective temperature (Xiong et al 1998).
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