Dynamical structure of the pulsating atmosphere of RR Lyrae

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 suitable 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 (∆ϕ = 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.

On the Use of Field RR Lyrae as Galactic Probes. I. The Oosterhoff Dichotomy Based on Fundamental Variables*

“…These variables are characterised by very large amplitudes and light curves showing a saw-tooth shape. The occurrence of nonlinear phenomena is strongly supported by the presence of a dip along the rising branch of the light curve and by solid spectroscopic evidence (Gillet 2013;Sneden et al 2017;Gillet et al 2019). The RRc variables are also affected by shocks, but the current evidence indicate that the shocks marginally affect their properties (Duan et al 2020;Benkő et al 2021).…”

Hydrogen and Helium Shock Phenomena during Rising Light in RR Lyrae Fundamental Mode Pulsators

“…The first, dubbed the "third apparition" (Preston 2011), is identified by a weak red-shifted Hα emission wing visible during about one third of the pulsation cycle following maximum light. Ad hoc explanations of this apparition have been advanced by Chadid & Preston (2013) and Gillet et al (2017Gillet et al ( , 2019. A second shock, identified by violet-shifted Hα emission during the "bump" preceding minimum light (Gillet & Crowe 1988), is discussed at length in the foregoing references.…”

Periodic Variable Stars Modulated by Time-Varying Parameters

“…Indeed, one of the longest-standing problems in stellar astrophysics is also one that specifically affects RR Lyrae stars: the so-called Blazhko effect (Blažko 1907). It consists in a long-term modulation of an RR Lyrae's light curve, over timescales ranging from a few to hundreds of days (for recent reviews, see Catelan & Smith 2015;Gillet et al 2019). The Blazhko effect is particularly common amongst fundamental-mode (ab-type) pulsators (e.g., Plachy et al 2019), but is also present, to a lesser extent, in first-overtone (c-type) RR Lyrae stars (e.g., Netzel et al 2018).…”

“…Over the decades since it was first described, the Blazhko effect has persistently defied theoretical explanations as to its cause (e.g., Gillet et al 2019). Gradual strengthening and weakening of turbulent convection in the stellar envelope (Stothers 2006), a 9:2 resonance between the fundamental and ninth-overtone radial modes (Buchler & Kolláth 2011), and interaction between fundamental and first-overtone modes in the "either-or" region of the instability strip (Gillet 2013) are the most recent candidates, but no consensus has yet been reached as to the root cause of the Blazhko effect, due in large part to the difficulties involved in the non-linear hydrodynamical modeling of the phenomenon.…”