Atmospheric dynamics in RR Lyrae stars

Abstract

Context. Discovered in 1907, the Blazhko effect is a modulation of the light variations of about half of the RR Lyr stars. It has remained unexplained for over 100 years, despite more than a dozen proposed explanations. Today it represents an ongoing challenge in variable-star research. Aims. We propose a new explanation. It is based on the observation that Blazhko stars seem to be located in the region of the instability strip where fundamental and first overtone modes are excited at the same time.Methods. An analysis of nonlinear and nonadiabatic pulsation models of RR Lyrae stars shows that a specific shock (called first overtone shock) may be generated by the perturbation of the fundamental mode by the transient first overtone. Results. The first overtone shock induces a sharp slowdown of the atmospheric layers during their infalling motion. This slowdown in turn affects the compression rate on the deep photospheric layers and the intensity of the κ-mechanism. After an amplification phase, the intensity of the main shock wave before the Blazhko maximum becomes high enough to provoke large radiative losses. These can be at least equal to 70% of the total energy flux of the shock, which induces a small decrease of the effective temperature at each pulsation cycle. In these conditions, when the intensity of the main shock reaches its highest critical value at the Blazhko maximum, it completely desynchronizes the motion of the phostospheric layers. At this point, the atmosphere relaxes and reaches a new synchronous state that occurs at the Blazhko minimum. Conclusions. The combined effects of these two shocks on the atmosphere cause the Blazhko effect. This effect can only exist if the first overtone mode is excited together with the fundamental mode. Because the involved physical mechanisms are essentially nonlinear (shocks, atmospheric dynamics, radiative losses, mode excitations), the Blazhko process is expected to be unstable and irregular. Consequently, the Blazhko process has a specific random nature that is in contrast with the pulsation of non-Blazhko stars.

Emission lines and shock waves in RR Lyrae stars

“…This observational information is very useful for modeling the main shock. If a shock amplification mechanism occurs in Blazhko stars (Gillet 2013), we expect that there are stronger shocks in Blazhko stars than in non-Blazhko stars. Today, observations suggest that the intensity of shock waves occurring in nonBlazhko stars is not strong enough to produce He II emission.…”

“…It seems to be the consequence of a superposition with the transient and very weak 1H-mode with the F-mode. According to Gillet (2013), it is possible that this shock should only be observed in Blazhko stars. Fokin et al (1996) did not detect this uncommon shock in their model devoted to the classical Cepheid star: δ Cephei.…”

“…Until now, only He II was detected in Blazhko stars. It should be noted that Gillet et al (2013) and Gillet (2013) mistakenly reported that no emission lines of helium were observed in non-Blazhko stars instead of saying only He II.…”

“…To explain the Blazhko effect, Gillet (2013) suggested that an amplification mechanism of the intensity of the main shock occurs between Blazhko minimum and maximum. Once the shock strength reaches a critical value (at Blazhko maximum), the shock desynchronizes the motion of photospheric layers.…”

“…Moreover, to explain the observed Blazhko modulations of the period, temperature, radius, etc., these authors must have assumed that there is an unknown "physical engine" in the atmosphere. In fact, Gillet (2013) suggested that it is simply the existence of a secondary shock, probably induced by the first overtone (Fokin & Gillet 1997), and radiative losses of the main shock, which are at the origin of the different observed modulations.…”