Emission lines and shock waves in RR Lyrae stars

Abstract

Context. Emission lines observed in radially pulsating stars are thought to be produced by atoms de-exciting after being excited by a shock wave that is traveling into and then compressing, heating, and accelerating the atmospheric gas. Aims. With the help of recent observations, we examine the origin of all the different types of emission lines of hydrogen and helium that appear during a pulsation cycle. Methods. To analyze the physical origin of emission lines, we used the different models of atmospheric dynamics of RR Lyrae stars that have been calculated so far. Results. In contrast to a recent explanation, we propose that the redshifted emission component of Hα, which occurs near the pulsation phase 0.3, is produced by the main shock. In this case, the emission is the natural consequence of the large extension of the expanding atmosphere. Therefore, this (weak) emission should only be observed in RR Lyrae stars for which the main shock will propagate far enough from the photosphere. It appears as a P-Cygni type profile. We estimate the shock front velocity during the shock propagation in the atmosphere and show that it decreases by 40% when the Hα emitting-shock passes from the photospheric level to the upper atmosphere. The Hα P-Cygni profile observed in long-period Cepheids also seems to be caused by the main shock wave. Although to date He II has only been detected in some Blazhko stars, a comprehensive survey of RR Lyrae stars is necessary to confirm this trend, so we can say that the most intense shocks will only be observed in Blazhko stars. Conclusions. The development of a model of atmospheric pulsation that takes the effects of 2D and 3D convection into account, seems to be a necessary step to fully quantify the effects of shock waves on the atmospheric dynamics of radially pulsating stars.

A helium P-Cygni profile in RR Lyrae stars?

“…Finally, because the shock is detached from the photosphere, the blueshifted emission component gradually disappears, and a P Cygni profile is formed. Gillet & Fokin (2014) suggested that the weak redshifted emission component occurring around the pulsation phase ϕ = 0.30 in RR Lyr can be also interpreted as a P-Cygni profile. The shock propagation toward the high atmosphere of the star could be the cause of this progressive and striking line profile change of the line profile.…”

“…If we assume that the D3 helium near the pulsation phase ϕ = 1.04 is built as a P-Cygni profile, then following Gillet & Fokin (2014), the shock front velocity is around 60 km s −1 . From the Hα profile Gillet & Fokin (2014) found that the shock velocity varies from 130 km s −1 at ϕ = 0.902 to 115 km s −1 at ϕ = 0.927.…”

“…Consequently, it is hard to know its exact phase of appearance. Gillet & Fokin (2014) were the first to suggest a physical mechanism to explain this weak redshifted emission. They assume that the shock wave is already sufficiently detached from the photosphere when the redshifted Hα line appears.…”

“…A third weak emission (the "third apparition"), occurring at phase 0.3 has been recently observed in the red absorption wing of the Hα profile by Preston (2011). Gillet & Fokin (2014) propose that the "third apparition" (a P-Cygni profile) is produced by the main shock when it is far enough from the photosphere. Thus, the emission is the natural consequence of the large extension of the expanding atmosphere.…”

“…Thus, the presence of helium emissions strongly depends on the intensity of the shock wave. In particular, to observe He II, the temperature behind the shock must be high enough to ionize He I. Consequently, the detection of He I and He II emissions provide fundamental informations about the shock intensity (Gillet & Fokin 2014). This is why Gillet et al (2013) define three hydrodynamic regimes: (a) the supersonic regime in which only hydrogen emissions are visible; (b) the weak hypersonic regime where the He I emission lines are also observed; and (c) the strong hypersonic regime with the additional observation of He II emission lines.…”