Gillet 1 , B. Mauclaire 2 , T. Garrel 3 , T. Lemoult 4 , Ph. Mathias 5 , T. Devaux 7 , H. Boussier 8 , D. Verilhac 9 , G. Brabant 10 , J. Desbordes 11 , O. Received: 14 October Accepted: 22 August It has been observed for the first time in in some RR Lyrae stars. The emission intensity is very weak, and its profile is a tiny persistent hump in the red side-line profile.
The presence of this third emission and its strength both seem to depend only marginally on the Blazhko phase. The physical origin of the emission is probably due to the infalling motion of the highest atmospheric layers, which compresses and heats the gas that is located immediately above the rising shock wave. The light curve exhibits a period of about 0.
In addition, the pulsation and Blazhko periods change slightly during their respective cycles, and the ephemerides therefore have to be updated regularly e. Preston et al. During this phase, Preston et al. These phenomena were interpreted by means of a shock wave propagation, following the work of Schwarzschild , who showed that a shock wave can produce these particular profiles in population II Cepheids.
In addition, during the Blazhko cycle, the critical zone in the stellar atmosphere where the wave breaks moves up and down. Although the code they used is purely radiative, convection is thought to affect the results only weakly since the helium ionization zone is located deeper in the envelope Xiong et al. The model considers a stationary shock wave within an homogeneous medium consisting solely of atomic hydrogen with the five first bound levels and the continuum, allowing a computation of the hydrogen emission line profiles.
The bump was interpreted by Hill as the consequence of a shock or collision between atmospheric layers. This means that an amplitude threshold may exist to induce this secondary shock. However, it should be noted that these authors did not explain how such a compression shock alone can produce redshifted emission. This weak emission should be observed when the main shock propagates far enough from the photosphere, that is, when the shock intensity is very high.
