Irregularities in atmospheric pulsations of RR Lyrae stars

From high spectral resolution observations of the Fe II line at 4923.921 ˚ A, it is shown that the star RR Lyrae shows significant differences, both in radial velocity and in line FWHM, from one pulsation cycle to the next. Two explanations are discussed. The first one is the multiperiodic character of the pulsation (Blazhko effect) which was recently detected (Chadid et al. 1999). In this case, the investigated variations would not be irregular but multiperiodic. Nevertheless, the intensity of the multiplet components seem too weak to fully reproduce the am- plitude of the observed "irregularities". The second mechanism is a dynamical interaction between the highest atmospheric la- yers and the deepest ones, which are traversed by strong outward shock waves. Nevertheless, because it is known that the shock wave intensities are modulated by the Blazhko effect, it is not excluded that the multiperiodic behavior of the pulsation can be in part at the origin of the observed variations. New observa- tions are required to determine the exact intensity value of all components. A comparison with a non-Blazhko star would be helpful.