In order to provide a detailed analysis of the RR Lyrae instability strip topology, nonlinear pulsating models have been extensively studied (Bono & Stellingwerf 1994). The effect of convection in photospheric regions has also been well studied (Bono & Marconi 1998; Feuchtinger & Dorfi 1998). Nevertheless, all these studies have not considered the detailed structure of the atmosphere, because the number of mass layers above the photosphere was not large enough to calculate line absorption profiles. Consequently, it was not possible to explain high-quality spectra of RR Lyrae that have been obtained with modern CCD-spectrographs such as the ELODIE spectrograph at the Haute-Provence Observatory.
Recently, Fokin & Gillet (1997), using non-linear non-adiabatic pulsating models have explained the line doubling phenomenon observed during very short intervals within some metallic absorption profiles (Chadid & Gillet 1996a). About 40-50 atmospheric mass layers above the photosphere were necessary for modeling the observed profiles. Although, in Lagrangian codes, the spatial resolution of the hydrogen ionization zone is usually too low to compute correctly the rapid variation of the gas parameters, the numerical results explain in a semi-quantitative manner the doubling phenomenon. In addition to this observational test, atmospheric models give a detailed description of the whole atmospheric structure, especially the number and the type of strong shock waves propagating throughout the mass layers. The highest atmospheric regions considered by these models have a relatively low density (log or -15 g/cm3 depending upon the pulsation phase). This means that hydrogen profiles such as H can be calculated (Fokin 1992).
Up until now, atmospheric pulsation models of RR Lyrae stars assume that the motion of layers, where metallic and hydrogen lines are formed, are strictly periodic. Although, the luminosity period of bright RR Lyrae stars is known to be constant to within a few seconds over 10 or 20 years, an appreciable fraction of RR Lyrae stars (around 30%) show noticeable variations in the shape of their luminosity and radial velocity curves over a period of about 100 pulsation cycles (This is the Blazhko effect). Consequently, a long term variation in the motion of the atmospheric layers should be expected. This is consistent with the variation of the intensity of the hydrogen line emission over the Blazhko period of RR Lyrae (Chadid & Gillet 1997) because it is directly related to the strength of shock waves.
Does the shock wave strength depend only on the Blazhko phase, or is a variation between two successive pulsation cycles possible? We know from previous studies (Hill 1972; Fokin 1992) that a secondary shock, called the early shock , is due to a "collision" between the free-falling outer atmospheric layers and the slower, upwardly moving photospheric layers. This shock has been detected observationally by the presence of a weak hydrogen emission (Gillet & Crowe 1988, Gillet et al. 1989) and by a broadening of the FWHM of metallic lines (Chadid & Gillet 1996b). This indicates that strong perturbations of the pulsation motion are present in the atmosphere and we can expect that dynamical effects, induced by the early shock, are not necessary completely relaxed when a new pulsation cycle starts again.
Using high-quality spectral observations, the goal of this paper is to investigate if the pulsation motion in the atmosphere of the brightest RR Lyrae star in the sky, RR Lyrae, is periodic or shows some irregularities. In Sect. 2 we describe the observations of the Fe II (4923.921Å) line profile. The presentation and the discussion of the radial velocity curves are given in Sect. 3 and those concerning the FWHM of the Fe II line in Sect. 4. Finally, a short discussion and some concluding remarks are given in Sects. 5 and 6 respectively.
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000