Time-resolved spectroscopy of the roAp star γ Equ

We report results of the spectroscopic monitoring of the roAp star gamma Equ with the ESO 3.6-m telescope. During 1.5 hours of observations a series of very high-resolution and high S/N spectra was obtained for this star in the 6138-6165 Angstrom spectral region. Short exposure times allowed us to resolve changes of gamma Equ line profiles due to the rapid pulsations and to follow profile variations over 5 oscillation cycles. From this unique observational material information on amplitudes and phase shifts of radial velocity (RV) variations was extracted for 29 lines of 17 individual ions. We confirmed that spectral lines of rare-earth elements (REE) have the largest pulsation amplitudes, reaching up to 0.8 km s(-1). Moreover, we detected a phase shift between RV variations of singly and doubly ionized REE, discovered significant RV shifts of weak Na I lines and analysed line profile variations of Pr III and Nd III spectral features. This rich observational material opens a possibility to obtain a detailed picture of the vertical stratification of chemical elements and extract the main characteristics of the pulsational mode(s). In our observations we did not find support for the existence of the unique dependence of RV amplitude on line strength suggested in earlier spectroscopic studies of roAp stars. Instead, we argue that the individuality of the variations of all ions is a result of the complex interplay between inhomogeneous vertical and horizontal distributions of chemical elements and individual pulsation modes of gamma Equ. We show that the extra line broadening observed in gamma Equ spectrum is most likely caused by pulsations. A detailed analysis of Pr III and Nd III line profile variations resulted in the estimate of l = 2 or 3, m = -l or -l + 1 and v(p) approximate to 10 km s(-1) for the p-mode of the main pulsation frequency.