Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements

摘要

We have investigated the structure of the pulsating atmosphere of one of thebest studied rapidly oscillating Ap stars, HD 24712. For this purpose weanalyzed spectra collected during 2001-2004. An extensive data set was obtainedin 2004 simultaneously with the photometry of the Canadian MOST mini-satellite.This allows us to connect directly atmospheric dynamics observed as radialvelocity variations with light variations seen in photometry. We directlyderived for the first time and for different chemical elements, respectivelyions, phase shifts between photometric and radial velocity pulsation maximaindicating, as we suggest, different line formation depths in the atmosphere.This allowed us to estimate for the first time the propagation velocity of apulsation wave in the outer stellar atmosphere of a roAp star to be slightlylower than the sound speed. We confirm large pulsation amplitudes (150-400 m/s)for REE lines and the Halpha core, while spectral lines of the other elements(Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did notfind different pulsation amplitudes and phases for the lines of rare-earthelements before and after the Balmer jump, which supports the hypothesis of REEconcentration in the upper atmosphere above the hydrogen line-forming layers.We also discuss radial velocity amplitudes and phases measured for individualspectral lines as tools for a 3D tomography of the atmosphere of HD 24712.