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

TL;DR

This study investigates the pulsating atmosphere of the roAp star HD 24712 through spectroscopic and photometric data, revealing phase shifts, pulsation wave velocity, and element-specific atmospheric dynamics.

Contribution

First-time measurement of phase shifts between photometry and radial velocities for different elements, estimating pulsation wave speed, and linking spectral line behavior to atmospheric structure in a roAp star.

Findings

Pulsation wave speed is slightly below sound speed.

Large pulsation amplitudes observed in REE lines and Halpha core.

No amplitude difference before and after the Balmer jump for REE lines.

Abstract

We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower 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 not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.