Pulsation in the atmosphere of the roAp star HD 24712 *,**
I. Spectroscopic observations and radial velocity measurements
1
Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria
2
Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 119017 Moscow, Russia e-mail: ryabchik@inasan.ru
3
Department of Astronomy and Space Physics, Uppsala University Box 515, 751 20 Uppsala, Sweden
4
European Southern Observatory, Casilla 19001, Santiago 19, Chile
5
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
6
Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
7
Dipartimento di Fisica ed Astronomia, Università di Catania, via S. Sofia 78, 95123 Catania, Italy
8
Tavrian National University, Simferopol, Ukraine
9
INAF – Telescopio Nazionale Galileo, PO Box 565, 38700 Santa Cruz de La Palma, Spain
Received:
8
September
2006
Accepted:
8
November
2006
Aims.We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712.
Methods.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.
Results.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-1) for REE lines and the Hα 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.
Key words: stars: atmospheres / stars: chemically peculiar / stars: individual: HD 24712 / stars: magnetic fields / stars: oscillations
Based on observations collected at the Canada-France-Hawaii Telescope (CFHT), at the Nordic Optical Telescope (NOT), at the European Southern Observatory, Paranal, Chile, (DDT-274.D-5011), at the Telescopio Nazionale Galileo (TNG), and from MOST, a Canadian Space Agency mission operated jointly by Dynacon, Inc., the University of Toronto Institute of Aerospace Studies, and the University of British Columbia, with assistance from the University of Vienna.
© ESO, 2007