EDP Sciences
Free access
Volume 374, Number 2, August I 2001
Page(s) 615 - 628
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20010726

A&A 374, 615-628 (2001)
DOI: 10.1051/0004-6361:20010726

Time-resolved spectroscopy of the roAp star $\gamma$ Equ

O. Kochukhov1 and T. Ryabchikova2, 3

1  Uppsala Astronomical Observatory, Box 515, 751 20 Uppsala, Sweden
2  Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 109017 Moscow, Russia
    e-mail: ryabchik@inasan.rssi.ru
3  Institute for Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
    e-mail: ryabchik@tycho.astro.univie.ac.at

(Received 7 December 2000 / Accepted 17 May 2001 )

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 Åspectral 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 NaI lines and analysed line profile variations of PrIII and NdIII 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 PrIII and NdIII line profile variations resulted in the estimate of $\ell=2$ or 3, $m=-\ell$ or $-\ell+1$ and vp $\approx10$ km s-1 for the p-mode of the main pulsation frequency.

Key words: stars: chemically peculiar -- stars: oscillations -- stars: individual: $\gamma$ Equ

Offprint request: O. Kochukhov, oleg@astro.uu.se

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