Abstract

Observations of rapid radial velocity variations of spectral lines in rapidly oscillating Ap (roAp) stars

O. Kochukhov¹, F. Kupka², J. D. Landstreet³, T. Ryabchikova^2,4, W. Weiss²

¹Uppsala Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden
²Institute for Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
³Department of Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
⁴Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 109017 Moscow, Russia

The roAp stars are late A and early F magnetic, chemically peculiar stars that exhibit low-amplitude rapid pulsations. These stars, which occur approximately where the instability strip crosses the main sequence, pulsate with one or several periods in the range of 5 to 15 minutes. The oscillations are non-radial p-modes of high order but low degree. The roAp stars pulsate with extremely low amplitudes, less than 0.02 m (in B light). More than 30 roAp stars are now known.
These stars have roughly dipolar magnetic fields with characteristic field strengths of one or a few kG. The field axis is inclined to the stellar rotation axis, and the pulsation axis appears to coincide with the field axis. Thus as the star rotates (typical periods are several days), the aspect of the field, and the pulsation amplitude, change. Furthermore, the mode or modes excited change with time, but not periodically. (One of the main mysteries of roAp stars is how the excited modes are selected by the star.) Mode identification in these stars is still extremely uncertain.
Most of the available data on the pulsations of these stars are from broad-band photometry. Recently, however, radial velocity (RV) variations with one or more of the pulsation frequencies have been detected in individual spectal lines of several of the brightest roAp stars (HD 24712 = HR 1217, HD 83368 = HR 3831, HD 128898 =

Cir, HD 137949 = 33 Lib, and HD 201601 =

Equ). The amplitudes are typically hundreds of ms^-1.
We have been searching for RV variations in the brighter roAp stars using the CFHT. In general, it is found that observable RV variations have much larger amplitude in lines of the ionized rare earths than in any other spectral lines: apparently these elements are concentrated by radiatively driven diffusion in the atmosphere layers where the oscillation amplitudes are largest. In this poster we report the discovery of periodic RV variations in rare earth lines of HD 176232 = 10 Aql with amplitudes of the order of 100 ms^-1, and the observation of larger amplitude variations in rare earth lines of HD 24712 and

Equ. Data such as ours should be of considerable use in identifying pulsation modes in these stars and eventually in probing the effects of interior atomic diffusion on their internal structure.