Variation of the line profile moments for stars pulsating in distorted oblique non-radial modes

Department of Astronomy and Space Physics, Uppsala University, 751 20 Uppsala, Sweden e-mail: oleg@astro.uu.se

Received: 3 January 2005
Accepted: 21 March 2005

Abstract

We derive expressions and develop a numerical technique for the analysis of the line profile moment variations for stars pulsating in oblique non-radial modes. This represents an extension of the widely used spectroscopic moment mode identification method to the oblique distorted pulsations observed in rapidly oscillating Ap stars. We demonstrate that a non-axisymmetric superposition of the pulsation and rotation velocity fields results in a qualitatively new behaviour of some of the line profile characteristics. It is found that for the majority of roAp stars the second moment varies with the pulsation frequency rather than with its first harmonic even for axisymmetric modes. We also identify pulsation observables which do not change during pulsation cycle but are modulated by the stellar rotation and can contribute to the variability of the stellar spectra averaged over many pulsation cycles. As an illustration of the new version of the moment technique, we compute rotational modulation of the pulsational changes of the line profile moments for the oblique axisymmetric dipolar pulsation modes with different parameters. It is also shown that a distortion of the oblique dipolar modes predicted by the recent theoretical studies of the stellar magneto-acoustic oscillations can be readily diagnosed through the moment analysis. In particular, the shape of the pulsation phase modulation for the radial velocity and the second moment is very sensitive to non-axisymmetric pulsation components, whereas the rotational modulation of the second moment amplitude is best suited to revealing axisymmetric magnetically induced distortion of pulsations.