TAU PEGASI - A FOURIER REPRESENTATION OF LINE-PROFILE VARIATIONS

The complex line-profile variations, probably caused by nonradial pulsations, in the spectrum of the rapidly rotating delta Scuti star tau Peg are analyzed with a novel two-dimensional Fourier transform technique. It resolves the oscillations in temporal frequency nu and apparent azimuthal order m. The two-dimensional Fourier spectrum resembles the (1, nu) diagrams generated for the Sun. Advantages of the technique are (1) both frequency and mode of the oscillations are determined directly; (2) complex variations in the line profiles are resolved into individual modes; (3) modes of both high and low degree are resolved. For tau Peg, we identify four apparent modes having Absolute value of m congruent-to 3, 7, 11, and 15. After correcting for rotation, most of the modes have frequencies of approximately 17 cycles day-1. Assuming T(eff) congruent-to 8000 K and L congruent-to 40 L., all of the modes fall within the range of frequencies theoretically predicted to be unstable.