The oscillation modes of θ² Tauri: Results from the 1992 MUSICOS campaign

We have analyzed a series of 619 spectra of θ² Tauri taken with four telescopes over four consecutive nights during the 1992 global MUSICOS campaign. Radial velocity variations provide information about the oscillation frequencies of low degree (0 ≤ ℓ ≤ 3), and line-profile variations provide information on modes of higher degree (3 ≤ ℓ ≤ 10). The radial velocities were derived with a cross-correlation technique. In addition to detecting several frequencies found photometrically (e.g., Breger et al. 1989), we have found two new frequencies, which implies that the oscillation spectrum of θ² Tau may not be stable. Variations within rotationally broadened absorption lines were transformed by a Fourier-Doppler imaging analysis into a map of apparent frequency versus apparent azimuthal order. From this two-dimensional Fourier representation we identify some seven oscillation modes using a genetic algorithm to explore parameter space. While we find good agreement between the detected frequencies and those predicted to be unstable based on the models of Dziembowski (1990), it is still not clear why only certain modes are selected.