2015A&A...573A.138B

Red giants are evolved stars that exhibit solar-like oscillations. Although a multitude of stars have been observed with space telescopes, only a handful of red giant stars were targets of spectroscopic asteroseismic observing projects. We search for solar-like oscillations in the two bright red giant stars γ Psc and θ¹ Tau from a time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power ν_max and the mean large frequency separation Δν for both stars. Seismic constraints on the stellar mass and radius will provide robust input for stellar modelling. The radial velocities of γ Psc and θ¹ Tau were monitored for 120 and 190-days, respectively. Nearly 9000 spectra were obtained. To reach accurate radial velocities, we used simultaneous thorium-argon and iodine-cell calibration of our optical spectra. In addition to the spectroscopy, we acquired interferometric observations of γ Psc for an independent estimate of the radius. We also analysed 22 days of observations of θ¹ Tau with the MOST satellite. The frequency analysis of the radial velocity data of γ Psc revealed an excess of oscillation power around 32µHz and a large frequency separation of 4.1±0.1µHz. θ¹ Tau exhibits oscillation power around 90µHz, with a large frequency separation of 6.9±0.2µHz. Scaling relations indicate that γPsc is a star of about 1M_☉ and 10R_☉. The object θ¹Tau appears to be a massive star of about 2.7M_☉ and 10R_☉. The radial velocities of both stars were found to be modulated on timescales much longer than the oscillation periods. The estimated radii from seismology are in agreement with interferometric observations and also with estimates based on photometric data. While the mass of θ¹ Tau is in agreement with results from dynamical parallaxes, we find a lower mass for γPsc than is found in the literature. The long periodic variability agrees with the expected timescales of rotational modulation.