Detection of solar-like oscillations in the bright red giant stars<i>γ</i>Piscium and<i>θ</i>¹ Tauri from a 190-day high-precision spectroscopic multi-site campaign

Abstract

Context. 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. Aims. We search for solar-like oscillations in the two bright red giant stars Psc and 1 Tau from a time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power max and the mean large frequency separation for both stars. Seismic constraints on the stellar mass and radius will provide robust input for stellar modelling. Methods. The radial velocities of Psc and 1 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 Psc for an independent estimate of the radius. We also analysed 22 days of observations of 1 Tau with the MOST satellite. Results. The frequency analysis of the radial velocity data of Psc revealed an excess of oscillation power around 32 Hz and a large frequency separation of 4.1 0.1 Hz. 1 Tau exhibits oscillation power around 90 Hz, with a large frequency separation of 6.9 0.2 Hz. Scaling relations indicate that Psc is a star of about 1 M and 10 R . The object 1 Tau appears to be a massive star of about 2.7 M and 10 R . The radial velocities of both stars were found to be modulated on timescales much longer than the oscillation periods. Conclusions. The estimated radii from seismology are in agreement with interferometric observations and also with estimates based on photometric data. While the mass of 1 Tau is in agreement with results from dynamical parallaxes, we find a lower mass for Psc than is found in the literature. The long periodic variability agrees with the expected timescales of rotational modulation.