Detection of solar-like oscillations in the bright red giant stars γ Piscium and θ¹ Tauri from a 190-day high-precision spectroscopic multi-site campaign^⋆

¹ Instituut voor Sterrenkunde, KU Leuven, 3001 Leuven, Belgium
e-mail: paul.beck@ster.kuleuven.be
² Okayama Astrophysical Observatory, National Astron. Obs. of Japan, Kamogata, Asakuchi, 719-0232 Okayama, Japan
³ Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁴ Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
⁵ Université de Toulouse; UPS-OMP; IRAP; CNRS; IRAP ; 57 avenue d’Azereix, BP 826, 65008 Tarbes, France
⁶ Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
⁷ Synergistic Exploitation of Atmospheric Data, Belgian Institute for Space Aeronomy, 1180 Brussels, Belgium
⁸ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁹ Instituto de Astrofsica de Andalucía, Glorieta de la Astronomía s/n, 18009 Granada, Spain
¹⁰ School of Physics and Astronomy, University of Birmingham, UK
¹¹ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, 152-8551 Tokyo, Japan
¹² Department of Physics & Astronomy, University of British Columbia, Vancouver, V6T 1Z1, Canada
¹³ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, 739-8526 Higashi-Hiroshima, Japan
¹⁴ OptCraft, 3-6-18 Higashi-Hashimoto, Midori-ku, 252-0144 Sagamihara, Japan

Received: 8 November 2013
Accepted: 16 July 2014

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 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.

Methods. 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.

Results. The frequency analysis of the radial velocity data of γ Psc revealed an excess of oscillation power around 32 μHz and a large frequency separation of 4.1 ± 0.1 μHz. θ¹ Tau exhibits oscillation power around 90 μHz, with a large frequency separation of 6.9 ± 0.2 μHz. Scaling relations indicate that γ Psc is a star of about 1 M_⊙ and 10 R_⊙. The object θ¹ 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 θ¹ Tau is in agreement with results from dynamical parallaxes, we find a lower mass for γ Psc than is found in the literature. The long periodic variability agrees with the expected timescales of rotational modulation.