Asteroseismology and interferometry of the red giant star ϵ Ophiuchi

¹ Instituut voor Sterrenkunde, Katholieke Universiteit, 200D Celestijnenlaan, 3001 Leuven, Belgium e-mail: anwesh@tifr.res.in
² Astronomy Department, Yale University, PO Box 208101, New Haven CT 06520-8101, USA
³ Homi Bhabha Centre for Science Education, TIFR, V. N. Purav Marg, Mankhurd, Mumbai 400088, India
⁴ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁵ Center for High Angular Resolution Astronomy, Georgia State University, PO Box 3965, Atlanta, Georgia 30302-3965, USA
⁶ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 Place Jules Janssen, 92195 Meudon, France
⁷ Institut d'Astrophysique Spatiale, CNRS/Université Paris XI UMR 8617, 91405 Orsay Cedex, France
⁸ GRAAL, Université Montpellier II, CNRS UMR 5024, 34095 Montpellier Cedex 05, France
⁹ Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Vienna, Austria
¹⁰ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver V6T 1Z1, Canada
¹¹ National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85719, USA

Received: 20 April 2009
Accepted: 10 June 2009

Abstract

The GIII red giant star ϵ Oph has been found to exhibit several modes of oscillation by the MOST mission. We interpret the observed frequencies of oscillation in terms of theoretical radial p-mode frequencies of stellar models. Evolutionary models of this star, in both shell H-burning and core He-burning phases of evolution, are constructed using as constraints a combination of measurements from classical ground-based observations (for luminosity, temperature, and chemical composition) and seismic observations from MOST. Radial frequencies of models in either evolutionary phase can reproduce the observed frequency spectrum of ϵ Oph almost equally well. The best-fit models indicate a mass in the range of 1.85 ± 0.05 with radius of 10.55 ± 0.15. We also obtain an independent estimate of the radius of ϵ Oph with highly accurate interferometric observations in the infrared K' band, using the CHARA/FLUOR instrument. The measured limb-darkened disk angular diameter of ϵ Oph is 2.961 ± 0.007 mas. Together with the Hipparcos parallax, this translates into a photospheric radius of R = 10.39 ± 0.07 . The radius obtained from the asteroseismic analysis matches the interferometric value quite closely even though the radius was not constrained during the modelling.