Detection of solar-like oscillations in the red giant star ϵ Ophiuchi by MOST spacebased photometry ^*

¹ Observatoire de Paris, LESIA, CNRS UMR 8109, Place Jules Janssen, 92195 Meudon, France e-mail: Caroline.Barban@obspm.fr
² Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
³ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver V6T 1Z1, Canada
⁴ Institut d'Astrophysique Spatiale, CNRS/Université Paris XI UMR 8617, 91405 Orsay Cedex, France
⁵ Astronomy Department, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
⁶ Department of Astronomy and Physics, St. Mary's University, Halifax, NS B3H 3C3, Canada
⁷ Département de Physique, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7, Canada
⁸ David Dunlap Observatory, University of Toronto, PO Box 360, Richmond Hill, ON L4C 4Y6, Canada
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 8 November 2006
Accepted: 13 March 2007

Abstract

Context.Solar-like oscillations have been discovered in a few red giants, including ϵ Oph, through spectroscopy. Acoustic modes around 60 μHz were clearly seen in this star, but daily aliasing of the groundbased data made it impossible to unambiguously isolate the p-mode frequencies in the eigenspectrum, and hence the correct value of the large spacing, to asteroseismically constrain the mass of this pulsating star.

Aims.We obtained about 28 days of contiguous high-precision photometry of ϵ Oph in May–June 2005 with the MOST (Microvariability & Oscillations of STars) satellite. The thorough time sampling removes the ambiguity of the frequency identifications based on the groundbased discovery data.

Methods.We identify equidistant peaks in the Fourier spectrum of the MOST photometry in the range where the p-modes were seen spectroscopically. Those peaks are searched by autocorrelation of the power spectrum to estimate the value of the large separation in the p-mode eigenspectrum. Having isolated the oscillation modes, we determine their mode parameters (frequency, amplitude and line width) by fitting the distribution of peaks to Lorentzian profiles.

Results.The clear series of equidistant peaks in the power spectrum, with amplitudes from about 30 to 130 ppm, are consistent with radial modes spaced by a mean value of (Hz. This large separation matches one of the two possibilities allowed by the groundbased observations thus constraining the stellar models to a much greater extent than previously possible. The line widths and Lorentzian fits indicate a rather short average mode lifetime:  days.