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

Context. Solar-like oscillations have been discovered in a few red giants, including epsilon Oph, through spectroscopy. Acoustic modes around 60 mu Hz 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 epsilon 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 (5.3 +/- 0.1) mu 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: (2.7(-0.8)(+0.6)) days.