Barban, C.; Matthews, J. M.; De Ridder, J.; Baudin, F.; Kuschnig, R.; Mazumdar, A.; Samadi, R.; Guenther, David B.; Moffat, A. F. J.; Rucinski, S. M.
Abstract:
Context. Solar-like oscillations have been discovered in a few red giants, including e 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 e 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) μHz. 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+0.6
−0.8) days.