Asteroseismology and interferometry of the red giant star ϵOphiuchi

The GIII red giant star ϵOph 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 ϵOph almost equally well. The best-fit models indicate a mass in the range of 1.85 ±0.05 ${M_\odot}$with radius of 10.55 ±0.15${R_\odot}$. We also obtain an independent estimate of the radius of ϵOph with highly accurate interferometric observations in the infrared K'band, using the CHARA/FLUOR instrument. The measured limb-darkened disk angular diameter of ϵOph is 2.961 ±0.007 mas. Together with the Hipparcosparallax, this translates into a photospheric radius of R= 10.39 ±0.07 ${R_\odot}$. The radius obtained from the asteroseismic analysis matches the interferometric value quite closely even though the radius was not constrained during the modelling.