Asteroseismology and interferometry of the red giant star $\epsilon$ Ophiuchi

The GIII red giantstarOph 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 ofOph almost equally well. The best- fit modelsindicate a mass in the range of 1.85 ± 0.05 Mwith radiusof 10.55 ± 0.15 R� . We also obtain an independent estimate of the radiusofOph with highly accurate interferometric observations in the infrared Kband, using the CHARA/FLUOR instrument. The measured limb-darkeneddisk angular diameterofOph is 2.961 ± 0.007 mas. Together with the Hipparcos parallax, this translates into a photospheric radiusof R = 10.39 ± 0.07 R� . The radiusobtained from the asteroseismic analysis matches the interferometric value quite closely even though the radiuswas not constrained during the modelling.