Asteroseismology and interferometry of the red giant star ɛ Ophiuchi

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