Asteroseismology and interferometry of the red giant star <i>ϵ</i> Ophiuchi

Abstract

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