2011A&A...529A.115M

We report on K-band VLTI/AMBER observations at medium spectral resolution (∼1500) of RS Capricorni, an M6/M7III semi-regular AGB star. From the spectrally dispersed visibilities, we measure the star diameter as a function of observing wavelength from 2.13 to 2.47-microns. We derive a Rosseland angular diameter of 7.95±0.07mas, which corresponds to an effective temperature of 3160±160K. We detect size variations of around 10% in the CO band heads, which are indicative of strong opacity effects of CO in the stellar photosphere. We also detect a linear increase in the size as a function of wavelength, beginning at 2.29-microns. Models of the stellar atmosphere, based on the mass of the star estimated from stellar-evolution models, predict CO-size effects of about half the sizes observed, and cannot reproduce the linear size increase with wavelength, redward of 2.29-microns. We are able to model this linear size increase with the addition of an extended water-vapor envelope around the star. However, we are unable to fit the data in the CO bandheads. Either the mass of the star is overestimated by the stellar-evolution models and/or there is an additional extended CO envelope in the outer part of the atmosphere. In any case, neither the water-vapor envelope nor the CO envelope can be explained using the current models.