JHK-band spectro-interferometry of T Cep with the IOTA interferometer

Our new IOTA(1) JHK-band beam combiner allows the simultaneous recording of spectrally dispersed J-, H- and K-band Michelson interferograms. In this paper we present our IOTA observations of the Mira star T Cep with this beam combiner (observations in June 2001; four baselines in the range of 14 m to 27 m). The beam combiner optics consists of an anamorphic cylindrical lens system and a prism. From the interferograms of T Cep we derive the visibilities and the J-, H-, and K-band uniform-disk diameters of 14.0+/-0.6 mas, 13.7+/-0.6 mas and 15.0+/-0.6 mas, respectively. Angular stellar filter radii and Rosseland radii are derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of different Mira star models(2,3). The available HIPPARCOS parallax (4.76+/-0.75 mas) of T Cep allows us to determine linear radii. For example, from the K-band visibility we derive a Rosseland radius of 329(-50)(+70) R-circle dot if we use the CLVs of the M-models(3) as fit functions. This radius is in good agreement with the theoretical M-model Rosseland radius of 315 R-circle dot. The comparison of measured stellar parameters (e.g. diameters, effective temperature, visibility shape) with theoretical parameters indicates whether any of the models is a fair representation of T Cep. The ratios of visibilities of different spectral channels can be measured with higher precision than absolute visibilities. Therefore, we use the visibility ratios V(lambda(1))/V(lambda(2)) to investigate the wavelength dependence of the stellar diameter. We find that the 2.03 mum uniform-disk diameter of T Cep is similar to1.26 times larger than the 2.26 mum uniform-disk diameter.