Thermal infrared stellar interferometry using single-mode guided optics: first scientific results on IOTA

We report on first scientific observations of a few bright late type stars by direct long baseline interferometry in the thermal infrared (3.4 to 4.1 microns) obtained with the TISIS (Thermal Infrared Stellar Interferometric Set-up) experiment of the IOTA (Infrared and Optical Telescope Array) interferometer. Beam combination is provided by a single-mode fluoride glass coupler optimized for operation in that wavelength domain and yielding visibility measurements with 2% typical relative accuracy. First precise estimations of uniform disk diameters for alpha Orionis, alpha Herculis, o Ceti and R Leonis are presented in the L band. Very large increases (50 to 70%) in apparent angular diameters have been found for the 2 Mira stars o Ceti and R Leonis with respect to previous measurements obtained at shorter infrared wavelengths and same luminosity phase. Extended optically thin close-by dust shells characterized by Infrared Spatial Interferometer (ISI) measurements are not found to play a significant role in the observed L band intensity distribution. Gas properties are likely to have a greater impact at these wavelengths. Our o Ceti interferometric observations look indeed in good agreement with the presence of very extended circumstellar gas layers (mostly H2O and SiO) derived from recent Infrared Space Observatory (ISO) thermal infrared spectral data.