Ten bright Miras and eight semi-regular variable giants and supergiants have been observed with the IOTA (Infrared and Optical Telescope Array) interferometer in the L' band (from 3.4 to 4.1 microns). Observations were carried out in March and November 2000 with the FLUOR/TISIS instrument. Variations in the diameter of R Leo are for the first time observed in the L' band, and our data show that the diameter of α Ori is remarkably stable. Important deviations from a uniform disk model are demonstrated for most of the Mira stars of our sample, and are particularly obvious for χ Cyg, R Cas and ο Cet. Observations of α Her carried out in March are consistent with previous estimates of its diameter published with the very first data of TISIS. The instrument has shown its reliability and the good quality of its data. The present results emphasize the importance of such data for a better comprehension of the circumstellar environment of evolved stars.
The calibration process of long baseline stellar interferometers requires the use of reference stars with accurately determined angular diameters. We present a catalog of 374 carefully chosen stars among the all-sky network of infrared sources provided by Ref. 1. The catalog benefits from a very good sky coverage and a median formal error on the angular diameters of only 1.2%. Besides, its groups together in a homogeneous handy set stellar coordinates, uniform and limb-darkened angular diameters, photometric measurements, and other parameters relevant to optical interferometry. In this paper, we describe the selection criteria applied to qualify stars as reference sources. Then, we discuss the catalog's statistical properties such as the sky coverage or the distributions of magnitudes and angular diameters. We study the number of available reference stars as a function of the baseline and the precision needed on the visibility measurements. Finally, we compare the angular diameters predicted in Ref. 1 with existing determinations in the literature, and find a very good agreement.
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 increase (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 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 thermal infrared spectral data.