Interferometric Angular Diameters of Mira Variables with theHubble Space Telescope

This paper presents the first successful interferometric observations of angular diameters with the astrometer Fine Guidance Sensor (FGS) aboard the Hubble Space Telescope , the only optical interferometer operating in space. We report single-epoch angular diameters in visible light for the two Mira-type variables, R Leonis and W Hydrae, along two orthogonal directions. Although the relatively large bandpass of the instrument will limit the use of the new diameters for improving on our knowledge of the atmospheric structure of Mira variables, the milliarcsecond precision of the FGS measurements show with unprecedented clarity that the atmospheres of the two stars do not have circular symmetry, with the "major" axis exceeding the "minor" axis by about 11% and 20% for R Leo and W Hya, respectively. There is also evidence of nonsymmetric brightness distributions over the stellar disks. Besides the intrinsic interest of these results, asymmetries measured in a wide optical band, such as that used in the present work, are of relevance for the interpretation of the HIPPARCOS astrometry of Mira variables.