INTERFEROMETRIC ANGULAR DIAMETERS OF MIRA VARIABLES WITH THE HUBBLE-SPACE-TELESCOPE

This paper presents the first successful interferometric observations of angular diameters with the astrometer Fine Guidance Sensor (FGS) ab oard the Hubble Space Telescope, the only optical interferometer opera ting in space. We report single-epoch angular diameters in visible lig ht for the two Mira-type variables, R Leonis and W Hydrae, along two o rthogonal directions. Although the relatively large bandpass of the in strument will limit the use of the new diameters for improving on our knowledge of the atmospheric structure of Mira variables, the milliarc second precision of the FGS measurements show with unprecedented clari ty that the atmospheres of the two stars do not have circular symmetry , with the ''major'' axis exceeding the ''minor'' axis by about 11% an d 20% for R Leo and W Hya, respectively. There is also evidence of non symmetric brightness distributions over the stellar disks. Besides the intrinsic interest of these results, asymmetries measured in a wide o ptical band, such as that used in the present work, are of relevance f or the interpretation of the HIPPARCOS astrometry of Mira variables.