CIRCUMSTELLAR ENVIRONMENTS .5. THE ASYMMETRIC CHROMOSPHERE AND DUST SHELL OF ALPHA-ORIONIS

We report new mid-infrared and radio images of the M supergiant alpha Orionis. The radio images, taken with MERLIN1 and the Very Large Array , resolve the chromosphere of the star at a wavelength of 6 cm, showin g that the radio-emitting region has between two and three times the d iameter of the optical photosphere. The infrared images, taken at the UK Infrared Telescope (UKIRT) using the Berkeley Infrared Camera, show that the dust shell is resolved, Images taken one year apart show gre at changes in morphology, which suggests that the dust shell is being asymmetrically illuminated by a stellar radiation field that is strong ly affected by the presence and evolution of spots on the stellar surf ace. We present a new model of the circumstellar environment of the st ar, which fits the infrared and radio images and the spectral energy d istribution (SED), and is consistent with recently reported Hubble Spa ce Telescope observations. The chromospheric structure is determined q uantitatively and the inner dust shell structure qualitatively with th is model. We find that the inner radius of the dust shell is approxima tely 0.5 arcsec, which disagrees with a number of interferometric meas urements that have resulted in an inner radius close to 1.0 arcsec, We are unable to explain this differenc.