Measurement at 11 micron wavelengths of the diameters of alpha Orionis and alpha Scorpii, and changes in effective temperature of alpha Orionis and very recent dust emission

The angular diameters of the stellar disks of a Ori and a Sco have been determined to be 56.6 +/- 1 and 44.4 +/- 2 mas, respectively, at 11 mu m using long-baseline interferometry. These values are about 10% larger than the most reliable diameters determined at visible wavelengths, and are consistent with predictions of significant visible limb darkening, but only minor infrared limb darkening. Variations in temperature and luminosity of alpha Ori during 1989-1995 have been studied by a combination of interferometry, optical photometry, and spectrophotometry. This shows that observed variations in rn, are primarily due to temperature changes. A recent episode of dust formation at about 0 ''.1 from the star has been detected; it is probably the result of a discrete mass-loss event of modest size.