DIRECT MEASUREMENT OF THE INNER RADIUS OF THE DUST SHELL AROUND THE COOL SUPERGIANT STAR ALPHA-SCORPII

We have measured high-precision 10 mu m image profiles of the supergiant star a Scorpii using a scanned linear array on a 3 m telescope. These image profiles have high signal-to-noise ratio and so can be used to study circumstellar structure with spatial resolution at and slightly finer than the diffraction limit of the telescope, FWHM similar to 0 ''.7. We find the inner radius of the dust shell to be about 0 ''.6, or 110 AU. At this circumstellar radius, the temperature of the dust is very low compared to the temperature at which silicate grains evaporate, indicating that grain condensation is impeded compared to equilibrium conditions if the dust shell morphology is constant in time. In this picture, grains may grow to a relatively large size soon after condensation. Alternatively, this large temperature discrepancy may suggest that dust formation is episodic, and the shell that we observed was originally formed closer to the star but has moved outward with the general circumstellar outflow of gas. In either case, for most published values of the mass-loss rate, our data can be used to argue that circumstellar dust grains around alpha Sco are predominantly large in size, so the size distribution seen in the general interstellar medium is not present at the time of grain formation.