CHARACTERISTICS OF DUST SHELLS AROUND 13 LATE-TYPE STARS

The spatial distribution of dust around a sample of 13 well-known late-type stars has been studied with the Infrared Spatial Interferometer (ISI) located at Mount Wilson. Comprised of two movable telescopes, currently used as a heterodyne interferometer at 11.15 mum, the ISI has obtained visibility curves of these stars at baselines up to 13 m. This is the first detailed observational study in the mid-infrared with sufficient resolution to determine the inner radii of the dust shells of such stars. Visibility curves obtained from three-dimensional radiative transfer models of the dust shells were used to make estimates of the inner radii, temperature, and the optical depth at 11 mum by chi2 fitting the visibility data. Results from the models are compared not only with the high spatial resolution data, but also with spectral data obtained by the IRAS Low Resolution Spectrometer. For stars in which the dust is resolved, estimates of the stellar diameters and temperatures were also made. Broadly speaking, two classes of stars have been found. One class has inner radii of their dust shells very close to the photospheres of the stars themselves (3-5 stellar radii) and at a higher temperature (approximately 1200 K) than previously measured. This class includes R Leo, VX Sgr, VY CMa, IK Tau, R Aqr, o Cet, and IRC + 10216. For the latter two, the visibility curves are shown to change with the luminosity phase of the star and new dust appears to form at still smaller radii during minimum luminosity. The second class of stars has dust shells with substantially larger inner radii and very little dust close to the stars, and includes alpha Ori, alpha Sco, alpha Her, chi Cyg, W Aql, and U Ori. This indicates sporadic production of dust with episodes of substantial dust production typically separated by a few decades.