THE DISTRIBUTION OF DUST AROUND ASYMPTOTIC GIANT BRANCH STARS

A sample of 13 Asymptotic Giant Branch stars (AGE stars) were observed in the submm continuum at 761 and 1100 mu m with the James Clerk Maxw ell Telescope (JCMT, Mauna Kea, Hawaii). Eleven sources were detected in at least one of the two wavelength bands. Mapping observations were attempted for five sources. Some of our sources were also measured at 1250 mu m by Walmsley et al. (1991)and good agreement was found excep t for three sources: IRC+10216, W Hya and OH26.5+0.6. The differences for IRC+10216, however, can be understood as caused by a rather steep density law in its dust shell; for W Hya there is evidence for submm v ariability. The radial dust distribution in the circumstellar envelope s was derived by combining the submm observations with near IR and IRA S observations obtained from the literature and by fitting the data wi th an optically thin and spherically symmetric dust shell model. In ad dition, for three sources the extended emission was modelled as a func tion of angular distance from the star. For five sources evidence was found for gradual changes in the mass loss rate. Four AGE stars have s ubmm fluxes in excess by a factor of 3-5 larger compared to what can b e explained with one dust shell. Various explanations are discussed: c ontributions of molecular lines, free-free emission, variability, diff erent dust emissivities at submm wavelengths and the possibility of a second dust shell from a previous mass loss phase or heating of the IS A? by the central star. None of these possibilities by themselves can explain the large submm excess. A combination of them may explain the observations but with the present data set it is not possible to chose between the various possibilities.