1995A&A...295..445V

A sample of 13 Asymptotic Giant Branch stars (AGB stars) were observed in the submm continuum at 761 and 1100µm with the James Clerk Maxwell 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 1250µm by Walmsley et al. (1991A&A...248..555W) and good agreement was found except 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 variability. The radial dust distribution in the circumstellar envelopes was derived by combining the submm observations with near IR and IRAS observations obtained from the literature and by fitting the data with an optically thin and spherically symmetric dust shell model. In addition, for three sources the extended emission was modelled as a function of angular distance from the star. For five sources evidence was found for gradual changes in the mass loss rate. Four AGB stars have submm fluxes in excess by a factor of 3-5 larger compared to what can be explained with one dust shell. Various explanations are discussed: contributions of molecular lines, free-free emission, variability, different dust emissivities at submm wavelengths and the possibility of a second dust shell from a previous mass loss phase or heating of the ISM 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.