A STUDY OF DUST SHELLS AROUND HIGH-LATITUDE SUPERGIANTS

A sample of 22 stars with infrared excess emission and many of which h ave supergiant-like spectra was surveyed in the submillimeter continuu m at 438,761 and 1100 mum using the James Clerk Maxwell Telescope (JCM T, Mauna Kea, Hawaii). Most of the sources are at high galactic latitu des but stars with similar properties at lower galactic latitudes are also included. For 16 sources it is believed that they are post Asympt otic Giant Branch stars (post-AGB stars) and 6 of them are detected. T he rest of the sample consists of 2 binaries, 1 Herbig Ae star, 1 Lumi nous Blue Variable and 2 sources for which the post-AGB nature is unce rtain. The observations are combined with optical, near infrared and I RAS data and are fitted with a spherically symmetric and optically thi n dust shell model. It is found that a large fraction of the excess em ission originates from relatively hot dust near the star. Only 2 sourc es lack such a hot dust component. For all stars with a hot dust compo nent it was found that the emission longward of 60 mum cannot be expla ined with only one dust shell. Several possible explanations are discu ssed and it is concluded that the extra excess at far IR and submm wav elengths is caused by a second colder and thus more distant dust compo nent. For the post-AGB stars this component is believed to be the remn ant AGB shell. The relatively low stellar temperatures (< 10, 000 K) i n combination with the relatively large ages of the remnant AGB shell (> 10(3) yr) confirm earlier suggestions that many of these stars are very slowly evolving towards higher temperatures, as expected for low mass post-AGB stars. Evidence was found that stars may stop losing mas s and evolve off the AGB at temperatures below 5,000 K (as assumed for the Schonberner tracks) causing a slower evolution towards higher tem peratures.