SEARCH FOR COOL CIRCUMSTELLAR MATTER IN THE URSAE-MAJORIS GROUP WITH ISO

We observed the mid- and far-infrared spectral energy distributions of 9 A-type stars from the 300 Myrs old Ursae Majoris group using the IS O satellite and the UKIRT telescope. We found that only 1 out of the 9 stars shows clear signature of circumstellar dust, and we derived an upper limit of 0.05 M-moon for the dust mass around the other stars. O ur results suggest that the relatively high incidence of Vega-like dis ks observed among A-type field stars in the solar neighbourhood by IRA S cannot be extrapolated to the rest of the Milky Way. The Vega phenom enon appears to be the exception rather than the rule.