SUBMILLIMETER STUDIES OF MAIN-SEQUENCE STARS

Vega, Fomalhaut, and Beta Pictoris are the most prominent examples of main-sequence stars surrounded by orbiting particulate matter which em its strongly at far-infrared wavelengths. We interpret JCMT maps of th e 800 mum emission from these three stars to indicate that they are no t ringed by large reservoirs of distant orbiting dust particles that a re too cold to have been detected by IRAS. We also searched for 800 mu m emission from stars in the Pleiades aid Ursa Majoris open clusters. In comparison with the mass of dust particles near T Tauri and Herbig Ae stars, the JCMT data indicate a decline in dust mass during the ini tial 3 x 10(8) yr that a star spends on the main sequence that is at l east as rapid as (time)-2. Others have suggested that the particles th at are observed to orbit Vega-like stars are not simply the decaying r emnants of ''primordial'' disks that formed at the same time as the st ars but, rather, the particulate matter is being replenished continual ly, most probably from objects such as comets and/or asteroids. We pre sent arguments in support of this model. By interpreting the dusty reg ions as extrasolar analogs of the Kuiper comet belt region that has be en proposed to lie between 20 and 50 AU from the Sun, we estimate that in the Kuiper belt the ratio of total mass carried by small particles to that carried by comets is orders of magnitude smaller than this ra tio is 1 AU from the Sun. If recent 800 mum opacities calculated by Po llack et al. (1993) are correct, then the particles with radii less th an 100 mum that dominate the far-IR fluxes measured by IRAS cannot ent irely account for the measured 800 mum fluxes at Vega, beta Pic, and F omalhaut; larger particles must be present also.