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 emits strongly at far-infrared wavelengths. We interpret JCMT maps of the 800 mum emission from these three stars to indicate that they are not ringed by large reservoirs of distant orbiting dust particles that are too cold to have been detected by IRAS. We also searched for 800 mum 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 initial 3 x 10(8) yr that a star spends on the main sequence that is at least as rapid as (time)-2. Others have suggested that the particles that are observed to orbit Vega-like stars are not simply the decaying remnants of ''primordial'' disks that formed at the same time as the stars but, rather, the particulate matter is being replenished continually, most probably from objects such as comets and/or asteroids. We present arguments in support of this model. By interpreting the dusty regions as extrasolar analogs of the Kuiper comet belt region that has been 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 ratio is 1 AU from the Sun. If recent 800 mum opacities calculated by Pollack et al. (1993) are correct, then the particles with radii less than 100 mum that dominate the far-IR fluxes measured by IRAS cannot entirely account for the measured 800 mum fluxes at Vega, beta Pic, and Fomalhaut; larger particles must be present also.