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Session 10 - Interstellar Medium and Star Formation.
Display session, Monday, June 08
Atlas Ballroom,
The process of planet formation in our solar system had a time scale of order 10^7 to 10^8 years according to present models. During that epoch our system was a strong emitter of thermal IR radiation from the large surface area of dust reprocessing solar flux. This second-generation dust would have been produced by collisions and activity of planetesimals stirred into crossing orbits by the growing planets. \beta Pictoris' and \alpha Lyrae's dust disks are believed to be in evolutionary states corresponding to this period of ``heavy bombardment". We used photometry from ISO at 25 and 60 \mum to compare the circumstellar environments of A stars in two young groups, the Sco-Cen association with age \sim10 Myr and IC 2602 with age \sim30 Myr. The samples of stars in the clusters were chosen to resemble each other in luminosity and temperature, and the two clusters are approximately equally distant from earth. Both groups are old enough that remnant protostellar disks with first-generation grains have most likely disappeared. We detect optically thin far-IR emission well in excess of photospheric flux around stars in both clusters, with a significant decrease in the average 60 \mum flux density between ages of 10 and 30 Myr. These data support hypotheses that stars commonly have planetesimal-debris disks after their original protostellar gas+dust disks disperse, and that the density of second-generation dust declines with time as the parent planetesimals accrete into planets.
