Debris Disks in Open Stellar Clusters

Abstract

Indirect searches for planets (such as radial velocity studies)show that their formation may be quite common. The planets are however too small and faint to be seen against the glare of their host stars; therefore, their direct detectionis limited to the nearest systems. Alternatively one can study planets by studying their "by-product" -- dust. We see raw material available for planets around young stars, anddebris dust around old stars betraying planet-induced activity. Dust has a larger surface area per unit mass compared with a large body; it can be spread over a largersolid angle, intercepting more starlight and emitting much more lightvia reprocessing. By studying dusty disks we can infer the presence of planets at larger distances.Here we present results of a survey conducted with the SpitzerSpace Telescope of debrisdisks in three open clusters. With ages of 30--100 Myrs, these clusters are old enough that the primordialdust should have accreted into planetesimals, fallen onto the star, or been blown away due to a numberof physical processes. The dust we observe must come from collisions or sublimation of larger bodies.The purpose of this study is to investigate the dustevolution in the terrestrial planet zone, analogous to the Zodiacal cloud in our Solar system. We are most sensitive to this zone becausethe peak of a 125 K black body radiation falls into the primary pass-band of our survey -- 24 micron. We investigate the fraction and amount of the infra-red excesses around intermediate- to solar-mass stars in open stellar clusterswith well defined ages. The results are analyzed in the context of disk studies at other wavelengths and ages, providing an understanding of the time-scale for diskdissipation and ultimately planet building and frequency.