Decay of planetary debris disks

We report new Spitzer 24 mum photometry of 76 main-sequence A-type stars. We combine these results with previously reported Spitzer 24 mum data and 24 and 25 mum photometry from the Infrared Space Observatory and the Infrared Astronomy Satellite. The result is a sample of 266 stars withmass close to 2.5 M-., all detected to at least the similar to 7 sigma level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K - 24 ( or 25) mum photospheric color and then compared all stars in the sample to that color. Because we have excluded stars with strong emission lines or extended emission ( associated with nearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emission more frequently and with higher fractional excess than do the older stars. However, as many as 50% of the younger stars do not show excess emission. The decline in the magnitude of excess emission, for those stars that show it, has a roughly t(0)/ time dependence, with t(0) similar to 150 Myr. If anything, stars in binary systems ( including Algoltype stars) and lambda Boo stars show less excess emission than the other members of the sample. Our results indicate that ( 1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10 - 60 AU region and ( 2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion end game, and that individual events often dominate the radiometric properties of a debris system. This latter behavior agrees generally with what we know about the evolution of the solar system, and also with theoretical models of planetary system formation.