New debris-disk candidates: 24 micron stellar excesses at 100 million years

Sixty-three members of the 100 Myr old open cluster M47 (NGC 2422) have been detected at 24 mum with the Spitzer Space Telescope. The Be star V 378 Pup shows an excess both in the near-infrared and at 24 mum (K-[24]=2.4 mag), probably due to free-free emission from the gaseous envelope. Seven other early-type stars show smaller excesses, K-[24]=0.6-0.9. Among late-type stars, two show large excesses: P922, a K1 V star with K-[24]=1.08+/-0.11, and P1121, an F9 V star with K-[24]-3.72+/-0.02. P1121 is the first known main-sequence star showing an excess comparable to that of beta Pic, which may indicate the presence of an exceptionally massive debris disk. It is possible that a major planetesimal collision has occurred in this system, consistent with the few hundred Myr timescales estimated for the clearing of the solar system.