Spitzer 24 μm Observations of Open Cluster IC 2391 and Debris Disk Evolution of FGK Stars

We present 24 � m Spitzer MIPS photometric observations of the � 50 Myr open cluster IC 2391. Thirty-four cluster members ranging in spectral type from B3 to M5 were observed in the central square degree of the cluster. Excesses indicative of debris disks were discovered around one A star, six FGK stars, and possibly one M dwarf. For the cluster members observed to their photospheric limit, we find a debris disk frequency of 10 þ17 � 3 % for B‐A stars and 31 þ13 � 9 % for FGK stars using a 15% relative excess threshold. Relative to a model of decaying excess frequency, thefrequencyofdebrisdisksaroundA-typestarsappearsmarginallylowforthecluster’sagewhilethatof FGKstars appears consistent. Scenarios that may qualitatively explain this result are examined. We conclude that planetesimal activity in the terrestrial region of FGK stars is common in thefirst � 50 Myr and decays on timescales of � 100 Myr. Despite luminosity differences, debris disk evolution does not appear to depend strongly on stellar mass. Subject headingg infrared: stars — open clusters and associations: individual (IC 2391) — planetary systems: protoplanetary disks