Spitzer 24 mu m observations of open cluster IC 2391 and debris disk evolution of FGK stars

We present 24 mu m Spitzer MIPS photometric observations of the similar to 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(-3)(+17)% for B - A stars and 31(-9)(+31)% for FGK stars using a 15% relative excess threshold. Relative to a model of decaying excess frequency, the frequency of debris disks around A-type stars appears marginally low for the cluster's age while that of FGK stars 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 the first similar to 50 Myr and decays on timescales of similar to 100 Myr. Despite luminosity differences, debris disk evolution does not appear to depend strongly on stellar mass.