2007ApJ...670..516G

We have surveyed the 30 Myr old cluster NGC 2547 for planetary debris disks using Spitzer. At 4.5-8 µm we are sensitive to the photospheric level down to mid-M stars (0.2 M_☉), and at 24 µm to early G stars (1.2 M_☉). We find only two to four stars with excesses at 8 µm out of ∼400-500 cluster members, resulting in an excess fraction ≲1% at this wavelength. By contrast, the excess fraction at 24 µm is ∼40% (for B-F types). Out of four late-type stars with excesses at 8 µm two marginal ones are consistent with asteroid-like debris disks. Among stars with strong 8 µm excesses one is possibly from a transitional disk, while another one can be a result of a catastrophic collision. Our survey demonstrates that the inner 0.1-1 AU parts of disks around solar-type stars clear out very thoroughly by 30 Myr of age. Comparing with the much slower decay of excesses at 24 and 70 µm, disks clear from the inside out, of order 10 Myr for the inner zones probed at 8 µm, compared with 100 or more megayears for those probed with the two longer wavelengths.