THE INCIDENCE OF DEBRIS DISKS AT 24μm AND 670 Myr

We use Spitzer Space Telescope 24 mu m data to search for debris disks among 122 AFGKM stars from the similar to 670 Myr clusters Hyades, Coma Ber, and Praesepe, utilizing a number of advances in data reduction and determining the intrinsic colors of main-sequence stars. For our sample, the 1 sigma dispersion about the main-sequence V-K-S, K-S-[24] locus is approximately 3.1%. We identify seven debris disks at 10% or more (>= 3 sigma confidence level) above the expected K-S-[24] for purely photospheric emission. The incidence of excesses of 10% or greater in our sample at this age is 5.7(-1.7)(+3.1)%. Combining with results from the literature, the rate is 7.8(-2.1)(+4.2)% for early-type (B9-F4) stars and 2.7(-1.7)(+3.3)% for solar-like (F5-K9) stars. Our primary sample has strict criteria for inclusion to allow comparison with other work; when we relax these criteria, three additional debris disks are detected. They are all around stars of solar-like type and hence reinforce our conclusion that disks around such stars are still relatively common at 670 Myr and are similar to the rate around early-type stars. The apparently small difference in decay rates between early-type and solar-like stars is inconsistent with the first-order theoretical predictions that the later type stellar disks would decay an order of magnitude more quickly than the earlier type ones.