Spitzer observations of the Orion OB1 association:: Second-generation dust disks at 5-10 Myr

We report new Spitzer observations of intermediate-mass stars in two regions of the Orion OB1 association located in the subassociations OB1a (similar to 10 Myr) and OB1b (similar to 5 Myr). In a representative sample of stars earlier than F5 of both stellar groups, we find a population of stars surrounded of debris disks, without excess in the IRAC bands and without emission lines in their optical spectra, but with a varying degree of 24 mu m excess. Comparing our samples with 24 mu m observations of intermediate-mass stars in other stellar groups, spanning a range of ages from 2.5 to 150 Myr, we find that debris disks are more frequent and have larger 24 mu m excess at 10 Myr (OB1a). This trend agrees with predictions of models of evolution of solids in the outer regions of disks (> 30 AU), where large icy objects (similar to 1000 km) begin to form at similar to 10Myr; the presence of these objects in the disk initiates a collisional cascade, producing enough dust particles to explain the relatively large 24 mu m excess observed in OB1a. The dust luminosity observed in the stellar groups older than 10 Myr declines roughly as predicted by collisional cascade models. Combining Spitzer observations, optical spectra, and 2MASS data, we found a new Herbig Ae/Be star (HD 290543) and a star (HD 36444) with a large 24 mu m excess, both in OB1b. This last object could be explained as a intermediate stage between HAeBe and true debris systems or as a massive debris disk produced by a collision between two large objects (> 1000 km).