THE LONG-LIVED DISKS IN THE η CHAMAELEONTIS CLUSTER

Abstract

We present IRS spectra and revised MIPS photometry for the 18 members of the\nEta Chamaeleontis cluster. Aged 8 Myr, the Eta Cha cluster is one of the few\nnearby regions within the 5-10 Myr age range, during which the disk fraction\ndecreases dramatically and giant planet formation must come to an end. For the\n15 low-mass members, we measure a disk fraction ~50%, high for their 8 Myr age,\nand 4 of the 8 disks lack near-IR excesses, consistent with the empirical\ndefinition of "transition'' disks. Most of the disks are comparable to\ngeometrically flat disks. The comparison with regions of different ages\nsuggests that at least some of the "transition" disks may represent the normal\ntype of disk around low-mass stars. Therefore, their flattened structure and\ninner holes may be related to other factors (initial masses of the disk and the\nstar, environment, binarity), rather than to pure time evolution. We analyze\nthe silicate dust in the disk atmosphere, finding moderate crystalline\nfractions (~10-30%) and typical grain sizes ~1-3 micron, without any\ncharacteristic trend in the composition. These results are common to other\nregions of different ages, suggesting that the initial grain processing occurs\nvery early in the disk lifetime (<1 Myr). Large grain sizes in the disk\natmosphere cannot be used as a proxy for age, but are likely related to higher\ndisk turbulence. The dust mineralogy varies between the 8-12micron and the\n20-30 micron features, suggesting high temperature dust processing and little\nradial mixing. Finally, the analysis of IR and optical data on the B9 star Eta\nCha reveals that it is probably surrounded by a young debris disk with a large\ninner hole, instead of being a classical Be star.\n