Silicate Dust in Evolved Protoplanetary Disks: Growth, Sedimentation, and Accretion

We present the Spitzer IRS spectra for 33 young stars in Tr 37 and NGC 7160.The sample includes the high- and intermediate-mass stars with MIPS 24 micronsexcess, the only known active accretor in the 12 Myr-old cluster NGC 7160, and19 low-mass stars with disks in the 4 Myr-old cluster Tr 37. We examine the 10microns silicate feature, present in the whole sample of low-mass star and in 3of the high- and intermediate-mass targets, and we find that PAH emission isdetectable only in the Herbig Be star. We analyze the composition and size ofthe warm photospheric silicate grains by fitting the 10 microns silicatefeature, and study the possible correlations between the silicatecharacteristics and the stellar and disk properties (age, SED slope, accretionrate, spectral type). We find indications of dust settling with age and of theeffect of turbulent enrichment of the disk atmosphere with large grains.Crystalline grains are only small contributors to the total silicate mass inall disks, and do not seem to correlate with any other property, except maybebinarity. We also observe that spectra with very weak silicate emission are atleast 3 times more frequent among M stars than among earlier spectral types,which may be an evidence of inner disk evolution. Finally, we find that 5 ofthe high- and intermediate-mass stars have SEDs and IRS spectra consistent withdebris disk models involving planet formation, which could indicate debris diskformation at ages as early as 4 Myr.Comment: 54 pages, 21 figure