A Comprehensive View of Circumstellar Disks in Chamaeleon I: Infrared Excess, Accretion Signatures, and Binarity

Abstract

We present a comprehensive study of disks around 81 young low-mass stars and\nbrown dwarfs in the nearby ~2-Myr-old Chamaeleon I star-forming region. We use\nmid-infrared photometry from the Spitzer Space Telescope, supplemented by\nfindings from ground-based high-resolution optical spectroscopy and adaptive\noptics imaging. We derive disk fractions of 52 (+/-6) % and 58 (+6/-7) % based\non 8-micron and 24-micron colour excesses, respectively, consistent with those\nreported for other clusters of similar age. Within the uncertainties, the disk\nfrequency in our sample of K3-M8 objects in Cha I does not depend on stellar\nmass. Diskless and disk-bearing objects have similar spatial distributions.\nThere are no obvious transition disks in our sample, implying a rapid timescale\nfor the inner disk clearing process; however, we find two objects with weak\nexcess at 3-8 microns and substantial excess at 24 microns, which may indicate\ngrain growth and dust settling in the inner disk. For a sub-sample of 35\nobjects with high-resolution spectra, we investigate the connection between\naccretion signatures and dusty disks: in the vast majority of cases (29/35) the\ntwo are well correlated, suggesting that, on average, the timescale for gas\ndissipation is similar to that for clearing the inner dust disk. The exceptions\nare six objects for which dust disks appear to persist even though accretion\nhas ceased or dropped below measurable levels. Adaptive optics images of 65 of\nour targets reveal that 17 have companions at (projected) separations of 10-80\nAU. Of the five <20 AU binaries, four lack infrared excess, possibly indicating\nthat a close companion leads to faster disk dispersal. The closest binary with\nexcess is separated by ~20 AU, which sets an upper limit of ~8 AU for the outer\ndisk radius. (abridged)\n