The Disk Population of the Chamaeleon I Star-forming Region

We present a census of circumstellar disks in the Chamaeleon I star-forming region. Using the Infrared Array Camera and the Multiband Imaging Photometer on board the Spitzer Space Telescope, we have obtained images of Chamaeleon I at 3.6, 4.5, 5.8, 8.0, and 24 μm. To search for new disk-bearing members of the cluster, we have performed spectroscopy on objects that have red colors in these data. Through this work, we have discovered four new members of Chamaeleon I with spectral types of M4, M6, M7.5, and L0. The first three objects are highly embedded (A_J ~ 5) and reside near known protostars, indicating that they may be among the youngest low-mass sources in the cluster (τ < 1 Myr ). The L0 source is the coolest known member of Chamaeleon I. Its luminosity implies a mass of 0.004-0.01 M_⊙, making it the least massive brown dwarf for which a circumstellar disk has been reliably detected. To characterize the disk population in Chamaeleon I, we have classified the infrared spectral energy distributions of the 203 known members that are encompassed by the Spitzer images. Through these classifications, we find that the disk fraction in Chamaeleon I is roughly constant at ~50% from 0.01 to 0.3 M_⊙. These data are similar to the disk fraction of IC 348, which is a denser cluster at the same age as Chamaeleon I. However, the disk fraction at Mgtrsim 1 M_⊙ is significantly higher in Chamaeleon I than in IC 348 (65% vs. 20%), indicating longer disk lifetimes in Chamaeleon I for this mass range. Thus, low-density star-forming regions like Chamaeleon I may offer more time for planet formation around solar-type stars than denser clusters.

Based on observations performed with the Magellan Telescopes at Las Campanas Observatory, Gemini Observatory, and the Spitzer Space Telescope. Gemini Observatory is operated by AURA under a cooperative agreement with the NSF on behalf of the Gemini partnership: the NSF (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina). Spitzer is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through contract 1256790 issued by JPL. Support for the IRAC instrument was provided by NASA through contract 960541 issued by JPL.