A Survey for Planetary-mass Brown Dwarfs in the Chamaeleon I Star-forming Region

We have performed a search for planetary-mass brown dwarfs in the Chamaeleon. I star-forming region using proper motions and photometry measured from optical and infrared images from the Spitzer Space Telescope, the Hubble Space Telescope, and ground-based facilities. Through near-IR spectroscopy at Gemini Observatory, we have confirmed six of the candidates as new late-type members of Chamaeleon I (>= M8). One of these objects, Cha J11110675-7636030, has the faintest extinction-corrected M-K among known members, which corresponds to a mass of 3-6 M-Jup according to evolutionary models. That object and two other new members have redder mid-IR colors than young photospheres at. <= M9.5, which may indicate the presence of disks. However, since those objects may be later than M9.5 and the mid-IR colors of young photospheres are ill-defined at those types, we cannot determine conclusively whether color excesses from disks are present. If Cha J11110675-7636030 does have a disk, it would be a contender for the least-massive known brown dwarf with a disk. Since the new brown dwarfs that we have found extend below our completeness limit of 6-10. M-Jup, deeper observations are needed to measure the minimum mass of the initial mass function in Chamaeleon. I.