Spectroscopy of Young Planetary Mass Candidates with Disks

It is now well established that many young brown dwarfs exhibitcharacteristics similar to classical T Tauri stars, including infrared excessfrom disks and emission lines related to accretion. Whether the same holds truefor even lower mass objects, namely those near and below the Deuterium-burninglimit, is an important question. Here we present optical spectra of sixisolated planetary mass candidates in Chamaeleon II, Lupus I and Ophiuchusstar-forming regions, recently identified by Allers and collaborators to harborsubstantial mid-infrared excesses. Our spectra, from ESO's Very Large Telescopeand New Technology Telescope, show that four of the targets have spectral typesin the ~M9-L1 range, and three of those also exhibit H_alpha. Theirluminosities are consistent with masses of ~5-15 M_{Jupiter} according tomodels of Chabrier, Baraffe and co-workers, thus placing these four objectsamong the lowest mass brown dwarfs known to be surrounded by circum-sub-stellardisks. Our findings bolster the idea that free-floating planetary mass objectscould have infancies remarkably similar to those of Sun-like stars and suggestthe intriguing possibility of planet formation around primaries whose massesare comparable to those of extra-solar giant planets. Another target appears tobe a brown dwarf (~M8) with prominent H_alpha emission, possibly arising fromaccretion. The sixth candidate is likely a background source, underlining theneed for spectroscopic confirmation.Comment: to appear in The Astrophysical Journal Letter