Discovery of Young Methane Dwarfs in the Rho Ophiuchi L 1688 Dark Cloud

We report the discovery of two methane dwarfs in the dark cloud L 1688 of the Rho Oph star-forming region. The two objects were among the T dwarf candidates with possible methane absorption and cool atmospheres, as diagnosed by infrared colors using deep WIRCam/CFHT HK plus CH4ON images, and IRAC/Spitzer c2d data. Follow-up spectroscopic observations with the FLAMINGOS-2/Gemini South confirmed the methane absorption at 1.6 micron. Compared with spectral templates of known T dwarfs in the field, i.e., of the old populations, Oph J162738-245240 (Oph-T3) is a T0/T1 type, whereas Oph J162645-241949 (Oph-T17) is consistent with a T3/T4 type in the H band but an L8/T1 in the K band. Compared with the BT-Settl model, both Oph-T3 and Oph-T17 are consistent with being cool, ~ 1000 K and ~ 900 K, respectively, and of low surface gravity, log(g) = 3.5. With an age no more than a couple Myr, these two methane dwarfs thereby represent the youngest T dwarfs ever confirmed. A young late L dwarf, OphJ162651-242110, was found serendipitously in our spectroscopic observations.