The Wide Brown Dwarf Binary Oph 1622-2405 and Discovery of a Wide, Low-Mass Binary in Ophiuchus (Oph 1623-2402): A New Class of Young Evaporating Wide Binaries?

We imaged five objects near the star-forming clouds of Ophiuchus with the Keck Laser Guide Star AO system. We resolved sources 11 (Oph 16222-2405) and 16 (Oph 16233-2402) from Allers and coworkers into binary systems. Source 11 is resolved into a 243 AU binary, the widest known for a very low mass (VLM) binary. The binary nature of source 11 was discovered first by Allers and independently here, during which we obtained the first spatially resolved R~2000 near-infrared (J and K) spectra, mid-IR photometry, and orbital motion estimates. We estimate for 11A and 11B gravities (logg>3.75), ages (5+/-2 Myr), luminosities [log(L/L_solar)=-2.77+/-0.10 and -2.96+/-0.10], and temperatures (T_eff=2375+/-175 K and 2175+/-175 K). We find self-consistent DUSTY evolutionary model (Chabrier and coworkers) masses of 17⁺⁴_-5 M_J and 14⁺⁶_-5 M_J, for 11A and 11B, respectively. Our masses are higher than those previously reported (13-15 M_J and 7-8 M_J) by Jayawardhana & Ivanov. Hence, we find that the system is unlikely a ``planetary mass binary,'' as do Luhman and coworkers, but it has the second lowest mass and lowest binding energy of any known binary. Oph 11 and Oph 16 belong to a newly recognized population of wide (>~100 AU), young (<10 Myr), roughly equal mass, VLM stellar and brown dwarf binaries. We deduce that ~6%+/-3% of young (<10 Myr) VLM objects are in such wide systems. However, only 0.3%+/-0.1% of old field VLM objects are found in such wide systems. Thus, young, wide, VLM binary populations may be evaporating, due to stellar encounters in their natal clusters, leading to a field population depleted in wide VLM systems.

Based on observations made with the Keck and Gemini North telescopes.