2MASS 22344161+4041387AB: A Wide, Young, Accreting, Low-mass Binary in the LkHa233 Group

Abstract

We report the discovery of a young, 0.16" binary, 2M2234+4041AB, found as the result of a Keck laser guide star adaptive optics imaging survey of young field ultracool dwarfs. Spatially resolved near-infrared photometry and spectroscopy indicate that the luminosity and temperature ratios of the system are near unity. From optical and near-infrared spectroscopy, we determine a composite spectral type of M6 for the system. Gravity-sensitive spectral features in the spectra of 2M2234+4041AB are best matched to those of young objects (~1 Myr old). A comparison of the Teff and age of 2M2234+4041AB to evolutionary models indicates the mass of each component is 0.10 (+0.075-0.04) Msun. Emission lines of H alpha in the composite optical spectrum of the system and Br gamma in spatially resolved near-IR spectra of the two components indicate that the system is actively accreting. Both components of the system have IR excesses, indicating that they both harbor circumstellar disks. Though 2M2234+4041AB was originally identified as a young field dwarf, it lies 1.5' from the well-studied Herbig Ae/Be star, LkHa 233. The distance to LkHa 233 is typically assumed to be 880 pc. It is unlikely 2M2234+4041AB could be this distant, as it would then be more luminous than any known Taurus objects of similar spectral type. We re-evaluate the distance to the LkHa 233 group and find a value of 325 (+72-50) pc, based on the Hipparcos distance to a nearby B3-type group member (HD 213976). 2M2234+4041AB is the first low-mass star to be potentially associated with the LkHa 233 group. At a distance of 325 pc, its projected physical separation is 51 AU, making it one of a growing number of wide, low-mass binaries found in young star-forming regions.

37 pages ApJ accepted

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