Abstract

Binary properties are an important diagnostic of the star and brown dwarf formation processes. While wide binaries appear to be rare in the substellar regime, recent observations have revealed Ophiuchus 162225-240515 (2MASS J16222521 - 2405139) as a likely young ultra-low-mass binary with an apparent separation of ≈240 AU. Here we present low-resolution near-infrared spectra of the pair from NTT/SOFI (R ∼ 600) and VLT/ISAAC (R ∼ 1400), covering the 1.0-2.5 μm spectral region. By comparing them to model atmospheres from Chabrier & Baraffe and Burrows et al., we confirm the surface temperatures to be TA = 2350 ± 150 K and TB = 2100 ± 100 K for the two components of the binary, consistent with earlier estimates from optical spectra. Using gravity-sensitive K I features, we find the surface gravity to be significantly lower than field dwarfs of the same spectral type, providing the best evidence so far that these objects are indeed young. However, we find that models are not sufficiently reliable to infer accurate ages/masses from surface gravity. Instead, we derive masses of MA = 13-4 +8MJ and MB = 10-4 +5 MJ for the two objects using the well-constrained temperatures and assuming an age of 1-10 Myr, consistent with the full range of ages reported for the Oph region. © 2006. The American Astronomical Society. All rights reserved.