2016AJ....151...46A

We present a spectroscopic analysis of the low-mass binary star system GJ 660.1AB, a pair of nearby M dwarfs for which we have obtained separated near-infrared spectra (0.9-2.5 µm) with the SpeX spectrograph. The spectrum of GJ 660.1B is distinctly peculiar, with a triangular-shaped 1.7 µm peak that initially suggests that it is a low-surface-gravity, young brown dwarf. However, we rule out this hypothesis and determine instead that this companion is a mild subdwarf (d/sdM7) based on the subsolar metallicity of the primary, [Fe/H] = -0.63±0.06. Comparison of the near-infrared spectrum of GJ 660.1B to two sets of spectral models yields conflicting results, with a common effective temperature of T_eff= 2550-2650 K, but alternately low surface gravity (log g = 4.4_–0.5^+0.5) and very low metallicity ([M/H] = -0.96_–0.24^+0.19), or high surface gravity (log g = 5.0-5.5) and slightly subsolar metallicity ([M/H] = -0.2_–0.19^+0.13). We conjecture that insufficient condensate opacity and excessive collision-induced H₂ absorption in the models bias them toward low surface gravities and a metallicity that is inconsistent with the primary and points toward improvements needed in the spectral modeling of metal-poor, very-low-mass dwarfs. The peculiar spectral characteristics of GJ 660.1B emphasize that care is needed when interpreting surface gravity features in the spectra of ultracool dwarfs.