Characterization of the Very Low Mass Secondary in the GJ 660.1AB System

We present 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 $\mu$m) with the SpeX spectrograph. The spectrum of GJ 660.1B is distinctly peculiar, with a triangular-shaped 1.7 $\mu$m peak that initially suggests it to be 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] = $-$0.63$\pm$0.06. Comparison of the near-infrared spectrum of GJ 660.1B to two sets of spectral models yields conflicting results, with a common effective temperature T$_{eff}$ = 2550-2650 K, but alternately low surface gravity ($\log{g}$ = 4.4$\pm$0.5 cgs) and very low metallicity ([M/H] = $-$0.96$^{+0.19}_{-0.24}$), or high surface gravity ($\log{g}$ = 5.0-5.5) and slightly subsolar metallicity ([M/H] =$-$0.20$^{+0.13}_{-0.19}$). We conjecture that insufficient condensate opacity and excessive collision induced H$_2$ absorption in the models bias them toward low surface gravities and a metallicity 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.