CHARACTERIZATION OF THE VERY-LOW-MASS SECONDARY IN THE GJ 660.1AB SYSTEM

Abstract

ABSTRACT 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] = − 0.20 − 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.