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

TL;DR

This study analyzes the low-mass binary GJ 660.1AB, revealing the secondary is a mild subdwarf with peculiar spectral features, highlighting challenges in modeling metal-poor ultracool dwarfs and interpreting their surface gravity indicators.

Contribution

It provides detailed spectroscopic characterization of GJ 660.1B, demonstrating discrepancies in spectral models and emphasizing the need for improved modeling of metal-poor, very-low mass stars.

Findings

GJ 660.1B is a mild subdwarf with [Fe/H] = -0.63

Spectral models show conflicting gravity and metallicity results

Spectral peculiarities highlight modeling challenges for metal-poor ultracool dwarfs

Abstract

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.