Defining a Spectral Metallicity Index for T dwarfs

As infrared imaging surveys become deeper and provide multi-epoch astrometry, a growing number of low-temperature, metal-poor, high-velocity thick disk and halo brown dwarfs are being identified in the Solar Neighborhood, including the first examples of “extreme” T subdwarfs (T ≤ 1400 K, [Fe/H] ≤ -1). The discovery of these objects necessitates new tools to define metallicity classes and quantify individual source metallicities. Guided by prior development of a “zeta” metallicity index for M dwarf optical spectra, we propose an equivalent metallicity index for T dwarfs based on near-infrared spectra. We evaluate combinations of previously-defined spectral indices on low-temperature spectral models incorporating subsolar metallicities, and on a sample of known T dwarfs and subdwarfs, including companions to metal-poor stars. We show that a calibrated index combining methane and water absorption features robustly disentangles temperature and metallicity effects, and reproduces the approximate metallicities of benchmark companions. We discuss the application and limitations of this T dwarf zeta index on current observations, and propose preliminary divisions for dT, d/sdT, sdT, and esdT metallicity classes.