Following the Lithium: Tracing Li-bearing Molecules across Age, Mass, and Gravity in Brown Dwarfs

Lithium is an important element for the understanding of ultracool dwarfs because it is lost to fusion at masses above ∼68MJ. Hence, the presence of atomic Li has served as an indicator of the nearby H-burning boundary at about 75MJ between brown dwarfs and very low mass stars. Historically, the “lithium test,” a search for the presence of the Li line at 670.8 nm, has been a marker if an object has a substellar mass. While the Li test could, in principle, be used to distinguish masses of later-type L–T dwarfs, Li is predominantly no longer found as an atomic gas but rather a molecular species such as LiH, LiF, LiOH, and LiCl in cooler atmospheres. The L- and T-type dwarfs are quite faint at 670 nm and thus challenging targets for high-resolution spectroscopy. But only recently have experimental molecular line lists become available for the molecular Li species, allowing molecular Li mass discrimination. Here we generated the latest opacity of these Li-bearing molecules and performed a thermochemical equilibrium atmospheric composition calculation of their abundances. Finally, we computed thermal emission spectra for a series of radiative–convective equilibrium models of cloudy and cloudless brown dwarf atmospheres (with Teff = 500–2400 K and log g = 4.0–5.0) to understand where the presence of atmospheric lithium-bearing species is most easily detected as a function of brown dwarf mass and age. After atomic Li, the best spectral signatures were found to be LiF at 10.5–12.5 μm and LiCl at 14.5–18.5 μm. Also, LiH shows a narrow feature at ∼9.38 μm.