Follow the Lithium: Signatures of Li-bearing Molecules with Age and Mass in Brown Dwarfs

Lithium is an important element for understanding of ultracool dwarfs because it is lost to fusion at masses above 70 M_Jup. Hence, the presence or absence of atomic Li has served as an indicator of the nearby H-burning boundary at about 75 M_Jup between brown-dwarfs and very low-mass stars. Historically the “Li test”, a search for the presence and strength of the Li line at 670.8 nm, has been a marker if an object has a substellar mass with stellar-like spectral energy distribution (e.g., a late-type M dwarf). With the explosion of brown dwarfs with spectral types L, T, and Y the classical Li-test is less applicable as it is obvious most of these objects are substellar. Despite this, the presence or absence of Li-bearing species can still serve as a mass indicator, distinguishing brown dwarfs that are more or less massive than the Li-burning limit. However, for later L and T type brown dwarfs Li is no longer found as an atomic gas, but rather a molecular species such as LiH, LiF, LiOH, and LiCl. Only recently have molecular line lists become available for such species. In this study, we generated a full spectral absorption cross-section of each of these Li-bearing molecules from the most recent line-lists for 75-4000 K and 10^-6-3000 bar, respectively. Then, thermochemical equilibrium atmospheric composition was computed to understand the abundance of these molecules in this same temperature and pressure domain, Finally, we compute thermal emission spectra for a series of cloudy radiative-convective equilibrium brown dwarf atmosphere models to understand where the presence or absence of atmospheric lithium-bearing species is most easily detected as a function of brown dwarf mass and age. After atomic Li, the best spectral signature was found to be LiCl. Our poster demonstrate how, by following the lithium, the most massive brown dwarfs can be recognized.