Follow the Lithium: The Correlation Between Li-bearing Molecules with 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 68 MJup. Hence, the presence or absence of atomic Li has served as an indicator of the nearby H-burning boundary at about 75 MJup 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). However, the Li test is not a reliable and straightforward probe to determine the mass and gravity of the ultracool object. In addition, for later T and Y type brown dwarfs, Li is no longer found as an atomic gas, but rather a molecular speciessuch as LiH, LiF, LiOH, and LiCl. Only recently have experimental 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. Then, thermochemical equilibrium atmospheric composition was performed to calculate the abundance of these molecules in this same temperature and pressure domain. Finally, we compute thermal emission spectra for a series of radiative-convective equilibrium modeling on cloudy and cloud-free brown dwarf atmospheres (with Teff=1000, 1500, 2000, 2400K) 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 LiF at 10.5-13 micron and LiCl at 15-18 micron. We predict that future missions such as James Webb Space telescopes could detect these LiF and LiCl spectral features. The detection of lithium compounds in cool objects would identify those with masses below the Lithium burning minimum mass, providing a new mass indicator for cool brown dwarfs.