text: 386,173,1400478101855817730,171674815,Mark Marley,"I want to highlight our new paper, led by @exoEhsan, with @NatashaBatalha and @ChannonVisscher about Lithium in brown dwarfs: <LINK> <LINK> In low mass stars and the most massive brown dwarfs Lithium is lost to fusion in the core. Li is a little bit easier to 'burn' than H but not as easy as deuterium. So while the minimum mass to steadily burn H is around 75 Jupiter masses, the limit for Li burning is around 65 M_J. So brown dwarfs and stars older than around 250 Myr and more massive than 65 MJ do not have Li visible in their atmospheres. There is a strong Li line in the optical that makes Li detection feasible. This led Rafael Rebolo in the 90s to suggest the ""Li test"" as a way of distinguishing brown dwarfs from stars. If you could detect Li you knew you had to have a brown dwarf on your hand since it would be below 65 MJ. For objects which were hot enough to be either this was helpful If you have a cooler object, say one that has CH4, you KNOW you don't have a star on your hands, so the Li test seems less important. Also atomic Li goes into various other molecules and is removed, thus muddying the waters. However there aren't many spectral gravity indicators for brown dwarfs and it would be nice to have some marker for the most massive objects, those between ~65 MJ and ~75 MJ. Missing Li would serve nicely, but you have the chemistry removing Li as well as the nuclear fires. So the question becomes, can we detect the other Li molecules that show up at lower Teff? If so then we have a new lithium test for identifying the most massive brown dwarfs. But we need molecular opacities for species such as LiH, LiF, LiOH, and LiCl. This is where Ehsan comes in. As part of his NPP postdoc at Ames Ehsan computed himself some of these opacities and compiled others so that we could investigate the detectability of the various Li species, allowing us to follow the lithium through the brown dwarf cooling sequence You can read the paper for details, but some of these other Li species should be detectable in the mid-IR. Unfortunately the molecular signatures are more subtle than the atomic Li feature, but the 30m telescopes and perhaps JWST should be able to search for them. I'm hopeful that someday Li could be used to resolve some puzzles, such as the mass of Gl 229 B. Special thanks to Channon for handling all of the Li-species chemistry for us. Our spectra were computed with PICASO.",https://arxiv.org/abs/2106.00781,"Lithium is an important element for the understanding of ultracool dwarfs because it is lost to fusion at masses above $\sim 68\, M_{\rm J}$. Hence, the presence or absence of atomic Li has served as an indicator of the nearby H-burning boundary at about $75\,M_{\rm J}$ between brown-dwarfs and very low-mass stars. Historically the ""Lithium 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). While the Li test could in principle also 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 their cooler atmospheres. L- and T-type brown dwarfs are also 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. In this study, we generated the latest opacity of each of these Li-bearing molecules and performed thermochemical equilibrium atmospheric composition calculation of the abundance of these molecules. Finally, we computed thermal emission spectra for a series of radiative-convective equilibrium models of cloudy and cloudless brown dwarf atmospheres (with $T_{\rm eff}=$ 500--2400~K, and $\log g$=4.0, 4.5, 5.0) 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 signatures were found to be LiF at $10.5-12.5$~\micron and LiCl at $14.5-18.5$ $\micron$. LiH also shows a narrow feature at $\sim 9.38$ $\micron$. ","Following the Lithium: Tracing Li-bearing Molecules Across Age, Mass,