FORMATION OF LITHIUM LINES IN VERY COOL DWARFS

We present LTE and NLTE results on the formation of Li I lines (lambda 6103, lambda 6708, and lambda 8126) in the atmospheres of solar metal licity dwarfs with effective temperatures in the range 5500 K to 2000 K. NLTE effects are governed by overionization of Li and by the interl ocking effects of energy levels. For stars with T-eff greater than or equal to 4000 K, we confirm previous findings by Magazzu et al. (1992) . NLTE corrections can lower the LTE Li abundances derived from strong Li lambda 6708 lines by up to 0.5 dex. Our computations using model a tmospheres with T-eff between 3000 K and 2000 K show that prominent Li I lines are formed. We give a set of line profiles, which support the feasibility of the Li test for brown dwarfs. The ionization-dissociat ion equilibrium for Li species was carefully considered. NLTE effects on the Li I lines of very cool dwarfs are found to be small, implying corrections to the LTE Li abundances lower than 0.1 dex. Several numer ical tests have been carried out to estimate the effects of chromosphe re-like structures on the formation of LiI lines. Our preliminary resu lts suggest that in the presence of very strong chromospheres, the lin e strengths are reduced.