LITHIUM LINES IN THE SPECTRA OF LATE M-DWARFS - THE EFFECTS OF CHROMOSPHERE-LIKE STRUCTURES

We study the effects of chromosphere-like structures in model atmosphe res of late M dwarfs on the formation of strong lithium resonance and subordinate lines in their spectra. Computations are performed for mod el atmospheres of the recently discovered young brown dwarfs Teide 1 a nd Calar 3. The formation of lithium lines is treated in the non-LTE a pproximation. The self-consistent system of statistical-balance and ra diative-transfer equations is solved for a 20-level lithium model atom . We show that (i) under typical conditions in the atmospheres of youn g brown dwarfs (T-eff approximate to 2700 K), the Li I lambda 6708 Ang strom resonance absorption doublet calculated by abandoning LTE is muc h less sensitive to the structure of the outer atmospheric layers than they are in the LTE case; (ii) the behavior of the source functions f or the Li I resonance lines depends on whether the blanketing effect a t the frequencies of radiative transitions of lithium is taken into ac count; and (iii) the profiles of the lithium lambda 8126 Angstrom subo rdinate doublet are essentially unaffected by the chromosphere-like st ructures in the outer atmospheres of young brown dwarfs.