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Spectroscopically determined Li abundances of metal-poor turn-off dwarfs areknown to be nearly constant (Spite plateau), but lower than the primordialvalue expected from the standard cosmological model. However, abundancedetermination by using conventional model atmospheres may not necessarily becorrect since the existence of high-temperature chromosphere even in very oldstars has been confirmed. The aim of this study is to examine how the extra UVflux possibly irradiated from the chromosphere could affect the formation ofthe Li I 6708 line, and whether or not its influence might lead to a solutionof the Li abundance discrepancy. A simple model chromosphere of a uniform thingray slab emitting only thermal radiation is assumed, characterized by opticalthickness and temperature. By taking into account this incident radiation inthe surface boundary condition, non-LTE calculations for Li are carried out tosee how the equivalent widths and the abundances are affected by theseparameters. If the parameters are appropriately chosen, the strength of the LiI 6708 line can be reduced by a factor of ~2-3 due to overionization caused byenhanced UV radiation, leading to a lowering of the abundance by ~0.3-0.5 dex,which is consistent with the discrepancy in question. Moreover, the observedslight metallicity-dependent trend of the plateau can also be reproduced. Thus,superficial underestimation of Li abundances due to considerable overionizationcaused by chromospheric radiation may be regarded as a ponderableinterpretation for the cosmological Li problem. The touchstone to verify thismodel would be to check the existence of significantly enhanced UV radiation inthese Spite plateau stars, which should be detected if this scenario is valid,although very few such UV spectrophotometric observations have been done todate.

Possibility of chromospheric back-radiation influencing the lithium line formation in Spite plateau stars