  Spectroscopically determined Li abundances of metal-poor turn-off dwarfs are
known to be nearly constant (Spite plateau), but lower than the primordial
value expected from the standard cosmological model. However, abundance
determination by using conventional model atmospheres may not necessarily be
correct since the existence of high-temperature chromosphere even in very old
stars has been confirmed. The aim of this study is to examine how the extra UV
flux possibly irradiated from the chromosphere could affect the formation of
the Li I 6708 line, and whether or not its influence might lead to a solution
of the Li abundance discrepancy. A simple model chromosphere of a uniform thin
gray slab emitting only thermal radiation is assumed, characterized by optical
thickness and temperature. By taking into account this incident radiation in
the surface boundary condition, non-LTE calculations for Li are carried out to
see how the equivalent widths and the abundances are affected by these
parameters. If the parameters are appropriately chosen, the strength of the Li
I 6708 line can be reduced by a factor of ~2-3 due to overionization caused by
enhanced 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 observed
slight metallicity-dependent trend of the plateau can also be reproduced. Thus,
superficial underestimation of Li abundances due to considerable overionization
caused by chromospheric radiation may be regarded as a ponderable
interpretation for the cosmological Li problem. The touchstone to verify this
model would be to check the existence of significantly enhanced UV radiation in
these Spite plateau stars, which should be detected if this scenario is valid,
although very few such UV spectrophotometric observations have been done to
date.
