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

Abstract

Context. Spectroscopically determined lithium abundances of metal-poor turn-off dwarfs are known to be nearly constant (Spite plateau), but manifestly 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. Aims. 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. Methods. 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-local thermodynamical equilibrium calculations for neutral Li atoms are carried out in order to see how the equivalent widths and the resulting abundances are affected by these parameters. Results. 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 an apparent 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 as a result of the change in atmospheric transparency. Conclusions. 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.