Non-LTE effects on the lead and thorium abundance determinations for cool stars

Abstract

Context. Knowing accurate lead abundances of metal-poor stars provides constraints on the Pb production mechanisms in the early Galaxy. Accurately deriving thorium abundances permits a nucleo-chronometric age determination of the star. Aims. We aim to improve the calculation of the Pb i and Th ii lines in stellar atmospheres based on non-local thermodynamic equilibrium (non-LTE) line formation, and to evaluate the influence of departures from LTE on Pb and Th abundance determinations for a range of stellar parameters by varying the metallicity from the solar value down to [Fe/H] =-3. Methods. We present comprehensive model atoms for Pb i and Th ii and describe calculations of the Pb i energy levels and oscillator strengths. Results. The main non-LTE mechanism for Pb i is the ultraviolet overionization. We find that non-LTE leads to systematically depleted total absorption in the Pb i lines and accordingly, positive abundance corrections. The departures from LTE increase with decreasing metallicity. Using the semi-empirical model atmosphere HM74, we determine the lead non-LTE abundance for the Sun to be log e[subscript Pb⊙] = 2.09, in agreement with the meteoritic lead abundance. We revised the Pb and Eu abundances of the two strongly r-process enhanced stars CS 31082-001 and HE 1523-0901 and the metal-poor stellar sample. Our new results provide strong evidence of universal Pb-to-Eu relative r-process yields during the course of Galactic evolution. The stars in the metallicity range-2.3 < [Fe/H] < -1.4 have Pb/Eu abundance ratios that are, on average, 0.51 dex higher than those of strongly r-process enhanced stars. We conclude that the s-process production of lead started as early as the time when Galactic metallicity had reached [Fe/H] =-2.3. The average Pb/Eu abundance ratio of the mildly metal-poor stars, with-1.4 = [Fe/H] =-0.59, is very close to the corresponding Solar System value, in line with the theoretical predictions that AGB stas with [Fe/H] ≃ -1 provided the largest contribution to the solar abundance of s-nuclei of lead. The departures from LTE for Th ii are caused by the pumping transitions from the levels with E[subscript] < 1 eV. Non-LTE leads to weakened Th ii lines and positive abundance corrections. Overall, the abundance correction does not exceed 0.2 dex when collisions with Hi atoms are taken into account in statistical equilibrium calculations. Keywords: line: formation / nuclear reactions; nucleosynthesis; abundances / Sun: abundances / stars: abundances / stars: atmospheres