Non-LTE effects on the lead and thorium abundance determinations for cool stars
1 Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
2 Institute of Astronomy, Russian Academy of Sciences, 119017 Moscow, Russia
3 Institute of Spectroscopy, Russian Academy of Sciences, 142190 Troitsk, Moscow region, Russia
4 Massachussetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
5 Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA
Received: 9 January 2012
Accepted: 6 February 2012
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 εPb, ⊙ = 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 stars 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 Eexc < 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 H i atoms are taken into account in statistical equilibrium calculations.
Key words: line: formation / nuclear reactions, nucleosynthesis, abundances / Sun: abundances / stars: abundances / stars: atmospheres
© ESO, 2012