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

¹ Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
e-mail: lyuda@usm.lmu.de
² Institute of Astronomy, Russian Academy of Sciences, 119017 Moscow, Russia
e-mail: lima@inasan.ru
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
e-mail: afrebel@mit.edu
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA

Received: 9 January 2012
Accepted: 6 February 2012

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 ε_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 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 E_exc  <  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.