NLTE ionization equilibrium of Nd II and Nd III in cool A and Ap stars

Abstract

The kinetic equilibrium of Nd II -Nd III in the atmospheres of A-type stars is investigated for the first time with a model atom containing 1651 levels of Nd II, 607 levels of Nd III, and the ground state of Nd, IV. NLTE leads to an overionization of Nd II resulting in weakening the Nd II lines at mild neodymium overabundances relative to the solar Nd abundance ([Nd/H] $\lt$ 2.5) and produces the opposite effect at the higher [Nd/H] values. NLTE abundance corrections grow with effective temperature and reach $\sim$0.6 dex at $\Teff$ = 9500 K. The Nd III lines are strengthened compared with LTE. NLTE abundance corrections range between -0.3 dex and -0.2 dex for $\Teff$ between 7500 K and 9500 K. Therefore NLTE effects may explain ionization discrepancies up to 0.8 – 0.9 dex, derived with the LTE approach.

NLTE effects are even larger for a stratified Nd abundance distribution compared with a homogeneous one resulting in positive NLTE abundance corrections up to 1.4 dex for the Nd II lines and negative ones as small as to -0.5 dex for the Nd III lines. The influence of uncertainty in the photoionization cross-sections on NLTE results is investigated. NLTE calculations were applied to Nd analyses in the atmospheres of roAp stars $\gamma$ Equ and HD 24712.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html