Abstract
We investigate the formation of NdII-III lines in the atmospheres of A-type stars with a comprehensive atomic model including 1651 levels of Nd II, 607 levels of Nd III and the ground state of Nd IV. NLTE leads to overionization of Nd II which weakens the NdII lines relative to the corresponding LTE line strengths at mild neodymium overabundance ([Nd/H] < 0.6 and amplifies them at higher [Nd/H] values. NLTE abundance corrections grow with the effective temperature and reach 0.6 dex at T eff = 9500 K for [Nd/H] = 2.5. The NdIII lines are strengthened compared with LTE in all cases, and NLTE abundance corrections lie between -0.3 dex and -0.2 dex for T eff between 7500 K and 9500 K. NLTE effects are larger for an inhomogeneous vertical abundance distribution compared with a homogeneous one resulting in positive NLTE abundance correction up to 1.3 dex for the NdII lines and in negative ones down to -0.5 dex for the Nd III lines. The neodymium distribution in the atmospheres of roAp stars y Equ and HD 24712 is deduced from NLTE analysis of the Nd II and Nd III lines and a strong evidence is found for the existence of enhanced Nd abundance layers above log τ 5000 = -3.
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