2005A&A...441..309M

We investigate the formation of NdII-III lines in the atmospheres of A-type stars with a comprehensive atomic model including 1651 levels of NdII, 607 levels of NdIII and the ground state of Ndiv. NLTE leads to overionization of NdII which weakens the NdII lines relative to the corresponding LTE line strengths at mild neodymium overabundance ([Nd/H]<2.5) and amplifies them at higher [Nd/H] values. NLTE abundance corrections grow with the effective temperature and reach 0.6dex at T_eff=9500K for [Nd/H]=2.5. The NdIII lines are strengthened compared with LTE in all cases, and NLTE abundance corrections lie between -0.3dex and -0.2dex for T_eff between 7500K and 9500K. NLTE effects are larger for an inhomogeneous vertical abundance distribution compared with a homogeneous one resulting in positive NLTE abundance correction up to 1.3dex for the NdII lines and in negative ones down to -0.5dex for the NdIII lines. The neodymium distribution in the atmospheres of roAp stars γ Equ and HD24712 is deduced from NLTE analysis of the Ndii and Ndiii lines and a strong evidence is found for the existence of enhanced Nd abundance layers above logτ₅₀₀₀=-3.