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

¹ Institut für Astronomie und Astrophysik der UniversitätMünchen, Scheinerstr. 1, 81679 München, Germany e-mail: lyuda@usm.lmu.de;lima@inasan.ru
² Institute of Astronomy, Russian Academy of Science, Pyatnitskaya 48, 119017 Moscow, Russia
³ Institute for Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁴ Institute of Spectroscopy, Russian Academy of Sciences, 142190 Troitsk, Moscow region, Russia

Received: 18 March 2005
Accepted: 20 May 2005

Abstract

We investigate the formation of Nd ii– 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 Nd ii 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 0.6 dex at T_eff = 9500 K for [Nd/H] = 2.5. The Nd iii 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 Nd ii lines and in negative ones down to -0.5 dex for the Nd iii lines. The neodymium distribution in the atmospheres of roAp stars γ 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 .