1. L. I. Mashonkina, T.A. Ryabchikova Institute of Astronomy RAS A. N. Ryabtsev Institute of Spectroscopy RAS NLTE ionization equilibrium of Nd II and Nd III in cool A and Ap stars.

2. Outline of the talk 1. Short Introduction 2. Models of Nd II/Nd III atoms and method of NLTE calculations 3. NLTE corrections 4. Application to Ap stars 5. Uncertainty of NLTE calculations

3. Term structure Nd II (1651 levels) Nd III (607 levels)

4. NLTE line formation Accelerated Lambda Iteration method, realized in the code DETAIL (Munich University) was used. The final model atom includes : 247 Nd II levels + 68 Nd III levels + Nd IV ground state. Energy levels : NIST, Blaise et al. (1984), our calc. Oscillator strengths: VALD + our calculations. Photoionization cross-sections: hydrogenic Collisional cross-sections: van Regemorter (1962) (allowed trans.), Ω = 1. (forbid. Trans.)

5. NLTE effects on Nd II and Nd III populations

6. NLTE corrections for Nd II and Nd III lines

7. Nd distribution roAp star γ Equ roAp star γ Equ b-factors NLTE Nd calculations in the atmosphere of

8. Comparison between the observed and computed line profiles. Nd II 5319 --- top Nd III 5294 --- bottom Observations -- black line, LTE calculations in stratified atmosphere -- blue dashed line NLTE calculations in stratified atmosphere -- red line roAp star γ Equ

9. Uncertainty of NLTE calculations NLTE abundance correction variations : photoionization cross-sections (stratified Nd) σ(ph-ion) : Δ NLTE = 1.07 - 1.31 (Nd II) and Δ NLTE = (-0.42) – (-0.48) (Nd III) σ(ph-ion) / 100 → NLTE effects decrease : Δ NLTE = 0.56 - 0.80 (Nd II) and Δ NLTE = (-0.26) – (-0.34) (Nd III) σ(ph-ion) x 100 → NLTE effects increase : Δ NLTE = 1.16 - 1.56 (Nd II) and Δ NLTE = (-0.41) – (-0.50) (Nd III) collisional cross-sections (homogeneous Nd) σ(coll) : Δ NLTE = 0.07 – (-0.04) (Nd II) and Δ NLTE = (-0.27) – (-0.31) (Nd III) σ(coll) x 10 → NLTE effects decrease : Δ NLTE = 0.05 – (-0.02) (Nd II) and Δ NLTE = (-0.17) – (-0.20) (Nd III)

10. Acknowledgements We are very grateful to : Prof. Thomas Gehren for providing the codes DETAIL and MAFAGS Presidium RAS Programme ''Nonstationary phenomena in astronomy'', RFBR grant number 04-02-16788, the Russian Federal Programme ''Astronomy'' for partial financial support. All NLTE calculations were performed using eridani computer of the Institute of Astronomy and Astrophysics of Munich University.