A study of rare earth elements in the atmospheres of chemically peculiar stars. PrIII and NdIII lines

We determine the abundances of Pr and Nd in the atmospheres of magnetic and non-magnetic chemically peculiar stars from the lines of rare earth elemen ts in the first and second ionization states. The computations for the magn etic stars take into account the influence of the magnetic field on line fo rmation. We studied the influence of errors in the stellar-atmosphere param eters and the atomic parameters of the spectral lines on the accuracy of ab undance determinations. Within the derived accuracy, ionization equilibrium is satisfied in the atmospheres of non-pulsating magnetic and non-magnetic stars (so that abundances derived separately from lines of first and secon d ions agree). For all the pulsating magnetic (roAp) stars studied, the abu ndances derived from lines of second ions are 1.0 to 1.7 dex higher than th ose derived from first ions. The violation of ionization equilibrium in the atmospheres of pulsating stars is probably due to, first, considerable enr ichment of Pr and Nd in the uppermost atmospheric layers, and second, a hig her location for the layer of enhanced elemental abundance in roAp stars th an in non-pulsating stars. Two objects from the list of non-pulsating magne tic stars, HD 62 140 and HD 115 708, exhibit anomalies of their Pr and Nd l ines characteristic of roAp stars. The differences in the rare earth anomal ies for the pulsating and non-pulsating peculiar stars can be used as a sel ection criterion for candidate roAp stars. (C) 2001 MAIK "Nauka/Interperiod ica ".