Twenty red giants with magnetic fields: a detailed analysis of their chemical composition

Abstract

ABSTRACT We determined the fundamental parameters and analysed the chemical composition for 20 G and K giants within 150 pc from the Sun, for which the magnetic fields Bmax from 0.3 to 98.6 G have been detected. Abundances of 18 chemical elements from lithium (Z = 3) to hafnium (Z = 72) are derived. From infrared CN molecular lines, the nitrogen abundance and the carbon isotope ratio 12C/13C are determined. The low values 12C/13C = 7–26 for most of the giants prove that they were passed through the deep convecting mixing during the first dredge-up (FDU) evolutionary phase. Lithium is detected in spectra of 16 of 20 giants studied. The presence of lithium in atmospheres of giants passed through the deep mixing during the FDU phase contradicts the theory. The pronounced correlation between the [N/C] and [N/O] values is found, which has an evolutionary nature. A comparison of this relation with theoretical models calculated for rotating stars shows that it reflects mainly the dependence of these two values on the initial rotational velocity. The total C + N + O abundance that should be constant during the star’s evolution is found to be correlated with [Fe/H]. This correlation can be explained partially by the known [O/Fe] versus [Fe/H] relation. The anticorrelation between the [El/Fe] and [Fe/H] values for rare earth (RE) elements is obtained. The relations for both the sum C + N + O and the RE elements reflect the initial chemical composition of the giants. No marked systematic differences in the element abundances are found between the magnetic and non-magnetic giants.