[N/C] Value as an Indicator of the Red Giant Evolution: Observed Differences between Magnetic and Non-Magnetic Giants

Abstract

Data concerning the [N/C] value, one of the most sensitive indicators of stellar evolution, have been analyzed for 20 red giants with magnetic fields in comparison with similar data for seven non-magnetic giants. For the most giants in question, the [N/C] value has showed a dependence on age \(t\) and mass \(M\). In particular, the [N/C] value for the most of magnetic giants decreases from 1.2 to 0.9 when \(\log t\) increases from 8.2 to 9.8 and \(M\) increases from 1 to 4 \({{M}_{ \odot }}\). For the most of magnetic giants, the [N/C] values have been shown to be lowered (up to 0.4 dex) as compared with non-magnetic giants with the same values of \(t\) or \(M\). Such differences in the [N/C] values between these two groups of stars have been supposed to be explained by different ways of stellar evolution of red giants with magnetic fields (on the main sequence (MS) stage, the fields could be strong) and giants without magnetic fields. In particular, the magnetic field can suppress the mixing process at the MS stage already and especially in the First Dredge-Up (FDU) phase, which leads to the [N/C] lowering in magnetic giants.