The [N/C] value as an indicator of the red giant evolution: observed differences between magnetic and non-magnetic giants

An analysis is performed of data concerning the [N/C] value, one of the most sensitive indicators of stellar evolution, for 20 red giants with magnetic fields in comparison with similar data for 7 non-magnetic giants. For the most giants in question the [N/C] value showed a dependence on the 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 logt increases from 8.2 to 9.8 and M increases from 1 to 4 M_⊙. It was shown that for the most of magnetic giants the [N/C] values are lowered (up to 0.4 dex) as compared with non-magnetic giants with the same values of t or M. The supposition was made that such differences in the [N/C] values between these two groups of stars are 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 on the MS stage already and especially in the First Dredge-Up (FDU) phase; that leads to the [N/C] lowering in magnetic giants.