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

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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.

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L. Lyubimkov

Crimean Astrophysical Observatory of the Russian Academy of Sciences

编辑信件的主要联系方式.
Email: lyub@craocrimea.ru
俄罗斯联邦, Nauchny

D. Poklad

Crimean Astrophysical Observatory of the Russian Academy of Sciences

Email: dmitry.crao@gmail.com
俄罗斯联邦, Nauchny

参考

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2. Fig. 1. The {} diagram for software giants. The evolutionary tracks of MIST [8] for masses M = 1, 2, 3, and 4M⊙ are shown; thick lines correspond to tracks with normal metallicity [Fe/H] = 0.0, thin lines to tracks with reduced metallicity [Fe/H] = –0.5. Here and in all subsequent figures, filled circles correspond to magnetic giants, open circles to non-magnetic giants. Some of the most interesting giants are marked.

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3. Fig. 2. Dependence of the age of software giants on their mass. The solid curve is an approximation obtained by the least squares method.

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4. Fig. 3. Dependence of the [N/C] value on the age of giants. The solid broken line represents the average dependence for 15 magnetic giants, the dashed line – the same for 6 non-magnetic giants.

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5. Fig. 4. Dependence of the [N/C] value on the mass of giants. The solid curve represents the average dependence for 15 magnetic giants, the dashed line – the same for 6 non-magnetic giants. The thick broken lines show the results of theoretical calculations for models of rotating stars at two values ​​of the initial relative rotation velocity (0.0 and 0.6).

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