Evolutionary Consequences

Let us briefly list some general evolutionary consequences of the presence of a magnetic field in comparison with models without magnetic field:

- Magnetic field is able to account for the internal solid body rotation of the Sun in agreement with helioseismic observations [172]. There are however other possible solutions.

- Magnetic field enforces an almost solid body rotation. As a matter of fact, an equilibrium state is quickly reached with a very small amount of differential rotation just enough to sustain the dynamo. The strong internal coupling of rotation by the magnetic field transmits the central spin-up to the stellar surface, which thus rotates faster during evolution.

- As rotation velocities keep higher, the losses of mass and angular momentum by massive stars are favored. This leads to final stages with lower rotation; however, this also depends on the evolutionary tracks followed by the stars. This may be an advantage to account for the relatively low rotation period of young pulsars (Sect. 28.6.2); for the interpretation of the precursor models for y-ray bursts (GRB), see Sect. 28.6.3.

- The higher velocity of meridional circulation in the models with magnetic field produces larger mixing than in a rotating model without magnetic field. The enrichment N/C reaches a factor of about 10 in a model of 15 M0 with an initial velocity of 300 km s-1 (Fig. 13.9). There is even a significant helium enhancement at the surface [369]. These results may however more depend on the specific dynamo and field intensity at work.

- The larger mixing increases the convective core in intermediate and massive stars, which in turn increases the MS lifetimes (by about 20% in the example given).

- Some layering may appear with successive magnetic and non-magnetic regions, with possible steps in chemical composition (Sect. 6.2).

In the context of the uncertainties discussed in Sect. 13.4.2, a clear result is that it is meaningful to further study the magnetic field in view of the importance of its evolutionary effects revealed by the above models.

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age [in million years]

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age [in million years]

Fig. 13.9 Evolution of the surface helium content 7s and of the ratios N/O, N/H and N/C (in mass fractions) for various 15 [email protected] models: the dotted line applies to model without rotation, the short broken line to model with rotation (Uini= 300 km s_1) but without magnetic fields, the continuous line to model with rotation ("0^= 300 km s_1) and magnetic fields. From the author and G. Meynet [369]

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age [in million years]

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age [in million years]

Fig. 13.9 Evolution of the surface helium content 7s and of the ratios N/O, N/H and N/C (in mass fractions) for various 15 [email protected] models: the dotted line applies to model without rotation, the short broken line to model with rotation (Uini= 300 km s_1) but without magnetic fields, the continuous line to model with rotation ("0^= 300 km s_1) and magnetic fields. From the author and G. Meynet [369]

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