Magnetic Fields throughout Stellar Evolution Proceedings IAU Symposium No. 302, 2013 P. Petit, M. Jardine & H. Spruit, eds. c  International Astronomical Union 2014 doi:10.1017/S1743921314001847 Stellar models of rotating, pre-main sequence low-mass stars with magnetic fields Luiz T. S. Mendes 1,3 , Nat´alia R. Landin 2,3 and Luiz P. R. Vaz 3 1 Depto. de Engenharia Eletrˆ onica, UFMG, 31270-901 Belo Horizonte, MG, Brazil email: luizt@cpdee.ufmg.br 2 UFV, Campus Florestal, 35690-000 Florestal, MG, Brazil email: nlandin@ufv.br 3 Depto. de F´ısica, UFMG, 31270-901 Belo Horizonte, MG, Brazil email: vaz@fisica.ufmg.br Abstract. We report our present efforts for introducing magnetic fields in the ATON stellar evolution code code, which now evolved to truly modifying the stellar structure equations so that they can incorporate the effects of an imposed, large-scale magnetic field. Preliminary results of such an approach, as applied to low-mass stellar models, are presented and discussed. Keywords. Stars : Magnetic fields – Stars : Evolution – Stars : Rotation. 1. Introduction and Method The mean-field α − Ω dynamo model has been the most explored theoretical tool for a better understanding of magnetic field generation in the Sun and probably also in sun-like stars. However, dynamo models are inherently 3-D, making them difficult to be integrated with 1-D stellar evolution codes. Fortunately, a method first proposed by Lydon & Sofia (1995, hereafter LS95) allows a self-consistent modification of the stellar structure equations in order to incorporate the effects of a large-scale magnetic field to 1-D stellar evolution codes. Here we report the application of that method to the ATON 2.3 evolution code and some preliminary results obtained with it. The LS95 method treats the magnetic field as a perturbation on the stellar structure equations, by means of a new state variable χ = B 2 /(8πρ) representing the magnetic energy density. The true 3-D nature of the corresponding magnetic pressure P χ is crudely simplified to represent only the radial dependence of the magnetic pressure. The transi- tion from the intrinsic 3-D magnetic field geometry to this 1-D approximation is described by a numerical factor γ , so that P χ =(γ − 1) χρ. The reader is referred to the LS95 work for full details regarding this technique. 2. The Models The main features of the ATON 2.3 evolution code such as opacities, diffusive mixing, overshooting, convection treatment and structural effects of rotation are described else- where (Ventura et al. 1998; Mendes et al. 1999). We computed rotating stellar models of 1 and 0.6 M  with solar chemical composition, mixing-length convection treatment with α =Λ/H P =1.5, and initial rotation rate obtained from Kawaler’s (1987) mass- radius and mass-moment of inertia relations for low-mass stars. A surface magnetic field strength < 100 G was adopted, that can be viewed as an “average” between the mean solar dipole field of 1 G and the kG values observed in T Tauri stars. The field scales throughout the stellar interior preserving the surface ratio between the magnetic and gas energy densities (D’Antona et al. 2000). 112 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921314001847 Downloaded from https://www.cambridge.org/core. IP address: 18.206.13.133, on 07 Jun 2020 at 10:51:22, subject to the Cambridge Core terms of use, available at