Estudos numéricos da viabilidade do dínamo de Tayler-Spruit em interiores estelares: o papel da rotação na instabilidade de Tayler

Abstract

The mean-field dynamo theory has been employed for decades in the study of the origin and maintenance of stellar magnetic fields. According to this theory, the dynamo is sustained through the shear between layers, related to the Ω-effect, and by helical motions induced by turbulence, associated with the α-effect. However, the applicability of the mean-field dynamo theory to stars with radiative envelopes, such as Ap/Bp stars, or to convectively stable regions, like the interior of solar-type stars, is limited. In this context, alternative theories such as the Tayler-Spruit dynamo emerge. In this approach, the α-effect in convectively stable zones would be driven by magnetohydrodynamic instabilities, such as the Tayler instability, where energy is transferred between different components of the magnetic field. In the first part of this work, we investigate the evolution of a purely toroidal magnetic field that is anti-symmetric with respect to the equator, confined in a convectively stable zone and, therefore, prone to Tayler instability. We used the EULAG-MHD code to perform non-linear three-dimensional simulations of a spherical layer whose stratification resembles that of the stable region located between the radiative and convective zones of the Sun. Our main focus was to examine how the evolution of this magnetic field is influenced, both in the short and long term, by the rotation rate of the model. The results indicate that rotation has the capacity to stabilize the magnetic field, delaying the growth of the instability. However, in all the models studied, the magnetic fields eventually become unstable, losing their original topology and later decaying due to turbulent diffusion. During this process, we observed the emergence of significant poloidal field components, as well as helicities, both kinetic and current. These results support the viability of Tayler instability in playing the role of the α-effect in the Tayler-Spruit dynamo. In the second part of this work, we coupled the EULAG-MHD and Pencil Code. This coupling allowed us to apply the test-field method (TFM), implemented in the Pencil Code, to calculate the dynamo coefficients in a test simulation performed with the EULAG-MHD code. The development of the code that enables this coupling was completed successfully , and the results obtained were consistent with previous studies using similar models. This tool opens up the possibility of exploring, in greater depth, the physical mechanisms involved in unstable processes, including the dynamo mechanism, in various simulations conducted with EULAG-MHD.