Dissertação de Mestrado #603 – Bonnie Romano Zaire – 04/07/2018

On the influence of the internal structure of pre-main-sequence stars in the magnetic field topology. A numerical study using the EULAG-MHD code.

Autor: Bonnie Romano Zaire

Banca Examinadora

Gustavo Andres Guerrero Eraso (orientador)

DF/UFMG

Profa. Si­lvia Helena Paixão Alencar

DF/UFMG

Prof. Ronald Dickman

DF/UFMG

Orientação

Gustavo Andres Guerrero Eraso

DF/UFMG

Resumo do Trabalho

Modern observational techniques have achieved detailed determination of stellar magnetic fields. This includes magnetic maps that represent the morphology of the field at the surface of the stars. For young pre-main-sequence stars (PMS), the results suggest that the development of a radiative core plays a fundamental role defining their magnetic field topology (Gregory et al. 2012). Understanding the plasma dynamics in the interior of these objects can provide relevant correlations between the internal structure and the mean flows and magnetic fields. In this work we study the magnetohydrodynamic processes occurring in the interior of PMS stars through global numerical simulations performed with the EULAG-MHD code. We performed two simulations whose stratifications mimic a 0.7 solar masses star at two different ages. The first one considers a fully convective envelope, corresponding to a 1.1Myr T Tauri star. The second one is a 14.0Myr T Tauri star with a radiative zone which spans up to 30% of the stellar radius. Both models are rotating with a 7 days period. We explored the development of mean flows and magnetic fields in these models resulting from rotating turbulent convection. The results of the simulations show the growing and evolution of a large-scale magnetic field driven by a dynamo mechanism. A mean-field dynamo analysis suggests that an αΩ dynamo is the ongoing process in the fully convective simulation, and that an αΩ dynamo operates in the partially convective simulation. A particularly interesting result is the large contribution of the small-scale current helicity to the alpha-effect in both models. In fact, this magnetic alpha-effect is the dominant dynamo source in the fully convective model. After the simulations have reached steady-state, the ratio between the magnetic energy rising from the large-scale poloidal field and the total magnetic field is 15% for the fully convective star, and 6% for the partially convective one. A quantitative analysis, based on spherical harmonic decomposition of the magnetic field, shows a dominant dipolar (l=1-6.0kG) and octupolar (l=3-7.7kG) field in the fully convective simulation, whereas in the partially convective simulation the modes l=3 and l=5 (with 22.7kG and 24.7kG, respectively) store the largest amount of magnetic energy. These results indicate lower complexity in the magnetic field of the youngest star, in agreement with the observations.