The Effect of Combined Magnetic Geometries on Thermally Driven Winds II: Dipolar, Quadrupolar and Octupolar Topologies

TL;DR

This study investigates how complex magnetic field geometries, including dipolar, quadrupolar, and octupolar components, influence stellar wind-driven angular momentum loss, revealing the dominance of the dipole component in most cases.

Contribution

It extends previous work by including octupolar fields and analyzing the combined effects of multiple magnetic geometries on stellar wind torque, providing a comprehensive torque formulation.

Findings

Dipole component generally dominates the spin-down torque.

The developed torque formula predicts simulation results within 20% accuracy.

Complex magnetic geometries significantly influence stellar wind behavior.

Abstract

During the lifetime of sun-like or low mass stars a significant amount of angular momentum is removed through magnetised stellar winds. This process is often assumed to be governed by the dipolar component of the magnetic field. However, observed magnetic fields can host strong quadrupolar and/or octupolar components, which may influence the resulting spin-down torque on the star. In Paper I, we used the MHD code PLUTO to compute steady state solutions for stellar winds containing a mixture of dipole and quadrupole geometries. We showed the combined winds to be more complex than a simple sum of winds with these individual components. This work follows the same method as Paper I, including the octupole geometry which increases the field complexity but also, more fundamentally, looks for the first time at combining the same symmetry family of fields, with the field polarity of the dipole and octupole geometries reversing over the equator (unlike the symmetric quadrupole). We show, as in Paper I, that the lowest order component typically dominates the spin down torque. Specifically, the dipole component is the most significant in governing the spin down torque for mixed geometries and under most conditions for real stars. We present a general torque formulation that includes the effects of complex, mixed fields, which predicts the torque for all the simulations to within 20% precision, and the majority to within ~5%. This can be used as an input for rotational evolution calculations in cases where the individual magnetic components are known.