Magnetic field generation in mergers of massive main sequence stars

TL;DR

This study uses 3D magnetohydrodynamics simulations to demonstrate that mergers of massive main-sequence stars can generate magnetic fields, which develop during mass transfer and form stable large-scale structures.

Contribution

The paper provides the first detailed simulation evidence that stellar mergers can produce magnetic fields in massive stars, highlighting a potential origin for observed stellar magnetism.

Findings

Magnetic fields are amplified during mass transfer in stellar mergers.

Magnetic energy saturates at an equipartition level during disruption.

Post-merger magnetic fields form stable, large-scale toroidal structures.

Abstract

Magnetic fields are found in many astrophysical objects, ranging from galaxy clusters to the interstellar medium of galaxies and neutron stars. Strong magnetic fields are also observed in massive stars, but it is still unclear how they are generated. There are different theories for their origin: the magnetic fields could be fossil fields from star formation, they could be generated in dynamo processes during stellar evolution, or they could be generated during a merger of two stars. Here, we show how magnetic fields are generated in simulations of mergers of massive main-sequence stars, conducted with the 3D magnetohydrodynamics code AREPO. In these simulations, a $9\,M_\odot$ and a $8\,M_\odot$ main sequence star merge, whereby the more massive star transfers mass to the less massive one until it is finally disrupted, forming a core-disk structure. During the initial mass transfer, magnetic fields start to be amplified in the accretion stream, saturating after the disruption at an equipartition level. Correspondingly, the magnetic energy starts being present at small scales, growing to larger scales during the merger. During the merger, the magnetic pressure is much smaller than the gas pressure, thus the dynamical impact of the magnetic fields on the merger is small. After the merger, both are similar in some regions, and the magnetic field shows a large-scale structure that is mostly toroidal and that is expected to be stable. Thus, our simulations show that magnetic fields in massive stars can originate from a merger of two main sequence stars.