Magnetic field generation in mergers of massive main sequence stars

TL;DR

This paper demonstrates, via 3D magnetohydrodynamic simulations with AREPO, that magnetic fields in massive stars can originate from the merger of two main-sequence stars. The authors track the amplification from tiny seed fields through Kelvin–Helmholtz and magnetorotational instabilities to a large-scale, predominantly toroidal field in a stable core–disk merger remnant, while showing that the magnetic pressure remains subdominant during the merger. The results indicate that merger remnants can acquire strong, organized magnetic fields that persist into the main sequence, providing a plausible pathway for magnetized massive stars like $ au$ Sco and connecting merger physics to observable stellar magnetism. The study also highlights the robustness of the magnetic amplification process against changes in resolution, initial separation, and seed field, and notes the relatively small dynamical impact of magnetic fields during the pre-merger phase and the limited dynamical mass loss during the event.

Abstract

Magnetic fields are found in many astrophysical objects, ranging from galaxy clusters to the interstellar medium of galaxies and magnetars. 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.

Figures (11)

• Figure 1: Initial model of the $8\,M_\odot$ MS star. The panels show from top to bottom radially averaged of profiles of the density, the Mach number and the difference in the hydrostatic equilibrium (gradient of pressure and gravitational force) at different times during the relaxation run.
• Figure 2: Evolution of orbital separation. The distance is computed between the centers of mass of both stars. The center of mass of each star is computed by using the corresponding passive scalar as a weight.
• Figure 3: Evolution of density over time for model 1. The upper row shows slices through the orbital plane ($x$--$y$), the lower row slices through a plane perpendicular to the orbital plane ($x$--$z$), connecting the cores of the two stars before they merge. The right-most panel includes a circle with radius $3\,R_\odot$ to roughly indicate the disk--core boundary.
• Figure 4: Evolution of a passive scalar over time for model 1. The passive scalar is initially 1 in the more massive star and 0 elsewhere. Slices are shown through the orbital plane ($x$--$y$). The right-most panel includes a circle with radius $3\,R_\odot$ to roughly indicate the disk--core boundary.
• Figure 5: Evolution of total magnetic energy (upper panel), ratio of magnetic to turbulent kinetic energy (middle panel), and magnetic energy in the cylindrical components of the magnetic field for Model 1 (lower panel).