Multipolar magnetic configuration: A trace of post-mergers events in circumstellar disks around FS CMa stars

Context. Observations suggest that the magnetic fields of disk-bearing stars can have non-dipolar configurations. However, the influence of these configurations on magnetospheric accretion remains poorly understood. Aims. We aim to simulate magnetospheric accretion by incorporating non-dipolar and strong magnetic field. Our model is informed by observations of IRAS 17449+2320, a post-merger belonging to the group of FS CMa stars, which indicate a dominant dipolar magnetic field with an additional quadrupole component. Methods. Using the PLUTO code, we conducted 2.5-D nonideal viscous-resistive ( α ν = 1 and α m = 1) magnetohydrodynamic simulations of star–disk magnetospheric interactions. We considered a thin accretion disk and a strong stellar magnetic field ( B ⋆ = 6.2 kG) under four configurations: pure dipole, pure quadrupole, dipole plus quadrupole, and dipole plus octupole. In the latter two cases, different magnetic polar strength ratios were explored. Results. For asymmetric magnetic field configurations, we find that accretion exhibits funnel streams below the midplane, indicating the dominance of the quadrupolar and octupolar components. In contrast, in dipolar configurations, we observe the formation of two symmetrical funnels with respect to the midplane. However, in the quadrupolar configuration, accretion is entirely confined to the disk midplane forming a cone-like pattern that leads to disk widening. Remarkably, the presence of a quadrupolar component gives rise to highly asymmetric substructures in the corona region. Conclusions. Multipolar stellar magnetic fields drive nonuniform accretion and lead to asymmetric density distributions in both the disk and corona. These results resemble observed features of some FS CMa post-mergers and Herbig Ae/Be stars, highlighting the critical role of magnetic field complexity in shaping circumstellar environments.