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

¹ Charles University, Faculty of Mathematics and Physics, Astronomical Institute., V Holešovičkách 747/2, 180 00, Prague 8, CZ-746 01 Opava, Czech Republic
² Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
³ Nicolaus Copernicus Superior School, College of Astronomy and Natural Sciences, Gregorkiewicza 3, 87-100 Toruń, Poland
⁴ Research Centre for Computational Physics and Data Processing, Institute of Physics, Silesian University in Opava, Bezručovo nám. 13, CZ-746 01 Opava, Czech Republic
⁵ Instituto de Astronomía, Universidad Nacional Autónoma de Mexico, A. P. 70-264, 04510 CDMX, México

^⋆ Corresponding author: a.moranchel@mff.cuni.cz

Received: 29 January 2025
Accepted: 23 August 2025

Abstract

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.