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

Abstract

Context: Observations suggest that magnetic fields of disk-bearing stars may have non-dipolar configurations. However, the influence of these configurations on magnetospheric accretion remains poorly understood. Aims: We aim to simulate magnetospheric accretion 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 conduct 2.5-D non-ideal viscous-resistive magnetohydrodynamical (MHD) simulations of star-disk magnetospheric interactions. We consider a thin accretion disk and strong stellar magnetic field ($B_\star= 6.2\mathrm{kG}$) under four configurations: pure dipole, pure quadrupole, dipole plus quadrupole, and dipole plus octupole. In the latter two cases, different magnetic polar strength ratios are 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 non-uniform 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.

Figures (8)

• Figure 1: Initial density (in units of $\rho_0$) and magnetic field lines (white curves) for model D (first panel), model Q (second panel), model O (third panel), model DQ (fourth) and model DO (fifth). The letters D, Q, O, DQ and DO represent dipolar, quadrupolar, octupolar, dipolar-quadrupolar and dipolar-octupolar configurations, respectively. In all cases, the sum of the different multipolar contributions to the stellar magnetic field is equal to $89.3$. Here, $r$ and $z$ represent the cylindrical coordinates, measured in units of the stellar radius.
• Figure 2: Density maps (in units of $\rho_0$) at $t=2T_0$ for a dipolar (D model) magnetic configuration (left panel) and for a quadrupolar (Q model) magnetic configuration (right panel). Blue solid and dashed lines on the left side of each panel show the angular momentum flux $\mathbf{f}_B$ carried by the field (see Eq. \ref{['eq:fb']}), with solid blue indicating positive flux and dashed blue indicating negative flux, while in the right side of each figure the yellow vectors show the velocity and the white lines represent the poloidal magnetic field lines. Lastly, inserts show a zoomed region of the density map for $R<R_\mathrm{co}$ where it is clearly seen how the gas follows different paths towards the star for the D and Q models. However, it is worth noting that, in both cases, the gas flow toward the star is symmetric about the midplane.
• Figure 3: Gas density maps for DQ models (see Table \ref{['tab:models_b']}), at $t=3T_0$. We overlapped the yellow vectors to show the direction of the gas velocity and the poloidal magnetic field lines in each case (solid white lines). Note that the gas follows the magnetic field lines forming only a twisted funnel flow that enters the star below the midplane of the disk in all cases. Here the inserts show a zoom region of the density maps contained within the corotation radius.
• Figure 4: Temporal evolution of the gas density (in logarithmic scale in units of $\rho_0$) for the model $\mathrm{DO}_\mathbf{13}$. Left panel corresponds to the time $t=3T_0$ and right panel corresponds to $t=6T_0$. The streamlines of the angular momentum fluxes $\mathbf{f}_B$ carried by the magnetic field are represented by the blue lines in the left side of each panel. While on the right side of each panel the velocity vectors are shown with yellow arrows. The inserts show a zoomed region of the density maps within $R<R_\mathrm{co}$.
• Figure 5: Gas density comparison between dipolar plus octupolar models in logarithmic scale in units of $\rho_0$ at $t=5T_0$. Left panel correspond to the model $\mathrm{DO_{17}}$, at $t=5T_0$, and right panel is for the model $\mathrm{DO_{13}}$ at the same time of evolution. The blue lines and inserts are the same as in Fig. \ref{['fig:Models_DO']}. The gas flow is compared at $t=5T_0$, when a quasi-steady state has been reached in the mixed dipolar-octupolar configurations.