2.5D magnetohydrodynamic models of circumstellar discs around FS CMa post-mergers – I. Non-stationary accretion stage

Abstract

ABSTRACT We investigate the dynamic evolution of the gaseous regions around FS CMa post-mergers. Owing to the slow rotation of the central B-type star, the dynamics is driven mainly by the magnetic field of the central star. Recent observations have allowed us to set realistic initial conditions, such as the magnetic field value ($B_\star \approx 6\times 10^{3}\, \mathrm{G}$), the mass of the central star ($M_\star =6\, \mathrm{M}_\odot$), and the initial disc density $\rho _{d0}\in [10^{-13}\, \mathrm{g\, cm^{-3}},10^{-11}\, \mathrm{g \, cm^{-3}}]$. We use the pluto code to perform 2.5D magnetohydrodynamic simulations of thin and thick disc models. Especially relevant for the interpretation of the observed properties of FS CMa post-mergers are the results for low-density discs, in which we find a jet emerging from the inner edge of the disc, as well as the formation of the so-called ‘hot plasmoid’ in the coronal region. Jets are probably detected as discrete absorption components in the resonance lines of FS CMa stars. Moreover, the magnetic field configuration in the low-density plasma region favours the appearance of magnetocentrifugal winds from the disc. The currents towards the star created by the magnetic field may explain accidentally observed material infall. The disc structure is significantly changed owing to the presence of the magnetic field. The magnetic field is also responsible for the formation of a hot corona, as observed in several FS CMa stars through the Raman lines. Our results are valid for all magnetic stars surrounded by a low-density plasma, that is, by some stars showing the B[e] phenomenon.