On the collimation properties of jets with finite Poynting flux launched from Keplerian accretion discs

Abstract

It is generally accepted that the launching of astrophysical jets requires a large-scale magnetic field threading a central object (black hole or star) and/or its surrounding accretion disc. However, the collimation mechanism far away from the central object has not yet been fully understood. In a previous work we investigated a mechanism in which the jet is self-collimated due to a dominant hoop stress. We ran numerical simulations in which a Jet-Emitting disc (JED) spans the entire lower computational boundary. Those were the first of their kind to showcase the steady recollimation shocks predicted by steady-state analytical studies of jets. However, the huge size of the JED prevented a complete study of the connection between the accelerating and asymptotic electric circuits, as well as the influence of the outer medium. We performed a set of axisymmetric ideal MagnetoHydroDynamics (MHD) non-relativistic jet simulations. In those, only the innermost region of the accretion disc is a jet-launching zone. The jets of finite radial extent in those simulations also produce steady recollimation shocks at large distances from the central object. Standing recollimation shocks are not a bias of self-similarity, but a generic feature of jets emitted from magnetized Keplerian accretion discs. They may produce observable features, such as a standing emission knots, a decrease of the rotation rate or a change in polarisation. We also recover previous results on the influence of external pressure on jet confinement, such as the relation between pressure profile and jet shape, and jet acceleration efficiency.

Figures (21)

• Figure 1: Sketch of the computational domain.
• Figure 2: Radial distributions of the rotation rate $\Omega_*$ of the magnetic surfaces at the ejection boundary for different values of the rotation at the axis $\Omega_{*_{\text{a}}}$. The JED follows a keplerian distribution $r^{-3/2}$ and is established from $r =1$ to $r=10$, with a SAD beyond $r =12$ assumed to launch no jet ($\Omega_*=0$) and a short continuous transition region between the two. The reference simulation O1 is the blue one, with $\Omega_{*_{\text{a}}}=0$ (see Table \ref{['tab:ParametresSimusTronquees']}).
• Figure 3: Radial distributions of the vertical magnetic field $B_z$ at the ejection boundary for different values of the external magnetic field $B_{\text{ext}}$ (note that in code units $B_{\text{d}} = 10$). The JED follows a Blandford & Payne distribution $r^{-5/4}$ and is established from $r =1$ to $r=10$, with a SAD beyond $r =12$ (see text and Fig. \ref{['fig:FrontiereOmegaTronque']}). The reference simulation O1 is the green one, with $B_{\text{ext}}=2\times 10^{-3}$ (see Table \ref{['tab:ParametresSimusTronquees']}).
• Figure 4: Snapshots of the reference simulation O1 at $t_{\text{end}}$. Left: Global view showing the logarithm of the FM mach number $n$ in background color, poloidal velocity vectors (black arrows) on field lines anchored at different radii ($r_0= 2; 4; 7; 15; 40; 80; 160; 320; 600; 1000; 1500$), isocontours (yellow lines) of the poloidal electric current and the FM critical surface (red dashed line). The colored points (blue, purple and green) show where the field line anchored at $r_0 = 4$ meets the three standing recollimation shocks. Right: Zoomed in view with the logarithm of the plasma density in background color. The white and green lines are poloidal magnetic field lines, the green ones being anchored on the disc at $r_0 \in [1.02;4]$. The red lines are the Alfvén (dotted) and FM (dashed) critical surfaces. The yellow lines show the poloidal electric circuit, flowing out of disc, from the JED ($z=0$, $r \in \left[ 1;10 \right]$) and outer transition zone ($z=0$, $r \in \left[ 10;12 \right]$) and closing into the central object ($R = \sqrt{r^2 + z^2} = 1$).
• Figure 5: Evolution of various cylindrical radii with altitude $z$ along the jet for simulation O1. In blue and orange are respectively the magnetic field lines and streamlines anchored on the disc $r_0 = 12$. In red is the radius $r_{\text{FM}}$ of the FM surface. In green is the radius $r_{\dot{M}_{\text{j}}}$ inside which the total ejected mass flux is located. For a YSO jet, this equates to a jet collimated down to a radius of about 70 au at 500 au from the disc.