Magnetised Winds in Transition Discs I: 2.5D Global Simulations

Abstract: Protoplanetary discs (PPDs) are cold, dense and weakly ionised environments that witness the planetary formation. Among these discs, transition discs (TDs) are characterised by a wide cavity in the dust and gas distribution. Despite this lack of material, many TDs strongly accrete onto their central star, possibly indicating that a mechanism is driving fast accretion in TDs cavities. The presence of radially extended 'dead zones' in PPDs recently revived the interest in magnetised disc winds (MDWs), where accretion is driven by a large magnetic field. We propose that TDs could be subject to similar winds, explaining their fast-accreting and long-lived cavities. We present the results of the first 2.5D global numerical simulations of TDs harbouring MDWs using the PLUTO code. We impose a cavity in the gas distribution and consider a power law distribution for the large-scale magnetic field strength. We assume weakly ionised discs subject to ambipolar diffusion, as expected in this range of densities and temperatures. Our simulated TDs reach a steady state with an inner cavity and an outer 'standard' disc. The accretion rates remain approximately constant through the entire discs, reaching $10^{-7}~M_\odot/\text{yrs}^{-1}$ for typical surface density values. The MDW launched from the cavity is more magnetised with a much larger lever arm than the MDW launched from the outer disc. The material in the cavity is accreted at sonic velocities, and the cavity itself is rotating at 70% of the Keplerian velocity due to the efficient magnetic braking imposed by the MDW. Overall, our cavity matches the dynamical properties of an inner jet emitting disc (JED) and of magnetically arrested discs (MADs) in black hole physics. Finally, kinematic diagnostics (wind speeds, orbital and accretion velocities) could disentangle classical photo-evaporation from MDW models.