On fan-shaped cold MHD winds from Keplerian accretion discs

TL;DR

This paper examines the conditions under which steady, fan-shaped cold MHD winds can be launched from thin Keplerian accretion discs, concluding that such winds require unrealistically high turbulence-driven transport coefficients, challenging their natural occurrence.

Contribution

It provides a detailed analysis of the steady-state MHD equations for fan-shaped winds, revealing the necessity of extremely high transport coefficients that are inconsistent with current turbulence models.

Findings

Steady fan-shaped winds need very high turbulent transport coefficients.

Current turbulence models cannot support the required wind launching conditions.

Numerical simulations align with the conclusion that such winds are unlikely to occur naturally.

Abstract

We investigate under which conditions cold, fan-shaped winds can be steadily launched from thin (Keplerian) accretion discs. Such winds are magneto-centrifugal winds launched from a thin annulus in the disc, along open magnetic field lines that fan out above the disc. In principle, such winds could be found in two situations: (1) at the interface between an inner Jet Emitting Disc, which is itself powering magneto-centrifugally driven winds, and an outer standard accretion disc; (2) at the interface between an inner closed stellar magnetosphere and the outer standard accretion disc. We refer to Terminal or T-winds to the former kind and to Magnetospheric or M-winds to the latter. The full set of resistive and viscous steady state MHD equations are analyzed for the disc (the annulus), which allow us to derive general expressions valid for both configurations. We find that, under the framework of our analysis, the only source of energy able to power any kind of fan-shaped winds is the viscous transport of rotational energy coming below the inner radii. Using standard local $\alpha$ prescriptions for the anomalous (turbulent) transport of angular momentum and magnetic fields in the disc, we derive the strength of the transport coefficients that are needed to steadily sustain the global configuration. It turns out that, in order for these winds to be dynamically relevant and explain observed jets, the disc coefficients must be far much larger than values expected from current knowledge of turbulence occurring inside proto-stellar discs. Either the current view on MHD turbulence must be deeply reconsidered or steady-state fan-shaped winds are never realized in Nature. The latter hypothesis seems to be consistent with current numerical simulations.