On fan-shaped cold MHD winds from Keplerian accretion discs

• Published in: Monthly notices of the Royal Astronomical Society Vol. 428; no. 1; pp. 307 - 320
• Main Authors: Ferreira, J., Casse, F.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 2013; Oxford University Press (OUP): Policy P - Oxford Open Option A
• Subjects: Astrophysics; Fluid mechanics; Magnetic fields; Physics; Turbulence; Turbulent flow; Viscosity; MHD; stars: mass-loss; stars: formation; ISM: jets and outflows; stars: pre-main-sequence; accretion, accretion discs
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sts012

We investigate under which conditions cold, fan-shaped winds can be steadily launched from thin (Keplerian) accretion discs. Such winds are magnetocentrifugal 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 magnetocentrifugally 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 as the former kind and to magnetospheric or M-winds as the latter. The full set of resistive and viscous steady-state magnetohydrodynamic (MHD) equations are analysed 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 α 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 protostellar 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.