Accretion and plasma outflow from dissipationless discs

TL;DR

This paper explores a model where cold, dissipationless accretion discs transfer angular momentum and energy to magnetized winds, producing highly efficient outflows that can exceed Keplerian velocities, relevant for low-luminosity astrophysical objects.

Contribution

It introduces a self-similar model of accretion and outflow with negligible viscosity, highlighting a mechanism for converting gravitational energy into energetic winds with near 100% efficiency.

Findings

Outflows carry away angular momentum and energy from the disc.

Accretion rate depends on radius as a power law influenced by the Alfvenic radius.

Final wind velocities can surpass local Keplerian velocities.

Abstract

We consider an extreme case of disc accretion onto a gravitating centre when the viscosity in the disc is negligible. The angular momentum and the rotational energy of the accreted matter is carried out by a magnetized wind outflowing from the disc. The outflow of matter from the disc occurs due to the Blandford & Payne(1982) centrifugal mechanism. The disc is assumed to be cold. Accretion and outflow are connected by the conservation of the energy, mass and the angular momentum. The basic properties of the outflow, angular momentum flux and energy flux per particle in the wind, do not depend on the details of the structure of the accretion disc. In the case of selfsimilar accretion/outflow, the dependence of the rate of accretion $\dot M$ in the disc depends on the disc radius $r$ on the law $\dot M \sim r^{{1\over2(\alpha^2-1)}}$, where $\alpha$ is a dimensionless Alfvenic radius. In the case of $\alpha \gg 1$, the accretion in the disc is provided by very weak matter outflow from the disc and the outflow predominantly occurs from the very central part of the disc. The solution obtained in the work provides mechanism which transforms the gravitational energy of the accreted matter into the energy of the outflowing wind with efficiency close to 100%. The final velocity can essentially exceed Kepler velocity at the site of the wind launch. This mechanism allows us to understand the nature of the astrophysical objects with low luminosity discs and energetic jet-like outflows.