Vol. 620
In section 2. Astrophysical processes

Turbulent and wind-driven accretion in dwarf novae threaded by a large-scale magnetic field

by N. Scepi, G. Lesur, G. Dubus, and M. Flock A&A 620, A49


The prescription most often employed in modeling accretion disks assumes that the turbulent stress can be phenomenologically related to the isotropic pressure by a single parameter, alpha, first introduced in the 1970s. Variations in this number are thought to drive the changes in mass accretion rates that explain the outbursts of dwarf novae with a higher value during outburst, and hence higher viscous torquing and increased accretion rates. This paper reports the results of 3D shearing box MHD simulations of standard disks with a range of vertically imposed magnetic fields (such as would be expected from a magnetized white dwarf whose field threads the disk). The authors find that even a weak field, as small as 1 G, drives a vertical outflow that is as effective as the magneto-rotational instability in transporting away angular momentum in the inter-outburst periods. They also find that the effective alpha is actually lower - for weak fields - than during the outburst, and that wind transport may also dominate the energy balance. The work creates the prospect of a sweeping rethink of the interdependence of outflow and inflow in all accreting systems.