Protodiscs around Hot Magnetic Rotator Stars

TL;DR

This paper models the formation and evolution of protodiscs around hot magnetic rotator stars, highlighting the role of magnetic fields, rotation profiles, and instabilities like MRI in disc formation.

Contribution

It introduces equations and solutions for protodisc structure considering differential rotation and magnetic effects, emphasizing MRI's role in disc formation.

Findings

Protodiscs form with super-Keplerian rotation and increase in density over time.

Magnetorotational instability can develop within hours or days in differentially rotating stars.

Viscosity from MRI may lead to quasi-steady disc formation despite centrifugal breakout.

Abstract

We develop equations and obtain solutions for the structure and evolution of a protodisc region that is initially formed with no radial motion and super-Keplerian rotation speed when wind material from a hot rotating star is channelled towards its equatorial plane by a dipole-type magnetic field. Its temperature is around $10^7$K because of shock heating and the inflow of wind material causes its equatorial density to increase with time. The centrifugal force and thermal pressure increase relative to the magnetic force and material escapes at its outer edge. The protodisc region of a uniformly rotating star has almost uniform rotation and will shrink radially unless some instability intervenes. In a star with angular velocity increasing along its surface towards the equator, the angular velocity of the protodisc region decreases radially outwards and magnetorotational instability (MRI) can occur within a few hours or days. Viscosity resulting from MRI will readjust the angular velocity distribution of the protodisc material and may assist in the formation of a quasi-steady disc. Thus, the centrifugal breakout found in numerical simulations for uniformly rotating stars does not imply that quasi-steady discs with slow outflow cannot form around magnetic rotator stars with solar-type differential rotation.