Models of hot star decretion disks

TL;DR

This paper models the physical behavior of circumstellar decretion disks around rapidly rotating massive stars, focusing on the effects of viscosity, temperature, and magnetic instabilities to understand their structure and evolution.

Contribution

It introduces a new two-dimensional hydrodynamic and magnetohydrodynamic simulation code to analyze disk properties and the role of magnetorotational instability in such disks.

Findings

Disk density and velocity profiles depend on temperature and viscosity distributions.

Magnetorotational instability can drive anomalous viscosity in the disks.

Outer regions of disks can extend significantly far from the star.

Abstract

Massive stars can during their evolution reach the phase of critical (or very rapid, near-critical) rotation when further increase in rotation rate is no longer kinematically allowed. The mass ejection and angular momentum outward transport from such rapidly rotating star's equatorial surface may lead to formation and supports further existence of a circumstellar outflowing (stellar decretion) disk. The efficient mechanism for the outward transport of the mass and angular momentum is provided by the anomalous viscosity. The outer supersonic regions of the disks can extend up to a significantly large distance from the parent star, the exact radial extension is however basically unknown, partly due to the uncertainties in radial variations of temperature and viscosity. We study in detail the behavior of hydrodynamic quantities, i.e., the evolution of density, radial and azimuthal velocity, and angular momentum loss rate in stellar decretion disks out to extremely distant regions. We investigate the dependence of these physical characteristics on the distribution of temperature and viscosity. We also study the magnetorotational instability, which we regard to be the source of anomalous viscosity in such outflowing disks and to some extent we provide the preliminary models of the two-dimensional radially-vertically correlated distribution of the disk density and temperature. We developed our own two-dimensional hydrodynamic and magnetohydrodynamic numerical code based on an explicit Eulerian finite difference scheme on staggered grid, including full Navier-Stokes viscosity. We use semianalytic approach to investigate the radial profile of magnetorotational instability, where on the base of the numerical time-dependent hydrodynamic model we analytically study the stability of outflowing disks submerged to the magnetic field of central star.