Mass and angular momentum loss via decretion disks

TL;DR

This paper investigates how mass and angular momentum are lost through decretion disks in rapidly rotating massive stars, highlighting differences from stellar winds and implications for stellar evolution and star formation.

Contribution

It introduces a detailed analysis of decretion disk mass loss, contrasting it with wind-driven loss, and provides parameterized laws for incorporating this into stellar evolution models.

Findings

Decretion disk mass loss can be significantly less than wind mass loss.

Outer disk radius critically influences angular momentum loss.

Metallicity and binary interactions affect disk-driven mass loss.

Abstract

We examine the nature and role of mass loss via an equatorial decretion disk in massive stars with near-critical rotation induced by evolution of the stellar interior. In contrast to the usual stellar wind mass loss set by exterior driving from the stellar luminosity, such decretion-disk mass loss stems from the angular momentum loss needed to keep the star near and below critical rotation, given the interior evolution and decline in the star's moment of inertia. Because the specific angular momentum in a Keplerian disk increases with the square root of the radius, the decretion mass loss associated with a required level of angular momentum loss depends crucially on the outer radius for viscous coupling of the disk, and can be significantly less than the spherical mass loss the spherical, wind-like mass loss commonly assumed in evolutionary calculations. We discuss the physical processes that affect the outer disk radius, including thermal disk outflow, and ablation of the disk material via a line-driven wind induced by the star's radiation. We present parameterized scaling laws for taking account of decretion-disk mass loss in stellar evolution codes, including how these are affected by metallicity, or by presence within a close binary and/or a dense cluster. Effects similar to those discussed here should also be present in accretion disks during star formation, and may play an important role in shaping the distribution of rotation speeds on the ZAMS.