Fast & slow winds from supergiants and Luminous Blue Variables

TL;DR

This paper models mass-loss rates and wind velocities of supergiants and LBVs, confirming observed velocity drops, revealing a larger bi-stability jump, and exploring implications for stellar evolution and supernova progenitors.

Contribution

It introduces a dynamical Monte Carlo method to predict wind properties, confirming observed phenomena and uncovering new insights into bi-stability effects and their impact on stellar evolution.

Findings

Confirmed the velocity drop around spectral type B1 at ~21,000 K.

Found the bi-stability jump to be larger than previously thought.

Predicted low wind velocities relevant for supernova progenitors.

Abstract

We predict quantitative mass-loss rates and terminal wind velocities for early-type supergiants and luminous blue variables (LBVs) using a dynamical version of the Monte Carlo radiative transfer method. First, the observed drop in terminal wind velocity around spectral type B1 is confirmed by the Monte Carlo method -- at the correct effective temperature of about 21 000 K. This drop in wind velocity is much steeper than would be expected from the drop in escape speed for cooler stars. The results may be particularly relevant for slow winds inferred for some High-Mass X-ray binaries. Second, the strength of the mass-loss bi-stability jump is found to be significantly larger than previously assumed. Not only could this make bi-stability braking more efficient in massive star evolution, but a rotationally-induced version of the bi-stability mechanism may now be capable of producing the correct density of outflowing disks around B[e] supergiants, although multi-dimensional modelling including the disk velocity structure is still needed. For LBVs, we find the bi-stability jump to become larger at higher metallicities, but perhaps surprisingly also larger at lower Eddington parameters. This may have consequences for the role of LBVs in the evolution of massive stars at different metallicities and Cosmic Epochs. Finally, our predicted low wind velocities may be important for explaining the slow outflow speeds of supernova type IIb/IIn progenitors, for which the direct LBV-SN link was first introduced.