Sub-mm free-free emission from the winds of massive stars in the age of ALMA

TL;DR

This paper models the sub-mm emission from massive star winds, highlighting the importance of wind acceleration and clumping, and demonstrates ALMA's capability to detect these effects through spectral index deviations.

Contribution

It introduces a new method to calculate thermal radio and sub-mm emission considering wind acceleration and clumping, enhancing understanding of mass-loss rates in massive stars.

Findings

Spectral index varies strongly between 100 GHz and 10,000 GHz.

ALMA can detect deviations caused by wind acceleration and clumping.

Both mechanisms significantly influence the interpretation of stellar mass-loss rates.

Abstract

The thermal radio and sub-mm emission from the winds of massive stars is investigated and the contribution to the emission due to the stellar wind acceleration region and clumping of the wind is quantified. Building upon established theory, a method for calculating the thermal radio and sub-mm emission using results for a line-driven stellar outflow according to Castor, Abbott & Klein (1975) is presented. The results show strong variation of the spectral index for 10 2 GHz < {\nu} < 10 4 GHz. This corresponds both to the wind acceleration region and clumping of the wind, leading to a strong dependence on the wind velocity law and clumping parameters. The Atacama Large Millimeter/sub-mm Array (ALMA) is the first observatory to have both the spectral window and sensitivity to observe at the high frequencies required to probe the acceleration regions of massive stars. The deviations in the predicted flux levels as a result of the inclusion of the wind acceleration region and clumping are sufficient to be detected by ALMA, through deviations in the spectral index in different portions of the radio/sub-mm spectra of massive stars, for a range of reasonable mass-loss rates and distances. Consequently both mechanisms need to be included to fully understand the mass-loss rates of massive stars.