Collective pulsational velocity broadening due to gravity modes as a physical explanation for macroturbulence in hot massive stars

TL;DR

This study proposes that macroturbulence in hot massive stars' atmospheres is caused by the collective effect of gravity-mode pulsations, offering a physical explanation supported by line profile simulations.

Contribution

It demonstrates that pulsational broadening can explain macroturbulence and provides diagnostics to distinguish between pulsational and turbulent broadening effects.

Findings

Pulsational broadening can mimic macroturbulence in line profiles.

Realistic pulsation amplitudes can produce velocities exceeding the sound speed.

Using pulsational models improves rotation velocity estimates.

Abstract

We aimed at finding a physical explanation for the occurrence of macroturbulence in the atmospheres of hot massive stars, a phenomenon found in observations since more than a decade but yet unexplained. We computed time series of line profiles for evolved massive stars broadened by rotation and by hundreds of low-amplitude nonradial gravity-mode pulsations which are predicted to be excited for evolved massive stars. In general, line profiles based on macrotubulent broadening can mimic those subject to pulsational broadening. In several cases, though, good fits require macroturbulent velocities that pass the speed of sound for realistic pulsation amplitudes. Moreover, we find that the rotation velocity can be seriously underestimated by using a simple parameter description for macroturbulence rather than an appropriate pulsational model description to fit the line profiles. We conclude that macroturbulence is a likely signature of the collective effect of pulsations. We provide line diagnostics and their typical values to decide whether or not pulsational broadening is present in observed line profiles, as well as a procedure to avoid an inaccurate estimation of the rotation velocity.