Observational consequences of turbulent pressure in the envelopes of massive stars

TL;DR

This paper explores how turbulent pressure in the sub-surface convection zones of massive stars influences observable phenomena, particularly macroturbulence, with implications for stellar structure and oscillations.

Contribution

It demonstrates the significant turbulent pressure fractions in massive star envelopes and links these to observable macroturbulent velocities, suggesting a physical connection.

Findings

Turbulent pressure can reach up to 5% in OB supergiants and 30% in cooler supergiants.

Structural changes due to turbulent pressure are minimal on evolutionary tracks.

Strong correlation between turbulent pressure fraction and macroturbulent velocities in stellar spectra.

Abstract

The major mass fraction of the envelope of hot luminous stars is radiatively stable. However, the partial ionisation of hydrogen, helium and iron gives rise to extended sub-surface convection zones in all of them. In this work, we investigate the effect of the pressure induced by the turbulent motion in these zones based on the mixing length theory, and search for observable consequences. We find that the turbulent pressure fraction can amount up to ~5% in OB supergiants, and to ~30% in cooler supergiants. The resulting structural changes are, however, not significantly affecting the evolutionary tracks compared to previous calculations. Instead, a comparison of macroturbulent velocities derived from high quality spectra of OB stars with the turbulent pressure fraction obtained in corresponding stellar models reveals a strong correlation of these two quantities. We discuss a possible physical connection, and conclude that turbulent pressure fluctuations may drive high-order oscillations, which - as conjectured earlier - manifest themselves as macroturbulence in the photospheres of hot luminous stars.