Relating turbulent pressure and macroturbulence across the HR diagram with a possible link to \gamma-Dor stars

TL;DR

This study links turbulent pressure fluctuations in stellar envelopes to observed macroturbulence and high-order oscillations, suggesting a connection to the gamma Doradus instability strip across the HR diagram.

Contribution

It demonstrates a strong correlation between turbulent pressure and macroturbulence, proposing turbulent pressure fluctuations as a driver for gamma Doradus pulsations.

Findings

Strong correlation between turbulent pressure and macroturbulence.

Turbulent pressure fluctuations may excite high-order stellar oscillations.

Location of high-order oscillations matches gamma Doradus instability strip.

Abstract

A significant fraction of the envelope of low- and intermediate-mass stars is unstable to convection, leading to sub-surface turbulent motion. Here, we consider and include the effects of turbulence pressure in our stellar evolution calculations. In search of an observational signature, we compare the fractional contribution of turbulent pressure to the observed macroturbulent velocities in stars at different evolutionary stages. We find a strong correlation between the two quantities, similar to what was previously found for massive OB stars. We therefore argue that turbulent pressure fluctuations of finite amplitude may excite high-order, high-angular degree stellar oscillations, which manifest themselves at the surface an additional broadening of the spectral lines, i.e., macroturbulence, across most of the HR diagram. When considering the locations in the HR diagram where we expect high-order oscillations to be excited by stochastic turbulent pressure fluctuations, we find a close match with the observational $\gamma$ Doradus instability strip, which indeed contains high-order, non-radial pulsators. We suggest that turbulent pressure fluctuations on a percentual level may contribute to the $\gamma$ Dor phenomenon, calling for more detailed theoretical modelling in this direction.