Micro-turbulence across the Hertzsprung-Russell diagram. Observational constrains for stars in the MW

TL;DR

This study compiles a comprehensive database of stellar parameters for over 1800 stars, revealing that micro-turbulence is a real physical effect likely caused by envelope convection and pulsations, impacting stellar modeling and understanding.

Contribution

It provides the first homogeneous, extensive dataset of stellar micro-turbulence parameters across a wide range of stars, enabling improved physical understanding and modeling.

Findings

Micro-turbulence is confirmed as a physical phenomenon.

Micro-turbulence likely arises from envelope convection and pulsations.

Neglecting micro-turbulence affects stellar rotation and mass estimates.

Abstract

We assemble a homogeneous database of precise and consistent determinations of effective temperature, surface gravity, projected rotational rate, and macro- and micro-turbulent velocities for over 1800 Galactic stars spanning spectral types O to K and luminosity classes I to V. By carefully minimizing biases due to target selection, data quality, and disparate analysis techniques, we carry out statistical tests and comparative analyses to probe potential dependencies between these parameters and micro-turbulence. Our findings indicate that photospheric micro-turbulence is a genuine physical phenomenon rather than a modelling artifact. A direct comparison between observed micro-turbulent velocities and corresponding theoretical predictions for the turbulent pressure fraction strongly suggests that this phenomenon most likely arises from photospheric motions driven by envelope convection zones, with an additional pulsational component likely operating in main-sequence B stars. We show that neglecting micro-turbulent broadening in Fourier transform analyses can partly explain the dearth of slow rotators and the scarcity of stars with extremely low macro-turbulent velocity. We argue that including micro-turbulent pressure in atmospheric modelling can significantly mitigate (even resolve) the mass discrepancy for less massive O stars. Our database offers a valuable resource for testing and refining theoretical scenarios, particularly those addressing puzzling phenomena in hot massive stars.