Collective velocity broadening from gravity waves as a plausible mechanism for macroturbulence in massive stars

Almost all massive stars end their lives as supernovae and form a black hole or neutron star, which consequently drive galactic evolution. However there are currently large uncertainties pertaining to massive star interiors already during the main sequence, such as their interior mixing and rotation. Asteroseismology of gravity waves in massive stars provides a unique insight of the physics beneath their opaque surface layers. All massive stars are predicted to excite gravity waves from core convection and from turbulent pressure fluctuations in their near-surface layers, which are extremely efficient at transporting angular momentum and mixing within stellar interiors. On the other hand, spectroscopy provides detailed constraints on the physics of stellar atmospheres, and reveals the importance of increasing macroturbulent broadening in the spectral lines of more massive stars. In this talk, I will discuss recent results based on the combination of high-precision TESS space photometry and high-resolution spectroscopy of massive stars. A clear relationship is found between the location of a massive star in the Hertzsprung-Russell diagram and the amplitudes and frequencies of stochastic photometric variability caused by gravity waves in the light curves of massive stars. Furthermore, the properties of the stochastic variability are strongly correlated with macroturbulent broadening in the spectral lines of massive stars. The common ensemble morphology for the stochastic low-frequency variability detected in TESS photometry and its relationship to macroturbulence is strong evidence for gravity waves being a plausible mechanism for macroturbulence in massive stars, as such waves are unique in providing the dominant tangential velocity field required to explain the observed spectroscopy. Finally, I will discuss the important similarities and differences between magnetic and non-magnetic massive stars in terms of their gravity wave signatures and measured macroturbulent velocities.