The ESO UVES/FEROS Large Programs of TESS OB pulsators

II. The physical origin of macroturbulence^⋆

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
³ Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010 6500 GL Nijmegen, The Netherlands

^⋆⋆ Corresponding author; nadya.serebriakova@kuleuven.be

Received: 19 July 2024
Accepted: 17 October 2024

Abstract

Context. Spectral lines of hot massive stars are known to exhibit large excess broadening in addition to rotational broadening. This excess broadening is often attributed to macroturbulence, whose physical origin is a matter of active debate in the stellar astrophysics community.

Aims. We aim to shed light on the physical origin of macroturbulent line broadening by looking into the statistical properties of a large sample of O- and B-type stars both in the Galaxy and the Large Magellanic Cloud (LMC).

Methods. We deliver newly measured macroturbulent velocities for 86 stars from the Galaxy in a consistent manner with 126 stars from the LMC. We composed a total sample of 594 stars with measured macroturbulent velocities by complementing our sample with archival data for the Galactic O- and B-type stars in order to gain better coverage of the parameter space. Furthermore, we computed an extensive grid of MESA models to compare, in a statistical manner, the predicted interior properties of stars (such as convection and wave propagation) with the inference of macroturbulent velocities from high-resolution spectroscopic observations.

Results. We find evidence for subsurface convective zones that formed in the iron opacity bump (FeCZ) being connected to observed macroturbulent velocities in hot massive stars. Additionally, we find the presence of two principally different regimes where, depending on the initial stellar mass, different mechanisms may be responsible for the observed excess line broadening.

Conclusions. Stars with initial masses above 30 M_⊙ exhibit macroturbulent velocities that are in line with FeCZ properties, as indicated by the trends in both observations and models. For stars below 12 M_⊙, alternative mechanisms are needed to explain macroturbulent broadening, such as internal gravity waves (IGWs). Finally, in the intermediate range between 12 and 30 M_⊙, IGWs tunnelling through subsurface convective layers combined with the presence of FeCZ-driven convection suggests that both processes could contribute to the observed macroturbulent velocities. This intermediate regime presents a region where the interplay between these two (or more) mechanisms remains to be fully understood.