Constraining general massive-star physics by exploring the unique properties of magnetic O-stars: Rotation, macroturbulence, and sub-surface convection

TL;DR

This study investigates how magnetic fields in O-stars influence their rotation, turbulence, and convection, revealing that magnetic suppression affects spectroscopic measurements and suggests a sub-surface convection origin for observed turbulence.

Contribution

It demonstrates the impact of magnetic fields on spectroscopic rotation estimates and links macroturbulence to sub-surface convection zones in massive stars.

Findings

Magnetic O-stars are predominantly slow rotators.

Macroturbulence can be overestimated due to line-broadening effects.

Sub-surface convection zones likely cause observed macroturbulence.

Abstract

A quite remarkable aspect of non-interacting O-stars with detected surface magnetic fields is that they all are very slow rotators. This paper uses this unique property to first demonstrate that the projected rotational speeds of massive, hot stars, as derived using current standard spectroscopic techniques, can be severely overestimated when significant "macroturbulent" line-broadening is present. This may, for example, have consequences for deriving the statistical distribution of rotation rates in massive-star populations, and for the use of these rates in stellar evolution models. It is next shown how such macroturbulence (seemingly a universal feature of hot, massive stars) is present in all but one of the magnetic O-stars, namely NGC 1624-2. Assuming then a simple model in which NGC 1624-2's exceptionally strong, large-scale magnetic field suppresses atmospheric motions down to layers where the magnetic and gas pressures are comparable, first empirical constraints on the formation depth of this enigmatic hot-star macroturbulence are derived. The results suggest an origin in the thin sub-surface convection zone of massive stars, consistent with a physical origin due to, e.g., stellar pulsations excited by the convective motions.