Letter to the Editor

Impact of the Tayler magnetic instability on the surface abundance of boron in massive stars

¹ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
³ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, B-1050 Brussels, Belgium

^⋆ Corresponding author: loukas.asatiani@etu.unige.ch

Received: 15 April 2025
Accepted: 23 June 2025

Abstract

Context. The surface abundances of massive stars show evidence of internal mixing, while asteroseismic data suggest that efficient angular momentum (AM) transport occurs in stellar interiors. It is of interest to find a consistent physical framework that is able to account for both of these effects simultaneously.

Aims. We investigate the impact of the Tayler magnetic instability on the surface abundance of boron in massive B-type stars as predicted by rotating stellar models accounting for the advective nature of meridional currents.

Methods. We used the Geneva stellar evolution code (GENEC) to compute models of 9, 12, and 15 M_⊙ stars at different rotational velocities and with and without magnetic fields. We compared the surface boron abundances predicted by these models with those of observed B-type stars to test the consistency between AM transport constrained by asteroseismology and chemical mixing constrained by boron abundances.

Results. We find that models with only hydrodynamic transport processes overestimate the amount of boron depletion for stars with high rotation rates, in disagreement with observational constraints. We show that this excessively high mixing efficiency is a direct consequence of the high degree of differential rotation predicted by purely hydrodynamic models. We thus conclude that, similarly to asteroseismic measurements, surface abundances of boron also indicate that a more efficient AM transport is needed in stellar radiative zones. We then studied the impact of the magnetic Tayler instability as a possible physical explanation to this issue. These magnetic models, which correctly reproduce asteroseismic constraints on AM transport, are found to be in good agreement with constraints on surface boron abundances, the evolutionary state, and the projected rotational velocity of moderately and fast-rotating B-type stars. We thus conclude that models accounting for the simultaneous impact of the magnetic Tayler instability and the advective nature of meridional currents offer a coherent and physically motivated theoretical framework that reproduces chemical mixing as revealed by boron abundances of fast-rotating stars and AM transport as constrained by asteroseismology. Finally, we note that at low rotational velocities (v sin i ≲ 50 km/s), models with magnetic fields do not predict sufficient depletion to be consistent with the observations, which are more compatible with non-magnetic models. This could suggest that the current prescriptions for transport by the Tayler instability somewhat overestimate the AM transport efficiency in slowly rotating B-type stars.