Letter to the Editor

Angular momentum transport through the baroclinic instability in intermediate-mass stars

¹ University of Geneva, Department of Astronomy, 51 Ch. Pegasi, CH-1290 Versoix, Switzerland
² Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
³ Astronomical Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 December 2025
Accepted: 5 March 2026

Abstract

Context. Models of rotating stars in which the transport of angular momentum (AM) is only driven by the shear instability and the meridional circulation tend to overestimate the internal degree of radial differential rotation, when compared to asteroseismic observations. This indicates that additional physical recipes are needed to accurately describe AM transport in radiative stellar layers.

Aims. We investigated the transport of AM in intermediate-mass stars through the baroclinic (BC) instability.

Methods. We implemented in the GENeva Evolution Code (GENEC) a formalism to account for the transport of AM through the BC instability. Numerical simulations were then performed for different initial rotation velocities assuming solid-body rotation on the zero-age main sequence, and both with and without this process in addition to the transport by the shear instability and meridional currents.

Results. We find that the inclusion of an additional AM transport process driven by the BC instability is able to counterbalance the creation of radial differential rotation during the evolution on the main sequence. The effects of the BC instability on the rotation profile appear to be stronger for models with a higher initial rotation velocity. The near-core rotation rates and surface rotation velocities predicted by models with the BC instability accurately reproduce the trend in the observed data available for main-sequence γ Doradus stars. AM transport by the BC instability is, however, not efficient enough in later evolutionary stages to reproduce the low core rotation rates observed in the central layers of red giant stars.

Conclusions. The BC instability appears to be an interesting candidate for the unknown process that transports AM in the interior of main-sequence intermediate-mass stars, in particular for γ Doradus stars. However, it is found to be insufficient to correctly account for the internal rotation of red giants.