Tayler instability in red giant cores: constraints from 3D direct numerical simulations

The stability of magnetic fields in radiative stellar interiors remains a key open problem, with important implications for the rotational and chemical evolution of low-mass stars. Recent asteroseismic observations of red giants reveal unexpectedly slow core rotations and signatures of strong internal magnetic fields, placing new constraints on angular momentum transport, possibly driven by magnetic instabilities. We investigate the Tayler instability – a kink-type instability of toroidal magnetic fields in stably stratified regions – using new 3D direct numerical simulations in spherical geometry. Our simulations adopt a consistent magnetohydrostatic background and include stable stratification and diffusive processes. First, in a non-rotating setup, we identify two unstable regimes: small-scale modes consistent with local (WKB) predictions, and large-scale global modes partially stabilized by stratification, in agreement with Bonanno & Urpin (2012). By combining these results with stellar evolution models, we constrain the critical toroidal field strengths for both the onset of the instability and the transition between regimes in red giant cores. Then, we present preliminary results including rotation, showing how the Tayler instability operates in rotating backgrounds and providing a first characterization of the resulting angular momentum transport.