The effect of variable stellar magnetic fields on the spin state of T Tauri stars

Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria

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Received: 1 December 2025
Accepted: 29 May 2026

Abstract

Understanding the stellar spin evolution of young stars is crucial for understanding the evolution of protoplanetary disks and, consequently, the formation of exoplanets. According to stellar spin models, T Tauri stars should evolve toward a spin equilibrium in which the external spin-down torque balances both the external spin-up (accretion) torque and the spin-up due to stellar contraction. A useful reference point along the way to this equilibrium is the “zero-torque state” (ZTS), at which only the external torques cancel out. Recent observations, however, have shown that the spin state of a considerable number of stars is shifted out of the spin equilibrium and the ZTS. We investigate the effects of variable stellar magnetic fields on the stellar spin state of T Tauri stars. The long-term decrease (0.1 − 2.0 Myr) in the stellar field strength due to the formation of a radiative core and short-term stellar magnetic cycles (10¹ − 10⁴ years) are taken into consideration. Both scenarios are studied using an implicit disk evolution code capable of modeling the magnetic star–disk interaction in the innermost disk and the stellar spin evolution self-consistently over long timescales. Temporal variations in the stellar magnetic field can significantly affect the stellar spin state of T Tauri stars. The strength of the effect on the stellar spin state depends on the relation between the timescale of the changing magnetic field, the spin-up timescale, and the viscous timescale of the accretion disk. A developing radiative core on a timescale shorter than the spin-up timescale has a strong effect on the spin state. Stellar magnetic cycles on timescales shorter than the viscous timescale of the inner disk have a weaker effect on the stellar spin state due to a slow back-reaction of the accretion disk. Our results can explain (at least) a part of the stars that are observed out of both states. Further theoretical and observational work is needed to connect accretion, stellar rotation, and magnetic properties in T Tauri stars.