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

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Original 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. [Abstract shortened for arXiv] 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.

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