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Modeling Stellar Activity-Rotation Relations in Unsaturated Cool Stars

Published onAug 18, 2020
Modeling Stellar Activity-Rotation Relations in Unsaturated Cool Stars

We apply a surface flux transport (SFT) model developed for the Sun to reconstruct the stellar activity-rotation relationship, LX/Lbol vs. Ro, observed for “unsaturated” cool stars (Rossby numbers Ro > about 0.1). This empirical flux transport model incorporates modulations of magnetic flux strength consistent with observed stellar activity cycles, as well as surface flux dynamics consistent with observed stellar relationships. We find that for stellar flux models corresponding to a range ~0.1 < (Ro/RoSun) < 2, the LX/Lbol vs. Ro relation matches the power-law behavior observed in the unsaturated regime of cool stars extremely well. Additionally, the magnetic activity cycles captured by the stellar simulations produce a spread about the power-law relation consistent with that observed in cool star populations, indicating that the observed spread may be caused by intrinsic variations resulting from cyclic stellar behavior. The success of our flux transport modeling in reproducing the observed activity relationship across a wide range of late-F, G, K, and M stars suggests that the photospheric magnetic fields of all unsaturated cool stars exhibit similar flux emergence and surface dynamic behavior, and may hint at possible similarities in stellar dynamo action across a broad range of stellar types.


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