Polar branches of stellar activity waves: dynamo models and observations

TL;DR

This paper investigates polar activity wave branches in stars, combining observations with mean-field dynamo models to understand their propagation, relation to stellar rotation laws, and implications for stellar internal dynamics.

Contribution

It unites observational data with dynamo theory, proposing models that explain polar activity branches and their connection to stellar rotation profiles.

Findings

Polar activity branches are more consistent with cylindrical rotation laws in fast rotators.

Dynamo models with two-layer structures better explain observed polar activity.

Observations can constrain internal stellar rotation profiles.

Abstract

[Abridged abstract:] Stellar activity data provide evidence of activity wave branches propagating polewards rather than equatorwards (the solar case). Stellar dynamo theory allows polewards propagating dynamo waves for certain governing parameters. We try to unite observations and theory, restricting our investigation to the simplest mean-field dynamo models. We suggest a crude preliminary systematization of the reported cases of polar activity branches. Then we present results of dynamo model simulations which contain magnetic structures with polar dynamo waves, and identify the models which look most promising for explaining the latitudinal distribution of spots in dwarf stars. Those models require specific features of stellar rotation laws, and so observations of polar activity branches may constrain internal stellar rotation. Specifically, we find it unlikely that a pronounced poleward branch can be associated with a solar-like internal rotation profile, while it can be more readily reproduced in the case of a cylindrical rotation law appropriate for fast rotators. We stress the case of the subgiant component of the active close binary HR 1099 which, being best investigated, presents the most severe problems for a dynamo interpretation. Our best model requires dynamo action in two layers separated in radius. Observations of polar activity branches provide valuable information for understanding stellar activity mechanisms and internal rotation, and thus deserve intensive observational and theoretical investigation. Current stellar dynamo theory seems sufficiently robust to accommodate the phenomenology.