Magnetic complexity as an explanation for bimodal rotation populations among young stars

TL;DR

This paper demonstrates that magnetic field complexity in young stars explains the bimodal distribution of their rotation rates by affecting angular momentum loss, supporting a physical basis for the Metastable Dynamo Model.

Contribution

It provides a physical explanation for the bimodal rotation populations by linking magnetic complexity to angular momentum loss in young stars.

Findings

Magnetic complexity sharply reduces mass and angular momentum loss.

Young rapid rotators have complex magnetic fields leading to inefficient spin-down.

Erosion of magnetic complexity triggers rapid transition to strong wind coupling.

Abstract

Observations of young open clusters have revealed a bimodal distribution of fast and slower rotation rates that has proven difficult to explain with predictive models of spin down that depend on rotation rates alone. The Metastable Dynamo Model proposed recently by Brown, employing a stochastic transition probability from slow to more rapid spin down regimes, appears to be more successful but lacks a physical basis for such duality. Using detailed 3D MHD wind models computed for idealized multipole magnetic fields, we show that surface magnetic field complexity can provide this basis. Both mass and angular momentum losses decline sharply with increasing field complexity. Combined with observation evidence for complex field morphologies in magnetically active stars, our results support a picture in which young, rapid rotators lose angular momentum in an inefficient way because of field complexity. During this slow spin-down phase, magnetic complexity is eroded, precipitating a rapid transition from weak to strong wind coupling.