Centrifugal breakout reconnection as the electron acceleration mechanism powering the radio magnetospheres of early-type stars

TL;DR

This paper proposes that centrifugal breakout-driven magnetic reconnection accelerates electrons in early-type star magnetospheres, explaining observed radio emissions and their dependence on stellar magnetic field and rotation.

Contribution

It introduces a novel model linking centrifugal breakout reconnection to electron acceleration, providing theoretical scalings that match empirical radio luminosity trends in magnetic B-stars.

Findings

CBO reconnection scalings align with observed radio luminosity.

A small, consistent energy conversion efficiency (~10^{-8}) is identified.

The model offers advantages over previous electromotive force explanations.

Abstract

Magnetic B-stars often exhibit circularly polarized radio emission thought to arise from gyrosynchrotron emission by energetic electrons trapped in the circumstellar magnetosphere. Recent empirical analyses show that the onset and strength of the observed radio emission scale with both the magnetic field strength and the stellar rotation rate. This challenges the existing paradigm that the energetic electrons are accelerated in the current sheet between opposite-polarity field lines in the outer regions of magnetised stellar winds, which includes no role for stellar rotation. Building on recent success in explaining a similar rotation-field dependence of H$\alpha$ line emission in terms of a model in which magnetospheric density is regulated by centrifugal breakout (CBO), we examine here the potential role of the associated CBO-driven magnetic reconnection in accelerating the electrons that emit the observed gyrosynchrotron radio. We show in particular that the theoretical scalings for energy production by CBO reconnection match well the empirical trends for observed radio luminosity, with a suitably small, nearly constant conversion efficiency $\epsilon \approx 10^{-8}$. We summarize the distinct advantages of our CBO scalings over previous associations with an electromotive force, and discuss the potential implications of CBO processes for X-rays and other observed characteristics of rotating magnetic B-stars with centrifugal magnetospheres.