H-alpha Emission as a Diagnostic of Plasma Transport Mechanics in Centrifugal Magnetospheres

Approximately one quarter of magnetic early (B5-B0) B-type type stars display Balmer line emission originating in centrifugal magnetospheres (CMs). Indeed, the CMs of these stars are uniquely detectable in all available magnetospheric diagnostics (radio synchrotron, NIR, Balmer emission, UV emission, and X-rays), and therefore represent an excellent opportunity for detailed study of magnetospheric plasmas. All CM host-stars are young, possess strong magnetic fields, and are rapid rotators, properties consistent with expectations from the Rigidly Rotating Magnetosphere model and evolutionary models incorporating magnetic braking. The nature of mass transport within CMs has been debated, with two competing scenarios: centrifugally driven magnetic reconnection due to overloading of the magnetic field by the stellar wind, and leakage arising from diffusion and drift of ions across magnetic field lines. We have conducted the first detailed study of the Halpha emission properties of the population of CM host stars. The results demonstrate that mass balancing must be accomplished by breakout. We find that emission strength is independent of mass-loss rate and can, to first order, be predicted simply by the strength of the equatorial magnetic field at the Kepler corotation radius. However, an apparent cutoff in the presence of emission lines at low luminosities may be indicative of diffusion/drift mechanisms becoming dominant in the weak-wind regime. We briefly point to potential implications of these results for X-ray and radio synchrotron emission.