How the breakout-limited mass in B-star centrifugal magnetospheres controls their circumstellar H-alpha emission

Abstract

Strongly magnetic B-type stars with moderately rapid rotation form `centrifugal magnetospheres' (CMs), from the magnetic trapping of stellar wind material in a region above the Kepler co-rotation radius. A longstanding question is whether the eventual loss of such trapped material occurs from gradual drift and/or diffusive leakage, or through sporadic `{\em centrifugal break out}' (CBO) events, wherein magnetic tension can no longer contain the built-up mass. We argue here that recent empirical results for Balmer-$α$ emission from such B-star CMs strongly favor the CBO mechanism. Most notably, the fact that the onset of such emission depends mainly on the field strength at the Kepler radius, and is largely {\em independent} of the stellar luminosity, strongly disfavors any drift/diffusion process, for which the net mass balance would depend on the luminosity-dependent wind feeding rate. In contrast, we show that in a CBO model the {\em maximum confined mass} in the magnetosphere is independent of this wind feeding rate, and has a dependence on field strength and Kepler radius that naturally explains the empirical scalings for the onset of H$α$ emission, its associated equivalent width, and even its line profile shapes. However, the general lack of observed Balmer emission in late-B and A-type stars could still be attributed to a residual level of diffusive or drift leakage that does not allow their much weaker winds to fill their CMs to the breakout level needed for such emission; alternatively this might result from a transition to a metal-ion wind that lacks the requisite Hydrogen.