How Centrifugal Breakout from Magnetic B-stars Controls the Onset of Their Hα Emission

How Centrifugal Breakout from Magnetic B-stars Controls the Onset of Their Hα Emission

S. Owocki, M. E. Shultz, A. ud-Doula, J. Sundqvist

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/diffusive leakage, or through sporadic '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 is largely 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 equilibrium mass in the magnetosphere is indeed 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 Hα emission. 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.

Proceedings of the Polish Astronomical Society, vol. 11, 131-139 (2020)

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