arXiv 2009.123592020-09-25astro-ph.SR

B星离心磁层中爆发极限质量如何控制其绕星H-α发射

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

Stanley P. Owocki, Matt E. Shultz, Asif ud-Doula, Jon O. Sundqvist, Richard H. D. Townsend, Steven R. Cranmer

备注：: 13 pages, 11 figures, accepted for MNRAS
DOI：: 10.1093/mnras/staa2325

摘要：

具有中等速度旋转的强磁性B型恒星形成“离心磁层”（CM），这是因为在开普勒共同旋转半径以上的区域中，恒星风物质被磁捕获。一个长期存在的问题是，这种捕获的材料的最终损失是由于逐渐漂移和/或扩散泄漏，还是由于偶发的“{ \em离心破裂}”（CBO）事件，其中磁张力不再包含累积的质量。我们在这里认为，最近的经验结果Balmer- $α$发射从这样的B星CM强烈支持CBO机制。最值得注意的是，这种发射的开始主要取决于开普勒半径处的场强，并且在很大程度上与恒星的光度{ \em无关}，这一事实强烈地不利于任何漂移/扩散过程，对于漂移/扩散过程，净质量平衡将取决于与光度相关的风馈送速率。与此相反，我们表明，在CBO模型中的{ \em最大约束质量}在磁层是独立的风喂养率，并依赖于磁场强度和开普勒半径，自然解释了经验缩放的发病H $α$发射，其相关的等效宽度，甚至其线轮廓形状。然而，在晚B型和A型恒星中普遍缺乏观测到的巴耳末辐射仍然可以归因于残余水平的扩散或漂移泄漏，这不允许它们的弱得多的风将它们的CM填充到这种辐射所需的突破水平;或者这可能是由于过渡到缺乏必要氢的金属离子风。

英文摘要：

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.

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