高加速反磁等离子体团：OB星风X射线产生的新机制
Highly Accelerated Diamagnetic Plasmoids: A New X-ray Production Mechanism for OB Stellar Winds
摘要：
观测到的X射线源的温度分布在OB恒星风，确定从高能量分辨率钱德拉观测，表明最高温度发生在恒星附近，然后稳步下降，通过风向外。为了解释这种意想不到的行为，我们提出了一个冲击模型的概念，利用一个众所周知的磁推进机制，表面喷射的“反磁性等离子体团”到一个分散的外部磁场。这会产生快速加速的独立结构，这些结构穿过环境风并形成弓形激波，弓形激波产生由等离子体团风相对速度确定的X射线温度范围。模型的自由参数是等离子体团的初始阿尔芬速度，初始等离子体β的外部介质，和外部场的发散率。这些都是通过拟合预测的弓形激波温度与观测到的OB超巨星X射线温度分布来确定的。我们发现，初始外部等离子体β的范围在0和2之间，并且假设径向减小的外部磁场强度与r^{-S}成比例，S的值在2和3之间。最重要的是，初始等离子体团阿尔芬速度被发现是很好的约束在一个值的0.6倍的终极速度，这似乎代表了所有正常OB星的上限。这个有趣的新的限制OB磁性能，来自钱德拉观测，强调需要进一步研究这些恒星的磁推进机制。
英文摘要：
The observed X-ray source temperature distributions in OB stellar winds, as determined from high energy resolution Chandra observations, show that the highest temperatures occur near the star, and then steadily decrease outward through the wind. To explain this unexpected behavior, we propose a shock model concept that utilizes a well-known magnetic propulsion mechanism; the surface ejection of "diamagnetic plasmoids" into a diverging external magnetic field. This produces rapidly accelerating self-contained structures that plow through an ambient wind and form bow shocks that generate a range in X-ray temperatures determined by the plasmoid-wind relative velocities. The model free parameters are the plasmoid initial Alfven speed, the initial plasma-beta of the external medium, and the divergence rate of the external field. These are determined by fitting the predicted bow shock temperatures with the observed OB supergiant X-ray temperature distribution. We find that the initial external plasma-beta has a range between 0 and 2, and the assumed radially-decreasing external magnetic field strength that scales as r^{-S} has a value of S lying between 2 and 3. Most importantly, the initial plasmoid Alfven speed is found to be well-constrained at a value of 0.6 times the terminal velocity, which appears to represent an upper limit for all normal OB stars. This intriguing new limit on OB magnetic properties, as derived from Chandra observations, emphasizes the need for further studies of magnetic propulsion mechanisms in these stars.
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