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arXiv 1907.03325astro-ph.SRastro-ph.GAastro-ph.HE
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FU Ori-type星与一般年轻恒星天体的X射线性质比较

A Comparison of the X-ray Properties of FU Ori-type Stars to Generic Young Stellar Objects

Michael A. Kuhn, Lynne A. Hillenbrand

备注：
17 pages, 6 figures, 3 tables. Accepted for publication in ApJ
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摘要：

像其他年轻的恒星天体（YSO）一样，FU Ori-type恒星被发现是强X射线发射体。然而，人们对这些恒星的爆发如何影响其X射线特性知之甚少。我们收集了来自XMM牛顿和钱德拉观测的16颗FU Ori恒星的可用X射线数据，其中包括盖亚17 bpi光学上升阶段的新XMM牛顿观测。在这些恒星中，有6颗至少被探测到一次，而10颗没有被探测到，我们计算了其固有X射线光度（$L_X$）作为等离子体温度（$kT$）和柱密度（$N_H$）的函数的上限。根据与猎户座星云团中低质量恒星样本的比较，检测到的FU Ori恒星往往比非爆发YSO的典型恒星更X射线明亮。在爆发初期和几十年后都观测到了具有高$L_X$的FU Ori星。我们使用的Kaplan-Meier估计调查是否较高的X射线光度为FU Ori星的特征或选择效应的结果，我们发现的差异是统计上显着的（$p<0.01$），即使考虑到非检测。相对于非爆发的YSO，FU Ori恒星的额外X射线亮度不能用吸积冲击来解释，因为观察到的等离子体温度很高。这表明，对于许多FU Ori恒星，要么1）爆发导致磁层的重组，以增强X射线发射的方式，或2）FU Ori爆发更有可能发生在具有最高静态X射线光度的YSO中。

英文摘要：

Like other young stellar objects (YSOs), FU Ori-type stars have been detected as strong X-ray emitters. However, little is known about how the outbursts of these stars affect their X-ray properties. We assemble available X-ray data from XMM Newton and Chandra observations of 16 FU Ori stars, including a new XMM Newton observation of Gaia 17bpi during its optical rise phase. Of these stars, six were detected at least once, while 10 were non-detections, for which we calculate upper limits on intrinsic X-ray luminosity ($L_X$) as a function of plasma temperature ($kT$) and column density ($N_H$). The detected FU Ori stars tend to be more X-ray luminous than typical for non-outbursting YSOs, based on comparison to a sample of low-mass stars in the Orion Nebula Cluster. FU Ori stars with high $L_X$ have been observed both at the onset of their outbursts and decades later. We use the Kaplan-Meier estimator to investigate whether the higher X-ray luminosities for FU Ori stars is characteristic or a result of selection effects, and we find the difference to be statistically significant ($p<0.01$) even when non-detections are taken into account. The additional X-ray luminosity of FU Ori stars relative to non-outbursting YSOs cannot be explained by accretion shocks, given the high observed plasma temperatures. This suggests that, for many FU Ori stars, either 1) the outburst leads to a restructuring of the magnetosphere in a way that enhances X-ray emission, or 2) FU Ori outbursts are more likely to occur among YSOs with the highest quiescent X-ray luminosity.

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