天鹅座蛋星云的微分自行测量：赤道外流的存在
Differential Proper-Motion Measurements of The Cygnus Egg Nebula: The Presence of Equatorial Outflow
摘要：
利用哈勃空间望远镜两历元光学数据对蛋状星云（RAFGL 2688，V1610 Cyg）进行了自行微分分析。首先，我们确定蛋星云的偏振特性受到星云中心区域较高光学深度的影响（即，尘埃球的半径约为1000天文单位），使星云分两步被照亮--直接的星光首先被引导到双极腔中，然后散射到星云的其余部分。然后，我们测量了当地的结构和签名同心弧的运动量，通过确定它们在7.25年的时间间隔的相对位移。根据我们的分析，不依赖于单次散射假设，我们得出结论，在过去的400年里，波瓣是通过沿着双极轴的线性扩张而被挖掘的，而同心弧是连续产生的，在过去的5,500年里，它以大约10公里/秒的速度径向向外移动，同心弧径向膨胀速度场有顺时针增大的趋势。人们对中心二元系统的质量损失调制进行了数值研究，预测了质量损失喷出物的螺旋激波尾流中存在这样一个纵坐标增加的膨胀速度场。因此，蛋星云可能代表了一个罕见的边缘上的情况下，联星调制的拱星环境，证实了以前的理论预测。
英文摘要：
We present the results of differential proper-motion analyses of the Egg Nebula (RAFGL 2688, V1610 Cyg) based on the archived two-epoch optical data taken with the Hubble Space Telescope. First, we determined that the polarization characteristics of the Egg Nebula is influenced by the higher optical depth of the central regions of the nebula (i.e., the "dustsphere" of about 1000 AU radius), causing the nebula illuminated in two steps -- the direct starlight is first channeled into bipolar cavities and then scattered off to the rest of the nebula. We then measured the amount of motion of local structures and the signature concentric arcs by determining their relative shifts over the 7.25 yr interval. Based on our analysis, which does not rely on the single-scattering assumption, we concluded that the lobes have been excavated by a linear expansion along the bipolar axis for the past 400 yr, while the concentric arcs have been generated continuously and moving out radially at about 10 km/s for the past 5,500 yr, and there appears to be a colatitudinally-increasing trend in the radial expansion velocity field of the concentric arcs. There exist numerical investigations into the mass-loss modulation by the central binary system, which predict such a colatitudinally-increasing expansion velocity field in the spiral-shock trails of the mass-loss ejecta. Therefore, the Egg Nebula may represent a rare edge-on case of the binary-modulated circumstellar environs, corroborating the previous theoretical predictions.
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