Full-Polarization Observations of OH Masers in Massive Star-forming Regions. II. Maser Properties and the Interpretation of Polarization
Full-Polarization Observations of OH Masers in Massive Star-forming Regions. II. Maser Properties and the Interpretation of Polarization
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大质量恒星形成区域 OH 脉泽的全偏振观测。
DOI：
10.1086/502650
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发表时间：
2006
期刊：
影响因子：
--
通讯作者：
M. Reid
中科院分区：
文献类型：
--
作者：
V. Fish;M. Reid
我们分析了全极化VLBA数据的基态OH脉泽在18个大质量的恒星形成区域以前提出的配套文件。我们确认以前看到的结果在几个单独的来源研究毫角秒角分辨率。OH脉泽常产生于超紧H Ⅱ区周围的激波中性气体中。从OH脉泽塞曼分裂导出的磁场在源尺度和~1015 cm脉泽团簇尺度上是高度有序的，这似乎是普遍的。OH脉泽在超致密H II区的寿命约为104年，然后突然关闭，而不是逐渐减弱。这些脉泽具有广泛的偏振特性。在一个极端（例如，W75 N），探测到π分量，脉泽斑的偏振位置角有一定的组织性。在另一个极端（例如，W51 e1/e2），几乎没有检测到线性偏振，并且发生部分去偏振。一个典型的源具有介于这两个极端之间的性质，在偏振位置角的分布中没有清晰的模式。这可以解释，如果法拉第旋转在一个典型的OH脉泽源是大的脉泽放大长度，但在一个单一的（e-折叠）增益长度小。增加或减少法拉第旋转的一个因素~5之间的不同来源可以解释所观察到的偏振特性的变化。纯π分量（理论上，100%线性偏振）很少见到。我们认为，几乎所有的π分量获得了显着量的圆偏振从低增益受激发射的σ分量的速度相干OH躺在沿着传播路径。
We analyze full-polarization VLBA data of ground-state OH masers in 18 massive star-forming regions previously presented in a companion paper. We confirm results previously seen in the few individual sources studied at milliarcsecond angular resolution. The OH masers often arise in the shocked neutral gas surrounding ultracompact H II regions. Magnetic fields as deduced from OH maser Zeeman splitting are highly ordered on the scale of a source and on the maser clustering scale of ~1015 cm, which appears to be universal. OH masers around ultracompact H II regions live ~104 yr before turning off abruptly, rather than weakening gradually. These masers have a wide range of polarization properties. At one extreme (e.g., W75 N), π-components are detected and the polarization position angles of maser spots show some organization. At the other extreme (e.g., W51 e1/e2), almost no linear polarization is detected and partial depolarization occurs. A typical source has properties intermediate to these two extremes, with no clear pattern in the distribution of polarization position angles. This can be explained if Faraday rotation in a typical OH maser source is large on a maser amplification length but small on a single (e-folding) gain length. Increasing or decreasing Faraday rotation by a factor of ~5 among different sources can explain the observed variation in polarization properties. Pure π-components (in theory, 100% linearly polarized) are seldom seen. We suggest that almost all π-components acquire a significant amount of circular polarization from low-gain stimulated emission of a σ-component from velocity-coherent OH lying along the propagation path.