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arXiv 1908.03421astro-ph.GA
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银河系冷团尘埃发射和偏振的合成观测

Synthetic observations of dust emission and polarisation of Galactic cold clumps

Mika Juvela, Paolo Padoan, Isabelle Ristorcelli, Veli-Matti Pelkonen

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Accepted to A&A (20 pages, 26 figures)
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摘要：

银河系冷团块的普朗克目录（PGCC）包含了超过13000个根据冷尘埃特征探测到的来源。它们被认为是由静止的、恒星形成前的和已经形成恒星的物体组成。我们从云模拟中提取了PGCC型物体，并研究了它们的物理和偏振特性。通过与PGCC星表的比较，可以进一步验证PGCC星表的性质，为星际介质的数值模拟提供有价值的检验。我们使用了几个MHD快照来定义我们模型的密度场。通过辐射转移计算获得了表面亮度和偏振的亚毫米图像。我们研究了合成冷团目录的统计数据，并研究了团极化分数p的变化。合成目录中的团大小，纵横比和温度与PGCC相似。通量和柱密度小了几倍。这不是因为尘埃不透明度增加，而是因为模型的平均气柱密度增加了2 ~ 3倍，接近N（H2）= 10^22 cm-2。当视线平行于平均磁场时，极化分数倾向于向团块中心增加，与观测相反。当场垂直时，极化分数倾向于朝向团块减小，但是$p$的下降很小（例如，从p~8%到p~7%）。磁场几何形状使模拟团块中的极化分数平均仅降低Δp~1%。朝向实际PGCC团块看到的较大下降表明致密介质中的晶粒排列的一些损失，例如由辐射扭矩机制预测的。统计研究无法量化PGCC团块尺度上的粉尘不透明度变化。

英文摘要：

The Planck Catalogue of Galactic Cold Clumps (PGCC) contains over 13000 sources detected based on their cold dust signature. They are believed to consist of a mixture of quiescent, pre-stellar, and already star-forming objects. We extracted PGCC-type objects from cloud simulations and examined their physical and polarisation properties. The comparison with the PGCC catalogue helps to characterise the PGCC sample and provides valuable tests for numerical simulations of interstellar medium. We used several MHD snapshots to define the density field of our models. Sub-millimetre images of the surface brightness and polarisation were obtained with radiative transfer calculations. We examined the statistics of synthetic cold clump catalogues and examined the variations of the clump polarisation fraction p. The clump sizes, aspect ratios, and temperatures in the synthetic catalogue are similar to the PGCC. The fluxes and column densities are smaller by a factor of a few. Rather than with an increased dust opacity, this could be explained by increasing the average column density of the models by a factor of two to three, close to N(H2)= 10^22 cm-2. When the line of sight is parallel to the mean magnetic field, the polarisation fraction tends to increase towards the clump centres, contrary to observations. When the field is perpendicular, the polarisation fraction tends to decrease towards the clumps, but the drop in $p$ is small (e.g. from p~8% to p~7%). Magnetic field geometry reduces the polarisation fraction in the simulated clumps by only Δp~1% on average. The larger drop seen towards the actual PGCC clumps suggests some loss of grain alignment in the dense medium, such as predicted by the radiative torque mechanism. The statistical study is not able to quantify dust opacity changes at the scale of the PGCC clumps.

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