On the origins of polarization holes in Bok globules
On the origins of polarization holes in Bok globules
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DOI：
10.1051/0004-6361/201527546
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发表时间：
2016-03
期刊：
影响因子：
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通讯作者：
R. Brauer;S. Wolf;S. Reissl
中科院分区：
文献类型：
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作者：
R. Brauer;S. Wolf;S. Reissl
上下文。对BOK球体的偏振观测经常显示出偏振度向其中心致密区域(极化空穴)方向的降低。这种行为通常被解释为由于高密度和高温度或可能复杂的磁场结构的角分辨率不足而导致的更多的不对准。目标。我们研究了在某些物理条件下，BOK球体中致密区域的极化发射是否有可能显著降低。例如，我们评估了光学深度效应和尘埃相的各种性质的影响。方法：研究方法。我们用辐射传输模型计算了一个解析的BOK球体模型的温度结构，并模拟了细长尘埃颗粒的偏振热发射。对于尘埃颗粒的对准，我们考虑了磁场，并考虑了辐射扭矩和内部对准。结果。除了通常的解释外，选定的温度和密度分布、尘埃相和磁场条件也能够显著降低BOK球体中致密区域的极化发射。考虑到亚mm/mm颗粒和现有BOK颗粒的典型柱状密度，光学厚度足够高，使偏振度降低高达{\Delta}P~10%。如果局限在密度最高的区域，粉尘颗粒长大到亚mm/mm尺寸和累积的石墨颗粒使极化程度分别降低高达P~10%和P~5%。然而，只有当石墨颗粒与磁场不对齐时，它们才会产生效果。
Context. Polarimetric observations of Bok globules frequently show a decrease in the degree of polarization towards their central dense regions (polarization holes). This behaviour is usually explained with increased disalignment owing to high density and temperature, or insufficient angular resolution of a possibly complex magnetic field structure. Aims. We investigate whether a significant decrease in polarized emission of dense regions in Bok globules is possible under certain physical conditions. For instance, we evaluate the impact of optical depth effects and various properties of the dust phase. Methods. We use radiative transfer modelling to calculate the temperature structure of an analytical Bok globule model and simulate the polarized thermal emission of elongated dust grains. For the alignment of the dust grains, we consider a magnetic field and include radiative torque and internal alignment. Results. Besides the usual explanations, selected conditions of the temperature and density distribution, the dust phase and the magnetic field are also able to significantly decrease the polarized emission of dense regions in Bok globules. Taking submm/mm grains and typical column densities of existing Bok globules into consideration, the optical depth is high enough to decrease the degree of polarization by up to {\Delta}P~10%. If limited to the densest regions, dust grain growth to submm/mm size and accumulated graphite grains decrease the degree of polarization by up to {\Delta}P~10% and {\Delta}P~5%, respectively. However, the effect of the graphite grains occurs only if they do not align with the magnetic field.