Magnetic Field Diagnostics Based on Far-Infrared Polarimetry: Tests Using Numerical Simulations
Magnetic Field Diagnostics Based on Far-Infrared Polarimetry: Tests Using Numerical Simulations
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DOI：
10.1086/323489
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发表时间：
2001-03
期刊：
影响因子：
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通讯作者：
F. Heitsch;E. Zweibel;M. Mac Low;P. Li;M. Norman
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文献类型：
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作者：
F. Heitsch;E. Zweibel;M. Mac Low;P. Li;M. Norman
如果不确定分子云磁场的结构和强度，就不能理解分子云形成恒星的动力学状态。对来自热排列尘埃颗粒的偏振远红外辐射的测量被用来绘制磁场的方向并估计其强度，但结果的准确性仍然存在疑问。为了评估该方法的可靠性，我们将其应用于超声速磁流体动力学湍流三维模拟得到的模拟远红外极化图，并将其估计值与模拟中已知的磁场强度进行了比较。我们研究了有限的望远镜分辨率和自重力对地图结构的影响。有限的观测分辨率影响了场的结构，因此可以完全抑制小规模的变化，从而给人一种非常均匀的场的印象。对钱德拉塞卡-费米法估计湍流介质中平均磁场的方法进行了测试，并提出了一种测量均方根场的扩展方法。对于分子云的典型场强，两种方法的结果都在2倍以内，而对于稍弱的场强，改进后的方法给出的估计更可靠。然而，单独使用这两种方法都不能很好地处理非常弱的场，其误差高达150倍。采用这两种方法的几何平均值，即使是最弱的场也能准确地估计在2.5倍以内。有限的望远镜分辨率导致了对所有方法的场强的系统性高估。我们讨论了造成这种高估的影响，并展示了如何提取底层（湍流）功率谱上的信息。
The dynamical state of star-forming molecular clouds cannot be understood without determining the structure and strength of their magnetic fields. Measurements of polarized far-infrared radiation from thermally aligned dust grains are used to map the orientation of the field and estimate its strength, but the accuracy of the results has remained in doubt. In order to assess the reliability of this method, we apply it to simulated far-infrared polarization maps derived from three-dimensional simulations of supersonic magnetohydrodynamical turbulence, and we compare the estimated values to the known magnetic field strengths in the simulations. We investigate the effects of limited telescope resolution and self-gravity on the structure of the maps. Limited observational resolution affects the field structure such that small-scale variations can be completely suppressed, thus giving the impression of a very homogeneous field. The Chandrasekhar-Fermi method of estimating the mean magnetic field in a turbulent medium is tested, and we suggest an extension to measure the rms field. Both methods yield results within a factor of 2 for field strengths typical of molecular clouds, with the modified version returning more reliable estimates for slightly weaker fields. However, neither method alone works well for very weak fields, missing them by a factor of up to 150. Taking the geometric mean of both methods estimates even the weakest fields accurately within a factor of 2.5. Limited telescope resolution leads to a systematic overestimation of the field strengths for all methods. We discuss the effects responsible for this overestimation and show how to extract information on the underlying (turbulent) power spectrum.