Size Limit of Superparamagnetic Inclusions in Dust Grains and Difficulty of Magnetic Grain Alignment in Protoplanetary Disks
Size Limit of Superparamagnetic Inclusions in Dust Grains and Difficulty of Magnetic Grain Alignment in Protoplanetary Disks
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尘埃颗粒中超顺磁性包裹体的尺寸限制与原行星盘磁颗粒排列的难度
DOI：
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发表时间：
2021
影响因子：
4.9
通讯作者：
Haifeng Yang
中科院分区：
文献类型：
--
作者：
Haifeng Yang
非球形粒子与磁场的排列是一个重要的问题，因为它为利用偏振尘埃热辐射探测磁场奠定了基础。在本文中，我们研究的可行性磁排列在原行星盘（PPD）。我们使用的对齐条件，拉莫尔进动应该是快的阻尼时标相比。我们首先表明，拉莫尔进动的时间尺度是一些3个数量级的阻尼时间比典型的PPD条件下的毫米级晶粒，使磁对准不太可能。超顺磁性包裹体（SPI）可以缩短进动时间，但减少因子强烈依赖于SPI团簇的大小，我们发现这受到所谓的“Néel弛豫过程”的限制。特别地，SPI的尺寸限制由SPI材料的所谓的“各向异性能量常数”设定，其描述了改变SPI的磁矩方向所需的能量势垒。对于最常见的含铁材料，我们发现最大SPI尺寸对应于103阶的拉莫尔进动时标的缩减因子。我们还发现，达到这个最大的减少因素，需要微调的SPI大小。最后，我们用一个简单的圆盘模型说明了SPI尺寸限制对尘埃颗粒磁性排列的影响，我们得出结论，即使有SPI，100 μm或更大的相对较大的颗粒也不太可能与磁场对齐。
Alignment of nonspherical grains with magnetic fields is an important problem, as it lays the foundation of probing magnetic fields with polarized dust thermal emissions. In this paper, we investigate the feasibility of magnetic alignment in protoplanetary disks (PPDs). We use an alignment condition that Larmor precession should be fast compared with the damping timescale. We first show that the Larmor precession timescale is some 3 orders of magnitude longer than the damping time for millimeter-sized grains under conditions typical of PPDs, making the magnetic alignment unlikely. The precession time can be shortened by superparamagnetic inclusions (SPIs), but the reduction factor strongly depends on the size of the SPI clusters, which we find is limited by the so-called “Néel’s relaxation process.” In particular, the size limit of SPIs is set by the so-called “anisotropic energy constant” of the SPI material, which describes the energy barrier needed to change the direction of the magnetic moment of an SPI. For the most common iron-bearing materials, we find maximum SPI sizes corresponding to a reduction factor of the Larmor precession timescale of order 103. We also find that reaching this maximum reduction factor requires fine-tuning on the SPI sizes. Lastly, we illustrate the effects of the SPI size limits on magnetic alignment of dust grains with a simple disk model, and we conclude that it is unlikely for relatively large grains of order 100 μm or more to be aligned with magnetic fields, even with SPIs.
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影响因子：
4.8
作者：
Zheyu Lin;Zhi-Yun Li;Haifeng Yang;L. Looney;I. Stephens;C. Hull
通讯作者：
Zheyu Lin;Zhi-Yun Li;Haifeng Yang;L. Looney;I. Stephens;C. Hull
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DOI：
10.3847/1538-4357/aaacd2
发表时间：
2018
期刊：
The Astrophysical Journal
影响因子：
--
作者：
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通讯作者：
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DOI：
10.3847/1538-4357/aa9edc
发表时间：
2018
期刊：
The Astrophysical Journal
影响因子：
--
作者：
Hoang, Thiem;Cho, Jungyeon;Lazarian, A.
通讯作者：
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DOI：
10.3847/1538-4357/aa998b
发表时间：
2017
期刊：
The Astrophysical Journal
影响因子：
--
作者：
Stephens, Ian W.;Yang, Haifeng;Li, Zhi-Yun;Looney, Leslie W.;Kataoka, Akimasa;Kwon, Woojin;Fernández-López, Manuel;Hull, Charles L.;Hughes, Meredith;Segura-Cox, Dominique
通讯作者：
Segura-Cox, Dominique