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arXiv 2108.10045astro-ph.GAastro-ph.SR
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用SOFIA/HAWC +观测的偏振热尘埃辐射研究M17中的磁场和尘埃

Studying magnetic fields and dust in M17 using polarized thermal dust emission observed by SOFIA/HAWC+

Thuong Duc Hoang, Nguyen Bich Ngoc, Pham Ngoc Diep, Le Ngoc Tram, Thiem Hoang, Wanggi Lim, Dieu D. Nguyen, Ngan Le, Nguyen Thi Phuong, Nguyen Fuda, Tuan Van Bui… 展开作者

Thuong Duc Hoang, Nguyen Bich Ngoc, Pham Ngoc Diep, Le Ngoc Tram, Thiem Hoang, Wanggi Lim, Dieu D. Nguyen, Ngan Le, Nguyen Thi Phuong, Nguyen Fuda, Tuan Van Bui, Kate Pattle, Gia Bao Truong Le, Hien Phan, Nguyen Chau Giang

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Submitted to Apj
发表信息：
The Astrophysical Journal 2022 April 10
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摘要：

我们报告了最高的空间分辨率测量磁场在M17使用热尘埃偏振的SOFIA/HAWC+中心在154 $μ$ m波长。利用Davis-Prasekhar-Fermi方法，我们发现$980 \pm 230\;μ$ G和$1665 \pm 885\;μ$ G分别在低密度区（M17-N）和高密度区（M17-S）存在强磁场。M17-N中的磁场形态可能模仿引力坍缩分子核中的磁场，而在M17-S中，磁场垂直于物质结构，并显示出柱状和不对称的沙漏形状。磁场强度的平均值被用来确定M17-N和M17-S的阿尔夫文马赫数（$\mathcal{M_A}$），结果表明它们是亚阿尔夫文马赫数，或磁场主导湍流。我们计算了质量通量比$λ$，并获得了M17-N的$λ=0.07$和M17-S的$0.28$。$λ$的次临界值与M17缺乏大质量恒星的形成相一致。为了研究尘埃物理，我们分析了尘埃极化分数$p$与热发射强度$I$、气柱密度$N({\rm H_2})$和尘埃温度$T_{\rm d}$之间的关系。偏振分数随着强度而减小，如$I^{-α}$与$α= 0.51$。极化分数也随着$N(\rm H_{2})$的增加而减小，这可以通过辐射扭矩（RAT）朝向具有较弱辐射场和/或磁场缠结的较密集区域的晶粒对准的减小来解释。极化分数随$T_{\rm d}$的增大先增大后减小，当$T_ {\rm d} > 50$ K.在M17-N中观察到的后一个特征，其中气体密度随着$T_{d}$而缓慢变化，与RAT破坏效应一致。

英文摘要：

We report the highest spatial resolution measurement of magnetic fields in M17 using thermal dust polarization taken by SOFIA/HAWC+ centered at 154 $μ$m wavelength. Using the Davis-Chandrasekhar-Fermi method, we found the presence of strong magnetic fields of $980 \pm 230\;μ$G and $1665 \pm 885\;μ$G in lower-density (M17-N) and higher-density (M17-S) regions, respectively. The magnetic field morphology in M17-N possibly mimics the fields in gravitational collapse molecular cores while in M17-S the fields run perpendicular to the matter structure and display a pillar and an asymmetric hourglass shape. The mean values of the magnetic field strength are used to determine the Alfvénic Mach numbers ($\mathcal{M_A}$) of M17-N and M17-S which turn out to be sub-Alfvénic, or magnetic fields dominate turbulence. We calculate the mass-to-flux ratio, $λ$, and obtain $λ=0.07$ for M17-N and $0.28$ for M17-S. The sub-critical values of $λ$ are in agreement with the lack of massive stars formed in M17. To study dust physics, we analyze the relationship between the dust polarization fraction, $p$, and the thermal emission intensity, $I$, gas column density, $N({\rm H_2})$, and dust temperature, $T_{\rm d}$. The polarization fraction decreases with intensity as $I^{-α}$ with $α= 0.51$. The polarization fraction also decreases with increasing $N(\rm H_{2})$, which can be explained by the decrease of grain alignment by radiative torques (RATs) toward denser regions with a weaker radiation field and/or tangling of magnetic fields. The polarization fraction tends to increase with $T_{\rm d}$ first and then decreases when $T_ {\rm d} > 50$ K. The latter feature seen in the M17-N, where the gas density changes slowly with $T_{d}$, is consistent with the RAT disruption effect.

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