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

• Published in: arXiv.org 2021-11
• Main Authors: Hoang, Thuong Duc, Nguyen, Bich Ngoc, Pham Ngoc Diep, Le Ngoc Tram, Thiem Hoang, Lim, Wanggi, Nguyen, Dieu D, Ngan Le, Nguyen Thi Phuong, Nguyen Fuda, Tuan Van Bui, Pattle, Kate, Gia Bao Truong Le, Phan, Hien, Nguyen Chau Giang
• Format: Article
• Language: English
• Subjects: Dust; Field strength; Gas density; Gravitational collapse; Magnetic fields; Massive stars; Morphology; Polarization; Spatial resolution; Tangling; Thermal emission
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2108.10045

We report the highest spatial resolution measurement of magnetic fields in M17 using thermal dust polarization taken by SOFIA/HAWC+ centered at 154 \(\mu\)m wavelength. Using the Davis-Chandrasekhar-Fermi method, we found the presence of strong magnetic fields of \(980 \pm 230\;\mu\)G and \(1665 \pm 885\;\mu\)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, \(\lambda\), and obtain \(\lambda=0.07\) for M17-N and \(0.28\) for M17-S. The sub-critical values of \(\lambda\) 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 \(I^{-\alpha}\) with \(\alpha = 0.51\). 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.