Studying Magnetic Fields and Dust in M17 Using Polarized Thermal Dust Emission Observed by SOFIA/HAWC+
D. Hoang, Nguyen Bich Ngoc, Pham Ngoc Diep, Le Ngoc Tram, Thiem Hoang, Kate Pattle, Wanggi Lim, Ngân Lê, Dieu D. Nguyen, Nguyễn Thị Phương, Nguyen Fuda, Tuan Van Bui, Bao Truong, Hien Phan, Nguyen Chau Giang
Abstract
Abstract We report on the highest spatial resolution measurement to date of magnetic fields (B-fields) in M17 using thermal dust polarization measurements taken by SOFIA/HAWC+ centered at a wavelength of 154 μ m. Using the Davis–Chandrasekhar–Fermi method, in which the polarization angle dispersion calculated using the structure function technique is the quantity directly observed by SOFIA/HAWC+, we found the presence of strong B-fields of 980 ± 230 and 1665 ± 885 μ G in the lower-density M17-N and higher-density M17-S regions, respectively. The B-field morphology in M17-N possibly mimics the fields in gravitationally collapsing molecular cores, while in M17-S the fields run perpendicular to the density structure. M17-S also displays a pillar feature and an asymmetric large-scale hourglass-shaped field. We use the mean B-field strengths to determine Alfvénic Mach numbers for both regions, finding that B-fields dominate over turbulence. We calculate the mass-to-flux ratio, λ , finding λ = 0.07 for M17-N and 0.28 for M17-S. These subcritical λ values are consistent with the lack of massive stars formed in M17. To study dust physics, we analyze the relationship between dust polarization fraction, p , emission intensity, I , gas column density, N (H 2 ), polarization angle dispersion function, S , and dust temperature, T d . p decreases with intensity as I − α with α = 0.51. p tends to first increase with T d , but then decreases at higher T d . The latter feature, seen in M17-N at high T d when N (H 2 ) and S decrease, is evidence of the radiative torque disruption effect.