Studying magnetic fields and dust in M17 using polarized thermal dust
  emission observed by SOFIA/HAWC+
  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 Alfv\'enic
Mach numbers ($\mathcal{M_A}$) of M17-N and M17-S which turn out to be
sub-Alfv\'enic, 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.
