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

Abstract

We report the highest spatial resolution measurement of magnetic fields in M17 using thermal dust polarization taken by SOFIA/HAWC+ centered at 154 $\hat{I}¼$m wavelength. Using the Davis-Chandrasekhar-Fermi method, we found the presence of strong magnetic fields of $980\pm 230\hat{I}¼$G and $1665\pm 885\hat{I}¼$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 (${\mathcal{M}}_{\mathcal{A}}$) 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, $\hat{I}»$, and obtain $\hat{I}»=0.07$ for M17-N and $0.28$ for M17-S. The sub-critical values of $\hat{I}»$ 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, $N({\mathrm{H}}_2)$, and dust temperature, $T_{\mathrm{d}}$. The polarization fraction decreases with intensity as $I^{-\hat{I}\pm }$ with $\hat{I}\pm =0.51$. The polarization fraction also decreases with increasing $N({\mathrm{H}}_2)$, 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 $T_{\mathrm{d}}$ first and then decreases when $T_{\mathrm{d}}>50$ 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.

Submitted to Apj