Near-Infrared Imaging Polarization Study of M 17

Abstract

Abstract We conducted wide-field ($ \sim$ 8$ '$$ \times$ 8$ '$ ) $ \textit{JHK}_{\rm s}$ imaging polarimetric observations toward the massive star-formation region M 17. The southern H II bar is identified as large-scale infrared reflection nebula (IRN) illuminated by OB stars in the cluster center, while the northern bar shows polarization originating from dichroic extinction. Three small-scale bipolar IRN with centro-symmetric polarization patterns were identified, and their illuminating sources, embedded young stellar objects (YSOs), are most likely the driving sources of bipolar outflows. $ \textit{JHK}_{\rm s}$ polarizations of point sources in M 17 show a wavelength dependence of the polarization of dichroic extinction in the general interstellar medium, indicating $ \alpha$$ \sim$ 1.8 for the empirical relation $ P$$ \propto$$ \lambda^{-\alpha}$ of interstellar polarization. The distribution of the polarization angle shows two peaks at 170$ ^{\circ}$ and 110$ ^{\circ}$ , indicating a non-uniform magnetic field structure in M 17. The direction of the magnetic field in the cluster region is south-north. However, the magnetic field in the H II region is roughly between 100$ ^{\circ}$ –140$ ^{\circ}$ , roughly perpendicular to the Galactic disk. Such a configuration indicates that the magnetic field in the cluster region is twisted from the primordial orientation in the cluster formation. The magnetic field in the southern bar has a total strength of $ \sim$ 230$ \ \mu$ G, and is 40$ ^{\circ}$ inclined to the plane of the sky. The outflows driven by YSOs in M 17 show a poor relation with the surrounding magnetic fields. This fact implies that the magnetic field in M 17 does not dominate the orientations of protostars formed inside.