The Submillimeter Polarization Spectrum of M17

TL;DR

This study presents submillimeter polarimetric observations of the M17 molecular cloud, revealing variations in polarization ratios linked to stellar wind exposure, supporting the radiative alignment torque model for grain alignment.

Contribution

First detailed submillimeter polarization measurements of M17 showing how polarization spectra vary with radiation exposure, supporting the RAT grain alignment model.

Findings

Polarization ratio varies across M17, lower near stellar wind regions.

Anti-correlation between polarization ratio and 21 cm to 450 μm intensity ratio.

Evidence supports higher grain alignment efficiency in radiation-exposed regions.

Abstract

We present 450 {\mu}m polarimetric observations of the M17 molecular cloud obtained with the SHARP polarimeter at the Caltech Submillimeter Observatory. Across the observed region, the magnetic field orientation is consistent with previous submillimeter and far-infrared polarization measurements. Our observations are centered on a region of the molecular cloud that has been compressed by stellar winds from a cluster of OB stars. We have compared these new data with previous 350 {\mu}m polarimetry and find an anti-correlation between the 450 to 350 {\mu}m polarization magnitude ratio and the ratio of 21 cm to 450 {\mu}m intensity. The polarization ratio is lower near the east end of the studied region where the cloud is exposed to stellar winds and radiation. At the west end of the region, the polarization ratio is higher. We interpret the varying polarization spectrum as evidence supporting the radiative alignment torque (RAT) model for grain alignment, implying higher alignment efficiency in the region that is exposed to a higher anisotropic radiation field.