Far-Infrared Polarization Spectrum of the OMC-1 Star-Forming Region
  We analyze the wavelength dependence of the far-infrared polarization
fraction toward the OMC-1 star forming region using observations from
HAWC+/SOFIA at 53, 89, 154, and 214 $\mu$m. We find that the shape of the
far-infrared polarization spectrum is variable across the cloud and that there
is evidence of a correlation between the slope of the polarization spectrum and
the average line-of-sight temperature. The slope of the polarization spectrum
tends to be negative (falling toward longer wavelengths) in cooler regions and
positive or flat in warmer regions. This is very similar to what was discovered
in $\rho$ Oph A via SOFIA polarimetry at 89 and 154 $\mu$m. Like the authors of
this earlier work, we argue that the most natural explanation for our falling
spectra is line-of-sight superposition of differing grain populations, with
polarized emission from the warmer regions and less-polarized emission from the
cooler ones. In contrast with the earlier work on $\rho$ Oph A, we do not find
a clear correlation of polarization spectrum slope with column density. This
suggests that falling spectra are attributable to variations in grain alignment
efficiency in a heterogeneous cloud consistent with radiative torques theory.
Alternative explanations in which variations in grain alignment efficiency are
caused by varying gas density rather than by varying radiation intensity are
disfavored.
