The Far-infrared Polarization Spectrum of ρ Ophiuchi A from HAWC+/SOFIA Observations

We report on polarimetric maps made with HAWC+/SOFIA toward ρ Oph A, the densest portion of the ρ Ophiuchi molecular complex. We employed HAWC+ bands C (89 μm) and D (154 μm). The slope of the polarization spectrum was investigated by defining the quantity ${{ \mathcal R }}_{{DC}}={p}_{D}/{p}_{C}$, where p C and p D represent polarization degrees in bands C and D, respectively. We find a clear correlation between ${{ \mathcal R }}_{{DC}}$ and the molecular hydrogen column density across the cloud. A positive slope (${{ \mathcal R }}_{{DC}}$ > 1) dominates the lower-density and well-illuminated portions of the cloud, which are heated by the high-mass star Oph S1, whereas a transition to a negative slope (${{ \mathcal R }}_{{DC}}$ < 1) is observed toward the denser and less evenly illuminated cloud core. We interpret the trends as due to a combination of (1) warm grains at the cloud outskirts, which are efficiently aligned by the abundant exposure to radiation from Oph S1, as proposed in the radiative torques theory; and (2) cold grains deep in the cloud core, which are poorly aligned owing to shielding from external radiation. To assess this interpretation, we developed a very simple toy model using a spherically symmetric cloud core based on Herschel data and verified that the predicted variation of ${{ \mathcal R }}_{{DC}}$ is consistent with the observations. This result introduces a new method that can be used to probe the grain alignment efficiency in molecular clouds, based on the analysis of trends in the far-infrared polarization spectrum.