Detection of Polarized Infrared Emission by Polycyclic Aromatic
  Hydrocarbons in the MWC 1080 Nebula
  Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in astrophysical
environments, as revealed by their pronounced emission features at 3.3, 6.2,
7.7, 8.6, 11.3, and 12.7 $\mu$m commonly ascribed to the C--H and C--C
vibrational modes. Although these features have long been predicted to be
polarized, previous searches for PAH polarization led to null or, at best,
tentative detections. Here we report the definite detection of polarized PAH
emission at 11.3 $\mu$m in the nebula associated with the Herbig Be star MWC
1080. We measure a polarization degree of 1.9$\pm$0.2\%, which is unexpectedly
high compared to models. This poses a challenge in the current understanding of
the alignment of PAHs, which is required to polarize the PAH emission but
thought to be substantially suppressed. PAH alignment with a magnetic field via
a resonance paramagnetic relaxation process may account for such a high level
of polarization.
