Polarization of Infrared Emission from Polycyclic Aromatic Hydrocarbons
  Polarized infrared emission from polycyclic aromatic hydrocarbons (PAHs) is
important for testing the basic physics of alignment of ultrasmall grains and
potentially offers a new way to trace magnetic fields. In this paper, a new
model of polarized PAH emission is presented, taking into account the effect of
PAH alignment with the magnetic field. The polarization level of PAH emission
features, for the different phases of the diffuse interstellar medium (ISM) is
discussed. We find that negatively charged smallest PAHs in the reflection
nebula can be excited to slightly suprathermal rotation due to enhanced ion
collisional excitation, which enhances the degree of PAH alignment and the
polarization level of PAH emission. The polarization level and polarization
angle predicted by our model including PAH alignment are supported by the first
detection of the polarization of $p\sim 2\%$ at 11.3 $\mu$m PAH feature from
MWC 1080 nebula by Zhang et al. The theoretical and observational progress
reveals that PAHs can be aligned with the magnetic field, resulting in a
moderate polarization level of spinning dust emission. Polarized infrared PAH
polarization would be useful for tracing the magnetic fields.
