On the collisional disalignment of dust grains in illuminated and shaded
  regions of IC 63
  Interstellar dust grain alignment causes polarization from UV to mm
wavelengths, allowing the study of the geometry and strength of the magnetic
field. Over last couple of decades observations and theory have led to the
establishment of the Radiative Alignment Torque (RAT) mechanism as leading
candidate to explain the effect. With a quantitatively well constrained theory,
polarization can be used not only to study the interstellar magnetic field, but
also the dust and other environmental parameters. Photo-dissociation Regions
(PDRs), with their intense, anisotropic radiation fields, consequent rapid $\rm
H_{2}$ formation, and high spatial density-contrast provide a rich environment
for such studies. Here we discuss an expanded optical, NIR, and mm-wave study
of the IC\,63 nebula, showing strong $\rm H_{2}$ formation-enhanced alignment
and the first direct empirical evidence for disalignment due to gas-grain
collisions using high-resolution $\rm HCO^{+}$(J=1-0) observations. We find
that relative amount of polarization is marginally anti-correlated with column
density of $\rm HCO^{+}$. However, separating the lines of sight of optical
polarimetry into those behind, or in front of, a dense clump as seen from
$\gamma$ Cas, the distribution separates into two well defined sets, with data
corresponding to \enquote{shaded} gas having a shallower slope. This is
expected if the decrease in polarization is caused by collisions since
collisional disalignment rate is proportional to R$_C\propto n\sqrt{T}$. Ratios
of the best-fit slopes for the \enquote{illuminated} and \enquote{shaded}
samples of lines of sight agrees, within the uncertainties, with the
square-root of the two-temperature H$_2$ excitation in the nebula seen by Thi
et al. (2009).
