Evidence of dust grain evolution from extinction mapping in the IC 63
  photodissociation region
  Photodissociation regions (PDRs) are parts of the ISM consisting of
predominantly neutral gas, located at the interface between H II regions and
molecular clouds. The physical conditions within these regions show variations
on very short spatial scales, and therefore PDRs constitute ideal laboratories
for investigating the properties and evolution of dust grains. We have mapped
IC 63 at high resolution from the UV to the NIR (275 nm to 1.6 $\mu$m), using
the Hubble Space Telescope WFC3. Using a Bayesian SED fitting tool, we
simultaneously derive a set of stellar ($T_\text{eff}$, $\log(g)$, distance)
and extinction ($A_V$, $R_V$) parameters for 520 background stars. We present
maps of $A_V$ and $R_V$ with a resolution of 25 arcsec based on these results.
The extinction properties vary across the PDR, with values for $A_V$ between
0.5 and 1.4 mag, and a decreasing trend in $R_V$, going from 3.7 at the front
of the nebula to values as low as 2.5 further in. This provides evidence for
evolution of the dust optical properties. We fit two modified blackbodies to
the MIR and FIR SED, obtained by combining the $A_V$ map with data from Spitzer
and Herschel. We derive effective temperatures (30 K and 227 K) and the ratio
of opacities at 160 $\mu$m to V band $\kappa_{160} / \kappa_V$ ($7.0 \times
10^{-4}$ and $2.9 \times 10^{-9}$) for the two dust populations. Similar fits
to individual pixels show spatial variations of $\kappa_{160} / \kappa_{V}$.
The analysis of our HST data, combined with these Spitzer and Herschel data,
provides the first panchromatic view of dust within a PDR.
