Spatial variation in temperature and density in the IC 63 PDR from $\rm
  H_{2}$ Spectroscopy
  We have measured the gas temperature in the IC 63 photodissociation region
(PDR) using the S(1) and S(5) pure rotation lines of molecular hydrogen with
SOFIA/EXES. We divide the PDR into three regions for analysis based on the
illumination from $\gamma$ Cas: "sunny," "ridge" and "shady." Constructing
rotation diagrams for the different regions, we obtain temperatures of
T$_{ex}$=$562^{+52}_{-43}$ K towards the "ridge" and T$_{ex}$=$495^{+28}_{-25}$
K in the "shady" side. The H$_2$ emission was not detected on the "sunny" side
of the ridge, likely due to the photo-dissociation of H$_2$ in this gas. Our
temperature values are lower than the value of T$_{ex}$=685$\pm$68 K using the
S(1), S(3), and S(5) pure rotation lines, derived by Thi et al. (2009) using
lower spatial-resolution ISO-SWS data at a different location of the IC 63 PDR.
This difference indicates that the PDR is inhomogeneous and illustrates the
need for high-resolution mapping of such regions to fully understand their
physics. The detection of a temperature gradient correlated with the extinction
into the cloud, points to the ability of using H$_2$ pure rotational line
spectroscopy to map the gas temperature on small scales. We used a PDR model to
estimate the FUV radiation and corresponding gas densities in IC 63. Our
results shows the capability of SOFIA/EXES to resolve and provide detailed
information on the temperature in such regions.
