Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey
  observations
  Photon-dominated regions (PDRs) are expected to show a layered structure in
molecular abundances and emerging line emission, which is sensitive to the
physical structure of the region as well as the UV radiation illuminating it.
We aim to study this layering in the Orion Bar, a prototypical nearby PDR with
a favorable edge-on geometry. We present new maps of 2 by 2 arcminute fields at
14-23 arcsecond resolution toward the Orion Bar in the SO 8_8-9_9, H2CO
5_(1,5)-4_(1,4), 13CO 3-2, C2H 4_(9/2)-3_(7/2) and 4_(7/2)-3_(5/2), C18O 2-1
and HCN 3-2 transitions. The data reveal a clear chemical stratification
pattern. The C2H emission peaks close to the ionization front, followed by H2CO
and SO, while C18O, HCN and 13CO peak deeper into the cloud. A simple PDR model
reproduces the observed stratification, although the SO emission is predicted
to peak much deeper into the cloud than observed while H2CO is predicted to
peak closer to the ionization front than observed. In addition, the predicted
SO abundance is higher than observed while the H2CO abundance is lower than
observed. The discrepancies between the models and observations indicate that
more sophisticated models, including production of H2CO through grain surface
chemistry, are needed to quantitatively match the observations of this region.
