DIGIT: Herschel and Spitzer spectro-imaging of SMM3 and SMM4 in Serpens
  We report on spectro-imaging observations employing Spitzer IRS and Herschel
PACS, aiming to constrain the physical conditions around SMM3 and SMM4 in
Serpens. The combined power of both instruments provides an almost complete
wavelength coverage between 5 and 200 micron at an angular resolution of 10".
We detect line emission from all major molecular (H2, CO, H2O and OH) and many
atomic ([OI], [CII], [FeII], [SiII] and [SI]) coolants. Line emission tends to
peak at distances of 10 - 20" from the protostellar sources, at positions of
known outflow shocks. The only exception is [CII] which likely traces a PDR
excited from the neighboring source SMM6. Excitation analysis indicates that H2
and CO originate from gas at two distinct rotational temperatures of 300 K and
1000 K, while H2O and OH emission corresponds to rotational temperatures of 100
- 200 K. The morphological and physical association between CO and H2 suggests
a common excitation mechanism which allows direct comparisons between the two
molecules. The CO/H2 abundance ratio varies from 10^-5 in the warm gas up to
10^-4 in the hotter regions. The occurrence of J-shocks is suggested by the
strong atomic/ionic (except for [CII]) emission as well as a number of line
ratio diagnostics. Both C- and J-shocks can account for the observed molecular
emission, however J-shocks are strongly advocated by the atomic emission and
provide simpler and more homogeneous solutions for CO and H2. C-shocks describe
better the emission from H2O and OH. The variations in the CO/H2 abundance
ratio for gas at different temperatures can be interpreted by their reformation
rates in dissociative J-type shocks, or the simultaneous influence of both C
and J shocks.
