Molecules in the CepE-mm jet: evidence for shock-driven photochemistry?
  The chemical composition of protostellar jets and its origin are still badly
understood. More observational constraints are needed to make progress. With
that objective, we have carried out a systematic search for molecular species
in the jet of Cep E-mm, a template for intermediate-mass Class 0 protostars,
associated with a luminous, high-velocity outflow. We made use of an unbiased
spectral line survey in the range 72-350 GHz obtained with the IRAM 30m
telescope, complementary observations of the CO $J$=3-2 transition with the
JCMT, and observations at 1" angular resolution of the CO $J$=2-1 transition
with the IRAM Plateau de Bure interferometer. In addition to CO, we have
detected rotational transitions from SiO, SO, H$_2$CO, CS, HCO$^{+}$ and HCN. A
strong chemical differentiation is observed in the southern and northern lobes
of the jet. Radiative transfer analysis in the Large Velocity Gradient
approximation yields typical molecular abundances of the order of $10^{-8}$ for
all molecular species other than CO. Overall, the jets exhibit an unusual
chemical composition, as CS, SO and H$_2$CO are found to be the most abundant
species, with a typical abundance of (3-4)$\times 10^{-8}$. The transverse size
of the CO jet emission estimated from interferometric observations is about
1000 au, suggesting that we are detecting emission from a turbulent layer of
gas entrained by the jet in its propagation and not the jet itself. We propose
that some molecular species could be the signatures of the specific
photochemistry driven by the UV radiation field generated in the turbulent
envelope.
