Mapping water in protostellar outflows with Herschel: PACS and HIFI
  observations of L1448-C
  We investigate on the spatial and velocity distribution of H2O along the
L1448 outflow, its relationship with other tracers, and its abundance
variations, using maps of the o-H2O 1_{10}-1_{01} and 2_{12}-1_{01} transitions
taken with the Herschel-HIFI and PACS instruments, respectively. Water emission
appears clumpy, with individual peaks corresponding to shock spots along the
outflow. The bulk of the 557 GHz line is confined to radial velocities in the
range \pm 10-50 km/s but extended emission associated with the L1448-C extreme
high velocity (EHV) jet is also detected. The H2O 1_{10}-1_{01}/CO(3-2) ratio
shows strong variations as a function of velocity that likely reflect different
and changing physical conditions in the gas responsible for the emissions from
the two species. In the EHV jet, a low H2O/SiO abundance ratio is inferred,
that could indicate molecular formation from dust free gas directly ejected
from the proto-stellar wind. We derive averaged Tkin and n(H2) values of about
300-500 K and 5 10^6 cm-3 respectively, while a water abundance with respect to
H2 of the order of 0.5-1 10^{-6} along the outflow is estimated. The fairly
constant conditions found all along the outflow implies that evolutionary
effects on the timescales of outflow propagation do not play a major role in
the H2O chemistry. The results of our analysis show that the bulk of the
observed H2O lines comes from post-shocked regions where the gas, after being
heated to high temperatures, has been already cooled down to a few hundred K.
The relatively low derived abundances, however, call for some mechanism to
diminish the H2O gas in the post-shock region. Among the possible scenarios, we
favor H2O photodissociation, which requires the superposition of a low velocity
non-dissociative shock with a fast dissociative shock able to produce a FUV
field of sufficient strength.
