The shocked gas of the BHR71 outflow observed by Herschel: indirect
  evidence for an atomic jet
  In the BHR71 region, two low-mass protostars drive two distinguishable
outflows. They constitute an ideal laboratory to investigate the effects of
shock chemistry and the mechanisms that led to their formation. We aim to
define the morphology of the warm gas component of the BHR 71 outflow and at
modelling its shocked component. We present the first far infrared Herschel
images of the BHR71 outflow in the CO(14-13), H$_2$O (2$_{21}$-1$_{10}$),
H$_2$O (2$_{12}$-1$_{01}$) and [OI] 145 $\mu$m, lines, revealing the presence
of several knots of warm, shocked gas associated with fast outflowing gas. In
two of these knots we performed a detailed study of the physical conditions by
comparing a large set of transitions from several molecules to a grid of shock
models. Herschel lines ratios in the outflow knots are quite similar, showing
that the excitation conditions of the fast moving gas do not change
significantly within the first $\sim$ 0.068 pc of the outflow, apart at the
extremity of the southern blue-shifted lobe that is expanding outside the
molecular cloud. Rotational diagram, spectral line profile and LVG analysis of
the CO lines in knot A show the presence of two gas components: one extended,
cold ($T\sim$80 K) and dense ($n$(H$_2$) = 3$\times$10$^5$-4$\times$10$^6$
cm$^{-3}$) and another compact (18 arcsec), warm ($T$ = 1700-2200 K) with
slightly lower density ($n$(H$_2$) = (2-6)$\times$10$^4$ cm$^{-3}$). In the two
brightest knots (where we performed shock modelling) we found that H$_2$ and CO
are well fitted with non-stationary (young) shocks. These models, however,
significantly underestimate the observed fluxes of [OI] and OH lines, but are
not too far off those of H$_2$O, calling for an additional, possibly
dissociative, J-type shock component. Our modelling indirectly suggests that an
additional shock component exists, possibly a remnant of the primary jet
