First spectrally-resolved H$_2$ observations towards HH 54 / Low H$_2$O
  abundance in shocks
  Context: Herschel observations suggest that the H$_2$O distribution in
outflows from low-mass stars resembles the H$_2$ emission. It is still unclear
which of the different excitation components that characterise the mid- and
near-IR H$_2$ distribution is associated with H$_2$O. Aim: The aim is to
spectrally resolve the different excitation components observed in the H$_2$
emission. This will allow us to identify the H$_2$ counterpart associated with
H$_2$O and finally derive directly an H$_2$O abundance estimate with respect to
H$_2$. Methods: We present new high spectral resolution observations of H$_2$
0-0 S(4), 0-0 S(9), and 1-0 S(1) towards HH 54, a bright nearby shock region in
the southern sky. In addition, new Herschel-HIFI H$_2$O (2$_{12}$$-$1$_{01}$)
observations at 1670~GHz are presented. Results: Our observations show for the
first time a clear separation in velocity of the different H$_2$ lines: the 0-0
S(4) line at the lowest excitation peaks at $-$7~km~s$^{-1}$, while the more
excited 0-0 S(9) and 1-0 S(1) lines peak at $-$15~km~s$^{-1}$. H$_2$O and
high-$J$ CO appear to be associated with the H$_2$ 0-0 S(4) emission, which
traces a gas component with a temperature of 700$-$1000 K. The H$_2$O abundance
with respect to H$_2$ 0-0 S(4) is estimated to be
$X$(H$_2$O)$<$1.4$\times$10$^{-5}$ in the shocked gas over an area of
13$^{\prime\prime}$. Conclusions: We resolve two distinct gas components
associated with the HH 54 shock region at different velocities and excitations.
This allows us to constrain the temperature of the H$_2$O emitting gas
($\leq$1000 K) and to derive correct estimates of H$_2$O abundance in the
shocked gas, which is lower than what is expected from shock model predictions.
