  Aims. We present a detailed study of the infrared line emission (1-200
micron) in the Herbig-Haro object HH54. Our database comprises: high- (R~9000)
and low- (R~600) resolution spectroscopic data in the near-infrared band (1-2.5
micron); mid-infrared spectrophotometric images (5-12 micron); and, far-IR
(45-200 micron, R~200) spectra acquired with the ISO satellite. As a result, we
provide the detection of and the absolute fluxes for more than 60 molecular
features (mainly from H2 in the near- and mid-infrared and from H2O, CO and OH
in the far-infrared) and 23 ionic lines. Methods. The H2 lines, coming from
levels from v=0 to v=4 have been interpreted in the context of a state-of-art
shock code, whose output parameters are adopted as input to a Large Velocity
Gradient computation in order to interpret the FIR emission of CO, H2O and OH.
Results. The H2 emission can be interpreted as originating in either
steady-state J-type shocks or in quasi-steady J-type shocks with magnetic
precursor. However, our multi-species analysis shows that only a model of a
J-type shock with magnetic precursor (v_shock=18 km/s, n_H=10^4 cm^-3, B=100
microG, age=400 yr) can account for both the observed H2 emission and the CO
and H2O lines. Such a model predicts a H2O abundance of ~ 7 10^-5, in agreement
with estimations from other shock models for outflows associated with low mass
protostars. We can exclude the possibility that the observed atomic lines arise
in the same shock as the molecular lines, and give arguments in favour of the
presence of a further high-velocity, fully dissociative shock component in the
region. Finally, in view of the forthcoming spectroscopic facilities on board
of the Herschel satellite, we provide predictions for H2O lines considered to
be the most suitable for diagnostic purposes.

Astrophysics