Aims . We present a detailed study of the infrared line emission ( 1-200 \mu m ) in the Herbig-Haro object HH54 .
Our database comprises : high- ( R \sim 9000 ) and low- ( R \sim 600 ) resolution spectroscopic data in the near-infrared band ( 1-2.5 \mu m ) ; mid-infrared spectrophotometric images ( 5-12 \mu m ) ; and , far-IR ( 45-200 \mu m , R \sim 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 H _ { 2 } in the near- and mid-infrared and from H _ { 2 } O , CO and OH in the far-infrared ) and 23 ionic lines . Methods . The H _ { 2 } 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 , H _ { 2 } O and OH . Results . The H _ { 2 } 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 _ { \mathrm { shock } } =18 km s ^ { -1 } , n _ { \mathrm { H } } =10 ^ { 4 } cm ^ { -3 } , B=100 \mu G , age=400 yr ) can account for both the observed H _ { 2 } emission and the CO and H _ { 2 } O lines .
Such a model predicts a H _ { 2 } O abundance of \sim 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 H _ { 2 } O lines considered to be the most suitable for diagnostic purposes .