Molecular line emission in HH54: a coherent view from near to far infrared

Aims.We present a detailed study of the infrared line emission (1-200 μm) 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 μm); mid-infrared spectrophotometric images (5-12 μm); and, far-IR (45-200 μm, 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-the-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^-1, n_H = 10⁴ cm^-3, B = 100 μG, 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 of 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.