Thermal H2O emission from the Herbig-Haro flow HH 54

The first detection of thermal water emission from a Herbig-Haro object is presented. The observations were performed with the LWS (Long Wavelength Spectrograph) aboard ISO (Infrared Space Observatory). Besides H 2 O, rotational lines of CO are present in the spectrum of HH 54. These high-J CO lines are used to derive the physical model parameters of the FIR (far-infrared) molecular line emitting regions. This model fits simultaneously the observed OH and H 2 O spectra for an OH abundance X(OH)=10 -6 and a water vapour abundance X(H 2 O)=10 -5 . At a distance of 250pc, the total CO, OH and H 2 O rotational line cooling rate is estimated to be 1.3x10 -2 L ⊙ , which is comparable to the mechanical luminosity generated by the 10km s -1 shocks, suggesting that practically all of the cooling of the weak-shock regions is done by these three molecular species alone.