Thermal H_2_O 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^Lsun_, which is comparable to the mechanical luminosity generated by the 10km/s shocks, suggesting that practically all of the cooling of the weak-shock regions is done by these three molecular species alone