The ISO spectroscopic view of the HH 24-26 region

We report the results of an investigation, performed with the ISO spectrome ters (LWS and SWS), on the star forming region associated with the Herbig-H aro objects HH 24-25 and 26. Low-resolution LWS spectra (45-197 mu m) were observed towards the HH24MMS, HH25MMS and HH26IR sources as well as the HH2 6IR outflow. In addition, SWS scans of the pure H-2 rotational lines toward s HH24MMS were acquired. Emission from [O I] 63 mu m and [C II] 158 mu m ap pears widespread while molecular transitions of carbon monoxide and water v apour were detected only towards HH25MMS and the blue lobe of HH26IR. From the analysis of the observed emission we deduce that the gas towards HH24MM S and HH25MMS is excited at densities similar to 10(6) cm(-3) and temperatu res ranging from 650 to 1400 K in HH24MMS and from 150 to 550 K in HH25MMS. Along the blue lobe of the HH26IR outflow, a more diffuse (n(H2) similar t o 10(4) cm(-3)) and warm (T similar to 1800 K) gas is found. Both the molec ular (CO, H2O and H-2) and atomic ([O I]) emission in the three sources can be interpreted as due to shock excitation, and a mixture of both C- and J- type shocks are required to reproduce most of the observed characteristics of the spectra. The derived water abundances (3 10(-7) -9 10(-6)) are lower than expected in warm shock excited gas, a result which has also been foun d in other similar regions investigated with ISO. The total cooling derived from the gas component traced by the FIR lines is always of the same order or larger than the cooling due to the molecular hydrogen as traced by the H-2 2.12 mu m line; although this latter could be underestimated if the dus t extinction is not negligible, however it is evident that a significant fr action of the energy released in the shocks is re-radiated away by the far infrared lines. Finally, the [C II] 158 mu m line intensities are rather co nstant at all of the observed positions, excluding the presence of strong p hoto-dissociation regions related to the nearby IR sources.