The ISO spectroscopic view of the HH 24–26 region

We report the results of an investigation, performed with the ISO spectrometers (LWS and SWS), on the star forming region associated with the Herbig-Haro objects HH 24–25 and 26. Low-resolution LWS spectra (45-197 μm) were observed towards the HH24MMS, HH25MMS and HH26IR sources as well as the HH26IR outflow. In addition, SWS scans of the pure H2 rotational lines towards HH24MMS were acquired. Emission from [O i] 63 μm and [C ii] 158 μm appears widespread while molecular transitions of carbon monoxide and water vapour were detected only towards HH25MMS and the blue lobe of HH26IR. From the analysis of the observed emission we deduce that the gas towards HH24MMS and HH25MMS is excited at densities ∼ 106 cm−3 and temperatures 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 (nH2 ∼ 104 cm−3) and warm (T ∼ 1800 K) gas is found. Both the molecular (CO, H2O and H2) and atomic ([O i]) emission in the three sources can be interpreted as due to shock excitation, and a mixture of both Cand 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 found 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 H2 2.12 μm line; although this latter could be underestimated if the dust extinction is not negligible, however it is evident that a significant fraction of the energy released in the shocks is re-radiated away by the far infrared lines. Finally, the [C ii]158μm line intensities are rather constant at all of the observed positions, excluding the presence of strong photo-dissociation regions related to the nearby IR sources. Send offprint requests to: M. Benedettini ([email protected]) ? Based on observations with ISO, an ESA project with instruments funded by ESA Member States and with the participation of ISAS and NASA