Strong H2O and high-J CO emission towards the Class 0 protostar L1448-mm

The spectrum of the Class 0 source L1448-mm has been measured over the wave length range extending from 6 to 190 mu m with the Long Wavelength Spectrom eter (LWS) and the Short Wavelength Spectrometer (SWS) on the Infrared Spac e Observatory (ISO). The far infrared spectrum is dominated by strong emiss ion from gaseous H2O and from CO transitions with rotational quantum number s J greater than or equal to 14; in addition, the H-2 pure rotational lines S(3), S(4) and S(5), the OH fundamental line at 119 mu m, as well as emiss ion from [OI] 63 mu m and [CII] 158 mu m are also observed. The strong CO a nd water emission can be consistently explained as originating in a warm ga s component at T similar to 700-1400 K and n(H2) similar to(3-50) 10(4) cm- (3), which fills about 0.2-2% of the similar to 75 " LWS field of view (cor responding, assuming a single emitting region, to a physical size of about (3-12)" or (0.5-2) 10(-2) pc at d = 300 pc). We derive an H2O/CO abundance ratio similar to 5, which, assuming a standard CO/H-2 abundance of 10(-4), corresponds to H2O/H-2 similar to 5 10(-4) This value implies that water is enhanced by about a factor similar to 10(3) with respect to its expected a bundance in the ambient gas. This is consistent with models of warm shocked regions which predict that most of the free atomic oxygen will be rapidly converted into water once the temperature of the post-shocked gas exceeds s imilar to 300 K. The relatively high density and compact size inferred for this emission may suggest an origin in the shocked region along the molecul ar jet traced by SiO and EHV CO millimeter line emission. Further support i s given by the fact that the observed enhancement in H2O can be explained b y shock conditions similar to those expected to produce the abundant SiO ob served in the region. L1448-mm shows the largest water abundance so far obs erved by ISO amongst young sources displaying outflow activity; we argue th at the occurrence of multiple shocks over a relatively short interval of ti me, like that evidenced in the surroundings of L1448-mm, could have contrib uted to enrich the molecular jet with a high H2O column density.