DETECTION OF THE 62 µm CRYSTALLINE H2O ICE FEATURE IN EMISSION TOWARD HH7 WITH ISO-LWS0

We report the detection of the 62 µm feature of crystalline water ice in emission towards the bow- shaped Herbig-Haro object HH7. Significant amounts of far infrared continuum emission are also de- tected between 10 and 200 µm, so that Herbig-Haro objects cease to be pure emission-line objects at FIR wavelengths. The formation of crystalline water ice mantles requires grain temperatures Tgr � > 100 K at the time of mantle formation, suggesting that we are seeing material processed by the HH7 shock front. The deduced ice mass is � 2×10−5 M⊙ corresponding to a water column density N(H2O)� 1018 cm−2; an estimate of the (H2O)/(H) abundance yields values close to the interstellar gas-phase oxygen abundance. The relatively high dust temperature and the copious amounts of gas-phase water needed to produce the observed quantity of crystalline water ice, suggest a scenario where both dissociative and non-dissociative shocks co-exist. The timescale for ice mantle formation is of the order of � 400 years, so that the importance of gas-phase water cooling as a shock diagnostic may be greatly diminished. Subject headings: (ISM:) dust, extinction — ISM: Herbig-Haro objects — ISM: individual (HH7) — ISM: lines and bands — infrared: ISM: continuum — infrared: ISM: lines and bands