Detection of the 62 Micron Crystalline H2O Ice Feature in Emission toward HH 7 with the Infrared Space Observatory Long-Wavelength Spectrometer

We report the detection of the 62 μm feature of crystalline water ice in emission toward the bow-shaped Herbig-Haro object HH 7. Significant amounts of far-infrared continuum emission are also detected between 10 and 200 μm, so that Herbig-Haro objects cease to be pure emission-line objects at far-infrared 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 HH 7 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 in which both dissociative and nondissociative shocks coexist. The timescale for ice mantle formation is of the order of ~400 yr, so that the importance of gas-phase water cooling as a shock diagnostic may be greatly diminished.