Detection of Water in the Shocked Gas Associated with IC 443: Constraints on Shock Models

We have used the Submillimeter Wave Astronomy Satellite (SWAS) to observe the ground-state 1₁₀ → 1₀₁ transition of ortho-H₂O at 557 GHz in three of the shocked molecular clumps associated with the supernova remnant IC 443. We also observed simultaneously the 487 GHz line (3,1 → 3,2) of O₂, the 492 GHz line (³P₁ → ³P₀) of C I, and the 550 GHz line (J = 5 → 4) of ¹³CO. We detected the H₂O, C I, and ¹³CO lines toward the shocked clumps B, C, and G. In addition, ground-based observations of the J = 1 → 0 transitions of CO and HCO⁺ were obtained. Assuming that the shocked gas has a temperature of 100 K and a density of 5 × 10⁵ cm^-3, we derive SWAS beam-averaged ortho-H₂O column densities of 3.2 × 10¹³, 1.8 × 10¹³, and 3.9 × 10¹³ cm^-2 in clumps B, C, and G, respectively. Combining the SWAS results with our ground-based observations, we derive a relative abundance of ortho-H₂O to CO in the postshock gas of between 2 × 10^-4 and 3 × 10^-3. On the basis of our results for H₂O, published results of numerous atomic and molecular shock tracers, and archival Infrared Space Observatory (ISO) observations, we conclude that no single shock type can explain these observations. However, a combination of fast J-type shocks (~100 km s^-1) and slow C-type shocks (~12 km s^-1) or, more likely, slow J-type shocks (~12-25 km s^-1) can most naturally explain the postshock velocities and the emission seen in various atomic and molecular tracers. Such a superposition of shocks might be expected as the supernova remnant overtakes a clumpy interstellar medium. The fast J-type shocks provide a strong source of ultraviolet radiation, which photodissociates the H₂O in the cooling (T ≤ 300 K) gas behind the slow shocks and strongly affects the slow C-type shock structure by enhancing the fractional ionization. At these high ionization fractions, C-type shocks break down at speeds ~10-12 km s^-1, while faster flows will produce J-type shocks. Our model favors a preshock gas-phase abundance of oxygen not in CO that is depleted by a least a factor of 2, presumably as water ice on grain surfaces. Both freezeout of H₂O and photodissociation of H₂O in the postshock gas must be significant to explain the weak H₂O emission seen by SWAS and ISO from the shocked and postshock gas.