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

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

Feb 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005apj...620..758s&link_type=abstract

The Astrophysical Journal, Volume 620, Issue 2, pp. 758-773.

Astronomy and Astrophysics

Astronomy

37

Ism: Abundances, Ism: Clouds, Ism: Molecules, Radio Lines: Ism, Shock Waves, Ism: Supernova Remnants

Scientific paper

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.

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