From HifiISMWiki

Shock regions

Observations of the interaction of blast waves with the surrounding atomic and molecular gas provide an excellent tool to support our theoretical understanding of molecular shocks.

Figure 1: Velocity-integrated emission of CO(2-1) in the supernova remnant IC443, outlining the structure of the molecular gas compressed by the SNR blast wave. Superimposed is the integrated H₂ emission, which nicely traces the interaction layer between the blast wave and the molecular gas.

• In a jump (J) shock all the heating occurs in a very small region and the gas subsequently cools in a hot post-shock relaxation layer. Thus J-shocks are mostly dissociative.
• In continuous (C) shocks, parameters vary smoothly from pre- to post-shock conditions. Heating occurs in an extended layer where heating and cooling processes compete. The temperature is lower and C-shocks are more non-dissociative. They will be prominent in the vibration-rotation transition of H₂ and the high-excitation molecular lines of CO and H₂O.

By directly observing the chemical composition and the main cooling species with HIFI and PACS, the critical information on the state of the shocked gas can be obtained. HIFI's high spectral resolving power will be crucial to provide the kinematic information that is needed to disentangle the emission of the shocked gas from the emission of the quiescent cloud.

HIFI will also allow to address the chemistry across the shocked gas layer, in particular the puzzling chemistry of H₂O and OH, indicated by observations of 1720 MHz OH maser emission in the post-shocked gas but a relative weakness of the H₂O rotational emissions.

Multi-transition observations will allow the temperature of the emitting water to be probed, and will indicate whether the water abundance is uniformly suppressed at all gas temperatures.