Shock Chemistry

Hydrodynamic and magnetohydrodynamic (MHD) shocks are important in the time-dependent chemistry of the diffuse interstellar medium. The time scales are very 

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Shock compression of solids V shock chemistry with applications to meteorite impacts / editors, Lee Davison, Yasuyuki Horie, Toshimori Sekine. 

Shock Chemistry with Applications to Meteorite Impaets... 

Various observational tests of the relative importance of shock chemistry compared with steady-state chemistry in diffuse clouds are possible ... 

The chemistry in this gas is driven in various ways. Initially, the weak ( 10 km a" -) reverse shock at the cavity boundary induces a characteristic neutral atom-siolecule chesiistry in the hot post-shock phase. The ionization in this accumulated wind is relatively high, and reactions with ions stodify the shock chemistry products. When the gas is cool, ion-molecule chemistry, familiar from earlier studies of dark clouds, plays its part. However, insufficient time is available for steady state to be achieved (this takes > 10 yr cf. Millar and Nejad 1985) because the heavy molecules such as CO, H2O, NH3 etc. are accreted on to the grain surface and lost from the gas phase. 

While the abundances of most neutral species are not much affected by enhanced rate coefficients, those of some protonated species such as HCS do increase. The detection of H30 has been discussed in some detail by Mamon et al. (1988) here we simply note that in the case of enhanced rate coefficients the HCSVCS abundance ratio is - 0.6 while the observed upper limit in OH231.8+4.2 is 0.54 (Morris et al. 1988), although one must remember that, in this object, the observed line-widths may indicate that shock chemistry is important. 

Chemistry in thepost-MHD shock environment is dominated by the separation between neutral and ionic species, the former being less affected than the latter by the magnetic field. In consequence, the reaction sites are calculated to show abundance stratification depending on the reaction channels. Since the fronts may be broad enough to be spatially resolved, it is possible to study the diffuse-phase ISM shock chemistry observationally in some detail. 

Because of the very low densities and temperatures prevalent in the interstellar medium, molecules cannot be formed by the same processes that produce them on earth. Astrochemistry appears to be dominated by three chemical reaction regimes ion-molecule, grain surface, and shock chemistry. 

Strong shocks produced by the expanding ionized envelopes of massive stars and supernova remnants heat and compress the interstellar medium, leading to conditions ripe for many high-temperature chemical reactions. Like grain surface chemistry, reactions within the shock chemistry environment are difficult to simulate, but are progressing toward a physical framework that can be compared to observations. While OH and H2O are prominent products of ion-molecule chemistry as well as shock chemistry, SiO, and SiS are predominently produced in shocks. 

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