2000ApJ...529L.111C

We consider the gas-phase chemistry of CO₂ molecules in active regions. We show that CO₂ molecules evaporated from dust in hot cores cannot be efficiently destroyed and are in fact copiously produced in cooler gas. When CO₂-rich ices are sputtered in strong MHD shock waves, the increase in atomic hydrogen, due to H₂ dissociation by ion-neutral streaming, means that CO₂ can be depleted by factors of ∼500 from its injected abundance. We find that a critical shock speed exists at higher preshock densities below which CO₂ molecules can be efficiently sputtered but survive in the postshock gas. These calculations offer an explanation for the low gas/solid CO₂ ratios detected by the Infrared Space Observatory in star-forming cores as being due to shock destruction followed by partial reformation in warm gas. The presence of high abundances of CO₂ in the strongly shocked Galactic center clouds Sgr B2 and Sgr A also find a tentative explanation in this scenario. Shock activity plays an important role in determining the chemistry of star-forming regions, and we suggest that most hot cores are in fact shocked cores.