Carbon dioxide in star-forming regions

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 efficie ntly destroyed and an in fact copiously produced in cooler gas. When CO2-ri ch ices are sputtered in strong MHD shock waves, the increase in atomic hyd rogen, due to H-2, dissociation by ion-neutral streaming, means that CO2 ca n 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. Th ese calculations offer an explanation for the low gas/solid CO2 ratios dete cted by the infrared Space Observatory in star-forming cores as being due t o shock destruction followed by partial reformation in warm gas. The presen ce 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-form ing regions, and we suggest that most hot cores are in fact shocked cores.