A Chemical Route to the Formation of Water in the Circumstellar Envelopes around Carbon-rich Asymptotic Giant Branch Stars: Fischer-Tropsch Catalysis

Fischer-Tropsch catalysis has been suggested as a means of driving hydrocarbon chemistry in oxygen-rich regions such as the protosolar nebula. In addition to producing hydrocarbons, Fischer-Tropsch catalysis also produces water, and it is therefore possible that such processes could account for the recent observations of water in the circumstellar envelope of the carbon-rich asymptotic giant branch star IRC +10216. We present models of the envelope of IRC +10216 and demonstrate that Fischer-Tropsch catalysis on the surfaces of small metallic grains can produce large quantities of water. The abundance of water produced depends on the number density of iron grains that can act as catalysts. If a few percent (0.5%-5%) of the available iron exists as metallic iron or iron/nickel alloy, then Fischer-Tropsch catalysis can produce sufficient gaseous water to account for the observations.