Carbon monoxide formation in SN 1987A

The formation of carbon monoxide in the supernova SN 1987A at 200 days is investigated. Rather than the one-zone models that have previously been employed, this work utilizes a radially dependent radiative transport model to compute the temperature and ionization structure and the destruction of CO and CO+ by ultraviolet radiation The CO profile is computed assuming chemical equilibrium. Two models for the density and abundance profiles are examined: model 10H (unmixed) and model 10HMM (mixed) of Pinto & Woosley. Sensitivity to adopted rates and temperature as it might be affected by CO cooling is examined. The models give a total mass of similar to 10(-4) M-. at 200 days for the unmixed model, which is comparable with the estimated observed abundance by Spyromilio et al. but nearly 2 orders of magnitude less than the estimated observed abundance and that computed in thermal-chemical models by Liu & Dalgarno. The effect of different model assumptions and results concerning ionization structure and radiative transfer are discussed. We confirm that CO+ is not expected to be produced in significant amounts and that the amount of CO is sensitive to the degree of the mixing of the composition of various elements present in the ejecta. The microscopically mixed model for SN 1987A does not create sufficient CO, giving only similar to 10(-6) M, because of destruction of CO by charge exchange. The mixed model might make substantial amounts of CO if the rate for HeH+ + H --> H-2(+) +He were substantially increased, leading to more H-2, CH, and OH as intermediaries in the formation of CO.