New Theoretical Results Concerning the Interstellar Abundance of Molecular Oxygen

The low abundance of molecular oxygen in cold cores of interstellar clouds poses a continuing problem to modelers of the chemistry of these regions. In chemical models O2 is formed principally by the reaction between O and OH, which has been studied experimentally down to 39 K. It remains possible that the rate coefficient of this reaction at 10 K is considerably less than its measured value at 39 K, which might inhibit the production of O2 and possibly bring theory and observation closer together over a wider range of times. Two theoretical determinations of the rate coefficient for the O + OH reaction at temperatures down to 10 K have been undertaken recently; both results show that the rate coefficient is indeed lower at 10 K than at 39 K, although they differ in the magnitude of the decrease. Here we show, using gas-phase models, how the calculated interstellar O2 abundance in cold cores is affected by a decrease in the rate coefficient. We also consider its effect on other species. Our major finding is that for standard O-rich abundances, the calculated abundance of O2 in cold cores is sufficiently low to explain observations only at early times regardless of the value of k1 in the range investigated here. For C-rich abundances, on the other hand, late-time solutions can also be possible.