A search for MgNC and A1NC in TMC-1: The status of metals in dark cloud cores

Heavy metals in the gas phase are important in controlling the ionization of a cold dark cloud and hence the chemistry and finally the cloud evolution. Two of us (S. P. and R. C. D.) earlier suggested a fast radiative association mechanism to form MgNC and AlNC in TMC-1. We have conducted a sensitive search with the Green Bank Telescope (GBT ) for both species in TMC-1. The upper limit for the fractional abundance of MgNC is 1.2 × 10^-11. We have run detailed models of the chemistry, which include high and low metal content, three values of C/O, and three values of density. The models include the time dependence up to 1 × 10⁸ yr. Most of the models predict that we should have seen MgNC. We compare the conditions in TMC-1 with those for IRC +10216, in which MgNC and AlNC are both detected. We conclude that in IRC +10216 the chemistry favors a high abundance of both atomic Mg⁺ and cyanopolyynes, the two reactants that form MgNC, and that these reactants are less favored in TMC-1.