Chemical diagnostics for gas-phase metal depletion in cold astrophysical environments

Negative results from searches for metal-containing molecules (MCMs) in dense interstellar (IS) clouds, and current models of IS cloud chemistry, suggest severe depletion of gas-phase metals. To properly interpret the observational results, however, the true gas-phase reactivity of metal-containing species must be known: non-detection of a MCM is useful only if its chemistry is well-understood. Here we suggest that the MCMs sought to date in IS clouds either lack viable formation processes or have an intrinsically short lifetime, severely limiting both their expected abundance and their usefulness as "fingerprints" for gas-phase metals. We explore efforts to develop more detailed models of metal chemistry in IS clouds and circumstellar envelopes, with a view to predicting detectable gas-phase markers for metals in such environments.