Searches for interstellar and circumstellar metal oxides and chlorides

The authors have made sensitive searches for millimetre wave emission lines from MgO, TiO, ClO and CCl in interstellar clouds and circumstellar envelopes. Although several lines were detected in the course of the observations, none of the above molecules were found. The authors discuss this result and conclude that it is consistent with the current knowledge of interstellar and circumstellar chemistry, elemental depletions and the composition of cosmic dust particles. It is possible that metallic atoms may be present in gas-phase molecules other than those for which the authors have searched and it is suggested that, in particular, SiH, and possibly MgH, may be reasonably abundant in interstellar clouds.