Search for interstellar and stellar iron monoxide

A search for the lowest energy rotational transition (J = 5--4) of iron monoxide (FeO) near 153 GHz in the lowest and first excited vibrational states has been conducted toward a variety of objects. FeO was not detected. Our upper limit on the FeO column density in Orion and other warm molecular clouds is N(FeO)<2 x 1012 cm-2. In Orion, <2 x 107 of the cosmic iron abundance can be in the form of FeO and [FeO]/[H2]<10-11. It is likely that most Fe is incorporated into interstellar dust grains. The remaining fraction in the gas phase probably favors atomic iron rather than FeO