A search for interstellar and stellar iron monoxide

It is reported that FeO could not be detected in a search for the lowest-energy, or 5-4, rotational transition of this compound near 153 GHz in the lowest and first excited vibrational states. The objects toward which the search was conducted include Orion KL, IC 443, IRC + 10216, U Her, and Nova Cyg 75, among others. The upper limit found for the FeO column density in Orion and other warm molecular clouds is less than 2 x 10 to the 12th/sq cm. It is considered probable that most Fe is incorporated into interstellar dust grains, with the remaining fraction in the gas phase probably favoring atomic iron rather than FeO.