Astronomical Search and Laboratory Spectroscopy of the FeCO Radical

The millimeter and submillimeter spectrum of the FeCO radical in the 3 Σ - ground electronic state has been observed in the laboratory. The radical was generated by a DC glow discharge in a mixture of Fe(CO) 5 and argon. A total of 33 rotational transitions in the three spin sublevels with Ω = 1, 0 were observed in the frequency region between 223 and 372 GHz. The observed frequencies were analyzed using a Hamiltonian for the 3 Σ - state, and the rotational, centrifugal distortion, spin-spin coupling, and spin-rotation coupling constants were determined. Based on these constants, we obtained the rest frequencies in the 2 and 3 mm regions of FeCO and searched for this radical toward the circumstellar envelope of IRC +10216 and the star-forming regions W3(OH), DR 21, and Sgr B2 with the Instituto de Radioastronomia Milimetrica 30 m radio telescope. The FeCO lines were not detected with an rms noise of 3-7 mK in four sources. The upper limit on the FeCO column density was estimated to be 3 × 10 13 cm -2 in IRC +10216.