2002ApJ...566L.109W

We have observed the J=5-4 ground-state transition of FeO at a frequency of 153 GHz toward a selection of Galactic sources. Toward the Galactic center source Sgr B2, we see weak absorption at approximately the velocity of other features toward this source (62 km.s^–1 LSR). Toward other sources, the results were negative, as they were also for MgOH (3-2) and FeC (6-5). We tentatively conclude that the absorption seen toward Sgr B2 is due to FeO in the hot (∼500 K), relatively low density, absorbing gas known to be present in this line of sight. This is the first (albeit tentative) detection of FeO or any iron-containing molecule in the interstellar gas. Assuming the observed absorption to be due to FeO, we estimate the [FeO]/[SiO] to be of the order of or less than 0.002, and the [FeO]/[H₂] of the order of 3x10^–11. This is compatible with our negative results in other sources. Our results suggest that the iron liberated from grains in the shocks associated with Sgr B2 remains atomic and is not processed into molecular form.