Tracing the Path of Iron: Detection of FeC (X ³Δ_i) in the Circumstellar Envelope of IRC+10216

Iron is one of the more abundant refractory elements with a cosmic abundance of 3.2 × 10^-5. It is thought to play a pivotal role in the formation of rocky, Earth-like exoplanets. It is also common in meteorites and in interstellar dust grains, as pure iron and in minerals such as schreibersite, (Fe,Ni)P₃. The connection between solid-state and gas-phase iron in the interstellar medium (ISM) is not well-understood, and the progression to Fe-containing planetary bodies is uncertain. Gas-phase atomic iron is known to be present in stellar photospheres, star-forming regions, and planetary nebulae, but molecular forms of iron remain elusive. For over 50 years, only one iron-bearing molecule has been conclusively identified in the ISM: FeCN. The species was found in the circumstellar envelope of the late-type carbon star IRC+10216. Here we present the interstellar identification of a second iron-containing molecule, FeC (X ³Δ_i). The radical was also observed in the envelope of IRC+10216, using the Arizona Radio Observatory (ARO) Submillimeter Telescope (SMT) at 1.3 mm and the ARO 12-m telescope at 2 mm. The J = 4→3, J = 5→4, and J = 6→5 rotational transitions of FeC were detected; these are the three lowest energy lines in the ground state spin-orbit ladder, Ω = 3. FeC was found to have a fractional abundance, relative to H₂, of 6 × 10^-11, with a shell distribution extending out to 500 R_* (~ 10ʺ). In contrast, FeCN has an abundance of 8 × 10^-11 in IRC+10216. These data suggest that much of the iron is sequestered into dust grains in circumstellar shells, setting the stage for condensation into larger objects.