Elusive Iron: Detection of the FeC Radical (X 3Δ i ) in the Envelope of IRC+10216

Abstract

A new interstellar molecule, FeC ( X ^3 Δ _i ), has been identified in the circumstellar envelope of the carbon-rich asymptotic giant branch star IRC+10216. FeC is the second iron-bearing species conclusively observed in the interstellar medium, in addition to FeCN, also found in IRC+10216. The J = 4 → 3, 5 → 4, and 6 → 5 rotational transitions of this free radical near 160, 201, and 241 GHz, respectively, were detected in the lowest spin–orbit ladder, Ω = 3, using the Submillimeter Telescope of the Arizona Radio Observatory (ARO) for the 1 mm lines and the ARO 12 m at 2 mm. Because the ground state of FeC is inverted, these transitions are the lowest energy lines. The detected features exhibit slight U shapes with LSR velocities near V _LSR ≈ −26 km s ^−1 and linewidths of Δ V _1/2 ≈ 30 km s ^−1 , line parameters characteristic of IRC+10216. Radiative transfer modeling of FeC suggests that the molecule has a shell distribution with peak radius near 300 R _* (∼6″) extending out to ∼500 R _* (∼10″) and a fractional abundance, relative to H _2 , of f ∼ 6 × 10 ^−11 . The previous FeCN spectra were also modeled, yielding an abundance of f ∼ 8 × 10 ^−11 in a larger shell situated near 800 R _* . These distributions suggest that FeC may be the precursor species for FeCN. Unlike cyanides and carbon-chain molecules, diatomic carbides with a metallic element are rare in IRC+10216, with FeC being the first such detection.

Keywords

• Astrochemistry
• Evolved stars
• Circumstellar envelopes
• Interstellar molecules
• Radio astronomy
• Asymptotic giant branch stars