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Despite its importance in planet formation and biology¹, phosphorus has been identified only in the inner 12 kpc of the Galaxy^2–19. The study of this element has been hindered in part by unfavourable atomic transitions^2,4,20. Phosphorus is thought to be created by neutron capture on²⁹Si and³⁰Si in massive stars^20,21, and released into the interstellar medium by Type II supernova explosions^2,22. However, models of galactic chemical evolution must arbitrarily increase the supernovae production²³to match observed abundances. Here we present the detection of gas-phase phosphorus in the Outer Galaxy through millimetre spectra of PO and PN. Rotational lines of these molecules were observed in the dense cloud WB89-621, located 22.6 kpc from the Galactic Centre²⁴. The abundances of PO and PN in WB89-621 are comparable to values near the Solar System²⁵. Supernovae are not present in the Outer Galaxy²⁶, suggesting another source of phosphorus, such as ‘Galactic Fountains’, where supernova material is redistributed through the halo and circumgalactic medium²⁷. However, fountain-enriched clouds are not found at such large distances. Any extragalactic source, such as the Magellanic Clouds, is unlikely to be metal rich²⁸. Phosphorus instead may be produced by neutron-capture processes in lower mass asymptotic giant branch stars²⁹which are present in the Outer Galaxy. Asymptotic giant branch stars also produce carbon²¹, flattening the extrapolated metallicity gradient and accounting for the high abundances of C-containing molecules in WB89-621.

A new interstellar molecule, FeC (X³Δ_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. TheJ= 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 nearV_LSR≈ −26 km s⁻¹and linewidths of ΔV_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 300R_*(∼6″) extending out to ∼500R_*(∼10″) and a fractional abundance, relative to H₂, off∼ 6 × 10⁻¹¹. The previous FeCN spectra were also modeled, yielding an abundance off∼ 8 × 10⁻¹¹in a larger shell situated near 800R_*. 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.

The millimeter-wave spectrum of the SiP radical (X²Π_i) has been measured in the laboratory for the first time using direct-absorption methods. SiP was created by the reaction of phosphorus vapor and SiH₄in argon in an AC discharge. Fifteen rotational transitions (J+ 1 ←J) were measured for SiP in the Ω = 3/2 ladder in the frequency range 151–533 GHz, and rotational, lambda doubling, and phosphorus hyperfine constants determined. Based on the laboratory measurements, SiP was detected in the circumstellar shell of IRC+10216, using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory at 1 mm and 2 mm, respectively. Eight transitions of SiP were searched: four were completely obscured by stronger features, two were uncontaminated (J= 13.5 → 12.5 and 16.5 → 15.5), and two were partially blended with other lines (J= 8.5 → 7.5 and 17.5 → 16.5). The SiP line profiles were broader than expected for IRC+10216, consistent with the hyperfine splitting. From non-LTE radiative transfer modeling, SiP was found to have a shell distribution with a radius ∼300R_*, and an abundance, relative to H₂, off∼ 2 × 10⁻⁹. From additional modeling, abundances of 7 × 10⁻⁹and 9 × 10⁻¹⁰were determined for CP and PN, respectively, both located in shells at 550–650R_*. SiP may be formed from grain destruction, which liberates both phosphorus and silicon into the gas phase, and then is channeled into other P-bearing molecules such as PN and CP.