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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.

 

Abstract Observations of HCN and HCO + have been carried out toward 13 planetary nebulae (PNe) using the facilities of the Arizona Radio Observatory (ARO). These nebulae represent a wide range of morphologies and ages (∼2000–28,000 yr). For both molecules, the J = 1 → 0 transitions at 88–89 GHz and the J = 3 → 2 lines at 265–267 GHz were measured, together with CO lines ( J = 1 → 0, 2 → 1, and 3 → 2, depending on the source), using the ARO 12 m and Submillimeter Telescopes. HCN and HCO + were detected with at least one transition in 10 nebulae: He 2-459, Hu 1-1, K3-52, K3-65, M1-8, M1-40, M1-59, M2-53, M4-17, and NGC 6445. HCO + was additionally identified via two transitions in Na 2. Some observed line profiles were complex, with multiple velocity components tracing varied outflows. From radiative transfer modeling, column densities were established for HCN and HCO + : N tot (HCN) = 0.005–1.1 × 10 14 and N tot (HCO + ) = 0.008–9.5 × 10 13 cm −2 . Gas densities of n (H 2 ) ∼ 10 5 –10 7 cm −3 were also determined for all PNe. Fractional abundances with respect to H 2 , calculated using CO as a proxy, are f (HCN) ∼ 0.2–1.5 × 10 −7 and f (HCO + ) ∼ 0.3–5.1 × 10 −8 . The abundances of HCN and HCO + did not significantly vary with nebular age to 28,000 yr. Combined with previous observations, at least 30 PNe contain HCN and/or HCO + , indicating that polyatomic molecules are common constituents of these objects. The data strongly support a scenario where dense ejecta from PNe seed the interstellar medium with molecular material.