A graph showing the distribution of phosphorus within the galaxy, between 0 and 25 kpc from the galactic center, as determined from stellar and interstellar observations. Detections due to atomic transitions in the photosphere of stars are indicated by green stars (from ref. 2 and references therein), in diffuse clouds by purple circles (from ref. 16 and references therein), and in planetary nebulae by purple circles (from ref. 16 and references therein). Indicated by a blue triangle, magenta circle and cyan color. Diamonds mark the identification of the circumstellar envelope from molecular clouds and molecular lines. The red square indicates the location of WB89-621. The orange X represents the solar system. APOGEE survey findings are not displayed by potential contamination. WB89-621 is the only known source of all forms of phosphorus above 12 kpc.
Despite the importance of phosphorus in planet formation and biology, phosphorus has only been identified within the interior 12 kpc of galaxy 2-19. Research on this element has been partially hampered by unfavorable atomic transitions.
Phosphorus is thought to be produced by neutron capture in 29Si and 30Si of massive stars and released into the interstellar medium during type II supernova explosions. But models of the chemical evolution of galaxies require an arbitrary increase in supernova production to match observed abundances.
Here we present the detection of gas-phase phosphorus in outer galaxies using millimeter-wave spectra of PO and PN. These molecular rotation lines were observed in the dense cloud WB89-621, located 22.6 kpc from the galactic center. The PO and PN abundances of WB89-621 are comparable to values near the solar system. Supernovae are absent from outer galaxies, suggesting other sources of phosphorus, such as galactic fountains, where supernovae material is redistributed through halos and the circumgalactic medium.
But clouds rich in fountains are not found very far away. Extragalactic sources like the Magellanic Clouds are unlikely to be metal-rich. Instead, phosphorus may be produced by neutron capture processes in low-mass asymptotic giant branch stars in outer galaxies. Asymptotic giant branch stars also produce carbon-21, flattening the extrapolated metallicity gradient and accounting for the high abundance of C-containing molecules in WB89-621.
Abundances of PN and PO and phosphorus atoms as a function of distance from the galactic center. Molecular weight versus H2 is plotted versus RGC (kpc) for AFGL 5142, G + 0.693-0.03, W3(OH), W51, L1157, Orion-KL, B1-a, NGC 1333-IRAS 3, Ser . SMM1, L723 (reference), and WB89-621. Atom abundances from the sources in Figure 1 are also shown. Abundances of PO, PN, and atomic P are shown in red, black, and blue, and uncertainties are estimated in 3σ. From 8.5 kpc to 22.6 kpc, the abundances of PO and PN decrease by a factor of 1.5 and 2.3, respectively, with almost no decrease in outer galaxies.
LA Cole May, KR Gold, LM Giulis
PMID: 37938703 PMCID: PMC10632128 DOI: 10.1038/s41586-023-06616-1
Phosphorus-containing molecules PO and PN at the edge of the galaxynature (open access)
Astrobiology, astrochemistry
