2021ApJ...906...55B

The PO molecule (X²Π_r) has been detected toward Orion-KL via its J = 2.5 - 1.5 transition near 109 GHz using the 12 m telescope of the Arizona Radio Observatory. This transition consists of a quartet of lines, generated by lambda-doubling and phosphorus hyperfine interactions, creating a distinct pattern of doublets of doublets. All four features (F = 3 - 2, e, f and 2 - 1, e, f ) were detected. The line profiles measured were relatively broad, with ΔV_1/2 ∼ 21-25 km s^–1, and V_LSR ∼ 10 km s^–1, clearly identifying the molecule as arising from the Orion "plateau" region. The J = 2 - 1 transition of PN (X¹Σ) at 94 GHz was also measured, and its line profile indicates a plateau origin in Orion, as found previously by Ziurys. Therefore, PO and PN arise from the same gas. There was no evidence of a "hot core" or "ridge" component in either molecule. The column densities determined for PO and PN in the Orion plateau are N_tot ≃ 5.4(0.2) x 10¹³ and 2.0(0.4) x 10¹³ cm^–2, respectively, with corresponding abundances, relative to H₂, of f ∼ 1.6(0.1) x 10^–10 and 6.1(0.6) x 10^–11, resulting in PO/PN ∼ 3. The PO and PN line profiles also resemble those of SiO and SiS, suggesting that the phosphorus molecules are produced by shocks in the plateau outflow. The observed PO/PN ratio in Orion-KL is very close in value to that measured in other warm molecular clouds, indicating a common synthesis pathway for these two molecules in outflows caused by star formation.