Detection of PO in Orion-KL: Phosphorus Chemistry in the Plateau Outflow

Abstract

Abstract The PO molecule (X 2Πr) 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 (X 1Σ) 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) × 1013 and 2.0(0.4) × 1013 cm−2, respectively, with corresponding abundances, relative to H2, of f ∼ 1.6(0.1) × 10−10 and 6.1(0.6) × 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.