The phosphorus monoxide radical, PO, has been detected with the ARO 10 m SMT in four lines of two rotational transitions in the millimeter-wave region:
E. D. Tenenbaum, N. J. Woolf, and L. M. Ziurys,
Identification of Phosphorus Monoxide (//X// ²Π_r) in VY Canis Majoris: Detection of the First P–O Bond in Space
Astrophys. J. 666, L29–L32 (2007).
The J = 5.5 – 4.5 and 6.5 – 5.5 transitions were detected in emission near 240 and 284 GHz with resolved splitting of the λ-doubling. The number of observed lines is probably the minimum number for the unambiguous detection of a diatomic molecule. The abundance is about the same as for the PN molecule. Interestingly, it is the first molecule to be detected in an oxygen-rich circumstellar envelope prior to anywhere else in space.

E. De Beck, T. Kaminski, N. A. Patel, K. H. Young, C. A. Gottlieb, K. M. Menten, and L. Decin
reported on
PO and PN in the Wind of the Oxygen-rich AGB Star IK Tauri
Astron. Astrophys. 558, Art. No. A132 (2013).
Several lines of four rotational transitions were detected with modest to good signal-to-noise ratio using the IRAM 30 m telescope and the SMA. PN was considerably more abundant than PO.

V. M. Rivilla, F. Fontani, M. T. Beltrán, A. Vasyunin, P. Caselli, J. Martín-Pintado, and R. Cesaroni
reported on
First Detections of the Key Prebiotic Molecule PO in Star-forming Regions
Astrophys. J. 826, Art. No. 161 (2016).
Three transitions where observed with the IRAM 30 m telescope toward W51 and W3(OH). Hyperfine splitting and λ-doubling were resolved. Signal was detected in all cases with no or only marginal blending in some lines and pronounced to severe blending for others. PO was in both cases more abundant than PN.

B. Lefloch, C. Vastel, S. Viti, I. Jimenez-Serra, C. Codella, L. Podio, C. Ceccarelli, E. Mendoza, J.R.D. Lepine, R. Bachiller,
reported on
Phosphorus-bearing Molecules in Solar-type Star Forming Regions: First PO Detection
Mon. Not. R. Astron. Soc. 462, 3937–3944 (2016).
Several lines of PO were detected with the IRAM 30 m telescope toward L1157-B1 with an abundance ratio to PN of 3 : 1.

S. H. J. Wallström, T. Danilovich, H. S. P. Müller, C. A. Gottlieb, S. Maes, M. Van de Sande, L. Decin, A. M. S. Richards, A. Baudry, J. Bolte, T. Ceulemans, F. De Ceuster, A. de Koter, I. El Mellah, M. Esseldeurs, S. Etoka, D. Gobrecht, E. Gottlieb, M. Gray, F. Herpin, M. Jeste, D. Kee, P. Kervella, T. Khouri, E. Lagadec, J. Malfait, L. Marinho, I. McDonald, K. M. Menten, T. J. Millar, M. Montargès, J. A. Nuth, J. M. C. Plane, R. Sahai, L. B. F. M. Waters, K. T. Wong, J. Yates, and A. Zijlstra
reported on
ATOMIUM: Molecular Inventory of 17 Oxygen-rich Evolved Stars Observed with ALMA
Astron. Astrophys. 681, Art. No. A50 (2024).
The authors detect in their ALMA measurements in Band 6 vibrationally excited PO toward AH Sco, IRC+10011, IRC−10529, R Hya, and S Pav. The Ω = 3/2, J = 11/2 − 9/2, F = 6 − 5 and 5 − 4 lines near 239.9 GHz with unresolved Λ-doubling were detected near the v = 0, Ω = 1/2, J = 11/2 − 9/2, F = 6 − 5 and 5 − 4 e-lines near 240.0 GHz; the f-lines were not covered.

Contributor(s): H. S. P. Müller; 08, 2007; 08; 2016; 09, 2024