Identification of Phosphorus Monoxide (X²Π_r) in VY Canis Majoris: Detection of the First P-O Bond in Space

A new interstellar molecule, PO (X²Π_r), has been detected toward the envelope of the oxygen-rich supergiant star VY Canis Majoris (VY CMa) using the Submillimeter Telescope of the Arizona Radio Observatory. The J = 5.5 → 4.5 and J = 6.5 → 5.5 rotational transitions of PO at 240 and 284 GHz were observed, each of which consisted of well-defined lambda-doublets. The line profiles are roughly parabolic in shape, analogous to PN, and suggest that this species arises from the spherical wind in VY CMa, as opposed to the collimated blue- and redshifted outflows. Comparison of line intensities indicates that PO arises from a confined source roughly 1'' in extent, with a column density of N_tot ≃ 2.8 × 10¹⁵ cm^-2, which corresponds to a fractional abundance of f ~ 9 × 10^-8, relative to H₂. Consequently, PO and PN have similar concentrations in VY CMa, a result not predicted by either LTE or kinetic models of circumstellar chemistry. These phosphorus compounds may arise from shock-induced reactions in this active envelope. Phosphorus monoxide is the first interstellar molecule detected that contains a P–O bond, a moiety essential in biochemical compounds. It is also the first new species to be identified in an oxygen-rich, as opposed to a carbon-rich, circumstellar envelope.