Identification of Phosphorus Monoxide ( X 2 Π r ) in VY Canis Majoris: Detection of the First P–O Bond in Space

Abstract

A new interstellar molecule, PO (X-2 IIr), 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 similar or equal to 2.8 x 10(15) cm(-2), which corresponds to a fractional abundance of f similar to 9 x 10(-8), relative to H-2. 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.