Constraining phosphorus chemistry in carbon- and oxygen-rich circumstellar envelopes: Observations of PN, HCP, and CP

Millimeter-wave observations of PN, CP, and HCP have been carried out toward circumstellar envelopes of evolved stars using the Arizona Radio Observatory (ARO). HCP and PN have been identified in the carbon-rich source CRL 2688 via observations at 1 mm using the Submillimeter Telescope (SMT) and 2-3 mm with the Kitt Peak 12 m. An identical set of measurements were carried out toward IRC +10216, as well as observations of CP at 1 mm. PN was also observed toward VY Canis Majoris (VY CMa), an oxygen-rich supergiant star. The PN and HCP line profiles in CRL 2688 and IRC +10216 are roughly flat topped, indicating unresolved, optically thin emission; CP, in contrast, has a distinct "U" shape in IRC +10216. Modeling of the line profiles suggests abundances, relative to H₂, of f(PN) ∼ (3-5) × 10 ^_9and f(HCP) ∼ 2 × 10^-7 in CRL 2688, about an order of magnitude higher than in IRC +10216. In VY CMa, f(PN) is ∼4 × 10^-8. The data in CRL 2688 and IRC +10216 are consistent with LTE formation of HCP and PN in the inner envelope, as predicted by theoretical calculations, with CP a photodissociation product at larger radii. The observed abundance of PN in VY CMa is a factor of 100 higher than LTE predictions. In IRC +10216, the chemistry of HCP/CP mimics that of HCN/CN and suggests an N ₂ abundance of f ∼ 1 × 10^-7. The chemistry of phosphorus appears active in both carbon- and oxygen-rich envelopes of evolved stars.