2025A&A...696A.170S

Context. Phosphorus plays an essential role in the chemistry of living organisms, being present in several fundamental biomolecules. The investigation of chemical reactions taking place in different astronomical environments involving phosphorus-containing molecules is essential for understanding how these species are produced and destroyed. Ultimately, it can help unravel the pathways that lead to important prebiotic molecules.
Aims. Phosphorus monoxide (PO) and phosphorus nitride (PN) are key reservoirs of phosphorus in the Interstellar Medium (ISM). Understanding their reaction mechanisms and accurately predicting rate coefficients are crucial for modelling phosphorus chemistry in space. This work presents a computational study of the CPN system to identify viable reaction pathways involving atom-diatom collisions and to explore a potential destruction route for PN in the ISM. We also evaluate the role of several neutral-neutral reactions involving PO and PN in chemical models simulating interstellar environments.
Methods. In this work we explore the potential energy landscape of the C(³P) + PN(¹Σ⁺), N(⁴S) + CP(²Σ⁺) and P(⁴S) + CN(²Σ⁺) reactions by performing high-accuracy ab initio calculations and provide their rate coefficients over a wide range of temperatures. The temperature-dependent rate coefficients were fitted to the modified Arrhenius equation: k(T) = α(T/300)^β^exp(-γ/T). An updated chemical network for P-bearing species was used to model the time-dependent abundances and reaction contributions of P, PO, PN, and PH (phosphinidene) during the chemical evolution of diffuse and translucent clouds and dense clouds.
Results. The only neutral-neutral reaction capable of destroying PN without an activation energy seems to be the PN + C one. We have also shown that reactions between CP and N can yield CN and PN barrierlessly. Chemical models indicate that PO is a crucial species driving the gas-phase formation of PN. Typically, PO/PN ratios exceed 1, though their chemistry is influenced by photon- and cosmic-ray-induced processes. Over time in simulated dense clouds, neutral-neutral reactions such as PO + N, PH + N, P + OH, and PH+O play a significant role in determining the relative abundances of PO and PN.