Formation of phosphorus monoxide (PO) in the interstellar medium: insights from quantum-chemical and kinetic calculations

In recent years, phosphorus monoxide (PO) -- an important molecule for prebiotic chemistry -- has been detected in star-forming regions and in the comet 67P/Churyumov-Gerasimenko. These studies have revealed that, in the interstellar medium, PO is systematically the most abundant P-bearing species, with abundances that are $sim$1-3 times greater than those derived for phosphorus nitride (PN), the second most abundant P-containing molecule. The reason why PO is more abundant than PN remains still unclear. Experimental studies with phosphorus in the gas phase are not available, probably because of the difficulties in dealing with its compounds. Therefore, the reactivity of atomic phosphorus needs to be investigated using reliable computational tools. To this end, state-of-the-art quantum-chemical computations have been employed to evaluate accurate reaction rates and branching ratios for the P + OH $rightarrow$ PO + H and P + H$_2$O $rightarrow$ PO + H$_2$ reactions in the framework of a master equation approach based on ab-initio transition state theory. The hypothesis that OH and H${_2}$O can be potential oxidizing agents of atomic phosphorus is based on the ubiquitous presence of H${_2}$O in the ISM. Its destruction then produces OH, which is another very abundant species. While the reaction of atomic phosphorus in its gound state with water is not a relevant source of PO because of emerged energy barriers, the P + OH reaction represents an important formation route of PO in the interstellar medium. Our kinetic results show that this reaction follow an Arrhenius behavior, and thus its rate coefficients alpha=2.28$times$10$^{-10}$ cm${^3}$ molecule$^{-1}$ s$^{-1}$, beta=0.16 and gamma=0.37 K increase by increasing the temperature.