The reaction of P+ with H2O (D2O): a pathway for the formation of PO and PO+ in the ISM and circumstellar envelopes

Context. Interstellar phosphorus shows uncertain depletion patterns and an unidentified main reservoir, with only a few P-bearing molecules detected despite their key astrochemical and astrobiological relevance. In particular, recent observations of PO+ with a high abundance with respect to PO suggest that ion–molecule chemistry may play a crucial role, motivating a reassessment of its formation pathways in the ISM and circumstellar envelopes. Aims. The role of the reaction between P+ and water (D2O) for the synthesis of PO+ and POH+ is explored in a joint experimental and theoretical study. Furthermore, new possible routes leading to the conversion of i) POH+ into PO (via a non-dissociative proton- transfer-reaction with ammonia) and of ii) PO into PN (via the adiabatic barrierless reaction of N(4S) with PO are proposed. Methods. The reaction P+ plus D2O is studied experimentally by measuring absolute cross sections (CSs) and branching ratios (BR), as a function of collision energy. Experiments are supported by a theoretical investigation combining high-level electronic structure calculations of the multidimensional triplet and singlet potential energy surfaces with a kinetic investigation to derive BRs and channel-specific rate constants as a function of temperature in the 10-5000 K range. Results. The reaction P+ + D2O leads mostly to POD+ plus D (BR=90%) with PO+ plus D2 being a minor channel (BR=10%). From the total reaction CS as a function of collision energy, estimates for the rate constant as a function of temperature have been obtained, with values ranging from 1.2 × 10−9 cm3s−1 (at 10 K) to 7.5 × 10−10 cm3s−1 (at 5000 K). The proton transfer reactions between POH+ and NH3 is found to be efficient with rate constants in the range 1.1 − 2.7 × 10−9 cm3 s−1 molecule−1. Conclusions. The reaction of P+ with water should be considered in astrochemical models where phosphorus can be released in the gas phase as a cation, from the energetic processing of icy interstellar grains due to shocks. The reaction leads to POH+ as the main reaction product and relevant interstellar isomer, an important precursor for PO formation not only via dissociative recombination with electrons, but also by proton transfer to NH3. The direct formation of PO+ as a secondary channel can explain the high PO+/PO ratio detected in the ISM.