Title: Collisional excitation of PO + by para-H₂: potential energy surface, scattering calculations, and astrophysical applications

Authors: F . T onolo , L. Bizzocchi , V. M. Rivilla , F. Lique , M. Melosso , C. Puzzarini

DOI: https://doi.org/10.1093/mnras/stad3140

Date: 13/10/2023

Working Group: WG5

Grant Period: 2

Grant Period Goal (number): GAGP-1, GAPG-7

Covered deliverables from the MoU (number): 5.5.3, 5.6.3, 5.7.3

Countries involved: Italy, France, Spain

Number of female/young/ITC coauthors: 2

Abstract: We report the derivation of rate coefficients for the rotational (de-)excitation of PO⁺ induced by collisions with H2. The calculations were performed on a 4D potential energy surface, obtained on top of highly accurate ab initio energy points. Preliminary tests pointed out the low influence of the coupling between j = 0 and the higher rotational levels of H₂ on the cross- sections values, thus allowing to neglect the rotational structure of H₂. On this basis, state-to-state collisional rate coefficients were derived for temperatures ranging from 5 to 200 K. Radiative transfer calculations have been used to model the recent observation of PO⁺ in the G + 0.693–0.027 molecular cloud, in order to evaluate the possible impact of non-LTE models on the determination of its physical conditions. The derived column density was found to be approximately ∼3.7×10¹¹cm⁻², which is 60% (a factor of ∼1.7) smaller than the previously LTE-derived value. Extensive simulations show that PO⁺ low- j rotational lines exhibit maser behaviour at densities between 10⁴ and 10⁶ cm⁻³, thus highlighting the importance of a proper treatment of the molecular collisions to accurately model PO⁺ emissions in the interstellar medium.