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

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 × 1011 cm−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.