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 four-dimensional potential energy surface, obtained on top of highly accurate abinitio 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 behavior 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.
@article{arxiv.2310.08052,
title = {Collisional excitation of PO$^+$ by $para$-H$_2$},
author = {F. Tonolo and L. Bizzocchi and V. M. Rivilla and F. Lique and M. Melosso and C. Puzzarini},
journal= {arXiv preprint arXiv:2310.08052},
year = {2023}
}