Collisional excitation of PO${}^{+}$ by $para$-H${}_2$

Abstract

We report the derivation of rate coefficients for the rotational (de-)excitation of PO${}^{+}$ induced by collisions with H${}_2$. The calculations were performed on a four-dimensional 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 H${}_2$ on the cross sections values, thus allowing to neglect the rotational structure of H${}_2$. 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 $\sim 3.7\times 10^{11}$ cm${}^{-2}$, which is 60\% (a factor of $\sim 1.7$) smaller than the previously LTE-derived value. Extensive simulations show that PO${}^{+}$ low-$j$ rotational lines exhibit maser behavior at densities between $10^4$ and $10^6$ cm${}^{-3}$, thus highlighting the importance of a proper treatment of the molecular collisions to accurately model PO${}^{+}$ emissions in the interstellar medium.