Title: Collisional excitation of PO+ by para-H2 : potential energy surface, scattering calculations, and astrophysical applications
Authors: F. Tonolo, L. Bizzocchi, V. M. Rivilla, F. Lique , M. Melosso, C. Puzzarini
Date: 2023-10-13
Working Group: 5
Grant Period: 2
Grant Period Goal (number): 7
Covered deliverables from the MoU (number): 1.3.2
Countries involved: Italy, Spain, France
Number of female/young/ITC coauthors: 2/2/0
Is the publication open access?: Yes
Is the publication co-lead by a YRI?: Yes
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 × 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.
