Rotational excitation of H$_3$O$^+$ cations by para-H$_2$: improved collisional data at low temperatures

TL;DR

This study provides improved collisional excitation data for H$_3$O$^+$ by para-H$_2$, significantly affecting astrophysical models of interstellar water and oxygen chemistry.

Contribution

The paper introduces highly accurate state-to-state collisional data for H$_3$O$^+$ with para-H$_2$, covering low temperatures and relevant energy levels, enhancing astrophysical excitation modeling.

Findings

New collisional rates significantly alter predicted emission line brightness.

Revised data can improve estimates of hydronium column density in interstellar clouds.

Impact on excitation temperatures can be up to a factor of 3.

Abstract

The hydronium cation plays a crucial role in interstellar oxygen and water chemistry. While its spectroscopy was extensively investigated earlier, the collisional excitation of H$_3$O$^+$ is not well studied yet. In this work we present state-to-state collisional data for rotational de-excitation of both ortho- and para-H$_3$O$^+$ due to para-H$_2$ impact. The cross sections are calculated within the close-coupling formalism using our recent, highly accurate rigid-rotor potential energy surface for this collision system. The corresponding thermal rate coefficients are calculated up to 100 K. For para-H$_3$O$^+$ the lowest 20 rotation-inversion states were considered in the calculations, while for ortho-H$_3$O$^+$ the lowest 11 states are involved (up to $j\leq5$), so all levels with rotational energy below 420 K (292 cm$^{-1}$) are studied. In order to analyse the impact of the new collisional rates on the excitation of H$_3$O$^+$ in astrophysical environments radiative transfer calculations are also provided. The most relevant emission lines from an astrophysical point of view are studied, taking into account the transitions at 307, 365, 389 and 396 GHz. We show that our new collisional data have a non-negligible impact (from a few percents up to about a factor of 3) on the brightness and excitation temperatures of H$_3$O$^+$, justifying the revision of the physical conditions in the appropriate astrophysical observations. The calculated rate coefficients allow one to recalculate the column density of hydronium in interstellar clouds, which can lead to a better understanding of interstellar water and oxygen chemistry.