The effect of water and electron collisions in the rotational excitation of HF in comets

TL;DR

This study provides new rate coefficients for the rotational excitation of HF by water and electrons, crucial for modeling cometary comas, highlighting the importance of both collision types due to HF's large dipole moment.

Contribution

First quantum statistical calculations of HF excitation rates by water and electrons across relevant temperatures for cometary environments.

Findings

Water and electron collisions significantly influence HF excitation in comets.

Both collision types must be considered for accurate non-LTE modeling.

Rate coefficients enable better interpretation of HF observations in cometary atmospheres.

Abstract

We present the first set of rate coefficients for the rotational excitation of the 7 lowest levels of hydrogen fluoride (HF) induced by collision with water molecules, the dominant collider in cometary comas, in the 5-150 K temperature range. The calculations are performed with a quantum statistical approach from an accurate rigid rotor ab initio interaction potential. Rate coefficients for excitation of HF by electron-impact are also computed, within the Born approximation, in the 10-10,000 K temperature range. These rate coefficients are then used in a simplified non-local thermodynamic equilibrium (non-LTE) model of a cometary coma that also includes solar radiative pumping and radiative decay. We investigate the range of H2O densities that lead to non-LTE populations of the rotational levels of HF. We show that to describe the excitation of HF in comets, considering collisions with both water molecules and electrons is needed as a result of the large dipole of HF.