Revisiting the gas-phase chemical rate coefficients at high temperatures in CLOUDY

TL;DR

This paper addresses the challenge of accurately modeling gas-phase reaction rate coefficients at high temperatures in astrophysical simulations, proposing a method to prevent unphysical divergence and implementing it in the CLOUDY code.

Contribution

It introduces a new approach to prevent divergence of reaction rates at high temperatures and integrates this into the CLOUDY spectral synthesis code.

Findings

Prevents unphysical large reaction rates at high temperatures.

Ensures realistic predictions across a temperature range from CMB to 10^10 K.

Improves the reliability of astrochemical modeling in diverse environments.

Abstract

A two-body gas-phase reaction rate coefficient can be given by the usual Arrhenius-type formula which depends on temperature. The UMIST Database for Astrochemistry is a widely used database for reaction rate coefficients. They provide fittings for coefficients valid over a particular range of temperatures. The permissible upper-temperature limits vary over a wide range: from 100 K to 41000K. A wide range of temperatures occurs in nature; thus, it requires evaluating the rate coefficients at temperatures outside the range of validity. As a result, a simple extrapolation of the rate coefficients can lead to unphysically large values at high temperatures. These result in unrealistic predictions. Here we present a solution to prevent the gas-phase reaction coefficients from diverging at a very high temperature. We implement this into the spectral synthesis code CLOUDY which operates over a wide range of temperatures from CMB to 10$^{10}$ K subject to different astrophysical environments.