CF+ excitation in the interstellar medium

Benjamin Desrousseaux,François Lique,Javier R Goicoechea,Richard Dawes,Ernesto Quintas-Sánchez

doi:10.1051/0004-6361/202039823

Abstract

The detection of CF+ in interstellar clouds potentially allows astronomers to infer the elemental fluorine abundance and the ionization fraction in ultraviolet-illuminated molecular gas. Because local thermodynamic equilibrium (LTE) conditions are hardly fulfilled in the interstellar medium (ISM), the accurate determination of the CF+ abundance requires one to model its non-LTE excitation via both radiative and collisional processes. Here, we report quantum calculations of rate coefficients for the rotational excitation of CF+ in collisions with para- and ortho-H2 (for temperatures up to 150 K). As an application, we present non-LTE excitation models that reveal population inversion in physical conditions typical of ISM photodissociation regions (PDRs). We successfully applied these models to fit the CF+ emission lines previously observed toward the Orion Bar and Horsehead PDRs. The radiative transfer models achieved with these new rate coefficients allow the use of CF+ as a powerful probe to study molecular clouds exposed to strong stellar radiation fields.

Highlights

Fluorine is one of the most reactive species in the interstellar medium (ISM; Neufeld et al 2005; Neufeld & Wolfire 2009), the chemistry of the most abundant F-bearing molecules, HF and CF+, can be accurately described by a few chemical reactions that depend on the amount of F atoms, H2 molecules, and C+ ions (Neufeld et al 2005)
Because local thermodynamic equilibrium (LTE) conditions are hardly fulfilled in the interstellar medium (ISM), the accurate determination of the CF+ abundance requires one to model its non-LTE excitation via both radiative and collisional processes
We present non-LTE excitation models that reveal population inversion in physical conditions typical of ISM photodissociation regions (PDRs)

Summary

Introduction

Fluorine is one of the most reactive species in the interstellar medium (ISM; Neufeld et al 2005; Neufeld & Wolfire 2009), the chemistry of the most abundant F-bearing molecules, HF and CF+, can be accurately described by a few chemical reactions that depend on the amount of F atoms, H2 molecules, and C+ ions (Neufeld et al 2005). We report quantum calculations of rate coefficients for the rotational excitation of CF+ in collisions with para- and ortho-H2 (for temperatures up to 150 K).

Results

Conclusion