CF⁺ excitation in the interstellar medium

¹ LOMC – UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 1123, 76063 Le Havre Cedex, France
e-mail: benjamin.desrousseaux@univ-lehavre.fr
² Instituto de Física Fundamental (CSIC). Calle Serrano 121-123, 28006 Madrid, Spain
³ Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409, USA

Received: 1 November 2020
Accepted: 24 November 2020

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.