Rotational equilibrium of C₂ in diffuse interstellar clouds

II. Formation-excitation at work?^★

¹ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
² Université Paris-Cité, Paris, France
³ University of Toledo, Department of Physics and Astronomy, Toledo, OH 43606, USA
⁴ University of Texas at Austin, Department of Astronomy, Austin, TX 78712, USA

^★★ Corresponding author: Jacques.Lebourlot@obspm.fr; steven.federman@utoledo.edu

Received: 26 March 2025
Accepted: 10 May 2025

Abstract

Context. Recent spectroscopic measurements have revealed absorption from higher rotational levels in C₂ than previous observations. These improvements are accompanied by the availability of updated radiative and collisional data.

Aims. We revisit the density and radiation field intensity diagnostics provided by the observations of many rotational levels of interstellar C₂ and extensive molecular information.

Methods. We built an excitation model of C₂ without spatial structure, including levels up to J = 34 where updated radiative and collisional excitation data are introduced as well as excitation by chemical formation.

Results. We confirm the importance of the recent collisional excitation rate coefficients of C₂ by molecular H₂. We show that the new higher level observations cannot be explained by the standard balance between collisional excitation and radiative transitions. We propose that chemical excitation at formation provides a plausible mechanism to explain the observed high excitation ofC₂. In addition, it allows us to lift the degeneracy of the density over radiation field strength parameter in the excitation model.

Conclusions. A 0D model remains limited and it is highly desirable to use a full Photon Dominated Region (PDR) model, which includes all excitation processes introduced here and full chemical and thermal balance.