Spectroscopic investigation of interstellar hydrogenated carbon clusters

Infrared and optical spectra of C₂₄H_n populations

¹ Laboratoire de Chimie et Physique Quantiques (LCPQ), Fédération FeRMI, CNRS & Université Toulouse [UT3], 31062 Toulouse, France
e-mail: aude.simon@irsamc.ups-tlse.fr
² Departamento de Química, Universidad Autónoma de Madrid, Módulo 13, 28049 Madrid, Spain
³ Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Université Paris-Saclay, 91405 Orsay, France

Received: 15 November 2022
Accepted: 12 January 2023

Abstract

Context. The assignment of the mid-infrared (mid-IR) emission features and plateaus observed in C-rich and H-rich regions of the interstellar medium (ISM) is still debated. Such mid-IR assignments must also be related to their contribution to the extinction curve in our galaxy and to the ultraviolet (UV) bump.

Aims. The aim of this work is to investigate the influence of hydrogenation rate on the mid-IR spectra of populations of carbon clusters in order to constraint the n_H/n_C ratios in regions of the ISM where carbon is an important component. Their potential contribution to the extinction curve and in particular to the UV bump is also investigated.

Methods. The absorption IR and optical spectra of tens of thousands of C₂₄H_n (n = 0, 6, 12, 18, 24) isomers classified into structural families – namely flakes, branched, pretzels, and cages – were computed using the density functional based tight binding electronic structure method and its time-dependent version, respectively. Final spectra were obtained by averaging the spectra of many individual isomers.

Results. The shapes and the relative intensities of the bands centered at ~3.25 and 3.40 µm and assigned to the C–H stretch of sp² and sp³ carbon atoms, respectively, present a clear dependence on the n_H/n_C ratio. From a comparison with the astronomical spectrum from the Orion bar H2S1, the most interesting emitting candidates would pertain to the flakes population; this is the most energetically favorable family of clusters, possessing a high content of five and six carbon rings and being mostly planar, with no sp¹ carbon atoms and with a n_H/n_C ratio of lower than 0.5. The same conclusion is drawn when comparing the computed IR features in the [4–20 µm] region with the observed plateaus from some C-rich and H-rich planetary nebulae objects of the Small Magellanic Cloud. The contribution of the same family could be considered for the UV bump. When n_H/n_C increases, only a contribution to the high-energy part of the continuum due to σ → π^✶ excitations can reasonably be considered.

Conclusions. These results bring some constraints on the structural features and on the n_H/n_C ratio of the hydrogenated carbon populations emitting in the mid-IR domain in interstellar objects such as protoplanetary and reflection nebulae. The flakes population, with a low n_H/n_C ratio, is an interesting candidate for the carbon population emitting in these regions, but not for that absorbing in the diffuse ISM. None of the populations studied in the present work can account for the UV bump, but they would contribute to a broad extinction rise in this domain. The computed features reported in this article could be used to interpret future astronomical data provided by the James Webb Space Telescope.