Small Carbon Chains and Rings in Photo-Dominated Regions

Abstract

Carbon is the fourth most abundant element in the interstellar medium (ISM), and also the most versatile for building molecules. Carbon chemistry can therefore be considered as the core of interstellar chemistry. Of the nearly 130 molecules now observed in various sources, about 75% have at least one carbon atom, while one fourth are hydrocarbons. These molecules, initially reported in circumstellar shells (e.g. [8], [1]), are now observed in many other media, ranging from molecular dark clouds (up to C₈H, [4]) to the diffuse gas (C₂, C₃, C₄ - [13], [2] -, C₂H and c-C₃H₂- [10]). A natural question therefore arises: if carbon chains are present in the diffuse ISM, what does happen in the Photon-Dominated Regions (hereafter PDRs)? As in diffuse clouds, the chemical processes are dominated by the radiation but the gas is denser. Knowledge about the distribution of carbon chains and rings in PDRs is however yet scarce and mostly limited to the works of [6] and [7], who reported observations of C₂H at some positions in NGC7023, the Orion Bar and NGC7027. In this work we extend the study of hydrocarbons in PDRs through an extensive inventory of carbon chains and rings of up to six carbon atoms.