Is acetylene essential for carbon dust formation?

TL;DR

This study investigates whether acetylene (C2H2) is a universal precursor for carbon dust formation in astrophysical environments, emphasizing the importance of accurate photoreaction rates and finding that C2H2 abundance varies significantly across different conditions.

Contribution

The paper provides a detailed analysis of the chemical pathways for carbon dust formation, challenging the assumption that acetylene is always a key precursor in irradiated environments.

Findings

C2H2 formation is highly sensitive to physical parameters.

In some dust-producing sources, C2H2 cannot reach significant abundances.

Multiple dust-formation channels likely exist.

Abstract

We have carried out an investigation of the chemical evolution of gas in different carbon-rich circumstellar environments. Previous studies have tended to invoke terrestrial flame chemistries, based on acetylene (C2H2) combustion to model the formation of carbon dust, via Polycyclic Aromatic Hydrocarbons (PAHs). In this work we pay careful attention to the accurate calculation of the molecular photoreaction rate coefficients to ascertain whether there is a universal formation mechanism for carbon dust in strongly irradiated astrophysical environments. A large number of possible chemical channels may exist for the formation of PAHs, so we have concentrated on the viability of the formation of the smallest building block species, C2H2, in a variety of carbon-rich stellar outflows. C2H2 is very sensitive to dissociation by UV radiation. This sensitivity is tested, using models of the time-dependent chemistry. We find that C2H2 formation is sensitive to some of the physical parameters and that in some known sources of dust-formation it can never attain appreciable abundances. Therefore multiple (and currently ill-defined) dust-formation channels must exist.