Is acetylene essential for carbon dust formation?

Abstract

We have carried out an investigation of the chemical evolution of gas in different carbonrich circumstellar environments. Previous studies have tended to invoke terrestrial flame chemistries, based on acetylene (C 2 H 2 ) 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, C 2 H 2 , in a variety of carbon-rich stellar outflows. C 2 H 2 is very sensitive to dissociation by UV radiation. This sensitivity is tested, using models of the time-dependent chemistry. We find that C 2 H 2 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.

Optical and infrared emission of H ii complexes as a clue to the PAH life cycle

“…However, the genesis of these particles is still far from being understood. It is generally believed that PAHs are synthesised in hot dense outflows (∼ 1000 K) of carbon-rich AGB stars (Gail & Sedlmayr 1987;Latter 1991;Cherchneff, Barker & Tielens 1992;Dhanoa & Rawlings 2014) or in novae (Bode & Evans 2008). While theoretical calculations apparently confirm this * E-mail: khramtsova@inasan.ru possibility, firm observational evidence is still scarce.…”

Acetylene ν1+ν5 (ΠuSuas-David1, Linnartz2, Bouwman3 2022Journal of Molecular Spectroscopy0000View full textAdd to dashboardCite

“…On Earth, this species plays a major role in combustion chemistry, particularly in the synthesis of polycyclic aromatic hydrocarbons (PAHs) [1,2] and in the formation of soot [3]. Similar chemical mechanisms are also thought to take place in astrophysical environments such as circumstellar envelopes of AGB stars [4][5][6][7][8] from which dust, PAHs and other complex hydrocarbon species are expelled into the interstellar medium. Accurate spectroscopic data of acetylene associated with thermally populated vibrational levels are required to characterize the dynamics and chemical evolution of such environments.…”

ExoMol molecular line lists – XXXVII. Spectra of acetylene

“…It has been detected on comets such as Hyakotake (Brooke et al 1996), Halley, and 67P/Churyumov-Gerasimenko (Le Roy et al 2015). Even further into the galactic neighbourhood, acetylene appears in star forming regions (Ridgway et al 1976;van Dishoeck et al 1998;Rangwala et al 2018), is speculated to be an important constituent of clouds in the upper atmospheres of brown dwarfs and exoplanets Bilger et al 2013;Madhusudhan et al 2016;Oppenheimer et al 2013;Shabram et al 2011), and is thought to play an important role in dust formation (Dhanoa & Rawlings 2014) and AGB star evolution and atmospheric composition (Jørgensen et al 2000;Cernicharo 2004;Gautschy-Loidl et al 2004;Loidl et al 1999;Aringer et al 2009), providing a major source of opacity in cool carbon stars (Rinsland et al 1982;Gautschy-Loidl et al 2004). For example, C2H2 was detected in the carbon star Y CVn by Goebel et al (1978) and in the low-mass young stellar object IRS 46 by Lahuis et al (2005).…”