Astronomical chemical evolution from graphene to polycyclic aromatic hydrocarbon reproducing observed infrared spectrum

TL;DR

This paper models the chemical evolution from graphene to PAH molecules in space, reproducing observed infrared spectra by first principles calculations and proposing a formation pathway involving supernovae, proton sputtering, and photo-ionization.

Contribution

It introduces a detailed chemical evolution pathway from graphene to PAH molecules in space, supported by first principles calculations and IR spectrum modeling.

Findings

Calculated IR spectrum matches 11 of 13 observed peaks.

Proposed formation pathway involves supernova nucleation, proton sputtering, and photo-ionization.

Model suggests central stars may be 4-7 times heavier than the Sun.

Abstract

Interstellar ubiquitous infrared spectrum (IR) due to polycyclic aromatic hydrocarbon (PAH) was observed in many astronomical dust clouds. A capable astronomical chemical evolution path from graphene to PAH was studied based on the first principles calculation. Step 1 is a nucleation of nano-carbon after supernova by super-cooling at expanding helium sphere. As a typical model, graphene molecule (C )24 having coronene skeleton with seven carbon hexagons was tried.Step 2 is a proton sputtering and passivation on ejected graphene molecule. Slow speed proton with energy less than 4.3eV makes hydrogenation, Graphene molecule (C )24 was transformed to PAH (C24H12). Higher speed proton having sufficient energy larger than 18.3 eV could make a void in a molecule as like C23H12. Resulted structure was a combination of two carbon pentagons and five hexagons. Step 3 is photo-ionization of those molecules by high energy photon. Electrons are removed to make a molecule to cation. Model molecule (C23H12) became mono-cation (C23H12)+, di-cation (C23H12)2+ and so on. Typical energy difference between such cation was 6.5 and 10.8 eV. If the light source has a nature of black-body radiation, effective temperature will be 18000K ~ 24000K, which suggested that central light source star may have 4 to 7 times heavier than our sun. Finally, theoretical IR spectrum was obtained. Especially in case of (C23H12)2+, calculated emission spectrum revealed that among 13 major peaks, 11 peaks could correlate with ubiquitous observed IR one.