Astrochemical Evolution Step From Acenaphthylene C12H8 To Pure Carbon C12 Around A Herbig Ae Young Star

TL;DR

This study investigates the astrochemical evolution of acenaphthylene molecules around a Herbig Ae star, revealing dehydrogenation to pure carbon and spectral signatures relevant to observed star spectra.

Contribution

It provides first principles quantum chemical calculations of PAH ionization and dehydrogenation, linking molecular evolution to infrared spectral observations in star environments.

Findings

Polycyclic hydrocarbon remains stable up to ionization n=6.

Dehydrogenation occurs at ionization n=7, forming pure carbon (C12).

Infrared spectra match observed star spectra with specific ionized molecules.

Abstract

Astrochemical evolution step of polycyclic aromatic hydrocarbon (PAH) around a Herbig Ae young star was analyzed using the first principles quantum chemical calculation. For simplicity, model molecule was selected to be acenaphthylene (C12H8) with hydrocarbon one pentagon combined with two hexagons. In a protoplanetary disk, molecules are illuminated by high energy photon from the central star and ionized to be cation (C12H8)n+ . Calculation shows that from n=0 to 6, molecule keeps its polycyclic hydrocarbon configuration. Whereas, at ionization step n=7, there occurs dehydrogenation of (C12H8) to pure carbon (C12). Such polycyclic pure carbon (PPC) would be attacked again by photons. At a stage of eighth ionization (C12)8+, there occur decomposition to aliphatic carbon chains, C9, C2, and mono carbon C1. Infrared spectra (IR) of those steps were calculated to identify observed spectra . Carrier molecules of Herbig Ae star WW Vul and HD145263 were identified by a combination of (C12H8)2+ and (C12H8)1+. Also, IR of HD37357 could be explained by (C12H8)2+, (C12H8)3+, and (C12H8)1+. Pure carbon molecules play an important role in many stars. IR of HD37258 was analyzed by a mixture of pure carbon (C12)2+, hydrocarbon (C12H8)2+ and neutral (C12H8)0+. Also, complex spectrum of HD38120 was analyzed by (C12)2+, (C12H8)2+ and (C12H8)3+. Acenaphthylene related molecules are just a typical example. We should apply various size molecules to understand total view around a new born star.