Polycyclic Aromatic Hydrocarbons in Dense Cloud Chemistry

• Published in: The Astrophysical journal Vol. 680; no. 1; pp. 371 - 383
• Main Authors: Wakelam, Valentine, Herbst, Eric
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 10.06.2008; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Molecular process; ISM: individual (L134N, TMC-1 (CP)); Polycyclic aromatic hydrocarbons; ISM: molecules; ISM: abundances; molecular processes; Abundance; Dense cloud; astrochemistry
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/587734

Virtually all detailed gas-phase models of the chemistry of dense interstellar clouds exclude polycyclic aromatic hydrocarbons (PAHs). This omission is unfortunate because from the few studies that have been done on the subject, it is known that the inclusion of PAHs can affect the gas-phase chemistry strongly. We have added PAHs to our network to determine the role they play in the chemistry of cold dense cores. Initially, only the chemistry of neutral and negatively charged PAH species was considered, since it was assumed that positively charged PAHs are of little importance. Subsequently, this assumption was checked and confirmed. In the models presented here, we include radiative attachment to form PAH super(-), mutual neutralization between PAH anions and small positively charged ions, and photodetachment. We also test the sensitivity of our results to changes in the size and abundance of the PAHs. Our results confirm that the inclusion of PAHs changes many of the calculated abundances of smaller species considerably. In TMC-1, the general agreement with observations is significantly improved, unlike in L134N. This may indicate a difference in PAH properties between the two regions. With the inclusion of PAHs in dense cloud chemistry, high-metal elemental abundances give a satisfactory agreement with observations. As a result, we do not need to decrease the observed elemental abundances of all metals, and we do not need to vary the elemental C/O ratio in order to produce large abundances of carbon species in TMC-1 (CP).