Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry

TL;DR

This study incorporates polycyclic aromatic hydrocarbons (PAHs) into dense cloud chemistry models, revealing their significant impact on molecular abundances and improving agreement with observations in certain regions.

Contribution

It introduces PAHs into dense cloud chemical models, exploring their effects and sensitivity, which was previously overlooked in such models.

Findings

Including PAHs improves model-observation agreement in TMC-1.

PAHs significantly alter the predicted abundances of small molecules.

High-metal elemental abundances can explain observations without metal depletion.

Abstract

Virtually all detailed gas-phase models of the chemistry of dense interstellar clouds exclude polycyclic aromatic hydrocarbons (PAH's). This omission is unfortunate because from the few studies that have been done on the subject, it is known that the inclusion of PAH's can affect the gas-phase chemistry strongly. We have added PAH's to our network to determine the role they play in the chemistry of cold dense cores. In the models presented here, we include radiative attachment to form PAH-, 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 PAH's. Our results confirm that the inclusion of PAH's changes many of the calculated abundances of smaller species considerably. In TMC-1, the general agreement with observations is significantly improved contrary to L134N. This may indicate a difference in PAH properties between the two regions. With the inclusion of PAH's 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).