C$_{60}$ in Photodissociation Regions

TL;DR

This study investigates the presence, formation, and excitation of C$_{60}$ fullerenes in photodissociation regions, revealing that C$_{60}$ forms through dehydrogenation processes influenced by local physical conditions, shocks, and PDR age.

Contribution

It provides new observational insights into C$_{60}$ formation mechanisms and its relation to PAHs in interstellar environments, emphasizing the role of dehydrogenation and environmental factors.

Findings

C$_{60}$ emits in cool PDRs with dust temperatures of 20-40 K

C$_{60}$ presence is uncorrelated with PAH emission spatially

C$_{60}$ abundance increases within some PDRs, but not universally

Abstract

Recent studies have confirmed the presence of buckminsterfullerene (C$_{60}$) in different interstellar and circumstellar environments. However, several aspects regarding C$_{60}$ in space are not well understood yet, such as the formation and excitation processes, and the connection between C$_{60}$ and other carbonaceous compounds in the interstellar medium, in particular polycyclic aromatic hydrocarbons (PAHs). In this paper we study several photodissociation regions (PDRs) where C$_{60}$ and PAHs are detected and the local physical conditions are reasonably well constrained, to provide observational insights into these questions. C$_{60}$ is found to emit in PDRs where the dust is cool ($T_d = 20-40$ K) and even in PDRs with cool stars. These results exclude the possibility for C$_{60}$ to be locked in grains at thermal equilibrium in these environments. We observe that PAH and C$_{60}$ emission are spatially uncorrelated and that C$_{60}$ is present in PDRs where the physical conditions (in terms of radiation field and hydrogen density) allow for full dehydrogenation of PAHs, with the exception of Ced 201. We also find trends indicative of an increase in C$_{60}$ abundance within individual PDRs, but these trends are not universal. These results support models where the dehydrogenation of carbonaceous species is the first step towards C$_{60}$ formation. However, this is not the only parameter involved and C$_{60}$ formation is likely affected by shocks and PDR age.