Abstract
Laboratory studies play a crucial role in understanding the chemical nature of the interstellar medium (ISM), but the disconnect between experimental timescales and the timescales of reactions in space can make a direct comparison between observations, laboratory, and model results difficult. Here we study the survival of reactive fragments of the polycyclic aromatic hydrocarbon (PAH) coronene, where individual C atoms have been knocked out of the molecules in hard collisions with He atoms at stellar wind and supernova shockwave velocities. Ionic fragments are stored in the DESIREE cryogenic ion-beam storage ring where we investigate their decay for up to one second. After 10 ms the initially hot stored ions have cooled enough so that spontaneous dissociation no longer takes place at a measurable rate; a majority of the fragments remain intact and will continue to do so indefinitely in isolation. Our findings show that defective PAHs formed in energetic collisions with heavy particles may survive at thermal equilibrium in the interstellar medium indefinitely, and could play an important role in the chemistry in there, due to their increased reactivity compared to intact or photo-fragmented PAHs.
Highlights
Laboratory studies play a crucial role in understanding the chemical nature of the interstellar medium (ISM), but the disconnect between experimental timescales and the timescales of reactions in space can make a direct comparison between observations, laboratory, and model results difficult
The blue and green curves show the raw output from our molecular dynamics (MD) simulations, where one or more C atoms have been removed from the coronene molecule, with different resolutions
We have shown that highly reactive fragments of polycyclic aromatic hydrocarbon (PAH) molecules damaged by the knockout of carbon atoms in collisions with energetic particles can remain stable on indefinitely long, i.e., astronomical, timescales in the gas phase
Summary
Laboratory studies play a crucial role in understanding the chemical nature of the interstellar medium (ISM), but the disconnect between experimental timescales and the timescales of reactions in space can make a direct comparison between observations, laboratory, and model results difficult. The most distinct fingerprint of these processes are fragments that have lost a single carbon atom, such as C59 and C23Hx fragments from C60 and coronene (C24H12) precursors, respectively[26,30] In addition to these processes being observed with isolated molecules in the gas phase, knockout-driven fragmentation has been shown to induce effective bond-forming reactions in cold, loosely bound clusters of such molecules that serve as laboratory analogs to small interstellar grains[33,34,35]. These grains may be processed by energetic particles or radiation to form PAHs and fullerenes[36,37] as part of the carbon cycle of the ISM2,38. Previous experimental studies have demonstrated that fragments produced by carbon knockout are stable on microsecond timescales[26,27], but until now it has not been possible to follow the fragments on radiative-cooling timescales (milliseconds or longer) in laboratory experiments
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