Desorption of polycyclic aromatic hydrocarbons by cosmic rays

Abstract

Context. The rate of sputtering and release of condensed species is an important aspect of interstellar chemistry, as is photodesorption for the most volatile species, because in the absence of such mechanisms the whole gas phase would have to condense in times often shorter than the lifetime of the considered medium, in particular for dense clouds. The recent detection of cyclic aromatic molecules by radioastronomy requires an understanding of the potential mechanisms supporting the rather high abundances observed. Aims. We perform experiments to advance our understanding of the sputtering yield due to cosmic rays for very large carbonaceous species in the solid phase. Methods. Thin films of perylene and coronene were deposited on a quartz cell microbalance and exposed to a 1.5 MeV N+ ion beam at the Laboratoire de Physique des 2 Infinis Irène Joliot-Curie (IJCLab, Orsay, France) and a 230 MeV 48Ca10+ ion beam at the GSI Helmholtzzentrum für Schwerionenforschung (GSI, Darmstadt, Germany). The mass loss was recorded as a function of the fluence for the N+ beam. The microbalance response was calibrated using Fourier transform infrared (FTIR) reflectance measurements of the produced films. In addition, the destruction cross-section of the same species was measured with the 48Ca10+ ion beam by in situ monitoring of the evolution of the infrared spectra of the bombarded films. Results. We deduced the sputtering yield for perylene and coronene and their radiolysis destruction cross-sections. Combining these results with a cosmic ray astrophysical spectrum, we discuss the impact on the possible abundance that may originate from the sputtering of dust grains with these molecules as well as from polycyclic aromatic molecules when they are trapped in ice mantles.

Photocleavage of Aliphatic C–C Bonds in the Interstellar Medium

“…Likewise, it provides a pathway for the formation of dienes in these environments by the UV photoprocessing of carbonaceous mantles that accrete on dust particles in the early stages of the evolution of DMCs (Jones et al 2013;Murga et al 2023). These newly formed mantles consist of aliphatic-rich material (Ysard et al 2015;Jones 2016;Murga et al 2023) and we speculate that if C 3 H 6 and small dienes are photoformed on the surface of grains, they can desorb through nonthermal processes (Fredon et al 2021;Dartois et al 2022;Del Fré et al 2023) to be incorporated into the gas phase. The UVinduced formation of C 3 H 6 and dienes might therefore be essential to understanding the synthesis of small PAHs in dark clouds.…”

“…However, our results point toward a different scenario, as we have shown that C-C photocleavage is favored over C-H bond breaking. Therefore, hydrogen depletion in carbonaceous dust grains is primarily a result of the photolysis of aliphatic C-C bonds, which produces small molecular fragments that might subsequently desorb through nonthermal processes (Fredon et al 2021;Dartois et al 2022;Del Fré et al 2023) carrying hydrogen toward the gas phase and provoking an effective dehydrogenation of the carbon grains.…”

Indene energetic processing in ice mantles in the interstellar medium

“…Nonthermal desorption mechanisms have been proposed and are being currently investigated to account for their presence in the gas phase. These mechanisms include electron promoted desorption (Marchione & McCoustra 2016), X ray induced desorption (Torres-Díaz et al 2023;Kalvāns 2021), desorption induced by exothermic reactions (Garrod et al 2007;Vasyunin & Herbst 2013) and, notably, UV photodesorption (Öberg et al 2009;Cruz-Diaz et al 2017) and cosmic ray (CR) sputtering (Dartois et al 2017(Dartois et al , 2022Pino et al 2019). Overall, COMs are expected to be highly diluted in the ice mantles of dust grains and on many occasions their gas-phase concentrations should thus be given by the desorption rate of the dominant ice molecules.…”

“…Overall, COMs are expected to be highly diluted in the ice mantles of dust grains and on many occasions their gas-phase concentrations should thus be given by the desorption rate of the dominant ice molecules. A recent model estimation by Dartois et al (2022), assuming pure water ice as well as CR sputtering as the prevailing desorption mechanism, suggests that for the typical conditions of a dense cloud like TMC-1 the gasto-solid fraction of a typical small aromatic hydrocarbon (they used naphtalene in their calculations) would be just ≈10 −3 -10 −4 . In the case of TMC-1, which is a comparatively young object of 0.5-1 Myr (Navarro-Almaida et al 2021;Chen et al 2022;Cernicharo et al 2022), the depletion of hypothetical indene formed in the gas phase might not be so high, but it is still expected to be significant.…”