Coulomb Explosion of Polycyclic Aromatic Hydrocarbons Induced by Heavy Cosmic Rays: Carbon Chains Production Rates

摘要

Cosmic Rays (CR) process the matter of the Interstellar Medium. Suchenergetic processing not only modifies the interstellar matter but also injectschemical species in the gas phase. In this work, we study the effect of the CRon the astrophysical polycyclic aromatic hydrocarbons (PAH). For events inwhich many electrons are stripped out from the PAH by interaction with a heavycosmic ray particle, coulomb explosion takes place and carbon chains areproduced. The fragments production rates of carbon chains are of particularinterest for astrophysical models. We computed PAH multi-ionization crosssections with an Independent Atom and Electron collisional model. We introducedand used a model to predict the fragmentation pattern for the coulombexplosion. Experimental measurements on small hydrocarbons, C$_{60}$ and PAHswere used to set confidence intervals on the calculations results. The carbon chains production rates were calculated using different CR fluxesand elemental compositions, to account for the variations expected in variousastrophysical environments. A range of PAH sizes and compactness were alsoexplored. The PAH lifetime with respect to a standard interstellar CR flux(corresponding to an H$_2$ ionization rate of $zeta pprox$6.10$^{-17}$s$^{-1}$) is found to be in the order of a few billion years. Theproduction rates of interstellar carbon chains containing around 5-15 carbonatoms are in the order of few to many tens of percent of the H$_2$ ionizationrate $zeta$. The exact rate value relies on the nature of the PAH and on theCR composition. In diffuse medium, with ten percent of the available cosmiccarbon locked in PAHs, this process leads to carbon chain fractional abundancesat steady state, in the range of $10^{-15}$-$10^{-14}$, with a confidenceinterval of about one order of magnitude. It reaches $10^{-13}$ in quiescentdense clouds.