Polycyclic aromatic hydrocarbon processing in interstellar shocks

TL;DR

This study investigates how interstellar shock waves impact polycyclic aromatic hydrocarbons (PAHs), revealing that high-velocity shocks destroy PAHs and affect their emission, with implications for understanding dust survival in space.

Contribution

The paper provides the first detailed calculations of ion and electron collisions with PAHs in shock waves, quantifying destruction thresholds and PAH lifetimes in interstellar environments.

Findings

PAHs with 50 carbon atoms are destroyed above 100 km/s shock velocity.

Larger PAHs with 200 carbon atoms are destroyed at velocities ≥125 km/s.

PAH lifetimes in the Galaxy are estimated to be a few hundred million years.

Abstract

Context: PAHs appear to be an ubiquitous interstellar dust component but the effects of shocks waves upon them have never been fully investigated. Aims: To study the effects of energetic (~0.01-1 keV) ion (H, He and C) and electron collisions on PAHs in interstellar shock waves.Methods: We calculate the ion-PAH and electron-PAH nuclear and electronic interactions, above the threshold for carbon atom loss from a PAH, in 50-200 km/s shock waves in the warm intercloud medium. Results: Interstellar PAHs (Nc = 50) do not survive in shocks with velocities greater than 100 km/s and larger PAHs (Nc = 200) are destroyed for shocks with velocities greater/equal to 125 km/s. For shocks in the ~75 - 100 km/s range, where destruction is not complete, the PAH structure is likely to be severely denatured by the loss of an important fraction (20-40%) of the carbon atoms. We derive typical PAH lifetimes of the order of a few x10^8 yr for the Galaxy. These results are robust and independent of the uncertainties in some key parameters that have yet to be well-determined experimentally. Conclusions: The observation of PAH emission in shock regions implies that that emission either arises outside the shocked region or that those regions entrain denser clumps that, unless they are completely ablated and eroded in the shocked gas, allow dust and PAHs to survive in extreme environments.