Polycyclic aromatic hydrocarbon processing in interstellar shocks
  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.
