Polycyclic aromatic hydrocarbon processing in interstellar shocks

Abstract

<i>Context. <i/>PAHs appear to be an ubiquitous interstellar dust component but the effects of shocks waves upon them have never been fully investigated.<i>Aims. <i/>We study the effects of energetic (<i>≈<i/>0.01-1 keV) ion (H, He and C) and electron collisions on PAHs in interstellar shock waves.<i>Methods. <i/>We calculate the ion-PAH and electron-PAH nuclear and electronic interactions, above the threshold for carbon atom loss from a PAH, in km s<sup>-1<sup/> shock waves in the warm intercloud medium.<i>Results. <i/>Interstellar PAHs ( = 50) do not survive in shocks with velocities greater than 100 km s<sup>-1<sup/> and larger PAHs ( = 200) are destroyed for shocks with velocities <i>≥<i/>125 km s<sup>-1<sup/>. For shocks in the <i>≈<i/>75-100 km s<sup>-1<sup/> 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 10<sup>8<sup/> 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.<i>Conclusions. <i/>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.