Polycyclic aromatic hydrocarbon processing in a hot gas
  Context: PAHs are thought to be a ubiquitous and important dust component of
the interstellar medium. However, the effects of their immersion in a hot
(post-shock) gas have never before been fully investigated. Aims: We study the
effects of energetic ion and electron collisions on PAHs in the hot post-shock
gas behind interstellar shock waves. Methods: We calculate the ion-PAH and
electron-PAH nuclear and electronic interactions, above the carbon atom loss
threshold, in H II regions and in the hot post-shock gas, for temperatures
ranging from 10^3 to 10^8 K. Results: PAH destruction is dominated by He
collisions at low temperatures (T < 3x10^4 K), and by electron collisions at
higher temperatures. Smaller PAHs are destroyed faster for T < 10^6 K, but the
destruction rates are roughly the same for all PAHs at higher temperatures. The
PAH lifetime in a tenuous hot gas (n_H ~ 0.01 cm^-3, T ~ 10^7 K), typical of
the coronal gas in galactic outflows, is found to be about thousand years,
orders of magnitude shorter than the typical lifetime of such objects.
Conclusions: In a hot gas, PAHs are principally destroyed by electron
collisions and not by the absorption of X-ray photons from the hot gas. The
resulting erosion of PAHs occurs via C_2 loss from the periphery of the
molecule, thus preserving the aromatic structure. The observation of PAH
emission from a million degree, or more, gas is only possible if the emitting
PAHs are ablated from dense, entrained clumps that have not yet been exposed to
the full effect of the hot gas.
