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 } -10 ^ { 8 } K .

Results : PAH destruction is dominated by He collisions at low temperatures ( T < 3 \times 10 ^ { 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 } \approx 0.01 cm ^ { -3 } , T \approx 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 .