  Clusters of polycyclic aromatic hydrocarbons (PAHs) have been proposed as
candidates for evaporating very small grains, which are thought to be
precursors of free-flying PAHs. Evaporation rates have been calculated so far
only for species containing up to a few 100 C atoms, whereas interstellar PAH
clusters could contain up to ~1000 C atoms. We present a method that
generalises the calculation of the statistical evaporation rate of large PAH
clusters and provides rates for species containing up to ~1000 C-atoms. The
evaporation of non-rotating neutral homo-molecular PAH clusters containing up
to 12 molecules from a family of highly symmetric compact PAHs is studied.
Statistical calculations were performed and completed with molecular dynamics
simulations at high internal energies to provide absolute values for the
evaporation rate and distributions of kinetic energy released. The calculations
used explicit atom-atom Lennard-Jones potentials in the rigid molecule
approximation. A new method is proposed to take both inter- and intra-molecular
vibrations into account. Without any parameter adjustment, the calculated
evaporation rates agree well with available experimental data. We find that the
non-rotation assumption has a limited impact on the evaporation rates. The
photostability of PAH clusters increases dramatically with the size of
molecules in the clusters, and to a lesser extent with the number of molecules
in the clusters. For values of the UV radiation field that are typical of the
regions where evaporating very small grains are observed, the smallest clusters
in this study (~50 C-atoms) are found to be quickly photo-evaporated, whereas
the largest clusters (~1000 C-atoms) are photostable. Our results support the
idea that large PAH clusters are good candidates for evaporating very small
grains.
