SIMBAD references

We present a model for dense clumps in photoionized regions in the 'cometary plasmon' regime. We consider that as these plasmons accelerate outwards from the external ionizing photon source (as a result of the back-reaction of the photoevaporated wind from the clumps), they move into regions of decreasing external ionizing photon fluxes (resulting from the geometrical dilution of the ionizing radiation, and possibly also from absorption due to intervening material). We find the presence of two regimes: a 'vanishing plasmon' regime, in which the clump rapidly accelerates, compresses, and becomes fully photoionized without travelling far from the photon source, and an 'expanding plasmon' regime, in which the clump travels over a considerable radial domain, and expands as a result of the lower impinging ionizing photon fluxes (before eventually decreasing in size and becoming fully photoionized). Interestingly, the cometary clumps of the Helix nebula are at the boundary between these two regimes, and therefore correspond to clumps that travel over a relatively extended radial domain while keeping an approximately constant size.