In this paper, we present a model that can be used to calculate the IR emission spectrum of a population of PAH-like species. Taking account of the photophysical properties of small neutral and ionized species, this model enables to consider, for the first time, not only the intensities of the bands but also their profiles. Such an approach puts stronger constraints on the population of emitting species.
The results that are presented in this paper are very encouraging. They show
that a molecular model can account for the relative band intensities as well
as for the profiles of the 6.2 and 11.3 
m bands. Several points remain
however to be elucidated. First, the profile of the 3.3 m band which
appears to be more symmetric in the observations than in the
calculations. Second and probably more important the apparent lack of spectral
diversity in the 6.2 and 11.3 m bands. This certainly gives new
information on the interstellar population. However, more studies have to be
performed before definitive conclusions can be made. We have discussed the
particular case of the planetary nebula IRAS 21282+5050 but the model has to be
applied to many other objects. A recent study by Verstraete et al. (2001) concerned the reflection nebula NGC 2023 and the
photodissociation region M17 SW. Also, this work emphasises the need to study
the photophysical properties of PAH-like systems containing typically 100
carbon atoms. This is one of the goal of PIRENEA (Joblin et al. 2002), a new experimental set-up
which is now being developed at the Centre d'Étude Spatiale des Rayonnements in
Toulouse.
Acknowledgements
We thank the referee Xander Tielens for helping us to increase the impact that the paper may have for the interpretation of the AIB spectrum.
Copyright ESO 2002