1994A&A...289..524B

We present observations of the planetary nebula BD+30 3639 in the 3.3µm feature and nearby continuum. The dust envelope is clearly resolved as a circular shell surrounding the exciting star. The 3.3µm emission correlates closely with the spatially resolved HII region, and a large fraction (≃70%) of the feature emission detected is coming from lines of sight toward the ionized region. The feature emission has a diffuse component distributed all over the ionized ring with several maxima which may originate from neutral or protected condensations. The distribution of the 3.3µm feature and the continuum emission is analyzed to determine the variations of the emissivity and abundance of the 3.3µm emitters under the PAH hypothesis.The abundance of the 3.3µm feature emitting particles is shown to rise steadily inside the ionized region toward the outer edge where it is maximum and 1 to 1.5 times that proposed by Desert et al. for the diffuse ISM. Most of the PAH emitting the 3.3µm feature are located outside the ionized shell but still, a significant fraction of the PAH (>11%) is found inside the HII region. We discuss these results in the frame of the PAH model, with particular emphasis on the formation and destruction of these molecules in the circumstellar environment. The comparison with the HII/MC interface in M17 indicates that PAH destruction or alteration caused by the high radiation fields in HII regions has to be balanced by faster recombination processes in the case of dense objects as BD+30 3639. Destruction through Coulomb Dissociation of PAH dications is consistent with the observations, if stability of large PAH dications is significantly larger than that of benzene. Dehydrogenation through Photo-Termo Dissociation may also explain the observed variations in BD+30 3639, provided relatively small PAH are present in the HII region (≃20 C atoms). The PAH abundances derived appear to be low for this type of objects to provide directly a large fraction of the PAH observed in the ISM. The relevance of this result for the formation of PAH molecules in the ISM is discussed.