The aim of this proposal is to use ISOCAM to study the distribution
 of Unidentified InfraRed Bands (UIRBs) carriers in compact Galactic
 HII regions. Polycyclic Aromatic Hydrocarbon (PAH) molecules are
 thought to be these carriers, responsible of a serie of bands
 observed in the infrared between 3 and 20 microns.
 The idea is to study the evolution of the carriers composition (size,
 dehydrogenation) and distribution (excitation/destruction) with the
 properties of the considered HII regions (spectral type of the
 exciting star, evolutionary stage).
 A previous work (Zavagno et al., 1992) has shown that the emission in
 the 7.7, 8.6 and 11.3 micron bands can account for 50% of the 12
 micron flux in compact HII regions. Only few very bright HII
 regions (Orion Bar, Bregman et al., 1989,, ApJ 344, 791 - Geballe
 et al., 1989, ApJ 341, 278) have been observed from the ground.
 Presently, it is not clear to know how that dust is exactly
 distributed in classical compact HII regions. Can those molecules
 survive in the ionized medium and if so, how their spatial
 distribution looks like in it? How far is the extension of that
 dust component behind the ionization front? Answer those questions
 requires the ISOCAM sensitivity in UIRB filters. Such studies
 cannot be leaded from the ground due to the poor sensitivity of the
 available cameras in such filters.
 We have selected a sample of 8 compact visible HII regions for which
 the spectral type of the exciting star and the distance are known
 and for which visible and radio data are available in order to link
 the observed properties of these bands carriers to the physical
 properties of the ionized region.