2001A&A...371..312Z

We present 3-12µm ISOCAM observations of the five Galactic compact HII regions Sh 61, Sh 138, Sh 152, Sh 156 and Sh 186. The unidentified infrared bands (UIBs) centred at 3.3, 6.2, 7.7, 8.6 and 11.2µm - and underlying continuum - are imaged using the SW1, SW2, LW4, LW6 and LW8 filters. Images are also obtained at 5.985, 6.911, 8.222, 10.520 and 12.000µm using the circular variable filter (CVF). We show that the 5.985µm emission represents a true continuum reference for the 6.2µm band, allowing a derivation of this band's properties. Due to uncertainties in the continuum estimates, only lower limits can be given for the 3.3, 7.7 and 11.3µm band fluxes. These limits agree with previous results found in the literature. The distribution of the bands coincide. The 3.3µm emission is not observed in high extinction zones, suggesting a lower temperature of the carriers and/or a higher abundance of larger molecules in those zones. The 6.2µm band emission peaks outside the ionized zone, in the photodissociation region. The 6.2µm band luminosity correlates with the far UV field intensity, suggesting a UV excitation. We also find a correlation between the spatial distribution of the 6.2µm band emission and zones of strong 2.122µm H₂ emission due to ultraviolet fluorescence. This suggests that both emissions are due to UV excitation. The 6.2µm emission is slightly closer to the exciting star. This suggests that the band carriers survive in the Hi zone. The 12µm emission traces the continuum emission from very small grains, when present, and follows well the distribution of UIB emission. This suggests a link between the two emission carriers. The emission peak observed on the star in Sh 61 and Sh 156 indicates that the continuum from very small grains dominates the emission in highly excited regions.