ISOCAM 3-12 μm imaging of five galactic compact Hii regions

Abstract

We present 3-12 μm ISOCAM observations of the five Galactic compact H ii 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 11.2 μm -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 12.000 μm using the circular variable filter (CVF). We show that the 5.985 μm emission represents a true continuum reference for the 6.2 μm 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 11.3 μm band fluxes. These limits agree with previous results found in the literature. The distribution of the bands coincide. The 3.3 μm 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 6.2 μm band emission peaks outside the ionized zone, in the photodissociation region. The 6.2 μm band luminosity correlates with the far UV field intensity, suggesting a UV excitation. We also find a correlation between the spatial distribution of the 6.2 μm band emission and zones of strong 2.122 μm H2 emission due to ultraviolet fluorescence. This suggests that both emissions are due to UV excitation. The 6.2 μm emission is slightly closer to the exciting star. This suggests that the band carriers survive in the H i zone. The 12 μm 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.