Infrared photometric properties of inner and outer parts of HII regions

The fact that infrared ring nebulae (IRRNe) are frequently associated with HII regions provides us with an opportunity to study dust at the interface between ionized and neutral gas. In this paper, we analyze the associated infrared (IR) radiation in the range from 8 to 500 μm in the outer and inner parts of 32 IRRNe showing a round shape. We aim to determine the morphology of these objects and possible dust evolution processes based on comparing IR radiation towards the ionized and neutral regions. We calculate six slopes between adjacent wavelengths in their spectral energy distributions to trace the difference in physical conditions inside and outside the ionized regions. Using data on these 32 objects, we demonstrate that their morphology is likely 3D spherical rather than 2D plane-like. The slope between 70 and 160 μm is the most appropriate tracer of dust temperature in the outer envelope. The larger 8-to-24 μm intensity ratio is associated with smaller intensities at mid-IR, indicating that {polycyclic aromatic hydrocarbons (PAHs) may indeed be generated due to larger grain destruction. These data are important for the subsequent theoretical modeling, and determining the dust evolution in HII regions and their envelopes. (paper)