# Infrared Photometric Properties of Inner and Outer Parts of HII regions

## TL;DR

This study investigates the infrared emission properties of inner and outer parts of 32 IR ring nebulae associated with HII regions, revealing dust temperature, morphology, and potential dust evolution processes at the interface of ionized and neutral gas.

## Contribution

It provides new insights into the 3D spherical morphology of IRRNs and introduces spectral slope analysis as a tool to study dust evolution in HII regions.

## Key findings

- Morphology of IRRNs is likely 3D spherical.
- The 70-160 μm slope effectively traces dust temperature.
- Higher 8-to-24 μm ratio indicates PAH generation from grain destruction.

## Abstract

The fact that infrared ring nebulae (IRRNs) are frequently associated with HII regions, provides us the opportunity to study dust at the interface between ionized and neutral gas. In this paper, we analyze the infrared radiation in the range from 8 to 500~$\mu$m in outer and inner parts of 32 IRRNs showing round shape. We aim to determine the morphology of these objects and possible dust evolution processes on the base of the comparison of 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 the physical conditions inside and outside ionized regions. Using the data on these 32 objects we show that their morphology is likely 3D spherical rather than 2D plane-like. The slope between 70 and 160~$\mu$m is the most appropriate tracer of the dust temperature in the outer envelope. The larger 8-to-24~$\mu$m intensity ratio is associated with smaller intensities at mid-IR indicating that the 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.