tokens: As described in Sections 3.6 and 3.7 , the H 2 and PAH emission, indicating the location of the PDR, encircles the western border of the H ii region. A curved morphology of the radio continuum emission in the western periphery further suggests a stalling effect for the ionized gas against the PDR in that direction. As shown by Pellegrini et al. ( 2012 ), the [S ii ]/[O iii ] ratio map can be used to distinguish density-bounded, radiation-bounded, and blister-type H ii regions. Pellegrini et al. ( 2012 ) found that the optically thick nebula with highly ionized gas (low [S ii ]/[O iii ] ratio; appears dark in ratio map) is usually surrounded by an ionization transition zone with a higher [S ii ]/[O iii ] ratio (appears as lighter gray scale). We made an [S ii ]/[O iii ] ratio map of the central part (∼10′ × 10′) of the Sh2-112 H ii region, which is shown in Figure 8 (bottom left panel). The ratio map consists of bright and dark features. We notice a pronounced ionization transition zone (bright region) toward the south of the massive star, where H α emission is traced. This ionization transition zone appears to be surrounded by a relatively diffuse feature in light gray. In the southwestern direction, the ratio of [S ii ]/[O iii ] reveals the presence of ionized gas (dark feature) in the region, which is spatially correlated with the IBL. The distribution of the ionized hydrogen, H 2 , and 13 CO emission indicates that the IBL structure is seen edge-on. An example of this kind of edge-on geometry is the Orion bar, which is located in a blister-type H ii region, the Orion Nebula (Goicoechea et al. 2018 ). In our case, the bright S ii emission could actually be on the surface of the H ii cavity. Therefore, from the above analysis, it seems that Sh2-112 is an example of a blistered H ii region that is ionization-bounded on the high-density sides (western direction) and density-bounded on the low-density sides (eastern direction). The intensity gradient of the radio emission (see 610 MHz emission contours in Figure 7 (b)) supports that the ionized gas is escaping from the H ii region toward the low-density ISM in the eastern direction. Based on radial velocity measurements of the H α (i.e., −9.7 km s −1 ) and CO (i.e., −12.2 km s −1 ) emissions, Israel ( 1978 ) also suggested that the ionized gas in the Sh2-112 region is probably undergoing a champagne flow. Thus, the structure of the radio continuum, dust extinction, 13 CO, and [S ii ]/[O iii ] ratio maps; the spatial distribution of the young stars; and the NIR-to-MIR images of the H ii region indicate that Sh2-112 is a blistered H ii region in which the ionized gas is possibly undergoing a champagne flow. In Figure 9 , we show the AKARI 160 μ m image superimposed by Planck 353 GHz contours (left panel) and a simple geometry of the H ii region (right panel). Based on the distribution of the cold dust emission and young stars, we suggest that the massive star possibly formed on the eastern
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