2021ApJ...922..207B

We report an observational study of the Galactic H II region Sh 2-305/S305 using the [C II] 158 µm line data, which are used to examine the gas dynamics and structure of photodissociation regions. The integrated [C II] emission map at [39.4, 49.5] km s^–1 spatially traces two shell-like structures (i.e., inner and outer neutral shells) having a total mass of ∼565 M_☉. The inner neutral shell encompasses an O9.5V star at its center and has a compact ring-like appearance. However, the outer shell is seen with more extended and diffuse [C II] emission, hosting an O8.5V star at its center, and surrounds the inner neutral shell. The velocity channel maps and position-velocity diagrams confirm the presence of a compact [C II] shell embedded in the diffuse outer shell, and both the shells seem to expand with v_exp∼1.3 km s^–1. The outer shell appears to be older than the inner shell, hinting that these shells are formed sequentially. The [C II] profiles are examined toward S305, which are either double peaked or blue skewed and have the brighter redshifted component. The redshifted and blueshifted components spatially trace the inner and outer neutral shell geometry, respectively. The ionized, neutral, and molecular zones in S305 are seen adjacent to one another around the O-type stars. The regularly spaced dense molecular and dust clumps (mass ∼10-10³ M_☉) are investigated around the neutral shells, which might have originated as a result of gravitational instability in the shell of collected materials.