Probing gas kinematics and PDR structure around O-type stars in Sh 2-305 HII region

TL;DR

This study uses [CII] 158 μm line observations to analyze gas dynamics and structure of photodissociation regions around O-type stars in the Sh 2-305 HII region, revealing expanding shells and their properties.

Contribution

It provides the first detailed analysis of gas kinematics and PDR structure around O-type stars in Sh 2-305 using [CII] emission data, identifying sequentially formed expanding shells.

Findings

Identification of two shell-like structures with distinct ionizing stars.

Confirmation of shell expansion with velocity ~1.3 km/s.

Detection of dense molecular clumps possibly formed by gravitational instability.

Abstract

We report an observational study of the Galactic HII region Sh 2-305/S305 using the [CII] 158 $\mu$m line data, which are used to examine the gas dynamics and structure of photodissociation regions. The integrated [CII] 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 $\sim$565 M$_\odot$. The inner neutral shell encompasses an O9.5V star at its centre and has a compact ring-like appearance. However, the outer shell is seen with more extended and diffuse [CII] emission, hosting an O8.5V star at its centre and surrounds the inner neutral shell. The velocity channel maps and position-velocity diagrams confirm the presence of a compact [CII] shell embedded in the diffuse outer shell, and both the shells seem to expand with $v_{\rm exp}\sim$1.3 km s$^{-1}$. The outer shell appears to be older than the inner shell, hinting that these shells are formed sequentially. The [CII] 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 $\sim$10-10$^{3}$ M$_{\odot}$) are investigated around the neutral shells, which might have originated due to gravitational instability in the shell of collected materials.