Star formation around the HII region Sh2-235

TL;DR

This study investigates star formation around HII region S235, revealing that the expansion of S235 likely triggered the formation of young stellar clusters within a molecular shell, supporting multiple star formation mechanisms.

Contribution

It provides a detailed analysis of molecular gas kinematics and distribution, demonstrating the role of HII region expansion in star formation and highlighting a mixture of triggered scenarios.

Findings

Young clusters are embedded in a molecular shell around S235.

Gas kinematics support triggered star formation via multiple mechanisms.

Molecular gas distribution is clumpy, affecting interpretation of star formation processes.

Abstract

We present a picture of star formation around the HII region Sh2-235 (S235) based upon data on the spatial distribution of young stellar clusters and the distribution and kinematics of molecular gas around S235. We observed 13CO(1-0) and CS(2-1) emission toward S235 with the Onsala Space Observatory 20-m telescope and analysed the star density distribution with archival data from the 2MASS survey. Dense molecular gas forms a shell-like structure at the south-eastern part of S235. The young clusters found with 2MASS data are embedded in this shell. The positional relationship of the clusters, the molecular shell and the HII region indicates that expansion of S235 is responsible for the formation of the clusters. The gas distribution in the S235 molecular complex is clumpy, which hampers interpretation exclusively on the basis of the morphology of the star forming region. We use data on kinematics of molecular gas to support the hypothesis of induced star formation, and distinguish three basic types of molecular gas components. The first type is primordial undisturbed gas of the giant molecular cloud, the second type is gas entrained in motion by expansion of the HII region (this is where the embedded clusters were formed), and the third type is a fast-moving gas, which might have been accelerated by winds from the newly formed clusters. The clumpy distribution of molecular gas and its kinematics around the HII region implies that the picture of triggered star formation around S235 can be a mixture of at least two possibilities: the "collect-and-collapse" scenario and the compression of pre-existing dense clumps by the shock wave.