Molecular clouds in supershells: A case study of three objects in the walls of GSH 287+04-17 and GSH 277+00+36

We present an in-depth case study of three molecular clouds associated with the walls of the Galactic supershells GSH 287+04-17 and GSH 277+00+36. These clouds have been identified in previous work as examples in which molecular gas is either being formed or destroyed due to the influence of the shells. ¹²CO(J = 1-0), ¹³CO(J = 1-0), and C¹⁸O(J = 1-0) mapping observations with the Mopra telescope provide detailed information on the distribution and properties of the molecular gas, enabling an improved discussion of its relationship to the wider environment in which it resides. We find that massive star formation is occurring in molecular gas likely formed in situ in the shell wall, at a Galactic altitude of ∼ 200pc. This second-generation star formation activity is dominating its local environment, driving the expansion of a small H II region which is blistering out of the atomic shell wall. We also find new morphological evidence of disruption in two smaller entrained molecular clouds thought to pre-date the shells. We suggest that at the present post-interaction epoch, the lifetime of this surviving molecular material is no longer strongly determined by the shells themselves.