Investigating the role of shocks, thermal instability and gravity in molecular cloud formation

Using the MG code we performed hydrodynamical simulations to study the relative importance between the thermal instability (TI), gravity and shock impact in the process of molecular cloud formation out of an initially quiescent atomic medium. A total of five scenarios were considered in this Thesis: an un-shocked scenario, two planar shock-cloud scenarios in the small cloud regime in Chapter 3, and two supernova-cloud scenarios in the large cloud regime in Chapter 4. In both regimes the shock was introduced when the cloud was in a “pre-TI” warm atomic state prior to any noticeable action of the TI, and a “post-TI” state after cold and dense clumps have formed. This allowed us to study the effects of the presence/absence of clumps on the resulting dynamics. In both shock-cloud scenarios, the planar shock significantly disrupted the clouds and the dynamics were shock dominated. The clouds showed evidence of local gravitational collapse on a 5.16 Myr timescale, which was not seen in our un-shocked clouds. In the pre-TI supernova-cloud case, the ambient pressure dropped below the cloud’s after 1 Myr, causing the cloud to expand. Prior to this, the TI was triggered behind the transmitted shock which appeared to form low density clumps, but these did not collapse like in the shock-cloud scenarios. The post-TI supernova had negligible impact on the cloud, and after ∼ 3.5 Myr it was almost indistinguishable from the un-shocked cloud - the pressure drop after 1 Myr had no effect either. Some clumps coalesced and merged due to the impact of the transmitted shock, but this did not aid in collapsing any clumps.