Cosentino, Giuliana; (2019) Physical and Chemical Processes in Cloud-Cloud Collisions: Star Formation in the Making. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Infrared Dark Clouds (IRDCs) are cold and dense regions in space, considered as the birthplace of high-mass stars and stellar clusters. Recent studies support the idea that the physical mechanisms leading to the formation of these IRDCs may also trigger high-mass star formation within the clouds. Footprints of such formation mechanisms may be stored in the molecular gas kinematics and chemical content of IRDCs. Hence, it is crucial to investigate IRDCs to search for signatures of their formation process. In this thesis, we analyse the spatial extent and kinematic structure of the shock tracer Silicon Monoxide (SiO) to identify signatures of one particular IRDCs formation scenario, cloud-cloud collisions, toward the nine IRDCs G018.82−00.28, G019.27+00.07, G028.37+00.07, G028.53−00.25, G028.67+00.13,G034.43+00.24,G034.77−00.55,G038.95−00.47, G053.11+00.05, at different evolutionary stages. Among the IRDCs, the extended SiO emission de- tected toward G034.77−00.55 probes the MHD shock wave associated with the collision between the cloud and a flow of molecular gas pushed toward the IRDC by the nearby supernova remnant W44. The shocked gas traces the internal time- dependent structure of the MHD shock, here directly observed for the first time. Toward the IRDCs G018.82−00.28 and G038.95−00.47, the widespread SiO emis- sion may be partially due to on-going star formation activity and partially associated with the interaction between the clouds and nearby galactic bubbles. The SiO emission detected toward IRDCs G019.27+00.07, G028.37+00.07, G028.53−00.25, G034.43+00.24 and G053.11+00.05, known to be actively forming stars, is likely associated with on-going star formation activity. Finally, no significant SiO emission is found toward the IRDC G028.67+00.13. With our results, we suggest that cloud-cloud collisions triggered by stellar feedback may represent a good laboratory to investigate massive star formation in its early stages, although they likely do not represent the main formation mechanism of massive stars.

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