Triggered Fragmentation in Self-Gravitating Protoplanetary discs: Cooling, Mass Movement and Instability

Abstract Previous three-dimensional hydrodynamical simulations of gravitationally unstable discs have shown that the formation of a single fragment can trigger the formation of subsequent fragments. This behaviour was attributed to changes in the surface mass density caused by the interaction between the first fragment and the disc material. This study reanalyses those simulations to see if the surface mass density was the sole driver. Our results reveal that both surface mass density and sound speed can contribute to the formation of additional fragments. Beyond the general cooling typical of such discs, the inwards movement of cool material from the outer disc, driven by the formation of the first fragment, can enhance fragmentation in the inner regions. We also identify that interactions between the midplane gas and the cooler upper layers of the disc can facilitate additional cooling. Triggered fragmentation can create a unique planet-formation environment by redistributing material across the disc, potentially leading to chemically distinct fragments from those formed in-situ.