Before fragmentation: from uniform clouds to stellar clusters

Abstract

Our study of the early stages of fragmentation in massive clumps has revealed a striking anomaly: $\sim15\%$ of the CoCoA sample exhibit no fragmentation at all. These sources could represent the initial conditions before core formation. We investigate whether these sources are indeed at the onset of core formation, and whether gravity or turbulence dominates their early fragmentation. We study the transition from uniform gas to active cluster formation, and compare contrasting predictions of turbulence- and gravity-dominated models of clump fragmentation through ALMA observations of three massive clumps spanning this phase. The fragmented source exhibits higher average $n$(H$_2$) and a core-to-envelope density contrast 5-6 times larger than the minimally fragmented sources. All sources display pc-scale velocity differences comparable to their linewidths and gravitational velocities. This suggests that the linewidth is dominated by the large-scale motions, and that the latter are driven by gravity. The high CH$_3$OH abundances and the mass build-up timescales align with astrochemical models where complex molecules form at intermediate densities before the appearance of cores. All independent diagnostics in our analysis are consistent with the minimally fragmented clumps being an early evolutionary phase, potentially marking the onset of core fragmentation. If turbulence were the primary driver of fragmentation, sources with comparable Mach numbers should exhibit similar structural properties. Instead, we observe significant variations in density contrast and fragmentation. These findings challenge turbulence-supported models and lend support to hierarchical, gravity-driven models of clump fragmentation, where large-scale ordered motions account for a significant fraction of the observed linewidths and drive the evolution of the clump toward increasingly dense and fragmented states.