Before fragmentation: From uniform clouds to stellar clusters

Abstract

Our systematic study of the early stages of fragmentation in massive clumps has revealed a striking anomaly: sim15% of the Cold Cores with ALMA (CoCoA) survey sample exhibit no fragmentation at all. These sources could represent the initial conditions before core formation. We set out to investigate whether these unfragmented clumps are indeed at the onset of core and cluster formation and whether gravity or turbulence dominates their early fragmentation. To understand the nature of these extreme sources, we studied the transition from uniform gas to active cluster formation and compared contrasting predictions of turbulence- and gravity-dominated models of clump fragmentation. We used ALMA observations of three massive clumps spanning this phase (from two minimally fragmented clumps to one significantly fragmented source). Our analysis includes direct gas-phase volume density estimates from CH_3OH$(2_K - 1_K)$ line ratios (independent of dust properties, temperature, and geometry) and unfiltered kinematics of the dense gas (CH_3OH and mathrm N_2H^+ J=1-0) and a comparison between the mass build-up and chemical timescales derived from methanol abundances. The fragmented source exhibits higher average volume densities and a core-to-envelope density contrast five to six times larger than the minimally fragmented sources. All sources display parsec-scale velocity differences comparable to their linewidths, and both the velocity differences and the linewidths are similar to the gravitational velocity. This suggests that the linewidth is dominated by the large-scale systematic motions and that the latter are driven by gravity. We also estimated high CH_3OH abundances (∼10 -9 ) across the clumps. These abundances and the inferred mass build-up timescales align with astrochemical models where complex molecules form at intermediate densities before the appearance of cores. All independent physical and chemical diagnostics in our analysis are consistent with a scenario where the minimally fragmented clumps represent an early evolutionary phase that potentially marks the onset of core fragmentation driven by gravitational collapse. If turbulence were the primary driver of fragmentation, sources with comparable Mach numbers (mathcal M ≈ 6-8 in our sample) should exhibit similar structural properties. Instead, we observed significant variations in density contrast and fragmentation. These findings challenge purely 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 towards increasingly dense and fragmented states.