ALMAGAL VI. The spatial distribution of dense cores during the evolution of cluster-forming massive clump

Abstract

High-mass stars and star clusters form from the fragmentation of massive dense clumps driven by gravity, turbulence, and magnetic fields. The ALMAGAL project observed $\sim 1000$ clumps at $\sim$1000\,au resolution, enabling a statistically significant characterization of this process across a large range of clump physical parameters and evolutionary stages. In this work, we investigated the spatial distribution of dense cores in the 514 massive, potentially cluster-forming, clumps hosting at least 4 cores, to trace fragmentation's initial conditions and early evolution. We used quantitative descriptors, evaluated against the clump bolometric luminosity-to-mass ratio as an indicator of evolution. Core separations were measured with the minimum spanning tree method (MST) and compared with the Jeans gravitational fragmentation theory. We used the $Q$ parameter and the mass segregation ratio, ${\Lambda }_{MSR}$, to evaluate whether cores have specific arrangements or differences depending on their masses. ALMAGAL cores are usually arranged in elliptical groups with an axis ratio $e\sim 2.2$, but $e\ge$5 is also observed. A single characteristic core separation per clump is found in $\sim 76$% of cases, but signatures of multiple fragmentation lengths not rare. Typical core separations are compatible with the clump-averaged thermal Jeans length, ${\lambda }_J^{th}$, though a population, typical of low-fragmented/young clumps, has wider separations with $l\approx 3\times {\lambda }_J^{th}$. The core separation decreases on average from $l\sim 22000$ au in younger systems to $l\sim 7000$ au in more evolved ones. Cores are typically distributed in fractal-type subclusters, with centrally concentrated patterns appearing only at later stages, but without a progressive evolutionary transition. Finally, mass segregation is found in 110 systems, with its occurrence increasing with evolution.

arXiv comment32 pages, 22 figures, accepted for publication in the Astronomy & Astrophysics journal, abstract adapted from current version

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