ALMA-IMF XXII. Role of core subfragmentation in the IMF origin: Hierarchical fragmentation cascade and CMF in W43-MM1

Abstract

We aim to predict how the currently observed top-heavy CMF in the massive protocluster W43-MM1 evolves due to core subfragmentation. We used getsf to extract sources in 5 ALMA images of W43-MM1 at 3mm, with a spatial resolution ranging from 14kau to 270au. Then, we applied FAMILY, a graph-theory-based analysis tool, to create and characterize networks of nested sources in W43-MM1. We compared the hierarchical fragmentation cascade of W43-MM1 to those measured in NGC2264 and in synthetic images of a MHD protocluster. Assuming self-similarity, we measure a small fractality index of F3D = 1.19 +/-0.10 in W43-MM1, which means that, on average, a cloud structure will fragment into only 1.19 fragments each time the physical scale decreases by a factor of two. In line with values measured above the core scale in the NGC 2264 and synthetic protoclusters, the W43-MM1 fractality index increases by ~30% at larger scales. We also estimate an imbalanced mass partition between siblings, with 2/3 of the mass of siblings at a given scale belonging to the dominant sibling. The mass transfer efficiency, computed from one physical scale to another, is high and corresponds to a CFE from 2.4kau cores to 200au seeds of ~16%. Based on the measured fractality and efficiency values, the gravo-turbulent model predicts that its fragmentation below ~14kau is not driven by turbulence but by gravity. Using these parameters and the measured mass partition, we demonstrate that the seed mass function, from which the IMF emerges, has a high-mass end which remains top-heavy. Therefore, based on our current assumptions, core subfragmentation in W43-MM1, and perhaps more broadly in massive Galactic protoclusters, plays a minimal role in shaping the high-mass slope of the IMF.