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

TL;DR

This study investigates how core subfragmentation influences the initial mass function in the massive protocluster W43-MM1, revealing a hierarchical fragmentation process with limited impact on the IMF's high-mass end.

Contribution

It provides a detailed analysis of hierarchical core fragmentation in W43-MM1, quantifying fractality, mass partition, and efficiency, and assesses its role in shaping the IMF.

Findings

W43-MM1 has a low fractality index of 1.19, indicating limited fragmentation per scale decrease.

Mass partitioning is imbalanced, with two-thirds of sibling mass in the dominant fragment.

Fragmentation below 14 kau is gravity-driven, not turbulence, and the high-mass IMF remains top-heavy.

Abstract

The gravo-turbulent fragmentation of the interstellar medium is expected to create a hierarchical cascade of cloud structures, crossing the scales from core to disk. We aim to predict how the currently observed top-heavy core mass function (CMF) in the massive protocluster W43-MM1 evolves due to core subfragmentation. We used the getsf algorithm to extract sources in five ALMA images of W43-MM1 at 3 mm, with a spatial resolution ranging from 14 kau to 270 au. 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 the NGC 2264 protocluster and in synthetic images of an Orion-like protocluster simulated by magneto-hydrodynamical calculations. Assuming self-similarity, we measure a small fractality index of mathcal 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. We 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 core formation efficiency (CFE) from 2400 au to 200 au of ~16%. Based on the fractality and efficiency values measured in W43-MM1, the gravo-turbulent model by Thomasson et al. predicts that its fragmentation below ~14 kau is not driven by turbulence but by gravity. Using these parameters and the measured mass partition, we demonstrate that the fragment mass function, from which the the initial mass function (IMF) emerges, has a high-mass end which remains top-heavy. Therefore, core subfragmentation in W43-MM1, and perhaps more broadly in massive Galactic protoclusters, plays a minimal role in the IMF origin.