Transition of the initial mass function in the metal-poor environments

We study star cluster formation in a low-metallicity environment using three-dimensional hydrodynamic simulations. Starting from a turbulent cloud core, we follow the formation and growth of protostellar systems with different metallicities ranging from 10-6 to 0.1 Z⊙. The cooling induced by dust grains promotes fragmentation at small scales and the formation of low-mass stars with M∗ ~0.01-0.1 M⊙. While the number of low-mass stars increases with metallicity, when Z/Z⊙ ≲ 10-5, the stellar mass distribution is still top-heavy for Z/Z⊙ ≲ 10-2 compared to the Chabrier initial mass function (IMF). In these cases, star formation begins after the turbulent motion decays and a single massive cloud core monolithically collapses to form a central massive stellar system. The circumstellar disc preferentially feeds the mass to the central massive stars, making the mass distribution top-heavy. When Z/Z⊙ = 0.1, collisions of the turbulent flows promote the onset of the star formation and a highly filamentary structure develops owing to efficient fine-structure line cooling. In this case, the mass supply to the massive stars is limited by the local gas reservoir and the mass is shared among the stars, leading to a Chabrier-like IMF. We conclude that cooling at the scales of the turbulent motion promotes the development of the filamentary structure and works as an important factor leading to the present-day IMF.