On the Initial Conditions for Star Formation and the IMF

TL;DR

This paper explores how the structure of turbulent, self-gravitating clouds influences the initial mass function (IMF) of stars, emphasizing the role of density PDFs, cloud structure, and core fragmentation in shaping stellar masses.

Contribution

It introduces a model linking cloud density PDFs and structure to the origin of the stellar IMF, including the impact of core fragmentation and the formation of brown dwarfs.

Findings

Density PDFs develop extended tails due to self-gravity.

The IMF may originate from a power-law distribution of cloud structures.

Core fragmentation introduces an independent mass function affecting the low-mass end.

Abstract

Density probability distribution functions (PDFs) for turbulent self-gravitating clouds should be convolutions of the local log-normal PDF, which depends on the local average density rho-ave and Mach number M, and the probability distribution functions for rho-ave and M, which depend on the overall cloud structure. When self-gravity drives a cloud to increased central density, the total PDF develops an extended tail. If there is a critical density or column density for star formation, then the fraction of the local mass exceeding this threshold becomes higher near the cloud center. These elements of cloud structure should be in place before significant star formation begins. Then the efficiency is high so that bound clusters form rapidly, and the stellar initial mass function (IMF) has an imprint in the gas before destructive radiation from young stars can erase it. The IMF could arise from a power-law distribution of mass for cloud structure. These structures should form stars down to the thermal Jeans mass MJ at each density in excess of a threshold. The high-density tail of the PDF, combined with additional fragmentation in each star-forming core, extends the IMF into the Brown Dwarf regime. The core fragmentation process is distinct from the cloud structuring process and introduces an independen core fragmentation mass function (CFMF). The CFMF would show up primarily below the IMF peak.