On the emergent System Mass Function: the contest between accretion and fragmentation

TL;DR

This paper introduces a new model for star cluster evolution, combining turbulent fragmentation and competitive accretion, explaining the formation of the high-mass tail of the System Mass Function with a power-law slope.

Contribution

It presents a novel integrated model that explains the development of the System Mass Function through turbulent fragmentation and accretion, matching observed distributions.

Findings

The SMF develops a Salpeter slope at high masses due to accretion.

Turbulent fragmentation creates low-mass seed proto-systems consuming up to 50% of available mass.

The model accurately reproduces the Chabrier SMF with specific seed proto-system parameters.

Abstract

We propose a new model for the evolution of a star cluster's System Mass Function (SMF). The model involves both turbulent fragmentation and competitive accretion. Turbulent fragmentation creates low-mass seed proto-systems (i.e. single and multiple protostars). Some of these low-mass seed proto-systems then grow by competitive accretion to produce the high-mass power-law tail of the SMF. Turbulent fragmentation is relatively inefficient, in the sense that the creation of low-mass seed proto-systems only consumes a fraction, $\sim 23\%$ (at most $\sim 50\%$), of the mass available for star formation. The remaining mass is consumed by competitive accretion. Provided the accretion rate onto a proto-system is approximately proportional to its mass ($dm/dt \propto m$), the SMF develops a power-law tail at high masses with the Salpeter slope ($\sim -2.3$). If the rate of supply of mass accelerates, the rate of proto-system formation also accelerates, as appears to be observed in many clusters. However, even if the rate of supply of mass decreases, or ceases and then resumes, the SMF evolves homologously, retaining the same overall shape, and the high-mass power-law tail simply extends to ever higher masses until the supply of gas runs out completely. The Chabrier SMF can be reproduced very accurately if the seed proto-systems have an approximately log-normal mass distribution with median mass $\sim 0.11 {\rm M}_{_\odot}$ and logarithmic standard deviation $\sigma_{\log_{10}(M/{\rm M}_\odot)}\sim 0.47$).