SIRIUS project II: a new tree-direct hybrid code for smoothed particle hydrodynamics/N-body simulations of star clusters

TL;DR

This paper introduces a new hybrid simulation code for star cluster formation that accurately models gas and stars without gravitational softening, revealing insights into cluster dynamics and binary formation.

Contribution

The development of ASURA+BRIDGE, a novel tree-direct hybrid code that simulates star cluster formation with realistic mass functions and no gravitational softening.

Findings

Artificial dense cores disappear without softening.

Star clusters form via mergers of smaller clumps.

Higher binary fractions among more massive stars.

Abstract

Star clusters form via clustering star formation inside molecular clouds. In order to understand the dynamical evolution of star clusters in their early phase, in which star clusters are still embedded in their surrounding gas, we need an accurate integration of individual stellar orbits without gravitational softening in the systems including both gas and stars, as well as modeling individual stars with a realistic mass function. We develop a new tree-direct hybrid smoothed particle hydrodynamics/N-body code, ASURA+BRIDGE, in which stars are integrated using a direct N-body scheme or PeTar, a particle-particle particle-tree scheme code, without gravitational softening. In ASURA+BRIDGE, stars are assumed to have masses randomly drawn from a given initial mass function. With this code, we perform star-cluster formation simulations starting from molecular clouds without gravitational softening. We find that artificial dense cores in star-cluster centers due to the softening disappear when we do not use softening. We further demonstrate that star clusters are built up via mergers of smaller clumps. Star clusters formed in our simulations include some dynamically formed binaries with the minimum semi-major axes of a few au, and the binary fraction is higher for more massive stars.