Formation of metal-free binaries: Impact of H$_{2}$ line cooling and CIE cooling

TL;DR

This study explores how collision-induced emission (CIE) cooling influences the formation and characteristics of metal-free binary stars during primordial star formation, highlighting differences in protostar mass distribution and binary properties.

Contribution

It provides the first detailed comparison of protostellar formation with and without CIE cooling, revealing its impact on binary mass accretion and orbital eccentricity.

Findings

CIE cooling increases maximum mass accretion rates for protobinaries.

Models with CIE cooling form more compact, eccentric binaries.

Number of fragments decreases with higher turbulence levels.

Abstract

During primordial star formation, the main cooling channel is provided by H$_{2}$ and super-molecules, such as H$_{2}$ or H$_{2}$, at sufficiently high densities. When the latter form at $n_{\rm H}$ $\geq$ $10^{14}$~cm$^{-3}$, collision-induced emission (CIE) provides efficient gas cooling. We investigate how CIE cooling affects the formation of metal-free binaries comparing simulations with and without this process. Irrespective of the cooling mechanism, we find a typical protostellar mass range between 0.01 to 100 M$_{\odot}$. However, models with only H$_{2}$ line cooling produce a greater number of low-mass protostars which exhibit stronger variations in their radial velocities than the high-mass protostars. Similarly, in models with both H$_{2}$ cooling and CIE cooling, significant variations in the radial velocities are found for protostars in the intermediate mass range. The initial number of fragments $N_{\rm max}$ decreases with increasing strength of turbulence. Cooling via super-molecules lets the most massive protobinaries (MMPBs) efficiently accrete mass. The maximum mass accretion rate $\dot M_{\rm max}$ for the MMPBs is more than an order of magnitude higher in the presence of CIE cooling than for pure H$_{2}$ line cooling. As a result, compact binaries with a semi-major axis as small as 3.57 au may form through the H$_{2}$ $-$ H$_{2}$ cooling channel. Our results indicate that in addition to the MMPBs most population III (Pop. III) binaries should be in eccentric i.e. non-circular orbits. This provides an important connection to the eccentric binaries reported in previous studies, which were found to exhibit rich temporal accretion signals during their evolution.