Evolution of Extremely Soft Binaries in Dense Star Clusters: On the Jupiter Mass Binary Objects

TL;DR

This paper develops an analytical model for the evolution of extremely soft binary systems, like binary planets and brown dwarfs, in dense star clusters, revealing universal semi-major axis distributions and implications for observed Jupiter-mass binaries.

Contribution

It extends the Hut & Bahcall formalism to derive ionization cross-sections for extreme mass ratio binaries, providing new insights into their orbital evolution and formation scenarios.

Findings

Low-mass soft binaries follow a universal power-law semi-major axis distribution between a^{-8/3} and a^{-5/3}.

Candidate Jupiter-mass binaries in the Trapezium cluster likely form through ejection mechanisms, but at high formation rates.

In-situ formation of binary brown dwarfs with specific semi-major axes can explain observed populations.

Abstract

Star-forming regions, characterized by dense environments, experience frequent encounters that significantly influence binary systems, leading to their hardening, softening, or ionization. We extend the Hut \& Bahcall formalism to derive an analytical expression for the ionization cross-section in extreme mass ratio binary systems, allowing us to investigate the orbital evolution and population dynamics of binary planets and binary brown dwarfs in star clusters, while considering ongoing binary system formation. Our findings reveal that for low-mass soft binaries, the semi-major axis distribution asymptotes to a universal power law between $\propto a^{-8/3}$ and $\propto a^{-5/3}$ over the derived ionization timescale. We also discuss the implications of our results for the candidate Jupiter-mass binary objects putatively reported in the Trapezium cluster. We demonstrate that if their existence is verified, they likely form continuously with a spectrum proportional to $a^{1}$, aligning better with the ejection mechanism than with the in-situ formation mechanism, which predicts a distribution roughly proportional to $a^{-1}$. However, this implies an impractically high ejection formation rate. Alternatively, if these objects are binary brown dwarfs, continuous in-situ formation ($\propto a^{-1}$) with an initial minimal semi-major axis around 20 AU and a formation rate of 100 Myr$^{-1}$ plausibly matches the observed number of single objects, binary number, binary fraction, and semi-major axis distribution.