Creation/destruction of ultra-wide binaries in tidal streams

TL;DR

This paper investigates a new formation mechanism for ultra-wide binary stars through chance entrapment in tidal streams, revealing their properties, disruption processes, and potential as probes of Galactic halo clumpiness.

Contribution

It introduces a novel formation pathway for ultra-wide binaries via tidal stream entrapment and characterizes their statistical properties and disruption mechanisms.

Findings

Ultra-wide binaries form via chance entrapment in tidal streams.

Their semi-major axis distribution follows a power law with exponent 1/2.

Disruption by tidal fluctuations follows Opik's law with a peak that decreases over time.

Abstract

This paper uses statistical and $N$-body methods to explore a new mechanism to form binary stars with extremely large separations ($> 0.1\,{\rm pc}$), whose origin is poorly understood. Here, ultra-wide binaries arise via chance entrapment of unrelated stars in tidal streams of disrupting clusters. It is shown that (i) the formation of ultra-wide binaries is not limited to the lifetime of a cluster, but continues after the progenitor is fully disrupted, (ii) the formation rate is proportional to the local phase-space density of the tidal tails, (iii) the semimajor axis distribution scales as $p(a)d a\sim a^{1/2}d a$ at $a\ll D$, where $D$ is the mean interstellar distance, and (vi) the eccentricity distribution is close to thermal, $p(e)d e= 2 e d e$. Owing to their low binding energies, ultra-wide binaries can be disrupted by both the smooth tidal field and passing substructures. The time-scale on which tidal fluctuations dominate over the mean field is inversely proportional to the local density of clumps. Monte-Carlo experiments show that binaries subject to tidal evaporation follow $p(a)d a\sim a^{-1}d a$ at $a\gtrsim a_{\rm peak}$, known as \"Opik's law, with a peak semi-major axis that contracts with time as $a_{\rm peak}\sim t^{-3/4}$. In contrast, a smooth Galactic potential introduces a sharp truncation at the tidal radius, $p(a)\sim 0$ at $a\gtrsim r_t$. The scaling relations of young clusters suggest that most ultra-wide binaries arise from the disruption of low-mass systems. Streams of globular clusters may be the birthplace of hundreds of ultra-wide binaries, making them ideal laboratories to probe clumpiness in the Galactic halo.