[Paper Review] Triple common envelope evolution: Circumstellar triples

Hila Glanz, Hagai B. Perets|arXiv (Cornell University)|Mar 31, 2020

Stellar, planetary, and galactic studies8 references4 citations

TL;DR

This paper presents the first hydrodynamical simulations of triple common envelope (TCE) evolution in circumstellar triples, where a compact binary is engulfed by a giant star's envelope. TCE leads to slower in-spiral, enhanced mass ejection, and aspherical remnants due to energy/momentum extraction from the inner binary, resulting in binary mergers, disruption, or ejection—offering a pathway to compact binaries, stellar mergers, and transient sources in electromagnetic and gravitational-wave bands.

ABSTRACT

The dynamical evolution of triple stellar systems could give rise to the formation of compact binaries and induce binary mergers. Common envelope (CE) evolution, which plays a major role in the evolution of compact binary systems, can similarly play a key role in the evolution of triples. Here we use hydrodynamical simulations coupled with few-body dynamics to provide the first detailed models of triple common envelope (TCE) evolution. We focus on the circumstellar case, where the envelope of an evolved giant engulfs a compact binary orbiting the giant (inner-binary), which then in-spirals into the core of the evolved star. Through our exploratory modeling we find several possible outcomes of such TCE: (1) The merger of the binary inside the third star's envelope; (2) The binary disruption of the in-spiraling binary following its plunge, leading to a chaotic triple dynamics of the stellar-core and the two components of the former disrupted binary. The chaotic evolution typically leads to the in-spiral and merger of at least one of the former binary components with the core, and sometimes to the ejection of the second, or alternatively its further now-binary common-envelope evolution. The in-spiral in TCE leads to overall slower in-spiral, larger mass ejection and the production of more aspherical remnant, compared with a corresponding binary case of similar masses, due to the energy/momentum extraction from the inner-binary. We expect TCE to play a key role in producing various types of stellar-mergers and unique compact binary systems, and potentially induce transient electromagnetic and gravitational-wave sources.

Motivation & Objective

• To investigate the dynamical evolution of triple stellar systems where a compact binary is engulfed by a giant star's envelope, leading to triple common envelope (TCE) evolution.
• To understand how the presence of a third star alters the in-spiral and outcome of common envelope evolution compared to binary systems.
• To model the energy and momentum exchange between the inner binary and the surrounding envelope, and its impact on ejection efficiency and remnant asphericity.
• To explore the formation pathways of compact binary systems and stellar mergers via TCE, including potential electromagnetic and gravitational-wave transients.

Proposed method

• Coupled hydrodynamical simulations with few-body dynamics to model the evolution of triple systems during common envelope phases.
• Focused on the circumstellar configuration, where the inner binary orbits within the envelope of an evolved giant star.
• Tracked energy and momentum transfer from the in-spiraling inner binary to the envelope, influencing ejection efficiency and remnant structure.
• Simulated the outcomes of inner binary in-spiral, including possible mergers, disruption, and subsequent chaotic dynamics of the core and binary components.
• Compared TCE outcomes to standard binary common envelope evolution for similar masses to isolate the effects of the third body.
• Analyzed the asphericity of the final remnant and mass ejection efficiency to assess observational signatures.

Experimental results

Research questions

• RQ1 What are the dominant dynamical outcomes of triple common envelope evolution in circumstellar triples?
• RQ2 How does the presence of a third star alter the in-spiral timescale and mass ejection efficiency compared to binary common envelope evolution?
• RQ3 What role does energy and momentum extraction from the inner binary play in shaping the final remnant and ejection properties?
• RQ4 Can TCE lead to the formation of compact binary systems or stellar mergers, and what are the associated transient signatures?
• RQ5 What are the conditions under which the inner binary is disrupted, and what are the subsequent dynamical fates of its components?

Key findings

• Triple common envelope evolution leads to slower in-spiral compared to binary common envelope evolution due to energy and momentum extraction from the inner binary.
• TCE results in larger mass ejection and more aspherical remnant structures than comparable binary systems.
• The in-spiral of the inner binary can lead to its merger within the core of the giant star, forming a single merged object.
• Inner binary disruption is a common outcome, triggering chaotic dynamics between the core and the two former binary components.
• Chaotic evolution typically results in the in-spiral and merger of at least one former binary component with the core.
• In some cases, the second former binary component is ejected, while in others it undergoes further common envelope evolution as a new binary.