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The origin and evolution of wide Jupiter Mass Binary Objects in young stellar clusters

by Simon Portegies Zwart, Erwan Hochart

Submission summary

Authors (as registered SciPost users): Simon Portegies Zwart
Submission information
Preprint Link:  (pdf)
Date submitted: 2024-03-14 16:21
Submitted by: Portegies Zwart, Simon
Submitted to: SciPost Astronomy
Ontological classification
Academic field: Astronomy
  • Earth and Planetary Astrophysics
Approaches: Theoretical, Computational


The recently observed population of 540 free-floating Jupiter-mass objects, including 40 dynamically soft pairs, and two triples, in the Trapezium cluster have raised interesting questions on their formation and evolution. We test various scenarios for the origin and survivability of these free floating Jupiter-mass objects and Jupiter-mass Binary Objects (JuMBOs) in the Trapezium cluster. The numerical calculations are performed by direct $N$-body integration of the stars and planets in the Trapezium cluster starting with a wide variety of planets in various configurations. We discuss four models: $\mathcal{SPP}$, in which selected stars have two outer orbiting Jupiter-mass planets; $\mathcal{SPM}$, where selected stars are orbited by Jupiter-mass planet-moon pairs; $\mathcal{ISF}$ in which \jumbos\, form in situ with the stars, and $\mathcal{FFC}$, where we introduce a population of free-floating single Jupiter-mass objects, but no initialised binaries. Models $\mathcal{FFC}$ and $\mathcal{SPP}$ fail to produce enough \jumbos. Models $\mathcal{SPM}$ can produce sufficient free-floaters and \jumbos\/, but requires unusually wide orbits for the planet-moon system around the star. The observed \jumbos\ and free-floating Jupiter-mass objects in the Trapezium cluster are best reproduced if they formed in pairs and as free-floaters together with the other stars in a smooth (Plummer) density profile with a virial radius of $\sim 0.5$\,pc. A fractal (with fractal dimension 1.6) stellar density distribution also works, but requires relatively recent formations ($\apgt 0.2$\,Myr after the other stars formed) or a high ($\apgt 50$\%) initial binary fraction. This would make the primordial binary fraction of \jumbos\, even higher than the already large observation fraction of $\sim 8$\,\% (42/540). The fraction of \jumbos\, will continue to drop with time, and the lack of \jumbos\ in Upper Scorpius could then result in its higher age, causing more \jumbos\, to be ionized. We then also predict that the interstellar density of Jupiter-mass objects (mostly singles with some $\sim 2$\% lucky surviving binaries) is $\sim 0.05$\, per pc$^{-3}$ (or around 0.24 per star).

Author comments upon resubmission

The comments were already submitted separately in a response on the SciPost website.

List of changes

>> Not really a weakness, but several effects (GR, SN, ...) were
>> neglected, which is well justified and does not lower the quality
>> and strength of the results.

Indeed, we decided to keep the simulations as simple as possible,
excluding all possible processes that are, in our opinion, not (or less)

>> The authors provide a through numerical study on the formation
>> process of JuMBOs in young stellar clusters and compare their
>> findings with observational data.
>> They define four different initial models for young stellar
>> clusters and two different models to distribute the initial JMOs
>> and JuMBOs. They find for one model (very good agreement with the
>> observations if JMOs and JuMBOs formed already early and for
>> another model if the objects formed late in the cluster.
>> The observational data of the JuMBOs and JMOs in the Trapezium
>> cluster are best reproduced in the simulations if they formed
>> pairwise and as free-floaters together with the other stars in a
>> Plummer sphere with a density profile and a virial radius of ∼ 0.5
>> pc.

>> p2 in enumeration of discovered objects: add a space between 6 and M_jup


>> p3 in sentence "Observed JuMBOs (...)": remove comma after 25 au

>> p3 in sentence "The hardest JuMBO (...)": fix 17 MJup to 17 M_Jup

>> p4 in model calculations: add e to Salpter


>> p7 in sect 4.1, sentence "There are several systematic trends in
>> terms of cluster density that depend on ones choice of a Plummer
>> (...): replace ones with one's


>> and p7 next sentence, replace JBO's with JBOs.


>> p11 in sect 4.4, after paragraph ends with "(...) properties":
>> remove '++++++' signs


>> p16 in sentence "If not just statistics or observational bias, this
>> could indicate some curious formation mechanism. ": the term
>> "curious" seems inapt, please replace it with "unknown",
>> "different", or "unexplored"...

changed to unexplored

>> p16: please replace 10 M_Jupobjects -> 10 M_Jup-objects, and 1
>> M_\odotstar -> 1 M_\odot-star, ...


>> p17 in sect. 5:should'nt -> should not


>> p19 in last but one paragraph:
>> in "(...)if the encounter velocity 0.8 v_*/v_1": please define v_*


>> same page last sentence:
>> please remove comma between star and would in "(...) 1 M_\odot star, would (...)"


>> p20 heading of section 5.3: replace "if" with "of"


>> p22: last sentence on page 22 (which is continued on p23) is not
complete or has no meaning, please rephrase.

hmm indeed, a weard sentence. Removed part of it.

>> p27: first paragraph: replace popultion with population


>> same page, last paragraph: replace in situe with in situ


Current status:
Refereeing in preparation

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