Orbital statistics of multiple systems formed from small-N subclusters

Ambrose, Hannah E. and Whitworth, A. P. ORCID: https://orcid.org/0000-0002-1178-5486 2025. Orbital statistics of multiple systems formed from small-N subclusters. Monthly Notices of the Royal Astronomical Society , staf1232. 10.1093/mnras/staf1232

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Abstract

We use numerical N-body experiments to explore the statistics of multiple systems formed in small-N subclusters, i.e. the distributions of orbital semi-major axis, a, orbital eccentricity, e, mass ratio, q, mutual orbital inclination, θ, and ejection velocity, υej. The stars in a subcluster are evolved as if they are the fragmentation products of a single isolated prestellar core from which most of the natal gas has already been dispersed, and there are no correlations between the stars’ initial positions and velocities. Two parameters are particularly important: the number of stars in the subcluster, N, and the fraction of kinetic energy in ordered rotation, αrot. Increasing N has the effect of systematically decreasing the semi-major axes of the tighter orbits, but has very little effect on the semi-major axes of the wider orbits. The main effect of αrot is to regulate the distribution of mutual orbital inclinations, with αrot ∼ 0.5 producing a distribution of orbital inclinations for triple systems which is consistent with observed values. Triples frequently form in high-inclination orbits without the assistance of von Zeipel-Lidov-Kozai cycles. Our previous work demonstrated that subclusters with mass segregation, moderate rotation, and typically N = 4 or 5 stars produced the best fit to the multiplicity statistics (proportions of singles, binaries, triples, etc.). Here we show that these parameters also reproduce the orbital statistics (distributions of orbital semi-major axis, a, orbital eccentricity, e, mass ratio, q, mutual orbital inclination, θ, and ejection velocity, υej). For the best-fit parameters, 21( ± 1)% of subclusters produce more than one multiple system.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Physics and Astronomy
Publisher:	Oxford University Press (OUP)
ISSN:	0035-8711
Date of First Compliant Deposit:	4 August 2025
Last Modified:	04 Aug 2025 12:30
URI:	https://orca.cardiff.ac.uk/id/eprint/180240

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