Different dynamical ages for the two young and coeval LMC star clusters, NGC 1805 and NGC 1818, imprinted on their binary populations

Aaron M. Geller, Richard De Grijs, Chengyuan Li, Jarrod R. Hurley

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19 Citations (Scopus)
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The two Large Magellanic Cloud star clusters, NGC 1805 and NGC 1818, are approximately the same chronological age (∼30 Myr), but show different radial trends in binary frequency. The F-type stars (1.3-2.2 M⊙) in NGC 1818 have a binary frequency that decreases toward the core, while the binary frequency for stars of similar mass in NGC 1805 is flat with radius, or perhaps bimodal (with a peak in the core). We show here, through detailed N-body modeling, that both clusters could have formed with the same primordial binary frequency and with binary orbital elements and masses drawn from the same distributions (defined from observations of open clusters and the field of our Galaxy). The observed radial trends in binary frequency for both clusters are best matched with models that have initial substructure. Furthermore, both clusters may be evolving along a very similar dynamical sequence, with the key difference that NGC 1805 is dynamically older than NGC 1818. The F-type binaries in NGC 1818 still show evidence of an initial period of rapid dynamical disruptions (which occur preferentially in the core), while NGC 1805 has already begun to recover a higher core binary frequency, owing to mass segregation (which will eventually produce a distribution in binary frequency that rises only toward the core, as is observed in old Milky Way star clusters). This recovery rate increases for higher-mass binaries, and therefore even at one age in one cluster, we predict a similar dynamical sequence in the radial distribution of the binary frequency as a function of binary primary mass.

Original languageEnglish
Article number11
Pages (from-to)1-11
Number of pages11
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 20 May 2015
Externally publishedYes

Bibliographical note

Copyright 2015 The American Astronomical Society. First published in The Astrophysical Journal, 805(1), 11, 2015. The original publication is available at http://dx.doi.org/10.1088/0004-637X/805/1/11, published by IOP Publishing. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.


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