Journal Article

The early expansion of cluster cores

N. Bastian, M. Gieles, S. P. Goodwin, G. Trancho, L. J. Smith, I. Konstantopoulos and Yu. Efremov

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 389, issue 1, pages 223-230
Published in print September 2008 | ISSN: 0035-8711
Published online August 2008 | e-ISSN: 1365-2966 | DOI:
The early expansion of cluster cores

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The observed properties of young star clusters, such as the core radius and luminosity profile, change rapidly during the early evolution of the clusters. Here we present observations of six young clusters in M51 where we derive their sizes using Hubble Space Telescope (HST) imaging and ages using deep Gemini-North spectroscopy. We find evidence for a rapid expansion of the cluster cores during the first 20 Myr of their evolution. We confirm this trend by including data from the literature of both Galactic and extragalactic embedded and young clusters, and possible mechanisms (rapid gas removal, stellar evolutionary mass loss and internal dynamical heating) are discussed. We explore the implications of this result, focussing on the fact that clusters were more concentrated in the past, implying that their stellar densities were much higher and relaxation times (trelax) correspondingly shorter. Thus, when estimating if a particular cluster is dynamically relaxed (i.e. when determining if a cluster's mass segregation is due to primordial or dynamical processes), the current relaxation time is only an upper limit, with trelax likely being significantly shorter in the past.

Keywords: open clusters and associations: general; galaxies: individual: M51; galaxies: star clusters

Journal Article.  6275 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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