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19 91Apjs. . .76. .185E the Astrophysical Journal Supplement The Astrophysical Journal Supplement Series, 76:185-214, 1991 May .185E © 1991. The American Astronomical Society. All rights reserved. Printed in U.S.A. .76. 91ApJS. THE STRUCTURE AND EVOLUTION OF RICH STAR CLUSTERS IN THE LARGE MAGELLANIC CLOUD 19 Rebecca A. W. Elson Bunting Institute, Radcliffe College; and Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Received 1990 March 19; accepted 1990 September 14 ABSTRACT Surface brightness profiles and color-magnitude diagrams are presented for 18 rich star clusters in the Large Magellanic Cloud (LMC), with ages ~ 107-109 yr. The profiles of the older clusters are well represented by models with a King-like core. The profiles of many of the younger clusters show departures from such models in the form of bumps, sharp “shoulders,” and central dips. These features persist in profiles derived from images from which the bright stars have been subtracted; they therefore appear to reflect real substructure within the clusters. There is an upper limit to the radii of the cluster cores, and this upper limit increases with age from ^ 1 pc for the youngest clusters, to ^6 pc for the oldest ones. This trend probably reflects expansion of the cores driven by mass loss from evolving stars. Recent models of cluster evolution predict that the cores should expand at a rate that depends on the slope of the initial mass function ( IMF). In the context of these models, the data favor an IMF for most of the clusters with a slope slightly flatter than the Salpeter value (for the range of stellar masses 0.4-14M©), but with significant cluster-to-cluster variations. If the clusters have undergone violent relaxation, then the small cores of the youngest ones may imply formation from relatively “cool” initial conditions, while the sharp shoulders favor “warmer” conditions. An alternative interpretation of the shoulders is that they are signa- tures of merging subcondensations. It seems likely that the high ellipticity observed in many of the young LMC clusters is due, at least in part, to the presence of these subcondensations. Such substructure will be erased as the clusters evolve, and this might account for the general rounder appearance of the older clusters. Finally, since the younger clusters are not relaxed through two-body encounters, their current structure should provide a good guide for selecting initial conditions for theoretical models of globular cluster evolution. Subject headings: clusters: open — galaxies: Magellanic Clouds — photometry — stars: stellar statistics 1. INTRODUCTION project, which complements that of EFF, was to investigate Gravitational collapse in the cores of globular clusters, a whether a relation between core radius and age was evident in a phenomenon expected to occur during the later stages of clus- larger sample of clusters with more accurate measurements of ter evolution, has provoked considerable interest over the past core radii. A synopsis of the results was given by Elson, Free- decade (see Elson, Hut, & Inagaki 1987 for a review). Several man, & Lauer ( 1989 ) : the trend of increasing core radius with surveys of globular clusters in the Galaxy, and of the older age did indeed persist. Furthermore, recent models of cluster clusters in the Magellanic Clouds, have been undertaken to evolution by Chemoff & Weinberg ( 1990) predict just such determine the proportion and properties of clusters with “col- core expansion due to mass loss from stellar evolution. lapsed” cores (Djorgovski & King 1986; Mateo 1987; Meylan This paper presents the full set of data on which the results of & Djorgovski 1987). Relatively little has been done to explore Elson et al. are based, including color-magnitude diagrams and the processes through which a core is initially established in a surface brightness profiles for the 18 clusters in their sample. cluster, and the evolution of that core prior to collapse. Such The observations are described in § 2, and the results are dis- studies could contribute to our understanding of the evolution cussed in § 3, and summarized in § 4. of A-body systems, the formation of globular clusters, and per- haps the conditions prevailing in the halos of galaxies at the 2. OBSERVATIONS epoch when globular clusters formed. CCD images of 18 of the richest clusters in the LMC with Elson, Fall, & Freeman ( 1987, hereafter EFF) determined ages ~107-109 yr, were obtained to provide a sequence of surface brightness profiles for ten rich young clusters in the “snapshots,” to investigate the evolution of the inner parts of Large Magellanic Cloud (LMC) using star counts from photo- these globular-like clusters. (The estimated masses of the clus- 4 5 graphic plates. They found that the outer parts of the clusters ters are ^10 -10 Mo;EFF).The clusters are listed in Table 1. showed little or no tidal truncation and had surface brightness Images in the B and V passbands were obtained on 1988 Jan- p(r) which varied as /¿(r) oc r~T, with 7 æ 2.6. The youngest uary 12-14, using the 1 m telescope at Siding Spring Observa- clusters in their sample had the smallest cores while the older tory, and the MSSSO coated GEC chip No. 2; the Limages are clusters had larger ones. There were too few clusters, and the shown in Figures l<2-ls (Pis. 26-35). At F/8 the image scale core radii were too crude, to determine whether this repre- was 0?56 pixel-1. Integration times were short to avoid satura- sented a real trend, but the possibility that clusters are born tion (200 s in F and 400 s in 2?), and limiting magnitudes were with very small cores which then go through a phase of expan- B ^ Væ 18. Seeing ranged from ~2"-5" which, while poor, sion was intriguing. The primary motivation for the present was adequate for determining cluster profiles. Preliminary re- 185 © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E PLATE 26 .76. 91ApJS. 19 © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E PLATE 27 .76. 91ApJS. 19 o Ûh \c Fig. Elson (see 76, 185) © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E PLATE 28 .76. 91ApJS. 19 1/ Fig. le - FlG Elson (see 76, 185) © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E PLATE 29 .76. 91ApJS. 19 Elson (see 76, 185) © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E PLATE 30 .76. 91ApJS. 19 1, . G n \h Fra. Elson (see 76, 185) © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E .76. 91ApJS. 19 Ik Fig. Elson (see 76, 185) © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E PLATE 32 .76. 91ApJS. 19 S o £ © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E PLATE 33 .76. 91ApJS. 19 \o Fig. \n Fig. Elson (see 76, 185) © American Astronomical Society • Provided by the NASA Astrophysics Data System 91ApJS. 19 IG p \p FKj. Elson (see 76, 185) © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E PLATE 35 .76. 91ApJS. 19 Is Fig. Elson (see 76, 185) © American Astronomical Society • Provided by the NASA Astrophysics Data System .185E 186 ELSON Vol. 76 TABLE 1 .76. Cluster Parameters NGC Number E(B- V) Age rJ,V) rJ<B) FWHM(F) FWHM(B) rÁV) (4) (10) 91ApJS. (1) (2) (3) (5) (6) (7) (8) (9) 19 NGC 1711 0.16 7.4 16.5 6.7 ± 0.5 7.4 ± 0.5 2.2 3.1 5.7 6.2 NGC 1755 0.12 7.5 17.0 7.1 ±0.6 7.2 ± 0.5 3.2 3.5 5.8 5.9 NGC 1818 0.10 7.3 16.5 9.4 ± 1.4 9.5 ± 1.3 3.5 3.9 7.9 7.9 NGC 1831 0.10 (8.5) 18.5 15.7 ±0.9 15.1 ±0.5 3.3 3.7 14.2 13.4 NGC 1850 0.15 7.5 16.5 10.2 ± 0.7 10.4 ± 0.5 3.1 3.8 8.8 8.8 NGC 1855 0.12 7.5 16.5 10.7 ± 0.9 11.3 ±0.9 4.1 5.1 9.0 9.3 NGC 1866 0.10 (8.1) 17.0 13.6 ±0.4 14.7 ± 0.4 4.4 5.0 11.7 12.5 NGC 1868 0.07 (8.7) 19.5 6.0 ± 0.3 6.4 ± 0.3 2.0 2.3 5.1 5.3 NGC 1872 0.13 7.6 17.5 >5.6 >6.1 2.0 2.4 >4.7 >5.1 NGC 2002 0.12 7.2 16.5 3.5 ± 0.4 6.0 ± 1.5 3.9 4.2 ^3.5 <6.0 NGC 2004 0.06 7.3 16.0 5.6 ± 0.7 6.1 ±0.7 3.7 3.6 4.3 4.7 NGC 2100 0.24 7.2 16.0 8.2 ± 0.7 8.4 ± 0.6 3.9 4.5 6.7 6.7 NGC 2156 0.10 7.6 18.0 7.1 ±2.4 5.9 ± 0.5 1.8 2.1 6.4 4.9 NGC 2157 0.10 7.6 17.5 9.6 ± 1.3 9.2 ± 0.8 2.2 2.9 8.6 7.9 NGC 2159 0.10 7.6 18.0 8.1 ± 1.0 8.6 ± 1.1 2.0 2.3 7.2 7.6 NGC 2164 0.10 7.7 17.0 7.4 ± 0.4 7.9 ± 0.5 4.6 5.4 5.7 5.9 NGC 2172 0.10 7.6 18.0 10.0 ± 1.4 10.4 ± 1.4 5.3 5.7 8.0 8.3 NGC 2214 0.10 7.6 17.5 10.5 ± 0.7 10.8 ± 0.7 5.3 5.7 8.5 8.6 Notes.—Col.
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