Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989a%26a...211....9b&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 211, no. 1, Feb. 1989, p. 9-24.
Astronomy and Astrophysics
Astrophysics
18
Astrometry, Magellanic Clouds, Star Clusters, Astronomical Photography, Automation, Ellipticity, Image Analysis, Open Clusters, Photographic Plates, Schmidt Telescopes
Scientific paper
We have developed a technique for automatically identifying clusters of stars in the Magellanic Clouds from the use of objective, machine-produced data. The method is based on scans made with the COSMOS high-speed, photographic-plate scanning machine at the Royal Observatory, Edinburgh on plates taken with the UK 1.2 m Schmidt Telescope. We estimate that our technique is nearly 99 complete down to an angular size limit of ˜0.'36 (corresponding to ˜5 pc at the distance to the Magellanic Clouds). The method can be extended down to ˜4 pc, although at this level the incompleteness becomes more problematic.
In this paper, the technique is applied to a 6 square degree region towards the North-Eastern end of the bar of the Large Magellanic Cloud (LMC), resulting in the detection of 284 clusters down to the above size limit of 4 pc. Our study has quadrupled the number of clusters recorded in this region of the LMC.
The size distribution for the clusters identified is asymmetric with a mode of 8 pc and mean of 11 pc. We demonstrate that although the LMC clusters are systematically larger than the Galactic open clusters the form of the distribution of sizes is similar, suggesting that cluster formation mechanisms are similar in the two galaxies. The scale shift in cluster sizes is most probably a result of the larger Jean's mass in the LMC, although underestimation of the sizes of Galactic clusters cannot, at this time, be ruled out.
The LMC clusters have a larger mean ellipticity than Galactic clusters, in agreement with the results of other authors. We find no strong difference in ellipticity between large and small clusters, although there is some indication that the major clusters (those with diameters >20 pc) possess a lower mean value for ellipticity than their smaller counterparts.
Evidence is found for an anti-correlation between the direction of the major axis orientations of the clusters and the major axis of the bar of the LMC, although the effect is weaker for the larger clusters. We also find correlations in the orientations of nearest neighbouring clusters, in the sense that nearest-neighbours show a tendency to be aligned with their major axes parallel. Observational arguments are presented suggesting that this latter alignment effect is more likely to be due to the fragmentation of neighbouring clusters from common initial gas clouds than to tidal interaction.
Bhatia Rajiv K.
MacGillivray Harvey T.
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