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4 Acta Astronomica

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The Optical Gravitational Lensing Experiment. Age of Star Clusters from the SMC

100%

Pietrzyński G. , Udalski A.

tom Vol. 49, No. 2

157--164

We present determination of age of clusters from 2.4 square degree region of the SMC bar. The photometric data were taken from the BVI maps of the SMC and catalog of clusters in this galaxy obtained during the OGLE-II microlensing survey. For 93 well populated SMC clusters their age is derived with the standard procedure of isochrone fitting. The distribution of age of cluster from the SMC is presented. It indicates either non-uniform process of cluster formation or very effective disruption of clusters.

The Optical Gravitational Lensing Experiment. Ages of about 600 Star Clusters from the LMC

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Pietrzyński G. , Udalski A.

tom Vol. 50, No. 3

337--354

We present results of age determination based on the standard procedure of isochrone fitting for about 600 star clusters younger than about 1.2 Gyr from the central parts of the LMC. Comparison of age distributions of star clusters from the LMC and the SMC shows that cluster formation histories are different in these galaxies. The age distribution of the LMC clusters reveals bursty nature of cluster formation in this galaxy, with contrast to the relatively uniform distribution of cluster ages in the SMC. We detected three extended peaks in the distribution of ages of LMC clusters, located at about 7 Myr, 125 Myr and 0.8 Gyr. All detected peaks have complex structure. While the structure of the youngest and the oldest peaks may be spurious due to accuracy of age determination, in the middle peak two evident sub-peaks at 100 Myr and 160 Myr are clearly seen. Similar peaks are seen in the distribution of ages of clusters from the SMC, which indicates that increased cluster formation ratio during these periods might be caused by the last encounter between the Magellanic Clouds. The sample of clusters from the LMC is overabundant in very young (<10 Myr) and old (>400 Myr) clusters, and underabundant in clusters having ages in the range from 80 to 15 Myr with respect to the studied sample of clusters from the SMC.

The Optical Gravitational Lensing Experiment. Multiple Cluster Candidates in the Large Magellanic Cloud

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Pietrzyński G. , Udalski A.

tom Vol. 50, No. 3

355--367

We present 100 multiple cluster candidates selected from the OGLE catalog of star clusters in the Large Magellanic Cloud. Statistical analysis shows that the significant fraction of these objects may constitute physical systems. Coeval ages of 102 components of multiple objects suggest their common origin. 53 components have very different ages. The comparison of the population of multiple clusters candidates from the SMC and the LMC shows that: a) distributions of sizes and ages of multiple and single clusters from the Magellanic Clouds are very similar, b) the difference of sizes of components of a given system is small, c) in both distributions of separation of multiple clusters from the LMC and SMC two peaks are seen at about 9 pc and 15 pc, d) both age distributions reveal peaks around 100 Myr, which may be connected with the last encounter of the LMC and the SMC.

The Optical Gravitational Lensing Experiment. Multiple Cluster Candidates in the Small Magellanic Cloud

100%

Pietrzyński G. , Udalski A.

tom Vol. 49, No. 2

165--169

We present the list of potential multiple star clusters from the central part of the SMC. Presented systems were selected from the catalog of star clusters from the SMC. We find 23 suspected cluster pairs and 4 triple systems. The statistical analysis suggests that many of them may constitute physical systems. Size, equatorial coordinates and age of presented clusters are given. Age of clusters which form five pairs and one triple system is coeval suggesting common origin of these objects.