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The Optical Gravitational Lensing Experiment. Catalog of Star Clusters from the Large Magellanic Cloud

We present the catalog of star clusters found in the area of about 5.8 square degree in the central regions of the Large Magellanic Cloud. It contains data for 745 clusters. 126 of them are new objects. For each cluster equatorial coordinates, radius, approximate number of members and cross-identification are provided. Photometric data for all clusters presented in the catalog and Atlas consisting of finding charts and color-magnitude diagrams are available electronically from the OGLE Internet archive.


Introduction
The Optical Gravitational Lensing Experiment (OGLE) is a long term observing project with the main goal to provide information on dark unseen matter using microlensing events (Paczyński 1986). Detailed description of the project can be found in Udalski, Kubiak and Szymański (1997).
The main observing targets of the second phase of the survey, OGLE-II, include large parts of the central bars of the SMC and LMC. Photometry collected in the standard BVI bands for millions of stars located in dense and poorly observed so far central regions of these galaxies provides an ideal material for many side projects.
One of the sub-projects of the OGLE-II survey aims at searching for and analyzing properties of star clusters in the Magellanic Clouds. In Pietrzyński et al. (1998, hereafter Paper I) the catalog of 273 clusters from the central parts of the SMC was published. In this paper we present the catalog of clusters from the observed regions in the LMC.
OGLE catalogs of star clusters enable many further detailed studies of properties of star clusters in the Magellanic Clouds. Results concerning the system of star clusters in the SMC were already published. Ages of 93 objects from the catalog of clusters from the SMC were derived using standard procedure of isochrone fitting by Pietrzyński and Udalski (1999a). The multiple cluster candidates were selected and listed in Pietrzyński and Udalski (1999b). Similar data for clusters from the LMC will be published in the forthcoming papers.
Large number observations collected in the course of the OGLE-II survey provides also an unique opportunity to explore populations of variable stars in star clusters. In the first paper of the series on variable stars located in the regions of star clusters 127 eclipsing systems in optical coincidence with star clusters in the SMC were presented by Pietrzyński and Udalski (1999c). The lists of Cepheids located in the close neighborhood of the Magellanic Cloud clusters were published by Pietrzyński and Udalski (1999d). Further papers containing lists as well as more detailed studies of variable stars from clusters of the Magellanic Clouds observed by the OGLE collaboration will follow.
Bearing in mind the potential usefulness of the photometric data of presented star clusters we have decided to make them publicly available. They are accessible from the OGLE Internet archive.

Previous Searches
Many efforts have been done for searching for star clusters in the LMC since Shapley and Mohr (1932) identified clusters in this galaxy for the first time. The large atlas of clusters covering most of the LMC was published by Hodge and Wright (1967). This catalog contains 1146 objects discovered until 1967, by Lynga and Westerlund (1963), Shapley and Lindsay (1963) and Hodge and Sexton (1966). Lamberts (1982) cataloged objects based on the ESO survey. This atlas provided coordinates for many star clusters, however majority of them were already known. Olszewski et al. (1988) found additional 156 clusters outside the area covered by the Hodge and Wright catalog. Hodge (1980Hodge ( , 1988 performed deep search for clusters in the LMC using 4-m telescope plates. He reported 387 new clusters in 15 small regions.

Observations
All observations presented in this paper were collected during the second phase of the OGLE microlensing survey with the 1.3 m Warsaw telescope at the Las Campanas Observatory, Chile, which is operated by the Carnegie Institution of Washington. The telescope was equipped with 2048 × 2048 CCD detector working in driftscan mode. The gain and readout noise were 3.8 e − /ADU and 5.4 e − , respectively. Details of the system can be found in Udalski, Kubiak and Szymański (1997).
Observations were conducted in 26 slightly overlapping fields with the size of about 14. ′ 2 × 57 ′ each, which gave the total coverage of about 5.8 square degree. Table 1 lists acronyms of the observed fields with equato-T a b l e 1 rial coordinates of their centers. Collected images were reduced with the standard OGLE data pipeline. Accuracy of transformations to the standard system was 0.01 − 0.02 mag. For more details about data reduction and transformation procedures the reader is referred to the paper with description of BVI photometric maps of the SMC . Quality of the data collected for the LMC is similar to that from the SMC. BVI maps of the LMC will be released in the near future (Udalski et al. in preparation).

Search for Clusters
Visual searches are subjective and, especially in crowded stellar background may lead to many spurious detections. The observed area in the LMC is relatively dense and large. In order to obtain the objective list of cluster candidates we performed an automatic, algorithmic search. Similar technique as presented in Zaritsky, Harris and Thompson (1997) was applied. This algorithm was already successfully used by us in searching for clusters in the SMC (Paper I). Detailed description of this algorithm can be found in Paper I. In short, each of 26 driftscans was divided into square boxes and stars were counted inside each of them. Because of the different size of the potential clusters, three sets of such density maps with boxes of 10 × 10, 20 × 20 and 30 × 30 pixels (4.1 × 4.1, 8.2 × 8.2, 12.3 × 12.3, arcsec respectively) were constructed. In order to remove background variations from our density maps the "unsharp masking" procedure was used. The suspected clusters were selected as the concentration of at least four pixels above a given threshold from the unsharp masked images. Three different detection thresholds of 4, 3 and 2σ of all pixels of the map were used. Then, all candidates were carefully examined and many of them were rejected due to proximity to bright overexposed stars or the edge of the frame. Our procedure resulted in detection of 745 objects.
The catalog was compared with the list of clusters presented by Bica et al. (1999). 619 objects turned out to be common to both these catalogs. Table 2 contains description of acronyms used in our catalog.

Coordinates and Angular Sizes of Clusters
The equatorial coordinates of clusters were obtained in the identical manner as in Paper I. Their accuracy depends on cluster richness and ranges from 2 arcsec for compact populous clusters to 10 arcsec for loose faint ones. Angular sizes of clusters were derived based on density profiles obtained from star counts performed in consecutive annuli around their adopted centers. In most cases counts obtained in relatively large distance from the cluster centers allowed to reliably define stellar background and derive precise dimensions. Many profiles show, however, significant fluctuations after the main drop of the stellar density. Such a behavior may be caused by presence of extended halos around clusters or background density fluctuations. As in the case of Paper I we decided to define two different kinds of cluster radii. One accounts for the presence of the above-mentioned fluctuations and represents the radius of entire cluster. The second one is defined as the distance from the cluster center to the main drop of the stellar density and it is useful for defining the boundary between the regions where the cluster or field stars are more frequent. and four digit number. In columns 2 and 3 we list the equatorial coordinates of cluster center and in column 4 the OGLE name of the field in which a given cluster was detected. Columns 5, 6, 7 and 8 provide radius, core radius, crude number of cluster members and cross-identification (see Table 2 for explanation of acronyms used in column 8). Remarks given in column 1 have the following meaning : (1) -cluster contains bright star, (2) -object is located close to the edge of the frame or bad columns. Crude number of members was calculated by subtraction of the mean stellar background from the number of stars counted in the radius of the cluster. Stellar background was averaged from independent counts in four regions around the cluster. While for the more populous clusters this number quite reasonably approximate number of stars in the cluster, its meaning is "several" when it is below a dozen or so (typically for small clusters located in high stellar background regions).

Content of the Catalog
Appendix presents a few pages of the Atlas of star clusters from the LMC. It consists of the finding chart and color-magnitude diagrams (CMD) of each cluster. Presented CMDs were not cleaned for field stars so in the case of clusters located in dense stellar regions and objects with small number of stars they can be contaminated by field stars. One can perform subtraction of field stars from cluster CMDs when the OGLE photometric maps of the LMC are released (Udalski et al. in preparation). Full version of Atlas is available electronically from the OGLE Internet archive.

Summary
We present the catalog of clusters found in the 5.8 square degrees area of the LMC, based on the BVI observations collected in the course of the OGLE-II microlensing project. The automatic, algorithmic procedure similar to that used in searching for clusters in the SMC (Paper I) resulted in detection of 745 objects. 126 of them are the new ones. For all of them the equatorial coordinates, radius, approximate number of members and cross-identification with previous catalogs are provided.