Abstract

We present Washington system CT1 color-magnitude diagrams of 13 star clusters and their surrounding fields that lie in the outer parts of the LMC disk (r > 4 degrees), as well as a comparison inner cluster. The total area covered is large (2/3 deg(2)), allowing us to study the clusters and their fields individually and in the context of the entire Galaxy. Ages are determined by means of the magnitude difference delta T-1 between the giant branch clump and the turnoff, while metallicities are derived from the location of the giant and subgiant branches as compared with fiducial star clusters. This yields a unique data set in which ages and metallicities for both a significant sample of dusters and their fields are determined homogeneously. We find that in most cases the stellar population of each star cluster is quite similar to that of the held where it is embedded, sharing its mean age and metallicity. The old population (t greater than or equal to 10 Gyr) is detected in most fields as a small concentration of stars on the horizontal branch blueward and faintward of the prominent clump. Three particular fields present remarkable properties: (1) The thus-far unique cluster ESO 121-SC03 at approximate to 9 Gyr has a surrounding held that shares the same properties (which, in turn, is also unique, in that such a dominant old-field component is not present elsewhere-at least not significantly in the fields as yet studied). (2) The field surrounding the far eastern intermediate-age cluster OHSC 37 is noteworthy in that we do not detect any evidence of LMC stars: it is essentially a Galactic foreground field. We can thus detect the LMC held out to greater than 11 degrees (the deprojected distance of ESO 121-SC03), or similar to 11 kpc, but not to 13 degrees (similar to 13 kpc), despite the presence of clusters at this distance. (3) In the northern part of the LMC disk, the fields of SL 388 and SL 509 present color-magnitude diagrams with a secondary clump approximate to 0.45 mag fainter than the dominant intermediate-age clump, suggesting a stellar population component located behind the LMC disk at a distance comparable to that of the SMC. Possibly we are witnessing a depth effect in the LMC, and the size of the corresponding structure is comparable to the size of a dwarf galaxy. The unusual spatial location of the cluster OHSC 37 and the anomalous properties of the SL 388 and SL 509 fields might be explained as debris from previous LMC interactions with the Galaxy and/or the SMC. The mean metallicity derived for the intermediate-age outer disk clusters is [[Fe/H]] = -0.66, and for their surrounding fields [[Fe/H]] = -0.56. These values are significantly lower than those found by Olszewski et al. for a sample of clusters of similar age but are in good agreement with several recent studies. A few clusters stand out in the age-metallicity relation, in that they are intermediate-age clusters at relatively low metallicity [[Fe/H] approximate to - 1).