Ages and Metallicities of LMC Clusters and Their Surrounding Fields

GEISLER, DOUG; BICA, EDUARDO; DOTTORI, HORACIO; CLARIA OLMEDO, JUAN JOSÉ; PIATTI, ANDRÉS E.; SANTOS JR, JOÂO F.C.

Proc. IAU Symp. 190

ASP Conf. Series

Lugar: San Francisco; Año: 1999 p. 431 - 433

We report the results of a CMD survey of 25 candidate old LMC clusters uncovered by previous investigations. Washington system C,T1 photometry was obtained on the CTIO 0.9m. For almost all of the sample, we reached the turnoff in a total of <1h of integration time. Ages based on the magnitude difference delta T1 between the giant branch clump and the turnoff revealed that no new old clusters were found. The candidates turned out to be of intermediate age (1-3 Gyr). We also study the age distribution of intermediate age and old clusters using both our data and CMDs from the literature. We confirm previous indications that there was apparently no cluster formation in the LMC during the period from 3-9 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at ~ 1.5 Gyrs ago. Our results suggest that there are few, if any, genuine old clusters in the LMC left to be found. We also determine ages for the surrounding fields, as well as metallicities for both the clusters and fields from the color of the giant branch compared to fiducial clusters. In most cases the stellar population of each cluster is quite similar to that of the field where it is embedded. In the northern part of the LMC disk the fields of SL388 and SL509 have a secondary clump ~ 0.45 mag fainter than the dominant clump, suggesting a component located behind the LMC at a distance comparable to that of the SMC. The mean metallicity for the intermediate age outer disk clusters is somewhat lower than found by Olszewski et al. (1991, AJ, 101, 515) for a sample of clusters of similar age, but is in good agreement with several recent studies. A few clusters stand out in the age--metallicity relation in the sense that they are intermediate age clusters at relatively low metallicity ([Fe/H] ~ -1).