CT1 Photometry of Old Star Cluster Candidates in the Large Magellanic Cloud

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

San Juan

Congreso; Reunión Anual de la Asoc. Argentina de Astronomía; 1997

There are only a handful of known star clusters in the LMC that are genuinely old, i.e. of similar age to the globular star clusters in the Milky Way. Recently, Bica et al. (1996, ApJS, 102, 57) have uncovered a large number of old star cluster candidates in the LMC from integrated UBV photometry. There are also several clusters from the study of Olszewski et al. (1991, AJ, 101, 515) which have low metallicities based on calcium triplet spectroscopy and therefore also are good candidates for old LMC clusters. We have obtained Washington C,T1 photometry for nearly all of these candidates using the CTIO 0.9m telescope. Age estimates based on the magnitude difference δT1 between the giant branch clump and the turnoff, calibrated using standard clusters, revealed that no new old clusters were found (although we cannot rule out old age for 2 of the very crowded bar clusters). All of the other candidates turn out to be intermediate-age clusters. We discuss several reasons why integrated photometry can confuse intermediate-age clusters with genuine old clusters. We strongly confirm previous indications that there was no cluster formation in the LMC during the period from 3-8 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at about 1.5 Gyrs ago.