HST COLOR-MAGNITUDE DIAGRAMS OF 6 OLD GLOBULAR-CLUSTERS IN THE LMC

We report on HST observations of six candidate old globular clusters i n the Large Magellanic Cloud (LMC): NGC 1754, 1835, 1898, 1916, 2005 a nd 2019. Deep exposures with the F555W and F814W filters provide us wi th colour-magnitude diagrams that reach to an apparent magnitude in V of similar to 25, well below the main-sequence turn-off. These particu lar clusters are associated with significantly high LMC field star den sities and care was taken to subtract the field stars from the cluster colour-magnitude diagrams accurately. In two cases there is significa nt variable reddening across at least part of the image, but only for NGC 1916 does the differential reddening preclude accurate measurement s of the CMD characteristics. The morphologies of the colour-magnitude diagrams match well those of Galactic globular clusters of similar me tallicity. All sir; have well-developed horizontal branches, while fou r clearly have stars on both sides of the RR Lyrae gap. The abundances obtained from measurements of the height of the red giant branch abov e the level of the horizontal branch are 0.3 dex higher, on average, t han previously measured spectroscopic abundances. Detailed comparisons with Galactic globular cluster fiducials show that all six clusters a re old objects, very similar in age to classical Galactic globulars su ch as M5, with little age spread among the clusters. This result is co nsistent with ages derived by measuring the magnitude difference betwe en the horizontal branch and main-sequence turn-off. We also find a si milar chronology by comparing the horizontal branch morphologies and a bundances with the horizontal branch evolutionary tracks of Lee, Demar que & Zinn. Our results imply that the LMC formed at the same time as the Milky Way Galaxy.