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Session 98 - Globular Clusters.
Oral session, Friday, January 09
In the Milky Way Galaxy, the ages of the globular clusters have played a key role in establishing the age of the Universe and testing galaxy formation scenarios. Similar measurements in other galaxies are needed, however, to establish that the properties of the Milky Way globular clusters are not unique. We present the results of WFPC2 observations obtained with the Hubble Space Telescope of five old LMC globular clusters: NGC 1754, NGC 1835, NGC 1898, NGC 2005, and NGC 2019. A sixth cluster, NGC 1916, was also observed, but is excluded from the present analysis because of the difficulty of handling its differential reddening. Through profile-fitting photometry of the cluster stars, we have produced V,(V-I) color-magnitude diagrams reaching \sim1.5 magnitudes below the main-sequence turnoff region. The ages of the clusters have been measured through a relative comparison with empirical Milky Way globular cluster fiducials having similar abundances and through fits to model isochrones. Using a comparison cluster of the appropriate abundance from the set M3, M5, M13, M55, and M92, we find that the LMC clusters have, on average, age differences of 1.0 \pm 0.6 Gyr compared to their Milky Way counterparts. Assuming that the distances and reddenings for the Milky Way comparison clusters are known, we use the relative offsets needed to match the LMC and Milky Way clusters to calculate distances and reddenings for the LMC clusters. We then find consistent model isochrone fits with ages of 13 Gyr for NGC 1754 and NGC 1898, 15 Gyr for NGC 2005 and NGC 2019, and 16 Gyr for NGC 1835, with internal errors of \sim \pm 1 Gyr. A free fit to the isochrones, allowing reddening, distance, and abundance to vary, yields ages of 14 \pm 3 Gyr for the LMC clusters, where the error in age reflects the uncertainty in the parameters and the difficulty in fitting the models.
Program listing for Friday