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D. R. Alves (STScI, UC Davis \& LLNL), C. Alcock, R. A. Allsman, T. S. Axelrod, A. Basu, A. C. Becker, D.P. Bennett, K. H. Cook, A.J. Drake, K. C. Freeman, M. Geha, K. Griest, L.J. King, M.J. Lehner, S.L. Marshall, D. Minniti, B.A. Peterson, P. Popowski, M.R. Pratt, P.J. Quinn, A.W. Rodgers, C.W. Stubbs, W. Sutherland, A. Tomaney, T. Vandehei, D.L. Welch (MACHO Collaboration), MACHO Collaboration
We present a 9 million star color-magnitude diagram (9M~CMD) of the Large Magellanic Cloud (LMC) bar. The 9M~CMD reveals a complex superposition of different age and metallicity stellar populations. Young LMC stellar populations are prominent in the 9M~CMD. Of these, the red and blue supergiants are potentially useful probes of the late stages of evolution in intermediate mass stars. Old LMC stellar populations are also evident in the 9M~CMD. These are used to reconstruct the evolution of the LMC during cosmologically interesting epochs. We first build a plausible model for the old LMC populations consistent with features observed in the 9M~CMD. We choose the 1.5 Gyr old cluster NGC~411 and the ancient globular cluster M3, with metal abundances of [Fe/H] = -0.7 and -1.5 dex respectively, as good representations of the giant branch and horizontal branch (HB) stars. The evolved asymptotic giant branch appears bimodal, which supports a model of two discrete older populations in the LMC field. We conclude the old populations in the LMC bar are likely a mix similar to NGC~411 and M3. Next, we infer the old and low metallicity LMC field population has a red HB morphology, which implies this population formed ~2 Gyr after the truly ancient LMC clusters formed. We find the surface density profile of this old LMC field population (traced by RRab variable stars) is exponential, favoring a disk-like rather than spheroidal distribution. We conclude the LMC disk formed ~10 Gyr ago, at the same time the Milky Way disk formed.