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Session 115 - Star Clusters & Dwarf Galaxies.
Oral session, Saturday, January 10
Jefferson,
We present a composite 9 million star color-magnitude diagram (9M CMD) of the LMC bar using calibrated MACHO project photometry. The large number of stars reveal numerous low-level features, many of which can be attributed to short-lived stages of stellar evolution. The star formation and chemical enrichment history of the LMC is investigated, with additional discussion of the \sim10^5 LMC variable stars in the MACHO database. Example features are intermediate mass supergiant sequences, multiple sequences of AGB stars, a bump on the RGB, and possible evidence for extended HB evolution. The 9M CMD is interpreted with theoretical isochrones, CMDs of LMC clusters, an HST CMD, and artificial CMDs. The latter include the effects of differential reddening and blending. The intermediate mass supergiants, including Cepheids, suggest a variable recent star formation history. The RGB and multiple AGB sequences are consistent with two superposed populations; an ancient population with [Fe/H] \approx -1.6 dex, and a few Gyr-old population with [Fe/H] \approx -0.7 dex. The location of the bump on the RGB constrains the age and metallicity of the younger population; no other RGB bump is observed. The possibility of an intervening stellar population in the Galactic halo is probed with the 9M CMD and archival data. Galactic star count model predictions agree well with foreground stars observed in the 9M CMD. We predict the planetary nebulae and carbon stars associated with an hypothetical intervening population. Lastly, we discuss the 9M CMD and MACHO microlensing detection efficiencies.
Alves dedicates this paper to Alex Rodgers, who greatly influenced this work and will be fondly remembered.
