The 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud

Mathematics – Logic

Scientific paper

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Macho Project, Stellar Populations, Galactic Evolution

Scientific paper

The MACHO Project is a microlensing survey experiment that monitors the brightnesses of millions of stars nightly. The primary target for survey observations is the Large Magellanic Cloud, a small galaxy in orbit around the Milky Way. Every star monitored is represented by a time-series of two-color measurements in a massive photometry database. The MACHO Project database presently contains more photometric measurements than previously recorded in the history of astronomy. We describe the calibration of MACHO Project instrumental photometry data and derive transformations to the Kron-Cousins V and R standard system. This allows for proper comparison with all other observational and theoretical data on this system. The highest precision calibrations are for 9 million stars in the central, bar region of the Large Magellanic Cloud. We present a composite 9 million star color-magnitude diagram of the Large Magellanic Cloud bar. The 9 million star color-magnitude diagram reveals a complex superposition of different age and metallicity stellar populations. In order to probe these populations, We assemble a broad base of theoretical stellar evolution and stellar pulsation results, extant observational data for suitable template clusters in the Galaxy and Magellanic Clouds, and new samples of rare variable stars in the Large Magellanic Cloud. Clues to the formation and evolution of the Large Magellanic Cloud are recorded in the oldest stellar populations. We find evidence for significant numbers of old and metal-poor stars. This population is distributed in a classic exponential disk. We show this old disk population likely formed after the oldest clusters in the Large Magellanic Cloud, a sequence of events analogous to the Milky Way. We conclude the disk of the Large Magellanic Cloud formed contemporaneously with the Milky Way disk. Thus, different mass galactic disks may form at similar cosmological epochs, a new clue to the poorly understood physical processes governing the evolution of galaxies in the early Universe.

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