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

Abstract

We present a 9 million star color-magnitude diagram (9M CMD) of the LMC bar. The 9M CMD reveals a complex superposition of different age and metallicity stellar populations, with important stellar evolutionary phases occurring over 3 orders of magnitude in number density. First, we count the non-variable supergiants, the associated Cepheids, and measure the effective temperatures defining the instability strip. Lifetime predictions of stellar evolution theory are tested, with implications for the origin of low-luminosity Cepheids. The highly-evolved AGB stars have a bimodal distribution in brightness, which we interpret as discrete old populations (>1 Gyr). The faint AGB may be metal-poor and very old. We identify the clusters NGC 411 and M3 as templates for the admixture of old stellar populations. However, there are indications that the old and metal-poor field population has a red HB morphology: the RR Lyraes lie on the red edge of the instability strip, the AGB-bump is very red, and the ratio of AGB-bump stars to RR Lyraes is quite large. If the HB second parameter is age, the old and metal-poor field population likely formed after the oldest clusters. Lifetime predictions of stellar evolution theory lead us to associate a significant fraction of the red HB clump giants with the same old and metal-poor population producing the RR Lyraes and the AGB-bump. In this case, compared to the age-dependent luminosity predictions of stellar evolution theory, the red HB clump is too bright relative to the RR Lyraes and AGB-bump. Last, the surface density profile of RR Lyraes is fit by an exponential, favoring a disk-like rather than spheroidal distribution. We conclude that the age of the LMC disk is probably similar to the age of the Galactic disk. (ABRIDGED)

to appear in the Astronomical Journal, 49 pages, 12 figures, aaspp4.sty

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