Red Clump Morphology as Evidence Against an Intervening Stellar Population as the Primary Source of Microlensing Toward the LMC

Submitted to: Astron. J. Abstract: We examine the morphology of the color-magnitude diagram (CMD) for core helium-burning (red clump) stars to test the recent suggestion by Zaritsky & Lin (1997) that an extension of the red clump in the Large Magellanic Cloud (LMC) toward brighter magnitudes is due to an intervening population of stars that is responsible for a significant fraction of the observed microlensing toward the LMC. Using our own high resolution CCD photometry of several fields across the LMC, we confirm the presence of this additional red clump feature, but conclude that it is caused by stellar evolution rather than a foreground population. We do this by demonstrating that the feature (1) is present in all our LMC fields and in some fields extends to slightly fainter as well as brighter magnitudes compared to the primary clump, (2) is in precise agreement with the location of the blue loops in the isochrones of intermediate age red clump stars with the metallicity and age of the LMC, and (3) is present in the Hipparcos CMD for the solar neighborhood where an intervening population cannot be invoked. Finally, we show that current understanding of stellar evolution and LMC star formation history is consistent with the features seen not only in our data, but also the LMC CMD of Zaritsky & Lin.