Abstract
(V, V - I) color-magnitude diagrams (CMDs) are presented for the four nearby globular clusters M3, M13, M5, and M92. These clusters span the range -1.3 > [Fe/H] > -2.3, and the new CMDs are intended for use as templates against which to compare the outer halo clusters observed with Hubble Space Telescope F555W and F814W filters. The calibration to the system of Landolt appears to be secure to 1%–2%. Comparing M3 with M13, it seems unlikely that the clusters differ in age by the 2–3 Gyr required to explain the difference in their horizontal-branch (HB) morphology purely as an age effect. If the HB difference is to be attributed to a single agent, we propose that a difference in Y of ~0.05 (with M13 having the larger Y) would most economically explain the differences in the main-sequence turnoff and HB morphology between the clusters.
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