Constraints on Helium Enhancement in the Globular Cluster M3 (NGC 5272):
  The Horizontal Branch Test
  It has recently been suggested that the presence of multiple populations
showing various amounts of helium enhancement is the rule, rather than the
exception, among globular star clusters. An important prediction of this helium
enhancement scenario is that the helium-enhanced blue horizontal branch (HB)
stars should be brighter than the red HB stars which are not helium-enhanced.
In this Letter, we test this prediction in the case of the Galactic globular
cluster M3 (NGC 5272), for which the helium-enhancement scenario predicts
helium enhancements of > 0.02 in virtually all blue HB stars. Using
high-precision Stroemgren photometry and spectroscopic gravities for blue HB
stars, we find that any helium enhancement among most of the cluster's blue HB
stars is very likely less than 0.01, thus ruling out the much higher helium
enhancements that have been proposed in the literature.
