Direct Evidence for an Enhancement of Helium in Giant Stars in Omega
  Centauri
  The double main sequence identified in the globular cluster Omega Centauri
has been interpreted using isochrones to indicate a large variation in the
abundance of helium. If true, a helium enhancement carries strong implications
for the chemical and stellar evolutionary history of this cluster. However,
only indirect measures currently support this conjecture. We report the
discovery of a variation in the line strength of the near-infrared He I 10830 A
transition in twelve similar red giants in Omega Centauri observed with PHOENIX
on Gemini-S. Abundances of these stars derived from Magellan/MIKE spectra taken
at Las Campanas Observatory show that the helium transition is not detected in
the most metal-poor population ([Fe/H] < -1.8), yet is present in the majority
of stars with [Fe/H] >= -1.8. These observations give the first direct evidence
for an enhancement of helium in Omega Centauri. The appearance of helium
appears better correlated with increased [Al/Fe] and [Na/Fe] abundances than as
a function of [Fe/H], giving observational support to the presence of
high-temperature H-burning in a prior generation of stars.
