Omega Centauri: weak MgH-band in red giants directly trace the helium
  content
  High spectral resolution and high signal-to-noise ratio optical spectra of
red giants in the globular cluster Omega Centauri are analysed for stellar
parameters and chemical abundances of 15 elements including helium by either
line equivalent widths or synthetic spectrum analyses. The simultaneous
abundance analysis of MgH and Mg lines adopting theoretical photospheres and a
combination of He/H$-$ratios proved to be the only powerful probe to evaluate
helium abundances of red giants cooler than 4400 K, wherein otherwise helium
line transitions (He {\scs I} 10830 and 5876 \AA) present for a direct spectral
line analysis. The impact of helium-enhanced model photospheres on the
resulting abundance ratios are smaller than 0.15 dex, in agreement with past
studies. The first indirect spectroscopic helium abundances measured in this
paper for the most metal-rich cluster members reveal the discovery of seven
He-enhanced giants ($\Delta$$Y=+$0.15$\pm$0.04), the largest such sample found
spectroscopically to date. The average metallicity of $-$0.79$\pm$0.06 dex and
abundances for O, Na, Al, Si, Ca, Ti, Ni, Ba, and La are consistent with values
found for the red giant branch (RGB-a) and subgiant branch (SGB-a) populations
of Omega Centauri, suggesting an evolutionary connection among samples. The
He-enhancement in giants is associated with larger $s$-process elemental
abundances, which correlate with Al and anticorrelate with O. These results
support the formation of He-enhanced, metal-rich population of Omega Centauri
out of the interstellar medium enriched with the ejecta of fast rotating
massive stars, binaries exploding as supernovae and asymptotic giant branch
(AGB) stars.
