The effect of helium-enhanced stellar populations on the
  ultraviolet-upturn phenomenon of early-type galaxies
  Recent observations and modeling of globular clusters with multiple
populations strongly indicate the presence of super helium-rich subpopulations
in old stellar systems. Motivated by this, we have constructed new population
synthesis models with and without helium-enhanced subpopulations to investigate
their impact on the UV-upturn phenomenon of quiescent early-type galaxies. We
find that our models with helium- enhanced subpopulations can naturally
reproduce the strong UV-upturns observed in giant elliptical galaxies assuming
an age similar to that of old globular clusters in the Milky Way. The major
source of far-UV (FUV) flux, in this model, is relatively metal- poor and
helium-enhanced hot horizontal branch stars and their progeny. The Burstein et
al. (1988) relation of the F U V - V color with metallicity is also explained
either by the variation of the fraction of helium-enhanced subpopulations or by
the spread in mean age of stellar populations in early-type galaxies.
