Evolution of the UV upturn in cluster galaxies: Abell 1689
  We have measured the strength of the UV upturn for red sequence galaxies in
the Abell~1689 cluster at $z=0.18$, reaching to or below the $L^*$ level and
therefore probing the general evolution of the upturn phenomenon. We find that
the range of UV upturn strengths in the population as a whole has not declined
over the past 2.2 Gyrs. This is consistent with a model where hot horizontal
branch stars, produced by a Helium-enriched population, provide the required UV
flux. Based on local counterparts, this interpretation of the result implies
Helium abundances of at least 1.5 times the primordial value for this HB
population, along with high formation and assembly redshifts for the galaxies
and at least a subset of their stellar populations.
