Nucleosynthesis in helium-enriched asymptotic giant branch models:
  Implications for Heavy Element Enrichment in Omega Centauri
  We investigate the effect of helium enrichment on the evolution and
nucleosynthesis of low-mass asymptotic giant branch (AGB) stars of 1.7Msun and
2.36Msun with a metallicity of Z=0.0006 ([Fe/H] = -1.4). We calculate
evolutionary sequences with the primordial helium abundance (Y = 0.24) and with
helium-enriched compositions (Y = 0.30, 0.35, 0.40). For comparison we
calculate models of the same mass but at a lower metallicity Z=0.0003 ([Fe/H] =
-1.8) with Y=0.24. Post-processing nucleosynthesis calculations are performed
on each of the evolutionary sequences to determine the production of elements
from hydrogen through to bismuth. Elemental surface abundance predictions and
stellar yields are presented for each model. The models with enriched helium
have shorter main sequence and AGB lifetimes, and enter the AGB with a more
massive hydrogen exhausted core than the primordial helium model. The main
consequences are 1) low-mass AGB models with enhanced helium will evolve more
than twice as fast, giving them the chance to contribute sooner to the chemical
evolution of the forming globular clusters, and 2) the stellar yields will be
strongly reduced relative to their primordial helium counterparts. An increase
of Delta Y = 0.10 at a given mass decreases the yields of carbon by up to ~60%,
of fluorine by up to 80%, and decreases the yields of the s-process elements
barium and lanthanum by about 45%. While the yields of first s-process peak
elements strontium, yttrium and zirconium decrease by up to 50%, the yields of
rubidium either do not change or increase.
