Bayesian Analysis of Two Stellar Populations in Galactic Globular
  Clusters III: Analysis of 30 Clusters
  We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival
ACS Treasury observations of 30 Galactic Globular Clusters to characterize two
distinct stellar populations. A sophisticated Bayesian technique is employed to
simultaneously sample the joint posterior distribution of age, distance, and
extinction for each cluster, as well as unique helium values for two
populations within each cluster and the relative proportion of those
populations. We find the helium differences among the two populations in the
clusters fall in the range of ~0.04 to 0.11. Because adequate models varying in
CNO are not presently available, we view these spreads as upper limits and
present them with statistical rather than observational uncertainties. Evidence
supports previous studies suggesting an increase in helium content concurrent
with increasing mass of the cluster and also find that the proportion of the
first population of stars increases with mass as well. Our results are examined
in the context of proposed globular cluster formation scenarios. Additionally,
we leverage our Bayesian technique to shed light on inconsistencies between the
theoretical models and the observed data.
