Chemical Inhomogeneities Amongst First Population Stars in Globular
  Clusters: Evidence for He variations
  Spreads in light element abundances among stars (a.k.a. multiple populations)
are observed in nearly all globular clusters. One way to map such chemical
variations using high-precision photometry is to employ a suitable combination
of stellar magnitudes in the F275W, F336W, F438W, and F814W filters (the so
called "chromosome map"), to maximise the separation between the different
multiple populations. For each individual cluster its chromosome map separates
the so-called first population -with metal abundance patterns typical of field
halo stars- from the second population, that displays distinctive abundance
variations among a specific group of light-elements. Surprisingly, the
distribution of first population stars in chromosome maps of several -but not
all- clusters has been found to be more extended than expected from purely
observational errors, suggesting a chemically inhomogeneous origin. We consider
here three clusters with similar metallicity ([Fe/H]~-1.3) and different
chromosome maps, namely NGC 288, M 3 and NGC 2808, and argue that the first
population extended distribution (as observed in two of these clusters) is due
to spreads of the initial helium abundance and possibly a small range of
nitrogen abundances as well. The presence of a range of initial He and N
abundances amongst stars traditionally thought to have homogeneous composition,
plus the fact that these spreads appear only in some clusters, challenge the
scenarios put forward so far to explain the multiple population phenomenon.
