Radial distributions of sub-populations in the globular cluster M15: a
  more centrally concentrated primordial population
  We examine the radial distributions of stellar populations in the globular
cluster (GC) M15, using HST/WFC3 photometry of red giants in the
nitrogen-sensitive F343N-F555W color. Surprisingly, we find that giants with
"primordial" composition (i.e., N abundances similar to those in field stars)
are the most centrally concentrated within the WFC3 field. We then combine our
WFC3 data with SDSS u, g photometry and find that the trend reverses for radii
>1' (3 pc) where the ratio of primordial to N-enhanced giants increases
outwards, as already found by Lardo et al. The ratio of primordial to enriched
stars thus has a U-shaped dependency on radius with a minimum near the
half-light radius. N-body simulations show that mass segregation might produce
a trend resembling the observed one, but only if the N-enhanced giants are
~0.25 Mo less massive than the primordial giants, which requires extreme He
enhancement (Y~0.40). However, such a large difference in Y is incompatible
with the negligible optical color differences between primordial and enriched
giants which suggest Delta Y < 0.03 and thus a difference in turn-off mass of
Delta M < 0.04 Mo between the different populations. The radial trends in M15
are thus unlikely to be of dynamical origin and presumably reflect initial
conditions, a result that challenges all current GC formation scenarios. We
note that population gradients in the central regions of GCs remain poorly
investigated and may show a more diverse behavior than hitherto thought.
