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 Hubble Space Telescope/Wide Field Camera 3 (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 Sloan Digital Sky Survey u, g photometry and find that the trend reverses for radii greater than or similar to 1' (3 pc) where the ratio of primordial to N-enhanced giants increases outward, 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 similar to 0.25 M-circle dot less massive than the primordial giants, which requires extreme He enhancement (Y greater than or similar to 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 less than or similar to 0.03 and thus a difference in turn-off mass of Delta M less than or similar to 0.04 M-circle dot 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