The Hubble Space Telescope UV Legacy Survey of Galactic Globular
  Clusters. IX. The Atlas of Multiple Stellar Populations
  We use high-precision photometry of red-giant-branch (RGB) stars in 57
Galactic globular clusters (GCs), mostly from the `Hubble Space Telescope (HST)
UV Legacy Survey of Galactic globular clusters', to identify and characterize
their multiple stellar populations. For each cluster the pseudo two-color
diagram (or `chromosome map') is presented, built with a suitable combination
of stellar magnitudes in the F275W, F336W, F438W and F814W filters that
maximizes the separation between multiple populations. In the chromosome map of
most GCs (Type I clusters), stars separate in two distinct groups that we
identify with the first (1G) and the second generation (2G). This
identification is further supported by noticing that 1G stars have primordial
(oxygen-rich, sodium-poor) chemical composition, whereas 2G stars are enhanced
in sodium and depleted in oxygen. This 1G-2G separation is not possible for a
few GCs where the two sequences have apparently merged into an extended,
continuous sequence. In some GCs (Type II clusters) the 1G and/or the 2G
sequences appear to be split, hence displaying more complex chromosome maps.
These clusters exhibit multiple SGBs also in purely optical color-magnitude
diagrams, with the fainter SGB joining into a red RGB which is populated by
stars with enhanced heavy-element abundance. We measure the RGB width by using
appropriate colors and pseudo-colors. When the metallicity dependence is
removed, the RGB width correlates with the cluster mass. The fraction of 1G
stars ranges from ~8% to ~67% and anticorrelates with the cluster mass,
indicating that incidence and complexity of the multiple population phenomenon
both increase with cluster mass.
