The HST Large Programme on NGC6752. II. Multiple populations at the
  bottom of the main sequence probed in NIR
  Historically, multiple populations in Globular Clusters (GCs) have been
mostly studied from ultraviolet and optical filters down to stars that are more
massive than ~0.6 solar masses. Here we exploit deep near-infrared (NIR)
photometry from the Hubble Space Telescope to investigate multiple populations
among M-dwarfs in the GC NGC6752. We discovered that the three main populations
(A, B and C), previously observed in the brightest part of the color-magnitude
diagram, define three distinct sequences that run from the main-sequence (MS)
knee towards the bottom of the MS (~0.15 solar masses). These results, together
with similar findings on NGC2808, M4, and omega Centauri, demonstrate that
multiple sequences of M-dwarfs are common features of the color-magnitude
diagrams of GCs. The three sequences of low-mass stars in NGC6752 are
consistent with stellar populations with different oxygen abundances. The range
of [O/Fe] needed to reproduce the NIR CMD of NGC6752 is similar to the oxygen
spread inferred from high-resolution spectroscopy of red-giant branch (RGB)
stars. The relative numbers of stars in the three populations of M-dwarfs are
similar to those derived among RGB and MS stars more massive than ~0.6 solar
masses. As a consequence, the evidence that the properties of multiple
populations do not depend on stellar mass is a constraint for the formation
scenarios.
