HST observations of the globular cluster NGC 6402 (M14) and its peculiar
  multiple populations
  We present Hubble Space Telescope (HST) photometric results for NGC 6402, a
highly reddened very luminous Galactic globular cluster (GC). Recent
spectroscopic observations of its red giant stars have shown a quite peculiar
behavior in the chemistry of its multiple populations. These results have
prompted UV and optical HST observations aimed at obtaining the cluster's
"Chromosome map" (ChM), an efficient tool to classify GCs and characterize
their multiple populations. We find that the discontinuity in the abundance
distributions of O, Mg, Al and Na inferred from spectroscopy is more nuanced in
the ChM, which is mostly sensitive to nitrogen. Nevertheless, photometry in
optical bands reveals a double main sequence, indicating a discontinuity in the
helium content of the populations. The population with the largest chemical
anomalies (extreme) peaks at a helium mass fraction Y~0.31. This helium content
is consistent with results from the analysis of the distribution of
horizontal-branch stars and the spectrophotometry of the red giants. The ChM
and the color magnitude diagrams are compared with those in NGC 2808, a
prototype GC with helium abundances up to Y > 0.35, and both confirm that NGC
6402 does not host stellar populations with such extreme helium content.
Further, the ChM reveals the presence of a group of stars with larger
metallicity, thus indicating that NGC 6402 is a Type II cluster. The modalities
of formation of the multiple populations in NGC 6402 are briefly surveyed, with
main attention on the Asymptotic Giant Branch and Supermassive star models, and
on possible clusters' merging.
