Discovery of extended main sequence turn offs in Galactic open clusters
  By far, the color-magnitude diagrams (CMDs) of Galactic open clusters have
been considered proto-types of single stellar populations. By using photometry
in ultraviolet and optical bands we discovered that the nearby young cluster
NGC6705 (M11) exhibits an extended main-sequence turn off (eMSTO) and a
broadened main-sequence (MS). This is the first evidence of multiple stellar
populations in a Galactic open cluster. By using high-resolution VLT spectra we
provide direct evidence that the multiple sequences along the CMD correspond to
stellar populations with different rotation rates. Specifically, the blue MS is
formed of slow-rotating stars, while red-MS stars are fast rotators. Moreover,
we exploit photometry from Gaia DR2 to show that three Galactic open clusters,
namely NGC2099, NGC2360, and NGC2818, exhibit the eMSTO, thus suggesting that
it is a common feature among these objects. Our previous work on the Large
Magellanic Cloud star cluster NGC1818 shows that slowly and fastly-rotating
stars populate the blue and red MS observed in its CMD. The similarities
between M11 and the young clusters of the Magellanic Clouds suggest that
rotation is responsible for the appearance of multiple populations in the CMDs
of both Milky Way open clusters and Magellanic Clouds young clusters.
