Discovery of Extended Main Sequence Turnoffs in Galactic Open Clusters

Abstract

Abstract The color–magnitude diagrams (CMDs) of Galactic open clusters are widely considered to be the prototypes of single stellar populations. By using photometry in ultraviolet and optical bands we discovered that the nearby young cluster NGC 6705 (M11) exhibits an extended main-sequence turnoff (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 Very Large Telescope (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 (bMS) is formed of slow-rotating stars, while red-MS (rMS) stars are fast rotators. Moreover, we exploit photometry from Gaia data release 2 (DR2) to show that three Galactic open clusters, namely NGC 2099, NGC 2360, and NGC 2818, exhibit the eMSTO, thus suggesting that it is a common feature among these objects. Our previous work on the Large Magellanic Cloud star cluster NGC 1818 shows that slowly and rapidly rotating stars populate the bMS and rMS observed in its CMD. The similarities between M11 and the young clusters of the Magellanic Clouds (MCs) suggest that rotation is responsible for the appearance of multiple populations in the CMDs of both Milky Way open clusters and MCs young clusters.