Discovery of extended main sequence turn offs in Galactic open clusters

TL;DR

This study reveals that Galactic open clusters, previously thought to be simple stellar populations, exhibit extended main sequence turn offs caused by stars with different rotation rates, indicating multiple stellar populations.

Contribution

First evidence of multiple stellar populations in a Galactic open cluster linked to stellar rotation differences, expanding understanding of cluster evolution.

Findings

NGC6705 (M11) shows extended main-sequence turn off and broadened main sequence.

Spectroscopic data confirms different rotation rates among stellar populations.

Multiple clusters (NGC2099, NGC2360, NGC2818) also exhibit eMSTO features.

Abstract

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.