Extended main sequence turnoffs in open clusters as seen by Gaia -- II. The enigma of NGC 2509

Abstract

We investigate the morphology of the colour-magnitude diagram (CMD) of the open cluster NGC 2509 in comparison with other Galactic open clusters of similar age using Gaia photometry. At $\sim 900$ Myr Galactic open clusters in our sample all show an extended main sequence turn off (eMSTO) with the exception of NGC 2509, which presents an exceptionally narrow CMD. Our analysis of the Gaia data rules out differential extinction, stellar density, and binaries as a cause for the singular MSTO morphology in this cluster. We interpret this feature as a consequence of the stellar rotation distribution within the cluster and present the analysis with MIST stellar evolution models that include the effect of stellar rotation on which we based our conclusion. In particular, these models point to an unusually narrow range of stellar rotation rates ($\Omega /{\Omega }_{\mathrm{crit},\mathrm{ZAMS}}=[0.4,0.6]$) within the cluster as the cause of this singular feature in the CMD of NGC 2509. Interestingly, models that do not include rotation are not as good at reproducing the morphology of the observed CMD in this cluster.

9 pages, 7 figures

References in corpus (6)

• A double main sequence turn-off in the rich star cluster NGC 1846 in the Large Magellanic Cloud
• Rotating Stellar Models Can Account for the Extended Main Sequence Turnoffs in Intermediate Age Clusters
• NGC 1866: First Spectroscopic Detection of Fast Rotating Stars in a Young LMC Cluster
• Discovery of Extended Main Sequence Turn-offs in Four Young Massive Clusters in the Magellanic Clouds
• Discrepancies in the ages of young star clusters; evidence for mergers?
• The central dynamics of M3, M13, and M92: Stringent limits on the masses of intermediate-mass black holes

Cited by in corpus (9)

• Painting a portrait of the Galactic disc with its stellar clusters
• How stellar rotation shapes the colour magnitude diagram of the massive intermediate-age star cluster NGC 1846
• TESS observations of the Pleiades cluster: a nursery for delta Scuti stars
• UOCS VII. Blue Straggler Populations of Open Cluster NGC 7789 with UVIT/AstroSat
• Photometric and Kinematic study of the three intermediate age open clusters NGC 381, NGC 2360, and Berkeley 68
• Binary-driven stellar rotation evolution at the main-sequence turn-off in star clusters
• The VVV Open Cluster Project. Near-infrared sequences of NGC6067, NGC6259, NGC4815, Pismis18, Trumpler23, and Trumpler20
• A binary star sequence in the outskirts of the disrupting Galactic open cluster UBC 274
• Multiple populations of Hbeta emission line stars in the Large Magellanic Cloud cluster NGC1971