Stellar rotation and the extended main sequence turnoff in the open cluster NGC 5822

Abstract

The origin of extended main sequence turnoffs (eMSTOs) in intermediate-age (1-3 Gyr) clusters is one of the most intriguing questions in current star cluster research. Unlike the split main sequences found in some globular clusters. which are caused by bimodal populations in age and/or chemical abundances, eMSTOs are believed to be owing to stellar rotation. We present a spectroscopic survey of MSTO stars in a nearby, intermediate-age (\unit[0.9]{Gyr}), low mass ($\sim 1.7\times 10^3{M}_{\odot }$) Galactic open cluster, NGC 5822. We derive a clean sample of member stars based on \textit{Gaia} proper motions and parallaxes and confirm the existence of an eMSTO. Using medium-resolution ($R\sim 4000$) Southern African Large Telescope (SALT) spectra, we derive the rotational velocities of 24 member stars (representing 20\% completeness around the eMSTO region) and find that the loci of the main sequence stars in the eMSTO region show a clear correlation with the projected rotational velocities in the sense that fast rotators are located on the red side of the eMSTO and slow rotators are found on the blue side. By comparison with a synthetic cluster model, we show that the stellar rotational velocities and the eMSTO of NGC 5822 can be well reproduced and we conclude that stellar rotation is the main cause of the eMSTO in NGC 5822.

13 pages, 7 figures, 1 table. Accepted for publication in ApJ

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• Revealing the Field Sub-subgiant Population Using a Catalog of Active Giant Stars and Gaia EDR3
• Variability in the Massive Open Cluster NGC 1817 from K2: A Rich Population of Asteroseismic Red Clump, Eclipsing Binary, and Main Sequence Pulsating Stars
• The role of binarity and stellar rotation in the split main sequence of NGC 2422
• The extended Main-Sequence Turnoff of the Milky Way open cluster Collinder 347
• Binary-driven stellar rotation evolution at the main-sequence turn-off in star clusters
• On aging star clusters using red supergiants independent of the fraction of interacting binary stars
• The solar abundance problem and eMSTOs in clusters
• UVIT-HST-Gaia-VISTA study of KRON 3 in the Small Magellanic Cloud: A cluster with an extended red clump in UV
• A stringent upper limit on Be star fractions produced by binary interaction