Investigating Unusually Extended Main Sequence Turnoff of the Galactic Open Cluster NGC 3532

Abstract

Extended main-sequence turnoffs are observed in the color-magnitude diagrams of some young and intermediate-age star clusters. Here we report the case study of NGC 3532. Among the cluster member candidates identified using the ML-MOC algorithm on Gaia DR3 sources, we found that fast rotators are significantly redder. Stars near the main-sequence turnoff exhibit a multi-modal distribution in the Gaia broadening velocity ($v_{\rm broad}$). A comparison with relevant literature suggests binaries being typically interspersed with fast rotators, often manifesting as slow to moderate rotators. Additionally, chemically peculiar stars are predominantly located in the bluer regions of the turnoff, with those in the fast-rotator region having large values of Gaia re-normalised unit weight error, indicating plausible companions. Our analysis suggests multiple channels responsible for angular momentum loss to account for the extended turnoff.

Figures (2)

• Figure 1: (a): Gaia CMD of NGC 3532, fitted with a non-rotating isochrone (magenta solid line) of $\log{\rm (Age/year)}= 8.52$, distance of 490 pc, $A_{\text{v}} = 0.07$, and $[Fe/H] = 0.0$. The cyan dashed line shows an equal-mass binary track shifted by $G -0.75$ mag. The inset zooms in to the MSTO region. (b): CMD of the MSTO stars, color-coded by the $v_{\text{broad}}$ measurements. Grey dots show all the cluster members. The black open circles show eclipsing binaries Gaia_var2023. ACV$\mid$CP$\mid$MCP$\mid$ROAM$\mid$ROAP$\mid$SXARI variables Gaia_var2023 having RUWE$< 1.21$ are shown as black pentagons, and those having RUWE$> 1.21$ as orange triangles. Also marked are eclipsing binaries and spectroscopic binaries Gavras_kv_2023, X-ray sources Chen2023, and $H_{\alpha}$ sources He2025.
• Figure 2: (Left) $v_{\text{broad}}$ distribution of MSTOs of the cluster having G $< 11.8$ mag. The black solid line represents the fitted multi-modal distribution. The red, green, and orange solid lines represent the fitted Gaussians for individual subsets of the distribution. (Right) Cumulative radial distributions of fast-rotating MSTOs, i.e., $v_{\text{broad}} > 100$ km/s (red solid line), slow-rotating MSTOs having $v_{\text{broad}} < 100$ km/s (blue solid line), high mass-ratio binaries with q $> 0.6$ within primary masses of 0.31 -- 1.5 M$_{\odot}$ (orange solid line), and MS within masses of 0.31 -- 1.5 M$_{\odot}$ (black solid line).