Unraveling eMSTO in NGC 2355: Stellar Rotation and Binaries

TL;DR

Addresses the origin of the extended Main Sequence Turn-off (eMSTO) in NGC 2355 by combining Gaia DR3 astrometry/photometry with Gaia-ESO spectroscopy to measure $v \sin i$ and its color dependence. The analysis finds a positive color-rotation correlation ($r = 0.48$, $p = 0.01$) with blue-eMSTO stars rotating more slowly ($v \sin i$ around $89.1 \pm 4.5$ km s$^{-1}$) and red-eMSTO stars rotating faster ($v \sin i$ around $138.1 \pm 5.3$ km s$^{-1}$), plus a central concentration of fast rotators. The spatial distribution challenges tidal-locking in binaries as the primary driver and instead supports star-disk interactions during the pre-main-sequence, with greater disk destruction in dense cores leading to spin-up. Overall, the work provides observational evidence that rotational spreads from early disk evolution drive the eMSTO in an MW open cluster, with gravity darkening explaining the CMD color spread.

Abstract

The extended Main Sequence Turn-off (eMSTO) in the open cluster NGC 2355 is investigated using precise astrometry and photometry from Gaia DR3 and spectroscopic data from the Gaia-ESO Survey. We find a clear positive correlation between the rotational velocity (v sin i) and color of eMSTO stars, supporting the role of stellar rotation and gravity darkening in causing the observed color spread. Contrary to predictions from binary tidal-locking scenarios, spatial distribution analysis reveals that fast-rotating stars are preferentially concentrated in the cluster's central regions. This finding provides the observational evidence favoring star-disk interactions during the pre-main-sequence phase, rather than tidal locking in binaries, as the primary mechanism responsible for the origin of rotational spread in eMSTO stars.

Figures (3)

• Figure 1: Color-magnitude diagram for NGC 2355. Gray points represent cluster members. eMSTO stars with measured $v \sin i$) are overplotted and color-coded by their $v \sin i$ values. The lime-green line shows the fiducial MS. The blue dashed rectangle highlights the eMSTO region in the upper MS.
• Figure 2: The correlation between pseudo color $\Delta$G$_{\rm BP}-G_{\rm RP}$ and $v \sin i$ for eMSTO stars. The red points ($v \sin i < 50$ km s$^{-1}$) are slow-rotating outliers.
• Figure 3: Left panel: Spatial distribution of blue-eMSTO and red-eMSTO main-sequence stars relative to the cluster center. The density contours illustrate the concentration of each stellar population. Right panel: Cumulative radial distributions of the blue-eMSTO and red-eMSTO main-sequence populations.