Unraveling eMSTO in NGC 2355: Stellar Rotation and Binaries

TL;DR

This study investigates the cause of the extended Main Sequence Turn-off in NGC 2355, providing evidence that stellar rotation and star-disk interactions, rather than binary tidal locking, are the main factors.

Contribution

It offers observational evidence linking stellar rotation and pre-main-sequence star-disk interactions to the eMSTO phenomenon, challenging binary tidal-locking models.

Findings

Positive correlation between rotation velocity and star color in eMSTO stars

Fast rotators are centrally concentrated in the cluster

Supports star-disk interactions over binary tidal locking as the cause

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.