A systematic study of effects of stellar rotation, age spread and binaries on color-magnitude diagrams with extended main-sequence turn-offs

TL;DR

This study systematically investigates how stellar rotation, age spread, and binary stars influence the peculiar extended main-sequence turn-offs in star cluster color-magnitude diagrams, providing insights into their relative roles and effects.

Contribution

It offers a comprehensive analysis of the combined effects of rotation, age spread, and binaries on CMDs, clarifying their roles in producing eMSTOs and eRCs in star clusters.

Findings

Rotation can partially explain eMSTOs in clusters younger than 2 Gyr.

An age spread of 200-500 Myr reproduces extended turn-offs in clusters under 2.5 Gyr.

Binaries influence MSTO spread mainly in clusters younger than 0.5 Gyr.

Abstract

Stellar rotation, age spread and binary stars are thought to be three most possible causes of the peculiar color-magnitude diagrams (CMDs) of some star clusters, which exhibit extended main-sequence turn-offs (eMSTOs). It is far from getting a clear answer. This paper studies the effects of three above causes on the CMDs of star clusters systematically. A rapid stellar evolutionary code and a recently published database of rotational effects of single stars have been used, via an advanced stellar population synthesis technique. As a result, we find a consistent result for rotation to recent works, which suggests that rotation is able to explain, at least partially, the eMSTOs of clusters, if clusters are not too old ($<$ 2.0\,Gyr). In addition, an age spread of 200 to 500\,Myr reproduces extended turn-offs for all clusters younger than 2.5\,Gyr, in particular, for those younger than 2.2\,Gyr. Age spread also results in extended red clumps (eRCs) for clusters younger than 0.5\,Gyr. The younger the clusters, the clearer the eRC structures. Moreover, it is shown that binaries (including interactive binaries) affect the spread of MSTO slightly for old clusters, but they can contribute to the eMSTOs of clusters younger than 0.5\,Gyr. Our result suggests a possible way to disentangle the roles of stellar rotation and age spread, i.e., checking the existence of CMDs with both eMSTO and eRC in clusters younger than 0.5\,Gyr.