Apparent Age Spreads in Clusters and the Role of Stellar Rotation

TL;DR

This paper demonstrates that apparent age spreads in star clusters' colour-magnitude diagrams can be explained by stellar rotation effects, which extend the main sequence lifetime of stars, mimicking age differences.

Contribution

The study introduces models incorporating stellar rotation to explain extended main sequence turn-offs without requiring actual age spreads.

Findings

Rotating stars stay on the main sequence longer, creating extended turn-offs.

The apparent age spread correlates with cluster age, supporting rotation as the cause.

Observed age spreads may be due to stellar rotation velocity distributions, not true age differences.

Abstract

We use the Geneva Syclist isochrone models that include the effects of stellar rotation to investigate the role that rotation has on the resulting colour-magnitude diagram (CMD) of young and intermediate age clusters. We find that if a distribution of rotation velocities exists within the clusters, rotating stars will remain on the main sequence (MS) for longer, appearing to be younger than non-rotating stars within the same cluster. This results in an extended main sequence turn-off (eMSTO) that appears at young ages ($\sim30$~Myr) and lasts beyond 1~Gyr. If this eMSTO is interpreted as an age spread, the resulting age spread is proportional to the age of the cluster, i.e. young clusters ($<100$~Myr) appear to have small age spreads (10s of Myr) whereas older clusters ($\sim1$~Gyr) appear to have much larger spreads, up to a few hundred Myr. We compare the predicted spreads for a sample of rotation rates to observations of young and intermediate age clusters, and find a strong correlation between the measured 'age spread' and the age of the cluster, in good agreement with models of stellar rotation. This suggests that the 'age spreads' reported in the literature may simply be the result of a distribution of stellar rotation velocities within clusters.