The effects of rotation on the main-sequence turnoff of intermediate-age massive star clusters

TL;DR

This study investigates how stellar rotation influences the main-sequence turnoff in intermediate-age star clusters, revealing that rotation can mimic age spreads and affect the interpretation of cluster CMDs, especially between 0.8 and 2.2 Gyr.

Contribution

It introduces a detailed analysis of rotation effects on MSTOs considering stellar lifetime changes, clarifying conflicting previous results and explaining observed features in star cluster CMDs.

Findings

Rotation causes MSTO shifts similar to 200 Myr age spreads.

Rotation effects are significant in clusters aged 0.8-2.2 Gyr.

Rotation does not produce extended MSTOs in younger clusters.

Abstract

The double or extended main-sequence turnoffs (MSTOs) in the color-magnitude diagram (CMD) of intermediate-age massive star clusters in the Large Magellanic Cloud are generally interpreted as age spreads of a few hundred Myr. However, such age spreads do not exist in younger clusters (i.e., 40-300 Myr), which challenges this interpretation. The effects of rotation on the MSTOs of star clusters have been studied in previous works, but the results obtained are conflicting. Compared with previous works, we consider the effects of rotation on the MS lifetime of stars. Our calculations show that rotating models have a fainter and redder MSTO with respect to non-rotating counterparts with ages between about 0.8 and 2.2 Gyr, but have a brighter and bluer MSTO when age is larger than 2.4 Gyr. The spread of the MSTO caused by a typical rotation rate is equivalent to the effect of an age spread of about 200 Myr. Rotation could lead to the double or extended MSTOs in the CMD of the star clusters with ages between about 0.8 and 2.2 Gyr. However, the extension is not significant; and it does not even exist in younger clusters. If the efficiency of the mixing were high enough, the effects of the mixing would counteract the effect of the centrifugal support in the late stage of evolution; and the rotationally induced extension would disappear in the old intermediate-age star clusters; but younger clusters would have an extended MSTO. Moreover, the effects of rotation might aid in understanding the formation of some "multiple populations" in globular clusters.