The exclusion of a significant range of ages in a massive star cluster

Abstract

Stars spend most of their lifetimes on the main sequence in the Hertzsprung-Russell diagram. The extended main-sequence turnoff regions -containing stars leaving the main sequence after having spent all of the hydrogen in their cores -found in massive (more than a few tens of thousands of solar masses), intermediateage (about one to three billion years old) star clusters [1][2][3][4][5][6][7][8] are usually interpreted as evidence of cluster-internal age spreads of more than 300 million years 2, 4, 5 , although young clusters are thought to quickly lose any remaining star-forming fuel following a period of rapid gas expulsion on timescales of order 10 7 years 9, 10 . Here we report that the stars beyond the main sequence in the two billionyear-old cluster NGC 1651, characterized by a mass of ∼ 1.7 × 10 5 solar masses 3 , can be explained only by a single-age stellar population, even though the cluster has clearly extended main-sequence turn-off region. The most plausible explanation for the extended main-sequence turn-offs invokes the presence of a population of rapidly rotating stars, although the secondary effects of the prolonged stellar lifetimes associated with such a stellar-population mixture are as yet poorly understood. From preliminary analysis of previously obtained data, we find that similar morphologies are apparent in the Hertzsprung-Russell diagrams of at least five additional intermediate-age star clusters 2,3,5, 11 , suggesting that an extended main-sequence turn-off does not necessarily imply the presence of a significant cinternal age dispersion.We obtained archival Hubble Space Telescope/Wide Field Camera-3 observations of the NGC 1651 field in the F475W ("B") and F814W ("I") broad-band filters (Methods). The corresponding colourmagnitude diagram, that is, the observational counterpart of the Hertzsprung-Russell diagram, is shown in Fig. 1. When stars have exhausted their core hydrogen supply, hydrogen fusion continues in a shell outside the stellar core. At this stage, stars leave the main sequence and evolve onto the subgiant branch. The colour-magnitude diagram of NGC 1651 exhibits a clearly extended main-sequence turnoff and a very narrow subgiant branch. This is surprising, given the corresponding, far-reaching implications for our interpretation of such extended turn-offs in the context of star cluster evolution.Star clusters more massive than a few tens thousands of solar masses were, until recently, considered single-generation ("simple") stellar populations. It was thought that all of their member stars had formed approximately simultaneously from molecular gas originally confined to a small volume of space. As a consequence, all cluster stars would thus have similar ages, a very narrow range in chemical composition and individual stellar masses that followed the initial mass function, that is, the stellar mass distribution at the time of star birth. In the past decade, however, consensus has emerged that massive star clusters are no longer ideal simple stellar populations [12][13][...

A young cluster with an extended main-sequence turnoff: confirmation of a prediction of the stellar rotation scenario

“…First, we took a cut across the MSTO, perpendicular to the isochrones and assigned an age to each star in this box based on its proximity to the nearest isochrone. This is similar to what was done in Goudfrooij et al (2014; hereafter G14), Li et al (2014; and Bastian & Niederhofer (2015). The resulting age distribution is shown in Fig.…”

The morphology of the sub-giant branch and red clump reveal no sign of age spreads in intermediate-age clusters

“…Hence, it appears that some effect is causing stars to move red-ward in colour space at the MSTO. NGC 1651 displays the same behaviour, with an inferred age of ∼ 2 Gyr, suggesting that it is a general property of the eMSTO intermediate age clusters, and not an age effect (Li et al 2014b). This is the behaviour predicted if stellar rotation was the cause of the eMSTO (Bastian & de Mink 2009;Yang et al 2013).…”

“…Here we use the same technique as Li et al (2014b), studying the width of the SGB in two intermediate age LMC clusters that display the eMSTO phenomenon, namely, NGC 1806 & NGC 1846. In § 2 we present the observations, while in § 3 we analyse the CMDs of the two clusters, testing the hypothesis of significant age spreads within the clusters.…”

Controversial age spreads from the main sequence turn-off and red clump in intermediate-age clusters in the LMC

“…This spread is much smaller than would be inferred from the MSTO region (∼450 Myr). Li et al (2014) therefore conclude that the extended MSTO is not due to a prolonged star formation. Bastian & Niederhofer (2015) extend the study of Li et al (2014) to NGC 1806 and NGC 1846, two ∼1.4 Gyr old LMC clusters, which also show an extended MSTO.…”

“…Li et al (2014) analyze the sub-giant branch (SGB) of NGC 1651, a ∼2 Gyr old cluster in the LMC that shows an extended MSTO. They show that the SGB stars mostly follow the youngest isochrone that covers the spread in the MSTO (1.74 Gyr) with a spread in the SGB of 80 Myr at most.…”