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

Abstract

A recent surprise in stellar cluster research, made possible through the precision of Hubble Space Telescope photometry, was that some intermediate age (1 − 2 Gyr) clusters in the Large and Small Magellanic Clouds have main sequence turn-off (MSTO) widths that are significantly broader than would be expected for a simple stellar population (SSP). One interpretation of these extended MSTOs (eMSTOs) is that age spreads of the order of ∼ 500 Myr exist within the clusters, radically redefining our view of stellar clusters, which are traditionally thought of as single age, single metallicity stellar populations. Here we test this interpretation by studying other regions of the CMD that should also be affected by such large age spreads, namely the width of the sub-giant branch (SGB) and the red clump (RC). We study two massive clusters in the LMC that display the eMSTO phenomenon (NGC 1806 & NGC 1846) and show that both have SGB and RC morphologies that are in conflict with expectations if large age spreads exist within the clusters. We conclude that the SGB and RC widths are inconsistent with extended star-formation histories within these clusters, hence age spreads are not likely to be the cause of the eMSTO phenomenon. Our results are in agreement with recent studies that also have cast doubt on whether large age spreads can exist in massive clusters; namely the failure to find age spreads in young massive clusters, a lack of gas/dust detected within massive clusters, and homogeneous abundances within clusters that exhibit the eMSTO phenomenon.

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

“…The results that we obtained in this work differ from the findings by Li et al (2014) and Bastian & Niederhofer (2015). These studies analyzed the morphology of the SGB and the red clump of NGC 1651, NGC 1806, and NGC 1846 and compared it to expected findings if the MSTO was due to an age spread.…”

“…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. They use similar techniques to Li et al (2014), but also take into account the morphology of the red clump.…”

“…In contrast to the studies by Li et al (2014) and Bastian & Niederhofer (2015), they argue that the position of the SGB should not be used (because of uncertainties in the stellar models) but rather that the width is a better indicator of age spreads. The authors find that the width of the SGB of these clusters does agree with an extended SFH using the Marigo et al (2008) set of isochrones.…”

“…Li et al (2014) and Bastian & Niederhofer (2015) compare the distributions of stars that populate the SGB and the red clump with expected findings for an SSP and an extended SFH. We continue these studies by performing a quantitative study of the SFH of the clusters in our sample.…”

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.…”