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

Abstract

Most star clusters at an intermediate age (1−2 Gyr) in the Large and Small Magellanic Clouds show a puzzling feature in their color−magnitude diagrams (CMD) that is not in agreement with a simple stellar population. The main sequence turn-off of these clusters is much broader than expected from photometric uncertainties. One interpretation of this feature is that age spreads of the order of 200−500 Myr exist within individual clusters, although this interpretation is highly debated. Such large age spreads should affect other parts of the CMD, which are sensitive to age, as well. In this study, we analyze the CMDs of a sample of 12 intermediateage clusters in the Large Magellanic Cloud that all show an extended turn-off using archival optical data taken with the Hubble Space Telescope. We fit the star formation history of the turn-off region and the red clump region independently. We find that in most cases, the age spreads inferred from the red clumps are smaller than those that result from the turn-off region. However, the age ranges that result from the red clump region are broader than expected for a single age. Only two out of 12 clusters in our sample show a red clump which seems to be consistent with a single age. As our results are ambiguous, by fitting the star formation histories to the red clump regions, we cannot ultimately tell if the extended main sequence turn-off feature is the result of an age spread or not. However, we do find that the width of the extended main sequence turn-off feature is correlated with the age of the clusters in a way which would be unexplained in the so-called age spread interpretation, but which may be expected if stellar rotation is the cause of the spread at the turn-off.

Star clusters: Anything but simple

“…Clusters younger than 1 billion years or those older than 2.5 billion years are not affected because of the complex interplay between the location of δ Scuti stars on the main sequence and its age-dependent overlap with the instability strip. This fresh insight is eerily similar to the results from a recent independent analysis which considered the apparent internal cluster age spread implied by the extent of the main-sequence turn-off as a function of cluster age, reaching a maximum at an age of 1.5-1.7 billion years 7 . The results of Salinas and co-workers are intriguing and offer significant food for thought.…”

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

“…As noted in previous works on the intermediate age clusters (Li et al 2014;Niederhofer et al 2016), the postmain sequence distribution of stars (e.g., the SGB) in some clusters does not appear to be consistent with an age spread within the cluster, instead being narrower and also concentrated towards the 'young' end of the distribution expected from the MSTO (although see Goudfrooij et al 2015 for an alternative view). The SGB does appear to be narrow in the SYCLIST models including rotation at this age, but this will be investigated in more detail in a future work.…”

The Overlooked Role of Stellar Variability in the Extended Main Sequence of LMC Intermediate-Age Clusters

“…-the color spread is a function of cluster age resembling the pattern discovered by Niederhofer et al (2016) , -no color spread will be introduced at the red clump,…”

“…The pseudo-age spread introduced at the MSTO is measured with the parallelogram approach introduced in the previous Section, centering it in each corresponding isochrone. Niederhofer et al (2016) used data from Goudfrooij et al (2014) to argue that age spreads in the LMC clusters increase as function of cluster age, reaching a maximum at 1.5-1.7 Gyr, and then decreasing after that age. Figure 4 shows the change in the FWHM of the pseudo age distributions as function of model age.…”

“…Niederhofer et al 2016), the impact of the latter, controlled by the incidence and the amplitude distribution of δ Sct , will be the easiest to properly assess once high spatial resolution time series images of these clusters become available.…”