Apparent age spreads in clusters and the role of stellar rotation

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 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 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 (∼30Myr) and lasts beyond 1Gyr. 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 (<100Myr) appear to have small age spreads (tens of Myr) whereas older clusters (∼1Gyr) appear to have much large 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 cluster

Apparent Age Spreads in Clusters and the Role of Stellar Rotation

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

A high fraction of Be stars in young massive clusters: evidence for a large population of near-critically rotating stars

Recent photometric analysis of the colour-magnitude diagrams (CMDs) of young massive clusters (YMCs) have found evidence for splitting in the main sequence and extended main sequence turn-offs, both of which have been suggested to be caused by stellar rotation. Comparison of the observed main sequence splitting with models has led various authors to suggest a rather extreme stellar rotation distribution, with a minority (10−30\%) of stars with low rotational velocities and the remainder (70−90\%) of stars rotating near the critical rotation (i.e., near break-up). We test this hypothesis by searching for Be stars within two YMCs in the LMC (NGC 1850 and NGC 1856), which are thought to be critically rotating stars with decretion disks that are (partially) ionised by their host stars. In both clusters we detect large populations of Be stars at the main sequence turn-off (∼30−60\% of stars), which supports previous suggestions of large populations of rapidly rotating stars within massive clusters

No evidence for younger stellar generations within the intermediate-age massive clusters NGC 1783, NGC 1806 and NGC 411

Recently, Li et al. claimed to have found evidence for multiple generations of stars in the intermediate-age clusters NGC 1783, NGC 1806 and NGC 411 in the Large and Small Magellanic Clouds. Here we show that these young stellar populations are present in the field regions around these clusters and are not likely associated with the clusters themselves. Using the same data sets, we find that the background subtraction method adopted by the authors does not adequately remove contaminating stars in the small number Poisson limit. Hence, we conclude that their results do not provide evidence of young generations of stars within these clusters

The search for multiple populations in Magellanic Cloud Clusters IV: Coeval multiple stellar populations in the young star cluster NGC 1978

We have recently shown that the $\sim2$ Gyr old Large Magellanic Cloud star cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance variations in [N/Fe]. These can be seen as a splitting or spread in the sub-giant and red giant branches (SGB and RGB) when certain photometric filter combinations are used. Due to its relative youth, NGC 1978 can be used to place stringent limits on whether multiple bursts of star-formation have taken place within the cluster, as predicted by some models for the origin of multiple populations. We carry out two distinct analyses to test whether multiple star-formation epochs have occurred within NGC 1978. First, we use UV CMDs to select stars from the first and second population along the SGB, and then compare their positions in optical CMDs, where the morphology is dominantly controlled by age as opposed to multiple population effects. We find that the two populations are indistinguishable, with age differences of $1\pm20$ Myr between them. This is in tension with predictions from the AGB scenario for the origin of multiple populations. Second, we estimate the broadness of the main sequence turnoff (MSTO) of NGC 1978 and we report that it is consistent with the observational errors. We find an upper limit of $\sim$65 Myr on the age spread in the MSTO of NGC 1978. This finding is in conflict with the age spread scenario as origin of the extended MSTO in intermediate age clusters, while it fully supports predictions from the stellar rotation model

The VMC Survey. XXXII. Pre-main-sequence populations in the Large Magellanic Cloud

Context Detailed studies of intermediate- and low-mass pre-main-sequence (PMS) stars outside the Galaxy have so far been conducted only for small targeted regions harbouring known star formation complexes. The VISTA Survey of the Magellanic Clouds (VMC) provides an opportunity to study PMS populations down to solar masses on a galaxy-wide scale. Aims Our goal is to use near-infrared data from the VMC survey to identify and characterise PMS populations down to ∼ 1 M� across the Magellanic Clouds. We present our colour−magnitude diagram method, and apply it to a ∼ 1.5 deg2 pilot field located in the Large Magellanic Cloud. Methods The pilot field is divided into equal-size grid elements. We compare the stellar population in every element with the population in nearby control fields by creating Ks/(Y−Ks) Hess diagrams; the observed density excesses over the local field population are used to classify the stellar populations. Results Our analysis recovers all known star formation complexes in this pilot field (N 44, N 51, N 148, and N 138) and for the first time reveals their true spatial extent. In total, around 2260 PMS candidates with ages . 10 Myr are found in the pilot field. PMS structures, identified as areas with a significant density excess of PMS candidates, display a power-law distribution of the number of members with a slope of −0.86 ± 0.12. We find a clustering of the young stellar populations along ridges and filaments where dust emission in the far-infrared (FIR) (70 µm – 500 µm) is bright. Regions with young populations lacking massive stars show a lower degree of clustering and are usually located in the outskirts of the star formation complexes. At short FIR wavelengths (70 µm, 100 µm) we report a strong dust emission increase in regions hosting young massive stars, which is less pronounced in regions populated only by less massive (. 4 M�) PMS stars

Searching for globular cluster chemical anomalies on the main sequence of a young massive cluster

The spectroscopic and photometric signals of the star-to-star abundance variations found in globular clusters seem to be correlated with global parameters like the cluster’s metallicity, mass, and age. Understanding this behaviour could bring us closer to the origin of these intriguing abundance spreads. In this work we use deep HST photometry to look for evidence of abundance variations in the main sequence of a young massive cluster NGC 419 (∼105 M⊙, ∼1.4 Gyr). Unlike previous studies, here we focus on stars in the same mass range found in old globulars (∼0.75–1 M⊙), where light elements variations are detected. We find no evidence for N abundance variations among these stars in the Un − B and U − B colour–magnitude diagrams of NGC 419. This is at odds with the N variations found in old globulars like 47 Tuc, NGC 6352, and NGC 6637 with similar metallicity to NGC 419. Although the signature of the abundance variations characteristic of old globulars appears to be significantly smaller or absent in this young cluster, we cannot conclude if this effect is mainly driven by its age or its mass

Age as a Major Factor in the Onset of Multiple Populations in Stellar Clusters

It is now well established that globular clusters (GCs) exhibit star-to-star light-element abundance variations (known as multiple stellar populations, MPs). Such chemical anomalies have been found in (nearly) all the ancient GCs (more than 10 Gyr old) of our Galaxy and its close companions, but so far no model for the origin of MPs is able to reproduce all the relevant observations. To gain new insights into this phenomenon, we have undertaken a photometric Hubble Space Telescope survey to study clusters with masses comparable to that of old GCs, where MPs have been identified, but with significantly younger ages. Nine clusters in the Magellanic Clouds with ages between $\sim$ 1.5-11 Gyr have been targeted in this survey. We confirm the presence of multiple populations in all clusters older than 6 Gyr and we add NGC 1978 to the group of clusters for which MPs have been identified. With an age of $\sim$ 2 Gyr, NGC 1978 is the youngest cluster known to host chemical abundance spreads found to date. We do not detect evident star-to-star variations for slightly younger massive clusters ($\sim$ 1.7 Gyr), thus pointing towards an unexpected age dependence for the onset of multiple populations. This discovery suggests that the formation of MPs is not restricted to the early Universe and that GCs and young massive clusters share common formation and evolutionary processes

The VMC survey XXVIII. Improved measurements of the proper motion of the Galactic globular cluster 47 Tucanae

We use deep multi-epoch point-spread function (PSF) photometry taken with the Visible and Infrared Survey Telescope for Astronomy (VISTA) to measure and analyze the proper motions of stars within the Galactic globular cluster 47 Tucanae (47 Tuc, NGC 104). The observations are part of the ongoing near-infrared VISTA survey of the Magellanic Cloud system (VMC). The data analyzed in this study correspond to one VMC tile, which covers a total sky area of 1.77 deg(2). Absolute proper motions with respect to similar to 9070 background galaxies are calculated from a linear regression model applied to the positions of stars in 11 epochs in the K-s filter. The data extend over a total time baseline of about 17 months. We found an overall median proper motion of the stars within 47 Tuc of (mu alpha cos(delta), mu(delta)) = (+5.89 +/- 0.02 (statistical) +/- 0.13 (systematic), -2.14 +/- 0.02 (statistical) +/- 0.08 (systematic)) mas yr(-1), based on the measurements of similar to 35 000 individual sources between 5’ and 42’ from the cluster center. We compared our result to the proper motions from the newest US Naval Observatory CCD Astrograph Catalog (UCAC5), which includes data from the Gaia data release 1. Selecting cluster members (similar to 2700 stars), we found a median proper motion of (mu(alpha)cos(delta), mu(delta)) = (+5.30 +/- 0.03 (statistical) +/- 0.70 (systematic), -2.70 +/- 0.03 (statistical) +/- 0.70 (systematic)) mas yr(-1). Comparing the results with measurements in the literature, we found that the values derived from the VMC data are consistent with the UCAC5 result, and are close to measurements obtained using the Hubble Space Telescope. We combined our proper motion results with radial velocity measurements from the literature and reconstructed the orbit of 47 Tuc, finding that the cluster is on an orbit with a low ellipticity and is confined within the inner similar to 7.5 kpc of the Galaxy. We show that the use of an increased time baseline in combination with PSF-determined stellar centroids in crowded regions significantly improves the accuracy of the method. In future works, we will apply the methods described here to more VMC tiles to study in detail the kinematics of the Magellanic Clouds