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This work aims to analyze some of the polluters proposed in the self-enrichment scenarios put forward to explain the multiple populations in globular clusters (GCs), extending previous studies. Three scenarios with different polluter stars were tested: asymptotic giant branch stars (AGBs), high-mass interacting binaries (IBs), and fast rotating massive stars (FRMSs). With abundance data available from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey and ΔY estimates from precise Hubble Space Telescope (HST) photometry, twenty-six clusters were studied, increasing the number of clusters in previous studies by more than a factor of three. We also included the study of the abundances of N, C, Mg, and Al, extending previous studies that mainly focused on the abundances of He, O, and Na. In addition, we constructed an empirical model to test whether one could explain the chemical signatures of the "enriched" population of GC stars with a fixed source and dilution process based on empirical data. In agreement with work by other authors, we found that the proposed polluters can generally predict the qualitative abundance patterns in GC stars and in some cases quantitatively predict some elements, but in most cases when we compare the model yields with the observations, we find that they cannot explain the entire set of observed abundance patterns. The empirical model succeeds in reproducing the abundances of Al for a given ΔY (and vice versa), showing that there is a direct relationship between Al and He, with one increasing proportionally to the other. However, the empirical model fails to reproduce the observed abundances of Na and N, in agreement with the results of previous works. The observed decoupling between the maximum abundances of CNO-cycle elements such as N and Na with those of Al and He provides new information and constraints for future models and could take us a step closer to understanding the origin of the peculiar abundance variations of GC stars. [ABSTRACT FROM AUTHOR]

We present ESO/VLT FORS2 low resolution spectroscopy of red giant branch stars in three massive intermediate age (∼1.7–2.3 Gyr) star clusters in the Large Magellanic Cloud. We measure CH and CN index bands at 4300 Å  and 3883 Å, as well as [C/Fe] and [N/Fe] abundance ratios for 24, 21, and 12 member stars of NGC 1978, NGC 1651, and NGC 1783, respectively. We find a significant intrinsic spread in CN in NGC 1978 and NGC 1651, a signal of multiple stellar populations (MPs) within the clusters. On the contrary, we report a null CN spread in NGC 1783 within our measurement precision. For NGC 1978, we separated the two populations in the CN distribution and we translated the CN spread into an internal N variation Δ[N/Fe] = 0.63 ± 0.49 dex. For NGC 1651 and NGC 1783, we put upper limits on the N abundance variations of Δ[N/Fe] ≤ 0.2, 0.4 dex, respectively. The spectroscopic analysis confirms previous results from HST photometry, where NGC 1978 was found to host MPs in the form of N spreads, while slightly younger clusters (e.g. NGC 1783, < 2 Gyr old) were not, within the limits of the uncertainties. It also confirms that intermediate age massive clusters show lower N abundance variations with respect to the ancient globular clusters, although this is in part due to the effect of the first dredge up at these stellar masses, as recently reported in the literature. We stress the importance of future studies to estimate the initial N abundance variations, free of stellar evolutionary mixing processes, by observing unevolved stars in young clusters. [ABSTRACT FROM AUTHOR]

Nearly all of the well-studied ancient globular clusters (GCs), in the Milky Way and in nearby galaxies, show star-to-star variations in specific elements (e.g. He, C, N, O, Na, and Al), known as 'multiple populations' (MPs). However, MPs are not restricted to ancient clusters, with massive clusters down to ∼2 Gyr showing signs of chemical variations. This suggests that young and old clusters share the same formation mechanism but most of the work to date on younger clusters has focused on N variations. Initial studies even suggested that younger clusters may not host spreads in other elements beyond N (e.g. Na), calling into question whether these abundance variations share the same origin as in the older GCs. In this work, we combine Hubble Space Telescope (HST) photometry with Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) spectroscopy of a large sample of red giant branch (RGB) stars (338) in the Large Magellanic Cloud cluster NGC 1978, the youngest globular to date with reported MPs in the form of N spreads. By combining the spectra of individual RGB stars into N-normal and N-enhanced samples, based on the 'chromosome map' derived from HST , we search for mean abundance variations. Based on the NaD line, we find a Na difference of Δ[Na/Fe] = 0.07 ± 0.01 between the populations. While this difference is smaller than typically found in ancient GCs (which may suggest a correlation with age), this result further confirms that the MP phenomenon is the same, regardless of cluster age and host galaxy. As such, these young clusters offer some of the strictest tests for theories on the origin of MPs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Recent studies have revealed that the multiple populations (MPs) phenomenon does not occur only in ancient and massive Galactic globular clusters (GCs), but it is also observed in external galaxies, where GCs sample a wide age range with respect to the Milky Way. However, for a long time, it was unclear whether we were looking at the same phenomenon in different environments or not. The first evidence that the MPs phenomenon is the same regardless of cluster age and host galaxy came out recently, when an intermediate-age cluster from the Small Magellanic Cloud, Lindsay 1, and a Galactic GC have been directly compared. By complementing those data with new images from the Hubble Space Telescope (HST), we extend the comparison to two clusters of different ages: NGC 2121 (∼2.5 Gyr) and NGC 1783 (∼1.5 Gyr), from the Large Magellanic Cloud. We find a clear correlation between the RGB (red giant branch) width in the pseudo-colour C F 275 W, F 343 N, F 438 W and the age of the cluster itself, with the older cluster having larger σ(C F 275 W, F 343 N, F 438 W )RGB and vice versa. Unfortunately, the σ values cannot be directly linked to the N-abundance variations within the clusters before properly taking account the effect of the first dredge-up. Such HST data also allow us to explore whether multiple star formation episodes occurred within NGC 2121. The two populations are indistinguishable, with an age difference of only 6 ± 12 Myr and an initial helium spread of 0.02 or lower. This confirms our previous results, putting serious constraints on any model proposed to explain the origin of the chemical anomalies in GCs. [ABSTRACT FROM AUTHOR]

Stars in globular clusters (GCs) lose a non-negligible amount of mass during their post-main-sequence evolution. This material is then expected to build up a substantial intracluster medium (ICM) within the GC. However, the observed gas content in GCs is a couple of orders of magnitude below these expectations. Here, we follow the evolution of this stellar wind material through hydrodynamical simulations to attempt to reconcile theoretical predictions with observations. We test different mechanisms proposed in the literature to clear out the gas such as ram-pressure stripping by the motion of the GC in the Galactic halo medium and ionization by UV sources. We use the code ramses  to run 3D hydrodynamical simulations to study for the first time, the ICM evolution within discretized multimass GC models including stellar winds and full radiative transfer. We find that the inclusion of both ram pressure and ionization is mandatory to explain why only a very low amount of ionized gas is observed in the core of GCs. The same mechanisms operating in ancient GCs that clear the gas could also be efficient at younger ages, meaning that young GCs would not be able to retain gas and form multiple generations of stars as assumed in many models to explain 'multiple populations'. However, this rapid clearing of gas is consistent with observations of young massive clusters. [ABSTRACT FROM AUTHOR]

Observed mass-to-light ratios (M / L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. We have derived radial velocities for 1622 stars located in the centres of 59 Milky Way GCs – 12 of which have no previous kinematic information – using integral-field unit data from the WAGGS project. Using N -body models, we determine dynamical masses and M / L V for the studied clusters. Our sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M / L up to [Fe/H] ∼ −0.1 dex. We find that metal-rich clusters have M / L V more than two times lower than what is predicted by simple stellar population models. This confirms that the discrepant M / L –[Fe/H] relation remains a serious concern. We explore how our findings relate to previous observations, and the potential causes for the divergence, which we conclude is most likely due to dynamical effects. [ABSTRACT FROM AUTHOR]

We investigate the binary content of the two stellar populations that coexist in the globular cluster NGC 3201. Previous studies of binary stars in globular clusters have reported higher binary fractions in their first populations (P1, having field-like abundances) compared to their second populations (P2, having anomalous abundances). This is interpreted as evidence for the latter forming more centrally concentrated. In contrast to previous studies, our analysis focusses on the cluster centre, where comparable binary fractions between the populations are predicted because of short relaxation times. However, we find that even in the centre of NGC 3201, the observed binary fraction of P1 is higher (23:1 ± 6:2)% compared to (8:2 ± 3:5)% in P2. Our results are difficult to reconcile with a scenario in which the populations only differ in their initial concentrations, but instead suggests that the populations also formed with different fractions of binary stars. [ABSTRACT FROM AUTHOR]

We combine MUSE spectroscopy and Hubble Space Telescope ultraviolet (UV) photometry to perform a study of the chemistry and dynamics of the Galactic globular cluster Messier 80 (M80, NGC 6093). Previous studies have revealed three stellar populations that vary not only in their light-element abundances, but also in their radial distributions, with the concentration decreasing with increasing nitrogen enrichment. This remarkable trend, which sets M80 apart from other Galactic globular clusters, points towards a complex formation and evolutionary history. To better understand how M80 formed and evolved, revealing its internal kinematics is key. We find that the most N-enriched population rotates faster than the other two populations at a 2 σ confidence level. While our data further suggest that the intermediate population shows the least amount of rotation, this trend is rather marginal (1−2 σ). Using axisymmetric Jeans models, we show that these findings can be explained from the radial distributions of the populations if they possess different angular momenta. Our findings suggest that the populations formed with primordial kinematical differences. [ABSTRACT FROM AUTHOR]

Context. Galactic globular clusters (GCs) are now known to host multiple populations displaying particular abundance variations. The different populations within a GC can be well distinguished following their position in the pseudo two-colors diagrams, also referred to as "chromosome maps". These maps are constructed using optical and near-UV photometry available from the Hubble Space Telescope (HST) UV survey of GCs. However, the chemical tagging of the various populations in the chromosome maps is hampered by the fact that HST photometry and elemental abundances are both only available for a limited number of stars. Aims. The spectra collected as part of the MUSE survey of globular clusters provide a spectroscopic counterpart to the HST photometric catalogs covering the central regions of GCs. In this paper, we use the MUSE spectra of 1115 red giant branch (RGB) stars in NGC 2808 to characterize the abundance variations seen in the multiple populations of this cluster. Methods. We used the chromosome map of NGC 2808 to divide the RGB stars into their respective populations. We then combined the spectra of all stars belonging to a given population, resulting in one high signal-to-noise ratio spectrum representative of each population. Results. Variations in the spectral lines of O, Na, Mg, and Al are clearly detected among four of the populations. In order to quantify these variations, we measured equivalent width differences and created synthetic populations spectra that were used to determine abundance variations with respect to the primordial population of the cluster. Our results are in good agreement with the values expected from previous studies based on high-resolution spectroscopy. We do not see any significant variations in the spectral lines of Ca, K, and Ba. We also do not detect abundance variations among the stars belonging to the primordial population of NGC 2808. Conclusions. We demonstrate that in spite of their low resolution, the MUSE spectra can be used to investigate abundance variations in the context of multiple populations. [ABSTRACT FROM AUTHOR]

In our HST photometric survey, we have been searching for multiple stellar populations (MPs) in Magellanic Clouds (MCs) massive star clusters which span a significant range of ages (∼1.5–11 Gyr). In the previous papers of the series, we have shown that the age of the cluster represents one of the key factors in shaping the origin of the chemical anomalies. Here, we present the analysis of four additional clusters in the MCs, namely Lindsay 38, Lindsay 113, NGC 2121, and NGC 2155, for which we recently obtained new UV HST observations. These clusters are more massive than ∼104 M⊙ and have ages between ∼2.5 and ∼6 Gyr, i.e. located in a previously unexplored region of the cluster age/mass diagram. We found chemical anomalies, in the form of N spreads, in three out of four clusters in the sample, namely in NGC 2121, NGC 2155, and Lindsay 113. By combining data from our survey and HST photometry for three additional clusters in the Milky Way (namely 47 Tuc, M15, and NGC 2419), we show that the extent of the MPs in the form of N spread is a strong function of age, with older clusters having larger N spreads with respect to the younger ones. Hence, we confirm that cluster age plays a significant role in the onset of MPs. [ABSTRACT FROM AUTHOR]

We report the spectroscopic discovery of abundance spreads (i.e. multiple populations) in the ∼2 Gyr old cluster in the LMC, Hodge 6. We use low-resolution VLT FORS2 spectra of 15 member stars in the cluster to measure their CN and CH band strengths at ≃3883 and 4300 Å, respectively, as well as [C/Fe] and [N/Fe] abundances. We find a subpopulation of two stars that are enriched in nitrogen, and we conclude that this subpopulation is evidence of multiple populations in Hodge 6. This is the second ∼2 Gyr old cluster (the first being NGC 1978 in the LMC) to show multiple populations and the first spectroscopic detection of MPs in a cluster of this age. This result is interesting as it hints at a possible relationship between the disappearance of extended main sequence turn-offs in clusters younger than ∼2 Gyr and the onset of multiple populations at ∼2 Gyr, which should be explored further. [ABSTRACT FROM AUTHOR]

Intermediate-age star clusters in the Magellanic Clouds harbour signatures of the multiple stellar populations long thought to be restricted to old globular clusters. We compare synthetic horizontal branch models with Hubble Space Telescope photometry of clusters in the Magellanic Clouds, with age between ∼2 and ∼10 Gyr, namely NGC 121, Lindsay 1, NGC 339, NGC 416, Lindsay 38, Lindsay 113, Hodge 6, and NGC 1978. We find a clear signature of initial helium abundance spreads (Δ Y) in four out of these eight clusters (NGC 121, Lindsay 1, NGC 339, NGC 416) and we quantify the value of Δ Y. For two clusters (Lindsay 38, Lindsay 113), we can only determine an upper limit for Δ Y, whilst for the two youngest clusters in our sample (Hodge 6 and NGC 1978) no conclusion about the existence of an initial He spread can be reached. Our Δ Y estimates are consistent with the correlation between maximum He abundance spread and mass of the host cluster found in Galactic globular clusters. This result strengthens the emerging view that the formation of multiple stellar populations is a standard process in massive star clusters, not limited to a high-redshift environment. [ABSTRACT FROM AUTHOR]

We present the results of a spectroscopic study of the intermediate age (≈6.5 Gyr) massive cluster Kron 3 in the Small Magellanic Cloud. We measure CN and CH band strengths (at ⋍3839 and 4300 Å, respectively) using VLT FORS2 spectra of 16 cluster members and find a sub-population of five stars enriched in nitrogen. We conclude that this is evidence for multiple populations in Kron 3, the fourth intermediate age cluster, after Lindsay 1, NGC 416 and NGC 339 (ages 6-8 Gyr), to display this phenomenon originally thought to be a unique characteristic of old globular clusters. At ≈6.5 Gyr this is one of the youngest clusters with multiple populations, indicating that the mechanism responsible for their onset must operate until a redshift of at least 0.75, much later than the peak of globular cluster formation at redshift ~3. [ABSTRACT FROM AUTHOR]

This is the second paper in our series about the search for multiple populations in Magellanic Cloud star clusters using the Hubble Space Telescope. Here we report the detection of multiple stellar populations in the colour-magnitude diagrams of the intermediate-age clusters Lindsay 1, NGC 416 and NGC 339. With ages between 6.0 and 7.5 Gyr, these clusters are the youngest ones in which chemical abundance spreads have been detected so far. This confirms that the appearance of multiple populations is not restricted to only ancient globular clusters, but may also be a common feature in clusters as young as 6 Gyr. Our results are in agreement with a recent spectroscopic study of Lindsay 1. We found that the fraction of enriched stars in NGC 416 is ~45 per cent whereas it is ~25 per cent in NGC 339 and ~36 per cent in Lindsay 1. Similar to NGC 121, these fractions are lower than the average value for globular clusters in the Milky Way. [ABSTRACT FROM AUTHOR]

Studies during the last decade have revealed that nearly all globular clusters (GCs) host multiple populations (MPs) of stars with a distinctive chemical patterns in light elements. No evidence of such MPs has been found so far in lower mass (<~ 104 M☉) open clusters nor in intermediate-age (1-2 Gyr) massive (>105 M☉) clusters in the Local Group. Young massive clusters (YMCs) have masses and densities similar to those expected of young GCs in the early Universe, and their near-infrared spectra are dominated by the light of red super giants (RSGs). The spectra of these stars may be used to determine the cluster's abundances, even though the individual stars cannot be spatially resolved from one another. We carry out a differential analysis between the Al lines of YMC NGC 1705: 1 and field Small Magellanic Cloud RSGs with similar metallicities. We exclude at high confidence extreme [Al/Fe] enhancements similar to those observed in GCs like NGC 2808 or NGC 6752. However, smaller variations cannot be excluded. [ABSTRACT FROM AUTHOR]

M2 has been claimed to possess three distinct stellar components that are enhanced in iron relative to each other. We use equivalent width measurements from 14 red giant branch stars from which Yong et al. detect a ∼0.8 dex wide, trimodal iron distribution to redetermine the metallicity of the cluster. In contrast to Yong et al., which derive atmospheric parameters following only the classical spectroscopic approach, we perform the chemical analysis using three different methods to constrain effective temperatures and surface gravities. When atmospheric parameters are derived spectroscopically, we measure a trimodal metallicity distribution, that well resembles that by Yong et al. We find that the metallicity distribution from Fe II lines strongly differs from the distribution obtained from Fe I features when photometric gravities are adopted. The Fe I distribution mimics the metallicity distribution obtained using spectroscopic parameters, while the Fe II shows the presence of only two stellar groups with metallicity [Fe/H] ≃ -1.5 and -1.1 dex, which are internally homogeneous in iron. This finding, when coupled with the high-resolution photometric evidence, demonstrates that M2 is composed by a dominant population (∼99 per cent) homogeneous in iron and a minority component (∼1 per cent) enriched in iron with respect to the main cluster population. [ABSTRACT FROM AUTHOR]

Some formation scenarios that have been put forward to explain multiple populations within globular clusters (GCs) require that the young massive cluster have large reservoirs of cold gas within them, which is necessary to form future generations of stars. In this paper, we use deep observations taken with Atacama Large Millimeter/submillimeter Array (ALMA) to assess the amount of molecular gas within three young (50-200 Myr) massive (~106 M⊙) clusters in the Antennae galaxies. No significant CO(3-2) emission was found associated with any of the three clusters. We place upper limits for the molecular gas within these clusters of ~1 × 105 M⊙ (or <9 percent of the current stellar mass). We briefly review different scenarios that propose multiple episodes of star formation and discuss some of their assumptions and implications. Our results are in tension with the predictions of GC formation scenarios that expect large reservoirs of cool gas within young massive clusters at these ages. [ABSTRACT FROM AUTHOR]

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 and 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. [ABSTRACT FROM AUTHOR]

One of the leading models for the formation of multiple stellar populations within globular clusters is the ‘fast rotating massive star’ (FRMS) scenario, where the ejecta of rapidly rotating massive stars is mixed with primordial material left over from the star formation process, to form a second generation of stars within the decretion discs of the high-mass stars. A requirement of this model, at least in its current form, is that young massive (i.e. proto-globular) clusters are not able to eject the unused gas and dust from the star formation process from the cluster for 20–30 Myr after the formation of the first generation of stars, i.e. the cluster remains embedded within the gas cloud in which it forms. Here, we test this prediction by performing a literature search for young massive clusters in nearby galaxies, which have ages less than 20 Myr that are not embedded. We report that a number of such clusters exist, with masses near or significantly above 106 M⊙, with ages between a few Myr and ∼15 Myr, suggesting that even high-mass clusters are able to clear any natal gas within them within a few Myr after formation. Additionally, one cluster, Cluster 23 in ESO 338-IG04, has a metallicity below that of some Galactic globular clusters that have been found to host multiple stellar populations, mitigating any potential effect of differences in metallicity in the comparison. The clusters reported here are in contradiction to the expectations of the FRMS scenario, at least in its current form. [ABSTRACT FROM PUBLISHER]

Scenarios that invoke multiple episodes of star formation within young globular clusters (GCs) to explain the observed chemical and photometric anomalies in GCs require that clusters can retain the stellar ejecta of the stars within them and accrete large amounts of gas from their surroundings. Hence, it should be possible to find young massive clusters in the local Universe that contain significant amounts (>10 per cent) of the cluster mass of gas and/or dust within them. Recent theoretical studies have suggested that clusters in the Large Magellanic Cloud (LMC) with masses in excess of 104 M⊙ and ages between 30 and ∼300 Myr should contain such gas reservoirs. We have searched for H i gas within 12 LMC (and 1 Small Magellanic Cloud) clusters and also for dust using Spitzer 70 and 160 μm images. No clusters were found to contain gas and/or dust. While two of the clusters have H i at the same (projected) position and velocity, the gas does not appear to be centred on the clusters, but rather part of nearby clouds or filaments, suggesting that the gas and cluster are not directly related. This lack of gas (<1 per cent of the stellar mass) is in strong tension with model predictions, and may be due to higher stellar feedback than has been previously assumed or due to the assumptions used in the previous calculations. [ABSTRACT FROM PUBLISHER]

Currently there are two competing scenarios to explain the origin of the stellar population in globular clusters (GCs). The main difference between them is whether or not multiple events of star formation took place within GCs. In this paper, we present the star formation history (SFH) of Cluster 1, a massive young cluster in NGC 34 ( ∼ 107M⊙). We use DynBaS (Dynamical Basis Selection), a spectrum fitting algorithm, to retrieve the SFH and find that Cluster 1 is consistent with a single stellar population of solar metallicity with an age of 100 ± 30 Myr and a mass of 1.9 ± 0.4 × 107M⊙. These results are in conflict with the expectations/predictions of the scenarios that invoke extended or multiple episodes within 30–100 Myr of the initial star formation burst in young massive clusters. [ABSTRACT FROM AUTHOR]

Cites a study published in 'Mon. Not. R. Astrom. Soc.,' which focuses on star cluster formation and evolution in starburst galaxies-II. Physical conditions conducive for formation of higher mass star structures; Significance of young compact star clusters for providing information about galaxy type globular clusters formation and evolution.

A significant number of intermediate age clusters (1–2 Gyr) in the Magellanic Clouds appear to have multiple stellar populations within them, derived from bimodal or extended main-sequence turn-offs. If this is interpreted as an age spread, the multiple populations are separated by a few hundred million years, which would call into question the long-held notion that clusters are simple stellar populations. Here, we show that stellar rotation in stars with masses between 1.2 and can mimic the effect of a double or multiple population, whereas in actuality only a single population exists. The two main causes of the spread near the turn-off are the effects of stellar rotation on the structure of the star and the inclination angle of the star relative to the observer. Both effects change the observed effective temperature, hence colour, and flux of the star. In order to match observations, the required rotation rates are 20–50 per cent of the critical rotation, which are consistent with observed rotation rates of similar mass stars in the Galaxy. We provide scaling relations which can be applied to non-rotating isochrones in order to mimic the effects of rotation. Finally, we note that rotation is unlikely to be the cause of the multiple stellar populations observed in old globular clusters, as low-mass stars are not expected to be rapid rotators. [ABSTRACT FROM AUTHOR]