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We used data from the near-infrared VISTA survey of the Magellanic Cloud system (VMC) to measure proper motions (PMs) of stars within the Small Magellanic Cloud (SMC). The data analysed in this study comprise 26 VMC tiles, covering a total contiguous area on the sky of ~40 deg$^2$. Using multi-epoch observations in the Ks band over time baselines between 13 and 38 months, we calculated absolute PMs with respect to ~130,000 background galaxies. We selected a sample of ~2,160,000 likely SMC member stars to model the centre-of-mass motion of the galaxy. The results found for three different choices of the SMC centre are in good agreement with recent space-based measurements. Using the systemic motion of the SMC, we constructed spatially resolved residual PM maps and analysed for the first time the internal kinematics of the intermediate-age/old and young stellar populations separately. We found outward motions that point either towards a stretching of the galaxy or stripping of its outer regions. Stellar motions towards the North might be related to the "Counter Bridge" behind the SMC. The young populations show larger PMs in the region of the SMC Wing, towards the young Magellanic Bridge. In the older populations, we further detected a coordinated motion of stars away from the SMC in the direction of the Old Bridge as well as a stream towards the SMC., 20 pages, 20 figures, accepted for publication in MNRAS

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 A, respectively, as well as [C/Fe] and [N/Fe] abundances. We find a sub-population of 2 stars that are enriched in nitrogen, and we conclude that this sub-population 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., 10 pages, accepted for publication in MNRAS

We study the morphology of the stellar periphery of the Magellanic Clouds in search of substructure using near-infrared imaging data from the VISTA Hemisphere Survey (VHS). Based on the selection of different stellar populations using the ($J-K_\mathrm{s}$, $K_\mathrm{s}$) colour-magnitude diagram, we confirm the presence of substructures related to the interaction history of the Clouds and find new substructures on the easter side of the LMC disc which may be owing to the influence of the Milky Way, and on the northern side of the SMC, which is probably associated to the ellipsoidal structure of the galaxy. We also study the luminosity function of red clump stars in the SMC and confirm the presence of a bi-modal distance distribution, in the form of a foreground population. We find that this bi-modality is still detectable in the eastern regions of the galaxy out to a 10 deg distance from its centre. Additionally, a background structure is detected in the North between 7 and 10 deg from the centre which might belong to the Counter Bridge, and a foreground structure is detected in the South between 6 and 8 deg from the centre which might be linked to the Old Bridge., 20 pages, 19 figures, accepted for publication in MNRAS

The eastern region of the Small Magellanic Cloud (SMC) is found to have a foreground stellar sub-structure, which is identified as a distance bimodality (12 kpc apart) in the previous studies using Red Clump (RC) stars. Interestingly, studies of Red giant branch (RGB) stars in the eastern SMC indicate a bimodal radial velocity (RV) distribution. In this study, we investigate the connection between these two bimodal distributions to better understand the nature and origin of the foreground stellar sub-structure in the eastern SMC. We use the Gaia EDR3 astrometric data and archival RV data of RGB stars for this study. We found a bimodal RV distribution of RGB stars (separated by 35 - 45 km/s) in the eastern and south-western (SW) outer regions. The observed proper motion values of the lower and higher RV RGB components in the eastern regions are similar to those of the foreground and main-body RC stars respectively. This suggests that the two RGB populations in the eastern region are separated by a similar distance as those of the RC stars, and the RGB stars in the lower RV component are part of the foreground sub-structure. Based on the differences in the distance and RV of the two components, we estimated an approximate time of formation of this sub-structure as 307+/-65 Myr ago. This is comparable with the values predicted by simulations for the recent epoch of tidal interaction between the Magellanic Clouds. Comparison of the observed properties of RGB stars, in the outer SW region, with N-body simulations shows that the higher RV component in the SW region is at a farther distance than the main body, indicating the presence of a stellar Counter-Bridge in the SW region of the SMC., Comment: Accepted for publication in MNRAS, 11 pages, 6 figures

Recent observational studies identified a foreground stellar sub-structure traced by red clump (RC) stars (~ 12 kpc in front of the main body) in the eastern regions of the Small Magellanic Cloud (SMC) and suggested that it formed during the formation of the Magellanic Bridge (MB), due to the tidal interaction of the Magellanic Clouds. Previous studies investigated this feature only up to 4.0 deg from the centre of the SMC due to the limited spatial coverage of the data and hence could not find a physical connection with the MB. To determine the spatial extent and properties of this foreground population, we analysed data from the Gaia data release 2 (DR2) of a ~ 314 sq. deg region centred on the SMC, which cover the entire SMC and a significant portion of the MB. We find that the foreground population is present only between 2.5 deg to ~ 5-6 deg from the centre of the SMC in the eastern regions, towards the MB and hence does not fully overlap with the MB in the plane of the sky. The foreground stellar population is found to be kinematically distinct from the stellar population of the main body with ~ 35 km/s slower tangential velocity and moving to the North-West relative to the main body. Though the observed properties are not fully consistent with the simulations, a comparison indicates that the foreground stellar structure is most likely a tidally stripped counterpart of the gaseous MB and might have formed from the inner disc (dominated by stars) of the SMC. A chemical and 3D kinematic study of the RC stars along with improved simulations, including both tidal and hydro-dynamical effects, are required to understand the offset between the foreground structure and MB., Accepted for publication in MNRAS

The Magellanic Clouds are a nearby pair of interacting dwarf galaxies and satellites of the Milky Way. Studying their kinematic properties is essential to understanding their origin and dynamical evolution. They have prominent tidal features and the kinematics of these features can give hints about the formation of tidal dwarfs, galaxy merging and the stripping of gas. In addition they are an example of dwarf galaxies that are in the process of merging with a massive galaxy. The goal of this study is to investigate the kinematics of the Magellanic Bridge, a tidal feature connecting the Magellanic Clouds, using stellar proper motions to understand their most recent interaction. We calculated proper motions based on multi-epoch $K_{s}$-band aperture photometry, which were obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA), spanning a time of 1-3 yr, and we compared them with $Gaia$ Data Release 2 (DR2) proper motions. We tested two methods for removing Milky Way foreground stars using $Gaia$~DR2 parallaxes in combination with VISTA photometry or using distances based on Bayesian inference. We obtained proper motions for a total of 576,411 unique sources over an area of $23$ deg$^{2}$ covering the Magellanic Bridge including mainly Milky Way foreground stars, background galaxies, and a small population of possible Magellanic Bridge stars ($, Comment: 17 pages, 12 figures, accepted for publication in A&A

Studies of young stellar objects (YSOs) in the Galaxy have found that a significant fraction exhibit photometric variability. However, no systematic investigation has been conducted on the variability of extragalactic YSOs. Here we present the first variability study of massive YSOs in a $\sim 1.5\,\mathrm{deg^2}$ region of the Large Magellanic Cloud (LMC). The aim is to investigate whether the different environmental conditions in the metal-poor LMC ($\sim$ 0.4-0.5 Z_sun) have an impact on the variability characteristics. Multi-epoch near-infrared (NIR) photometry was obtained from the VISTA Survey of the Magellanic Clouds (VMC) and our own monitoring campaign using the VISTA telescope. By applying a reduced $\chi^2$-analysis, stellar variability was identified. We found 3062 candidate variable stars from a population of 362 425 stars detected. Based on several Spitzer studies, we compiled a sample of high-reliability massive YSOs: a total of 173 massive YSOs have NIR counterparts ($K_{\mathrm{s}}\sim 18.5\,$mag) in the VMC catalogue, of which 39 display significant ($>3\sigma$) variability. They have been classified as eruptive, fader, dipper, short-term variable and long-period variable YSOs based mostly on the appearance of their $K_{\mathrm{s}}$ band light curves. The majority of YSOs are aperiodic, only five YSOs exhibit periodic lightcurves. The observed amplitudes are comparable or smaller than those for Galactic YSOs (only two Magellanic YSOs exhibit $\Delta K_{\mathrm{s}}>1\,$mag), not what would have been expected from the typically larger mass accretion rates observed in the Magellanic Clouds., Comment: Accepted for publication in MNRAS, 32 pages, 24 figures

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 ($\sim30-60$\% of stars), which supports previous suggestions of large populations of rapidly rotating stars within massive clusters., Comment: 6 pages, 3 figures, accepted for publication in MNRAS

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 ($\sim 1.5-11$ 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 $\sim 10^4M_{\odot}$ and have ages between $\sim 2.5-6$ 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 3 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., Accepted for publication in MNRAS. 12 pages, 9 figures

The Magellanic Clouds are nearby dwarf irregular galaxies whose morphologies show different properties when traced by different stellar populations, making them an important laboratory for studying galaxy morphologies. We study the morphology of the Magellanic Clouds using data from the VISTA survey of the Magellanic Clouds system (VMC). We used about $10$ and $2.5$ million sources across an area of $\sim105$ deg$^2$ and $\sim42$ deg$^2$ towards the Large and Small Magellanic Cloud (LMC and SMC), respectively. We estimated median ages of stellar populations occupying different regions of the near-infrared ($J-K_\mathrm{s}, K_\mathrm{s}$) colour-magnitude diagram. Morphological maps were produced and detailed features in the central regions were characterised for the first time with bins corresponding to a spatial resolution of $0.13$ kpc (LMC) and $0.16$ kpc (SMC). In the LMC, we find that main sequence stars show coherent structures that grow with age and trace the multiple spiral arms of the galaxy, star forming regions become dimmer as we progress in age, while supergiant stars are centrally concentrated. Intermediate-age stars, despite tracing a regular and symmetrical morphology, show central clumps and hints of spiral arms. In the SMC, young main sequence stars depict a broken bar. Intermediate-age populations show signatures of elongation towards the Magellanic Bridge that can be attributed to the LMC-SMC interaction $\sim200$ Myr ago. They also show irregular central features suggesting that the inner SMC has also been influenced by tidal interactions., Accepted for publication in MNRAS, 20 pages, 12 figures and 2 tables

Detailed studies of intermediate/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. Our goal is to use near-infrared data from the VMC survey to identify and characterise PMS populations down to ~1 M_sun across the Magellanic Clouds. We present our colour-magnitude diagram method, and apply it to a ~1.5 deg^2 pilot field located in the Large Magellanic Cloud. The pilot field is divided into equally-sized grid elements. We compare the stellar population in every element with the population in nearby control fields by creating K_s/(Y-K_s) Hess diagrams; the observed density excesses over the local field population are used to classify the stellar populations. Our analysis recovers all known star formation complexes in this pilot field (N44, N51, N148 and N138) and for the first time reveals their true spatial extent. In total, around 2260 PMS candidates with ages $\lesssim$ 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 micron - 500 micron) is bright. Regions with young populations lacking massive stars show a lesser degree of clustering and are usually located in the outskirts of the star formation complexes. At short FIR wavelengths (70 micron, 100 micron) we report a strong dust emission increase in regions hosting young massive stars, which is less pronounced in regions populated only by less massive ($\lesssim$ 4 M_sun) PMS stars., Comment: Accepted by A&A, 17 pages, 17 Figures, 3 Tables

We present the first spatially resolved map of stellar proper motions within the central ($\sim$3.1 $\times$ 2.4 kpc) regions of the Small Magellanic Cloud (SMC). The data used for this study encompasses four tiles from the ongoing near-infrared VISTA survey of the Magellanic Clouds system and covers a total contiguous area on the sky of $\sim$6.81 deg$^2$. Proper motions have been calculated independently in two dimensions from the spatial offsets in the $K_s$ filter over time baselines between 22 and 27 months. The reflex motions of $\sim$33~000 background galaxies are used to calibrate the stellar motions to an absolute scale. The resulting catalog is composed of more than 690 000 stars which have been selected based on their position in the $(J-K_s, K_s)$ color-magnitude diagram. For the median absolute proper motion of the SMC, we find ($\mu_{\alpha}\mathrm{cos}(\delta)$, $\mu_{\delta}$) = (1.087 $\pm$ 0.192 (sys.) $\pm$ 0.003 (stat.), $-$1.187 $\pm$ 0.008 (sys.) $\pm$ 0.003 (stat.)) mas yr$^{-1}$, consistent with previous studies. Mapping the proper motions as a function of position within the SMC reveals a non uniform velocity pattern indicative of a tidal feature behind the main body of the SMC and a flow of stars in the South-East moving predominantly along the line-of-sight., Comment: 4 pages, 2 figures, accepted for publication in A&A

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., 11 pages, 10 figures, accepted for publication on MNRAS

The Magellanic Clouds are nearby dwarf irregular galaxies that represent a unique laboratory for studying galaxy interactions. Their morphology and dynamics have been heavily influenced by their mutual interactions as well as with their interaction(s) with the Milky Way. We use the VISTA near-infrared YJKs survey of the Magellanic Clouds system (VMC) in combination with stellar partial models of the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC) and the Milky Way to investigate the spatial distribution of stellar populations of different ages across the Magellanic Clouds. In this contribution, we present the results of these studies that allow us to trace substructures possibly related to the interaction history of the Magellanic Clouds., Comment: 4 pages, 2 figures, Poster contribution, IAU Symposium 344, Dwarf Galaxies: from the Deep Universe to the Present, (Eds) S. Stierwalt & K. McQuinn

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., 14 pages, 12 figures. Accepted for publication in MNRAS

We present the third paper about our ongoing HST survey for the search for multiple stellar populations (MPs) within Magellanic Cloud clusters. We report here the analysis of NGC 419, a $\sim 1.5$ Gyr old, massive ($\gtrsim 2 \times 10^5 \, {\rm M_{\odot}}$) star cluster in the Small Magellanic Cloud (SMC). By comparing our photometric data with stellar isochrones, we set a limit on [N/Fe] enhancement of $\lesssim$+0.5 dex and hence we find that no MPs are detected in this cluster. This is surprising because, in the first two papers of this series, we found evidence for MPs in 4 other SMC clusters (NGC 121; Lindsay 1, NGC 339, NGC 416), aged from 6 Gyr up to $\sim 10-11$ Gyr. This finding raises the question whether age could play a major role in the MPs phenomenon. Additionally, our results appear to exclude mass or environment as the only key factors regulating the existence of a chemical enrichment, since all clusters studied so far in this survey are equally massive ($\sim 1-2 \times 10^5 \, {\rm M_{\odot}}$) and no particular patterns are found when looking at their spatial distribution in the SMC., Comment: MNRAS, accepted. 9 pages, 7 figures

Using broadband photometry from the Hubble Space Telescope in combination with Very Large Telescope narrowband Halpha observations of the nearby spiral galaxy NGC 300, we explore a method for estimating the escape fractions of hydrogen-ionizing photons from HII regions within this galaxy. Our goal in this concept study is to evaluate the spectral types of the most massive stars using the broadband data and estimating their ionizing photon output with the help of stellar atmosphere models. A comparison with the Halpha flux that gives the amount of ionized gas in the HII region provides a measure of the escape fraction of ionizing photons from that region. We performed some tests with a number of synthetic young clusters with varying parameters to assess the reliability of the method. However, we found that the derived stellar spectral types and consequently the expected ionizing photon luminosity of a region is highly uncertain. The tests also show that on one hand we tended to overestimate the integrated photon output of a region for young ages and low numbers of stars, and on the other hand we mostly underestimated the combined ionizing luminosity for a large stellar number and older cluster ages. We conclude that the proposed method of using stellar broadband photometry to infer the leakage of ionizing photons from HII regions is highly uncertain and dominated by the errors of the resulting stellar spectral types. Therefore this method is not suitable. Stellar spectra are needed to reliably determine the stellar types and escape fractions. Studies to this end have been carried out for the Magellanic Clouds., 10 pages, 10 figures, accepted for publication in A&A

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 would be expected from photometric uncertainties. One interpretation of this feature is that age spreads of the order 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 intermediate-age 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 with two different theoretical isochrone models. We find that in most of the cases, the age spreads inferred from the red clumps are smaller than the ones resulting from the turn-off region. However, the age ranges resulting from the red clump region are broader than would be 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 not unambiguous, we can not ultimately tell if the extended main sequence turn-off feature is due to an age spread, or not, by fitting the star formation histories to the red clump regions. However, we 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 "age spread" interpretation, but which may be expected if stellar rotation is the cause of the spread at the turn-off., Comment: 17 pages, 18 figures, accepted for publication in A&A

We use deep multi-epoch 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 here corresponds to one VMC tile and covers a total sky area of 1.77 deg^2. Absolute proper motions with respect to ~9070 background galaxies are calculated from a linear regression model applied to the stellar positions in 11 epochs in the Ks filter. The data extend over a total time baseline of ~17 months. We found a median proper motion of the stars within 47 Tuc of (mu_a cos(d), mu_d) = (+5.89 +/- 0.02(stat) +/- 0.13(sys), -2.14 +/- 0.02(stat) +/- 0.08(sys)) mas/yr, based on the measurements of ~35000 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 (~2700 stars) we found a median proper motion of (mu_a cos(d), mu_d) = (+5.30 +/- 0.03(stat) +/- 0.70(sys), -2.70 +/- 0.03(stat) +/- 0.70(sys)) mas/yr. The values derived from the VMC data are consistent with the UCAC5 result, and are close to literature measurements obtained using HST. We combined our results with literature radial velocity measurements and reconstructed the orbit of 47 Tuc, finding that the cluster is on a low-ellipticity orbit and is confined within the inner ~7.5 kpc of the Galaxy. We show that the use of an increased time baseline combined with PSF-determined stellar centroids in crowded regions significantly improves the accuracy of the method. In future works, we will apply this method to more VMC tiles to study in detail the kinematics of the Magellanic Clouds., Comment: 16 pages, 15 figures, accepted for publication in A&A

We present Hubble Space Telescope photometry of NGC1850, a ~100 Myr, ~10^5 Msun cluster in the Large Magellanic Cloud. The colour magnitude diagram clearly shows the presence of an extended main sequence turnoff (eMSTO). The use of non-rotating stellar isochrones leads to an age spread of ~40 Myr. This is in good agreement with the age range expected when the effects of rotation in MSTO stars are wrongly interpreted in terms of age spread. We also do not find evidence for multiple, isolated episodes of star-formation bursts within the cluster, in contradiction to scenarios that invoke actual age spreads to explain the eMSTO phenomenon. NGC 1850 therefore continues the trend of eMSTO clusters where the inferred age spread is proportional to the age of the cluster. While our results confirm a key prediction of the scenario where stellar rotation causes the eMSTO feature, direct measurements of the rotational rate of MSTO stars is required to definitively confirm or refute whether stellar rotation is the origin of the eMSTO phenomenon or if it is due to an as yet undiscovered effect., MNRAS letters, in press

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% whereas it is ~25% in NGC 339 and ~36% in Lindsay 1. Similar to NGC 121, these fractions are lower than the average value for globular clusters in the Milky Way., Comment: 7 pages, 6 figures, accepted for publication in MNRAS

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.

We started a photometric survey using the WFC3/UVIS instrument onboard the Hubble Space Telescope to search for multiple populations within Magellanic Cloud star clusters at various ages. In this paper, we introduce this survey. As first results of this programme, we also present multi-band photometric observations of NGC 121 in different filters taken with the WFC3/UVIS and ACS/WFC instruments. We analyze the colour-magnitude diagram (CMD) of NGC 121, which is the only "classical" globular cluster within the Small Magellanic Cloud. Thereby, we use the pseudo-colour C_(F336W,F438W,F343N)=(F336W-F438W)-(F438W-F343N) to separate populations with different C and N abundances. We show that the red giant branch splits up in two distinct populations when using this colour combination. NGC 121 thus appears to be similar to Galactic globular clusters in hosting multiple populations. The fraction of enriched stars (N rich, C poor) in NGC 121 is about 32% +/- 3%, which is lower than the median fraction found in Milky Way globular clusters. The enriched population seems to be more centrally concentrated compared to the primordial one. These results are consistent with the recent results by Dalessandro et al. (2016). The morphology of the Horizontal Branch in a CMD using the optical filters F555W and F814W is best produced by a population with a spread in Helium of Delta(Y) =0.025+/-0.005., Comment: 12 pages, 9 figures, accepted for publication in MNRAS

The presence of extended main-sequence turn-off (eMSTO) regions in intermediate-age star clusters in the Large and Small Magellanic Clouds is often interpreted as resulting from extended star-formation histories (SFHs), lasting $\geq$ 300 Myr. This strongly conflicts with the traditional view of the dominant star-formation mode in stellar clusters, which are thought of as single-aged stellar populations. Here we present a test of this interpretation by exploring the morphology of the subgiant branch (SGB) of NGC 411, which hosts possibly the most extended eMSTO among all known intermediate-age star clusters. We show that the width of the NGC 411 SGB favours the single-aged stellar population interpretation and rules out an extended SFH. In addition, when considering the red clump (RC) morphology and adopting the unproven premise that the widths of all features in the colour--magnitude diagram are determined by an underlying range in ages, we find that the SFH implied is still very close to that resulting from a single-aged stellar population, with a minor fraction of stars scattering to younger ages compared with the bulk of the population. The SFHs derived from the SGB and RC are both inconsistent with the SFH derived from the eMSTO region. NGC 411 has a very low escape velocity and it has unlikely undergone significant mass loss at an early stage, thus indicating that it may lack the capacity to capture most of its initial, expelled gas from stellar evolutionary processes, a condition often required for extended SFHs to take root., 11 pages, 11 figures, MNRAS accepted

The colour-magnitude diagrams of some intermediate-age clusters (1-2 Gyr) star clusters show unexpectedly broad main-sequence turnoffs, raising the possibility that these clusters have experienced more than one episode of star formation. Such a scenario predicts the existence of an extended main sequence turn off (eMSTO) only in clusters with escape velocities above a certain threshold ($>15$ km s$^{-1}$), which would allow them to retain or accrete gas that eventually would fuel a secondary extended star-formation episode. This paper presents a test of this scenario based on the study of the young and massive cluster NGC 7252: W3. We use the HST photometry from WFPC2 and WFC3 images obtained with UV and optical filters, as well as MagE echellette spectrograph data from the Las Campanas Clay 6.5m telescope, in order to construct the observed UV/optical SED of NGC 7252: W3. The observations are then compared with synthetic spectra based on different star formation histories consistent with those of the eMSTO clusters. We find that the SED of this cluster is best fitted by a synthetic spectrum with a single stellar population of age $570^{+70}_{-62}$ Myr and mass $1.13^{+0.14}_{-0.13}\times 10^8$ M$_\odot$, confirming earlier works on NGC 7252: W3. We also estimate the lower limit on the central escape velocity of 193 km s$^{-1}$. We rule out extended star-formation histories, like those inferred for the eMSTO clusters in the Magellanic Clouds, at high confidence. We conclude that the escape velocity of a cluster does not dictate whether a cluster can undergo extended periods of star formation., 14 pages, 10 figures, accepted for publication in MNRAS

Recent discoveries have put the picture of stellar clusters being simple stellar populations into question. In particular, the color-magnitude diagrams of intermediate age (1-2 Gyr) massive clusters in the Large Magellanic Cloud (LMC) show features that could be interpreted as age spreads of 100-500 Myr. If multiple generations of stars are present in these clusters then, as a consequence, young (, 19 pages, 33 figures, accepted for publication in A&A

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 ($, 6 pages, 4 figures, accepted for publication in MNRAS

Using Science Verification data from the Atacama Large Millimeter/Submillimeter Array (ALMA), we have identified and imaged five rotational transitions (J=5-4 and J=6-5) of the three silicon monoxide isotopologues 28SiO v=0, 1, 2 and 29SiO v=0 and 28Si18O v=0 in the frequency range from 214 to 246 GHz towards the Orion BN/KL region. The emission of the ground-state 28SiO, 29SiO and 28Si18O shows an extended bipolar shape in the northeast-southwest direction at the position of Radio Source I, indicating that these isotopologues trace an outflow (~18 km/s, P.A. ~50deg, ~5000 AU in diameter) that is driven by this embedded high-mass young stellar object (YSO). Whereas on small scales (10-1000 AU) the outflow from Source I has a well-ordered spatial and velocity structure, as probed by Very Long Baseline Interferometry (VLBI) imaging of SiO masers, the large scales (500-5000 AU) probed by thermal SiO with ALMA reveal a complex structure and velocity field, most likely related to the effects of the environment of the BN/KL region on the outflow emanating from Source I. The emission of the vibrationally-excited species peaks at the position of Source I. This emission is compact and not resolved at an angular resolution of ~1.5" (~600 AU at a distance of 420 pc). 2-D Gaussian fitting to individual velocity channels locates emission peaks within radii of 100 AU, i.e. they trace the innermost part of the outflow. A narrow spectral profile and spatial distribution of the v=1 J=5-4 line similar to the masing v=1 J=1-0 transition, provide evidence for the most highly rotationally excited (frequency > 200 GHz) SiO maser emission associated with Source I known to date. The maser emission will enable studies of the Source I disk-outflow interface with future ALMA longest baselines., Accepted to A&A, 11 pages, 13 figures

Even after decades of research, the origin of multiple stellar populations in globular clusters remains enigmatic. The question as to whether the galaxy environment plays a role in their formation remains unanswered. To that extent, we analysed two classical (> 10 Gyr old) Large Magellanic Cloud globular clusters namely, NGC 1786 and NGC 1898, using imaging data from Hubble Space Telescope to compare and contrast them with ancient Galactic globular clusters to assess systematic differences that might exist between their abundance variations. We calculated their Red Giant Branch width, subtracted the effect of metallicity and compared it with the available data on Galactic globular clusters by plotting them against initial and current cluster mass. We see that the two clusters follow the same general trend as that of the Galactic globular clusters and Galactic globular clusters from different progenitors follow the same general trend as one another, indicating that the galaxy environment may only play a minor role in the formation of multiple stellar populations within globular clusters., Accepted for publication in MNRAS. 20 pages, 14 figures, 2 Appendices

We have derived high spatial resolution metallicity maps covering $\sim$42 deg$^2$ across the Small Magellanic Cloud (SMC) in an attempt to understand its metallicity distribution and gradients up to a radius of $\sim$ 4$^{\circ}$. Using the near-infrared VISTA Survey of the Magellanic Clouds, our data cover a thrice larger area compared with previous studies. We identify red giant branch (RGB) stars in spatially distinct $Y, (Y-K_{\rm s})$ colour--magnitude diagrams. In any of our selected subregions, the RGB slope is used as an indicator of the average metallicity, based on calibration to metallicity using spectroscopic data. The metallicity distribution across the SMC is unimodal and can be fitted by a Gaussian distribution with a peak at [Fe/H] = $-$0.97 dex ($\sigma$[Fe/H] = 0.05 dex). We find evidence of a shallow gradient in metallicity ($-0.031 \pm 0.005$ dex deg$^{-1}$) from the galactic centre to radii of 2$^{\circ}$--2.5$^{\circ}$, followed by a flat metallicity trend from $\sim$ 3.5$^{\circ}$ to 4$^{\circ}$. We find that the SMC's metallicity gradient is radially asymmetric. It is flatter towards the East than to the West, hinting at mixing and/or distortion of the spatial metallicity distribution (within the inner 3$^{\circ}$), presumably caused by tidal interactions between the Magellanic Clouds., Comment: 16 pages, 15 figures, 1 table, Accepted for publication in MNRAS on 15 Jul 2020

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 ($\sim10^5$ M$_{\odot}$, $\sim1.4$ Gyr). Unlike previous studies, here we focus on stars in the same mass range found in old globulars ($\sim0.75-1$ M$_{\odot}$), where light elements variations are detected. We find no evidence for N abundance variations among these stars in the $Un-B$ and $U-B$ CMD 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., Comment: 9 pages, 8 figures, accepted for publication in MNRAS