Magellanic Cloud Clusters Research Articles

The Role of Cluster Mass in the Multiple Populations of Galactic and Extragalactic Globular Clusters

Abstract Studies based on Hubble Space Telescope (HST) photometry in F275W, F336W, and F438W have shown that the incidence and complexity of multiple populations (MPs) in globular clusters (GCs) depend on cluster mass. This result, however, is based on nearby, low-reddening Galactic GCs, for which we can obtain accurate F275W photometry. In this work, we exploit HST photometry in F336W, F438W, and F814W to characterize MPs in 68 Galactic and extragalactic GCs by using the pseudocolor C F336W,F438W,F814W, which is more efficient in terms of observation time than the pseudocolor C F275W,F336W,F438W adopted in previous works. We first analyze the Galactic GCs alone and find that the red giant branch (RGB) color width strongly correlates with [Fe/H]. After removing the dependence from metallicity, we obtain a strong correlation with cluster mass, thus confirming previous findings. We then exploit the RGB width to compare MPs in Galactic and extragalactic GCs. Similarly to Galactic GCs, the RGB width of Magellanic Cloud (MC) GCs correlates with cluster mass after removing the dependence from metallicity. This fact demonstrates that cluster mass is the main factor affecting the properties of MPs. The MC clusters exhibit, on average, narrower RGBs than Galactic GCs with similar mass and metallicity. We suggest that this difference is a signature of stellar mass loss in Galactic GCs. As an alternative, the mass–RGB–width relation would depend on the host galaxy. Finally, we use ground-based photometry to investigate Terzan 7 through the C U,B,I pseudocolor, ground-based analogs of C F336W,F438W,F814W and find that this cluster is consistent with a simple population.

Size diversity of old Large Magellanic Cloud clusters as determined by internal dynamical evolution

The distribution of size as a function of age observed for star clusters in the Large Magellanic Cloud (LMC) is very puzzling: young clusters are all compact, while the oldest systems show both small and large sizes. It is commonly interpreted as due to a population of binary black holes driving a progressive expansion of cluster cores. Here we propose, instead, that it is the natural consequence of the fact that only relatively low-mass clusters have formed in the last ~3 Gyr in the LMC and only the most compact systems survived and are observable. The spread in size displayed by the oldest (and most massive) clusters, instead, can be explained in terms of initial conditions and internal dynamical evolution. To quantitatively explore the role of the latter, we selected a sample of five coeval and old LMC clusters with different sizes, and we estimated their dynamical age from the level of central segregation of blue straggler stars (the so-called dynamical clock). Similarly to what found in the Milky Way, we indeed measure different levels of dynamical evolution among the selected coeval clusters, with large-core systems being dynamically younger than those with small size. This behaviour is fully consistent with what expected from internal dynamical evolution processes over timescales mainly set by the structure of each system at formation.

An extragalactic chromosome map: the intermediate-age SMC cluster Lindsay 1

ABSTRACT The discovery of star-to-star abundance variations (a.k.a. multiple populations – MPs) within globular clusters (GCs), which are generally not found in the field or in lower mass open clusters, has led to a search for the unique property of GCs that allow them to host this phenomenon. Recent studies have shown that MPs are not limited to the ancient GCs but are also found in massive clusters with ages down to (at least) 2 Gyr. This finding is important for understanding the physics of the MP phenomenon, as these young clusters can provide much stronger constraints (e.g. on potential age spreads within the clusters) than older ones. However, a direct comparison between ancient GCs and intermediate clusters has not yet been possible due to the different filters adopted in their studies. Here we present new Hubble Space Telescope UV photometry of the 7.5 Gyr, massive Small Magellanic Cloud cluster, Lindsay 1, in order to compare its pseudo colour–colour diagram to that of Galactic GCs. We find that they are almost identical and conclude that the MPs phenomenon is the same, regardless of cluster age and host galaxy.

The search for multiple populations in Magellanic Clouds clusters – V. Correlation between cluster age and abundance spreads

ABSTRACT 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.

Extended Main-sequence Turnoffs in the Double Cluster h and χ Persei: The Complex Role of Stellar Rotation

Abstract Using Gaia Data Release 2 photometry, we report the detection of extended main-sequence turnoff (eMSTO) regions in the color–magnitude diagrams (CMDs) of the ∼14 Myr old double clusters h and χ Persei (NGC 869 and NGC 884). We find that stars with masses below ∼1.3 M ⊙ in both h and χ Persei populate narrow main sequences (MSs), while more massive stars define the eMSTO, closely mimicking observations of young Galactic and Magellanic Cloud clusters (with ages older than ∼30 Myr). Previous studies based on clusters older than ∼30 Myr found that rapidly rotating MS stars are redder than slow rotators of similar luminosity, suggesting that stellar rotation may be the main driver of the eMSTO. By combining photometry and projected rotational velocities from the literature of stars in h and χ Persei, we find no obvious relation between the rotational velocities and colors of non-emission-line eMSTO stars, in contrast with what is observed in older clusters. Similar to what is observed in Magellanic Cloud clusters, most of the extremely rapidly rotating stars, identified by their strong Hα emission lines, are located in the red part of the eMSTOs. This indicates that stellar rotation plays a role in the color and magnitude distribution of MSTO stars. By comparing the observations with simulated CMDs, we find that a simple population composed of coeval stars that span a wide range of rotation rates is unable to reproduce the color spread of the cluster’s MSs. We suggest that variable stars, binary interactions, and stellar rotation affect the eMSTO morphology of these very young clusters.

Multiple stellar populations: From old Milky Way globulars to young star clusters

AbstractI present the latest results from our group about the multiple stellar populations in the old Milky Way globular clusters (GCs) and in the young systems both in the Magellanic Clouds and in the Milky Way. For the ancient GCs in our Galaxy I summarize the chemical properties of the stellar populations as observed on the chromosome map. Both Type I and Type II GCs are discussed. For the youngest clusters I will briefly report our latest spectroscopic analysis on the Large Magellanic Cloud cluster NGC 1818 and the Galactic open cluster M 11, which supports the co-existence of stellar populations with different rotation rates.

Helium enrichment in intermediate-age Magellanic Clouds clusters: towards an ubiquity of multiple stellar populations?

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 (delta(Y)) in four out of these eight clusters (NGC 121, Lindsay 1, NGC 339, NGC 416) and we quantify the value of delta(Y). For two clusters (Lindsay 38, Lindsay 113) we can only determine an upper limit for delta(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 delta(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.

Blue Straggler Stars beyond the Milky Way. IV. Radial Distributions and Dynamical Implications

Blue straggler stars have been proposeed as powerful indicators to measure the dynamical state of Galactic globular clusters. Here we examine for the first time if this framework of blue straggler stars as dynamical clocks, which was specifically developed for Galactic globular clusters, may also hold for younger globular clusters in the Large Magellanic Cloud. Using Hubble Space Telescope observations, we study seven Large Magellanic Cloud star clusters with ages between ~700 Myr and 7 Gyr. We confirm that our sample clusters are consistent with the empirical correlation previously derived for Galactic globular clusters, within a modest tolerance. We suggest that to further examine if blue straggler stars can measure the dynamical state of their host clusters over an extended range of dynamical ages, more studies of dynamically older Magellanic Cloud clusters are required. We discuss the physical implications of our results in terms of their central, dimensionless King potential, as well as the initial retention fraction of black holes.

Search for a Metallicity Spread in the Multiple Population Large Magellanic Cloud Cluster NGC 1978

Abstract We report on the spread of [Fe/H] values in the massive Large Magellanic Cloud cluster NGC 1978, recently confirmed to harbor multiple populations of nearly the same age. We used accurate Strömgren photometry of carefully selected cluster red giant branch stars along with a high-dispersion spectroscopy-based calibration of the metallicity-sensitive index m 1. Once we accounted for the photometry quality, assessed from extensive artificial star tests to trace the photometric uncertainties as a function of the position of the cluster’s center as well as the stellar brightness, and those from the metallicity calibration, we found that NGC 1978 exhibits a small metallicity spread of 0.035 dex (±0.019–0.023), depending on whether stars with individual σ[Fe/H] ≤ 0.15 dex or those located in the cluster’s outer areas are considered. Such a spread in [Fe/H] is consistent with a cluster formation model with self-enrichment, if mass loss higher than 90% due to stellar evolutionary and galactic tidal effects is assumed. Nevertheless, scenarios in which the apparent [Fe/H] variation reflects CN abundance anomalies or less extreme mass-loss models with environmentally dependent self-enrichment should not be ruled out.

Metallicity estimates of young clusters in the Magellanic Clouds from Strömgren photometry of supergiant stars

We present results obtained from Stromgren photometry of 13 young (~30-220 Myr) Magellanic Cloud (MC) clusters, most of them lacking in the literature from direct metallicity measurements. We derived for them [Fe/H] values from a high-dispersion spectroscopy-based empirical calibration of the Stromgren metallicity sensitive index m1 for yellow and red supergiants (SGs). Particular care was given while estimating their respective uncertainties. In order to obtain the mean cluster metallicities, we used [Fe/H] values of selected SGs for which we required to be located within the cluster radii, placed in the expected SG region in the cluster colour-magnitude diagrams, and with [Fe/H] values within the FWHM of the observed cluster metallicity distributions. The resulting metallicities for nearly 75 per cent of the cluster sample agree well with the most frequently used values of the mean MCs' present-day metallicities. The remaining clusters have mean [Fe/H] values that fall near the edge of the MC present-day metallicity distributions. When comparing the cluster metallicities with their present positions, we found evidence that supports the claimed recent interaction of the MCs with the Milky Way, that could have caused that some clusters were scattered from their birthplaces. Indeed, we show examples of clusters with metal contents typical of the galaxy inner regions placed outward them. Likewise, we found young clusters, at present located in the inner regions of both MCs, formed out of gas that has remained unmixed since several Gyr ago.

Extended Main-sequence Turnoff as a Common Feature of Milky Way Open Clusters

Abstract We present photometric analysis of 12 Galactic open clusters and show that the same multiple-population phenomenon observed in Magellanic Clouds (MCs) is present in nearby open clusters. Nearly all the clusters younger than ∼2.5 Gyr of both MCs exhibit extended main-sequence turnoffs (eMSTOs) and all the cluster younger than ∼700 Myr show broadened/split main sequences (MSs). High-resolution spectroscopy has revealed that these clusters host stars with a large spread in the observed projected rotations. In addition to rotation, internal age variation is indicated as possibly responsible for the eMSTOs, making these systems the possible young counterparts of globular clusters with multiple populations. Recent work has shown that the eMSTO+broadened MSs are not a peculiarity of MCs clusters. Similar photometric features have been discovered in a few Galactic open clusters, challenging the idea that the color–magnitude diagrams (CMDs) of these systems are similar to single isochrones and opening new windows to explore the eMSTO phenomenon. We exploit photometry+proper motions from Gaia DR2 to investigate the CMDs of open clusters younger than ∼1.5 Gyr. Our analysis suggests that: (i) 12 open clusters show eMSTOs and/or broadened MSs, which cannot be due to either field contamination or binaries; (ii) split/broadened MSs are observed in clusters younger than ∼700 Myr, while older objects display only an eMSTO, similarly to MCs clusters; (iii) the eMSTO, if interpreted as a pure age spread, increases with age, following the relation observed in MCs clusters and demonstrating that rotation is responsible for this phenomenon.

Different Stellar Rotations in the Two Main Sequences of the Young Globular Cluster NGC 1818: The First Direct Spectroscopic Evidence*

Abstract We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC 1818 (age ∼40 Myr) in the Large Magellanic Cloud. Our photometric survey of Magellanic Clouds clusters has revealed that NGC 1818, similar to other young objects with ages ≲600 Myr, displays not only an extended MS turnoff (eMSTO), as observed in intermediate-age clusters (age ∼1–2 Gyr), but also a split MS. The most straightforward interpretation of the double MS is the presence of two stellar populations: a sequence of slowly rotating stars lying on the blue-MS (bMS) and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS (rMS). We report the first direct spectroscopic measurements of projected rotational velocities for the double MS, eMSTO, and Be stars of a young cluster. The analysis of line profiles includes non-local thermodynamic equilibrium effects, required for correctly deriving values. Our results suggest that: (i) the mean rotation for bMS and rMS stars is =71 ± 10 km s−1 (σ = 37 km s−1) and = 202 ± 23 km s−1 (σ = 91 km s−1), respectively; (ii) eMSTO stars have different , which are generally lower than those inferred for rMS stars, and (iii) as expected, Be stars display the highest values. This analysis supports the idea that distinct rotational velocities play an important role in the appearance of multiple stellar populations in the color–magnitude diagrams of young clusters, and poses new constraints on the current scenarios.

An Unexpected Detection of Bifurcated Blue Straggler Sequences in the Young Globular Cluster NGC 2173∗

Abstract The bifurcated patterns in the color–magnitude diagrams of blue straggler stars (BSSs) have attracted significant attention. This type of special (but rare) pattern of two distinct blue straggler sequences is commonly interpreted as evidence that cluster core-collapse-driven stellar collisions are an efficient formation mechanism. Here, we report the detection of a bifurcated blue straggler distribution in a young Large Magellanic Cloud cluster, NGC 2173. Because of the cluster’s low central stellar number density and its young age, dynamical analysis shows that stellar collisions alone cannot explain the observed BSSs. Therefore, binary evolution is instead the most viable explanation of the origin of these BSSs. However, the reason why binary evolution would render the color–magnitude distribution of BSSs bifurcated remains unclear.

Multiple stellar populations in Magellanic Cloud clusters – VI. A survey of multiple sequences and Be stars in young clusters

The split main sequences (MSs) and extended MS turnoffs (eMSTOs) detected in a few young clusters have demonstrated that these stellar systems host multiple populations differing in a number of properties such as rotation and, possibly, age.We analyze Hubble Space Telescope photometry for thirteen clusters with ages between ~40 and ~1000 Myrs and of different masses. Our goal is to investigate for the first time the occurrence of multiple populations in a large sample of young clusters. We find that all the clusters exhibit the eMSTO phenomenon and that MS stars more massive than ~1.6 solar masses define a blue and red MS, with the latter hosting the majority of MS stars. The comparison between the observations and isochrones suggests that the blue MSs are made of slow-rotating stars, while the red MSs host stars with rotational velocities close to the breakup value. About half of the bright MS stars in the youngest clusters are H-alpha emitters. These Be stars populate the red MS and the reddest part of the eMSTO thus supporting the idea that the red MS is made of fast rotators. We conclude that the split MS and the eMSTO are a common feature of young clusters in both Magellanic Clouds. The phenomena of a split MS and an eMSTO occur for stars that are more massive than a specific threshold which is independent of the host-cluster mass. As a by-product, we report the serendipitous discovery of a young SMC cluster, GALFOR1.

Blue straggler stars beyond the Milky Way: a non-segregated population in the Large Magellanic Cloud cluster NGC 2213

Using the high-resolution observations obtained by the Hubble Space Telescope, we analyzed the blue straggler stars (BSSs) in the Large Magellanic Cloud cluster NGC 2213. We found that the radial distribution of BSSs is consistent with that of the normal giant stars in NGC 2213, showing no evidence of mass segregation. However, an analytic calculation carried out for these BSSs shows that they are already dynamically old, because the estimated half-mass relaxation time for these BSSs is significantly shorter than the isochronal age of the cluster. We also performed direct N-body simulations for a NGC 2213-like cluster to understand the dynamical processes that lead to this none-segregated radial distribution of BSSs. Our numerical simulation shows that the presence of black hole subsystems inside the cluster centre can significantly affect the dynamical evolution of BSSs. The combined effects of the delayed segregation, binary disruption and exchange interactions of BSS progenitor binaries may result in this none-segregated radial distribution of BSSs in NGC 2213.

Stars caught in the braking stage in young Magellanic Cloud clusters

The color-magnitude diagrams of many Magellanic Cloud clusters (with ages up to 2 billion years) display extended turnoff regions where the stars leave the main sequence, suggesting the presence of multiple stellar populations with ages which may differ even by hundreds million years (Mackey et al. 2008, Milone et al. 2009, Girardi et al. 2011). A strongly debated question is whether such an extended turnoff is instead due to populations with different stellar rotations (Girardi et al. 2011, Goudfrooij et al. 2011, Rubele et al. 2013, Li et al. 2014). The recent discovery of a `split' main sequence in some younger clusters (about 80--400Myr) added another piece to this puzzle. The blue (red) side of the main sequence is consistent with slowly (rapidly) rotating stellar models (D'Antona et al. 2015, Milone et al. 2016, Correnti et al. 2017, Milone et al 2016), but a complete theoretical characterization of the observed color-magnitude diagram appeared to require also an age spread (Correnti et al. 2017). We show here that, in three clusters so far analyzed, if the blue main sequence stars are interpreted with models that have been always slowly rotating, they must be about 30% younger than the rest of the cluster. If they are instead interpreted as stars initially rapidly rotating, but that have later slowed down, the age difference disappears, and "braking" also helps to explain the apparent age differences of the extended turnoff. The age spreads in Magellanic Cloud clusters are a manifestation of rotational stellar evolution. Observational tests are suggested.

Observational hints of a real age spread in the young LMC star cluster NGC 1971

Abstract We report that the serendipitous young Large Magellanic Cloud cluster NGC 1971 exhibits an extended main-sequence turn-off (eMSTO) possibly originated mostly by a real age spread. We used CT1 Washington photometry to produce a colour–magnitude diagram (CMD) with the fiducial cluster features. From its eMSTO, we estimated an age spread of ∼170 Myr (observed age range 100–280 Myr), once observational errors, stellar binarity, overall metallicity variations and stellar rotation effects were subtracted in quadrature from the observed age width.

The search for multiple populations in Magellanic Cloud Clusters – III. No evidence for multiple populations in the SMC cluster NGC 419

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.

The search for multiple populations in Magellanic Cloud clusters – II. The detection of multiple populations in three intermediate-age SMC clusters

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.

Multiple stellar populations in Magellanic Cloud clusters – V. The split main sequence of the young cluster NGC 1866

One of the most unexpected results in the field of stellar populations of the last few years is the discovery that some Magellanic Cloud globular clusters younger than ∼400 Myr exhibit bimodal main sequences (MSs) in their colour–magnitude diagrams (CMDs). Moreover, these young clusters host an extended main-sequence turn-off (eMSTO) in close analogy with what is observed in most ∼1–2 Gyr old clusters of both Magellanic Clouds. We use high-precision Hubble Space Telescope photometry to study the young star cluster NGC 1866 in the Large Magellanic Cloud. We discover an eMSTO and a split MS. The analysis of the CMD reveals that (i) the blue MS is the less populous one, hosting about one-third of the total number of MS stars; (ii) red MS stars are more centrally concentrated than blue MS stars; (iii) the fraction of blue MS stars with respect to the total number of MS stars drops by a factor of ∼2 in the upper MS with mF814W ≲ 19.7. The comparison between the observed CMDs and stellar models reveals that the observations are consistent with ∼200 Myr old highly rotating stars on the red MS, with rotation close to critical value, plus a non-rotating stellar population spanning an age interval between ∼140 and 220 Myr, on the blue MS. Noticeable, neither stellar populations with different ages only, nor coeval stellar models with different rotation rates, properly reproduce the observed split MS and eMSTO. We discuss these results in the context of the eMSTO and multiple MS phenomenon.