Showing papers by "Nate Bastian published in 2020"

Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations

Abstract: Globular clusters (GCs) formed when the Milky Way experienced a phase of rapid assembly. We use the wealth of information contained in the Galactic GC population to quantify the properties of the satellite galaxies from which the Milky Way assembled. To achieve this, we train an artificial neural network on the E-MOSAICS cosmological simulations of the co-formation and co-evolution of GCs and their host galaxies. The network uses the ages, metallicities, and orbital properties of GCs that formed in the same progenitor galaxies to predict the stellar masses and accretion redshifts of these progenitors. We apply the network to Galactic GCs associated with five progenitors: {\it Gaia}-Enceladus, the Helmi streams, Sequoia, Sagittarius, and the recently discovered, `low-energy' GCs, which provide an excellent match to the predicted properties of the enigmatic galaxy `Kraken'. The five galaxies cover a narrow stellar mass range [$M_\star=(0.6{-}4.6)\times10^8~{\rm M}_\odot$], but have widely different accretion redshifts ($z_{\rm acc}=0.57{-}2.65$). All accretion events represent minor mergers, but Kraken likely represents the most major merger ever experienced by the Milky Way, with stellar and virial mass ratios of $r_{M_\star}=1$:$31^{+34}_{-16}$ and $r_{M_{\rm h}}=1$:$7^{+4}_{-2}$, respectively. The progenitors match the $z=0$ relation between GC number and halo virial mass, but have elevated specific frequencies, suggesting an evolution with redshift. Even though these progenitors likely were the Milky Way's most massive accretion events, they contributed a total mass of only $\log{(M_{\rm \star,tot}/{\rm M}_\odot)}=9.0\pm0.1$, similar to the stellar halo. This implies that the Milky Way grew its stellar mass mostly by in-situ star formation. We conclude by organising these accretion events into the most detailed reconstruction to date of the Milky Way's merger tree.

How stellar rotation shapes the colour-magnitude diagram of the massive intermediate-age star cluster NGC 1846

Abstract: We present a detailed study of stellar rotation in the massive 1.5 Gyr old cluster NGC 1846 in the Large Magellanic Cloud. Similar to other clusters at this age, NGC 1846 shows an extended main sequence turn-off (eMSTO), and previous photometric studies have suggested it could be bimodal. In this study, we use MUSE integral-field spectroscopy to measure the projected rotational velocities (vsini) of around 1400 stars across the eMSTO and along the upper main sequence of NGC 1846. We measure vsini values up to ~250 km/s and find a clear relation between the vsini of a star and its location across the eMSTO. Closer inspection of the distribution of rotation rates reveals evidence for a bimodal distribution, with the fast rotators centred around vsini = 140 km/s and the slow rotators centred around vsini = 60 km/s. We further observe a lack of fast rotating stars along the photometric binary sequence of NGC 1846, confirming results from the field that suggest that tidal interactions in binary systems can spin down stars. However, we do not detect a significant difference in the binary fractions of the fast and slowly rotating sub-populations. Finally, we report on the serendipitous discovery of a planetary nebula associated with NGC 1846.

The accreted nuclear clusters of the Milky Way

Abstract: A number of the massive clusters in the halo, bulge, and disc of the Galaxy are not genuine globular clusters (GCs) but instead are different beasts altogether. They are the remnant nuclear star clusters (NSCs) of ancient galaxies since accreted by the Milky Way. While some clusters are readily identifiable as NSCs and can be readily traced back to their host galaxy (e.g. M54 and the Sagittarius Dwarf galaxy), others have proven more elusive. Here, we combine a number of independent constraints, focusing on their internal abundances and overall kinematics, to find NSCs accreted by the Galaxy and trace them to their accretion event. We find that the true NSCs accreted by the Galaxy are: M54 from the Sagittarius Dwarf, ω Centari from Gaia-Enceladus/Sausage, NGC 6273 from Kraken, and (potentially) NGC 6934 from the Helmi Streams. These NSCs are prime candidates for searches of intermediate-mass black holes (BHs) within star clusters, given the common occurrence of galaxies hosting both NSCs and central massive BHs. No NSC appears to be associated with Sequoia or other minor accretion events. Other claimed NSCs are shown not to be such. We also discuss the peculiar case of Terzan 5, which may represent a unique case of a cluster–cluster merger.

Weighing stars from birth to death: mass determination methods across the HRD

Abstract: The mass of a star is the most fundamental parameter for its structure, evolution, and final fate It is particularly important for any kind of stellar archaeology and characterization of exoplanets There exists a variety of methods in astronomy to estimate or determine it In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants For all methods uncertainties and restrictions will be discussed We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between $[03,2]\%$ for the covered mass range of $M\in [01,16]\,\msun$, $75\%$ of which are stars burning hydrogen in their core and the other $25\%$ covering all other evolved stages We close with a recommendation how to combine various methods to arrive at a "mass-ladder" for stars

Where did the globular clusters of the Milky Way form? Insights from the E-MOSAICS simulations

Abstract: Globular clusters (GCs) are typically old, with most having formed at z >~ 2. This makes understanding their birth environments difficult, as they are typically too distant to observe with sufficient angular resolution to resolve GC birth sites. Using 25 cosmological zoom-in simulations of Milky Way-like galaxies from the E-MOSAICS project, with physically-motivated models for star formation, feedback, and the formation, evolution, and disruption of GCs, we identify the birth environments of present-day GCs. We find roughly half of GCs in these galaxies formed in-situ (52.0 +/- 1.0 per cent) between z ~ 2 - 4, in turbulent, high-pressure discs fed by gas that was accreted without ever being strongly heated through a virial shock or feedback. A minority of GCs form during mergers (12.6 +/- 0.6 per cent in major mergers, and 7.2 +/- 0.5 per cent in minor mergers), but we find that mergers are important for preserving the GCs seen today by ejecting them from their natal, high density interstellar medium (ISM), where proto-GCs are rapidly destroyed due to tidal shocks from ISM substructure. This chaotic history of hierarchical galaxy assembly acts to mix the spatial and kinematic distribution of GCs formed through different channels, making it difficult to use observable GC properties to distinguish GCs formed in mergers from ones formed by smooth accretion, and similarly GCs formed in-situ from those formed ex-situ. These results suggest a simple picture of GC formation, in which GCs are a natural outcome of normal star formation in the typical, gas-rich galaxies that are the progenitors of present-day galaxies.

Predicting accreted satellite galaxy masses and accretion redshifts based on globular cluster orbits in the E-MOSAICS simulations

Abstract: The ages and metallicities of globular clusters (GCs) are known to be powerful tracers of the properties of their progenitor galaxies, enabling their use in determining the merger histories of galaxies. However, while useful in separating GCs into individual accretion events, the orbits of GC groups themselves have received less attention as probes of their progenitor galaxy properties. In this work, we use simulations of galaxies and their GC systems from the E-MOSAICS project to explore how the present-day orbital properties of GCs are related to the properties of their progenitor galaxies. We find that the orbits of GCs deposited by accretion events are sensitive to the mass and merger redshift of the satellite galaxy. Earlier mergers and larger galaxy masses deposit GCs at smaller median apocentres and lower total orbital energy. The orbital properties of accreted groups of GCs can therefore be used to infer the properties of their progenitor galaxy, though there exists a degeneracy between galaxy mass and accretion time. Combining GC orbits with other tracers (GC ages, metallicities) will help to break the galaxy mass/accretion time degeneracy, enabling stronger constraints on the properties of their progenitor galaxy. In situ GCs generally orbit at lower energies (small apocentres) than accreted GCs, however they exhibit a large tail to high energies and even retrograde orbits (relative to the present-day disc), showing significant overlap with accreted GCs. Applying the results to Milky Way GCs groups suggests a merger redshift $z \sim 1.5$ for the Gaia Sausage/Enceladus and $z>2$ for the `low-energy'/Kraken group, adding further evidence that the Milky Way had two significant mergers in its past.

The mass fraction of halo stars contributed by the disruption of globular clusters in the E-MOSAICS simulations

Abstract: Globular clusters (GCs) have been posited, alongside dwarf galaxies, as significant contributors to the field stellar population of the Galactic halo. In order to quantify their contribution, we examine the fraction of halo stars formed in stellar clusters in the suite of 25 present-day Milky Way-mass cosmological zoom simulations from the E-MOSAICS project. We find that a median of $2.3$ and $0.3$ per cent of the mass in halo field stars formed in clusters and GCs, defined as clusters more massive than $5\times 10^3$ and $10^5~M_{\odot}$, respectively, with the $25$-$75$th percentiles spanning $1.9$-$3.0$ and $0.2$-$0.5$ per cent being caused by differences in the assembly histories of the host galaxies. Under the extreme assumption that no stellar cluster survives to the present day, the mass fractions increase to a median of $5.9$ and $1.8$ per cent. These small fractions indicate that the disruption of GCs plays a sub-dominant role in the build-up of the stellar halo. We also determine the contributed halo mass fraction that would present signatures of light-element abundance variations considered to be unique to GCs, and find that clusters and GCs would contribute a median of $1.1$ and $0.2$ per cent, respectively. We estimate the contributed fraction of GC stars to the Milky Way halo, based on recent surveys, and find upper limits of $2$-$5$ per cent (significantly lower than previous estimates), suggesting that models other than those invoking strong mass loss are required to describe the formation of chemically enriched stellar populations in GCs.

The globular cluster system mass–halo mass relation in the E-MOSAICS simulations

Abstract: Linking globular clusters (GCs) to the assembly of their host galaxies is an overarching goal in GC studies. The inference of tight scaling relations between GC system properties and the mass of both the stellar and dark halo components of their host galaxies are indicative of an intimate physical connection, yet have also raised fundamental questions about how and when GCs form. Specifically, the inferred correlation between the mass of a GC system (Mgc) and the dark matter halo mass (Mhalo) of a galaxy has been posited as a consequence of a causal relation between the formation of dark matter mini-haloes and GC formation during the early epochs of galaxy assembly. We present the first results from a new simulation of a cosmological volume ($L=34.4$~cMpc on a side) from the E-MOSAICS suite, which includes treatments of the formation and evolution of GCs within the framework of a detailed galaxy formation model. The simulated Mgc-Mhalo relation is linear for halo masses $>5\times10^{11}~Msun$, and is driven by the hierarchical assembly of galaxies. Below this halo mass, the simulated relation features a downturn, which we show is consistent with observations, and is driven by the underlying stellar mass-halo mass relation of galaxies. Our fiducial model reproduces the observed Mgc-Mstar relation across the full mass range, which we argue is more physically relevant than the Mgc-Mhalo relation. We also explore the physical processes driving the observed constant value of $Mgc / Mhalo \sim 5\times10^{-5}$ and find that it is the result of a combination of cluster formation physics and cluster disruption.

On the origin of the bimodal rotational velocity distribution in stellar clusters: rotation on the pre-main sequence

Abstract: We address the origin of the observed bimodal rotational distribution of stars in massive young and intermediate age stellar clusters. This bimodality is seen as split main sequences at young ages and also has been recently directly observed in the $Vsini$ distribution of stars within massive young and intermediate age clusters. Previous models have invoked binary interactions as the origin of this bimodality, although these models are unable to reproduce all of the observational constraints on the problem. Here we suggest that such a bimodal rotational distribution is set up early within a cluster's life, i.e., within the first few Myr. Observations show that the period distribution of low-mass ($\la 2 M_\odot$) pre-main sequence (PMS) stars is bimodal in many young open clusters and we present a series of models to show that if such a bimodality exists for stars on the PMS that it is expected to manifest as a bimodal rotational velocity (at fixed mass/luminosity) on the main sequence for stars with masses in excess of $\sim1.5$~\msun. Such a bimodal period distribution of PMS stars may be caused by whether stars have lost (rapid rotators) or been able to retain (slow rotators) their circumstellar discs throughout their PMS lifetimes. We conclude with a series of predictions for observables based on our model.

Extended main sequence turnoffs in open clusters as seen by Gaia - II. The enigma of NGC 2509

Abstract: We investigate the morphology of the colour-magnitude diagram (CMD) of the open cluster NGC 2509 in comparison with other Galactic open clusters of similar age using Gaia photometry. At $\sim900$ Myr Galactic open clusters in our sample all show an extended main sequence turn off (eMSTO) with the exception of NGC 2509, which presents an exceptionally narrow CMD. Our analysis of the Gaia data rules out differential extinction, stellar density, and binaries as a cause for the singular MSTO morphology in this cluster. We interpret this feature as a consequence of the stellar rotation distribution within the cluster and present the analysis with MIST stellar evolution models that include the effect of stellar rotation on which we based our conclusion. In particular, these models point to an unusually narrow range of stellar rotation rates ($\Omega/\Omega_{\rm{crit,ZAMS}} = [0.4, 0.6]$) within the cluster as the cause of this singular feature in the CMD of NGC 2509. Interestingly, models that do not include rotation are not as good at reproducing the morphology of the observed CMD in this cluster.

The binary content of multiple populations in NGC 3201

Abstract: 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 focuses on the cluster centre, where comparable binary fractions between the populations are predicted because of the 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 where the populations only differ in their initial concentrations, but instead suggests that the populations also formed with different fractions of binary stars.

Chromosome maps of young LMC clusters: an additional case of coeval multiple populations

Abstract: 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 ($\sim$2.5Gyr) and NGC 1783 ($\sim$1.5Gyr), from the Large Magellanic Cloud. We find a clear correlation between the RGB width in the pseudo-colour $C_{F275W,F343N,F438W}$ and the age of the cluster itself, with the older cluster having larger $\sigma(C_{F275W,F343N,F438W})^{RGB}$ and vice-versa. Unfortunately, the $\sigma$ 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$\pm$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.

The WAGGS project-III. Discrepant mass-to-light ratios of Galactic globular clusters at high metallicity

Abstract: 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 1,622 stars located in the centres of 59 Milky Way GCs - twelve of which have no previous kinematic information - using integral-field unit data from the WAGGS project. Using N-body models, we then determine dynamical masses and M/L ratios 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 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.

The peculiar kinematics of the multiple populations in the globular cluster Messier 80 (NGC 6093)

Abstract: 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 not only vary in their light-element abundances, but also in their radial distributions, with concentration decreasing with increasing nitrogen enrichment. This remarkable trend, which sets M80 apart from the 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 sigma confidence level. While our data further suggest that the intermediate population shows the least amount of rotation, this trend is rather marginal (1 - 2 sigma). 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.

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

Abstract: 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.

Photometric characterization of multiple populations in star clusters: the impact of the first dredge-up

Abstract: The existence of star-to-star light-element abundance variations (multiple populations, MPs) in massive Galactic and extragalactic star clusters older than about 2 Gyr is by now well established. Photometry of red giant branch (RGB) stars has been and still is instrumental in enabling the detection and characterization of cluster MPs, through the appropriate choices of filters, colours and colour combinations, that are mainly sensitive to N and --to a lesser degree-- C stellar surface abundances. An important issue not yet properly addressed is that the translation of the observed widths of the cluster RGBs to abundance spreads must account for the effect of the first dredge-up on the surface chemical patterns, hence on the spectral energy distributions of stars belonging to the various MPs. We have filled this gap by studying theoretically the impact of the dredge-up on the predicted widths of RGBs in clusters hosting MPs. We find that for a given initial range of N abundances, the first dredge up reduces the predicted RGB widths in N-sensitive filters compared to the case when its effect on the stellar spectral energy distributions is not accounted for. This reduction is a strong function of age and has also a dependence on metallicity. The net effect is an underestimate of the initial N-abundance ranges from RGB photometry if the first dredge-up is not accounted for in the modelling, and also the potential determination of spurious trends of N-abundance spreads with age.

Linking globular cluster formation at low and high redshift through the age-metallicity relation in E-MOSAICS

Abstract: We set out to compare the age–metallicity relation (AMR) of massive clusters from Magellanic Cloud mass galaxies in the E-MOSAICS suite of numerical cosmological simulations with an amalgamation of observational data of massive clusters in the Large and Small Magellanic Clouds (LMC/SMC). We aim to test if: (i) star cluster formation proceeds according to universal physical processes, suggestive of a common formation mechanism for young-massive clusters (YMCs), intermediate-age clusters (IACs), and ancient globular clusters (GCs); (ii) massive clusters of all ages trace a continuous AMR; and (iii) the AMRs of smaller mass galaxies show a shallower relation when compared to more massive galaxies. Our results show that, within the uncertainties, the predicted AMRs of L/SMC-mass galaxies with similar star formation histories to the L/SMC follow the same relation as observations. We also find that the metallicity at which the AMR saturates increases with galaxy mass, which is also found for the field star AMRs. This suggests that relatively low-metallicity clusters can still form in dwarfs galaxies. Given our results, we suggest that ancient GCs share their formation mechanism with IACs and YMCs, in which GCs are the result of a universal process of star cluster formation during the early episodes of star formation in their host galaxies.

Leveraging HST with MUSE - I. Sodium abundance variations within the 2-Gyr-old cluster NGC 1978

Abstract: 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, Al), known as "multiple populations" (MPs). However, MPs are not restricted to ancient clusters, with massive clusters down to $\sim2$ 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 HST photometry with VLT/MUSE spectroscopy of a large sample of 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 $\Delta$[Na/Fe]$=0.07\pm0.01$ 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.

Searching for multiple populations in the integrated light of the young and extremely massive clusters in the merger remnant NGC 7252

Abstract: Recent work has shown that the properties of multiple populations within massive stellar clusters (i.e., in the extent of their abundance variations as well as the fraction of stars that show the anomalous chemistry) depend on the mass as well as the age of the host cluster. Such correlations are largely unexpected in current models for the formation of multiple populations and hence provide essential insight into their origin. Here we extend or previous study into the presence or absence of multiple populations using integrated light spectroscopy of the $\sim600$~Myr, massive ($\sim10^7 - 10^8$~\msun) clusters, W3 and W30, in the galactic merger remnant, NGC 7252. Due to the extreme mass of both clusters, the expectation is that they should host rather extreme abundance spreads, manifested through, e.g., high mean [Na/Fe] abundances. However, we do not find evidence for a strong [Na/Fe] enhancement, with the observations being consistent with the solar value. This suggests that age is playing a key role, or alternatively that multiple populations only manifest below a certain stellar mass, as the integrated light at all ages above $\sim100$~Myr is dominated by stars near or above the main sequence turn-off.

The loss of the intracluster medium in globular clusters

Abstract: 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 intra-cluster 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 ionisation by UV sources. We use the code ramses to run 3D hydrodynamical simulations to study for the first time the ICM evolution within discretised multi-mass GC models including stellar winds and full radiative transfer. We find that the inclusion of both ram-pressure and ionisation is mandatory to explain why only a very low amount of ionised 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.

Time-domain Study of the Young Massive Cluster Westerlund 2 with the Hubble Space Telescope. I

Abstract: Time-domain studies of pre-main-sequence (PMS) stars have long been used to investigate star properties during their early evolutionary phases and to trace the evolution of circumstellar environments. Historically these studies have been confined to the nearest, low-density, star-forming regions. We used the Wide Field Camera 3 on board the Hubble Space Telescope to extend, for the first time, the study of PMS variability to one of the few young massive clusters in the Milky Way, Westerlund 2. Our analysis reveals that at least one-third of the intermediate- and low-mass PMS stars in Westerlund 2 are variable. Based on the characteristics of their light curves, we classified ~11% of the variable stars as weak-line T Tauri candidates, ~52% as classical T Tauri candidates, ~5% as dippers, and ~26% as bursters. In addition, we found that 2% of the stars below 6 M ⊙ (~6% of the variables) are eclipsing binaries, with orbital periods shorter than 80 days. The spatial distribution of the different populations of variable PMS stars suggests that stellar feedback and UV radiation from massive stars play an important role in the evolution of circumstellar and planetary disks.

Leveraging HST with MUSE: II. Na-abundance variations in intermediate age star clusters

Abstract: Ancient ($>$10 Gyr) globular clusters (GCs) show chemical abundance variations in the form of patterns among certain elements, e.g. N correlates with Na and anti-correlates with O. Recently, N abundance spreads have also been observed in massive star clusters that are significantly younger than old GCs, down to an age of $\sim$2 Gyr. However, so far N has been the only element found to vary in such young objects. We report here the presence of Na abundance variations in the intermediate age massive star clusters NGC 416 ($\sim$6.5 Gyr old) and Lindsay 1 ($\sim$7.5 Gyr old) in the Small Magellanic Cloud, by combining HST and ESO-VLT MUSE observations. Using HST photometry we were able to construct ''chromosome maps'' and separate sub-populations with different N content, in the red giant branch of each cluster. MUSE spectra of individual stars belonging to each population were combined, resulting in high signal-to-noise spectra representative of each population, which were compared to search for mean differences in Na. We find a mean abundance variation of $\Delta$[Na/Fe]$=0.18\pm0.04$ dex for NGC 416 and $\Delta$[Na/Fe]$=0.24\pm0.05$ dex for Lindsay 1. In both clusters we find that the population that is enhanced in N is also enhanced in Na, which is the same pattern to the one observed in ancient GCs. Furthermore, we detect a bimodal distribution of core-helium burning Red Clump (RC) giants in the UV colour magnitude diagram of NGC 416. A comparison of the stacked MUSE spectra of the two RCs shows the same mean Na abundance difference between the two populations. The results reported in this work are a crucial hint that star clusters of a large age range share the same origin: they are the same types of objects, but only separated in age.

The globular cluster system mass-halo mass relation in the E-MOSAICS simulations

Abstract: Linking globular clusters (GCs) to the assembly of their host galaxies is an overarching goal in GC studies. The inference of tight scaling relations between GC system properties and the mass of both the stellar and dark halo components of their host galaxies are indicative of an intimate physical connection, yet have also raised fundamental questions about how and when GCs form. Specifically, the inferred correlation between the mass of a GC system (Mgc) and the dark matter halo mass (Mhalo) of a galaxy has been posited as a consequence of a causal relation between the formation of dark matter mini-haloes and GC formation during the early epochs of galaxy assembly. We present the first results from a new simulation of a cosmological volume ($L=34.4$~cMpc on a side) from the E-MOSAICS suite, which includes treatments of the formation and evolution of GCs within the framework of a detailed galaxy formation model. The simulated Mgc-Mhalo relation is linear for halo masses $>5\times10^{11}~Msun$, and is driven by the hierarchical assembly of galaxies. Below this halo mass, the simulated relation features a downturn, which we show is consistent with observations, and is driven by the underlying stellar mass-halo mass relation of galaxies. Our fiducial model reproduces the observed Mgc-Mstar relation across the full mass range, which we argue is more physically relevant than the Mgc-Mhalo relation. We also explore the physical processes driving the observed constant value of $Mgc / Mhalo \sim 5\times10^{-5}$ and find that it is the result of a combination of cluster formation physics and cluster disruption.

Time-domain study of the young massive cluster Westerlund 2 with the Hubble Space Telescope. I

Abstract: Time-domain studies of pre-main sequence stars have long been used to investigate star properties during their early evolutionary phases and to trace the evolution of circumstellar environments. Historically these studies have been confined to the nearest, low-density, star forming regions. We used the Wide Field Camera 3 on board of the Hubble Space Telescope to extend, for the first time, the study of pre-main sequence variability to one of the few young massive clusters in the Milky Way, Westerlund 2. Our analysis reveals that at least 1/3 of the intermediate and low-mass pre-main sequence stars in Westerlund 2 are variable. Based on the characteristics of their light curves, we classified ~11% of the variable stars as weak-line T-Tauri candidates, ~ 52% as classical T-Tauri candidates, ~ 5% as dippers and ~26% as bursters. In addition, we found that 2% of the stars below 6Mo (~6% of the variables) are eclipsing binaries, with orbital periods shorter than 80 days. The spatial distribution of the different populations of variable pre-main sequence stars suggests that stellar feedback and UV-radiation from massive stars play an important role on the evolution of circumstellar and planetary disks.