Multiple Population Phenomenon Research Articles

Cracking the Relation between Mass and 1P Star Fraction of Globular Clusters. I. Present-day Cluster Masses as a First Tool

The phenomenon of multiple stellar populations is exacerbated in massive globular clusters, with the fraction of first-population (1P) stars a decreasing function of the cluster present-day mass. We decipher this relation in far greater detail than has been done so far. We assume (i) a fixed stellar mass threshold for the formation of second-population (2P) stars, (ii) a power-law scaling F1P∝mecl−1 between the mass m ecl of newly formed clusters and their 1P star fraction F 1P, and (iii) a constant F 1P over time. The F 1P(m ecl) relation is then evolved up to an age of 12 Gyr for tidal field strengths representative of the entire Galactic halo. The 12 Gyr old model tracks cover the present-day distribution of Galactic globular clusters in the (mass, F 1P) space extremely well. The distribution is curtailed on its top right side by the scarcity of clusters at large Galactocentric distances and on its bottom left side by the initial scarcity of very high-mass clusters and dynamical friction. Given their distinct dissolution rates, “inner” and “outer” model clusters are offset from each other, as observed. The locus of Magellanic Clouds clusters in the (mass, F 1P) space is as expected for intermediate-age clusters evolving in a gentle tidal field. Given the assumed constancy of F 1P, we conclude that 2P stars do not necessarily form centrally concentrated. We infer a minimum mass of 4 · 105 M ⊙ for multiple-population clusters at secular evolution onset. This high-mass threshold severely limits the number of 2P stars lost from evolving clusters, thereby fitting the low 2P star fraction of the Galactic halo field.

JWST uncovers helium and water abundance variations in the bulge globular cluster NGC 6440

We used ultra-deep observations obtained with the NIRCam aboard the James Webb Space Telescope to explore the stellar population of NGC 6440: a typical massive, obscured, and contaminated globular cluster formed and orbiting within the Galactic bulge. Leveraging the exceptional capabilities of this camera, we sampled the cluster down to about five magnitudes below the main sequence turn-off in the (mF115W, mF115W − mF200W) colour–magnitude diagram. After carefully accounting for differential extinction and contamination by field interlopers, we find that the main sequence splits into two branches, each above and below the characteristic knee. By comparing the morphology of the colour–magnitude diagram with a suitable set of isochrones, we argue that the upper main sequence bi-modality is likely due to the presence of a He-enriched stellar population with a helium spread of ΔY = 0.04. The lower main sequence bi-modality can be attributed to variations in the abundance of water (i.e., oxygen) with Δ[O/Fe] ∼ −0.4. This is the first evidence of both helium and oxygen abundance variations in a globular cluster purely based on JWST observations. These results open the window for future in-depth investigations of the multiple population phenomenon in clusters located in the Galactic bulge, which were previously unfeasible with near-UV observations, due to prohibitive reddening and crowding conditions.

The chemical compositions of multiple stellar populations in the globular cluster NGC 2808

ABSTRACT Pseudo two-colour diagrams or Chromosome maps (ChM) indicate that NGC 2808 host five different stellar populations. The existing ChMs have been derived by the Hubble Space Telescope photometry, and comprise of stars in a small field of view around the cluster centre. To overcome these limitations, we built a ChM with U, B, I photometry from ground-based facilities that disentangle the multiple stellar populations of NGC 2808 over a wider field of view. We used spectra collected by GIRAFFE@VLT in a sample of 70 red giant branch and seven asymptotic giant branch (AGB) stars to infer the abundances of C, N, O, Al, Fe, and Ni, which combined with literature data for other elements (Li, Na, Mg, Si, Ca, Sc, Ti, Cr, and Mn), and together with both the classical and the new ground-based ChMs, provide the most complete chemical characterization of the stellar populations in NGC 2808 available to date. As typical of the multiple population phenomenon in globular clusters, the light elements vary from one stellar population to another; whereas the iron peak elements show negligible variation between the different populations (at a level of ≲0.10 dex). Our AGB stars are also characterized by the chemical variations associated with the presence of multiple populations, confirming that this phase of stellar evolution is affected by the phenomenon as well. Intriguingly, we detected one extreme O-poor AGB star (consistent with a high He abundance), challenging stellar evolution models that suggest that highly He-enriched stars should avoid the AGB phase and evolve as AGB-manqué star.

Multiple stellar populations in the high-temperature regime: Potassium abundances in the globular cluster M 54 (NGC 6715)

Among the multiple stellar populations in globular clusters (GCs) the very high-temperature H-burning regime, able to produce elements up to potassium, is still poorly explored. Here we present the first abundance analysis of K in 42 giants of NGC 6715 (M 54) with homogeneous abundances of light elements previously derived in our FLAMES survey. Owing to the large mass and low metallicity, a large excess of K could be expected in this GC, which is located in the nucleus of the Sagittarius dwarf galaxy. We actually found a spread in [K/Fe] spanning about 1 dex, with [K/Fe] presenting a significant anti-correlation with [O/Fe] ratios, regardless of the metallicity component in M 54. Evidence for a K–Mg anti-correlation also exists, but this is statistically marginal because of the lack of very Mg-poor stars in this GC. We found, however, a strong correlation between K and Ca. These observations clearly show that the K enhancement in M 54 is probably due to the same network of nuclear reactions generating the phenomenon of multiple stellar populations, at work in a regime of very high temperature. The comparison with recent results in ω Cen is hampered by an unexplained trend with the temperatures for K abundances from optical spectroscopy, and somewhat by a limited sample size for infrared APOGEE data. There are few doubts, however, that the two most massive GCs in the Milky Way host a K–Mg anti-correlation.

Multiple Populations in Star Clusters

We review the multiple population (MP) phenomenon of globular clusters (GCs): i.e., the evidence that GCs typically host groups of stars with different elemental abundances and/or distinct sequences in photometric diagrams. Most Galactic and extragalactic clusters exhibit internal variations of He, C, N, O, Na, and Al. They host two distinct stellar populations: the first population of stars, which resemble field stars with similar metallicities, and one or more second stellar populations that show the signature of high-temperature H-burning. In addition, a sub-sample of clusters hosts stellar populations with different heavy-element abundances. The MP origin remains one of the most puzzling, open issues of stellar astrophysics. We summarize the scenarios for the MP formation and depict the modern picture of GCs and their stellar populations along with the main evolutionary phases. We show that the MP behavior dramatically changes from one cluster to another and investigate their complexity to define common properties. We investigate relations with the host galaxy, the parameters of the host clusters (e.g., GC’s mass, age, orbit), and stellar mass. We summarize results on spatial distribution and internal kinematics of MPs. Finally, we review the relation between MPs and the so-called second-parameter problem of the horizontal-branch morphology of GCs and summarize the main findings on the extended main sequence phenomenon in young clusters.

Confirmation of a metallicity spread amongst first population stars in globular clusters

Stars in massive star clusters exhibit intrinsic variations in some light elements (the multiple populations phenomenon) that are difficult to explain in a fully coherent formation scenario. In recent years, high quality Hubble Space Telescope (HST) photometry has led to the characterisation of the global properties of these multiple populations in an unparalleled level of detail. In particular, the colour-(pseudo)colour diagrams known as ‘chromosome maps’ have been proven to be very efficient at separating cluster stars with a field-like metal abundance distribution (first population) from an object with distinctive light-element abundance anti-correlations (second population). The unexpected wide colour ranges covered by the first population group – traditionally considered to have a uniform chemical composition – in the chromosome maps of the majority of the investigated Galactic globular clusters have recently been attributed to intrinsic metallicity variations up to ∼0.30 dex from the study of subgiant branch stars in two metal-rich Galactic globular clusters by employing appropriate HST filter combinations. On the other hand, high-resolution spectroscopy of small samples of first populations stars in the globular clusters NGC 3201 and NGC 2808 – both displaying extended sequences of first population stars in their chromosome maps – have provided conflicting results thus far, with a spread of metal abundance detected in NGC 3201 but not in NGC 2808. We present here a new method that employs HST near-UV and optical photometry of red giant branch stars to confirm these recent results independently. Our approach was firstly validated using observational data for M 2, a globular cluster hosting a small group of first population stars with an enhanced (by ≃0.5 dex) metallicity with respect to the main component. We then applied our method to three clusters that cover a much larger metallicity range and that have well populated, extended first population sequences in their chromosome maps, namely M 92, NGC 2808, and NGC 6362. We confirm that metallicity spreads are present among first population stars in these clusters, thus solidifying the case for the existence of unexpected variations up to a factor of two of metal abundances in most globular clusters. We also confirm the complex behaviour of the mean metallicity (and metallicity range) differences between first and second population stars.

APOGEE-2S Mg–Al anti-correlation of the metal-poor globular cluster NGC 2298

We present detailed elemental abundances and radial velocities of stars in the metal-poor globular cluster (GC) NGC 2298, based on near-infrared high-resolution (R ∼ 22 500) spectra of 12 members obtained during the second phase of the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) at Las Campanas Observatory as part of the seventeenth Data Release (DR 17) of the Sloan Digital Sky Survey IV (SDSS-IV). We employed the Brussels Automatic Code for Characterizing High accuracy Spectra (BACCHUS) software to investigate abundances for a variety of species including α elements (Mg, Si, and Ca), the odd-Z element Al, and iron-peak elements (Fe and Ni) located in the innermost regions of NGC 2298. We find a mean and median metallicity [Fe/H] = −1.76 and −1.75, respectively, with a star-to-star spread of 0.14 dex, which is compatible with the internal measurement errors. Thus, we find no evidence for an intrinsic [Fe/H] abundance spread in NGC 2298. The typical α-element enrichment in NGC 2298 is overabundant relative to the Sun, and it follows the trend of other metal-poor GCs. We confirm the existence of an Al-enhanced population in this cluster, which is clearly anti-correlated with Mg, indicating the prevalence of the multiple-population phenomenon in NGC 2298.

The formation of globular clusters as a case of overcooling

ABSTRACT Driven by recent observational findings, we select massive interactive binaries as the most suitable among the existing candidates for producing the chemical patterns typical of multiple populations of Galactic globular clusters. Still, to avoid supernova contamination, we are further driven to endorse the notion that above a critical mass stars fail to produce supernova events, but rather eventually sink into black holes without ejecting much energy and heavy metals. This assumption has the attractive implication of suppressing star formation feedback for some 5–10 million yr, in practice leading to runaway star formation, analog to overcooling that in absence of feedback would have turned most baryons into stars in the early Universe. Under such conditions, multiple episodes of star formation, incorporating binary star ejecta from previous episodes, appear to be unavoidable, thus accounting for the ubiquity of the multiple population phenomenon in globular clusters.

Lithium abundances as a tracer of asymptotic giant branch star pollution in the globular cluster NGC 6752

ABSTRACT This paper presents a chemical abundance analysis of 217 stars in the metal-poor globular cluster NGC 6752, distributed from the turn-off to the lower red giant branch. Al and Li abundances were derived through spectral synthesis applied to spectra collected with FLAMES, in both GIRAFFE and UVES modes. The work aims to gain insight into the nature of the polluter(s) responsible for the abundance variations and the C-N, Na-O, Al-Mg anticorrelations associated with the multiple-population phenomenon. We found a plateau at A(Li) = 2.33 ± 0.06 dex in unevolved stars, with the average Li content decreasing continuously down to ∼1.25 dex at the bottom of the red giant branch. As expected in the classic anticorrelation scenario, we found stars low in Al and high in Li abundance, and stars high in Al and low in Li. However, in addition, we also found evidence of Al-rich, second-generation stars with high Li content. This finding suggests the need for Li production, known to happen in intermediate-mass (∼4–8 M⊙) asymptotic giant branch stars through the Cameron–Fowler mechanism. It is worth noting that the Li abundance observed in Al-rich stars never exceeds that in Al-poor stars.

CAPOS: The bulge Cluster APOgee Survey

We performed the first detailed spectral analysis of red giant members of the relatively high-metallicity globular cluster (GC) Tononzintla 2 (Ton 2) using high-resolution near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2), obtained as part of the bulge Cluster APOgee Survey. We investigated chemical abundances for a variety of species including the light, odd-Z, α-, Fe-peak, and neutron-capture elements from high S/N spectra of seven giant members. The derived mean cluster metallicity is [Fe/H] = −0.70 ± 0.05, with no evidence for an intrinsic metallicity spread. Ton 2 exhibits a typical α-enrichment that follows the trend for high-metallicity Galactic GCs, similar to that seen in 47 Tucanae and NGC 6380. We find a significant nitrogen spread (> 0.87 dex), and a large fraction of nitrogen-enriched stars that populate the cluster. Given the relatively high-metallicity of Ton 2, these nitrogen-enriched stars are well above the typical Galactic levels, indicating the prevalence of the multiple-population phenomenon in this cluster that also contains several stars with typical low first-generation N abundances. We also identify the presence of [Ce/Fe] abundance spread in Ton 2, which is correlated with the nitrogen enhancement, indicating that the s-process enrichment in this cluster has likely been produced by relatively low-mass asymptotic giant branch stars. Furthermore, we find a mean radial velocity of the cluster, −178.6 ± 0.86 km s−1, with a small velocity dispersion, 2.99 ± 0.61 km s−1, which is typical of GCs. We also find a prograde bulge-like orbit for Ton 2 that appears to be radial and highly eccentric. Finally, the considerably nitrogen-enhanced population observed in Ton 2, combined with its dynamical properties, makes this object a potential progenitor for the nitrogen-enriched field stars identified so far toward the bulge region at similar metallicity.

APOGEE-2S view of the globular cluster Patchick 125 (Gran 3)

We present detailed elemental abundances, radial velocity, and orbital elements for Patchick 125, a recently discovered metal-poor globular cluster (GC) in the direction of the Galactic bulge. Near-infrared high-resolution (R ∼ 22 500) spectra of two members were obtained during the second phase of the Apache Point Observatory Galactic Evolution Experiment at Las Campanas Observatory as part of the sixteenth Data Release (DR 16) of the Sloan Digital Sky Survey. We investigated elemental abundances for four chemical species, including α- (Mg, Si), Fe-peak (Fe), and odd-Z (Al) elements. We find a metallicity covering the range from [Fe/H] = −1.69 to −1.72, suggesting that Patchick 125 likely exhibits a mean metallicity ⟨[Fe/H]⟩ ∼ −1.7, which represents a significant increase in metallicity for this cluster compared to previous low-resolution spectroscopic analyses. We also found a mean radial velocity of 95.9 km s−1, which is ∼21.6 km s−1 higher than reported in the literature. The observed stars exhibit an α-enrichment ([Mg/Fe] ≲ 0.20, and [Si/Fe] ≲ +0.30) that follows the typical trend of metal-poor GCs. The aluminum abundance ratios for the present two member stars are enhanced in [Al/Fe] ≳ +0.58, which is a typical enrichment characteristic of the so-called ‘second-generation’ of stars in GCs at similar metallicity. This supports the possible presence of the multiple-population phenomenon in Patchick 125, as well as its genuine GC nature. Further, Patchick 125 shows a low-energy, low-eccentric (< 0.4) and retrograde orbit captured by the inner Galaxy, near the edge of the bulge. We confirm that Patchick 125 is a genuine metal-poor GC, which is currently trapped in the vicinity of the Milky Way bulge.

Expanding the Time Domain of Multiple Populations: Evidence of Nitrogen Variations in the ∼1.5 Gyr Old Star Cluster NGC 1783

Abstract We present the result of a detailed analysis of Hubble Space Telescope UV and optical deep images of the massive and young (∼1.5 Gyr) stellar cluster NGC 1783 in the Large Magellanic Cloud. This system does not show evidence of multiple populations (MPs) along the red giant branch (RGB) stars. However, we find that the cluster main sequence (MS) shows evidence of a significant broadening (50% larger than what is expected from photometric errors) along with hints of possible bimodality in the MP sensitive (m F343N − m F438W, m F438W) color–magnitude diagram (CMD). Such an effect is observed in all color combinations including the m F343N filter, while it is not found in the optical CMDs. This observational evidence suggests we might have found light-element chemical abundance variations along the MS of NGC 1783, which represents the first detection of MPs in a system younger than 2 Gyr. A comparison with isochrones including MP-like abundances shows that the observed broadening is compatible with a N abundance enhancement of Δ([N/Fe]) ∼ 0.3. Our analysis also confirms previous results about the lack of MPs along the cluster RGB. However, we find that the apparent disagreement between the results found on the MS and the RGB is compatible with the mixing effects linked to the first dredge up. This study provides new key information about the MP phenomenon and suggests that star clusters form in a similar way at any cosmic age.

Integrated Photometry of Multiple Stellar Populations in Globular Clusters

Abstract Evidence that multiple populations (MPs) are common properties of globular clusters (GCs) has accumulated over the past decades from clusters in the Milky Way and in its satellites. This finding has revived research into GCs, and suggested that their formation at high redshift must have been a much more complex phenomenon than imagined before. However, most information on MPs is limited to nearby GCs. The main limitation is that most studies of MPs rely on resolved stars, posing a major challenge to the investigation of the MP phenomenon in distant galaxies. Here we search for integrated colors of old GCs that are sensitive to the MP phenomenon. To do this, we exploit integrated magnitudes of simulated GCs with MPs, and multiband Hubble Space Telescope photometry of 56 Galactic GCs, where MPs are widely studied, and characterized as part of the UV Legacy Survey of Galactic GCs. We find that both integrated C F275W,F336W,F438W and m F275W − m F814W colors strongly correlate with the iron abundance of the host GC. To second order, the pseudo two-color diagram built with these integrated colors is sensitive to the MP phenomenon. In particular, once the dependence on cluster metallicity is removed, the color residuals depend on the maximum internal helium variation within GCs and on the fraction of second-generation stars. This diagram, which we define here for Galactic GCs, has the potential to detect and characterize MPs from integrated photometry of old GCs, thus providing the possibility to extend their investigation outside the Local Group.

APOGEE-2S Discovery of Light- and Heavy-element Abundance Correlations in the Bulge Globular Cluster NGC 6380

Abstract We derive abundance ratios for nine stars in the relatively high-metallicity bulge globular cluster NGC 6380. We find a mean cluster metallicity between [Fe/H] = −0.80 and −0.73, with no clear evidence for a variation in iron abundances beyond the observational errors. Stars with strongly enhanced [N/Fe] abundance ratios populate the cluster and are anticorrelated with [C/Fe], trends that are considered a signal of the multiple-population phenomenon in this cluster. We detect an apparent intrinsic star-to-star spread (≳0.27 dex) in the slow neutron-capture process element (s-element) Ce ii. Moreover, the [Ce/Fe] abundance ratio exhibits a likely correlation with [N/Fe], and a somewhat weaker correlation with [Al/Fe]. If confirmed, NGC 6380 could be the first high-metallicity globular cluster where a N–Ce correlation is detected. Furthermore, this correlation suggests that Ce may also be an element involved in the multiple-population phenomenon. Currently, a consensus interpretation for the origin of the this apparent N–Ce correlation in high-metallicity clusters is lacking. We tentatively suggest that it could be reproduced by different channels—low-mass asymptotic giant branch stars in the high-metallicity regime or fast-rotating massive stars (“spinstars”), due to the rotational mixing. It may also be the cumulative effect of several pollution events including the occurrence of peculiar stars. Our findings should guide stellar nucleosynthesis models, in order to understand the reasons for its apparent exclusivity in relatively high-metallicity globular clusters.

CAPOS: The bulge Cluster APOgee Survey

We present results from a study of 15 red giant members of the intermediate-metallicity globular cluster (GC) FSR 1758 using high-resolution, near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2) that were obtained as part of CAPOS (the bulge Cluster APOgee Survey). Since its very recent discovery as a massive GC in the bulge region, evoking the name Sequoia, this has been an intriguing object with a highly debated origin, and initially led to the suggestion of a purported progenitor dwarf galaxy of the same name. In this work, we use new spectroscopic and astrometric data to provide additional clues as to the nature of FSR 1758. Our study confirms the GC nature of FSR 1758, and as such we report the existence of the characteristic N-C anticorrelation and Al-N correlation for the first time. We thereby reveal the existence of the multiple-population phenomenon, similar to that observed in virtually all GCs. Furthermore, the presence of a population with strongly enriched aluminum makes it unlikely that FSR 1758 is the remnant nucleus of a dwarf galaxy because Al-enhanced stars are uncommon in dwarf galaxies. We find that FSR 1758 is slightly more metal rich than previously reported in the literature; this source has a mean metallicity [Fe/H] between −1.43 to −1.36, depending on the adopted atmospheric parameters and a scatter within observational error, again pointing to its GC nature. Overall, the α-enrichment (≳ + 0.3 dex), Fe-peak (Fe, Ni), light (C, N), and odd-Z (Al) elements follow the trend of intermediate-metallicity GCs. Isochrone fitting in the Gaia bands yields an estimated age of ∼11.6 Gyr. We used the exquisite kinematic data, including our CAPOS radial velocities and Gaia eDR3 proper motions, to constrain the N-body density profile of FSR 1758, and found that it is as massive (∼2.9 ± 0.6 × 105 M⊙) as NGC 6752. We confirm a retrograde and eccentric orbit for FSR 1758. A new examination of its dynamical properties with the GravPot16 model favors an association with the Gaia-Enceladus-Sausage accretion event. Thus, paradoxically, the cluster that gave rise to the name of the Sequoia dwarf galaxy does not appear to belong to this specific merging event.

APOGEE spectroscopic evidence for chemical anomalies in dwarf galaxies: The case of M 54 and Sagittarius

We present evidence for globular cluster stellar debris in a dwarf galaxy system (Sagittarius; Sgr) based on an analysis of high-resolution H-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. We add [N/Fe], [Ti/Fe], and [Ni/Fe] abundance ratios to the existing sample of potential members of M 54; this is the first time that [N/Fe] abundances are derived for a large number of stars in M 54. Our study reveals the existence of a significant population of nitrogen- (with a large spread, ≳1 dex) and aluminum-enriched stars with moderate Mg depletions in the core of the M 54+Sgr system, which share the light element anomalies characteristic of second-generation globular cluster stars (GC), thus tracing the typical phenomenon of multiple stellar populations seen in other Galactic GCs at similar metallicity, confirming earlier results based on the Na-O anti-correlation. We further show that most of the stars in M 54 exhibit different chemical patterns evidently not present in Sgr field stars. Furthermore, we report the serendipitous discovery of a nitrogen-enhanced extra-tidal star that has GC second-generation-like chemical patterns for which both chemical and kinematic evidence are commensurate with the hypothesis that the star has been ejected from M 54. Our findings support the existence of chemical anomalies associated with likely tidally shredded GCs that are in dwarf galaxies in the Local Group. These findings motivate future searches for such bona fide stars along other known Milky Way streams.

Excess of Ca (and Sc) produced in globular cluster multiple populations: a first census in 77 Galactic globular clusters

Multiple stellar populations in globular clusters (GCs) are distinct by their different abundances of light elements. The abundance anti-correlations point towards a nucleosynthesis origin due to high-temperature H burning, but it remains to be assessed which type of stars altered primordial abundances in GCs. In particular, the regime at very high temperature that shapes the variations in potassium as well as calcium and scandium, which has been detected in a few notable cases such as NGC 2419 and NGC 2808, is still poorly explored. We started a systematic search for excess of Ca (and Sc) in GC stars with respect to the level of unmodified field stars. This method has recently been proven to be highly efficient in revealing the outcome of the proton-capture reactions at very high temperatures. Statistically robust evidence of such excess was found in a small number of GCs (NGC 4833, NGC 6715, NGC 6402, NGC 5296, NGC 5824, and NGC 5139/ωCentauri) that join the previously known two clusters. For the first time we show that NGC 4833 is likely to host anti-correlated K and Mg abundances. All these GCs are among the most massive ones in the Galaxy. We found that the fraction of stars with Ca enhancement at 3σabove the field star distribution is a multivariate function of the GC mass and metallicity, as in other manifestations of the multiple population phenomenon in GCs. We argue that these alterations in only a few GCs can be reproduced by two different channels: either a class of ordinary stars, that is common to all GCs, acts only in particular environments, or an on-off mechanism is generated by the occurrence of a peculiar type of stars (or lack of such stars). Hot bottom-burning in asymptotic giant branch stars in the low-metallicity regime is a good candidate for the first class. Alternatively, a metallicity dependence is also expected for supermassive stars, which are predicted to preferentially form in massive GCs.

Multiple Stellar Populations along the Red Horizontal Branch and Red Clump of Globular Clusters

Abstract We exploit multiband Hubble Space Telescope photometry to investigate multiple populations (MPs) along the red horizontal branches (HBs) and red clumps of 14 metal-rich globular clusters (GCs), including 12 Milky Way GCs and the Magellanic Cloud GCs NGC 1978 and NGC 416. Based on appropriate two-color diagrams, we find that the fraction of first-generation (1G) stars in Galactic GCs correlates with cluster mass, confirming previous results based on red-giant branch (RGB) stars. Magellanic Cloud GCs show higher fractions of 1G stars than Galactic GCs with similar masses, thus suggesting that the environment affects the MP phenomenon. We compared and combined our population fractions based on the HB with previous estimates from the MS and RGB, and we used ground-based UBVI photometry (available for NGC 104, NGC 5927, NGC 6366, and NGC 6838) to extend the investigation over a wide field of view. All studied GCs are consistent with flat distributions of 1G and second-generation (2G) stars within ∼1′ from the cluster center except NGC 416, where the 2G is more centrally concentrated. 2G stars of NGC 104 and NGC 5927 are more centrally concentrated than the 1G stars, whereas the distribution is flat for NGC 6366 and NGC 6838. We discover that most of the analyzed GCs exhibit extended sequences of 1G stars along the red HB, not consistent with a simple population. The comparison between appropriate synthetic and observed CMDs reveals that these extended distributions are consistent with either star-to-star variation in helium or with an internal metallicity spread, recalling the inhomogeneity of 1G stars along the chromosome maps.

The enigmatic globular cluster UKS 1 obscured by the bulge: H-band discovery of nitrogen-enhanced stars

The presence of nitrogen-enriched stars in globular clusters provides key evidence for multiple stellar populations (MPs), as has been demonstrated with globular cluster spectroscopic data towards the bulge, disk, and halo. In this work, we employ the VVV Infrared Astrometric Catalogue (VIRAC) and the DR16 SDSS-IV release of the APOGEE survey to provide the first detailed spectroscopic study of the bulge globular cluster UKS 1. Based on these data, a sample of six selected cluster members was studied. We find the mean metallicity of UKS 1 to be [Fe/H] = −0.98 ± 0.11, considerably more metal-poor than previously reported, and a negligible metallicity scatter, typical of that observed by APOGEE in other Galactic globular clusters. In addition, we find a mean radial velocity of 66.1 ± 12.9 km s−1, which is in good agreement with literature values, within 1σ. By selecting stars in the VIRAC catalogue towards UKS 1, we also measure a mean proper motion of (μαcos(δ), μδ) = (−2.77 ± 0.23, −2.43 ± 0.16) mas yr−1. We find strong evidence for the presence of MPs in UKS 1, since four out of the six giants analysed in this work have strong enrichment in nitrogen ([N/Fe] ≳ +0.95) accompanied by lower carbon abundances ([C/Fe] ≲ −0.2). Overall, the light- (C, N), α- (O, Mg, Si, Ca, Ti), Fe-peak (Fe, Ni), Odd-Z (Al, K), and the s-process (Ce, Nd, Yb) elemental abundances of our member candidates are consistent with those observed in globular clusters at similar metallicity. Furthermore, the overall star-to-star abundance scatter of elements exhibiting the multiple-population phenomenon in UKS 1 is typical of that found in other global clusters (GCs), and larger than the typical errors of some [X/Fe] abundances. Results from statistical isochrone fits in the VVV colour-magnitude diagrams indicate an age of 13.10−1.29+0.93 Gyr, suggesting that UKS 1 is a fossil relic in the Galactic bulge.

Multiple populations in massive star clusters under the magnifying glass of photometry: theory and tools

The existence of star-to-star light-element abundance variations in massive Galactic and extragalactic star clusters has fairly recently superseded the traditional paradigm of individual clusters hosting stars with the same age, and uniform chemical composition. Several scenarios have been put forward to explain the origin of this multiple stellar population phenomenon, but so far all have failed to reproduce the whole range of key observations. Complementary to high-resolution spectroscopy, which has first revealed and characterized chemically the presence of multiple populations in Galactic globular clusters, photometry has been instrumental in investigating this phenomenon in much larger samples of stars --adding a number of crucial observational constraints and correlations with global cluster properties-- and in the discovery and characterization of multiple populations also in Magellanic Clouds' intermediate age clusters. The purpose of this review is to present the theoretical underpinning and application of the photometric techniques devised to identify and study multiple populations in resolved star clusters. These methods have played and continue to play a crucial role in advancing our knowledge of the cluster multiple population phenomenon, and promise to extend the scope of these investigations to resolved clusters even beyond the Local Group, with the launch of the James Webb Space Telescope.