Abundances Of Light Elements Research Articles

Big-Bang Nucleosynthesis and Primordial Lithium Abundance Problem

Prediction of the primordial abundances of elements in the big-bang nucleosynthesis (BBN) is one of the three strong evidences for the big bang model. Precise knowledge of the baryon-to-photon ratio of the Universe from observations of the anisotropies of cosmic microwave background radiation has made the Standard BBN a parameter-free theory. Although, there is a good agreement over a range of nine orders of magnitude between abundances of light elements deduced from observations and calculated in primordial nucleosynthesis, there remains a yet-unexplained discrepancy of $^7$Li abundance higher by a factor of $\sim 3$ when calculated theoretically. The primordial abundances depend on the astrophysical nuclear reaction rates and on three additional parameters, the number of light neutrino flavours, the neutron lifetime and the baryon-to-photon ratio in the universe. The effect of the modification of thirty-five reaction rates on light element abundance yields in BBN was investigated earlier by us. In the present work we have incorporated the most recent values of neutron lifetime and the baryon-to-photon ratio and further modified $^3$He($^4$He,$\gamma$)$^7$Be reaction rate which is used directly for estimating the formation of $^7$Li as a result of $\beta^+$ decay as well as the reaction rates for t($^4$He,$\gamma$)$^7$Li and d($^4$He,$\gamma$)$^6$Li. We find that these modifications reduce the theoretically calculated abundance of $^7$Li by $\sim 12\%$.

Breaking the dichotomy between typical and anomalous globular clusters: the case of NGC 3201

AbstractWe recently discovered that NGC 3201 has characteristics that set it outside the current twofold classification scheme for Galactic globular clusters (GCs). Most GCs are mono-metallic and show light-element abundance variations (e.g., Na-O and C-N anti-correlations); but a minority of clusters also present variations in Fe correlating with s-process element and C+N+O abundances, and they possess multiple C-N sequences. These anomalous GCs also have a broad sub-giant branch (SGB) and follow the same mass-size relation as dwarf galaxies possibly evolving into GCs. We now revealed that NGC 3201 belongs to neither group. It has multiple C-N sequences, but no broad SGB, no strong evidence of a Fe-spread, and it does not follow the mass-size relation.

Integrated spectroscopy of extragalactic Globular Clusters

AbstractIntegrated light (IL) spectroscopy enables studies of stellar populations beyond the Milky Way and its nearest satellites. In this paper, I will review how IL spectroscopy reveals essential information about globular clusters and the assembly histories of their host galaxies, concentrating particularly on the metallicities and detailed chemical abundances of the GCs in M31. I will also briefly mention the effects of multiple populations on IL spectra, and how observations of distant globular clusters help constrain the source(s) of light-element abundance variations. I will end with future perspectives, emphasizing how IL spectroscopy can bridge the gap between Galactic and extragalactic astronomy.

Supermassive stars as the origin of the multiple populations in globular clusters

AbstractGlobular clusters (GCs) display anomalous light-elements abundances (HeCNONaMgAl), resembling the yields of hot-hydrogen burning, but there is no consensus yet on the origin of these ubiquitous multiple populations. We present a model in which a super-massive star (SMS, ≳103 M⊙) forms via stellar collisions during GC formation and pollutes the intra-cluster medium. The growth of the SMS finds a balance with the wind mass loss rate, such that the SMS can produce a significant fraction of the total GC mass in processed material, thereby overcoming the so-called mass-budget problem that plagues other models. Because of continuous rejuvenation, the SMS acts as a ‘conveyer-belt’ of hot-hydrogen burning yields with (relatively) low He abundances, in agreement with empirical constraints. Additionally, the amount of processed material per unit of GC mass correlates with GC mass, addressing the specific mass budget problem. We discuss uncertainties and tests of this new self-enrichment scenario.

Constant light element abundances suggest that the extended P1 in NGC 2808 is not a consequence of CNO-cycle nucleosynthesis

Abstract Recent photometric results have identified a new population among globular cluster stars. This population, referred to as the ‘extended P1', has been suggested to be the manifestation of a new abundance pattern where the initial mass fraction of He changes among cluster stars that share the same CNO values. The current paradigm for the formation of the multiple stellar populations in globular clusters assumes that variations in He are the product of chemical ‘enrichment’ by the ashes of the CNO-cycle (which changes He and other elements like C, N and O simultaneously). We obtained MIKE@Magellan spectra of six giant stars in NGC 2808, a cluster with one of the strongest examples of the extended P1 population. We provide the first complete characterization of the light elements abundances for the stars along a significant range of the extended P1 photometric group. The stars from our sample appear to be homogeneous in C, N, O, Na, Mg and Al. The lack of a significant change in these products of the CNO-cycle suggests that unlike the rest of the populations identified to date, the photometric changes responsible for the extended P1 feature are a consequence of an alternative mechanism. Our measurements are consistent with the interpretations where the changes of the He mass fraction among these stars could be a consequence of p–p chain nucleosynthesis (which could increase the He in stars without affecting heavier elements). Having said that, direct measurements of He are necessary to conclude if variations of this element are present among extended P1 stars.

First constraint on the neutrino-induced phase shift in the spectrum of baryon acoustic oscillations

The existence of the cosmic neutrino background is a fascinating prediction of the hot big bang model. These neutrinos were a dominant component of the energy density in the early universe and, therefore, played an important role in the evolution of cosmological perturbations. The energy density of the cosmic neutrino background has been measured using the abundances of light elements and the anisotropies of the cosmic microwave background (CMB). A complementary and more robust probe is a distinct shift in the temporal phase of sound waves in the primordial plasma which is produced by fluctuations in the neutrino density and has recently been detected in the CMB. In this paper, we report on the first constraint on this neutrino-induced phase shift in the spectrum of baryon acoustic oscillations (BAO) of the BOSS DR12 data. Constraining the acoustic scale using Planck data while marginalizing over the effects of neutrinos in the CMB, we find a non-zero phase shift at greater than 95\% confidence. We also demonstrate the robustness of this result in simulations and forecasts. Besides providing a new test of the cosmic neutrino background, our work is the first application of the BAO signal to early universe physics and a non-trivial confirmation of the standard cosmological history.

Cosmological constraints on unstable particles: Numerical bounds and analytic approximations

Many extensions of the Standard Model predict large numbers of additional unstable particles whose decays in the early universe are tightly constrained by observational data. For example, the decays of such particles can alter the ratios of light-element abundances, give rise to distortions in the cosmic microwave background, alter the ionization history of the universe, and contribute to the diffuse photon flux. Constraints on new physics from such considerations are typically derived for a single unstable particle species with a single well-defined mass and characteristic lifetime. In this paper, by contrast, we investigate the cosmological constraints on theories involving entire ensembles of decaying particles --- ensembles which span potentially broad ranges of masses and lifetimes. In addition to providing a detailed numerical analysis of these constraints, we also formulate a set of simple analytic approximations for these constraints which may be applied to generic ensembles of unstable particles which decay into electromagnetically-interacting final states. We then illustrate how these analytic approximations can be used to constrain a variety of toy scenarios for physics beyond the Standard Model. For ease of reference, we also compile our results in the form of a table which can be consulted independently of the rest of the paper. It is thus our hope that this work might serve as a useful reference for future model-builders concerned with cosmological constraints on decaying particles, regardless of the particular model under study.

A Closer Look at Some Gas-phase Depletions in the ISM: Trends for O, Ge, and Kr versus , f(H2), and Starlight Intensity*

Abstract In a survey of archived ultraviolet spectra of 100 stars recorded by the echelle spectrograph of the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we measure the strengths of the weak absorption features of O i, Ge ii, and Kr i in the interstellar medium. Our objective is to undertake an investigation that goes beyond earlier abundance studies to see how these elements are influenced independently by three different environmental properties: (1) values of a generalized atomic depletion factor F * due to condensations onto dust grains (revealed here by the abundances of Mg and Mn relative to H), (2) the fraction of H atoms in the form of H2 f(H2), and (3) the ambient intensity I of ultraviolet starlight relative to an average value in our part of the Galaxy I 0. As expected, the gas-phase abundances of all three elements exhibit negative partial correlations with F *. The abundances of free O atoms show significant positive partial correlations with and , while Ge and Kr exhibit negative partial correlations with at marginal levels of significance. After correcting for these trends, the abundances of O relative to H show no significant variations with location, except for the already-known radial gradient of light-element abundances in the Milky Way. A comparison of Ge and O abundances revealed no significant regional enhancements or deficiencies of neutron-capture elements relative to α-process ones.

Homogeneous analysis of globular clusters from the APOGEE survey with the BACCHUS code

Aims. We seek to provide abundances of a large set of light and neutron-capture elements homogeneously analyzed that cover a wide range of metallicity to constrain globular cluster (GC) formation and evolution models. Methods. We analyzed a large sample of 885 GCs giants from the SDSS IV-Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. We used the Cannon results to separate the red giant branch and asymptotic giant branch stars, not only allowing for a refinement of surface gravity from isochrones, but also providing an independent H-band spectroscopic method to distinguish stellar evolutionary status in clusters. We then used the Brussels Automatic Code for Characterizing High accUracy Spectra (BACCHUS) to derive metallicity, microturbulence, macroturbulence, many light-element abundances, and the neutron-capture elements Nd and Ce for the first time from the APOGEE GCs data. Results. Our independent analysis helped us to diagnose issues regarding the standard analysis of the APOGEE DR14 for low-metallicity GC stars. Furthermore, while we confirm most of the known correlations and anticorrelation trends (Na-O, Mg-Al, C-N), we discover that some stars within our most metal-poor clusters show an extreme Mg depletion and some Si enhancement. At the same time, these stars show some relative Al depletion, displaying a turnover in the Mg-Al diagram. These stars suggest that Al has been partially depleted in their progenitors by very hot proton-capture nucleosynthetic processes. Furthermore, we attempted to quantitatively correlate the spread of Al abundances with the global properties of GCs. We find an anticorrelation of the Al spread against clusters metallicity and luminosity, but the data do not allow us to find clear evidence of a dependence of N against metallicity in the more metal-poor clusters. Conclusions. Large and homogeneously analyzed samples from ongoing spectroscopic surveys unveil unseen chemical details for many clusters, including a turnover in the Mg-Al anticorrelation, thus yielding new constrains for GCs formation/evolution models.

An investigation of C, N, and Na abundances in red giant stars of the Sculptor dwarf spheroidal galaxy

The origin of the star-to-star abundance variations found for the light elements in Galactic globular clusters (GGCs) is not well understood, which is a significant problem for stellar astrophysics. While the light element abundance variations are very common in globular clusters, they are comparatively rare in the Galactic halo field population. However, little is known regarding the occurrence of the abundance anomalies in other environments such as that of dwarf spheroidal (dSph) galaxies. Consequently, we have investigated the anti-correlation and bimodality of CH and CN band strengths, which are markers of the abundance variations in GGCs, in the spectra of red giants in the Sculptor dwarf spheroidal galaxy. Using spectra at the Na~D lines, informed by similar spectra for five GGCs (NGC 288, 1851, 6752, 6809 and 7099), we have also searched for any correlation between CN and Na in the Sculptor red giant sample. Our results indicate that variations analogous to those seen in GGCs are not present in our Sculptor sample. Instead, we find a weak positive correlation between CH and CN, and no correlation between Na and CN. We also reveal a deficiency in [Na/Fe] for the Sculptor stars relative to the values in GGCs, a result which is consistent with previous work for dSph galaxies. The outcomes reinforce the apparent need for a high stellar density environment to produce the light element abundance variations.

Earth's chondritic light rare earth element composition: Evidence from the Ce–Nd isotope systematics of chondrites and oceanic basalts

Combined Ce and Nd isotope ratios provide a time-integrated record of the light rare earth element (LREE) abundances of their source materials. Here, we present new high precision Ce isotope data for chondrites and basalts from ocean islands (OIB) and mid ocean ridges (MORB). The new Ce isotope ratios in chondritic meteorites define a precise new CHUR reference value. In Ce–Nd isotopic space, the MORB and OIB form a well-defined array that intersects with the Ce–Nd chondritic reference value. The simplest first-order explanation is that the bulk silicate Earth (BSE) has chondritic LREE and Ce–Nd isotope ratios. We show, however, that the intercept and slope of the Ce–Nd isotope mantle array depend on several factors. Perhaps most important are whether the BSE is chondritic and how closely the mantle average reflected in the MORB and OIB data corresponds to the Ce–Nd isotope ratio of the BSE. A significant difference between the accessible mantle's average Ce–Nd isotope ratio and that of BSE could result from the permanent storage of a considerable proportion of Earth's total LREE budget in the continental crust or potential isolated reservoirs. We show that the formation of isolated reservoirs either has a minor effect on the average Ce–Nd composition of the average mantle, or is geochemically and geodynamically implausible. If, due to formation of the continental crust, a significant shift in the average mantle's Ce–Nd isotope composition relative to BSE occurs, this shift is parallel to the Ce–Nd mantle array, and does not affect its chondritic intercept. The chondritic intercept of the Ce–Nd isotope mantle array therefore is strong evidence that BSE's relative LREE and Ce–Nd isotope composition is chondritic. However, an apparent difference between the accessible mantle's average Ce–Nd isotope ratio and that of BSE could also result if MORB and OIB do not sample the accessible mantle in a representative manner. Although we cannot entirely exclude the latter, it would require a fortuitous combination of factors to cause the observed chondritic intercept of the Ce–Nd isotope mantle array. We therefore conclude that the bulk silicate Earth has chondritic LREE and Ce–Nd isotope ratios.

Confirming the Presence of Second-population Stars and the Iron Discrepancy along the AGB of the Globular Cluster NGC 6752∗

Abstract Asymptotic giant branch (AGB) stars in the globular cluster NGC 6752 have been found to exhibit some chemical peculiarities with respect to the red giant branch (RGB) stars. A discrepancy between [Fe i/H] and [Fe ii/H] (not observed in RGB stars) has been detected adopting spectroscopic temperatures. Moreover, a possible lack of second-population stars along the AGB was claimed. The use of photometric temperatures based on (V − K) colors was proposed to erase this iron discrepancy. Also, ad hoc scenarios have been proposed to explain the absence of second-population AGB stars. Here we analyzed a sample of 19 AGB and 14 RGB stars of NGC 6752 observed with the spectrograph’s UVES. The two temperature scales agree very well for the RGB stars while for the AGB stars there is a systematic offset of ∼100 K. We found that even if the photometric temperatures alleviate the iron discrepancy with respect to the spectroscopic ones, a systematic difference between [Fe i/H] and [Fe ii/H] is still found among the AGB stars. An unexpected result is that the photometric temperatures do not satisfy the excitation equilibrium in the AGB stars. This suggests that standard 1D-LTE model atmospheres are unable to properly describe the thermal structure of AGB stars, at variance with the RGB stars. The use of photometric temperatures confirms the previous detection of second-population AGB stars in this cluster, with the presence of clear correlations/anticorrelations among the light element abundances. This firmly demonstrates that both first- and second-population stars evolve along the AGB of NGC 6752.

Progress on the BL2 beam measurement of the neutron lifetime.

A precise value of the neutron lifetime is important in several areas of physics, including determinations of the quark-mixing matrix element │V ud│, related tests of the Standard Model, and predictions of light element abundances in Big Bang Nucleosynthesis models. We report the progress on a new measurement of the neutron lifetime utilizing the cold neutron beam technique. Several experimental improvements in both neutron and proton counting that have been developed over the last decade are presented. This new effort should yield a final uncertainty on the lifetime of 1 s with an improved understanding of the systematic effects.

Chemical analysis of NGC 6528: one of the most metal-rich bulge globular clusters

Context. The bulge globular clusters (GCs) are key tracers of the bulge, a central and ancient component of our Galaxy. It is essential to understand their formation and evolution to study that of the bulge, as well as their relationship with the other Galactic GC systems (halo and disk GCs). High-resolution spectroscopy is a powerful tool for such studies, allowing us to obtain a detailed chemical characterization and kinematics of the clusters and to compare their chemical patterns with those of their halo and disk counterparts. Aims. Our main goals are to obtain detailed abundances for a sample of seven red giant members of NGC 6528 in order to characterize their chemical composition and study the relationship of this GC with the bulge, and with other bulge, halo, and disk GCs. Moreover, we analyze this cluster’s behavior associated with the multiple-populations phenomenon. Methods. We obtained the stellar parameters and chemical abundances of light elements (Na, Al), iron-peak elements (V, Cr, Mn, Fe, Co, Ni, Cu), α-elements (O, Mg, Si, Ca, Ti) and heavy elements (Zr, Ba, Eu) in seven red giant members of NGC 6528 using high-resolution spectroscopy from FLAMES-UVES. Results. In six stars of our sample we obtained a mean iron content of [Fe/H] = − 0.14 ± 0.03 dex, in good agreement with other studies. We found no significant internal iron spread. We detected one candidate variable star, which was excluded from the mean in iron content, and derived a metallicity in this star of [Fe/H] = − 0.55 ± 0.04 dex. Moreover, we found no extended O-Na anticorrelation but instead only an intrinsic Na spread. In addition, NGC 6528 does not exhibit a Mg-Al anticorrelation, and no significant spread in either Mg or Al. The α and iron-peak elements show good agreement with the bulge field star trend. The heavy elements are slightly dominated by the r-process. The chemical analysis suggests an origin and evolution similar to that of typical old Galactic bulge field stars. Finally, we find remarkable agreement in the chemical patterns of NGC 6528 and another bulge GC, NGC 6553, suggesting a similar origin and evolution.

Systematic effects and self-consistency in the primordial helium abundance determination

Observations of light element abundances (4He, D, 7Li) provide a powerful and independent test of the Standard Model of Big Bang Nucleosynthesis. The 4He abundance is measured most accurately among the light elements. This value is determined from observations of spectral lines from small-scale extragalactic sources (HII regions) in the optical and near infrared ranges. The intensity of the forbidden lines like [OIII] and [SII] allows to obtain the physical characteristics of the source, such as temperature and metallicity. Intensities of the 4He lines are used to determine the abundance of this element in the object. However, there are various systematic effects that should be taken in account for precise definition of this physical quantity. These include (1) interstellar reddening, (2) emissivities of 4He in different wavelengths, (3) underlying absorption for H and He lines, (4) temperature and (5) ionization structure HII regions, (6) collisional and fluorescent excitation of H and He lines. Incorrect accounting of these effects can lead to overestimation or underestimation of the primordial helium abundance Yp by several percent.At the moment, there are two independent groups of researchers dealing with this problem. The results for the calculation using the same data sample for these groups are as follows: Yp = 0.2551 ± 0.0022 (Izotov, Thuan, Guseva, 2014) and Yp = 0.2449 ± 0.0040 (Aver, Olive, Skillman, 2015). The last result is consistent with the result Yp = 0.2467 ± 0.0006 based on the study of the CMB anisotropy on (P.A.R. Ade et al., 2015). However, the values of Yp obtained by these groups do not coincide even within the margin of error, and at the moment there is no clear answer to the question of which of these two groups got correct result. In the paper, differences in the methods of processing data and obtaining the quantity of the primordial helium abundance are investigated, in particular, how the groups take into account the named systemic effects. An error analysis and comparative tests are performed.

The chemical composition of the oldest nearby open cluster Ruprecht 147

Context. Ruprecht 147 (NGC 6774) is the closest old open cluster, with a distance of less than 300 pc and an age of about 2.5 Gyr. It is therefore well suited for testing stellar evolution models and for obtaining precise and detailed chemical abundance information. Aims. We combined photometric and astrometric information coming from literature and the Gaia mission with very high-resolution optical spectra of stars in different evolutionary stages to derive the cluster distance, age, and detailed chemical composition. Methods. We obtained spectra of six red giants using HARPS-N at the Telescopio Nazionale Galileo (TNG). We also used European Southern Observatory (ESO) archive spectra of 22 main sequence (MS) stars, observed with HARPS at the 3.6 m telescope. The very high resolution (115 000) and the large wavelength coverage (about 380–680 nm) of the twin instruments permitted us to derive atmospheric parameters, metallicity, and detailed chemical abundances of 23 species from all nucleosynthetic channels. We employed both equivalent widths and spectrum synthesis. We also re-derived the cluster distance and age using Gaia parallaxes, proper motions, and photometry in conjunction with the PARSEC stellar evolutionary models. Results. We fully analysed those stars with radial velocity and proper motion/parallax in agreement with the cluster mean values. We also discarded one binary not previously recognised, and six stars near the MS turn-off because of their high rotation velocity. Our final sample consists of 21 stars (six giants and 15 MS stars). We measured metallicity (the cluster average [Fe/H] is +0.08, rms = 0.07) and abundances of light, α, Fe-peak, and neutron-capture elements. The Li abundance follows the expectations, showing a tight relation between temperature and abundance on the MS, at variance with M 67, and we did not detect any Li-rich giant. Conclusions. We confirm that Rup 147 is the oldest nearby open cluster. This makes it very valuable to test detailed features of stellar evolutionary models.

Search for an Intrinsic Metallicity Spread in Old Globular Clusters of the Large Magellanic Cloud

Abstract We report for the first time on the magnitude of the intrinsic [Fe/H] spread among 10 old globular clusters (GCs) of the Large Magellanic Cloud (LMC). Such spreads are merely observed in approximately 5% of the Milky Way GCs and recently gained more attention in theoretical models of GC evolution. We derived metallicities with a typical precision of 0.05 dex ≤ σ[Fe/H] ≤ 0.20 dex for an average of 14 red giant branch stars per GC from Strömgren photometry. The respective, metallicity-sensitive indices have been calibrated to precise and accurate high-dispersion spectroscopy. For all clusters, we found null [Fe/H] spreads with a typical uncertainty of 0.04 dex, with the possible exception of NGC 1786 that shows an intrinsic dispersion of 0.07 ± 0.04 dex. The mean, observed standard deviation of the derived metallicities for nearly 40% of our GC sample amounted to smaller than 0.05 dex. At present, we cannot exclude the fact that the remaining GCs also have intrinsic Fe-abundance variations in excess of 0.05 dex, but in order to significantly detect those, the measurement errors on individual [Fe/H]-values would need to be lowered to the 0.03–0.07 dex level. These findings suggest, along with those from ages and light element abundances, that the LMC GCs studied here are similar to the majority of Galactic GCs.

Ruprecht 106: A Riddle, Wrapped in a Mystery, inside an Enigma∗

Abstract Galactic globular clusters (GCs) show overwhelming photometric and spectroscopic evidence for the existence of multiple stellar populations. The question of whether or not there exists a GC that represents a true “simple stellar population” remains open. Here we focus on Ruprecht 106 (R106), a halo GC with [Fe/H] = −1.5 and [α/Fe] ≃0. A previous spectroscopic study found no sign of the Na–O anticorrelation among nine of its brightest red giants, which led to the conclusion that R106 is a true simple stellar population GC. Here we present new Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) photometry of R106 that, when combined with archival HST images spanning a 6 year baseline, allows us to create proper-motion cleaned color–magnitude diagrams spanning the ultraviolet (F336W) to the near-infrared (F814W). These data allow us to construct the pseudo-color that is sensitive to the presence of light-element abundance spreads. We find no evidence of a split along the red giant branch (RGB) in the diagram but the width of the RGB ( ) is marginally broader than expected from artificial star tests ( ). The observed spread in is smaller than any other Galactic GC studied to date. Our results raise important questions about the role of formation environment and primordial chemical composition in the formation of multiple stellar populations in GCs.

A tonalitic analogue to ancient detrital zircon

A zircon population from an Archean tonalite sample from southern West Greenland has been used as a source analogue in order to test common methods and approaches applied to ancient detrital zircon populations. Measurements of U-Th-Pb, oxygen and Lu-Hf isotopes as well as rare earth element and Ti concentrations were made in these zircon crystals and, where possible, in multiple areas within a single grain. The population is dominated by oscillatory zoned cores aged 3.82 Ga with an isotopically and compositionally distinct rim that formed at 3.59 Ga. We demonstrate that multiple age components may be erroneously inferred from within these oscillatory zoned zircon cores, both from the total population and within individual grains. This has bearing on other zircon-hosted geochemical systems, as temporal correlations may be incorrectly assigned. Oxygen and Lu-Hf isotope compositions are relatively consistent through the population with only a small number of outliers. Ranges in rare earth element and Ti abundances are evident from the total population, from which apparent inverse cooling trends may be inferred. Additionally, we show that even with enhanced filtering of Ti concentrations using light rare earth element abundances, crystallisation temperatures derived from zircon grains of a single, hand sample sized rock can yield both wide and bimodal results. Since even simple, single “source rock” zircon populations may, without careful scrutiny, portray artificially complex results, particular care must be taken in the interpretation of complex ancient detrital zircon populations.

Nuclear Star Clusters in Cosmological Simulations

Abstract We investigate the possible connection between the most massive globular clusters, such as ω Cen and M54, and nuclear star clusters (NSCs) of dwarf galaxies that exhibit similar spreads in age and metallicity. We examine galactic nuclei in cosmological galaxy formation simulations at z ≈ 1.5 to explore whether their age and metallicity spreads could explain these massive globular clusters. We derive structural properties of these nuclear regions, including mass, size, rotation, and shape. By using theoretical supernova yields to model the supernova enrichment in the simulations, we obtain individual elemental abundances for Fe, O, Na, Mg, and Al. Our nuclei are systematically more metal-rich than their host galaxies, which lie on the expected mass–metallicity relation. Some nuclei have a spread in Fe and age comparable to the massive globular clusters of the Milky Way, lending support to the hypothesis that NSCs of dwarf galaxies could be the progenitors of these objects. None of our nuclear regions contain the light element abundance spreads that characterize globular clusters, even when a large age spread is present. Our results demonstrate that extended star formation history within clusters, with metal pollution provided solely by supernova ejecta, is capable of replicating the metallicity spreads of massive globular clusters, but still requires another polluter to produce the light element variations.