Metal-poor Globular Clusters Research Articles

Imprints of interaction processes in the globular cluster system of NGC 3640

We present a wide-field study of the globular cluster systems (GCS) of the elliptical galaxy NGC 3640 and its companion NGC 3641, based on observations from Gemini Multi-Object Spectrograph/Gemini using g’r’i’ filters. NGC 3640 is a shell galaxy which presents a complex morphology, which previous studies have indicated as the sign of a recent ‘dry’ merger, although whether its nearest neighbour could have influenced these substructures remains an open question. In this work, we trace the spatial distribution of the globular clusters (GCs) as well as their colour distribution, finding a potential bridge of red (metal-rich) GCs that connects NGC 3640 to its less massive companion, and signs that the blue (metal-poor) GCs were spatially disturbed by the event that created the shells. While NGC 3640 presents a typical bimodal colour distribution, the GCS attributed to NGC 3641 appears unimodal, with most GCs presenting an intermediate colour in comparison to those closer to its companion.

Combined Gemini-South and HST photometric analysis of the globular cluster NGC 6558

Context. NGC 6558 is a low-galactic-latitude globular cluster projected in the direction of the Galactic bulge. Due to high reddening, this region presents challenges in deriving accurate parameters, which require meticulous photometric analysis. We present a combined analysis of near-infrared and optical photometry from multi-epoch high-resolution images collected with Gemini-South/GSAOI+GeMS (in the J and KS filters) and HST/ACS (in the F606W and F814W filters). Aims. We aim to refine the fundamental parameters of NGC 6558, utilising high-quality Gemini-South/GSAOI and HST/ACS photometries. Additionally, we intend to investigate its role in the formation of the Galactic bulge. Methods. We performed a meticulous differential reddening correction to investigate the effect of contamination from Galactic bulge field stars. To derive the fundamental parameters – age, distance, reddening, and the total-to-selective coefficient – we employed a Bayesian isochrone fitting. The results from high-resolution spectroscopy and RR Lyrae stars were implemented as priors. For the orbital parameters, we employed a barred Galactic mass model. Furthermore, we analysed the age-metallicity relation to contextualise NGC 6558 within the Galactic bulge’s history. Results. We studied the impact of two differential reddening corrections on the age derivation. When removing as much as possible of the Galactic bulge field star contamination, the isochrone fitting combined with synthetic colour-magnitude diagrams gives a distance of 8.41−0.10+0.11 kpc, an age of 13.0 ± 0.9 Gyr, and a reddening of E(B − V) = 0.34 ± 0.02. We derived a total-to-selective coefficient of RV = 3.2 ± 0.2 thanks to the simultaneous near-infrared–optical synthetic colour-magnitude diagram fitting, which, aside from errors, agrees with the commonly used value. The orbital parameters showed that NGC 6558 is confined within the inner Galaxy and it is not compatible with a bar-shape orbit, indicating that it is a bulge member. Assembling the old and moderately metal-poor ([Fe/H] ∼ −1.1) clusters in the Galactic bulge, we derived their age-metallicity relation with star formation starting at 13.6 ± 0.2 Gyr and effective yields of ρ = 0.05 ± 0.01 Z⊙. Conclusions. The old age derived for NGC 6558 is compatible with other clusters with similar metallicity and a blue horizontal branch in the Galactic bulge, which constitute the moderately metal-poor globular clusters. The age-metallicity relation shows that the starting age of star formation is compatible with the age of NGC 6558, and the chemical enrichment is ten times faster than the ex situ globular cluster branch.

Spatially resolving the AGB star V3 in the metal-poor globular cluster 47 Tuc with VLTI/GRAVITY

Context. Mass loss at the asymptotic giant branch (AGB) plays an important role not only in the final fates of stars, but also in the chemical evolution of galaxies. Nevertheless, the metallicity effects on AGB mass loss are not yet fully understood. Aims. We present spatially resolved observations of an AGB star, V3, in the metal-poor globular cluster 47 Tuc (NGC 104). Methods. The AGB star 47 Tuc V3 was observed using the GRAVITY instrument at ESO’s Very Large Telescope Interferometer (VLTI) at 2–2.45 μm, with a projected baseline length of up to 96 m. Results. The object 47 Tuc V3 has been spatially resolved and stands as the first to attempt to spatially resolve an individual star in a globular cluster. The uniform-disk fit to the observed data results in an angular diameter of ∼0.7 mas. Our modeling of the spectral energy distribution and near-infrared interferometric GRAVITY data suggests that the observed data can be explained by an optically thin dust shell with a 0.55 μm optical depth of 0.05–0.25, consisting of metallic iron grains, likely together with effects of the extended atmosphere of the central star. The dust temperature at the inner shell boundary is 500–800 K (corresponding to 23–90 stellar radii), significantly lower than observed in nearby oxygen-rich AGB stars. Radiation pressure on small (< 0.05 μm) iron grains is not sufficient to drive stellar winds. Therefore, iron grains may grow to larger sizes, even in the metal-poor environment. Alternatively, it is possible that the observed iron grain formation is a result of the mass outflow initiated by some other mechanism(s). Conclusions. The sensitivity and angular resolution of VLTI provides a new window onto spatially resolving individual stars in metal-poor globular clusters. This allows us to improve subsequent studies of the metallicity dependence of dust formation and mass loss.

Horizontal branch structure, age, and chemical composition for very metal-poor extragalactic globular clusters

ABSTRACT This paper presents the results of analysing the integrated light (IL) low-resolution spectra of globular clusters (GCs) in the M31 and Centaurus A groups of galaxies. The sample consists of eight very metal-poor GCs ($\rm [Fe/H]\le -2$ dex) with high signal-to-noise ratio spectra acquired with the telescopes: the 6-m SAO RAS (BTA), the Southern African Large (SALT), and the 6.5-m Magellan (MMT). We study the influence of contribution of the horizontal branch stars on the hydrogen Balmer line profiles in the IL spectra. By modelling the Balmer lines, as well as the metal lines in the observed spectra, we determine the optimum parameters of stellar evolution isochrones and, consequently, the parameters of the atmospheres of the cluster stars. For all the studied GCs, the parameters of horizontal branch stars set by the selected isochrones, the corresponding ages, and carbon abundances are presented for the first time. The abundances of several other elements (Mg, Ca, Ti, Cr, and Mn) were determined for five GCs for the first time. All the studied GCs have blue horizontal branches and are older than 10 Gyr. Their chemical abundances, with the exception of Mg and Mn, are in good agreement with the abundances of stars in the Galactic field. The reasons of low [Mg/Fe] and of high [Mn/Fe] are discussed. Study of the fundamental properties of stellar populations in old GCs facilitates a better understanding of the formation processes of their parent galaxies and nucleosynthesis in the early Universe.

Ages and metallicities of globular clusters in M81 using GTC/OSIRIS spectra

ABSTRACT We here present the results of an analysis of the optical spectroscopy of 42 globular cluster (GC) candidates in the nearby spiral galaxy M81 (3.61 Mpc). The spectra were obtained using the long-slit and multi-object spectroscopic modes of the OSIRIS instrument at the 10.4-m Gran Telescopio Canarias at a spectral resolution of ∼1000. We used the classical H β versus [MgFe]′ index diagram to separate genuine old GCs from clusters younger than 3 Gyr. Of the 30 spectra with continuum signal-to-noise ratio >10, we confirm 17 objects to be classical GCs (age >10 Gyr, −1.4 < [Fe/H] < −0.4), with the remaining 13 being intermediate-age clusters (1–7.5 Gyr). We combined age and metallicity data of other nearby spiral galaxies (≲18 Mpc) obtained using similar methodology like the one we have used here to understand the origin of GCs in spiral galaxies in the cosmological context. We find that the metal-poor ([Fe/H] < −1) GCs continued to form up to 6 Gyr after the first GCs were formed, with all younger systems (age <8 Gyr) being metal rich.

Study of Blue Metal-poor Stars Using UVIT/AstroSat

Blue metal-poor stars are main-sequence stars that are bluer and brighter than typical turn-off stars in metal-poor globular clusters. They are thought to have either evolved through post-mass transfer mechanisms as field blue straggler stars or have accreted from Milky Way dwarf satellite galaxies. It has been found that a considerable fraction of blue metal poor stars are binaries, possibly with a compact companion. We observed 27 blue metal poor stars using UV imaging telescope of AstroSat in two far-UV filters, F148W and F169M. In this work, we explain the possible formation channels of two stars, BMP17 and BMP37. We fit BMP17 to a single-component spectral energy distribution whereas BMP37 with a binary-component spectral energy distribution. As both of them are known SB1s, we suggest that the WD companion of BMP17 may have cooled down so that it is out of UV imaging telescope detection limit. On the other hand, we discover a normal mass white dwarf as the hot companion of BMP37, indicating mass transfer as the possible formation channel.

First asteroseismic analysis of the globular cluster M80: multiple populations and stellar mass-loss

ABSTRACT Asteroseismology provides a new avenue for accurately measuring the masses of evolved globular cluster (GC) stars. We present the first detections of solar-like oscillations in 47 red giant branch (RGB) and early asymptotic giant branch (EAGB) stars in the metal-poor GC M80; only the second with measured seismic masses. We investigate two areas of stellar evolution and GC science: multiple populations and stellar mass-loss. We detect a distinct bimodality in the EAGB mass distribution. We suggest that this could be due to sub-population membership. If confirmed in future work with spectroscopy, it would be the first direct measurement of a mass difference between sub-populations. A mass difference was not detected between the sub-populations in our RGB sample. We instead measured an average RGB mass of $0.782\pm 0.009~\mathrm{M}_{\odot }$, which we interpret as the average of the sub-populations. Differing mass-loss rates on the RGB have been proposed as the second parameter that could explain the horizontal branch morphology variations between GCs. We calculated an integrated RGB mass-loss separately for each sub-population: $0.12\pm 0.02~\mathrm{M}_{\odot }$ (SP1) and $0.25\pm 0.02~\mathrm{M}_{\odot }$ (SP2). Thus, SP2 stars appear to have enhanced mass-loss on the RGB. Mass-loss is thought to scale with metallicity, which we confirm by comparing our results to a higher metallicity GC, M4. Finally, our study shows the robustness of the Δν-independent mass scaling relation in the low-metallicity (and low surface gravity) regime.

The kinematics, metallicities, and orbits of six recently discovered Galactic star clusters with Magellan/M2FS spectroscopy

ABSTRACT We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters (Gran 3/Patchick 125, Gran 4, Garro 01, and LP 866) and two newly discovered open clusters (Gaia 9 and Gaia 10) at low Galactic latitudes. We measure line-of-sight velocities and stellar parameters ([Fe/H], log g, Teff, and [Mg/Fe]) from high-resolution spectroscopy centred on the Mg triplet and identify 20–80 members per star cluster. We determine the kinematics and chemical properties of each cluster and measure the systemic proper motion and orbital properties by utilizing Gaia astrometry. We find Gran 3 to be an old, metal-poor (mean metallicity of [Fe/H] = −1.83) globular cluster located in the Galactic bulge on a retrograde orbit. Gran 4 is an old, metal-poor ([Fe/H] = −1.84) globular cluster with a halo-like orbit that happens to be passing through the Galactic plane. The orbital properties of Gran 4 are consistent with the proposed LMS-1/Wukong and/or Helmi streams merger events. Garro 01 is metal-rich ([Fe/H] = −0.30) and on a near-circular orbit in the outer disc but its classification as an open cluster or globular cluster is ambiguous. Gaia 9 and Gaia 10 are among the most distant known open clusters at $R_{\mathrm{GC}}\sim 18,~21.2~\mathrm{\, kpc}$ and most metal-poor with [Fe/H] ∼−0.50, −0.34 for Gaia 9 and Gaia 10, respectively. LP 866 is a nearby, metal-rich open cluster ([Fe/H] = +0.10). The discovery and confirmation of multiple star clusters in the Galactic plane shows the power of Gaia astrometry and the star cluster census remains incomplete.

Field blue straggler stars: discovery of white dwarf companions to blue metal-poor stars using UVIT/AstroSat

ABSTRACT Blue metal-poor (BMP) stars are the main-sequence stars that appear bluer and more luminous than normal turn-off stars of metal-poor globular clusters. They are believed to be either field blue straggler stars (FBSS) formed via post-mass transfer mechanism or accreted from dwarf satellite galaxies of the Milky Way. A significant fraction of BMP stars are discovered to be potential binaries. We observed 27 BMP stars using UVIT/AstroSat in two FUV filters, F148W and F169M. We report the discovery of white dwarf (WD) companions of 12 BMP stars for the first time. The WD companions have estimated temperatures Teff ∼10 500–18 250 K, and masses 0.17–0.8 M⊙. Based on [Fe/H] and space velocity, we group the 12 BMP/FBSS stars as the thick disc (5) and halo (5), whereas two stars appear to be in-between. All the 5 thick disc BMP/FBSS have extremely low-mass (M < 0.2 M⊙) WDs as companions, whereas the 5 halo BMP/FBSS have low (0.2 M⊙ < M < 0.4 M⊙), normal (0.4 M⊙ < M < 0.6 M⊙), and high mass (M > 0.6 M⊙) WD companions. Our analysis suggests that at least ∼44 ${{\ \rm per\ cent}}$ of BMP stars are FBSS, and these stars hold the key to understand the details of mass transfer, binary properties, and chemical enrichment among the FBSS.

Multiple Stellar Populations in Metal-poor Globular Clusters with JWST: A NIRCam View of M92

Recent work on metal-intermediate globular clusters (GCs) with [Fe/H] = −1.5 and −0.75 has illustrated the theoretical behavior of multiple populations in photometric diagrams obtained with the JWST. These results are confirmed by observations of multiple populations among the M dwarfs of 47 Tucanae. Here we explore multiple populations in metal-poor GCs with [Fe/H] = −2.3. We take advantage of synthetic spectra and isochrones that account for the chemical composition of multiple populations to identify photometric diagrams that separate the distinct stellar populations of GCs. We derive high-precision photometry and proper motion for main-sequence (MS) stars in the metal-poor GC M92 from JWST and Hubble Space Telescope images. We identify a first-generation (1G) and two main groups of second-generation (2GA and 2GB) stars and investigate their kinematics and chemical composition. We find isotropic motions with no differences among the distinct populations. The comparison between the observed colors of the M92 stars and the colors derived by synthetic spectra reveals that the helium abundances of 2GA and 2GB stars are higher than those of the 1G by ΔY ∼ 0.01 and 0.04, respectively. The m F090W versus m F090W − m F277W color–magnitude diagram shows that below the knee MS stars exhibit a wide color broadening due to multiple populations. We constrain the amount of oxygen variation needed to reproduce the observed MS width, which is consistent with results on red giant branch stars. We conclude that multiple populations with masses of ∼0.1–0.8 M ⊙ share similar chemical compositions.

The dual nature of the tidal tails of NGC 5904 (M5)

ABSTRACT The tangential velocity dispersion of stars belonging to the Milky Way globular cluster’s tidal tails has recently been found from N-body simulations to be a parameter that distinguishes between cored and cuspy profiles of low-mass dwarf galaxy dark matter subhaloes where that globular cluster formed, and the in situ formation scenario. In this context, we discovered that M5’s tidal tails are composed by stars at two different metallicity regimes ([Fe/H] ∼ −1.4 and −2.0 dex). The more metal-rich tidal tail stars are of the same metal content than M5’s members and have a tangential velocity dispersion that coincides with the predicted value for a cuspy formation scenario (subhalo mass ∼ 109 M⊙). The more metal-poor stars, that are found along the entire M5 tidal tails and have similar distributions to their more metal-rich counterparts in the M5 colour–magnitude diagram and orbit trajectory, have a tangential velocity dispersion that refers to a cored subhalo (mass ∼ 109 M⊙), or an in situ formation scenario. In order to reconcile the dual distribution of M5 tidal tail stars, in kinematics and chemistry, we propose that M5 collided with another more metal-poor and less-massive globular cluster anytime before or after it was accreted into the Milky Way.

M92 (NGC 6341) Is a Metal-complex Globular Cluster with an Atypical Primordial Population

We present a multiple stellar population study of the metal-poor globular cluster (GC) M92 (NGC 6341), which is long known for the substantial metallicity dispersion, using our own photometric system. We find two groups with slightly different mean metallicities, the metal-poor (MP) stars with [Fe/H] hk = −2.412 ± 0.03 and the metal-rich (MR) ones with −2.282 ± 0.002. The MP constitutes about 23% of the total mass with a more central concentration. Our populational tagging based on the [C/Fe] ch and [N/Fe] nh provides the mean n(P):n(I):n(E) = 32.2:31.6:36.2 (±2.4), where P, I, and E denote the primordial, intermediate, and extreme populations, respectively. Our populational number ratio is consistent with those of others. However, the MP has a significantly different populational number ratio than the mean value, and the domination of the primordial population in the MP is consistent with observations of Galactic GCs that less massive GCs contain larger fractions of the primordial population. Structural and constituent differences between the MP and MR may indicate that M92 is a merger remnant in a dwarf galaxy environment, consistent with recent suggestions that M92 is a GC in a dwarf galaxy or a remnant nucleus of the progenitor galaxy. Discrepancy between our method and those widely used for the Hubble Space Telescope photometry exists in the primordial population. Significant magnesium and oxygen depletions of −0.8 and −0.3 dex, respectively, and helium enhancement of ΔY ≳ 0.03 are required to explain the presence of this abnormal primordial group. No clear explanation is available with limited information of detailed elemental abundances.

JWST/NIRCam Probes Young Star Clusters in the Reionization Era Sunrise Arc

Star cluster formation in the early universe and its contribution to reionization remains largely unconstrained to date. Here we present JWST/NIRCam imaging of the most highly magnified galaxy known at z ∼ 6, the Sunrise arc. We identify six young massive star clusters (YMCs) with measured radii spanning from ∼20 down to ∼1 pc (corrected for lensing magnification), estimated stellar masses of ∼106–7 M ⊙, and ages of 1–30 Myr based on SED fitting to photometry measured in eight filters extending to rest frame 7000 Å. The resulting stellar mass surface densities are higher than 1000 M ⊙ pc−2 (up to a few 105 M ⊙ pc−2), and their inferred dynamical ages qualify the majority of these systems as gravitationally bound stellar clusters. The star cluster ages map the progression of star formation along the arc, with two evolved systems (≳10 Myr old) followed by very young clusters. The youngest stellar clusters (<5 Myr) show evidence of prominent Hβ+[O iii] emission based on photometry with equivalent widths larger than >1000 Å rest frame and are hosted in a 200 pc sized star-forming complex. Such a region dominates the ionizing photon production with a high efficiency log(ξion[Hzerg−1])∼25.7 . A significant fraction of the recently formed stellar mass of the galaxy (10%–30%) occurred in these YMCs. We speculate that such sources of ionizing radiation boost the ionizing photon production efficiency, which eventually carves ionized channels that might favor the escape of Lyman continuum radiation. The survival of some of the clusters would make them the progenitors of massive and relatively metal-poor globular clusters in the local universe.

A Photometric Survey of Globular Cluster Systems in Brightest Cluster Galaxies

Hubble Space Telescope imaging for 26 giant early-type galaxies, all drawn from the MAST archive, is used to carry out photometry of their surrounding globular cluster (GC) systems. Most of these targets are Brightest Cluster Galaxies and their distances range from 24–210 Mpc. The catalogs of photometry, completed with DOLPHOT, are publicly available. The GC color indices are converted to [Fe/H] through a combination of 12 Gyr single stellar population models and direct spectroscopic calibration of the fiducial color index (F475W–F850LP). All of the resulting metallicity distribution functions (MDFs) can be accurately matched by bimodal-Gaussian functions. The GC systems in all of the galaxies also exhibit shallow metallicity gradients with projected galactocentric distance that average . Several parameters of the MDFs including the means, dispersions, and blue/red fractions are summarized. Perhaps the most interesting new result is the trend of blue/red GC fraction with galaxy mass, which connects with predictions from recent simulations of GC formation within hierarchical assembly of large galaxies. The observed trend reveals two major transition stages: for low-mass galaxies, the metal-rich (red) GC fraction f(red) increases steadily with galaxy mass, until halo mass M h ≃ 3 × 1012 M ⊙. Above this point, more than half the metal-poor (blue) GCs come from accreted satellites and f(red) starts declining. But above a still higher transition point near M h ≃ 1014 M ⊙, the data hint that f(red) may start to increase again because the metal-rich GCs also become dominated by accreted systems.

Globular cluster metallicity distributions in the E-MOSAICS simulations

ABSTRACT The metallicity distributions of globular cluster (GC) systems in galaxies are a critical test of any GC formation scenario. In this work, we investigate the predicted GC metallicity distributions of galaxies in the MOdelling Star cluster population Assembly In Cosmological Simulations within EAGLE (E-MOSAICS) simulation of a representative cosmological volume (L = 34.4 comoving Mpc). We find that the predicted GC metallicity distributions and median metallicities from the fiducial E-MOSAICS GC formation model agree well the observed distributions, except for galaxies with masses $M_\ast \sim 2 \times 10^{10} \, \rm {M}_{\odot }$, which contain an overabundance of metal-rich GCs. The predicted fraction of galaxies with bimodal GC metallicity distributions (37 ± 2 per cent in total; 45 ± 7 per cent for $M_\ast \gt 10^{10.5} \, \rm {M}_{\odot }$) is in good agreement with observed fractions ($44^{+10}_{-9}$ per cent), as are the mean metallicities of the metal-poor and metal-rich peaks. We show that, for massive galaxies ($M_\ast \gt 10^{10} \, \rm {M}_{\odot }$), bimodal GC distributions primarily occur as a result of cluster disruption from initially-unimodal distributions, rather than as a result of cluster formation processes. Based on the distribution of field stars with GC-like abundances in the Milky Way, we suggest that the bimodal GC metallicity distribution of Milky Way GCs also occurred as a result of cluster disruption, rather than formation processes. We conclude that separate formation processes are not required to explain metal-poor and metal-rich GCs, and that GCs can be considered as the surviving analogues of young massive star clusters that are readily observed to form in the local Universe today.

Ca triplet metallicities and velocities for 12 globular clusters toward the galactic bulge

Context. Globular clusters (GCs) are excellent tracers of the formation and early evolution of the Milky Way. The bulge GCs (BGCs) are particularly important because they can reveal vital information about the oldest in situ component of the Milky Way. Aims. Our aim is to derive the mean metallicities and radial velocities for 13 GCs that lie toward the bulge and are generally associated with this component. This region is observationally challenging because of high extinction and stellar density, which hampers optical studies of these and similar BGCs, making most previous determinations of these parameters quite uncertain. Methods. We used near-infrared low-resolution spectroscopy with the FORS2 instrument on the VLT to measure the wavelengths and equivalent widths of the CaII triplet (CaT) lines for a number of stars per cluster. We derived radial velocities, ascertained membership, and applied known calibrations to determine metallicities for cluster members, for a mean of 11 members per cluster. Unfortunately, one of our targets, VVV-GC002, which is the closest GC to the Galactic center, turned out not to have any members in our sample. Results. We derive mean cluster RV values to 3 km s−1, and mean metallicities to 0.05 dex. We find general good agreement with previous determinations for both metallicity and velocity. On average, our metallicities are 0.07 dex more metal-rich than those of Harris (2010, arXiv:1012.3224), with a standard deviation of the difference of 0.25 dex. Our sample has metallicities between −0.21 and −1.64, and the values are distributed between the traditional metal-rich BGC peak near [Fe/H] −0.5 and a more metal-poor peak around [Fe/H] −1.1, which has recently been identified. These latter are candidates for the oldest GCs in the Galaxy, if blue horizontal branches are present, and include BH 261, NGC 6401, NGC 6540, NGC 6642, and Terzan 9. Finally, Terzan 10 is even more metal-poor. However, dynamically, Terzan 10 is likely an intruder from the halo, possibly associated with the Gaia-Enceladus or Kraken accretion events. Terzan 10 is also confirmed as an Oosterhoff type II GC based on our results. Conclusions. The CaT technique is an excellent method for deriving mean metallicities and velocities for heavily obscured GCs. Our sample provides reliable mean values for these two key properties via spectroscopy of a significant number of members per cluster for this important yet previously poorly studied sample of BGCs. We emphasize that the more metal-poor GCs are excellent candidates for being ancient relics of bulge formation. The lone halo intruder in our sample, Terzan 10, is conspicuous for also having by far the lowest metallicity, and casts doubt on the possibility of any bona fide BGCs at metallicities below about −1.5.

Near-infrared observations of RR Lyrae and Type II Cepheid variables in the metal-rich bulge globular cluster NGC 6441

Context. NGC 6441 is a bulge globular cluster (GC) with an unusual horizontal branch morphology and a rich population of RR Lyrae (RRL) and Type II Cepheid (T2C) variables that is unexpected for its relatively high metallicity. Aims. Our goal is to characterize the pulsation properties of the population II RRL and T2C variables in this metal-rich GC and compare them with a sample of variables in more metal-poor clusters, as well as with theoretical predictions. Methods. We present JHKs time-series observations of 42 RRLs, eight T2Cs, and ten eclipsing binary candidate variables in NGC 6441 . The multi-epoch observations were obtained using the FLAMINGOS-2 instrument on the 8-m Gemini South telescope. Multiband data are used to investigate pulsation properties of the RRL and T2C variables, including their light curves, instability strip, period–amplitude diagrams, and period–luminosity and period–Wesenheit relations (PLRs and PWRs) in the JHKs filters. Results. The near-infrared (NIR) pulsation properties of RRL variables are well fitted with theoretical models that have canonical helium content and the mean-metallicity of NGC 6441 ([Fe/H] = −0.44 ± 0.07 dex). The helium-enhanced RRL models predict brighter NIR magnitudes and bluer colors than the observations of RRL in the cluster. We find that the empirical slopes of the RRL PLRs and PWRs in NGC 6441 are statistically consistent with those of RRLs in more metal-poor GCs, as well as the theoretical models. Therefore, we use theoretical calibrations of RRL period–luminosity–metallicity (PLZ) relations to simultaneously estimate the mean reddening, E(J − Ks)=0.26 ± 0.06 mag, and the distance, d = 12.67 ± 0.09 kpc, to NGC 6441. Our mean reddening value is consistent with an independent estimate from the bulge reddening map based on red clump stars. The RRL-based distance agrees well with similar literature measurements based on the PLZ relations, and the Gaia and other independent methods. Our distance and reddening values provide a very good agreement between the PLRs of T2Cs in NGC 6441 and those for RRLs and T2Cs in Galactic GCs that span a broad range of metallicity. Conclusions. We conclude that the NIR colour-magnitude diagrams, pulsation properties, and PLRs for RRLs and T2Cs in NGC 6441 are in good agreement with the predictions of RRL pulsation models with canonical helium content. This suggests that these population II variables are either not significantly helium enhanced, as previously thought, or the impact of such enhancement is smaller in NIR than the predictions of the pulsation models.

A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. III. Discrete Multicomponent Population-dynamical Models Based on the Jeans Equations

We present comprehensive multicomponent dynamical models of M54 (NGC 6715), the nuclear star cluster of the Sagittarius (Sgr) dwarf galaxy, which is undergoing a tidal disruption in the Milky Way halo. Previous papers in this series used a large MUSE mosaic data set to identify multiple stellar populations in the system and study their kinematic differences. Here, we use Jeans-based dynamical models that fit the population properties (mean age and metallicity), spatial distributions, and kinematics simultaneously. They provide a solid physical explanation for our previous findings. Population-dynamical models deliver a comprehensive view of the whole system, and allow us to disentangle the different stellar populations. We explore their dynamical interplay and confirm our previous findings about the build-up of Sgr’s nuclear cluster via contributions from globular cluster stars, Sgr inner field stars, and in situ star formation. We explore various parameterizations of the gravitational potential and show the importance of a radially varying mass-to-light ratio for the proper treatment of the mass profile. We find a total dynamical mass within M54's tidal radius (∼75 pc) of 1.60 ± 0.07 × 106 M ⊙ in excellent agreement with N-body simulations. Metal-poor globular cluster stars contribute about 65% of the total mass or 1.04 ± 0.05 × 106 M ⊙. Metal-rich stars can be further divided into young and intermediate-age populations, which contribute 0.32 ± 0.02 × 106 M ⊙ (20%) and 0.24 ± 0.02 × 106 M ⊙ (15%), respectively. Our population-dynamical models successfully distinguish the different stellar populations in Sgr’s nucleus because of their different spatial distributions, ages, metallicities, and kinematic features.

High-resolution near-infrared spectroscopy of globular cluster and field stars toward the Galactic bulge

Globular clusters (GCs) play an important role in the formation and evolution of the Milky Way. New candidates are continuously found, particularly in the high-extinction low-latitude regions of the bulge, although their existence and properties have yet to be verified. In order to investigate the new GC candidates, we performed high-resolution near-infrared spectroscopy of stars toward the Galactic bulge using the Immersion Grating Infrared Spectrometer (IGRINS) instrument at the Gemini-South telescope. We selected 15 and 10 target stars near Camargo 1103 and Camargo 1106, respectively, which have recently been reported as metal-poor GC candidates in the bulge. In contrast to the classical approaches used in optical spectroscopy, we determined stellar parameters from a combination of line-depth ratios and the equivalent width of a CO line. The stellar parameters of the stars follow the common trends of nearby APOGEE sample stars in a similar magnitude range. We also determined the abundances of Fe, Na, Mg, Al, Si, S, K, Ca, Ti, Cr, Ni, and Ce through spectrum synthesis. There is no clear evidence of a grouping in radial velocity – metallicity space that would indicate the characterization of either object as metal-poor GCs. This result emphasizes the necessity of follow-up spectroscopy for new GC candidates toward the bulge, although we cannot completely rule out a low probability that we only observed nonmember stars. We also note discrepancies between the abundances of Al, Ca, and Ti when derived from the H- versus the K-band spectra. Although the cause of this discrepancy is not clear, the effects of atmosphere parameters or nonlocal thermodynamic equilibrium are discussed. Our approach and results demonstrate that IGRINS spectroscopy is a useful tool for studying the chemical properties of stars toward the Galactic bulge with a statistical uncertainty in [Fe/H] of ∼0.03 dex, while the systematic error through uncertainties of atmospheric parameter determination, at ∼0.14 dex, is slightly larger than in measurements from optical spectroscopy.

Photometry and astrometry with JWST – I. NIRCam point spread functions and the first JWST colour–magnitude diagrams of a globular cluster

ABSTRACT As the James Webb Space Telescope (JWST) has become fully operational, early release data are now available to begin building the tools and calibrations for precision point-source photometry and astrometry in crowded cluster environments. Here, we present our independent reduction of NIRCam imaging of the metal-poor globular cluster M 92, which were collected under Director’s Discretionary Early Release Science programme ERS-1334. We derived empirical models of the point spread function (PSF) for filters F090W, F150W, F277W, and F444W, and find that these PSFs: (i) are generally undersampled (FWHM ∼ 2 pixel) in F150W and F444W and severely undersampled (FWHM ∼ 1 pixel) in F090W and F277W; (ii) have significant variation across the field of view, up to ∼15–20 per cent; and (iii) have temporal variations of ∼ 3–4 per cent across multi-epoch exposures. We deployed our PSFs to determine the photometric precision of NIRCam for stars in the crowded, central regions of M 92, measured to be at the ∼0.01 mag level. We use these data to construct the first JWST colour–magnitude diagrams of a globular cluster. Employing existing stellar models, we find that the data reach almost the bottom of the M 92 main sequence (∼0.1 M⊙), and reveal 24 white dwarf candidate members of M 92 in the brightest portion of the white dwarf cooling sequence. The latter are confirmed through a cross-match with archival HST UV and optical data. We also detect the presence of multiple stellar populations along the low-mass main sequence of M 92.