Doug Geisler

Context . The inner Milky Way hosts overlapping stellar populations (bar–bulge, inner thin and thick disks, and halo), thus complicating population assignments along the line of sight. A joint chemical–dynamical approach is required to isolate a clean field bulge sample. Metal-poor bulge stars are particularly valuable as they likely trace the earliest phases of chemical enrichment in the inner Galaxy. Aims . We characterize the α -element (Si, Mg) and selected Fe-peak abundances of a dynamically defined sample of bulge field stars, and contrast these trends with the inner-halo tail in the metal-poor regime. Methods . We analyzed metal-poor candidates from the Pristine Inner Galaxy Survey observed by the bulge Cluster APOGEE Survey. Using APOGEE/ASPCAP abundances (S/N ≥ 50), we integrated full 6D orbits in a barred Milky Way potential. Bulge membership was defined via orbital confinement using an apocenter cut and the high-density locus in the ( E _J , L _z ) plane, with uncertainties estimated using Monte Carlo simulations. We identified 98 stars as genuine bulge members. Results . The metallicity distribution spans −2.5 ≤ [Fe/H]≤−0.4, with a median [Fe/H] = −1.71, offset to higher metallicity than the inner halo (median [Fe/H] = −1.96). The sample follows a high- α sequence with slopes d[Si/Fe]/d[Fe/H] = −0.020 ^+0.012 _−0.051 and d[Mg/Fe]/d[Fe/H] = −0.097 ^+0.062 _−0.133 . Fe-peak tracers show [Ni/Fe]∼0, while [Mn/Fe] declines with [Fe/H] with a mild upturn at higher metallicity. We detect no significant [Si/Fe] gradients with R _apo or Z _max (+0.010 ^+0.018 _0.000 and +0.006 ^+0.022 _−0.011 dex kpc ⁻¹ , respectively). The results are insensitive to the assumed Ω _p , yielding indistinguishable memberships. Conclusions . The chemo-orbital evidence favors an in situ origin within the inner Galaxy for the metal-poor bulge field, enriched at early times and later rearranged by secular bar evolution, with a minor contribution from halo stars at the most metal-poor end.

The distinct stellar populations of the Galactic bulge can be disentangled through detailed analysis of their chemical abundances and kinematical properties. Recent studies have suggested that globular clusters (GCs) located in the Galactic bulge with metallicities around [Fe/H] ≈ −0.7 may represent some of the oldest systems in the Milky Way, potentially tracing the early spheroidal bulge. The coincidence of a metallicity peak at [Fe/H] ≈ −0.7 in both field stars and GCs, together with the presence of phosphorus-rich (P-rich) stars, may provide important clues to the nature of the first generations of stars formed in the Galaxy. In this work, we investigate the odd- Z elements Na, Al, and particularly P in the bulge GCs NGC 6539 ([Fe/H] ∼ −0.75) and NGC 6569 ([Fe/H] ∼ −0.85) using APOGEE spectra. We also examine the clusters Tonantzintla-1 and NGC 6316, which exhibit evidence of phosphorus enhancement. Our analysis confirms that NGC 6539 is a cluster of interest, with one clearly P-rich star, whereas NGC 6569 shows a lower level of P enhancement. This again suggests that there might have been an early bulge building block with the metallicity of [Fe/H] ∼ −0.75, which NGC 6539 would be part of. The observed abundance patterns indicate that the production of Na and Al is consistent with nucleosynthesis in massive stars. However, the origin of the phosphorus enrichment remains uncertain, suggesting that additional nucleosynthetic channels may be required to explain the observed abundances. These findings provide new constraints on the chemical evolution of the Galactic bulge and the nature of its earliest stellar populations.

During its evolution, the Milky Way (MW) incorporated numerous dwarf galaxies, particularly low-mass systems. The surviving dwarf galaxies orbiting the MW serve as exceptional laboratories for studying the unique properties of these systems. Their metalpoor environments and shallow gravitational potentials likely drive significant differences in star formation and star cluster properties compared to those in the MW. Using high-quality near-infrared spectra from the APOGEE survey, we determined abundances of Fe, C, N, O, Mg, Al, Si, Ca, Ti, Cr, Mn, Ni, and Ce for 74 stars in four MW satellite dwarf galaxies: Fornax, Sextans, Draco, and Carina. Our analysis reveals that the distribution of α elements (e.g., [Si/Fe]) strongly correlates with galaxy luminosity (and hence mass), underscoring the critical role of galaxy mass in shaping chemical evolution. These dwarf galaxies exhibit [Al/Fe]~ −0.5, which is comparable to those of the metal-poor stars in the MW. Additionally, we identified nitrogen-rich field stars in the Fornax dwarf galaxy, which display distinct metallicities compared to its known globular clusters (GCs). If these stars originated in GCs and subsequently escaped, their presence suggests we are observing relics of destroyed GCs, offering possible evidence of cluster disruption.

Context . This study presents detailed elemental abundances in the intermediate-metallicity bulge globular cluster Djorg 2 based on high-resolution near-infrared spectra of six (R ~ 22 500) members obtained through the bulge Cluster APOgee Survey (CAPOS). CAPOS is focused on the study of clusters within the Galactic bulge and uses the APOGEE-2S as part of the Sloan Digital Sky Survey IV (SDSS-IV). Aims . This study was undertaken to chemically explore this poorly studied cluster and to analyze it for the first time using the code BACCHUS, with the main objective of deriving the mean chemical abundances for a number of species and investigating the occurrence of multiple populations. Methods . We employed BACCHUS to provide the line-by-line elemental abundances of a variety of species, including α -elements (O, Mg, Si, Ca, and Ti), light elements (C and N), the odd-Z element Al, the s-process element (Ce), and iron-peak elements (Fe and Ni). Results . We found an average metallicity of [Fe/H] = –1.04 ± 0.06, without an indication of an intrinsic spread. The mean values for the other elements and their standard deviations are [C/Fe] = –0.35 ± 0.18, [N/Fe] = 0.38 ± 0.40, [O/Fe] = 0.22 ± 0.18, [Mg/Fe] = 0.38 ± 0.05, [Al/Fe] = 0.32 ±0.15, [Ca/Fe] = 0.21 ± 0.10, [Si/Fe] = 0.38 ± 0.05, [Ce/Fe] = +0.00 ± 0.06, [Ti/Fe] = +0.12 ± 0.08 and [Ni/Fe] = +0.09 ± 0.06. Conclusions . The typical α-element enrichment in Djorg2 follows the trend of other metal-rich globular clusters. We found evidence for intrinsic spreads in C, N and O, which furthermore show the C:N and N:O anticorrelations that are typical of globular clusters. Ce shows no intrinsic variation, and in particular no correlation with N or Al. We found that Djorg 2 moves in a very elongated and flat orbit, that always remains inside the bulge and does not move far away in altitude above the Galactic plane. Moreover, it moves in a retrograde direction (backward to the rotation of the galaxy) and is not trapped by the galactic bar.

Context . Bulge globular clusters are key to understanding the formation and chemical evolution of the ancient central component of our Galaxy. Thanks to CAPOS, the bulge Cluster APOgee Survey, we can mitigate the observational difficulties limiting access to these objects in the optical and investigate them in more detail in the near-IR. Aims . Our goal is to perform a rigorous abundance analysis on a large number of member stars in the metal-rich bulge globular cluster NGC 6304, using high-resolution spectra, in order to determine its detailed chemical composition and study the relationship of this globular cluster with its halo counterparts. In addition, we investigate chemical patterns that allow us to identify multiple populations. Methods . This analysis is based on spectroscopic data obtained by CAPOS, which uses the APOGEE-2S spectrograph to observe in the H band. The abundances of 17 elements (C, N, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Mn, Fe, Ni, and Ce) were derived using the BACCHUS code, using atmospheric parameters from both APOGEE’s ASPCAP pipeline and those derived independently from photometry ( Gaia and 2MASS) to cross validate our results with the ASPCAP values and provide a consistent chemical analysis. Results . We derived a mean iron abundance of [Fe/H] = −0.45 ± 0.05 using the ASPCAP stellar parameters, and [Fe/H] = −0.45 ± 0.08 when using photometric stellar parameters, with no evidence of an intrinsic metallicity spread. NGC 6304 shows a typical enhancement in α elements, with [ α /Fe] spec = +0.24 ± 0.07 and [ α /Fe] phot = +0.23 ± 0.08, similar to what is observed in other globular clusters. We find a significant spread in [N/Fe], with σ _spec = 0.54 and σ _phot = 0.46, along with a clear C–N anticorrelation. Furthermore, we detect a correlation of Ce with both N and Al, consistent with patterns observed in some metal-rich bulge globular clusters but not all. Conclusions . Our study provides the first comprehensive spectroscopic evidence for multiple populations in NGC 6304. We also find a significant star-to-star variation in Na, but a minimal variation in O, in concordance with trends found in other metal-rich bulge clusters. The absence of the Mg–Al anticorrelation supports the evidence that the MgAl cycle is not active in globular clusters at high metallicity. The observed correlation between Ce and both N and Al suggests that the enrichment of these elements may be driven by asymptotic giant branch stars, positioning Ce as an element involved in the multiple population phenomenon in metal-rich globular clusters. We find generally that abundances are consistent with those of bulge field stars of similar metallicity, suggesting a similar origin and chemical evolution.

Phosphorus-enhanced (P-rich; [P/Fe] ≳ +0.8) giants have been found among mildly metal-poor field stars, but in only one star in a globular cluster (GC), M4 (NGC 6121). Also, in a sample of bulge spheroid stars, some of them showed a moderate P enhancement in the range +0.5 < [P/Fe] < +1.0. In this paper we derive the P abundance of moderately metal-poor ([Fe/H] ≳ −1) GC stars, aiming to check if the phenomenon could be related to the unusual multiple stellar populations found in most GCs. Here we present the detection of moderately P-enhanced stars among two out of seven bulge GCs (Tonantzintla 1 and NGC 6316), with metallicities similar to those of the bulge-field P-rich stars. Using H-band high-resolution (R ∼ 22,500) spectra from the APOGEE-2 survey, we present the first high-resolution abundance analysis of [P/Fe] from the P I 16482.932 Å line in a sample of selected bulge GCs. We find that all P-rich stars tend to also be N-rich, which hints at the origin of P-rich stars as second-generation stars in GCs. However no other correlations of P and other elements are found, which are usually indicators of second-generation stars. Further studies with larger samples and comparisons with field stars will be needed before any firm conclusions are drawn.

We performed a detailed abundance analysis of ten red giant members of the heavily obscured bulge globular cluster HP 1 using high-resolution, high S/N, 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 (CAPOS). We investigated the chemical abundances for a variety of species, including the light (C, N), odd-Z (Al), α (O, Mg, Si, S, Ca, and Ti), Fe-peak (Ni, Fe), and neutron-capture (Ce) elements. The derived mean cluster metallicity is [Fe/H] = −1.15 ± 0.03, with no evidence of an intrinsic metallicity spread. HP 1 exhibits a typical α enrichment that follows the trend for similar metallicity Galactic globular clusters (GCs), such as NGC 288 and NGC 5904, although our [Si/Fe] abundances are relatively high. We find a significant nitrogen spread (~1 dex) and a large fraction of nitrogen-enhanced ([N/Fe] > +0.7) stars that populate the cluster. We also detect intrinsic star-to-star spreads in [C/Fe], [O/Fe], [Al/Fe], and [Ca/Fe], which are (anti)correlated with several chemical species, indicating the prevalence of the multiple-population phenomenon in HP 1. We uncover for the first time a possible correlation between Ca and Al, although the sample is small. The mean ⟨[Mg/Fe]⟩ = +0.29 and ⟨[Al/Fe]⟩ = +0.46 place HP 1 in the region dominated by in situ GCs, supporting the in situ nature of this cluster.

This work consists of the first detailed photometric study of Terzan 6, one of the least known globular clusters in the Galactic bulge. Through the analysis of high angular resolution and multiwavelength data obtained from adaptive optics corrected and space observations, we built deep, optical and near-infrared color-magnitude diagrams reaching ≈4 magnitudes below the main-sequence turnoff. Taking advantage of four different epochs of observations, we measured precise relative proper motions for a large sample of stars, from which cluster members have been solidly distinguished from Galactic field interlopers. A noncanonical reddening law (with R_V = 2.85) and high-resolution differential reddening map, with color excess variations up to δE(B − V) ≈ 0.8 mag, have been derived in the direction of the system. According to these findings, new values for the extinction and distance modulus have been obtained: E(B − V) = 2.36 ± 0.05 and (m − M)₀ = 14.46 ± 0.10 (corresponding to d = 7.8 ± 0.3 kpc), respectively. We also provide the first determinations of the cluster center and projected density profile from resolved star counts. The center is offset by more than 7″ to the east from the literature value, and the structural parameters obtained from the King model fitting to the density profile indicate that Terzan 6 is in an advanced stage of its dynamical evolution, with a large value for the concentration parameter (c = 1.94_−0.26^+0.24) and a small core radius (r_c = 2.6_−0.7^+0.09 arcsec). We also determined the absolute age of the system, finding t = 13 ± 1 Gyr, in agreement with the old ages found for the globular clusters in the Galactic bulge. From the redetermination of the absolute magnitude of the red giant branch bump and the recent estimate of the cluster global metallicity, we find that Terzan 6 nicely matches the tight relation between these two parameters drawn by the Galactic globular cluster population.

Context. Stars presently identified in the bulge spheroid are probably very old, and their abundances can be interpreted as due to the fast chemical enrichment of the early Galactic bulge. The abundances of the iron-peak elements are important tracers of nucleosynthesis processes, in particular oxygen burning, silicon burning, the weak s-process, and α-rich freeze-out. Aims. The aim of this work is to derive the abundances of V, Cr, Mn, Co, Ni, and Cu in 58 bulge spheroid stars and to compare them with the results of a previous analysis of data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Methods. We selected the best lines for V, Cr, Mn, Co, Ni, and Cu located within the H-band of the spectrum, identifying the most suitable ones for abundance determination, and discarding severe blends. Using the stellar physical parameters available for our sample from the DR17 release of the APOGEE project, we derived the individual abundances through spectrum synthesis. We then complemented these measurements with similar results from different bulge field and globular cluster stars, in order to define the trends of the individual elements and compare with the results of chemical-evolution models. Results. We verify that the H-band has useful lines for the derivation of the elements V, Cr, Mn, Co, Ni, and Cu in moderately metalpoor stars. The abundances, plotted together with others from high-resolution spectroscopy of bulge stars, indicate that: V, Cr, and Ni vary in lockstep with Fe; Co tends to vary in lockstep with Fe, but could be showing a slight decrease with decreasing metallicity; and Mn and Cu decrease with decreasing metallicity. These behaviours are well reproduced by chemical-evolution models that adopt literature yields, except for Cu, which appears to drop faster than the models predict for [Fe/H]<−0.8. Finally, abundance indicators combined with kinematical and dynamical criteria appear to show that our 58 sample stars are likely to have originated in situ.