Heitor Ernandes

Context. The formation of the Galactic bulge remains incompletely understood, with evidence pointing to different stellar populations, including a bar-driven component, an inner-disk population, and an older spheroidal component. Chemical abundances provide critical constraints on the origin of these populations, particularly for odd-Z elements such as Na and Al, as well as Mg, whose behaviour at high metallicity is still a matter of debate. Aims. We aim to investigate the presence of overabundances of Na, Mg, and Al in Galactic bulge stars, with particular emphasis on the metal-rich regime, and to evaluate their consistency with predictions from chemical evolution models. Methods. We re-derived the abundances of Na, Mg, and Al for a sample of 55 bulge red giants previously analysed in the literature. Our study is based on high-resolution UVES spectra obtained with the ESO Very Large Telescope and employs spectrum synthesis using the T URBOSPECTRUM code, along with updated atomic and molecular line lists. For consistency, we also re-derived the abundances of C, N, and O. Results. We find somewhat lower abundances of Mg and Al at the metal-rich end than previous studies, while a fraction of the metalrich stars still exhibit significant Na enhancements. These enhancements persist when different sets of stellar parameters are adopted, indicating that they are robust. Mg shows smoother trends with metallicity, broadly consistent with expectations. Conclusions. The presence of Na-enhanced stars at high metallicity is difficult to reconcile with standard chemical evolution models and suggests additional enrichment processes in the bulge, or a particular behaviour of stellar yields with metallicity. The Na enhancement could be due to metallicity-dependent yields from massive stars, not taken into account in available models, and/or enrichment by asymptotic giant branch stars, or due to second-generation stars evaporated from globular clusters, the latter option arising because for the metal-rich ([Fe∕H]>0) stars a Na-O anti-correlation appears to occur.

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.

. The Gaia-Sausage-Enceladus was the last major merger and central turning point in the Milky Way’s story. This event, comparable in mass to the Large Magellanic Cloud today, left behind significant debris that provides valuable insights into the assembly history of our Galaxy and the chemical evolution of dwarf galaxies. By examining the aftermath of the GSE merger, we can delve deeper into understanding how the Milky Way’s formation unfolded and how dwarf galaxies evolved chemically. Specifically, the distinct patterns of neutron capture elements such as Eu and Ba, along with Mg, offer clues about the star formation history. Through a comprehensive analysis of data compiled in the SAGA database, we investigated the Gaia Sausage-Enceladus’ star formation history. Elemental abundance ratios ([Eu/Mg], [Ba/Mg], and [Eu/Ba]) derived from this study, when compared with those of surviving Milky Way satellites, indicate that the GSE experienced a prolonged period of slow star formation, lasting over 2 Gyr, until it was eventually quenched by merging with the Milky Way. Consequently, these elemental signatures serve as a unique window into the complex history of both surviving and accreted satellites orbiting our Galaxy.

Context. The inner Galaxy is the most complex region of the Milky Way, comprising the early bulge, inner thin and thick discs, and inner halo stars; moreover, the formation of the bar caused transfer of gas and stars from the disc to the inner Galaxy. Moreover, accretion of dwarf galaxies took place along the Galaxy’s lifetime, merging with the original bulge. In this work, we sought to constrain the metal-rich stars of the earliest spheroidal bulge. Aims. With the aim of studying the oldest bulge stars, which show a distribution in a spheroid, we applied a selection based on kinematical and dynamical criteria, in the metal-rich range [Fe/H] >-0.8. This analysis complements our previous work on a symmetric sample with [Fe/H] <-0.8. Methods. We derived the individual abundances through spectral synthesis for the elements C, N, O, Al, P, S, K, Mn, and Ce using the stellar physical parameters available for our sample from Data Release 17 of the Apache Point Observatory Galactic Evolution Experiment (APOGEE DR17) project in the H band. We also compared the present results, together with literature data, with chemicalevolution models. Results. The abundances of the alpha elements Mg Si, and Ca, and iron-peak elements V, Cr, Co, and Ni from APOGEE DR17 follow the expected behaviour as compared with the chemical-evolution models. Mn shows the expected secondary behaviour. S and K show a large star-to-star spread, but remain broadly compatible with the model predictions. Phosphorus and cerium display a clear abundance excess around [Fe/H]~ -0.7 that is more pronounced than in our metal-poor sample, suggesting a distinctive chemical signature for the earliest bulge population. Diagnostic diagrams involving [Mg/Mn] versus [Al/Fe] and [Ni/Fe] versus [(C+N)/O] indicate an in situ origin of the bulk of the sample. At super-solar metallicities, a subset of stars shows enhanced K and Mn (and possibly S) together with low [Ce/Fe] ratios, hinting at enrichment processes linked to the nuclear disc and bar. These stars may therefore trace a chemically distinct population shaped by the unique dynamical and star formation conditions of the innermost Galaxy.

The assembly history of the Galactic bulge is intimately tied to the formation of the proto–Milky Way, yet reconstructing this early phase is difficult because mergers and secular evolution have erased most of its original structure. Among present-day stellar systems, only globular clusters retain the ancient signatures needed to trace these primordial building blocks. Here we present the most detailed characterization to date of Tonantzintla 2, a prime candidate for a relic of the Milky Way’s primordial bulge. It is a moderately metal-rich globular cluster projected onto the bulge that has remained largely unexplored despite its potential to constrain the early formation of the inner Milky Way. We derive its fundamental parameters using proper-motion-corrected Hubble Space Telescope WFC3 and Advanced Camera for Surveys photometry. By applying an isochrone fitting to very clean data, we obtain an age of 13.58 − 1.0 + 0.72 Gyr, a reddening E ( B – V ) = 1.44 ± 0.02, a metallicity [M/H] = −0.68 − 0.05 + 0.04 , and a heliocentric distance of d _⊙ = 7.38 0.08 0.13 kpc. A complementary chemical-abundance analysis of seven member stars from APOGEE high-resolution spectroscopy reveals an enrichment pattern consistent with an in situ origin. Tonantzintla 2 is among the oldest globular clusters studied in the literature, and the oldest so far analyzed in the Galactic bulge. Its age places a stringent constraint on the onset of the bulge formation, implying that star formation in the inner Galaxy began within ∼0.2 Gyr of the Big Bang and that Tonantzintla 2 represents an exceptional relic of the Milky Way’s earliest chemical enrichment.

The assembly history of the Galactic bulge is intimately tied to the formation of the proto-Milky Way, yet reconstructing this early phase is difficult because mergers and secular evolution have erased most of its original structure. Among present-day stellar systems, only globular clusters retain the ancient signatures needed to trace these primordial building blocks. Here we present the most detailed characterization to date of Tonantzintla 2, a prime candidate for a relic of the Milky Way's primordial bulge. It is a moderately metal-rich globular cluster projected onto the bulge that has remained largely unexplored despite its potential to constrain the early formation of the inner Milky Way. We derive its fundamental parameters using proper motion-corrected Hubble Space Telescope WFC3 and ACS photometry. By applying an isochrone fitting to very clean data, we obtain an age of 13.58 Gyr, a reddening E(B-V) = 1.44, a metallicity [M/H]=-0.68, and a heliocentric distance of d = 7.38 kpc. A complementary chemical-abundance analysis of seven member stars from APOGEE high-resolution spectroscopy reveals an enrichment pattern consistent with an in-situ origin. Tonantzintla 2 is among the oldest globular clusters studied in the literature, and the oldest so far analyzed in the Galactic bulge. Its age places a stringent constraint on the onset of the bulge formation, implying that star formation in the inner Galaxy began within ~0.2 Gyr of the Big Bang and that Tonantzintla 2 represents an exceptional relic of the Milky Way's earliest chemical enrichment.

Context . Differentiating between in situ and accreted populations in the Milky Way halo is a challenging task. Various kinematic spaces are often used to identify distinct accreted populations from the in situ Milky Way halo. However, this approach has limitations, especially at low orbital energies. To overcome this ambiguity, elemental abundances are typically used to distinguish between populations. However, for many elemental abundance ratios (e.g. [Mg/Fe]), it remains difficult to make this distinction at low metallicities ([Fe/H]~−1.5). Aluminium abundances, on the other hand, are empirically effective discriminators and separate accreted from in situ populations in the Milky Way halo, even at low metallicities and low orbital energies. Aims . We aim to test the discriminating power of [Al/Fe] using a well-studied sample of high-velocity stars in the solar vicinity with high-quality spectra. With these stars, we explore the ability of [Al/Fe] to separate in situ from accreted stars and test its limitations. Methods . We derived aluminium abundances from the Al I 3944 and 3961 Å lines for 45 stars observed in two ESO programmes, along with 11 stars with archival spectra. Aluminium abundances were determined using 1D LTE and 1D NLTE spectral synthesis and line-profile fitting. Results . We confirm that the halo low- α population (associated with accreted stars) systematically exhibits lower [Al/Fe] compared to halo high- α (in situ) stars. However, at low [Fe/H] ≈ −1.4, we identify three stars, previously classified as in situ or thick-disc stars, as having low [Al/Fe] < −0.3, indicating that they might actually be accreted. Conclusions . Aluminium abundances, when carefully measured and with NLTE effects taken into account, are effective tracers of the chemical history of halo stars within the [Fe/H] limits analysed in the present work (−1.5 ≲[Fe/H]≲ −0.5). They provide an independent constraint on origin, complementing α -element abundance trends, and help disentangle subpopulations within the accreted halo, especially in the metal-poor regime.

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.

(Abridged) The present-day spatial and kinematic distribution of stars in the Milky Way provides key constraints on its internal dynamics and evolutionary history. We select stars that are more metal-rich than the interstellar medium (ISM) at their guiding radius (the so-called Local Metal-Rich stars, LMR) and investigate their chemo-kinematics. Until recently, existing catalogues did not contain such targets in large quantities, but one can now select many millions of them by using Gaia photometric metallicities. Once selected, we investigate their kinematics and age distributions across the disc, and compare them to the stellar populations having the metallicity of the ISM. Compared to locally born stars with [M/H]=[M/H]_ISM, we find that LMR stars, at a given location, are always older (mean age up to 2 Gyr older) and with velocity dispersions similar or slightly higher. Furthermore, at a given [M/H], LMR stars are older at larger galactocentric radiii, reflecting the fact that they need time to migrate. Finally, whereas we do not find any correlation between the location of the spiral arms and the spatial density of LMR stars, we find that the mean stellar eccentricity and mean ages show smaller values where the spiral arms are. Our results confirm a well established theoretical result that has not yet been formally confirmed via observations on large datasets without modelling: churning is not significantly heating the Galactic disc. Furthermore, the age distribution of these stars rule-out any significant contribution from Galactic fountains as their origin, and confirm the effect of the spiral arms on them.

Chemical abundances of cobalt (Co; Z = 27) and copper (Cu; Z = 29) in bulge and halo stars are presented and compared with chemical evolution models. The aim is to distinguish if Co and Cu are dominantly produced by neutron-capture or the alpha-rich freeze-out processes. Neutron-capture can be identified by a secondary behaviour in the [X/Fe] vs. [Fe/H] plot, and alpha-rich freeze-out would give rather a primary behaviour.