Studying the composition and origin of the inner region of our Galaxy—the “Galactic bulge”—is crucial for understanding the formation and evolution of the Milky Way and other galaxies. We present new observational constraints based on a sample of around 18 000 stars in the inner Galaxy, combining Gaia DR3 RVS and APOGEE DR17 spectroscopy. Gaia-RVS complements APOGEE by improving the sampling of the metallicity, [Fe/H], in the −2.0 to −0.5 dex range. This work marks the first application of Gaia-RVS spectroscopy to the bulge region, enabled by a novel machine learning approach (hybrid-CNN) that derives stellar parameters from intermediate-resolution spectra with precision comparable to APOGEE’s infrared data. We performed full orbit integrations using a barred Galactic potential and applied orbital frequency analysis to disentangle the stellar populations in the inner Milky Way. For the first time, we are able to robustly identify the long-sought pressure-supported bulge traced by the field stars. We show this stellar population to be chemically and kinematically distinct from the other main components co-existing in the same region. The spheroidal bulge has a metallicity distribution function (MDF) peak at around −0.70 dex extending to solar values. It is dominated by a high-[α∕Fe] population with almost no dependency on metallicity, consistent with very rapid early formation predating the thick disc and the bar. We find evidence that the bar has influenced the dynamics of the spheroidal bulge, introducing a mild triaxiality and radial extension. We identify a group of stars on X4 orbits, likely native to the early spheroid, as this population mimics the chemistry of the spheroidal bulge, with a minor contamination from the more metal-poor ([Fe∕H]< −1.0) halo. We find the inner thick disc to be kinematically hotter (Vφ ≈ 125 km s−1) than the local thick disc. The disc, chemically distinct from the spheroidal bulge and bar, is predominantly metal-poor with an MDF peak at [Fe/H] ≈ −0.45 dex and includes a high fraction of stars with sub-solar [Fe/H] and intermediate [α/Fe]. In contrast to the spheroidal bulge, the [α/Fe] disc shows a steeper decline with [Fe/H], consistent with smaller star formation efficiency than that of the spheroidal bulge. Both the thick disc and the spheroidal bulge present vertical metallicity gradients. We find that the Galactic bar contains both metal-rich and metal-poor stars, as well as high and low [α/Fe] in nearly equal measure. However, their relative contributions vary significantly across different orbital families. The bar shows no strong metallicity trends in orbital extent or velocity dispersions and maintains a consistent elongated shape across all metallicities, indicating that it is a well-mixed dynamical structure. Despite their spatial overlap, the spheroidal bulge, thick disc, and bar occupy distinct regions in both phase space and chemical abundance, indicating separate formation pathways. The stars with [Fe/H]< −1.0 and crossing the Galactic bulge are comprised by accreted populations primarily (70%) belonging to the Gaia-Enceladus/Sausage (GES) merger with an MDF peak at −1.30 dex and possibly a secondary merger remnant with an MDF peak at −1.80 dex.

Aims. In this work, we exploit the most robust, old, and cosmology-independent age estimates of individual stars from Gaia DR3 to place a lower bound on the age of the Universe, t(U). These constraints can be used as an anchor point for any cosmological model, thus providing an upper limit to the Hubble constant H-0. Methods. Our primary stellar age catalog comprises 3000 of the oldest and most robustly measured main-sequence turn-off (MSTO) and subgiant branch (SGB) stars, with ages older than 12.5 Gyr and associated uncertainty below 1 Gyr. Stellar ages are derived via isochrone fitting using the Bayesian code StarHorse, spanning the uniform range 0-20 Gyr, not considering any cosmological prior knowledge on t(U). By applying a conservative cut in the Kiel diagram and strict quality cuts on both stellar parameters and posterior probability distribution shapes, and filtering out potential contaminants, we isolated a final sample of 160 bona fide stars, the most numerous sample of precise and reliable MSTO and SGB stars ages available to date. Results. The age distribution of the final sample peaks at 13.6 +/- 1.0 (stat) +/- 1.4 (syst) Gyr. Assuming a maximum formation redshift for these stars of z(f) = 20, corresponding to a formation delay of similar to 0.2 Gyr, we obtain a lower bound on t(U) of t(U) >= 13.8 +/- 1.0 (stat) +/- 1.4 (syst) Gyr. Considering the 10th percentile of the posterior probability distributions of the individual stars, we find that, at 90% confidence level, 70 stars favour t(U) > 13 Gyr, while none exceeds 14.1 Gyr. For this upper envelope to fall below 13 Gyr, a shift of nearly the full systematic error budget would be required, indicating that such low values are only attainable under very peculiar assumptions. Conclusions. This work presents the first statistically significant use of individual stellar ages as cosmic clocks, opening a new independent approach for cosmological studies. While this analysis already represents a significant step forward, future Gaia data releases will enable even larger and more precise stellar samples, further strengthening these constraints.

Context. Unresolved binaries in star clusters can bias stellar and cluster mass estimates. A proper treatment is therefore essential for studying the cluster dynamics and evolution. Aims. We develop a fast and robust framework for jointly deriving stellar masses and multiplicity statistics of member stars, together with optimal cluster parameters. Methods. We used Gaia DR3 parallaxes together with multi-band photometry of open cluster (OC) members to infer stellar masses and binary mass ratios through simulation-based inference (SBI), while iteratively fitting the cluster parameters. The validation of our SBI framework on simulated clusters demonstrates that the inclusion of infrared photometry significantly improves the detection of low mass-ratio binaries. The minimum mass-ratio threshold for reliably identifying unresolved binaries depends on the cluster properties and the available photometry, but typically lies below q = 0.5. Results. Applying our method to 42 well-populated OCs, we derived a catalogue of stellar masses and mass ratios for 27 201 stars achieving typical uncertainties of 0.08 in q and 0.01 M⊙ in the primary stellar mass. We analysed the archetype OCs M67 and NGC 2360 in detail, including mass segregation and mass-ratio distribution, and derived multiplicity fractions for the rest of the sample. We find evidence that the high mass ratio (q ≥ 0.6) binary fraction is strongly correlated with the age and weakly anti-correlated with the cluster metallicity. Furthermore, the variation in the binary fraction with stellar mass in OCs strongly agrees with the observed dependence for field stars with masses higher than ≳0.6 M⊙. Conclusions. Our work paves a path for future population-level investigations of multiplicity statistics and precision stellar masses in extended OC samples.

Context. Open clusters are among the most useful and widespread tracers of Galactic structure. The completeness of the Galactic open cluster census, however, remains poorly understood. Aims. For the first time ever, we aim to establish the selection function of an entire open cluster census, publishing our results as an open-source Python package for use by the community. Our work is valid for the Hunt & Reffert catalogue of clusters in Gaia DR3. Methods. We developed and open sourced our cluster simulator from our first work. Then, we performed 80 590 injection and retrievals of simulated open clusters to test the Hunt & Reffert catalogue’s sensitivity. We fitted a logistic model of cluster detectability that only depends on a cluster’s number of stars, median parallax error, Gaia data density, and a user-specified significance threshold. Results. We find that our simple model accurately predicts cluster detectability, with a 94.53% accuracy on our training data that is comparable to a machine-learning-based model with orders of magnitude more parameters. Our model itself offers numerous insights into why certain clusters are detected. We briefly used our model to show that cluster detectability depends on non-intuitive parameters, such as a cluster’s proper motion, and we show that even a modest 25 km/s boost to a cluster’s orbital speed can result in an almost 3× higher detection probability, depending on its position. In addition, we published our raw cluster injection and retrievals and cluster memberships, which could be used for a number of other science cases – such as estimating cluster-membership incompleteness. Conclusions. Using our results, selection effect-corrected studies are now possible with the open cluster census. Our work will enable a number of brand new types of study, such as detailed comparisons between the Milky Way’s cluster census and recent extragalactic cluster samples.

The high-precision Gaia Data Release 3 (DR3) enables the discovery of numerous open clusters (OCs) in the Milky Way, providing an excellent opportunity to search for blue straggler stars in OCs and investigate their formation and evolution in these environments. Using the member stars from literature OC catalogs, we visually inspected the color-magnitude diagram (CMD) of each cluster and selected cluster candidates that potentially host blue stragglers. We then reassessed cluster memberships using the pyUPMASK algorithm with Gaia DR3 and performed isochrone fitting to derive physical parameters for each cluster, including age, distance modulus, mean reddening, and metallicity. Finally, we empirically identified straggler stars based on their positions relative to the best-fitting isochrone, zero-age main sequence, and equal-mass binary sequence on the CMD. In total, we identified 272 new straggler stars in 99 OCs, comprising 153 blue stragglers, 98 probable blue stragglers, and 21 yellow stragglers. Compared to the reported blue straggler catalogs based on earlier Gaia data, our results increase the number of OCs with stragglers in the Milky Way by 22.2%, and the total number of blue stragglers by 11.2%.

Out of all the discovered open clusters (OCs) that are located in the inner part of the Galaxy, only a small fraction has been observed with high-resolution spectroscopy. An intriguing population of inner-disc OCs at relatively high altitudes (Z) from the Galactic plane remains poorly studied. There are few reliable detections of such OCs, and their occurrence rate, dynamical origin and survival mechanism remain uncertain. We perform a detailed spectroscopic analysis of UBC 1052, located at a cylindrical galactocentric radius R_GC = 6.1 kpc and Z = 340 pc, which stands out as the oldest and highest-|Z| inner-disc OC studied at high resolution to date. We used FLAMES/VLT to acquire high signal-to-noise ratio UVES spectra of four red clump (RC) members (G∼14 mag). From them we derived high-precision radial velocities (v_r) and local thermodynamic equilibrium chemical abundances for 23 elements through a strict line-by-line differential analysis, achieving a median precision in [X/H] of ≃0.06 dex for each star. The four RC stars have fully compatible chemical abundances, with [X/H] dispersions among them <0.03 dex for 20 elements. We also acquired GIRAFFE spectra of other candidate members and derived their v_r. We find that UBC 1052 has an age of 2.25±0.25 Gyr, a distance of 3.11±0.07 kpc, an extinction A_V =1.23 mag, a mean radial velocity of 34.0±0.6 km s^-1, and a slightly super-solar [Fe/H] = 0.05±0.01 dex. Such relatively low [Fe/H] at its R_GC suggests that UBC 1052 is a rare candidate for an inward-migrated OC in the inner disc. Its detailed abundance pattern (e.g. [Ba/Zr] and [Nd/Y]) shows some interesting features that appear to be unique in the current census of OCs studied at high resolution, making it an interesting object for potential strong chemical-tagging searches for already dispersed member stars. [Abridged]

Context. Unresolved binaries in star clusters can bias stellar and cluster mass estimates. A proper treatment is therefore essential for studying the cluster dynamics and evolution. Aims. We develop a fast and robust framework for jointly deriving stellar masses and multiplicity statistics of member stars, together with optimal cluster parameters. Methods. We used Gaia DR3 parallaxes together with multi-band photometry of open cluster (OC) members to infer stellar masses and binary mass ratios through simulation-based inference (SBI), while iteratively fitting the cluster parameters. The validation of our SBI framework on simulated clusters demonstrates that the inclusion of infrared photometry significantly improves the detection of low mass-ratio binaries. The minimum mass-ratio threshold for reliably identifying unresolved binaries depends on the cluster properties and the available photometry, but typically lies below q = 0.5. Results. Applying our method to 42 well-populated OCs, we derived a catalogue of stellar masses and mass ratios for 27 201 stars achieving typical uncertainties of 0.08 in q and 0.01 M⊙ in the primary stellar mass. We analysed the archetype OCs M67 and NGC 2360 in detail, including mass segregation and mass-ratio distribution, and derived multiplicity fractions for the rest of the sample. We find evidence that the high mass ratio (q ≥ 0.6) binary fraction is strongly correlated with the age and weakly anti-correlated with the cluster metallicity. Furthermore, the variation in the binary fraction with stellar mass in OCs strongly agrees with the observed dependence for field stars with masses higher than ≳0.6 M⊙. Conclusions. Our work paves a path for future population-level investigations of multiplicity statistics and precision stellar masses in extended OC samples.

Mapping the local and distant Universe is key to our understanding of it. For decades, the Sloan Digital Sky Survey (SDSS) has made a concerted effort to map millions of celestial objects to constrain the physical processes that govern our Universe. The most recent and fifth generation of SDSS (SDSS-V) is organized into three scientific "mappers": the Milky Way Mapper, which aims to chart the various components of the Milky Way and constrain its formation and assembly; the Black Hole Mapper, which focuses on understanding supermassive black holes in distant galaxies across the Universe; and the Local Volume Mapper, which uses integral field spectroscopy to map the ionized interstellar medium in the Local Group. This paper describes the scope and content for the nineteenth data release (DR19) of SDSS, which is the most substantial to date in SDSS-V. DR19 is the first to contain data from all three mappers. Additionally, we also describe nine value-added catalogs that enhance the science that can be conducted with the SDSS-V data. Finally, we discuss how to access SDSS DR19 and provide illustrative examples and tutorials.

Despite the fundamental importance of the star formation history (SFH) and the initial mass function (IMF) in the description of the Milky Way, their consistent and robust derivation is still elusive. Recent and accurate astrometry and photometry collected by the Gaia satellite provide the natural framework to consolidate these ingredients in our local Galactic environment. We aim to simultaneously infer the IMF and the SFH of the Galactic disc comparing Gaia data with the mock catalog resulting from the Besançon population synthesis model (BGM). Our goal is also to estimate the impact of the systematics present in current stellar evolutionary models on this inference. We use a new implementation of the BGM Fast Approximate Simulations (BGM FASt) framework to fit the seven million star Gaia DR3 all-sky G<13 color-magnitude diagram (CMD) to the most updated dynamically self-consistent BGM. Our derived SFH supports an abrupt decrease of the star formation approximately 1-1.5 Gyr ago followed by a significant enhancement with a wide plateau in the range 2-6 Gyr ago. A remarkable hiatus appears around 5-7 Gyr ago with a ∼1 Gyr shift depending on the set of stellar models. A complex and discrepant evolution at ages older than 8 Gyr deserves further investigation. Precise but discrepant values are found for the power-law indices of the IMF. For the range 0.5-1.53 M_⊙ the slope takes a value of α_2 = 1.45^+0.19_-0.12, while for masses larger than 1.53 M_⊙ we obtain α_3 = 1.98^+0.13_-0.05. The current implementation of the BGM FASt framework is ready to address executions fitting all-sky Gaia data up to 14-17 apparent limiting magnitude. This will naturally allow us to derive both a reliable SFH for the early epochs of the Galactic disc evolution and a precise slope for the IMF at low masses.

Context. As a well-known open cluster, NGC 2323 (also called M50) has been widely investigated for over a hundred years and has always been considered a classical single cluster. Aims. We studied the binary structure nature of NGC 2323 with Gaia Data Release 3 (DR3). Methods. We discovered the binary structure of NGC 2323 by the HDBSCAN algorithm based on the astrometric and photometric data from Gaia DR3. Then we employed the PETAR N-body code to investigate its binary structure origin. Results. We found that NGC 2323 consists of two distinct subclusters (NGC 2323-a and NGC 2323-b) that are still gravitationally bound. They have very close positions (three-dimensional Delta pos = 12.3 pc, sigma(Delta pos) = 3.4 pc) and similar tangential velocities (two-dimensional Delta V = 2.2 km s(-1), sigma(Delta V) = 0.02 km s(-1)). The best isochrone fitting ages of the two clusters are the same (158 Myr), further proving their possibly common origin. The numerical N-body simulation suggests that the less massive cluster is unlikely to be the cluster tidal tails created by the differential rotation of the Milky Way.

In this paper we aim to simulate realistic exoplanet populations across different regions of the MW by combining state-of-the-art cosmological simulations of our Galaxy with exoplanet formation models and observations. We model the exoplanet populations around single stars, using planet occurrence rates and multiplicity depending on stellar mass, metallicity, and planet type, and assign them physical parameters such as mass and orbital period. Focussing first on the solar vicinity, we find mostly metallicity-driven differences in the distributions of non-hosting and planet-hosting single stars. In our simulated solar neighbourhood, 52.5% of all planets are Earth-like (23% of them located in the Habitable Zone), 44% are super-Earths/Neptunes, and 3.5% are giant planets. A comparison with the census of Kepler exoplanets and candidates shows that, when taking into account the most relevant selection effects, we obtain a similar distribution of exoplanets compared to the observed population. However, we also detect significant differences in the exoplanet and host star distributions (e.g. more planets around F-type and red-giant stars compared to observations) that we attribute mostly to a too strong recent star formation and a too large disc scale height in the simulation, as well as to some caveats in our exoplanet population synthesis that will be addressed in future work. Extending our analysis to other regions of the simulated MW and to other simulated galaxies, we find that the relative percentages of planet types remain largely consistent as long as the simulated galaxy matches the morphology and mass of the MW. We have created a fast and flexible framework to produce exoplanet populations based on MW-like simulations that can easily be adapted to produce predictions for the yields of future exoplanet detection missions. (abridged)

Context . Exoplanet transit and radial-velocity surveys have allowed us to explore the exoplanet population in our Galactic surroundings. The planet populations in more remote areas of the Milky Way (MW) will become accessible with future instrumentation. Aims . In this paper, we aim to simulate realistic exoplanet populations across different regions of the MW by combining state-of-the-art cosmological simulations of our Galaxy with exoplanet formation models and observations. Methods . We model the exoplanet populations around simulated single stars, using planet occurrence rate and multiplicity depending on stellar mass, metallicity, and planet type, and assign them physical parameters such as mass and orbital period. Results . Focussing first on the solar vicinity, we find mostly metallicity-driven differences in the distributions of non-hosting and planet-hosting single stars. In our simulated solar neighbourhood, 52.5% of all planets are Earth-like (23% of them located in the Habitable Zone), 44% are super-Earths or Neptunes, and 3.5% are giant planets. A detailed comparison with the census of Kepler exoplanets and candidates shows that, when taking into account the most relevant selection effects, we obtain a similar distribution of exoplanets compared to the observed population. However, we also detect some significant differences in the exoplanet and host star distributions (e.g. more planets around F-type and red-giant stars compared to the observations) that we attribute mostly to a too strong recent star formation and a too large disc scale height in the simulation compared to the solar neighbourhood, as well as to some caveats in our exoplanet population synthesis that will be addressed in future work. Extending our analysis to other regions of the simulated MW and to other galaxies within the same suite of simulations, we find that the relative percentages of Earth-like, super-Earth or Neptunes, and giant planets remain largely consistent as long as the simulated galaxy matches the morphology and mass of the MW. Conclusions . We have created a fast and flexible framework to produce exoplanet populations based on MW-like simulations that can easily be adapted to produce predictions for the yields of future exoplanet detection missions.

Context. Open clusters have long been used as tracers of Galactic structure. However, without a selection function to describe the completeness of the cluster census, it is difficult to quantitatively interpret their distribution. Aims. We create a method to empirically determine the selection function of a Galactic cluster catalogue. We test it by investigating the completeness of the cluster census in the outer Milky Way, where old and young clusters exhibit different spatial distributions. Methods. We develop a method to generate realistic mock clusters as a function of their parameters, in addition to accounting for Gaia's selection function and astrometric errors. We then inject mock clusters into Gaia DR3 data, and attempt to recover them in a blind search using HDBSCAN. Results. We find that the main parameters influencing cluster detectability are mass, extinction, and distance. Age also plays an important role, making older clusters harder to detect due to their fainter luminosity function. High proper motions also improve detectability. After correcting for these selection effects, we find that old clusters are 2.97 +/- 0.11 times more common at a Galactocentric radius of 13 kpc than in the solar neighbourhood - despite positive detection biases in their favour, such as hotter orbits or a higher scale height. Conclusions. The larger fraction of older clusters in the outer Galaxy cannot be explained by an observational bias, and must be a physical property of the Milky Way: young outer-disc clusters are not forming in the outer Galaxy, or at least not with sufficient masses to be identified as clusters in Gaia DR3. We predict that in this region, more old clusters than young ones remain to be discovered. The current presence of old, massive outer-disc clusters could be explained by radial heating and migration, or alternatively by a lower cluster destruction rate in the anticentre.

The Milky Way serves as a template for understanding the formation and evolution of late-type massive disk galaxies since we can obtain detailed chemical and kinematic information for large samples of individual stars. However, the early formation of the disk and the dichotomy between the chemically thick and thin disks remain under intense debate. Some mechanisms have been proposed to explain the formation of this dichotomy, such as the injection of metal-poor gas by a gas-rich merger such as Gaia-Sausage Enceladus (GSE), or by cosmic gas filaments, radial migration, and the presence of star-forming clumps at high redshift ( z > 2). In this work, we combine astrometric data from the Gaia mission, chemical abundances from APOGEE and LAMOST spectroscopic surveys, and StarHorse ages to study the early stage of chemical dichotomy in the Milky Way disk. The Bayesian isochrone-fitting code StarHorse can estimate ages for thousands of stars in the solar neighborhood, being most reliable for main-sequence turnoff and subgiants, computing distances and extinction simultaneously. From these samples, we show that (i) there is an old thin disk population (>11 Gyr) that indicates a period of coformation between the thick and thin disks of the Milky Way before the GSE merger, i.e., the Galaxy itself could initiate the formation of a low- α disk without the need for a gas-rich merger, and (ii) this merger would have been important to stop the formation of stars in the thick disk.

Gravitational waves from black-hole merging events have revealed a population of extra-galactic BHs residing in short-period binaries with masses that are higher than expected based on most stellar evolution models - and also higher than known stellar-origin black holes in our Galaxy. It has been proposed that those high-mass BHs are the remnants of massive metal-poor stars. Gaia astrometry is expected to uncover many Galactic wide-binary systems containing dormant BHs, which may not have been detected before. The study of this population will provide new information on the BH-mass distribution in binaries and shed light on their formation mechanisms and progenitors. As part of the validation efforts in preparation for the fourth Gaia data release (DR4), we analysed the preliminary astrometric binary solutions, obtained by the Gaia Non-Single Star pipeline, to verify their significance and to minimise false-detection rates in high-mass-function orbital solutions. The astrometric binary solution of one source, Gaia BH3, implies the presence of a 32.70 \pm 0.82 M\odot BH in a binary system with a period of 11.6 yr. Gaia radial velocities independently validate the astrometric orbit. Broad-band photometric and spectroscopic data show that the visible component is an old, very metal-poor giant of the Galactic halo, at a distance of 590 pc. The BH in the Gaia BH3 system is more massive than any other Galactic stellar-origin BH known thus far. The low metallicity of the star companion supports the scenario that metal-poor massive stars are progenitors of the high-mass BHs detected by gravitational-wave telescopes. The Galactic orbit of the system and its metallicity indicate that it might belong to the Sequoia halo substructure. Alternatively, and more plausibly, it could belong to the ED-2 stream, which likely originated from a globular cluster that had been disrupted by the Milky Way.

Context. Open clusters provide valuable information on stellar nucleosynthesis and the chemical evolution of the Galactic disk, as their age and distances can be measured more precisely with photometry than in the case of field stars. Aims. Our aim is to study the chemical distribution of the Galactic disk using open clusters by analyzing the existence of gradients with Galactocentric distance, azimuth, or height from the plane and dependency with age. Methods. We used the high-resolution spectra ( R > 60 000) of 194 stars belonging to 36 open clusters to determine the atmospheric parameters and chemical abundances with two independent methods: equivalent widths and spectral synthesis. The sample was complemented with 63 clusters with high-resolution spectroscopy from literature. Results. We measured LTE abundances for 21 elements: α (Mg, Si, Ca, and Ti), odd-Z (Na and Al), Fe-peak (Fe, Sc, V, Cr, Mn, Co, Ni, Cu, and Zn), and neutron-capture (Sr, Y, Zr, Ba, Ce, and Nd). We also provide non-local thermodynamic equilibrium abundances for elements when corrections are available. We find inner disk young clusters enhanced in [Mg/Fe] and [Si/Fe] compared to other clusters of their age. For [Ba/Fe], we report an age trend flattening for older clusters (age < 2.5 Ga). The studied elements follow the expected radial gradients as a function of their nucleosynthesis groups, which are significantly steeper for the oldest systems. For the first time, we investigate the existence of an azimuthal gradient, finding some hints of its existence among the old clusters (age > Ga).

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 alpha-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 metal-poor 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.

We explore the kinematic and chemical properties of the Monoceros stellar overdensity by combining data from the Two Micron All Sky Survey, Wide-field Infrared Survey Explorer, APOGEE, and Gaia. Monoceros is a structure located toward the Galactic anticenter and close to the disk. We have identified that its stars have azimuthal velocity in the range of 200 < v _ϕ (km s ^−1 ) < 250. Combining their kinematics and spatial distribution, we designed a new method to select stars from this overdensity. This method allows us to easily identify the structure in both hemispheres and estimate their distances. Our analysis was supported by comparison with simulated data from the entire sky generated by the Galaxia code. Furthermore, we characterized, for the first time, the Monoceros overdensity in several chemical abundance spaces. Our results confirm its similarity to stars found in the thin disk of the Galaxy and suggest an in situ formation. Furthermore, we demonstrate that the southern and northern regions of Monoceros exhibit indistinguishable chemical compositions.

Context. Reliable stellar age estimates are fundamental for testing several problems in modern astrophysics, in particular since they set the timescales of Galactic dynamical and chemical evolution. Aims. In this study, we determine ages using only Gaia DR3 photometry and parallaxes, in combination with interstellar extinction maps, and spectroscopic metallicities and alpha abundances from the latest data release (DR8) of the LAMOST survey. In contrast with previous age estimates, we do not use spectroscopic effective temperatures or surface gravities, and thus we rely on the excellent precision and accuracy of the Gaia photometry. Methods. We use a new version of the publicly available SPInS code with improved features, including the on-the-fly computation of the autocorrelation time and the automatic convergence evaluation. Results. We determine reliable age estimates for 35 096 and 243 768 sub-giant and main-sequence turn-off stars in the LAMOST DR8 low- and medium-resolution surveys with typical uncertainties smaller than 10%. In addition, we successfully test our method on more than 4000 stars of 14 well-studied open and globular star clusters covering a wide range of ages, confirming the reliability of our age and uncertainty estimates.

The relative enrichment of s-process to $\alpha$-elements ([s/$\alpha$]) has been linked with age, providing a potentially useful avenue in exploring the Milky Way's chemical evolution. However, the age--[s/$\alpha$] relationship is non-universal, with dependencies on metallicity and current location in the Galaxy. In this work, we examine these chemical clock tracers across birth radii ($\rm \text{R}_\text{birth}$), recovering the inherent trends between the variables. We derive $\rm \text{R}_\text{birth}$ and explore the [s/$\alpha$]--age--$\rm \text{R}_\text{birth}$ relationship for 36,652 APOGEE DR17 red giant and 24,467 GALAH DR3 main sequence turnoff and subgiant branch disk stars using [Ce/Mg], [Ba/Mg], and [Y/Mg]. We discover that the age--[s/Mg] relation is strongly dependent on birth location in the Milky Way, with stars born in the inner disk having the weakest correlation. This is congruent with the Galaxy's initially weak, negative [s/Mg] radial gradient, which becomes positive and steep with time. We show that the non-universal relations of chemical clocks is caused by their fundamental trends with $\rm \text{R}_\text{birth}$ over time, and suggest that the tight age--[s/Mg] relation obtained with solar-like stars is due to similar $\rm \text{R}_\text{birth}$ for a given age. Our results are put into context with a Galactic chemical evolution model, where we demonstrate the need for data-driven nucleosynthetic yields.