The Milky Way bulge bar is composed of multiple populations. Using chemical and kinematical planes, we segregate six populations in a bulge bar sample observed by the APOGEE survey: two with bar-driven orbits, two with eccentric orbits, and two with low-eccentricity orbits, each composed of low- and high-[Mg/Fe] stars. Our sample spans −2.0 ≲ [Fe/H] ≲ +0.5 and Galactocentric distance R Gal < 6 kpc. We use chemical abundances from APOGEE DR17 for the elements Mg, Si, Ca, Al, K, Mn, Co, Ni, and Fe and from the BAWLAS catalog for Ce and Nd. We find that the low- and high-[Mg/Fe] stars with low-eccentricity orbits, which exhibit chemical and orbital characteristics similar to those of the low- and high-[α/Fe] disks, display slightly negative and positive metallicity gradients, respectively. This result for the low-[Mg/Fe], low-eccentricity stars indicates a break in the global thin-disk metallicity gradient. The high-eccentricity populations with both low and high [Mg/Fe] show approximately flat metallicity gradients. In general, the [X/H] gradients of all elements for all populations follow Fe, except for the neutron-capture elements Ce and Nd. For all elements, the high-[Mg/Fe] bar population shows a much steeper positive [X/H] gradient than the nearly flat gradient for the low-[Mg/Fe] bar stars. The positive [X/H] gradients observed among our high-[Mg/Fe] bar stars probably reflect an age variation along the peanut structure. This interpretation agrees with the N-body simulations. Such steep positive gradients have also been reported in some high-redshift (z ∼ 4–10) galaxies.

Classical Cepheids (CCs) have long been considered excellent tracers of the chemical evolution of the Milky Way’s young disk. We present a homogeneous, non-local thermodynamical equilibrium (NLTE) spectroscopic analysis of 401 Galactic CCs, based on 1351 high-resolution optical spectra, spanning Galactocentric distances from 4.6 to 29.3 kpc. Using PySME with MARCS atmospheres and state-of-the-art grids of NLTE departure coefficients, we derived the atmospheric parameters and abundances for key species tracing multiple nucleosynthetic channels (O, Na, Mg, Al, Si, S, Ca, Ti, Mn, Fe, and Cu). Our sample is the largest CC NLTE dataset to date and it achieves high internal precision, enabling the robust modelling of present-day thin-disk abundance patterns and radial gradients. We estimate abundance gradients using three analytic prescriptions (linear, logarithmic, bilinear with a break) within a Bayesian, outlier-robust framework. We also applied Gaussian process (GP) regression to capture non-parametric variations. We find that NLTE atmospheric parameters differ systematically from LTE determinations. Moreover, iron and most elemental abundance profiles are better described by non-linear behaviour rather than by single-slope linear models: logarithmic fits generally outperform simple linear models, while bilinear fits yield inconsistent break radii across elements. GP models reveal a consistent outer-disk flattening of [X/H] for nearly all studied elements. The [X/Fe] ratios are largely flat with Galactocentric radius, indicating coherent chemical scaling with iron across the thin disk, with modest positive offsets for Na and Al and mild declines for Mn and Cu. Finally, Cepheid kinematics confirm thin-disk orbits for the great majority of the sample. Comparisons with recent literature shows an overall agreement, while also highlighting NLTE-driven differences, especially in outer-disk abundances. These results provide tighter empirical constraints for chemo-dynamical models of the Milky Way and set the stage for future NLTE mapping with upcoming large spectroscopic surveys.

The evolutionary history of the Milky Way disk is imprinted in the ages, positions, and chemical compositions of individual stars. In this study, we derive the intrinsic density distribution of different stellar populations using the final data release of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. A total of 203,197 red giant branch stars are used to sort the stellar disk (R <= 20 kpc) into subpopulations of metallicity (Delta[M/H] = 0.1 dex), age ( Delta log(ageyr)=0.1 ), and alpha-element abundances ([alpha/M]). We fit the present-day structural parameters and density distribution of each stellar subpopulation after correcting for the survey selection function. The low-alpha disk is characterized by longer scale lengths and shorter scale heights, and is best fit by a broken exponential radial profile for each population. The high-alpha disk is characterized by shorter scale lengths and larger scale heights, and is generally well-approximated by a single exponential radial profile. These results are applied to produce new estimates of the integrated properties of the Milky Way from early times to the present day. We measure the total stellar mass of the disk to be 5.27-1.5+0.2x1010 M circle dot, and the average mass-weighted scale length is Rd = 2.37 +/- 0.2 kpc. The Milky Way's present-day color of (g - r) = 0.72 +/- 0.02 is consistent with the classification of a red spiral galaxy, although it has only been in the "green valley" region of the galaxy color-mass diagram for the last similar to 3 Gyr.

In the third APOKASC catalog, we present data for the complete sample of 15,808 evolved stars with APOGEE spectroscopic parameters and Kepler asteroseismology. We used 10 independent asteroseismic analysis techniques and anchor our system on fundamental radii derived from Gaia L and spectroscopic Teff. We provide evolutionary state, asteroseismic surface gravity, mass, radius, age, and the data used to derive them for 12,418 stars. This includes 10,036 exceptionally precise measurements, with median fractional uncertainties in nu max , Delta nu, mass, radius, and age of 0.6%, 0.6%, 3.8%, 1.8%, and 11.1%, respectively. We provide more limited data for 1624 additional stars that either have lower-quality data or are outside of our primary calibration domain. Using lower red giant branch (RGB) stars, we find a median age for the chemical thick disk of 9.14 +/- 0.05(ran) +/- 0.9(sys) Gyr with an age dispersion of 1.1 Gyr, consistent with our error model. We calibrate our red clump (RC) mass loss to derive an age consistent with the lower RGB and provide asymptotic GB and RGB ages for luminous stars. We also find a sharp upper-age boundary in the chemical thin disk. We find that scaling relations are precise and accurate on the lower RGB and RC, but they become more model dependent for more luminous giants and break down at the tip of the RGB. We recommend the use of multiple methods, calibration to a fundamental scale, and the use of stellar models to interpret frequency spacings.

The Galactic center and inner disk of the Milky Way contain complex stellar populations obscured by heavy dust extinction. To study their chemical composition, high-resolution near-infrared (near-IR) spectroscopy is necessary. Expanding the set of elements measurable in the near-IR, especially neutron-capture elements, improves our ability to trace nucleosynthesis and Galactic chemical evolution. This work aims to identify and characterize a spectral line suitable for determining rubidium (Rb) abundances. Rubidium is produced in roughly equal parts by the r- and s-processes. We analyze high-resolution (R = 45,000) Immersion GRating INfrared Spectrograph (or IGRINS) near-IR spectra of 40 M giants in the solar neighborhood, most observed with Gemini South. We perform spectral synthesis of the Rb i line at 15289.48 Å, using new log g f values and including an astrophysical calibration of the blending Fe i lines. The resulting [Rb/Fe] ratios are compared to other neutron-capture elements and interpreted with chemical evolution models. We demonstrate that the used Rb line is a reliable abundance indicator in M giants and the coolest K giants, but becomes too weak at higher temperatures. [Rb/Fe] shows a decreasing trend with metallicity, mirroring that of ytterbium (Yb), another mixed r-/s-process element. Our results agree with optical studies, validating the use of this near-IR line. Comparisons with chemical evolution models confirm that both s- and r-process sources are needed to explain the Rb trend. This work adds Rb to the list of elements measurable in high-resolution H- and K-band spectra, enabling studies of one more neutron-capture element in dust-obscured regions like the Galactic center and inner disk.

Context: Open clusters have been extensively used as tracers of Galactic chemical evolution, as their constituent stars possess shared characteristics, including age, Galactocentric radius, metallicity, and chemical composition. By examining the trends of elemental abundances with metallicity, age, and Galactocentric radius, valuable insights can be gained into the distribution and nucleosynthetic origins of chemical elements across the Galactic disk. The infrared domain in particular facilitates the observation of some elemental abundances that can be challenging or impossible to discern in the optical; for example, K and F.

Aims: The objective of this study is to derive the stellar parameters and elemental abundances of up to 23 elements in 114 stars spanning 41 open clusters using high-resolution infrared spectroscopy. In addition, the present study aims to examine the chemical evolution of the Galactic disk. This is achieved by investigating radial abundance gradients, variations in abundance between clusters, and the dependence of chemical abundances on the cluster age.

Methods: The spectra utilized in this study were obtained with the high-resolution near-infrared GIANO-B spectrograph at the Telescopio Nazionale Galileo. The derivation of stellar parameters and chemical abundances was achieved by employing the Python version of Spectroscopy Made Easy. In the H -band region, a combination of atomic and molecular features was utilized to constrain the stellar parameters, including OH, CN, and CO molecular lines, and Mg I, Si I, Ti I, Ti II, C I, and Fe I atomic lines.

Results: Abundances for up to 23 elements, C, N, F, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ce, Nd, and Yb, were derived and compared with available literature values where possible. Non-local thermodynamic equilibrium analysis was utilized for the elements C, Na, Mg, Al, Si, S, K, Ca, Ti, Mn, Fe, and Cu. For each element, Galactic trends were examined by analyzing both [X/Fe] and [X/H] as functions of [Fe/H], stellar age, and Galactocentric radius. In particular, the radial abundance gradient of Ytterbium is presented for the first time, thereby extending the observational constraints on heavy neutron-capture elements.

Conclusions: Radial abundance gradients for a wide range of elements in the Galactic disk are found, with [X/Fe] slopes ranging from −0.061 to +0.065 dex/kpc. The observed gradients are consistent with an inside-out formation scenario for the Galactic disk, wherein chemical enrichment proceeds from the inner regions to the outer ones over time. The observed [X/Fe] trends across multiple nucleosynthetic groups, including α elements, odd-Z elements, iron-peak elements, and neutron-capture elements such as Y, Ce, Nd, and Yb, reflect the diverse production sites and timescales associated with each group. In particular, the positive [Zn/H] and [Zn/Fe] gradients suggest a distinctive nucleosynthetic origin for Zn, possibly linked to metallicity-dependent yields. The positive gradient in [Yb/Fe] (0.065 ± 0.031 dex/kpc) provides significant new constraints on neutron-capture enrichment processes and the chemical evolution of the Galactic disk.

We performed a detailed spectroscopic analysis of three extremely metal-poor RR Lyrae stars, exploring uncharted territories at these low metallicities for this class of stars. Using high-resolution spectra acquired with HARPS-N at TNG, UVES at VLT, and PEPSI at LBT, and employing Non-Local Thermodynamic Equilibrium (NLTE) spectral synthesis calculations, we provide abundance measurements for Fe, Al, Mg, Ca, Ti, Mn, and Sr. Our findings indicate that the stars have metallicities of [Fe/H] = - 3.40 ± 0.05, - 3.28 ± 0.02, and - 2.77 ± 0.05 for HD 331986, DO Hya, and BPS CS 30317-056, respectively. Additionally, we derived their kinematic and dynamical properties to gain insights into their origins. Interestingly, the kinematics of one star (HD 331986) is consistent with the Galactic disc, while the others exhibit Galactic halo kinematics, albeit with distinct chemical signatures. We compared the [Al/Fe] and [Mg/Mn] ratios of the current targets with recent literature estimates to determine whether these stars were either accreted or formed in situ, finding that the adopted chemical diagnostics are ineffective at low metallicities ([Fe/H]≲- 1.5). Finally, the established horizontal branch evolutionary models, indicating that these stars arrive at hotter temperatures on the Zero-Age Horizontal Branch (ZAHB) and then transition into RR Lyrae stars as they evolve, fully support the existence of such low-metallicity RR Lyrae stars. As a consequence, we can anticipate detecting more of them when larger samples of spectra become available from upcoming extensive observational campaigns.

This study probes the chemical abundances of the neutron-capture elements cerium and neodymium in the inner Milky Way from an analysis of a sample of similar to 2000 stars in the Galactic bulge bar spatially contained within divided by X-Gal divided by < 5 kpc, divided by Y-Gal divided by < 3.5 kpc, and divided by Z(Gal)divided by < 1 kpc, and spanning metallicities between -2.0 less than or similar to [Fe/H] less than or similar to +0.5. We classify the sample stars into low- or high-[Mg/Fe] populations and find that, in general, values of [Ce/Fe] and [Nd/Fe] increase as the metallicity decreases for the low- and high-[Mg/Fe] populations. Ce abundances show a more complex variation across the metallicity range of our bulge-bar sample when compared to Nd, with the r-process dominating the production of neutron-capture elements in the high-[Mg/Fe] population ([Ce/Nd] < 0.0). We find a spatial chemical dependence of Ce and Nd abundances for our sample of bulge-bar stars, with low- and high-[Mg/Fe] populations displaying a distinct abundance distribution. In the region close to the center of the MW, the low-[Mg/Fe] population is dominated by stars with low [Ce/Fe], [Ce/Mg], [Nd/Mg], [Nd/Fe], and [Ce/Nd] ratios. The low [Ce/Nd] ratio indicates a significant contribution in this central region from r-process yields for the low-[Mg/Fe] population. The chemical pattern of the most metal-poor stars in our sample suggests an early chemical enrichment of the bulge dominated by yields from core-collapse supernovae and r-process astrophysical sites, such as magnetorotational supernovae.

Context. Carbon abundances in first-ascent giant stars are usually lower than those of their main-sequence counterparts. At moderate metallicities, stellar evolution of single stars cannot account for the existence of red-giant branch stars with enhanced carbon abundances. The phenomenon is usually interpreted as resulting from past mass transfer from an evolved binary companion now in the white dwarf evolutionary stage.

Aims. We aim to confirm the links between [C/O] enhancement, s-process element enhancement and binary fraction using large-scale catalogues of stellar abundances and probable binary stars.

Methods. We use a large data set from the 17^th data release of the SDSS-IV/APOGEE 2 survey to identify carbon-enhanced stars in the Galactic disk. We identify a continuum of carbon enrichment throughout three different sub-populations of disk stars and explore links between the degree of carbon enrichment and binary frequency, metallicity and chemical compositions.

Results. We verify a clear correlation between binary frequency and enhancement in the abundances of both carbon and cerium, lending support to the scenario whereby carbon-enhanced stars are the result of mass transfer by an evolved binary companion. In addition, we identify clustering in the carbon abundances of high-α disk stars, suggesting that those on the high metallicity end are likely younger, in agreement with theoretical predictions for the presence of a starburst population following the gas-rich merger of the Gaia-Enceladus/Sausage system.

The relative enrichment of s-process to α-elements ([s/α]) has been linked with age, providing a potentially useful avenue in exploring the Milky Way’s chemical evolution. However, the age–[s/α] 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 (⁠𝑅birth⁠), recovering the inherent trends between the variables. We derive 𝑅birth and explore the [s/α]–age–𝑅birth relationship for 36 652 APOGEE DR17 red giant and 24 467 GALAH DR3 main-sequence turn-off and subgiant branch disc 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 disc 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 𝑅birth over time, and suggest that the tight age–[𝑠/Mg] relation obtained with solar-like stars is due to similar 𝑅birth 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.