IAC Preprints

Gaia FGK benchmark stars Abundances of n-capture elements of the third version

2026-10-29

In the current era, in which an unprecedented wealth of data are available for the study of the Milky Way, Gaia benchmark stars (GBSs) have become an established reference and calibration sample. Studies of stellar structure and evolution and of the chemical history of our Galaxy generally rely on large spectroscopic surveys and their output catalogs. In this context, deriving precise and accurate stellar parameters and chemical abundances is of paramount importance. This study provides the determination of neutron-capture element abundances and extends the set of chemical abundances available for the third GBS release (GBSv3). Based on the compilation of high-resolution spectra assembled for GBSv3 and consistent with the spectral analysis adopted for the chemical abundances of GBSv3, we used the public iSpec code to derive heavy element abundances. We inferred homogeneous abundances of neutron-capture elements (Y, Zr, Mo, Ba, La, Ce, Pr, Nd, and Eu) across the GBSv3 sample using an in-depth line assessment tailored to different groups identified through a clustering algorithm that accounts for the diversity in stellar parameters and metallicities. This approach addresses key challenges in the spectral analysis of these elements, including the paucity of usable lines, weak line strengths, saturation effects, and sensitivity to atomic data. The assessment yielded reliable measurements, establishing an extended and robust reference scale in good agreement with the literature. This compilation of neutron-capture abundances is based on the GBS sample’s robust and accurate atmospheric parameters and the analysis of a large sample of stellar spectra per star, which provides a reliable and homogeneous spectral analysis. It also supports the use of chemical abundances as precise tracers of the Milky Way’s star formation history and chemical evolution and constitutes a legacy sample for the calibration of current and future spectroscopic surveys.

Stellar nucleosynthesis in the era of large surveys: S-process-polluted binaries in GALAH DR4

2026-10-29

Context. Binary interactions during the asymptotic giant branch phase can lead to the formation of chemically peculiar stars such as Ba-, CH-, and CEMP-s stars with overabundances of s-process elements as well as carbon. Only a few hundred of these stars have been subject to detailed chemical and/or dynamical studies, typically in non-homogeneous samples. Aims. We compile a systematic sample of s-process-polluted candidates using the fourth data release of the large spectroscopic survey GALAH. We also compare the chemical and orbital properties of these candidates with those of confirmed s-process-polluted stars to gain stronger evidence of their nature. Methods. GALAH DR4 uses neural networks and automatic spectral analysis methods as well as data of a lower spectral resolution than is normally used to characterise these objects. We therefore built a validation sample of s-rich stars before querying the survey, for which we obtained VLT/UVES and Mercator/HERMES high-resolution (>80 000) spectra and analysed them with traditional methods. We compared our stellar parameters and individual abundances with those of the survey and used this validation to define the thresholds that a star in GALAH DR4 must pass to be flagged as a good s-process-rich candidate. Based on our comparisons with the high-resolution complementary spectra, we defined thresholds on five quantities: [s/Fe] > 0.40 dex, [Y/Fe] > 0.22 dex, [Zr/Fe] > 0.26 dex, [Ba/Fe] > 0.19 dex, and [La/Fe] > 0.26 dex. Results. We identified 1059 stars in GALAH DR4 that are good candidates to be s-process-polluted stars, covering a broad parameter space (4000 K < Teff < 6000 K, 0.5 dex < log g < 5.0 dex, −2.0 dex < [Fe/H] < 0.5 dex). They share many similarities with the samples of confirmed s-rich stars, especially their ratios of heavy over light s-elements, which strengthens our confidence in the purity of the sample. When we cross-matched the sample with the literature, we found that only 7% of the candidates have measured orbital periods and eccentricities. This limits a full comparison with confirmed Ba and related star samples for now. However, their binary fraction is, as expected, higher than the one we found for the full GALAH DR4 catalogue. Conclusions. Our sample of candidates is almost five times larger than the number of currently confirmed polluted stars. This and the fact that it has been homogeneously treated by the GALAH survey opens very interesting avenues to compare nucleosynthesis models as well as binary evolution models.

Ionized Gas Kinematics and Star-forming Clumps in Extremely Metal-poor Galaxies

2026-10-22

Extremely Metal Poor (XMP) galaxies are the least chemically evolved objects in the local Universe and provide important constraints on galaxy formation in low-metallicity environments. This work investigates the ionised gas kinematics and star formation properties of XMP galaxies to identify disturbed or multi-component gas motions and explore their connection with star formation. We analyse long-slit spectroscopic observations of nine XMP galaxies obtained with the Intermediate Dispersion Spectrograph on the 2.5m Isaac Newton Telescope. The study uses the H$\alpha$ emission line to trace ionised gas and emission-line ratios to derive gas-phase metallicities. The kinematic analysis reveals that some galaxies exhibit ordered rotation, with kinematic centres not always coincident with the luminous star-forming heads. Most systems show disturbed, asymmetric, or declining velocity profiles, while localized distortions near bright star-forming heads suggest that massive clumps influence the surrounding gas dynamics. We derive dynamical masses for the dominant clumps and investigate the mass--metallicity and size--metallicity relations. The clumps exhibit low metallicities consistent with the XMP regime and comparable to those observed in low-mass dwarf galaxies. Three galaxies show counter-rotating or kinematically decoupled ionised-gas components, indicating complex gas kinematics and possible external gas acquisition. Our sample is predominantly composed of isolated systems, with metal-poor star formation concentrated in compact head regions. These properties may reflect gas accretion through mergers, interactions, or the acquisition of metal-poor gas from the intergalactic medium and cosmic filaments. Internal processes such as outflows, gas mixing, recycling, and stochastic star formation may also contribute to the observed properties.

An improvement to the spatial discretisation in the Astrophysical Weighted Particle Hydrodynamics method

2026-10-21

The discretisation of space in the Astrophysical Weighted Particle Hydrodynamics method is attained through a partition-of-unity-based cubature rule. The node (particle) weights in such rules must in general be obtained using approximations, for which only one has been thus far proposed in the literature. In this work, new approximations for these weights are proposed, their properties are studied, and their impact on the accuracy of the cubature rule and the overall method is assessed. Approximations for node centroids are also proposed and studied, and their possible applications are laid out. Additionally, for schemes in which the kernel-function compact-support size is set to be constant, rather than adaptive, weight and centroid approximations are provided as well. Finally, for the cubature rule arising from the weight approximation previously proposed in the literature, an asymptotic expression for its mean relative error is derived, and its usefulness for understanding the finite-sample properties of the cubature rule is showcased with some simple examples.

First L-type substellar object discovered in the vicinity of the embedded cluster NGC 2023

2026-10-15

We present the first substellar object in a star-forming region spectroscopically confirmed with Euclid, namely J054158.70-021031.4. This young and faint source is located close to the embedded open cluster NGC 2023. It has a spectral type of L3, a reddening of Av = 3 mag, proper motion consistent with the nearby cluster NGC 2024, and a magnitude of ∼24.5 in the Euclid optical band. At the distance of 403 pc, its inferred mass lies close to the planetary-mass boundary. We discovered the object by searching for ultracool dwarf candidates in the Euclid Early Release Observations. The search was based on photometric criteria, followed by the analysis of slitless 2D spectro-images in order to distinguish features that indicate the nature of these objects. This work demonstrates the potential of Euclid to unveil previously hidden populations of very low-mass objects, located in obscured regions where star formation is ongoing.