arXiv subjects

L. Berni

Publications and source records attributed to L. Berni.

7 recordsLinked to original sources

arXiv2026

Thorium and Europium production in the early Galaxy: Implications for nucleocosmochronology

The ages of the oldest stellar populations can be investigated through nucleocosmochronology, with the thorium-to-europium abundance ratio providing a promising chronometer. We assess the reliability of log(Th/Eu) as an age indicator by testing whether thorium and europium were co-produced in a constant ratio in metal-poor stars, and evaluate the uncertainties affecting stellar ages. We present new Th and Eu abundance measurements for eight TITANS reference stars observed with UVES at the VLT. Combining these measurements with homogeneous literature samples, we investigate the behaviour of log(Th/Eu) as a function of metallicity. We derive ages adopting a theoretical initial r-process production ratio and assess the effects of observational uncertainties and sample statistics. At low metallicity, the observed Th/Eu ratios show no significant dependence on metallicity after excluding stars with anomalous actinide abundances, supporting broadly uniform Th and Eu co-production in the early Galaxy. The derived ages are consistent with expectations for an old, metal-poor halo population, although individual uncertainties remain substantial. Higher spectral resolution is needed to improve Th measurements, while combining stars in coeval populations can provide additional statistical gains. Nucleocosmochronology offers an independent approach to dating the oldest stellar populations. Homogeneous samples of coeval stars, particularly in globular clusters, observed with future high-resolution multi-object spectrographs could provide stronger constraints on the early formation history of the Milky Way and the age of the Universe.

astro-ph.SR↗

arXiv2026

Orbital Parameters for 65 Wide Binary Systems: no evidence for gravitational anomaly at low acceleration

We use a Monte Carlo analysis to study whether wide binaries can be represented by bound Newtonian orbits, and, if this is the case, whether the distribution of the orbital parameters agrees with expectations. The potential orbits are classified according to the $χ^2$ probability of representing star coordinates, parallaxes, velocity components and masses. We apply this analysis to 44 systems previously published, to which we add 26 wide binaries, consisting of twin low-mass (M) stars. We find orbital solutions for 65 systems, and derive the distribution of their orbital parameters. For one system composed of M-type stars, no Newtonian bound solution can be found; however, this pair is a member of the young Argus association and therefore not a suitable target. The remaining four systems that cannot be fit were already identified as likely disrupted binaries. The uncertainty in the separation between the components, given by the parallax error, is by far the dominant error source, and the main limitation of the present analysis, affecting especially the determination of the orbits' inclination angle and phase. Newtonian dynamics describe well our sample. When dividing the sample into two subsamples, according to the mutual acceleration, the eccentricity and periastron distributions agree with theoretical expectations; inclination and phase are distorted by the parallax-limited separation, more strongly in the high-acceleration (37 systems) than in the low-acceleration ($A_N < 10^{-9} m s^{-2}$, 28 systems) subsample, which is opposite to what a gravitational signal would produce. In particular the ratio between observed velocity difference and circular velocity does not show any excess for the low acceleration sample. When parametrized as a boost $γ$ parameter to the velocity, $0.95 < γ<1.05 (1 σ)$, fully consistent with Newton also for the low acceleration sample.

astro-ph.SR↗

arXiv2026

Wide binaries without viable bound Newtonian orbits

Context. Wide binaries offer a unique opportunity to test gravity in the low acceleration regime, where deviations from Newtonian dynamics may appear. Aims. We use high-resolution VLT-ESPRESSO archival spectra to study 26 wide binaries with projected separations larger than 13,000 AU. By combining precise radial velocities with Gaia proper motions and parallaxes, we aim to test whether these systems are consistent with Newtonian gravity in the low acceleration regime. Methods. We use multiple radial velocity measurements and stellar parameters to remove systems affected by unresolved triples, chance alignments, or young systems. For the remaining binaries, we combine radial velocities (corrected for convective shift and gravitational redshift) with Gaia proper motions, parallaxes, and positions to attempt bound Newtonian orbital solutions Results. Fourteen of the 26 initial systems were discarded: 12 due to radial velocity variability indicating unresolved close binaries, one hosting a faint Gaia companion, and one too young. Of the remaining 12, nine can be fitted with bound orbital solution, while three show velocity differences too large to be reconciled with any bound Newtonian orbit. Conclusions. For the three systems that cannot be fitted with a bound orbit, repeated radial velocity observations allow us to confidently exclude, with one possible exception, unresolved triple stellar companions or massive close-in planets as causes. Given their likely large 3-dimensional separations, these binaries may have been dynamically perturbed or disrupted by stellar encounters or Galactic tides, and may no longer be gravitationally bound. This highlights how utmost caution must be applied when studying wide binaries as isolated systems.

astro-ph.SR↗

arXiv2026

Evidence for multiple crossings and stripping of Gaia-Enceladus/Sausage across the Milky Way

The accretion of Gaia-Enceladus/Sausage (GES) onto the Milky Way (MW) is one of the most prominent features of the Galactic halo revealed by the combination of the Gaia satellite and large spectroscopic surveys. This massive accretion largely contributes to the local stellar halo mass and was significant enough to alter the formation history and the morphology of the MW. In this work, we aim to analyse the selection of stars previously identified as belonging to GES with different kinematics and chemical properties to test the hypothesis of a two-phase accretion event. We apply several statistical tests to assess the significance of the separation between the two populations in GES. We then employ galactic chemical evolution models to investigate the origin of the chemical differences encountered in the analysis. We confirm the presence of two distinct populations, with consistently different dynamical and chemical properties. The low energy population seems to show higher overall abundances, whereas the high-energy one may be more metal-poor. We attribute this difference to the presence of at least two separate populations of stars within Gaia-Enceladus, likely associated with the innermost (low-energy) and outermost (high-energy) regions of the progenitor. The adopted models successfully reproduce the patterns in metallicity and [alpha/M] distributions in an inside-out scenario. Our analysis supports the presence of a former metallicity gradient in Gaia-Enceladus, and reinforces the interpretation of its accretion as a multi-passage event through the Milky Way disc.

astro-ph.GA↗

arXiv2025

Tracing the Early Milky Way with Globular Clusters: The Diagnostic Power of Neutron-Capture Elements

Globular clusters (GCs) are fundamental tracers of the early assembly of the Milky Way (MW). They formed in diverse environments -- including both our Galaxy and dwarf galaxies -- retaining chemical and dynamical signatures that encode their origins and the merger history of the Galaxy. Although significant progress has been made in characterising GC chemistry, most studies have focused on light, $α$-, and iron-peak elements. In contrast, neutron-capture (n-capture) elements remain sparsely investigated across the GC system, despite their unique ability to trace nucleosynthetic channels and star-formation timescales. A homogeneous and statistically robust mapping of n-process elements in a large sample of GCs would provide powerful constraints on their formation environments, chemical signatures of in situ and accreted systems, and refine our understanding of the early chemical evolution of the MW halo. Addressing this gap requires high-resolution, multiplexing, and blue-sensitive spectroscopy capable of accessing the full suite of n-capture diagnostics in several tens of stars per GC.

astro-ph.IM↗

arXiv2025

Exploring substructures in the Milky Way halo Neural networks applied to Gaia and APOGEE DR 17

The identification of stellar structures in the Galactic halo, including stellar streams and merger remnants, often relies on the dynamics of their constituent stars. However, this approach has limitations due to the complex dynamical interactions between these structures and their environment. Perturbations such as tidal forces exerted by the Milky Way, the potential escape of stars, and passages through the Galactic plane can result in the loss of dynamical coherence of stars in these structures. Consequently, relying solely on dynamics may be insufficient for detecting such disrupted or dispersed remnants. We combine chemistry and dynamics, integrated through a system of neural networks, to develop a clustering method for identifying accreted structures in the Galactic halo. We developed an integrated approach combining Siamese neural networks (SNNs), graph neural networks (GNNs), autoencoders, and the OPTICS algorithm to create a comprehensive procedure named CREEK. This method is designed to uncover stellar structures in the Galactic halo. Initially, CREEK was trained on known globular clusters (GCs) and then applied to the dataset to identify stellar streams. CREEK successfully recovered 80% of the GCs present in the APOGEE dataset, re-identified several known stellar streams, and identified a potential new stream. Additionally, within highly populated stellar structures, CREEK can identify substructures that exhibit distinct chemical compositions and orbital energies. This approach provides an objective data-driven method for selecting stars associated with streams and stellar structures in general.

astro-ph.GA↗

arXiv2025

Neutron-Capture Element Signatures in Globular Clusters: Insights from the Gaia-ESO Survey

Globular clusters (GCs) are key to understanding the formation and evolution of our Galaxy. While the abundances of light and Fe-peak elements in GCs have been widely studied, investigations into heavier, neutron-capture elements -- and their connection to multiple stellar populations and GC origins -- remain limited. In this work, we analysed the chemical abundances of neutron-capture elements in GCs to trace the Galactic halo and to explore possible links to the MP phenomenon. Our goal is to better constrain the nature of the polluters responsible for intracluster enrichment and to distinguish the origin of GCs through the chemical signature of neutron-capture elements. We examined 14 GCs from the Gaia-ESO Survey, spanning a wide metallicity range, [Fe/H] from -0.40 to -2.32, using a homogeneous methodology. We focused on the abundances of Y, Zr, Ba, La, Ce, Nd, Pr, and Eu, derived from FLAMES-UVES spectra. These were compared with predictions from a stochastic Galactic chemical evolution model. With the exception of Zr, the model broadly reproduces the observed trends in neutron-capture elements. In some GCs, we found strong correlations between hot H-burning products (Na, Al) and s-process elements, pointing to a shared nucleosynthesis site, e.g., asymptotic giant branch stars of different masses and/or fast-rotating massive stars. We also detect a distinct difference in [Eu/Mg] ratio between in-situ ($\langle$[Eu/Mg]$\rangle$ = 0.14 dex) and ex-situ ($\langle$[Eu/Mg]$\langle$ = 0.32 dex) GCs, highlighting their different enrichment histories. Finally, on average, Type II GCs (NGC 362, NGC 1261, and NGC 1851) showed a s-process element spread ratio between second- and first-generations about twice as large as those seen in Type I clusters.

astro-ph.GA↗