arXiv subjects

M. Racca

Publications and source records attributed to M. Racca.

3 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

3D NLTE Sodium abundances in late-type stars. Abundance corrections and synthetic spectra

Neutral sodium is an important tracer of the Galactic chemical evolution, a powerful diagnostic of different stellar populations, and the subject of detailed studies of exoplanet atmospheres via transmission spectroscopy. This work aims to study and quantify the errors in stellar analyses of Na I lines caused by the use of one-dimensional (1D) hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE). We studied the line formation of nine Na I lines in FGK dwarfs and giants via, for the first time, 3D non-LTE (NLTE) radiative transfer post-processing with the code Balder on 3D radiation hydrodynamic stellar atmospheres from the Stagger grid spanning Teff= 4000 to 6500 K, log g = 1.5 to 5.0, and [Fe/H]=-4 to +0.5. We find that the 3D NLTE abundance corrections relative to 1D LTE tend to be negative, and more positive than the corresponding 1D NLTE corrections. This reflects more efficient overionisation in the steeper temperature gradient of the 3D models. The corrections are typically less severe than -0.1 dex for weak lines, but become much larger for saturated lines in low-gravity giants (log g < 2.0), even reaching -0.7 dex. However, for the D resonance lines, the 3D NLTE corrections relative to 1D LTE become slightly positive at the lowest metallicities in our grid, typically around +0.05 dex at [Fe/H]=-4. We make our 3D NLTE grid, together with interpolation routines based on radial basis functions and fully connected feedforward neural networks, publicly available. This will enable more accurate determination of sodium abundances in present and forthcoming stellar spectroscopic surveys, particularly for metal-poor stars, as well as a better characterisation of the Na I D lines in exoplanet atmospheres.

astro-ph.SR↗

arXiv2025

Revisiting inelastic Cu+H collisions and the non-LTE Galactic evolution of copper

The Galactic evolution of copper remains poorly understood, partly due to the strong departures from local thermodynamic equilibrium (LTE) affecting Cu I lines. A key source of uncertainty in non-LTE modelling is the treatment of inelastic Cu+H collisions. We present new rate coefficients based on a combined asymptotic LCAO and free electron model approach, which show significant differences from previous calculations. Applying these updated rates to non-LTE stellar modelling, we find reduced line-to-line scatter and improved consistency between metal-poor dwarfs and giants. Our non-LTE analysis reveals a strong upturn in the [Cu/Fe] trend towards lower [Fe/H] < -1.7. We show that this may reflect the interplay between external enrichment of Cu-rich material of the Milky Way halo at low metallicities, and metallicity-dependent Cu yields from rapidly rotating massive stars. This highlights the unique diagnostic potential of accurate Cu abundances for understanding both stellar and Galactic evolution.

astro-ph.SR↗