Marta Molero

Context . The astrophysical site(s) of the rapid neutron-capture process ( r -process) remains uncertain, with competing scenarios such as neutron star mergers and magneto-rotational supernovae offering different predictions for the timing, frequency, and yield for heavy elements pollution. Galactic chemical evolution models provide an important tool to constrain these properties by comparing model predictions to the observed abundance of neutron-capture elements across different metallicities. Aims . Our aim is to explore, in a systematic and data-driven manner, the range of astrophysical conditions under which the r -process enrichment can reproduce the observed trends of multiple neutron-capture elements in the Milky Way. Rather than assuming a fixed physical site, we adopted a flexible parametric approach to assess whether a common set of r -process parameters can simultaneously explain the chemical evolution of several heavy elements of interest. Methods . We constructed a grid of one-infall homogeneous Galactic chemical evolution models varying four key parameters: the Eu r -process yield per event, the rate of r -process-producing events, the delay time before enrichment, and the progenitor star mass range. For each of the ∼1.5 × 10 ⁵ models, we computed the predicted [X/Fe] versus [Fe/H] trends for several neutron-capture elements, obtained by scaling Eu yields with the solar r -process pattern, and we evaluated the model performance using χ ² statistics. A multi-objective optimisation approach based on Pareto front analysis was used to identify models that best reproduce the observed abundance trends across multiple neutron-capture elements simultaneously. Results . The best models consistently favour short delay times (≤30 Myr), low-mass progenitors (∼20−25 M _⊙ ), and an effective Eu injection rate of ∼2 × 10 ⁻⁷ M _⊙ per event. Stars more massive than ∼80 M _⊙ are too rare to account for the observed r -process enrichment alone, and therefore they cannot be the only source. While heavier neutron-capture elements (Ba, La, Ce) can be simultaneously reproduced within the Pareto-optimal set, the lighter ones (Sr, Y, Zr) present stronger conflicts with Eu, leading to systematically larger distances from the ideal solution. This tension reflects the fact that the solar r -process scaling relation, used to extend Eu yields to other elements, becomes progressively less valid towards lighter neutron-capture elements. Conclusions . Our results suggest that no single class of r -process events under solar-scaled yields can simultaneously account for the chemical evolution of both light and heavy neutron-capture elements. Instead, at least two distinct scaling components appear necessary: one corresponding to the main r -process, similar to the solar and r -process rich star patterns, and another weak component with enhanced production of lighter r -process elements, similar to that observed in r -process poor stars.

Context. Europium (Eu) serves as a crucial tracer in studying the origin of rapid neutron-capture process (r-process) elements. For this reason, extensive efforts have been made in the last decade to model the chemical evolution of this element in the Galaxy. However, far less attention has been reserved thus far for Eu in different galaxies of the Local Group. Aims. By employing detailed and well-tested chemical evolution models, we investigate Eu enrichment across Local Group dwarf spheroidal galaxies, allowing for a direct comparison between model predictions for dwarf galaxies and the Milky Way. Methods. Building upon an r-process enrichment framework that successfully reproduces the observed Eu abundance patterns, as well as the supernova and compact binary merger rates in the Milky Way, we built chemical evolution models for the Sagittarius, Fornax, and Sculptor dwarf spheroidal galaxies. We used these models to test the enrichment scenario against the abundance patterns observed in these galaxies. Results. Models reproducing the Galactic Eu patterns significantly underestimate the [Eu/Fe] ratios observed in Local Group dwarfs. To address this ‘missing Eu’ problem, we estimated the Eu production rate needed to match the observations and explored the potential contributions either from prompt (core-collapse supernova like) or delayed (compact binary mergers) sources, assessing their compatibility with the Milky Way observables. Conclusions. The same r-process enrichment frameworks cannot simultaneously reproduce the Eu patterns both in the Milky Way and in dwarf galaxies. However, a scenario where additional Eu is provided by an increased production from delayed sources at low metallicity can theoretically reconcile the trends observed in the Milky Way and in Local Group dwarfs. This is because of the small discrepancies (≲0.1 dex) between model predictions and observations found in this case. Further studies targeting and modelling of neutron-capture elements in Local Group galaxies are needed to fill the gaps in our current understanding of this problem.

In exoplanet research, the focus is increasingly on identifying Earth analogs, planets similar in density and habitability potential. As the number of rocky exoplanets grows, parallel discussions have emerged on system architectures and Galactic environments that may support life, drawing comparisons to our own Earth. This has brought renewed attention to the concept of the Galactic Habitable Zone (GHZ) as a broader context for interpreting the diversity of planetary environments. This study is the first to use detailed chemical evolution models to investigate the impact of stellar migration, modeled through a parametric approach, on the GHZ. Our findings reveal that stellar migration significantly enhances the number of stars capable of hosting habitable planets in the outer Galactic regions, with an increase of up to a factor of five at 18 kpc relative to a baseline value of unity at 6 kpc. Furthermore, we explore a novel scenario where the presence of gas giant planets increases the probability for the formation of terrestrial ones. We find that this increased probability is higher in the inner Galactic disc, but is also mitigated by stellar migration. In particular, at the present time, the number of FGK stars hosting terrestrial planets with minimum habitability conditions in the ring centered at 4 kpc is approximately 1.4 times higher than in scenarios where gas giants are assumed to hinder the formation and evolution of Earth-like planets. Without stellar migration, this factor increases to 1.5. Even larger ratios are predicted for terrestrial planets orbiting retired A stars, reaching 2.8 in models with stellar migration and 3.3 in models without it.