arXiv subjects

F. Rizzuti

Publications and source records attributed to F. Rizzuti.

13 recordsLinked to original sources

arXiv2026

3D simulations of a complete convective silicon shell burning phase

We present 3D hydrodynamic simulations of a complete silicon shell burning phase until fuel exhaustion at the end of the evolution of a 14$M_\odot$ core-collapse supernova progenitor, using a reduced 25-isotope nuclear network. We investigate how realistic turbulent convection affects this burning phase, which has a more complicated set of nuclear reactions than previous burning phases. We find broad similarities between the 3D simulation and the 1D \textsc{MESA} model. However, due to more realistic feedback from the very stiff convective boundaries in the hydrodynamic simulations, the 3D simulation experiences lower convective boundary mixing (CMB) compared to 1D, and hence entrains less fresh fuel into the silicon shell. This leads to the silicon shell in the 3D model burning for roughly 800\,s shorter. We find that the nuclear burning timescales for the dominant reactions are faster than the mixing timescale, making this entire process a convective-reactive event. The angular-averaged energy generation profile shows a double-peaked structure, where the region between the positive peaks are close to zero, or are negative. We find that throughout the base of this silicon shell, many $(\alpha, p)$ and their inverse reactions are important. The forward and reverse rates are very similar, so slight fluctuations in the temperature cause regions to be either exoergic or endoergic, leading to a complicated energy generation evolution. This study presents an exploration using a single reduced nuclear network, however, due to the sensitivity of this burning phase future studies should investigate the impact of more complete nuclear networks.

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arXiv2026

Phlegethon: a fully compressible magnetohydrodynamic code for simulations in stellar astrophysics

We present PHLEGETHON, a fully compressible, Eulerian magnetohydrodynamic (MHD) code designed for multidimensional simulations in stellar astrophysics. The code uses a time-explicit, second-order, finite-volume method optimized to model a wide range of dynamical processes in stars, from very low-Mach-number turbulent convection in the cores of massive stars to supersonic flows in subsurface convection zones. PHLEGETHON employs low-dissipation Riemann solvers and a well-balanced method to accurately capture slow flows arising from strongly stratified media. The induction equation is solved using a staggered constrained-transport method to ensure divergence-free evolution of the magnetic field. The MHD equations are coupled to arbitrary nuclear reaction networks solved in a time-implicit approach, together with super-time-stepping for efficient treatment of thermal diffusion. Equations of state appropriate for stellar plasmas are available, accounting for partial ionization, electron degeneracy, and electron-positron pair production. The code is implemented in a compact and user-friendly manner, and it scales to tens of thousands of CPU cores using MPI-based domain decomposition. We perform several verification tests to demonstrate the accuracy and versatility of the code, and present simulations of magnetoconvection in a core-collapse supernova progenitor star. The rich variety of physical effects and numerical methods implemented in PHLEGETHON enables the code to model diverse multidimensional processes that play a crucial role in stellar-interior dynamics, such as reactive convection, convective boundary mixing, internal-wave excitation, and magnetic-field amplification mechanisms. Within a single framework, these phenomena can be investigated across a wide range of stellar evolutionary stages, from main-sequence stars to supernova progenitors. PHLEGETHON is publicly accessible online.

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arXiv2025

Lithium evolution in the Galactic thin disc from Main-Sequence and early Red-Giant-Branch stars

The role of novae as producers of galactic lithium has been suggested since the 1970s, and it has been reconsidered recently with the detection of $^7$Be in their outbursts. At the same time, stellar models are moving forward to comprehend the discrepancy between the primordial lithium abundance predicted by the standard Big Bang Nucleosynthesis theory and the measured value of old dwarf stars. In this work, we follow the evolution of $^7$Li in the galactic thin disc starting from a primordial value of A(Li)=2.69 dex and applying $^7$Li depletion corrections of the stellar model with overshooting to our chemical evolution models. We use the upper envelope of the observational data to constrain the models. In addition to the dwarf main sequence (MS) stars, our analysis includes, for the first time, the early red-giant-branch (RGB) stars. Besides the renowned Spite plateau of the MS stars at low metallicities, we also confirm the existence of a second A(Li) plateau of the early RGB stars, which can be explained by our model with the corrections from stellar models. Our best-fit model is obtained with an effective averaged $^7$Li yield $^{Li}Y_\mathrm{Nova}=2.34\times 10^{-5} M_\odot$ during the whole lifetime of a nova. This reinforces the possibility that novae are the main galactic $^7$Li source, together with the stellar models' ability to comprehend the "cosmological lithium problem" in this context.

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arXiv2025

Unlocking the mystery of Sr synthesis in the early Galaxy through analysis of barium isotopes in very metal-poor stars

We determine the contributions of the rapid (r) and slow (s) neutron capture processes to the Ba isotope mixture, along with Ba, Eu, and Sr NLTE abundances, in a sample of very metal-poor stars. The selected stars formed before the contribution from the main s-process in low- and intermediate-mass stars became significant. Some of our sample stars are enhanced in Sr, with [Sr/Ba] reaching up to 0.7. These stars gained their high Sr abundance from a poorly understood process, which may appear to be a weak s-process or a weak r-process. Our aim is to uncover the nature of this additional Sr source. The abundances derived from the resonance Ba II lines are influenced by the adopted Ba isotope mixture. We compute Ba isotope mixtures corresponding to different r- to s-process contributions and determine the corresponding abundances from the Ba II resonance lines in each sample star. Additionally, we determine Ba abundances from weak subordinate Ba II lines, which are unaffected by the adopted Ba isotope mixture. We then compare the Ba abundances derived from the subordinate lines with those from the Ba II resonance lines. We find a higher s-process contribution to Ba isotopes in stars with greater [Sr/Eu] and [Sr/Ba] overabundances, suggesting that the additional Sr synthesis was due to the early s-process occurring in massive stars. We estimate the [Sr/Ba] ratio produced by the early s-process and obtain [Sr/Ba]_earlyS = 1.1 +- 0.2. Regarding the potential synthesis of Sr and Ba in the i-process in massive stars, our results argue that there was no detectable contribution from this process within the error bars in our sample stars. In the early Galaxy, before significant main s-process enrichment, barium and strontium were produced primarily by the main r-process and the early s-process, which occurred in rapidly rotating massive stars.

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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.

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arXiv2025

Explaining the $^{12}\text{C}/^{13}\text{C}$ ratio in the Galactic halo: the contribution from shell mergers in primordial massive stars

Recent campaigns of observations have provided new measurements of the carbon isotopes in the most metal-poor stars of the Galaxy. These stars are so metal-poor that they could only have been enriched by one or few generations of massive progenitors. However, explaining the primary production of $^{13}$C and the low $^{12}$C/$^{13}$C ratio measured in these stars is challenging. Making use of the most up-to-date models for zero-metal and low-metallicity stars, we investigate the possible sources of $^{13}$C at low metallicity and verify whether massive stars could be the sole responsible for the $^{12}$C/$^{13}$C ratio observed in halo stars. We employ the stochastic model for Galactic chemical evolution GEMS to reproduce the evolution of CNO elements and $^{12}$C/$^{13}$C ratio, including the enrichment from rotating massive stars, some of which show the occurrence of H-He shell mergers. We find that stars without H-He shell mergers do not produce enough $^{13}$C to be compatible with the observations. Instead, the primary production by shell mergers and later ejection during the supernova explosion can explain 30 < $^{12}$C/$^{13}$C < 100. The observations are best reproduced assuming a large frequency of shell mergers. The $^{12}$C/$^{13}$C < 30 can only be reproduced assuming an outer layer ejection and no explosion, but requiring a larger production of $^{12}$C and $^{13}$C. Zero-metal and low-metallicity spinstars with H-He shell mergers appear as the most plausible scenario to explain the low $^{12}$C/$^{13}$C ratio in CEMP-no stars. The entire range of $^{12}$C/$^{13}$C values can be explained by assuming that some stars fully explode while others only eject their outer layers. Shell mergers should be also more frequent and productive, which is allowed by the current uncertainties in the treatment of convection in stellar modelling.

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arXiv2024

Chemical Evolution of R-process Elements in Stars (CERES). III. Chemical abundances of neutron capture elements from Ba to Eu

The chemical abundances of elements such as barium and the lanthanides are essential to understand the nucleosynthesis of heavy elements in the early Universe as well as the contribution of different neutron capture processes (for example slow versus rapid) at different epochs. The Chemical Evolution of R-process Elements in Stars (CERES) project aims to provide a homogeneous analysis of a sample of metal-poor stars ( [Fe/H]\<-1.5) to improve our understanding of the nucleosynthesis of neutron capture elements, in particular the r-process elements, in the early Galaxy. Our data consist of a sample of high resolution and high signal-to-noise ratio UVES spectra. The chemical abundances were derived through spectrum synthesis, using the same model atmospheres and stellar parameters as derived in the first paper of the CERES series. We measured chemical abundances or upper limits of seven heavy neutron capture elements (Ba, La, Ce, Pr, Nd, Sm, and Eu) for a sample of 52 metal-poor giant stars. We estimated through the mean shift clustering algorithm that at Ba/H =-2.4 and Fe/H =-2.4 a variation in the trend of X/Ba with X=La,Nd,Sm,Eu, versus Ba/H occurs. This result suggests that, for Ba/H $\<$$-2.4$, Ba nucleosynthesis in the Milky Way halo is primarily due to the $r$-process, while for Ba/H \<-2.4 the effect of the s-process contribution begins to be visible. In our sample, stars with Ba/Eu compatible with a Solar System pure r-process value (hereafter, r-pure) do not show any particular trend compared to other stars, suggesting r-pure stars may form in similar environments to stars with less pure r-process enrichments. Homogeneous investigations of high resolution and signal-to-noise ratio spectra are crucial for studying the heavy elements formation, as they provide abundances that can be used to test nucleosynthesis models as well as Galactic chemical evolution models.

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arXiv2024

High N/O ratio at high redshift as a result of a strong burst of star formation and differential galactic winds

Recent observations by JWST have revealed supersolar $^{14}$N abundances in galaxies at very high redshift. On the other hand, these galaxies show subsolar metallicity. The observed N/O ratios are difficult to reproduce in the framework of chemical evolution models for the Milky Way. Our aim is to reproduce these high N/O ratios with chemical evolution models assuming different histories of star formation triggering galactic winds coupled with detailed nucleosynthesis prescriptions for $^{14}$N, $^{12}$C, $^{16}$O and $^{56}$Fe. We compute several models for small galaxies ($10^{9}\text{ - }10^{10}\text{ M}_{\odot}$) with high star formation efficiency and strong galactic winds. These winds are assumed to be differential, carrying out mainly the products of the explosion of core-collapse supernovae. We find that only models with high star formation rates, normal initial mass function, and differential galactic winds can reproduce the observed chemical abundances. We also find that with the same assumptions about star formation and galactic winds, but with a very rapid formation resulting from fast gas infall, we can also reproduce the estimated ages of these objects. We find no necessity to invoke peculiar nucleosynthesis from Population III stars, very massive stars and supermassive stars.

astro-ph.GA↗

arXiv2024

3D simulations of convective shell Neon-burning in a massive star

The treatment of convection remains a major weakness in the modelling of stellar evolution with one-dimensional (1D) codes. The ever increasing computing power makes now possible to simulate in 3D part of a star for a fraction of its life, allowing us to study the full complexity of convective zones with hydrodynamics codes. Here, we performed state-of-the-art hydrodynamics simulations of turbulence in a neon-burning convective zone, during the late stage of the life of a massive star. We produced a set of simulations varying the resolution of the computing domain (from 1283 to 10243 cells) and the efficiency of the nuclear reactions (by boosting the energy generation rate from nominal to a factor of 1000). We analysed our results by the mean of Fourier transform of the velocity field, and mean-field decomposition of the various transport equations. Our results are in line with previous studies, showing that the behaviour of the bulk of the convective zone is already well captured at a relatively low resolution (2563), while the details of the convective boundaries require higher resolutions. The different boosting factors used show how various quantities (velocity, buoyancy, abundances, abundance variances) depend on the energy generation rate. We found that for low boosting factors, convective zones are well mixed, validating the approach usually used in 1D stellar evolution codes. However, when nuclear burning and turbulent transport occur on the same timescale, a more sophisticated treatment would be needed. This is typically the case when shell mergers occur.

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arXiv2023

3D stellar evolution: hydrodynamic simulations of a complete burning phase in a massive star

Our knowledge of stellar evolution is driven by one-dimensional (1D) simulations. 1D models, however, are severely limited by uncertainties on the exact behaviour of many multi-dimensional phenomena occurring inside stars, affecting their structure and evolution. Recent advances in computing resources have allowed small sections of a star to be reproduced with multi-D hydrodynamic models, with an unprecedented degree of detail and realism. In this work, we present a set of 3D simulations of a convective neon-burning shell in a 20 M$_\odot$ star run for the first time continuously from its early development through to complete fuel exhaustion, using unaltered input conditions from a 321D-guided 1D stellar model. These simulations help answer some open questions in stellar physics. In particular, they show that convective regions do not grow indefinitely due to entrainment of fresh material, but fuel consumption prevails over entrainment, so when fuel is exhausted convection also starts decaying. Our results show convergence between the multi-D simulations and the new 321D-guided 1D model, concerning the amount of convective boundary mixing to include in stellar models. The size of the convective zones in a star strongly affects its structure and evolution, thus revising their modelling in 1D will have important implications for the life and fate of stars. This will thus affect theoretical predictions related to nucleosynthesis, supernova explosions and compact remnants.

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arXiv2022

Realistic 3D hydrodynamics simulations find significant turbulent entrainment in massive stars

Our understanding of stellar structure and evolution coming from one-dimensional (1D) stellar models is limited by uncertainties related to multi-dimensional processes taking place in stellar interiors. 1D models, however, can now be tested and improved with the help of detailed three-dimensional (3D) hydrodynamics models, which can reproduce complex multi-dimensional processes over short timescales, thanks to the recent advances in computing resources. Among these processes, turbulent entrainment leading to mixing across convective boundaries is one of the least understood and most impactful. Here we present the results from a set of hydrodynamics simulations of the neon-burning shell in a massive star, and interpret them in the framework of the turbulent entrainment law from geophysics. Our simulations differ from previous studies in their unprecedented degree of realism in reproducing the stellar environment. Importantly, the strong entrainment found in the simulations highlights the major flaws of the current implementation of convective boundary mixing in 1D stellar models. This study therefore calls for major revisions of how convective boundaries are modelled in 1D, and in particular the implementation of entrainment in these models. This will have important implications for supernova theory, nucleosynthesis, neutron stars and black holes physics.

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arXiv2021

Dynamics in a stellar convective layer and at its boundary: Comparison of five 3D hydrodynamics codes

Our ability to predict the structure and evolution of stars is in part limited by complex, 3D hydrodynamic processes such as convective boundary mixing. Hydrodynamic simulations help us understand the dynamics of stellar convection and convective boundaries. However, the codes used to compute such simulations are usually tested on extremely simple problems and the reliability and reproducibility of their predictions for turbulent flows is unclear. We define a test problem involving turbulent convection in a plane-parallel box, which leads to mass entrainment from, and internal-wave generation in, a stably stratified layer. We compare the outputs from the codes FLASH, MUSIC, PPMSTAR, PROMPI, and SLH, which have been widely employed to study hydrodynamic problems in stellar interiors. The convection is dominated by the largest scales that fit into the simulation box. All time-averaged profiles of velocity components, fluctuation amplitudes, and fluxes of enthalpy and kinetic energy are within $\lesssim 3\sigma$ of the mean of all simulations on a given grid ($128^3$ and $256^3$ grid cells), where $\sigma$ describes the statistical variation due to the flow's time dependence. They also agree well with a $512^3$ reference run. The $128^3$ and $256^3$ simulations agree within $9\%$ and $4\%$, respectively, on the total mass entrained into the convective layer. The entrainment rate appears to be set by the amount of energy that can be converted to work in our setup and details of the small-scale flows in the boundary layer seem to be largely irrelevant. Our results lend credence to hydrodynamic simulations of flows in stellar interiors. We provide in electronic form all outputs of our simulations as well as all information needed to reproduce or extend our study.

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arXiv2021

Constraints on stellar rotation from the evolution of Sr and Ba in the Galactic halo

Recent studies show that the chemical evolution of Sr and Ba in the Galaxy can be explained if different production sites, hosting r- and s-processes, are taken into account. However, the question of unambiguously identifying these sites is still unsolved. Massive stars are shown to play an important role in the production of s-material if rotation is considered. In this work, we study in detail the contribution of rotating massive stars to the production of Sr and Ba, in order to explain their chemical evolution, but also to constrain the rotational behaviour of massive stars. A stochastic chemical evolution model was employed to reproduce the enrichment of the Galactic halo. We developed new methods for model-data comparison which help to objectively compare the stochastic results to the observations. We employed these methods to estimate the value of free parameters which describe the rotation of massive stars, assumed to be dependent on the stellar metallicity. We constrain the parameters using the observations for Sr and Ba. Employing these parameters for rotating massive stars in our stochastic model, we are able to correctly reproduce the chemical evolution of Sr and Ba, but also Y, Zr and La. The data supports a decrease of both the mean rotational velocities and their dispersion with increasing metallicity. Our results show that a metallicity-dependent rotation is a necessary assumption to explain the s-process in massive stars. Our novel methods of model-data comparison represent a promising tool for future galactic chemical evolution studies.

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