S-process nucleosynthesis in AGB stars with the full spectrum of turbulence scheme for convection

Abstract. This thesis describes SNUPPAT (s-process nucleosynthesis post processing code for Aton), a post-processing slow neutron-capture process (s-process) code developed by us for the stellar evolutionary code ATON. The aim is to provide ATON, which shows distinct key physical characteristics to other evolutionary codes, with the capability of following heavy element nucleosynthesis in lowto intermediate-mass stars. Meaning that we have to increase the number of species followed from 30 (ATON follows 30 species from hydrogen to 31P) to around 320 (from hydrogen to 210Po) at the very least. In the long term we hope that, through ATON nucleosynthesis predictions, we may open the way to a deeper understanding of the physics and evolution during the thermally pulsing asymptotic giant branch (TP-AGB) phase. This objective is realized by the creation from scratch of a nucleosynthesis code, explained at the beginning of this thesis, along with possible optimizations to some of the traditional numerical methods used for these kind of astrophysical problems. We also tackle the issue of mixing, which includes both the convective mixing as well as the convective overshooting responsible of, among other things, the formation of an effective 13C pocket, essential for nucleosynthesis in TP-AGB stars. Following the code description, we present SNUPPAT solar metallicity results in the form of final stellar surface abundances. We explore different stellar initial masses as well as variations of the extra-mixing parameter, which governs the convective overshooting behavior. These results are analyzed to connect them with the different physical processes taking place in the deeper layers of the AGB stars. Finally, we compare our results with those from other known s-processing numerical codes, finding a reasonably good agreement with at least one of them (specifically, the MONASH version of the Mount Stromlo Stellar Structure Program), which coincidentally is the nucleosynthesis code that better explains the currently limited observational information (Rb, Zr abundances) about these stars. We note the fact that ATON-SNUPPAT appears to generate hotter models with a low third dredge-up efficiency that forces us to increase the nucleosynthesis output in the helium intershell, in order to obtain comparable surface abundances. The consequence is that SNUPPAT predictions present, generally, the signs of a higher overall neutron exposure (a measure of the total neutron captures) in the final stellar surface abundances.En esta tesis describimos SNUPPAT (llamado as´ı por sus siglas en ingl´es sprocess nucleosynthesis post processing code for Aton), un c´odigo num´erico que hemos desarrollado de post-procesado para el c´alculo de la nucleos´ıntesis debida al proceso de captura lenta de neutrones (s-process) para el c´odigo de evoluci´on estelar ATON. El objetivo es proporcionar al c´odigo ATON, que se diferencia de otros c´odigos de evoluci´on estelar en ciertas caracter´ısticas f´ısicas clave, una forma de predecir la nucleos´ıntesis de elementos pesados en estrellas de masa baja e intermedia. Para ello necesitamos incrementar el n´umero de especies at´omicas cuya evoluci´on seguimos desde las 30 (desde el hidr´ogeno hasta el 31P) de ATON hasta unas 320 (desde el hidr´ogeno hasta el 210Po) como m´ınimo. A largo plazo estamos interesados en obtener una mayor comprensi´on de los procesos f´ısicos que se dan en el interior de las estrellas en la fase TP-AGB (del ingl´es thermally pulsing asymptotic giant branch). Para lograr este objetivo, hemos creado un c´odigo de nucleos´ıntesis desde cero, descrito al comienzo de esta tesis. Tambi´en hemos decidido incluir una serie de posibles optimizaciones a los m´etodos num´ericos tradicionales utilizados para este tipo de problemas astrof´ısicos. Para completar la descripci´on del c´odigo SNUPPAT tambi´en estudiamos c´omo mezclar correctamente las abundancias elementales en este tipo de estrellas. Esto incluye tanto la mezcla debida a la convecci´on como la debida a un caso de overshooting convectivo. Este ´ultimo es esencial para la formaci´on de un pocket de 13C, necesario para la nucleos´ıntesis en las estrellas en la fase de TP-AGB. Tras la descripci´on del c´odigo de nucleos´ıntesis, presentamos las predicciones a metalicidad solar de SNUPPAT en la forma de abundancias finales en la superficie de la estrella. Exploramos tanto con distintas masas iniciales (en el rango 3-6 M ) como con distintos par´ametros de overshooting. Seguidamente, analizamos estos resultados te´oricos para trazar una conexi´on entre las abundancias superficiales y los diferentes procesos f´ısicos que se dan en las capas m´as profundas de las estrellas AGB. Finalmente, comparamos nuestros resultados con los de otros c´odigos de nucleos´ıntesis de s-process conocidos. Al hacerlo, encontramos un acuerdo razonable con al menos uno de ellos (en concreto, los de la versi´on MONASH del Mount Stromlo Stellar Structure Program), precisamente los modelos de nucleos´ıntesis que mejor explican la limitada informaci´on observacional (abundancias de Rb, Zr) de la que se dispone hasta la fecha. Con esta comparaci´on concluimos que los modelos conjuntos de ATON y SNUPPAT parecen generar estrellas m´as calientes a la misma masa y con relativa baja eficiencia del tercer dragado. Este hecho nos obliga a incrementar la producci´on debida a la nucleos´ıntesis en las capas m´as internas para obtener resultados comparables en la superficie. Como consequencia de esto los modelos de SNUPPAT muestran, en general, signos de una mayor captura neutr´onica en las abundancias finales de la superficie estelar

Inhomogeneous Enrichment of Radioactive Nuclei in the Galaxy: Deposition of Live 53 Mn, 60 Fe, 182 Hf, and 244 Pu into Deep-sea Archives. Surfing the Wave?

© 2023. The Author(s). Published by the American Astronomical Society. This article is license under a Creative Commons license. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. https://creativecommons.org/licenses/by/4.0/While modeling the galactic chemical evolution (GCE) of stable elements provides insights to the formation history of the Galaxy and the relative contributions of nucleosynthesis sites, modeling the evolution of short-lived radioisotopes (SLRs) can provide supplementary timing information on recent nucleosynthesis. To study the evolution of SLRs, we need to understand their spatial distribution. Using a three-dimensional GCE model, we investigated the evolution of four SLRs: 53Mn, 60Fe, 182Hf, and 244Pu with the aim of explaining detections of recent (within the last ≈1–20 Myr) deposition of live 53Mn, 60Fe, and 244Pu of extrasolar origin into deep-sea reservoirs. We find that core-collapse supernovae are the dominant propagation mechanism of SLRs in the Galaxy. This results in the simultaneous arrival of these four SLRs on Earth, although they could have been produced in different astrophysical sites, which can explain why live extrasolar 53Mn, 60Fe, and 244Pu are found within the same, or similar, layers of deep-sea sediments. We predict that 182Hf should also be found in such sediments at similar depths.Peer reviewe

Stochastic Chemical Evolution of Radioactive Isotopes with a Monte Carlo Approach

Short-lived radionuclides (SLRs) with mean lives τ of a few to hundreds of Myr provide unique opportunities to probe recent nucleosynthesi events in the interstellar medium and the physical conditions in whic the Sun formed. Here we quantify the uncertainty in the predicte evolution of SLRs within a parcel of interstellar gas given th stochastic nature of stellar enrichment events. We assume that a enrichment progenitor is formed at every time interval γ. For eac progenitor, we randomly sample the delay time between its formation an its enrichment event, based on several delay-time distribution (DTD functions that cover a wide range of astrophysical sites. For each se of τ, γ, and DTD functions, we follow the abundances of SLRs for 15 Gy and repeat this process thousands of times to derive their probabilit distributions. For τ/γ ≳ 2, the distributions depend on the DT function, and we provide tabulated values and analytical expressions t quantify the spread. The relative abundance uncertainty reaches maximum of ∼60% for τ/γ = 1. For τ/γ ≲ 1, we provide the probability fo the SLR abundance to carry the signature of only one enrichment event which is greater than 50% when τ/γ ≲ 0.3. For 0.3 ≲ τ/γ ≲ 2, a smal number of events contributed to the SLR abundance. This case needs to b investigated with a separate statistical method. We find that a isolation time for the birth of the Sun of roughly 9─13 Myr i consistent with the observed abundances of 60Fe 107Pd, and 182Hf in the early solar system whe assuming τ/γ ∼ 3 for these isotope

Origin of Plutonium-244 in the Early Solar System

We investigate the origin in the early Solar System of the short-lived radionuclide 244Pu (with a half life of 80 Myr) produced by the rapid (r) neutron-capture process. We consider two large sets of r-process nucleosynthesis models and analyse if the origin of 244Pu in the ESS is consistent with that of the other r and slow (s) neutron-capture process radioactive nuclei. Uncertainties on the r-process models come from both the nuclear physics input and the astrophysical site. The former strongly affects the ratios of isotopes of close mass (129I/127I, 244Pu/238U, and 247Pu/235U). The 129I/247Cm ratio, instead, which involves isotopes of a very different mass, is much more variable than those listed above and is more affected by the physics of the astrophysical site. We consider possible scenarios for the evolution of the abundances of these radioactive nuclei in the galactic interstellar medium and verify under which scenarios and conditions solutions can be found for the origin of 244Pu that are consistent with the origin of the other isotopes. Solutions are generally found for all the possible different regimes controlled by the interval ($\delta$) between additions from the source to the parcel of interstellar medium gas that ended up in the Solar System, relative to decay timescales. If r-process ejecta in interstellar medium are mixed within a relatively small area (leading to a long $\delta$), we derive that the last event that explains the 129I and 247Cm abundances in the early Solar System can also account for the abundance of 244Pu. Due to its longer half life, however, 244Pu may have originated from a few events instead of one only. If r-process ejecta in interstellar medium are mixed within a relatively large area (leading to a short $\delta$), we derive that the time elapsed from the formation of the molecular cloud to the formation of the Sun was 9-16 Myr.Comment: 17 pages, 3 figures, published on Universe as part of the Special Issue "Nuclear Astrophysics in the Era of High Precision Astronomy

Enrichment of the Galactic disc with neutron-capture elements: Gd, Dy, and Th

The study of the origin of heavy elements is one of the main goals of nuclear astrophysics. In this paper, we present new observational data for the heavy r-process elements gadolinium (Gd, Z= 64), dysprosium (Dy, Z= 66), and thorium (Th, Z= 90) in a sample of 276 Galactic disc stars (-1.0 < [Fe/H] < + 0.3). The stellar spectra have a high resolution of 42 000 and 75 000, and the signal-to-noise ratio higher than 100. The LTE abundances of Gd, Dy, and Th have been determined by comparing the observed and synthetic spectra for three Gd lines (149 stars), four Dy lines (152 stars), and the Th line at 4019.13 angstrom (170 stars). For about 70 per cent of the stars in our sample, Gd and Dy are measured for the first time, and Th for 95 per cent of the stars. Typical errors vary from 0.07 to 0.16 dex. This paper provides the first extended set of Th observations in the Milky Way disc. Together with europium (Eu, Z= 63) data from our previous studies, we have compared these new observations with nucleosynthesis predictions and Galactic Chemical Evolution simulations. We confirm that [Gd/Fe] and [Dy/Fe] show the same behaviour of Eu. We study with GCE simulations the evolution of [Th/Fe] in comparison with [Eu/Fe], showing that unlike Eu, either the Th production is metallicity dependent in case of a unique source of the r-process in the Galaxy, or the frequency of the Th-rich r-process source is decreasing with the increase in [Fe/H]

Galactic Chemical Evolution of Radioactive Isotopes with an s-process Contribution

Analysis of inclusions in primitive meteorites reveals that several short-lived radionuclides (SLRs) with half-lives of 0.1-100 Myr existed in the early solar system (ESS). We investigate the ESS origin of 107Pd, 135Cs, and 182Hf, which are produced by slow neutron captures (the s-process) in asymptotic giant branch (AGB) stars. We modeled the Galactic abundances of these SLRs using the OMEGA+ galactic chemical evolution (GCE) code and two sets of mass- and metallicity-dependent AGB nucleosynthesis yields (Monash and FRUITY). Depending on the ratio of the mean-life τ of the SLR to the average length of time between the formations of AGB progenitors γ, we calculate timescales relevant for the birth of the Sun. If τ/γ ⪆ 2, we predict self-consistent isolation times between 9 and 26 Myr by decaying the GCE predicted 107Pd/108Pd, 135Cs/133Cs, and 182Hf/180Hf ratios to their respective ESS ratios. The predicted 107Pd/182Hf ratio indicates that our GCE models are missing 9%-73% of 107Pd and 108Pd in the ESS. This missing component may have come from AGB stars of higher metallicity than those that contributed to the ESS in our GCE code. If τ/γ ≲ 0.3, we calculate instead the time (T LE) from the last nucleosynthesis event that added the SLRs into the presolar matter to the formation of the oldest solids in the ESS. For the 2 M o˙, Z = 0.01 Monash model we find a self-consistent solution of T LE = 25.5 Myr

The RADIOSTAR Project

© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).Radioactive nuclei are the key to understanding the circumstances of the birth of our Sun because meteoritic analysis has proven that many of them were present at that time. Their origin, however, has been so far elusive. The ERC-CoG-2016 RADIOSTAR project is dedicated to investigating the production of radioactive nuclei by nuclear reactions inside stars, their evolution in the Milky Way Galaxy, and their presence in molecular clouds. So far, we have discovered that: (i) radioactive nuclei produced by slow (107Pd and 182Hf) and rapid (129I and 247Cm) neutron captures originated from stellar sources —asymptotic giant branch (AGB) stars and compact binary mergers, respectively—within the galactic environment that predated the formation of the molecular cloud where the Sun was born; (ii) the time that elapsed from the birth of the cloud to the birth of the Sun was of the order of 107 years, and (iii) the abundances of the very short-lived nuclei 26Al, 36Cl, and 41Ca can be explained by massive star winds in single or binary systems, if these winds directly polluted the early Solar System. Our current and future work, as required to finalise the picture of the origin of radioactive nuclei in the Solar System, involves studying the possible origin of radioactive nuclei in the early Solar System from core-collapse supernovae, investigating the production of 107Pd in massive star winds, modelling the transport and mixing of radioactive nuclei in the galactic and molecular cloud medium, and calculating the galactic chemical evolution of 53Mn and 60Fe and of the p-process isotopes 92Nb and 146Sm.Peer reviewedFinal Published versio