AGB Stars: Element Forges of the Universe

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Universe Letters".

Deadline for manuscript submissions: closed (30 June 2021) | Viewed by 20231

Special Issue Editors

INAF - Osservatorio Astronomico d'Abruzzo, Via Mentore Maggini snc, Loc. Collurania, 64100 Teramo TE, Italy
Interests: stellar evolution; nucleosynthesis; nuclear astrophysics; AGB stars
INAF–Osservatorio Astronomico di Roma, 00078 Monte Porzio Catone, Italy
Interests: stellar structure and evolution; stellar populations in clusters and galaxies
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Asymptotic giant branch (AGB) stars are marvelous cauldrons, in which light and heavy elements are synthesized (the latter via the slow neutron capture process, the s-process). Their large luminosities allow the detection of intermediate-age populations in distant systems, their electromagnetic emission dominating the infrared part of the spectrum. The low temperature characterizing their external layers allows the formation of clusters of molecules and the condensation of stellar dust (AGBs are in fact the main dust producers in the Universe). Their stellar ejecta are the product of the combined action of nuclear, physical, and chemical processes, representing thus exceptional laboratories to test our theoretical knowledge of stellar physics and nuclear astrophysics. Synergies between different communities (stellar observers, stellar modelers, nuclear experimentalists, and dust experts) are thus needed in order to untangle the secrets these stars still hold. This can be performed only through a shared vision of these objects.

The aim of this Special Issue is to collect contributions for a discussion on the observational, experimental, and theoretical aspects of AGB evolution. The topics to be discussed include stellar mixing processes, rotation, magnetic fields, presolar grains, molecule formation, dust production, spectroscopic observations, radiative transfer calculations, galactic chemical evolution, and experimental nuclear astrophysics.

The scope is to describe the state-of-the-art and perspectives in these research fields. Topics of interest include but are not limited to those described above.

Only letters and communications (about 5–10 pages) about original research and new experimental proposals will be considered for publication in this Special Issue.

Dr. Sergio Cristallo
Dr. Paolo Ventura
Guest Editors

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Published Papers (11 papers)

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Research

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12 pages, 496 KiB  
Article
AGB Stars and Their Circumstellar Envelopes: An Operative Approach to Computing Their Atmospheres
Universe 2021, 7(9), 340; https://doi.org/10.3390/universe7090340 - 10 Sep 2021
Viewed by 1196
Abstract
The study of AGB stars necessarily covers a wide range of topics, from the primary astronomical observations to their interpretation in terms of fundamental physics. All that requires proper ad hoc methodologies, among which numerical modeling of the outer layers of AGB stars [...] Read more.
The study of AGB stars necessarily covers a wide range of topics, from the primary astronomical observations to their interpretation in terms of fundamental physics. All that requires proper ad hoc methodologies, among which numerical modeling of the outer layers of AGB stars plays a paramount role. In this paper, we present an iterative sequential procedure, operative and physically sound, tailored to compute extended stellar atmospheres. It will constitute the backbone of the in fieri TEIDE package to be implemented into our VULCAN code. Such an improvement will allow us to compute more realistic models of the extended atmospheres of AGB stars taking into account important physical aspects that are neglected in the actual version of VULCAN. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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11 pages, 3396 KiB  
Article
The Ionization Energies of Dust-Forming Metal Oxide Clusters
Universe 2021, 7(7), 243; https://doi.org/10.3390/universe7070243 - 14 Jul 2021
Cited by 4 | Viewed by 1422
Abstract
Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, [...] Read more.
Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, represented by molecular clusters, is uncertain. We investigated the vertical and adiabatic ionization energies of four different metal-oxide clusters by means of density functional theory. They included clusters of magnesia (MgO)n, silicon monoxide (SiO)n, alumina (Al2O3)n, and titania (TiO2)n with stoichiometric sizes of n = 1–8. The magnesia, alumina, and titania clusters showed relatively little variation in their ionization energies with respect to the cluster size n: 7.1–8.2 eV for (MgO)n, from 8.9–10.0 eV for (Al2O3)n, and 9.3–10.5 eV for (TiO2)n. In contrast, the (SiO)n ionization energies decrease with size n, starting from 11.5 eV for n = 1, and decreasing to 6.6 eV for n = 8. Therefore, we set constraints on the stability limit for neutral metal-oxide clusters to persist ionization through radiation or high temperatures and for the nucleation to proceed via neutral–neutral reactions. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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10 pages, 510 KiB  
Article
s-Processing in Asymptotic Giant Branch Stars in the Light of Revised Neutron-Capture Cross Sections
Universe 2021, 7(7), 239; https://doi.org/10.3390/universe7070239 - 11 Jul 2021
Cited by 3 | Viewed by 2135
Abstract
Current AGB stellar models provide an adequate description of the s-process nucleosynthesis that occurs. Nonetheless, they still suffer from many uncertainties related to the modeling of the 13C pocket formation and the adopted nuclear reaction rates. For many important s-process isotopes, a [...] Read more.
Current AGB stellar models provide an adequate description of the s-process nucleosynthesis that occurs. Nonetheless, they still suffer from many uncertainties related to the modeling of the 13C pocket formation and the adopted nuclear reaction rates. For many important s-process isotopes, a best set of neutron-capture cross sections was recently re-evaluated. Using stellar models prescribing that the 13C pocket is a by-product of magnetic-buoyancy-induced mixing phenomena, s-process calculations were carried out with this database. Significant effects are found for a few s-only and branching point isotopes, pointing out the need for improved neutron-capture cross section measurements at low energy. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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17 pages, 734 KiB  
Article
Dust Production around Carbon-Rich Stars: The Role of Metallicity
Universe 2021, 7(7), 233; https://doi.org/10.3390/universe7070233 - 07 Jul 2021
Cited by 6 | Viewed by 1414
Abstract
Background: Most of the stars in the Universe will end their evolution by losing their envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase, enriching the interstellar medium of galaxies with heavy elements, partially condensed into dust grains formed in their extended [...] Read more.
Background: Most of the stars in the Universe will end their evolution by losing their envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase, enriching the interstellar medium of galaxies with heavy elements, partially condensed into dust grains formed in their extended circumstellar envelopes. Among these stars, carbon-rich TP-AGB stars (C-stars) are particularly relevant for the chemical enrichment of galaxies. We here investigated the role of the metallicity in the dust formation process from a theoretical viewpoint. Methods: We coupled an up-to-date description of dust growth and dust-driven wind, which included the time-averaged effect of shocks, with FRUITY stellar evolutionary tracks. We compared our predictions with observations of C-stars in our Galaxy, in the Magellanic Clouds (LMC and SMC) and in the Galactic Halo, characterised by metallicity between solar and 1/10 of solar. Results: Our models explained the variation of the gas and dust content around C-stars derived from the IRS Spitzer spectra. The wind speed of the C-stars at varying metallicity was well reproduced by our description. We predicted the wind speed at metallicity down to 1/10 of solar in a wide range of mass-loss rates. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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11 pages, 722 KiB  
Communication
First Results of the 140Ce(n,γ)141Ce Cross-Section Measurement at n_TOF
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Universe 2021, 7(6), 200; https://doi.org/10.3390/universe7060200 - 17 Jun 2021
Cited by 3 | Viewed by 3112
Abstract
An accurate measurement of the 140Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance [...] Read more.
An accurate measurement of the 140Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the 140Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in 140Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the 140Ce Maxwellian-averaged cross-section. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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15 pages, 684 KiB  
Article
Group II Oxide Grains: How Massive Are Their AGB Star Progenitors?
Universe 2021, 7(6), 175; https://doi.org/10.3390/universe7060175 - 01 Jun 2021
Cited by 5 | Viewed by 1362
Abstract
Presolar grains and their isotopic compositions provide valuable constraints to AGB star nucleosynthesis. However, there is a sample of O- and Al-rich dust, known as group 2 oxide grains, whose origin is difficult to address. On the one hand, the 17O [...] Read more.
Presolar grains and their isotopic compositions provide valuable constraints to AGB star nucleosynthesis. However, there is a sample of O- and Al-rich dust, known as group 2 oxide grains, whose origin is difficult to address. On the one hand, the 17O/16O isotopic ratios shown by those grains are similar to the ones observed in low-mass red giant stars. On the other hand, their large 18O depletion and 26Al enrichment are challenging to account for. Two different classes of AGB stars have been proposed as progenitors of this kind of stellar dust: intermediate mass AGBs with hot bottom burning, or low mass AGBs where deep mixing is at play. Our models of low-mass AGB stars with a bottom-up deep mixing are shown to be likely progenitors of group 2 grains, reproducing together the 17O/16O, 18O/16O and 26Al/27Al values found in those grains and being less sensitive to nuclear physics inputs than our intermediate-mass models with hot bottom burning. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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10 pages, 443 KiB  
Article
AGB Winds with Gas-Dust Drift in Stellar Evolution Codes
Universe 2021, 7(5), 113; https://doi.org/10.3390/universe7050113 - 21 Apr 2021
Cited by 3 | Viewed by 1217
Abstract
A significant fraction of new metals produced in stars enter the interstellar medium in the form of dust grains. Including dust and wind formation in stellar evolution models of late-stage low- and intermediate-mass stars provides a way to quantify their contribution to the [...] Read more.
A significant fraction of new metals produced in stars enter the interstellar medium in the form of dust grains. Including dust and wind formation in stellar evolution models of late-stage low- and intermediate-mass stars provides a way to quantify their contribution to the cosmic dust component. In doing so, a correct physical description of dust formation is of course required, but also a reliable prescription for the mass-loss rate. Here, we present an improved model of dust-driven winds to be used in stellar evolution codes and insights from recent detailed numerical simulations of carbon-star winds including drift (decoupling of dust and gas). We also discuss future directions for further improvement. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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13 pages, 381 KiB  
Article
AGB Stars and Their Circumstellar Envelopes. I. the VULCAN Code
Universe 2021, 7(4), 80; https://doi.org/10.3390/universe7040080 - 27 Mar 2021
Cited by 4 | Viewed by 1596
Abstract
The interplay between AGB interiors and their outermost layers, where molecules and dust form, is a problem of high complexity. As a consequence, physical processes like mass loss, which depend on the chemistry of the circumstellar envelope, are often oversimplified. The best candidates [...] Read more.
The interplay between AGB interiors and their outermost layers, where molecules and dust form, is a problem of high complexity. As a consequence, physical processes like mass loss, which depend on the chemistry of the circumstellar envelope, are often oversimplified. The best candidates to drive mass-loss in AGB stars are dust grains, which trap the outgoing radiation and drag the surrounding gas. Grains build up, however, is far from being completely understood. Our aim is to model both the physics and the chemistry of the cool expanding layers around AGB stars in order to characterize the on-going chemistry, from atoms to dust grains. This has been our rationale to develop ab initio VULCAN, a FORTRAN hydro code able to follow the propagation of shocks in the circumstellar envelopes of AGB stars. The version presented in this paper adopts a perfect gas law and a very simplified treatment of the radiative transfer effects and dust nucleation. In this paper, we present preliminary results obtained with our code. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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16 pages, 1087 KiB  
Article
Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis
Universe 2021, 7(2), 25; https://doi.org/10.3390/universe7020025 - 26 Jan 2021
Cited by 6 | Viewed by 2628
Abstract
The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M < 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution [...] Read more.
The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M < 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses Mini/M=2 and 3 for initial metallicities Z=0.001 and 0.002. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the 13C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a 13C-pocket size of at least ∼3 × 104 M, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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11 pages, 366 KiB  
Article
Studying Asymptotic Giant Branch Stars in the JWST Era
Universe 2020, 6(12), 223; https://doi.org/10.3390/universe6120223 - 26 Nov 2020
Viewed by 1402
Abstract
We explore the potential offered by the incoming launch of the James Webb Space Telescope, to study the stars evolving through the asymptotic giant branch (AGB) phase. To this aim we compare data of AGB stars in the Large Magellanic Cloud, taken with [...] Read more.
We explore the potential offered by the incoming launch of the James Webb Space Telescope, to study the stars evolving through the asymptotic giant branch (AGB) phase. To this aim we compare data of AGB stars in the Large Magellanic Cloud, taken with the IRS spectrograph, with the results from modelling of AGB evolution and dust formation in the wind. We find that the best diagrams to study M- and C-stars are, respectively, ([F770W]−[F2500W], [F770W]) and ([F770W]−[F1800W], [F1800W]). ([F770W]−[F2500W], [F770W]) turns out to be the best way of studying the AGB population in its entirely. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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Review

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11 pages, 582 KiB  
Review
The Missing Lead: Developments in the Lead (Pb) Discrepancy in Intrinsically s-Process Enriched Single Post-AGB Stars
Universe 2021, 7(11), 446; https://doi.org/10.3390/universe7110446 - 19 Nov 2021
Cited by 1 | Viewed by 1066
Abstract
Lead (Pb) is predicted to have large over-abundances with respect to other s-process elements in Asymptotic Giant Branch (AGB) stars, especially of low metallicities. However, our previous abundance studies of s-process enriched post-Asymptotic Giant Branch (post-AGB) stars in the Galaxy and [...] Read more.
Lead (Pb) is predicted to have large over-abundances with respect to other s-process elements in Asymptotic Giant Branch (AGB) stars, especially of low metallicities. However, our previous abundance studies of s-process enriched post-Asymptotic Giant Branch (post-AGB) stars in the Galaxy and the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. For the subset of post-AGB stars with low metallicities the determined upper limits based on detailed chemical abundance studies are much lower than what is predicted. Recent theoretical studies have pointed to the occurrence of the i-process to explain the observed chemical patterns, especially of Pb. A major development, in the observational context, is the release of the GAIA EDR3 parallaxes of the post-AGBs in the Galaxy, which has opened the gateway to systematically studying the sample of stars as a function of current luminosities (which can be linked to their initial masses). In this paper, we succinctly review the Pb discrepancy in post-AGB stars and present the latest observational and theoretical developments in this research landscape. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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