XIV Torino Workshop on AGB stars

10 Jun 2024, 09:00 → 14 Jun 2024, 14:00 UTC

Sala Gratton (INAF - Observatory of Rome)

 We are pleased to announce that the XIV edition of the Torino Workshop will take place at the Observatory of Rome (Italy), in the week 10-14 June, 2024.  

The conference, along the tradition of the previous editions of the Torino workshop, will be  focused on the physics and nucleosynthesis of asymptotic giant branch (AGB) stars and on several related topics. The meeting will offer a valuable opportunity to discuss the state of the art of AGB, post-AGB and super AGB modelling, the impact of current and future experimental efforts, above and below ground, to determine the rates of the relevant charged-particle and neutron-capture reactions, as well as the role of these stars in the chemical evolution of the Milky Way and other galaxies. The possibilities offered by present and oncoming observational facilities (such as JWST, VLT, LAMOST, ELT etc.), the study of dust formation in the wind of evolved stars and the meteoritic dust measurements, hot topics in the field of theoretical and nuclear astrophysics will be also addressed. 

The scope of the workshop is to bring together researchers from a variety of research fields, to address the current issues, and to discuss the future directions by interdisciplinary approaches. The conference program will include a series of oral presentations and poster sessions.

 

 

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Monday, 10 June

AGB structure, evolution and nucleosynthesis

1 The effects of induced magnetic mixing in AGB stars.

Asymptotic giant branch (AGB) stars synthesize half of the elements heavier than iron through the slow neutron capture process.
Despite the significant progresses in theory over the last few decades, many uncertainties still affect AGB models. The most notable example
is the mechanism responsible for the formation of the main neutron source in AGB stars, the so called 13C pocket.
Stringent constraints on the type and efficiency of mixing processes relevant to the s-process nucleosynthesis in AGB stars are provided by isotopic ratios in presolar grains, as well as from the latest spectroscopic observations of both intrinsic and extrinsic AGB stars.
We present recent results from AGB stellar models including the effects of mixing induced by magnetic fields. The comparison to extant observations suggests that magnetic instabilities may be at the origin of the 13C pocket in AGB stars, triggering future research aiming at better describing the physics governing these stars.

Speaker: Sergio Cristallo (Istituto Nazionale di Astrofisica (INAF))

2 Stellar evolution and nucleosynthesis in 3D hydrodynamic models of stars

Our understanding of stellar evolution and nucleosynthesis is limited by the uncertainties coming from the complex multi-dimensional processes in stellar interiors, such as convection and nuclear burning. 3D stellar models can improve this knowledge by studying realistic multi-D processes for a short timerange (minutes or hours). Recent advances in computing resources have enabled 3D stellar models to reproduce longer timescales and include nuclear reactions, making the simulations more realistic and allowing to study explicit nucleosynthesis.
In this talk, I will present results from 3D stellar simulations that include an explicit nuclear network for different burning phases in advanced massive stars. I will introduce the methods and limitations of multi-D stellar modelling, and describe the effects on the evolution of the stellar structure, discussing also the implications for stellar evolution, nucleosynthesis, and convection theory.

Speaker: Federico Rizzuti (Istituto Nazionale di Astrofisica (INAF))

3 Evolution and final fate of stars in the transition between AGB and Massive Stars

According to a standard initial mass function, stars in the range 7-12 Msun constitute $\sim$ 50$\%$ (by number) of the stars more massive than 7 Msun. Despite this, their evolutionary properties, particularly their final fate, remain mostly understudied. In this talk I will present some of the results published in our recent paper, where we discussed in details the evolutionary properties of solar metallicity, non rotating, stars in the range 7-15 Msun, from the pre-main sequence up to the pre-supernova stage or up to an advanced stage of the thermally pulsing phase, depending on the initial mass. Our findings revealed several key points: (1) the 7.00 Msun develops a degenerate CO core and evolves as a classical AGB star; (2) stars with initial mass M $\geq$ 9.22 Msun end their life as core collapse supernovae; (3) stars in the range 7.50 $<$M/Msun $<$ 9.20 develop a degenerate ONe core and evolve through the thermally pulsing SAGB phase; 4) stars in the mass range 7.50 $\leq$ M/Msun $\leq$ 8.00 end their life as hybrid CO/ONe- or ONe-WD; (5) stars with initial mass in the range 8.50 $\leq$ M/ Msun $\leq$ 9.20 most likely achieve the central densities in excess of the threshold value for the activation of the electron capture on 20Ne before losing the entire H-rich envelope and therefore may potentially explode as electron capture supernovae.

Speaker: Lorenzo Roberti (Konkoly Observatory, Research Centre for Astronomy and Earth Sciences)

10:40 Coffee break

4 The Role of the Third Dredge-up and Mass Loss in Shaping the Initial–Final Mass Relation of White Dwarfs

The initial-final mass relation (IFMR) plays a crucial role in understanding stellar structure and evolution by linking a star’s initial mass to the mass of the resulting white dwarf. This study explores the IFMR using full PARSEC evolutionary calculations supplemented with COLIBRI computations to complete the ejection of the envelope and obtain the final core mass. Recent works have shown that the supposed monotonicity of the IFMR is interrupted by a kink in the initial mass range M$_\mathrm{ini}$ ≈ 1.65 − 2.10M$_\odot$, due to the interaction between recurrent dredge-up episodes and stellar winds in carbon stars evolving on the thermally-pulsing asymptotic giant branch phase. To reproduce the IFMR non-monotonic behavior we investigate the role of convective overshooting efficiency applied to the base of the convective envelope (f$_\mathrm{env}$) and to the borders of the pulse-driven convective zone (f$_\mathrm{pdcz}$), as well as its interplay with mass loss. We compare our models to observational data and find that f$_\mathrm{env}$ must vary with initial mass in order to accurately reproduce the IFMR’s observed kink and slopes. We find some degeneracy between the overshooting parameters when only the IFMR information is used. Nonetheless, this analysis provides valuable insights into the internal mixing processes during the TP-AGB phase. Finally, we present chemical yields and ejecta calculated with our IFMR-calibrated models at solar metallicity.

Speaker: Francesco Addari (Scuola Internazionale Superiore di Studi Avanzati (SISSA), Istituto Nazionale di Astrofisica (INAF))

5 The initial-final mass relation from carbon stars in open clusters

Recently (Marigo et al. 2020-22), identified a kink in the initial-final mass relation around M$_i \simeq 1.65-2.10$ M$_\odot$, based on Gaia DR2 and EDR3 data for white dwarfs in open clusters aged 1.5-2.5 Gyr. Notably, wide dwarfs at this kink, all from NGC 7789, exhibit masses of $\simeq 0.70-0.74$ M$_\odot$, usually associated with stars of M$_i\simeq 3-4$ M$_\odot$. The above kink in the M$_i$ mass range coincides with the theorerically acepted solar metallicity lowest-mass stars evolving into carbon stars during the AGB phase. According to our explanation, these carbon stars likely underwent shallow third dredge-up events, resulting in low photospheric C/O ratios and, as a consequence, midle stellar winds. Under such conditions, the AGB lifetime is prolonged allowing for greater core mass growth beyond typical predictions.

We have analyzed chemically a few carbon stars belonging to open clusters with the above cluster ages. Our chemical analysis confirms that the carbon stars found within the kink exhibit markedly low photospheric C/O ratios and stellar winds, and the typical chemical composition expected for carbon stars of near solar metallicity, thus validating our theoretical predictions. However, we also show that this conclusion is strongly dependent on the derived stellar luminosity of these carbon stars.

Speaker: Carlos Abia (University of Granada)

6 The intermediate neutron capture process in AGB stars

The origin of trans-iron elements is not yet fully understood. In addition to the slow (s) and rapid (r) neutron capture processes, an intermediate neutron capture process (i-process) is thought to exist at neutron densities intermediate between the s- and r-processes. The chemical composition of the so-called r/s-stars support the existence of this process but the astrophysical site(s) hosting the i-process is (are) actively debated. The early asymptotic giant branch (AGB) phase of low-mass stars is a promising site. In this talk, I will focus on the development of the i-process in AGB stellar models of various masses and metallicities computed with the stellar evolution code STAREVOL. In particular, new results on the impact of overshooting and nuclear uncertainties will be discussed. The unique chemical fingerprint of these stars will also be presented and confronted with observations.

Speaker: Arthur Choplin (Université Libre de Bruxelles)

7 Exploring nucleosynthetic processes in a large sample of Barium stars using high resolution spectra

Barium (Ba) stars belong to binary systems where a former asymptotic giant branch (AGB, now a white dwarf) star polluted the less evolved companion, which became enriched with material produced through the slow neutron capture process (s process). The currently observed Ba star preserves the abundance pattern of the AGB, allowing us to test the imprints of the s process. Comparing different AGB nucleosynthetic models and Ba star abundances based on high-resolution spectra, we are able to constrain, for example, the effect of the initial rotation velocity and the nature of the neutron source. When comparing AGB models to the extended list of heavy element abundances available for a large homogeneous observational sample of 169 Ba stars, we could confirm that the polluting AGBs are of low mass (< 4 MSun). However, approximately 25% of the sample stars show anomalous abundance patterns, mainly at the first s-process peak (with higher Nb, Mo and/or Ru than the models), along with high W. The high W value is comparable to some post-AGB stars, and might indicate that we can identify different subgroups among the Ba star sample. Additional measurements could reveal the cause for these overabundances and can help to identify the underlying processes.

Speaker: Borbála Cseh (Konkoly Observatory, HUN-REN CSFK)

12:50 Lunch

8 S-Process Nucleosynthesis in and from AGB Stars

The nucleosynthetic slow neutron capture process (s-process) in AGB stars between ∼ 1 − 6 M⊙ is responsible for creating about half of the heavy elements in the universe. The s-process can be traced directly through AGBs, or indirectly through their binary companions (CEMP-s stars, Ba stars, CH stars), as thermally pulsing AGBs will dredge s-process material from the inter-shell to the surface. We present and study 10 AGB (intrinsic) stars and 10 (extrinsic) companions where mass transfer is important. Using high-resolution spectra, we derive atmospheric parameters and compute 1D LTE surface abundances, focusing on heavy elements created during the thermally pulsing AGB phase (C, N, Y, Zr, Nb, Mo, Ba, La, Ce, Nd, Pb), and the r-process element Eu. We compare our results to the FRUITY yields to constrain the masses of our AGB stars and their companions, and investigate correlations in abundance space using Gaussian mixture modelling. Through detailed stellar modelling, we constrain possible binary companion masses and other system parameters. This can help determine efficiencies of AGB wind mass transfer, and has implications for galactic chemical evolution as AGB stars deposit their material back into the ISM to seed further stellar generations.

Speaker: Alexander Dimoff (Max Planck Institute für Astronomie)

9 The impact of nuclear parameter and models uncertainties on the i-process nucleosynthesis in early AGB stars

The observed surface abundance distribution of Carbon-enhanced metal-poor (CEMP) r/s-stars suggests that these stars have been polluted by an intermediate neutron-capture process (the so-called i-process) occurring at intermediate neutron densities between the r- and s-processes. Triggered by the ingestion of protons inside a convective He-burning zone, the i-process could be hosted in several sites, a promising one being the early AGB phase of low-mass low-metallicity stars. The i-process remains however affected by many uncertainties including those of nuclear origin since it involves hundreds of nuclei for which reaction rates have not yet been determined experimentally.

We investigate both the parameter and model uncertainties associated with theoretical nuclear reaction rates of relevance during the i-process and explore their impact on the i-process elemental production, and subsequently on the surface enrichment, for low-mass low-metallicity stars during the early AGB phase.

We use the TALYS reaction code (Koning et al. 2023) to estimate both the model and parameter uncertainties affecting the photon strength function and the nuclear level densities, hence the radiative neutron capture rates. The impact of correlated uncertainties is estimated by considering different nuclear models, as detailed in Goriely et al. 2022. In contrast, the uncorrelated uncertainties associated with local variation of model parameters are estimated using a variant of the backward-forward Monte Carlo method to constrain the parameter changes to experimentally known cross-sections before propagating them consistently to the neutron capture rates of nuclei of i-process interest.

On such a basis, the STAREVOL code (Siess et al. 2006) is used to determine the impact of nuclear uncertainties on the i-process nucleosynthesis in a 1 M⊙ [Fe/H] = - 2.5 model star during the proton ingestion event in the early AGB phase. A large nuclear network of 1160 species coherently coupled to the transport processes is solved to follow the i-process nucleosynthesis. This study allows us to quantify the relative importance of parameters versus model uncertainties with respect to the surface abundances in AGB stars and to identify the reaction rates that would need to be better constrained in the future in order to improve our understanding of the i-process.

Speaker: Sébastien Martinet (Université Libre de Bruxelles)

10 From the s-process to the i-process: A new perspective on the chemical enrichment of extrinsic stars

Recently, there have been hints that an increasing number of extrinsic stars (barium, CH and CEMP) could be enriched in chemical elements produced by the i-process (characterised by neutron densities $N_n \sim 10^{14-15}$ cm$^{-3}$) rather than by the s-process ($N_n \sim 10^{8}$ cm$^{-3}$) .
Different isotopic mixtures are predicted for the s-process on the one hand, and for the r- or i-processes on the other. After a brief review of previous isotopic composition determinations, we report on our investigation, based on high-resolution, high signal-to-noise HERMES and UVES spectra and non-LTE line synthesis, of the barium isotopic composition of extrinsic stars enriched in heavy elements.
We also report on a sample of extrinsic stars uncovered in Gaia DR3. We discuss their properties in terms of chemical abundances and binarity, as well as the overlap with existing catalogues of extrinsic stars.

Speaker: Dr Sophie Van Eck (Université libre de Bruxelles)

11 Barium stars: Systematic deviations from the AGB models

Barium (Ba) stars are peculiarly enriched in s-process elements, even though they have not yet reached the AGB evolutionary phase. This enrichment can be explained by contamination from a companion star when that was an AGB star. Ba stars preserve this material for a long time and are easy to derive their high-resolution spectra, making them objects ideal for testing models of AGB star nucleosynthesis. We had developed a method with which we could identify the best-matching polluter AGB model for each Ba star abundances.

By comparing the identified AGB models with the Ba star abundances, our aim was to derive systematic deviations between the observations and models. We found that there is a systematic underproduction of some elements in the AGB models compared to the observations: in particular for Nb, Mo and Ru, which are just beyond the first s-process peak. This may imply that there is a neutron capture process (e.g. the i process) not yet included in the AGB models, which contributes to the abundance of these elements.

We investigated the correlations between the abundances, the residuals between the observationally derived and modelled abundances and the metallicity of the stars. Some correlations may be noted between the residuals of these elements, indicating a common source for the inability to fit with the models. There is a weak dependence on metallicity, which is the strongest for Nb, Mo and Ru. The offset of these three elements are also increasing with higher s-process abundances.

We have also examined the relation between the Zr and Nb abundances of these stars, since this can be used as a thermometer for the s process, assuming local nuclear equilibrium. However, the vast majority of stars would require a temperature higher than 400 MK to explain their abundances within this framework - this may also indicate that Zr itself is not produced solely in the steady-state s process.

Speaker: Blanka Világos (Konkoly Observatory (HUN-REN CSFK), Eötvös Loránd University)

15:50 Coffee break

Tuesday, 11 June

A variety of AGB stars: observational understandings