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 ⊙, Z = 0.01 Monash model we find a self-consistent solution of T LE = 25.5 Myr.

Mixing and Magnetic Fields in Asymptotic Giant Branch Stars in the Framework of FRUITY Models

In the last few years, the modeling of asymptotic giant branch (AGB) stars has been much investigated, both focusing on nucleosynthesis and stellar evolution aspects. Recent advances in the input physics required for stellar computations made it possible to construct more accurate evolutionary models, which are an essential tool to interpret the wealth of available observational and nucleosynthetic data. Motivated by such improvements, the FUNS stellar evolutionary code has been updated. Nonetheless, mixing processes occurring in AGB stars’ interiors are currently not well-understood. This is especially true for the physical mechanism leading to the formation of the 13C pocket, the major neutron source in low-mass AGB stars. In this regard, post-processing s-process models assuming that partial mixing of protons is induced by magneto-hydrodynamics processes were shown to reproduce many observations. Such mixing prescriptions have now been implemented in the FUNS code to compute stellar models with fully coupled nucleosynthesis. Here, we review the new generation of FRUITY models that include the effects of mixing triggered by magnetic fields by comparing theoretical findings with observational constraints available either from the isotopic analysis of trace-heavy elements in presolar grains or from carbon AGB stars and Galactic open clusters.

Underground Measurements of Nuclear Reaction Cross-Sections Relevant to AGB Stars

Nuclear reaction cross sections are essential ingredients to predict the evolution of AGB stars and understand their impact on the chemical evolution of our Galaxy. Unfortunately, the cross sections of the reactions involved are often very small and challenging to measure in laboratories on Earth. In this context, major steps forward were made with the advent of underground nuclear astrophysics, pioneered by the Laboratory for Underground Nuclear Astrophysics (LUNA). The present paper reviews the contribution of LUNA to our understanding of the evolution of AGB stars and related nucleosynthesis.

The Missing Lead: Developments in the Lead (Pb) Discrepancy in Intrinsically s-Process Enriched Single Post-AGB Stars

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.

Observational Evidence Points at AGB Stars as Possible Progenitors of CEMP-s and CEMP-r/s Stars*

The origin of enhanced abundance of heavy elements observed in the surface chemical composition of carbon-enhanced metal-poor (CEMP) stars still remains poorly understood. Here, we present detailed abundance analysis of seven CEMP stars based on high-resolution (R ∼ 50,000) spectra that reveal enough evidence of asymptotic giant branch (AGB) stars being possible progenitors for these objects. For the objects HE 0110−0406, HE 1425−2052, and HE 1428−1950, we present for the first time a detailed abundance analysis. Our sample is found to consist of one metal-poor ([Fe/H] < −1.0) and six very metal-poor ([Fe/H] < −2.0) stars with enhanced carbon and neutron-capture elements. We have critically analyzed the observed abundance ratios of [O/Fe], [Sr/Ba], and [hs/ls] and examined the possibility of AGB stars being possible progenitors. The abundance of oxygen estimated in the program stars is characteristic of AGB progenitors except for HE 1429−0551 and HE 1447+0102. The estimated values of [Sr/Ba] and [hs/ls] ratios also support AGB stars as possible progenitors. The locations of the program stars in the absolute carbon abundance A(C) versus [Fe/H] diagram, along with the Group I objects, hint at the binary nature of the object. We have studied the chemical enrichment histories of the program stars based on abundance ratios [Mg/C], [Sc/Mn], and [C/Cr]. Using [C/N] and 12C/13C ratios, we have examined whether any internal mixing had modified their surface chemical compositions. Kinematic analysis shows that the objects HE 0110−0406 and HE 1447+0102 are thick-disk objects and the remaining five objects belong to the halo population of the Galaxy.

Detached Shell Carbon Stars: Tracing Thermal Pulses on the Asymptotic Giant Branch

We consider whether the subset of carbon-rich asymptotic giant branch (AGB) stars that exhibit detached, expanding circumstellar shells may reveal the past histories of these stars as having undergone helium shell flashes (thermal pulses) on the AGB. We exploit newly available Gaia parallaxes and photometry, along with archival infrared photometry, to obtain refined estimates of the luminosities of all (12) known detached shell carbon stars. We examine the relationship between these luminosities and the estimated dynamical ages (ejection times) of the detached shells associated with the 12 stars, which range from ∼1000 to ∼30,000 yr. When arranged according to detached shell dynamical age, the (implied) luminosity evolution of the known detached shell carbon stars closely follows the predicted “light curves” of individual thermal pulses obtained from models of AGB stars. The comparison between data and models suggests that detached shell carbon stars are descended from ∼2.5 to 4.0 M ⊙ progenitors. We conclude that detached shell carbon stars may serve as effective tracers of the luminosity evolution of AGB thermal pulses.

Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3−2 emission around the oxygen-rich AGB star W Hya. Results. We find CO maser emission amplifying the stellar continuum with an optical depth τ ≈−0.55. The maser predominantly amplifies the limb of the star because CO J = 3−2 absorption from the extended stellar atmosphere is strongest towards the centre of the star. Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H2O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 μm and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 μm. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.

AGB Stars and Their Circumstellar Envelopes: An Operative Approach to Computing Their Atmospheres

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.