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The abundance scatter of heavier r-process elements ( Z ≥ 56 ) relative to Fe ([r/Fe]) in metal-poor stars preserves excellent information of the star formation history and provides important insights into the various situations of the Galactic chemical enrichment. In this respect, the upper and lower boundaries of [r/Fe] could present useful clues for investigating the extreme situations of the star formation history and the early Galactic chemical evolution. In this paper, we investigate the formation of the upper and lower boundaries of [r/Fe] for the gas clouds. We find that, for a cloud from which metal-poor stars formed, the formation of the upper limits of [r/Fe] is mainly due to the pollution from a single main r-process event. For a cloud from which metal-poor stars formed, the formation of the lower limits of [r/Fe] is mainly due to the pollution from a single SN II event that ejects primary Fe.

In this review, we discuss the impact of s-process nucleosynthesis in asymptotic giant branch stars on the enrichment of heavy elements. We review the main steps made on this subject in the last 40 years and discuss the importance of modelling the evolution of the abundances of such elements in our Milky Way. From the comparison between model results and observations, we can impose strong constraints on stellar nucleosynthesis, as well as on the evolution of the Milky Way.

We present an analysis of the relative abundance features of a number of chemical elements in the atmospheres of metal-rich ( ) Galactic-field RR Lyrae variable stars and the kinematic characteristics of these stars. We have previously shown that the relative abundances of some elements: magnesium, silicon, calcium, and to a greater extent of titanium, as well as yttrium and scandium in such stars are lower than in most other types of stars bearing similar metallicity. It is found here that some of these metal-rich RR Lyrae stars also have very low relative abundances of sodium, aluminum, and nickel. The orbital parameters of all the metal-rich RR Lyrae variables studied in this paper are typical of the Galactic thin or thick disk objects, however, the unusual chemical composition let us to suggest a possible extragalactic origin for some of them.

We briefly describe the large observational project MAGIC aimed at mass spectroscopic studies of stars of different subsystems of our Galaxy. The project was carried out on the Southern African Large Telescope (hereafter referred to as SALT) with efficient use of its capabilities, site’s climate, and spectroscopic instruments. We discuss in detail the issues of high-resolution spectroscopy when studying the chemical composition of Galactic Cepheids. Observations of these objects with HRS echelle spectrograph of SALT telescope started in 2016 and currently the number of such observations exceeds one hundred. Spectra were acquired in the medium-resolution mode (R ~36 500–39 000)with a high signalto-noise ratio (S/N ~50–220). All obtained echelle images acquired were reduced with a package that we developed based on standard MIDAS system. We describe standard reduction steps using the Cepheid RSNor as a test object. Based on the data of spectroscopic observations we determined the atmospheric parameters and the abundances of 31 chemical elements (36 neutral atoms and ions) whose absorption lines are observed in the spectrum of RSNor. We performed all computations using two methods and showed that echelle spectra acquired in the medium-resolution mode of HRS can be used to study the chemical composition of Cepheids with good accuracy and that the results agree with the results of other studies within the quoted errors.

Among the main science cases that have motivated the proposal of CUBES, a new high-resolution spectrograph for the Very Large Telescope at the European Southern Observatory, there is the study and the characterisation of the nucleosynthesis of beryllium. Classical novae have been proposed since the ’70s as one of the main factories of lithium in the Galaxy, but this hypothesis has been demonstrated on empirical basis only recently thanks to the direct identification of lithium in V1369 Cen and through the observations of the resonance transition of 7 Be II, the 7 Li parent, at 313.0 nm in the near-UV range. CUBES is then the ideal instrument to quantify the amount of 7 Be and therefore of 7 Li produced by the different novae types hosted in the different components of the Milky Way and also in its nearby satellite galaxies. As important by-product of high resolution spectroscopic observations obtained with CUBES, there are the study of the properties of nova ejecta abundances, the shocks evolution in novae and their connection with the high-energy emission observed in these transients, from satellites as Fermi and Swift.

A major source of uncertainty in AGB models is the partial-mixing process of hydrogen, required for the formation of the so-called 13C pocket. Among the attempts to derive a self-consistent treatment of this physical process, there are 2D and 3D simulations of magnetic buoyancy. The 13C pocket resulting from mixing induced by magnetic buoyancy extends over a region larger than those so far assumed, showing an almost flat 13C distribution and a negligible amount of 14N. Recently, it has been proved to be a good candidate to match the records of isotopic abundance ratios of s-elements in presolar SiC grains. However, up to date such a magnetic mixing has been applied in post-process calculations only, being never implemented in a stellar evolutionary code. Here we present new stellar models, performed with the 1-d hydrostatic FUNS evolutionary code, which include magnetic buoyancy. We comment the resulting s-process distributions and show preliminary comparisons to spectroscopic observations and pre-solar grains measurements.

The three most iron-poor stars known until now are also known to have peculiar enhancements of intermediate mass elements. Under the assumption that these iron-deficient stars reveal the nucleosynthesis result of Pop III stars, we show that a weak supernova model successfully reproduces the observed abundance patterns. Moreover, we show that the initial parameters of the progenitor, such as the initial masses and the rotational property, can be constrained by the model, since the stellar yields result from the nucleosynthesis in the outer region of the star, which is significantly affected by the initial parameters. The initial parameter of Pop III stars is of prime importance for the theoretical study of the early universe. Future observation will increase the number of such carbon enhanced iron-deficient stars, and the same analysis on the stars may give valuable information for the Pop III stars that existed in our universe.

The element boron belongs, together with lithium and beryllium, to a known trio of important elements for the study of evolutionary processes in low mass stars. Because B is the least fragile of this trio to be destroyed in the stellar interiors, it can be used to test if the Li enrichment is of planetary origin. Here, for the first time, boron lines are examined in the UV for four giants with different degrees of large Li enrichment by means of observations with the Hubble telescope. Two main results are found in our study. One is that to first approximation B abundances appear not to be in excess, invalidating the planet engulfment mechanism. The second one is that the two stars with very large Li abundances present emission lines indicating that quite strong active chromospheres are acting in these very Li-rich giants. These new results obtained from the UV complement our recent studies in the mid-IR (de la Reza et al. 2015) where strong emission-line features of organic material were found in the spectra of some Li-rich stars.

The earliest phases of Galactical chemical evolution and nucleosynthesis can be investigated by studying the old metal-poor stars. It has been recognized that a large fraction of metal-poor stars possess significant over-abundances of carbon relative to iron. Here we present the results of a 23-star homogeneously analyzed sample of metal-poor candidates from the Hamburg/ESO survey. We have derived abundances for a large number of elements ranging from Li to Pb. The sample includes four ultra metal-poor stars ([Fe/H] < −4.0), six CEMP-no stars, five CEMP-s stars, two CEMP-r stars and two CEMP-r/s stars. This broad variety of the sample stars gives us an unique opportunity to explore different abundance patterns at low metallicity.

We review some recent developments in theoretical studies on the connection between the progenitor systems of Type Ia supernovae (SNe Ia) and the explosion mechanisms. (1) DD-subCh: In the merging of double C+O white dwarfs (DD scenario), if the carbon detonation is induced near the white dwarf (WD) surface in the early dynamical phase, it could result in the (effectively) sub-Chandrasekhar mass explosion. (2) DD-Ch: If no surface C-detonation is ignited, the WD could grow until the Chandrasekhar mass is reached, but the outcome depends on whether the quiescent carbon shell burning is ignited and burns C+O into O+Ne+Mg. (3) SD-subCh: In the single degenerate (SD) scenario, if the He shell-flashes grow strong to induce a He detonation, it leads to the sub-Chandra explosion. (4) SD-Ch: If the He-shell flashes are not strong enough, they still produce interesting amounts of Si and S near the surface of the C+O WD before the explosion. In the Chandra mass explosion, the central density is high enough to produce electron capture elements, e.g., stable 58Ni. Observations of the emission lines of Ni in the nebular spectra provides useful diagnostics of the sub-Chandra vs. Chandra issue. The recent observations of relatively low velocity carbon near the surface of SNe Ia provide also an interesting constraint on the explosion models.