skip to main content


Search for: All records

Creators/Authors contains: "Thielemann, F-K"

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Context. The oldest stars in the Milky Way are metal-poor with [Fe/H] < −1.0, displaying peculiar elemental abundances compared to solar values. The relative variations in the chemical compositions among stars is also increasing with decreasing stellar metallicity, allowing for the pure signature of unique nucleosynthesis processes to be revealed. The study of ther-process is, for instance, one of the main goals of stellar archaeology and metal-poor stars exhibit an unexpected complexity in the stellar production of ther-process elements in the early Galaxy.

    Aims. In this work, we report the atmospheric parameters, main dynamic properties, and the abundances of four metal-poor stars: HE 1523-0901, HD 6268, HD 121135, and HD 195636 (−1.5 > [Fe/H] > −3.0).

    Methods. The abundances were derived from spectra obtained with the HRS echelle spectrograph at the Southern African Large Telescope, using both local and non-local thermodynamic equilibrium (LTE and NLTE) approaches, with the average error between 0.10 and 0.20 dex.

    Results. Based on their kinematical properties, we show that HE 1523-0901 and HD 195636 are halo stars with typical high velocities. In particular, HD 121135 displays a peculiar kinematical behaviour, making it unclear whether it is a halo or an accreted star. Furthermore, HD 6268 is possibly a rare prototype of very metal-poor thick disk stars. The abundances derived for our stars are compared with theoretical stellar models and with other stars with similar metallicity values from the literature.

    Conclusions. HD 121135 is Al-poor and Sc-poor, compared to stars observed in the same metallicity range (−1.62 > [Fe/H] > −1.12). The most metal-poor stars in our sample, HE 1523-0901, HD 6268, and HD 195636, exhibit anomalies that are better explained by supernova models from fast-rotating stellar progenitors for elements up to the Fe group. Compared to other stars in the same metal-licity range, their common biggest anomaly is represented by the low Sc abundances. If we consider the elements beyond Zn, HE 1523-0901 can be classified as an r-II star, HD 6268 as an r-I candidate, and HD 195636 and HD 121135 exhibiting a borderliner-process enrichment between limited-r and r-I star. Significant relative differences are observed between the r-process signatures in these stars.

     
    more » « less
    Free, publicly-accessible full text available July 1, 2025
  2. Abstract

    We present the state-of-the-art single-zone nuclear reaction networkWinNet, which is capable of calculating the nucleosynthetic yields of a large variety of astrophysical environments and conditions. This ranges from the calculation of the primordial nucleosynthesis, where only a few nuclei are considered, to the ejecta of neutron star mergers with several thousands of involved nuclei. Here we describe the underlying physics and implementation details of the reaction network. We additionally present the numerical implementation of two different integration methods, the implicit Euler method and Gears method, along with their advantages and disadvantages. We furthermore describe basic example cases of thermodynamic conditions that we provide together with the network and demonstrate the reliability of the code by using simple test cases. With this publication,WinNetwill be publicly available and open source at GitHub and Zenodo.

     
    more » « less
  3. Aims. Various nucleosynthesis studies have pointed out that the r -process elements in very metal-poor (VMP) halo stars might have different origins. By means of familiar concepts from statistics (correlations, cluster analysis, and rank tests of elemental abundances), we look for causally correlated elemental abundance patterns and attempt to link them to astrophysical events. Some of these events produce the r -process elements jointly with iron, while others do not have any significant iron contribution. We try to (a) characterize these different types of events by their abundance patterns and (b) identify them among the existing set of suggested r -process sites. Methods. The Pearson and Spearman correlation coefficients were used in order to investigate correlations among r -process elements (X,Y) as well as their relation to iron (Fe) in VMP halo stars. We gradually tracked the evolution of those coefficients in terms of the element enrichments [X/Fe] or [X/Y] and the metallicity [Fe/H]. This approach, aided by cluster analysis to find different structures of abundance patterns and rank tests to identify whether several events contributed to the observed pattern, is new and provides deeper insights into the abundances of VMP stars. Results. In the early stage of our Galaxy, at least three r -process nucleosynthesis sites have been active. The first two produce and eject iron and the majority of the lighter r -process elements. We assign them to two different types of core-collapse events, not identical to regular core-collapse supernovae (CCSNe), which produce only light trans-Fe elements. The third category is characterized by a strong r -process and is responsible for the major fraction of the heavy main r -process elements without a significant coproduction of Fe. It does not appear to be connected to CCSNe, in fact most of the Fe found in the related r -process enriched stars must come from previously occurring CCSNe. The existence of actinide boost stars indicates a further division among strong r -process sites. We assign these two strong r -process sites to neutron star mergers without fast black hole formation and to events where the ejecta are dominated by black hole accretion disk outflows. Indications from the lowest-metallicity stars hint at a connection with massive single stars (collapsars) forming black holes in the early Galaxy. 
    more » « less
  4. ABSTRACT

    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 Å (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].

     
    more » « less