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1. Correlations of r -process elements in very metal-poor stars as clues to their nucleosynthesis sites

   https://doi.org/10.1051/0004-6361/202141038  

   Farouqi, K. ; Thielemann, F.-K. ; Rosswog, S. ; Kratz, K.-L. ( July 2022 , Astronomy & Astrophysics)

   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. 

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2. Decoding the compositions of four bright r -process-enhanced stars

   https://doi.org/10.1093/mnras/stad2206  

   Saraf, Pallavi ; Prieto, Carlos Allende ; Sivarani, Thirupathi ; Bandyopadhyay, Avrajit ; Beers, Timothy C ; Susmitha, A ( July 2023 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT There has been a concerted effort in recent years to identify the astrophysical sites of the r-process that can operate early in the galaxy. The discovery of many r-process-enhanced (RPE) stars (especially by the R-process Alliance collaboration) has significantly accelerated this effort. However, only limited data exist on the detailed elemental abundances covering the primary neutron-capture peaks. Subtle differences in the structure of the r-process pattern, such as the relative abundances of elements in the third peak, in particular, are expected to constrain the r-process sites further. Here, we present a detailed elemental-abundance analysis of four bright RPE stars selected from the HESP–GOMPA survey. Observations were carried out with the 10-m class telescope Gran Telescopio Canarias (GTC), Spain. The high spectral signal-to-noise ratios obtained allow us to derive abundances for 20 neutron-capture elements, including the third r-process peak element osmium (Os). We detect thorium (Th) in two stars, which we use to estimate their ages. We discuss the metallicity evolution of Mg, Sr, Ba, Eu, Os, and Th in r-II and r-I stars, based on a compilation of RPE stars from the literature. The strontium (Sr) abundance trend with respect to europium (Eu) suggests the need for an additional production site for Sr (similar to several earlier studies); this requirement could be milder for yttrium (Y) and zirconium (Zr). We also show that there could be some time delay between r-II and r-I star formation, based on the Mg/Th abundance ratios. 

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3. Neutron star mergers and their impact on second generation star formation in the early universe

   https://doi.org/10.1093/mnras/stae397  

   Skinner, Danielle ; Wise, John H. ( February 2024 , Monthly Notices of the Royal Astronomical Society)

   The exact evolution of elements in the Universe, from primordial to heavier elements produced via the r-process, is still under scrutiny. The supernova deaths of the very first stars led to the enrichment of their local environments, and can leave behind neutron stars (NSs) as remnants. These remnants can end up in binary systems with other NSs, and eventually merge, allowing for the r-process to occur. We study the scenario where a single NS merger (NSM) enriches a halo early in its evolution to understand the impact on the second generation of stars and their metal abundances. We perform a suite of high-resolution cosmological zoom-in simulations using enzo where we have implemented a new NSM model varying the explosion energy and the delay time. In general, an NSM leads to significant r-process enhancement in the second generation of stars in a galaxy with a stellar mass of ∼105 M⊙ at redshift 10. A high explosion energy leads to a Population II (Pop II) mass fraction of 72 per cent being highly enhanced with r-process elements, while a lower explosion energy leads to 80 per cent being enhanced, but only 14 per cent being highly enhanced. When the NSM has a short delay time of 10 Myr, only 5 per cent of the mass fraction of Pop II stars is highly enhanced, while 64 per cent is highly enhanced for the longest delay time of 100 Myr. This work represents a stepping stone towards understanding how NSMs impact their environments and the metal abundances of descendant generations of stars.

    

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4. Origin of Plutonium-244 in the Early Solar System

   https://doi.org/10.3390/universe8070343  

   Lugaro, Maria ; Yagüe López, Andrés ; Soós, Benjámin ; Côté, Benoit ; Pető, Mária ; Vassh, Nicole ; Wehmeyer, Benjamin ; Pignatari, Marco ( July 2022 , Universe)

   We investigate the origin in the early Solar System of the short-lived radionuclide 244Pu (with a half life of 80 Myr) produced by the rapid (r) neutron-capture process. We consider two large sets of r-process nucleosynthesis models and analyse if the origin of 244Pu in the ESS is consistent with that of the other r and slow (s) neutron-capture process radioactive nuclei. Uncertainties on the r-process models come from both the nuclear physics input and the astrophysical site. The former strongly affects the ratios of isotopes of close mass (129I/127I, 244Pu/238U, and 247Pu/235U). The 129I/247Cm ratio, instead, which involves isotopes of a very different mass, is much more variable than those listed above and is more affected by the physics of the astrophysical site. We consider possible scenarios for the evolution of the abundances of these radioactive nuclei in the galactic interstellar medium and verify under which scenarios and conditions solutions can be found for the origin of 244Pu that are consistent with the origin of the other isotopes. Solutions are generally found for all the possible different regimes controlled by the interval (δ) between additions from the source to the parcel of interstellar medium gas that ended up in the Solar System, relative to decay timescales. If r-process ejecta in interstellar medium are mixed within a relatively small area (leading to a long δ), we derive that the last event that explains the 129I and 247Cm abundances in the early Solar System can also account for the abundance of 244Pu. Due to its longer half life, however, 244Pu may have originated from a few events instead of one only. If r-process ejecta in interstellar medium are mixed within a relatively large area (leading to a short δ), we derive that the time elapsed from the formation of the molecular cloud to the formation of the Sun was 9-16 Myr. 

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5. The detailed chemical abundance patterns of accreted halo stars from the optical to infrared

   https://doi.org/10.1093/mnras/stac518  

   Carrillo, Andreia ; Hawkins, Keith ; Jofré, Paula ; de Brito Silva, Danielle ; Das, Payel ; Lucey, Madeline ( April 2022 , Monthly Notices of the Royal Astronomical Society)

   Understanding the assembly of our Galaxy requires us to also characterize the systems that helped build it. In this work, we accomplish this by exploring the chemistry of accreted halo stars from Gaia-Enceladus/Gaia-Sausage (GES) selected in the infrared from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16. We use high resolution optical spectra for 62 GES stars to measure abundances in 20 elements spanning the α, Fe-peak, light, odd-Z, and notably, the neutron-capture groups of elements to understand their trends in the context of and in contrast to the Milky Way and other stellar populations. Using these derived abundances we find that the optical and the infrared abundances agree to within 0.15 dex except for O, Co, Na, Cu, and Ce. These stars have enhanced neutron-capture abundance trends compared to the Milky Way, and their [Eu/Mg] and neutron-capture abundance ratios (e.g. [Y/Eu], [Ba/Eu], [Zr/Ba], [La/Ba], and [Nd/Ba]) point to r-process enhancement and a delay in s-process enrichment. Their [α/Fe] trend is lower than the Milky Way trend for [Fe/H] > −1.5 dex, similar to previous studies of GES stars and consistent with the picture that these stars formed in a system with a lower rate of star formation. This is further supported by their depleted abundances in Ni, Na, and Cu abundances, again, similar to previous studies of low-α stars with accreted origins.

    

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