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1. The R-process Alliance: A Nearly Complete R-process Abundance Template Derived from Ultraviolet Spectroscopy of the R-process-enhanced Metal-poor Star HD 222925*

   https://doi.org/10.3847/1538-4365/ac5cbc  

   Roederer, Ian U. ; Lawler, James E. ; Den Hartog, Elizabeth A. ; Placco, Vinicius M. ; Surman, Rebecca ; Beers, Timothy C. ; Ezzeddine, Rana ; Frebel, Anna ; Hansen, Terese T. ; Hattori, Kohei ; et al ( June 2022 , The Astrophysical Journal Supplement Series)

   Abstract We present a nearly complete rapid neutron-capture process ( r -process) chemical inventory of the metal-poor ([Fe/H] = −1.46 ± 0.10) r -process-enhanced ([Eu/Fe] = +1.32 ± 0.08) halo star HD 222925. This abundance set is the most complete for any object beyond the solar system, with a total of 63 metals detected and seven with upper limits. It comprises 42 elements from 31 ≤ Z ≤ 90, including elements rarely detected in r -process-enhanced stars, such as Ga, Ge, As, Se, Cd, In, Sn, Sb, Te, W, Re, Os, Ir, Pt, and Au. We derive these abundances from an analysis of 404 absorption lines in ultraviolet spectra collected using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and previously analyzed optical spectra. A series of appendices discusses the atomic data and quality of fits for these lines. The r -process elements from Ba to Pb, including all elements at the third r -process peak, exhibit remarkable agreement with the solar r -process residuals, with a standard deviation of the differences of only 0.08 dex (17%). In contrast, deviations among the lighter elements from Ga to Te span nearly 1.4 dex, and they show distinct trends from Ga to Se, Nb through Cd, and In through Te. The r -process contribution to Ga, Ge, and As is small, and Se is the lightest element whose production is dominated by the r -process. The lanthanide fraction, log X La = −1.39 ± 0.09, is typical for r -process-enhanced stars and higher than that of the kilonova from the GW170817 neutron-star merger event. We advocate adopting this pattern as an alternative to the solar r -process-element residuals when confronting future theoretical models of heavy-element nucleosynthesis with observations. 

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2. r-Process elements from magnetorotational hypernovae

   https://doi.org/10.1038/s41586-021-03611-2  

   Yong, D. ; Kobayashi, C. ; Da Costa, G. S. ; Bessell, M. S. ; Chiti, A. ; Frebel, A. ; Lind, K. ; Mackey, A. D. ; Nordlander, T. ; Asplund, M. ; et al ( July 2021 , Nature)

   null (Ed.)
3. New Highly r-Process-Enhanced Halo Stars

   https://doi.org/10.1017/S1743921317006561  

   Beers, Timothy C. ; Holmbeck, Erika M. ; Placco, Vinicius M. ; Hansen, Terese T. ; Simon, Joshua D. ; Thompson, Ian ; Frebel, Anna ; Sakari, Charli M. ( July 2017 , Proceedings of the International Astronomical Union)

   Abstract There are roughly 25 very metal-poor (VMP; [Fe/H] < -2.0), highly r-process-enhanced (‘r-II’; [Eu/Fe] > + 1.0) stars currently known, discovered over the past 20+ years. These stars provide nearly pure signatures of r-process events early in the Galactic history. We are conducting a high-resolution follow-up survey of RAVE and other bright targets to identify a total of > 100 r-II stars. Our pilot runs on the du Pont 2.5-m at Las Campanas Observatory and the ARC 3.5-m at Apache Point Observatory have already identified up to fourteen new r-II stars. We are continuing our high-resolution follow-up efforts to constrain the astrophysical site(s) and nature of the r-process. 

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4. r-Process Contributions to Low-Metallicity Stars

   https://doi.org/10.1051/epjconf/202226009002  

   Thielemann, Friedrich-K. ; Farouqi, Khalil ; Rosswog, Stephan ; Kratz, Karl-Ludwig ( January 2022 , EPJ Web of Conferences)

   Liu, W. ; Wang, Y. ; Guo, B. ; Tang, X. ; Zeng, S. (Ed.)

   Various nucleosynthesis studies have pointed out that the rapid neutron capture r-process elements in very metal-poor (VMP) halo stars might have different origins. It has been known that an r-process can either be obtained in neutron-rich low Ye conditions or in high entropy environments [see e.g. 1–5], an overview over many investigations has appeared recently [6]. In the present article we analyze with statistical methods the observational abundance patterns from trans-Fe elements up to the actinides and come to the conclusion that four to five categories of astrophysical events must have contributed. These include the ejection of Fe and trans-Fe elements Sr, Y, Zr (continuing possibly beyond to slightly higher mass numbers) in category 0 events (hereafter "C0"), Fe and weak r-process contributions (including Eu in moderate to slightly larger but varying amounts) in CI and CII events, strong r-process abundance patterns with no or negligible (in comparison to solar) Fe production in CIIIa and CIIIb events, where category CIIIb shows a tendency for an actinide boost behavior. When comparing these categories with presently existing nucleosynthesis predictions, we suggest to identify them (despite remaining uncertainties) with regular core-collapse supernovae, quark deconfinement supernovae, magneto-rotational supernovae, neutron star mergers, and outflows from black hole accretion tori. 

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5. The R-Process Alliance: Abundance Universality among Some Elements at and between the First and Second R-Process Peaks*

   https://doi.org/10.3847/1538-4357/ac85bc  

   Roederer, Ian U. ; Cowan, John J. ; Pignatari, Marco ; Beers, Timothy C. ; Den Hartog, Elizabeth A. ; Ezzeddine, Rana ; Frebel, Anna ; Hansen, Terese T. ; Holmbeck, Erika M. ; Mumpower, Matthew R. ; et al ( September 2022 , The Astrophysical Journal)

   We present new observational benchmarks of rapid neutron-capture process (r-process) nucleosynthesis for elements at and between the first (A∼ 80) and second (A∼ 130) peaks. Our analysis is based on archival ultraviolet and optical spectroscopy of eight metal-poor stars with Se (Z= 34) or Te (Z= 52) detections, whoser-process enhancement varies by more than a factor of 30 (−0.22 ≤ [Eu/Fe] ≤ +1.32). We calculate ratios among the abundances of Se, Sr through Mo (38 ≤Z≤ 42), and Te. These benchmarks may offer a new empirical alternative to the predicted solar systemr-process residual pattern. The Te abundances in these stars correlate more closely with the lighterr-process elements than the heavier ones, contradicting and superseding previous findings. The small star-to-star dispersion among the abundances of Se, Sr, Y, Zr, Nb, Mo, and Te (≤0.13 dex, or 26%) matches that observed among the abundances of the lanthanides and thirdr-process-peak elements. The concept ofr-process universality that is recognized among the lanthanide and third-peak elements inr-process-enhanced stars may also apply to Se, Sr, Y, Zr, Nb, Mo, and Te, provided the overall abundances of the lighterr-process elements are scaled independently of the heavier ones. The abundance behavior of the elements Ru through Sn (44 ≤Z≤ 50) requires further study. Our results suggest that at least one relatively common source in the early Universe produced a consistent abundance pattern among some elements spanning the first and secondr-process peaks.

    

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