Neutron star mergers (NSMs) are promising astrophysical sites for the rapid neutron-capture ("r") process, but can their integrated yields explain the majority of heavy-element material in the Galaxy? One method to address this question implements a forward approach that propagates NSM rates and yields along with stellar formation rates and compares those results with observed chemical abundances of r-process-rich, metal-poor stars. In this work, we take the inverse approach by utilizing r-process-element abundance ratios of metal-poor stars as input to reconstruct the properties-especially the masses-of their neutron star (NS) binary progenitors. This novel analysis provides an independent avenue for studying the population of the original NS binary systems that merged and produced the r-process material now incorporated in Galactic metal-poor halo stars. We use ratios of elements typically associated with the limited-r-process and the actinide region to those in the lanthanide region (i.e., Zr/Dy and Th/Dy) to probe the NS masses of the progenitor merger. We find that NSMs can account for all r-process material in metal-poor stars that display r-process signatures, while simultaneously reproducing the present-day distribution of double-NS systems. Notably, with our model assumptions and the studied stellar sample, we postulate that the most r-process enhanced stars (the r-II stars) on their own would require progenitor NSMs of asymmetric systems that are distinctly different from present ones in the Galaxy. We also explore variations to the model and find that the predicted degree of asymmetry is most sensitive to the electron fraction of the remnant disk wind.

Of the variations in the elemental abundance patterns of stars enhanced with r-process elements, the variation in the relative actinide-to-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are accessed in r-process sites. We find that variations between relative stellar actinide abundances is most likely astrophysical in nature, owing to how neutron-rich the ejecta from an r-process event may be. Furthermore, if an r-process site is capable of generating variations in the neutron-richness of its ejected material, then only one type of r-process site is needed to explain all levels of observed relative actinide enhancements.

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title 'The R-Process Alliance: fourth data release from the search for r-process-enhanced stars in the Galactic halo.' (bibcode: 2020ApJS..249...30H) Copyright: Refer to CDS usage

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title 'The R-Process Alliance: spectroscopic follow-up of low-metallicity star candidates from the Best & Brightest Survey.' (bibcode: 2019ApJ...870..122P) Copyright: Refer to CDS usage

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title 'The R-process Alliance: The Peculiar Chemical Abundance Pattern of RAVE J183013.5-455510' (bibcode: 2020ApJ...897...78P) Copyright: Refer to CDS usage

We present new observational benchmarks of rapid neutron-capture process (r-process) nucleosynthesis for elements at and between the first (A similar to 80) and second (A similar to 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, whose r-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 system r-process residual pattern. The Te abundances in these stars correlate more closely with the lighter r-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 third r-process-peak elements. The concept of r-process universality that is recognized among the lanthanide and third-peak elements in r-process-enhanced stars may also apply to Se, Sr, Y, Zr, Nb, Mo, and Te, provided the overall abundances of the lighter r-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 second r-process peaks.

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.

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title 'The r-process Alliance: chemodynamically tagged groups of halo r-process-enhanced stars reveal a shared chemical-evolution history.' (bibcode: 2021ApJ...908...79G) Copyright: Refer to CDS usage

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title 'The R-Process Alliance: a nearly complete r-process abundance template derived from ultraviolet spectroscopy of the r-process-enhanced metal-poor star HD222925.' (bibcode: 2022ApJS..260...27R) Copyright: Refer to CDS usage

In situ angle-dispersive x-ray diffraction (ADXD) measurement by synchrotron beam under high pressure was performed and pressure-induced amorphization (PIA) and YO6 octahedral changes were investigated for both Y2O3/Eu3+ nanotubes and the bulk sample. The cubic structure of Y2O3/Eu3+ nanotubes transforms into an amorphous phase at a pressure of 21.9 GPa. Differential nano-effects in the radial and axial directions of nanotubes causes distinct compression behaviors for Y-O bonds. The variation in Y-O bonds of nanotubes exhibits disorder with pressure unlike that of bulk sample, which instead exhibits linear decreases. The YO6 octahedra of Y2O3/Eu3+ nanotubes are deformed in disorder under high pressure which abrogates the ordered long-distance octahedral arrangement, thus resulting in the amorphous transition.