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

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-processenhanced 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.

The Production of Actinides in Neutron Star Mergers

“…III assuming BNSMs are indeed the sites enriching these stars [73]. Notwithstanding the large theoretical and even larger observational uncertainties associated with the Pb abundance that precludes a definite answer now, it may be worthwhile to examine this in future with improved theoretical understanding as well as better observational data (e.g., [117]).…”

Observations of R-Process Stars in the Milky Way and Dwarf Galaxies

“…Ever since the discovery of the first r-process star, CS22892-052, it has been shown repeatedly that the derived abundance pattern of heavy neutron-capture elements in all these stars shows a universal behavior; that is the relative log (X) abundances (defined as log 10 (X) = log 10 (N X /N H ) + 12 = [X/H] + log (X)) for elements barium and above always show the exact same distinct pattern. This is illustrated in Figure 4 for the metal-poor r-process enhanced star HD 222925 (Roederer et al, 2022). For example, two stars with different metallicities would not share the same level of enrichment (higher metallicity stars tend to have higher overall abundances including those of neutron-capture abundances) but they would share the same relative abundance pattern.…”

“…As such, it has been the long-standing benchmark star for comparisons with essentially all observational and theoretical results. However, recently, the bright star Figure 5: Residuals of stellar r-process element abundances subtracted from the solar r-process pattern (Burris et al, 2000), for a variety of r-process enhanced (r-II) stars in the halo, the brightest star in the r-process dwarf galaxy Reticulum II (Ji & Frebel, 2018), and the r-process star HD 222925 (Roederer et al, 2022). The residuals are calculated to minimize the absolute deviation for 56 ≤ Z ≤ 66 (see Ji et al 2016b).…”

“…HD 222925 was discovered to be r-process-enhanced. Using both ground and spaced based observations it was possible to measure chemical abundances of 42 heavy neutron-capture elements from 31 ≤ Z ≤ 90 (Roederer et al, 2022). This makes HD 222925 the star with the most elements measured besides the Sun.…”

“…Going forward, HD 222925 could be used as the benchmark for all further comparisons related to testing the site of the r-process, especially given the large deviations in the first r-process peak. Table 1 provides multiple different determinations of the solar r-process residual as well as the measurements of HD 222925 (Prantzos et al, 2020;Roederer et al, 2022).…”