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-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.

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

“…Elements at the three r-process peaks are readily detectable in UV spectra (λ < 3100 Å) collected using spectrographs on the Hubble Space Telescope. These elements include Se at the first peak (e.g., Roederer 2012;Roederer & Lawler 2012;Roederer et al 2014b;Peterson et al 2020); Te at the second peak (e.g., Roederer et al 2012aRoederer et al , 2012bRoederer et al , 2016aRoederer et al , 2022; and osmium (Os, Z = 76), iridium (Ir, Z = 77), and platinum (Pt, Z = 78) at the third peak (e.g., Cowan et al 1996Cowan et al , 2005Sneden et al 1998;Den Hartog et al 2005;Barbuy et al 2011). Se and Te are more difficult to detect than Os, Ir, or Pt, because the Se I and Te I lines are found at shorter UV wavelengths (2000 < λ < 2400 Å) where longer exposure times are required to acquire sufficient signal-to-noise ratios.…”

“…1. We update the abundances to a common gf log( ) scale, with references given in Roederer et al (2022).…”

“…Old stars, on the other hand, retain heavy-element signatures dominated by individual r-process events. For example, 63 metals, including 42 r-process elements, have been detected in the metal-poor star HD 222925 (Roederer et al 2018(Roederer et al , 2022, which presents the most complete chemical inventory known for any object beyond the solar system.…”

“…Here we contribute to this debate by exploring the star-tostar dispersion among the abundances of 12 lighter r-process elements. Our analysis begins with selenium (Se, Z = 34), the lightest element with a substantial r-process contribution (Roederer et al 2022), and concludes at tellurium (Te, Z = 52), the heaviest element detectable with a mass less than barium (Ba, Z = 56) and the lanthanides. We introduce our stellar sample and present the abundances from the literature in Section 2.…”

The production of actinides in neutron star mergers

“…In particular, it may be used to constrain the amount of αdecay nuclei relevant for kilonova discussed in Section 3 assuming BNSMs are indeed the sites enriching these stars [94]. 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., [138]).…”