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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The R -Process Alliance: detailed chemical composition of an r -process enhanced star with UV and optical spectroscopy
We present a detailed chemical-abundance analysis of a highly r-process-enhanced (RPE) star, 2MASS J00512646-1053170, using high-resolution spectroscopic observations with Hubble Space Telescope/STIS in the UV and Magellan/MIKE in the optical. We determined abundances for 41 elements in total, including 23 r-process elements and rarely probed species such as Al ii, Ge i, Mo ii, Cd i, Os ii, Pt i, and Au i. We find that [Ge/Fe] = +0.10, which is an unusually high Ge enhancement for such a metal-poor star and indicates contribution from a production mechanism decoupled from that of Fe. We also find that this star has the highest Cd abundance observed for a metal-poor star to date. We find that the dispersion in the Cd abundances of metal-poor stars can be explained by the correlation of Cd i abundances with the stellar parameters of the stars, indicating the presence of NLTE effects. We also report that this star is now only the sixth star with Au abundance determined. This result, along with abundances of Pt and Os, uphold the case for the extension of the universal r-process pattern to the third r-process peak and to Au. This study adds to the sparse but growing number of RPE stars with extensive chemical-abundance inventories and highlights the need for not only more abundance determinations of these rarely probed species, but also advances in theoretical NLTE and astrophysical studies to reliably understand the origin of r-process elements.
more » « less- NSF-PAR ID:
- 10495820
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 529
- Issue:
- 3
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 1917-1940
- Size(s):
- p. 1917-1940
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Abstract The ages of the oldest stars shed light on the birth, chemical enrichment, and chemical evolution of the universe. Nucleocosmochronometry provides an avenue to determining the ages of these stars independent from stellar-evolution models. The uranium abundance, which can be determined for metal-poor
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ABSTRACT We present a comprehensive analysis of the detailed chemical abundances for a sample of 11 metal-poor, very metal-poor, and extremely metal-poor stars ([Fe/H] = −1.65 to [Fe/H] = −3.0) as part of the HESP-GOMPA (Galactic survey Of Metal Poor stArs) survey. The abundance determinations encompass a range of elements, including C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, and Ba, with a subset of the brighter objects allowing for the measurement of additional key elements. Notably, the abundance analysis of a relatively bright highly r-process-enhanced (r-II) star (SDSS J0019+3141) exhibits a predominantly main r-process signature and variations in the lighter r-process elements. Moreover, successful measurements of thorium in this star facilitate stellar age determinations. We find a consistent odd–even nucleosynthesis pattern in these stars, aligning with expectations for their respective metallicity levels, thus implicating Type II supernovae as potential progenitors. From the interplay between the light and heavy r-process elements, we infer a diminishing relative production of light r-process elements with increasing Type II supernova contributions, challenging the notion that Type II supernovae are the primary source of these light r-process elements in the early Milky Way. A chemodynamical analysis based on Gaia astrometric data and our derived abundances indicates that all but one of our program stars are likely to be of accreted origin. Additionally, our examination of α-poor stars underscores the occurrence of an early accretion event from a satellite on a prograde orbit, similar to that of the Galactic disc.
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Context. In recent years, theR -Process Alliance (RPA) has conducted a successful search for stars that are enhanced in elements produced by the rapid neutron-capture (r -)process. In particular, the RPA has uncovered a number of stars that are strongly enriched in lightr -process elements, such as Sr, Y, and Zr. These so-called limited-r stars were investigated to explore the astrophysical production site(s) of these elements.Aims. We investigate the possible formation sites for light neutron-capture elements by deriving detailed abundances for neutron-capture elements from high-resolution spectra with a high signal-to-noise ratio of three limited-r stars.Methods. We conducted a kinematic analysis and a 1D local thermodynamic equilibrium spectroscopic abundance analysis of three stars. Furthermore, we calculated the lanthanide mass fraction (X La) of our stars and of limited-r stars from the literature.Results. We found that the abundance pattern of neutron-capture elements of limited-r stars behaves differently depending on their [Ba/Eu] ratios, and we suggest that this should be taken into account in future investigations of their abundances. Furthermore, we found that theX Laof limited-r stars is lower than that of the kilonova AT2017gfo. The latter seems to be in the transition zone between limited-rX Laand that ofr -I andr -II stars. Finally, we found that unliker -I andr -II stars, the current sample of limited-r stars is largely born in the Galaxy and is not accreted.
