Abstract
The recent detection of a binary neutron star merger and the clear evidence of the decay of radioactive material observed in this event have, after 60 years of effort, provided an ...astrophysical site for the rapid neutron-capture (
r
-) process which is responsible for the production of the heaviest elements in our universe. However, observations of metal-poor stars with highly enhanced
r
-process elements have revealed abundance patterns suggesting that multiple sites may be involved. To address this issue, and to advance our understanding of the
r
-process, we have initiated an extensive search for bright (
V
< 13.5), very metal-poor (Fe/H < −2) stars in the Milky Way halo exhibiting strongly enhanced
r
-process signatures. This paper presents the first sample collected in the southern hemisphere using the echelle spectrograph on du Pont 2.5 m telescope at Las Campanas Observatory. We have observed and analyzed 107 stars with −3.13 < Fe/H < −0.79. Of those, 12 stars are strongly enhanced in heavy
r
-process elements (
r
-II), 42 stars show moderate enhancements of heavy
r
-process material (
r
-I), and 20 stars exhibit low abundances of the heavy
r
-process elements and higher abundances of the light
r
-process elements relative to the heavy ones (limited-
r
). This search is more successful at finding
r
-process-enhanced stars compared to previous searches, primarily due to a refined target selection procedure that focuses on red giants.
Abstract While it is now known that the mergers of double neutron star binary systems (NSMs) are copious producers of heavy elements, there remains much speculation about whether they are the sole or ...even principal site of rapid neutron-capture ( r -process) nucleosynthesis, one of the primary ways in which heavy elements are produced. The occurrence rates, delay times, and galactic environments of NSMs hold sway over estimating their total contribution to the elemental abundances in the solar system and the Galaxy. Furthermore, the expected elemental yields of NSMs may depend on the merger parameters themselves—such as their stellar masses and radii—which are not currently considered in many galactic chemical evolution models. Using the characteristics of the observed sample of double neutron star (DNS) systems in the Milky Way as a guide, we predict the expected nucleosynthetic yields that a population of DNSs would produce upon merger, and we compare that nucleosynthetic signature to the heavy-element abundance pattern of solar system elements. We find that with our current models, the present DNS population favors the production of lighter r -process elements, while underproducing the heaviest elements relative to the solar system. This inconsistency could imply an additional site for the heaviest elements or a population of DNSs much different from that observed today.
The rapid neutron-capture ("r-") process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process ...nucleosynthesis can reproduce abundances derived from observations with varying success, but so far they fail to account for the observed overenhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger (NSM) and explore whether varying levels of neutron-richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, β-decay, and mass model. For most cases, the actinides are overproduced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of an NSM are likely production sites for the formation of actinides, a significant contribution from another site or sites (e.g., the NSM accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars.
Abstract
With the most trans-iron elements detected of any star outside the solar system, HD 222925 represents the most complete chemical inventory among metal-poor stars enhanced with elements made ...by the rapid neutron capture (“
r
”) process. As such, HD 222925 may be a new “template” for the observational
r
-process, where before the (much higher-metallicity) solar
r
-process residuals were used. In this work, we test under which conditions a single site accounts for the entire elemental
r
-process abundance pattern of HD 222925. We found that several of our tests—with the single exception of the black hole–neutron star merger case—challenge the single-site assumption by producing an ejecta distribution that is highly constrained, in disagreement with simulation predictions. However, we found that ejecta distributions that are more in line with simulations can be obtained under the condition that the nuclear data near the second
r
-process peak are changed. Therefore, for HD 222925 to be a canonical
r
-process template likely as a product of a single astrophysical source, the nuclear data need to be reevaluated. The new elemental abundance pattern of HD 222925—including the abundances obtained from space-based, ultraviolet (UV) data—call for a deeper understanding of both astrophysical
r
-process sites and nuclear data. Similar UV observations of additional
r
-process–enhanced stars will be required to determine whether the elemental abundance pattern of HD 222925 is indeed a canonical template (or an outlier) for the
r
-process at low metallicity.
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.
We report the discovery of a new actinide-boost star, 2MASS J09544277+5246414, originally identified as a very bright (V = 10.1), extremely metal-poor (Fe/H = −2.99) K giant in the LAMOST survey, and ...found to be highly r-process-enhanced (r-II; Eu/Fe = +1.28), during the snapshot phase of the R-Process Alliance (RPA). Based on a high signal-to-noise ratio (S/N), high-resolution spectrum obtained with the Harlan J. Smith 2.7 m telescope, this star is the first confirmed actinide-boost star found by RPA efforts. With an enhancement of Th/Eu = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhanced r-II star yet discovered, and only the sixth metal-poor star with a measured uranium abundance (U/Fe = +1.40). Using the Th/U chronometer, we estimate an age of 13.0 4.7 Gyr for this star. The unambiguous actinide-boost signature of this extremely metal-poor star, combined with additional r-process-enhanced and actinide-boost stars identified by the RPA, will provide strong constraints on the nature and origin of the r-process at early times.
Abstract
We derive dynamical parameters for a large sample of 446
r
-process-enhanced (RPE) metal-poor stars in the halo and disk systems of the Milky Way, based on data releases from the
R
-Process ...Alliance, supplemented by additional literature samples. This sample represents more than a 10-fold increase in size relative to that previously considered by Roederer et al. and, by design, covers a larger range of
r
-process-element enrichment levels. We test a number of clustering analysis methods on the derived orbital energies and other dynamical parameters for this sample, ultimately deciding on application of the
HDBSCAN
algorithm, which obtains 30 individual chemodynamically tagged groups (CDTGs); 21 contain between 3 and 5 stars, and 9 contain between 6 and 12 stars. Even though the clustering was performed solely on the basis of their dynamical properties, the stars in these CDTGs exhibit
statistically significant similarities
in their metallicity (Fe/H), carbonicity (C/Fe), and neutron-capture element ratios (Sr/Fe, Ba/Fe, and Eu/Fe). These results demonstrate that the RPE stars in these CDTGs have likely experienced common chemical-evolution histories, presumably in their parent satellite galaxies or globular clusters, prior to being disrupted into the Milky Way’s halo. We also confirm the previous claim that the orbits of the RPE stars preferentially exhibit pericentric distances that are substantially lower than the present distances of surviving ultrafaint dwarf and canonical dwarf spheroidal galaxies, consistent with the disruption hypothesis. The derived dynamical parameters for several of our CDTGs indicate their association with previously known substructures, dynamically tagged groups, and RPE groups.
Abstract
Binary neutron star mergers (NSMs) have been confirmed as one source of the heaviest observable elements made by the rapid neutron-capture (
r
-) process. However, modeling NSM outflows—from ...the total ejecta masses to their elemental yields—depends on the unknown nuclear equation of state (EOS) that governs neutron star structure. In this work, we derive a phenomenological EOS by assuming that NSMs are the dominant sources of the heavy element material in metal-poor stars with
r
-process abundance patterns. We start with a population synthesis model to obtain a population of merging neutron star binaries and calculate their EOS-dependent elemental yields. Under the assumption that these mergers were responsible for the majority of
r
-process elements in the metal-poor stars, we find parameters representing the EOS for which the theoretical NSM yields reproduce the derived abundances from observations of metal-poor stars. For our proof-of-concept assumptions, we find an EOS that is slightly softer than, but still in agreement with, current constraints, e.g., by the Neutron Star Interior Composition Explorer, with
R
1.4
= 12.25 ± 0.03 km and
M
TOV
= 2.17 ± 0.03
M
⊙
(statistical uncertainties, neglecting modeling systematics).
Abstract
We report the discovery of RAVE J203843.2−002333, a bright (
V
= 12.73), very metal-poor (
= −2.91),
r
-process-enhanced (
= +1.64 and
= −0.81) star selected from the RAVE survey. This star ...was identified as a metal-poor candidate based on its medium-resolution (
R
∼ 1600) spectrum obtained with the KPNO/Mayall Telescope, and followed up with high-resolution (
R
∼ 66,000) spectroscopy with the
Magellan
/Clay Telescope, allowing for the determination of elemental abundances for 24 neutron-capture elements, including thorium and uranium. RAVE J2038−0023 is only the fourth metal-poor star with a clearly measured U abundance. The derived chemical abundance pattern exhibits good agreement with those of other known highly
r
-process-enhanced stars, and evidence suggests that it is not an actinide-boost star. Age estimates were calculated using U/X abundance ratios, yielding a mean age of 13.0 ± 1.1 Gyr.
The astrophysical production site of the heaviest elements in the universe remains a mystery. Incorporating heavy-element signatures of metal-poor, r-process-enhanced stars into theoretical studies ...of r-process production can offer crucial constraints on the origin of heavy elements. In this study, we introduce and apply the "actinide-dilution with matching" model to a variety of stellar groups, ranging from actinide-deficient to actinide-enhanced, to empirically characterize r-process ejecta mass as a function of electron fraction. We find that actinide-boost stars do not indicate the need for a unique and separate r-process progenitor. Rather, small variations of neutron richness within the same type of r-process event can account for all observed levels of actinide enhancements. The very low-Ye, fission-cycling ejecta of an r-process event need only constitute 10%-30% of the total ejecta mass to accommodate most actinide abundances of metal-poor stars. We find that our empirical Ye distributions of ejecta are similar to those inferred from studies of GW170817 mass ejecta ratios, which is consistent with neutron-star mergers being a source of the heavy elements in metal-poor, r-process-enhanced stars.
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