32 research outputs found
Total r-process Yields of Milky Way Neutron Star Mergers
While it is now known that double neutron star binary systems (DNSs) 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
DNSs hold sway over estimating their total contribution to the elemental
abundances in the Solar system and the Galaxy. Furthermore, the expected
elemental yield for DNSs may depend on the merger parameters themselves -- such
as their stellar masses and radii -- which is not currently considered in many
galactic chemical evolution models. Using the characteristics of the observed
sample of DNSs 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 the Solar system elements. We find that with our current models, the
present DNS population favors production of the 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.Comment: 12 pages, 6 figures, 2 table
Actinide-rich and Actinide-poor -Process Enhanced Metal-Poor Stars do not Require Separate -Process Progenitors
The astrophysical production site of the heaviest elements in the universe
remains a mystery. Incorporating heavy element signatures of metal-poor,
-process enhanced stars into theoretical studies of -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 -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 -process progenitor. Rather, small variations of neutron
richness within the same type of -process event can account for all observed
levels of actinide enhancements. The very low-, fission-cycling ejecta of
an -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 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, -process
enhanced stars.Comment: 14 pages, 11 figures, Submitted to Ap
Superheavy Elements in Kilonovae
As LIGO-Virgo-KAGRA enters its fourth observing run, a new opportunity to
search for electromagnetic counterparts of compact object mergers will also
begin. The light curves and spectra from the first "kilonova" associated with a
binary neutron star binary (NSM) suggests that these sites are hosts of the
rapid neutron capture ("") process. However, it is unknown just how robust
elemental production can be in mergers. Identifying signposts of the production
of particular nuclei is critical for fully understanding merger-driven
heavy-element synthesis. In this study, we investigate the properties of very
neutron rich nuclei for which superheavy elements () can be produced
in NSMs and whether they can similarly imprint a unique signature on kilonova
light-curve evolution. A superheavy-element signature in kilonovae represents a
route to establishing a lower limit on heavy-element production in NSMs as well
as possibly being the first evidence of superheavy element synthesis in nature.
Favorable NSMs conditions yield a mass fraction of superheavy elements is
at 7.5 hours post-merger. With this mass
fraction of superheavy elements, we find that kilonova light curves may appear
similar to those arising from lanthanide-poor ejecta. Therefore, photometric
characterizations of superheavy-element rich kilonova may possibly misidentify
them as lanthanide-poor events.Comment: 9 pages, 5 figure
Uranium Abundances and Ages of -process Enhanced Stars with Novel U II Lines
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
-process enhanced (RPE) stars, has been known to constitute one of the most
robust chronometers known. So far, U abundance determination has used a
U II line at \r{A}. Consequently, U abundance has been
reliably determined for only five RPE stars. Here, we present the first
homogeneous U abundance analysis of four RPE stars using two novel U II lines
at \r{A} and \r{A}, in addition to the canonical
\r{A} line. We find that the U II lines at \r{A}
and \r{A} are reliable and render U abundances in agreement with
the U abundance, for all the stars. We, thus, determine revised U
abundances for RPE stars, 2MASS J09544277+5246414, RAVE J203843.2-002333, HE
1523-0901, and CS 31082-001, using multiple U II lines. We also provide
nucleocosmochronometric ages of these stars based on the newly derived U, Th,
and Eu abundances. The results of this study open up a new avenue to reliably
and homogeneously determine U abundance for a significantly larger number of
RPE stars. This will, in turn, enable robust constraints on the
nucleocosmochronometric ages of RPE stars, which can be applied to understand
the chemical enrichment and evolution in the early Universe, especially of
-process elements.Comment: Resubmitted to Ap
The R-Process Alliance: Abundance Universality among Some Elements at and between the First and Second R-Process Peaks
We present new observational benchmarks of rapid neutron-capture process
(r-process) nucleosynthesis for elements at and between the first (A ~ 80) and
second (A ~ 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.Comment: 16 pages, 3 figures, including 4 appendices. Published in the
Astrophysical Journa
The R-Process Alliance: The Peculiar Chemical Abundance Pattern of RAVE J183013.5-455510
We report on the spectroscopic analysis of RAVE J183013.5-455510, an
extremely metal-poor star, highly enhanced in CNO, and with discernible
contributions from the rapid neutron-capture process. There is no evidence of
binarity for this object. At [Fe/H]=-3.57, this is one of the lowest
metallicity stars currently observed, with 18 measured abundances of
neutron-capture elements. The presence of Ba, La, and Ce abundances above the
Solar System r-process predictions suggest that there must have been a
non-standard source of r-process elements operating at such low metallicities.
One plausible explanation is that this enhancement originates from material
ejected at unusually fast velocities in a neutron star merger event. We also
explore the possibility that the neutron-capture elements were produced during
the evolution and explosion of a rotating massive star. In addition, based on
comparisons with yields from zero-metallicity faint supernova, we speculate
that RAVE J1830-4555 was formed from a gas cloud pre-enriched by both
progenitor types. From analysis based on Gaia DR2 measurements, we show that
this star has orbital properties similar to the Galactic metal-weak thick-disk
stellar population.Comment: Accepted for publication in Ap