39 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
Characterizing r-Process Sites through Actinide Production
© Published under licence by IOP Publishing Ltd. Of the variations in the elemental abundance patterns of stars enhanced with r-process elements, the variation in the relative actinide-To-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are accessed in r-process sites. We find that variations between relative stellar actinide abundances is most likely astrophysical in nature, owing to how neutron-rich the ejecta from an r-process event may be. Furthermore, if an r-process site is capable of generating variations in the neutron-richness of its ejected material, then only one type of r-process site is needed to explain all levels of observed relative actinide enhancements