144 research outputs found
Synthesis of thorium and uranium in asymptotic giant branch stars
The intermediate neutron capture process (i-process) operates at neutron
densities between those of the slow and rapid neutron-capture processes. It can
be triggered by the ingestion of protons in a convective helium-burning region.
One possible astrophysical site is low-mass low-metallicity asymptotic giant
branch (AGB) stars. We study here the possibility that actinides (particularly
Th and U) may be significantly synthesized through i-process nucleosynthesis in
AGB stars. We computed a 1 model at [Fe/H] with the
stellar evolution code STAREVOL. We used a nuclear network of 1160 species from
H to Cf coupled to the transport processes. During the proton ingestion event,
the neutron density goes up to cm. While most of the
nuclear flow cycles in the neutron-rich Pb-Bi-Po region, a non-negligible
fraction leaks towards heavier elements and eventually synthesizes actinides.
The surface enrichment in Th and U is subject to nuclear and astrophysical
model uncertainties that could be lowered in the future, in particular by a
detailed analysis of the nuclear inputs that affect the neutron capture rates
of neutron-rich isotopes between Pb and Pa. One stellar candidate that may
confirm the production of actinides by the i-process is the carbon-enhanced
metal-poor r/s star J0949-1617, which shows Th lines in its spectrum. Its
surface abundance is shown to be reasonably well reproduced by our AGB model.
Combined with cosmochronometry, this finding opens the way to dating the
i-process event and thus obtaining a lower limit for the age of CEMP-r/s stars.
Such a dating is expected to be accurate only if surface abundances of Th and U
can be extracted simultaneously. This work shows that actinides can be
synthesized in AGB stars through the i-process. As a consequence, the r-process
may not be the sole mechanism for the production of U and Th.Comment: 6 pages, 3 figures, accepted in A&
The intermediate neutron capture process: IV. Impact of nuclear model and parameter uncertainties
We investigate both the systematic and statistical uncertainties associated
with theoretical nuclear reaction rates of relevance during the i-process and
explore their impact on the i-process elemental production, and subsequently on
the surface enrichment, for a low-mass low-metallicity star during the early
AGB phase. We use the TALYS reaction code (Koning et al. 2023) to estimate both
the model and parameter uncertainties affecting the photon strength function
and the nuclear level densities, hence the radiative neutron capture rates. The
STAREVOL code (Siess et al. 2006) is used to determine the impact of nuclear
uncertainties on the i-process nucleosynthesis in a 1 [Fe/H] = -
2.5 model star during the proton ingestion event in the early AGB phase. A
large nuclear network of 1160 species coherently coupled to the transport
processes is solved to follow the i-process nucleosynthesis. We find that the
non-correlated parameter uncertainties lead the surface abundances
uncertainties of element with to range between 0.5 and 1.0 dex, with
odd- elements displaying higher uncertainties. The correlated model
uncertainties are of the same order of magnitude, and both model and parameter
uncertainties have an important impact on potential observable tracers such as
Eu and La. Both the correlated model and uncorrelated parameter uncertainties
need to be estimated coherently before being propagated to astrophysical
observables through multi-zone stellar evolution models. Many reactions are
found to affect the i-process predictions and will require improved nuclear
models guided by experimental constraints. Priority should be given to the
reactions influencing the observable tracers.Comment: Accepted: October 11, 2023 \\ 14 Pages, 14 Figures, 2 Table
Does the i-process operate at nearly solar metallicity?
A sample of 895 s-process-rich candidates has been found among the 454180
giant stars surveyed by LAMOST at low spectral resolution (R~1800). In a
previous study, taking advantage of the higher resolution (R~86 000) offered by
the the HERMES-Mercator spectrograph, we performed the re-analysis of 15 among
the brightest stars of this sample. Among these 15 program stars, having
close-to-solar metallicities, 11 showed mild to strong heavy element
overabundances. The nucleosynthesis process(es) at the origin of these
overabundances were however not questioned in our former study. We derive the
abundances in s- and r-process elements of the 15 targets in order to
investigate whether some stars also show an i-process signature, as sometimes
found in their lower metallicity counterparts (namely, the Carbon-Enhanced
Metal-Poor (CEMP)-rs stars). Abundances are derived from the high-resolution
HERMES spectra for Pr, Nd, Sm, and Eu, using the TURBOSPECTRUM radiative
transfer LTE code with MARCS model atmospheres. Using the new classification
scheme proposed in our recent study we find that two stars show overabundances
in both s- and r-process elements well above the level expected from the
Galactic chemical evolution, an analogous situation to the one of CEMP-rs stars
at lower metallicities. We compare the abundances of the most enriched stars
with the nucleosynthetic predictions from the STAREVOL stellar evolutionary
code and find abundances compatible with an i-process occurring in AGB stars.
Despite a larger number of heavy elements to characterize the enrichment
pattern, the limit between CEMP-s and CEMP-rs stars remains fuzzy. It is
however interesting to note that an increasing number of extrinsic stars are
found to have abundances better reproduced by an i-process pattern even at
close-to-solar metallicities.Comment: Accepted for publication in A&A, 9 pages, 9 figures including the two
in appendi
Dynamic Monitoring and Control of Irreversible Chronic Diseases with Application to Glaucoma
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149269/1/poms12975_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149269/2/poms12975-sup-0001-Appendix.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149269/3/poms12975.pd
Systematic study of the low-lying electric dipole strength in Sn isotopes and its astrophysical implications
The -ray strength functions (GSF) and nuclear level densities (NLD)
below the neutron threshold have been extracted for Sn
from particle- coincidence data with the Oslo method. The evolution of
bulk properties of the low-lying electric dipole response has been investigated
on the basis of the Oslo GSF data and results of a recent systematic study of
electric and magnetic dipole strengths in even-even Sn isotopes with
relativistic Coulomb excitation. The obtained GSFs reveal a resonance-like peak
on top of the tail of the isovector giant dipole resonance, centered at
8 MeV and exhausting 2\% of the classical Thomas-Reiche-Kuhn
(TRK) sum. In contrast to predictions of the relativistic quasiparticle
random-phase and time-blocking approximation calculations (RQRPA and RQTBA), no
monotonous increase in the total low-lying strength was observed in the
experimental data from Sn to Sn, demonstrating rather similar
strength distributions in these nuclei. The Oslo GSFs and NLDs were further
used as inputs to constrain the cross sections and Maxwellian-averaged cross
sections of reactions in the Sn isotopic chain using TALYS. The
obtained results agree well with other available experimental data and the
recommended values from the JINA REACLIB, BRUSLIB, and KADoNiS libraries.
Despite relatively small exhausted fractions of the TRK sum rule, the low-lying
electric dipole strength makes a noticeable impact on the radiative
neutron-capture cross sections in stable Sn isotopes. Moreover, the
experimental Oslo inputs for the SnSn
reactions were found to affect the production of Sb in the astrophysical
-process, providing new constraints on the uncertainties of the resulting
chemical abundances from multi-zone low-metallicity Asymptotic Giant Branch
stellar models.Comment: 27 pages, 14 pages. Submitted to Physical Review C journal on 13
November 202
Nuclear Level Density and -ray Strength Function of and the impact on the i-process
Proton- coincidences from reactions between
a beam and a deuterated polyethylene target have been
analyzed with the inverse Oslo method to find the nuclear level density (NLD)
and -ray strength function (SF) of . The
capture cross section has been calculated
using the Hauser-Feshbach model in TALYS using the measured NLD and SF
as constraints. We confirm that acts as a
bottleneck when relying on one-zone nucleosynthesis calculations. However, we
find that the impact of this reaction is strongly damped in multi-zone
low-metallicity AGB stellar models experiencing i-process nucleosynthesis.Comment: Submitted to Phys. Rev.
Grids of stellar models with rotation: V. Models from 1.7 to 120 Msun at zero metallicity
Understanding the nature of the first stars is key to understanding the early universe. With new facilities such as JWST we may soon have the first observations of the earliest stellar populations, but to understand these observations we require detailed theoretical models. Here we compute a grid of stellar evolution models using the Geneva code with the aim to improve our understanding of the evolution of zero-metallicity stars, with particular interest in how rotation affects surface properties, interior structure, and metal enrichment. We produce a range of models of initial masses (Mini) from 1.7 Msun to 120 Msun, focusing on massive models of 9 Msun 60 Msun reach critical rotation on the main sequence and experience mass loss. We find that rotational mixing strongly affects metal enrichment, but does not always increase metal production as we see at higher metallicities. This is because rotation leads to an earlier CNO boost to the H shell during He-burning, which may hinder metal enrichment depending on initial mass and rotational velocity. Electronic tables of this new grid of Population III models are publicly available
A direct measurement of the 17O(α,γ)21Ne reaction in inverse kinematics and its impact on heavy element production
During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorb neutrons thereby influencing the final heavy element abundances. At low metallicities, the high neutron capture rate of 16O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the 17O(α,n)20Ne reaction. The efficiency of this neutron recycling is determined by competition between the 17O(α,n)20Ne and 17O(α,γ)21Ne reactions. While some experimental data are available on the former reaction, no data exist for the radiative capture channel at the relevant astrophysical energies. The 17O(α,γ)21Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Laboratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV Ecm, reaching into the Gamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and 1.122 MeV Ecm have been extracted. The experimentally based reaction rate calculated represents a lower limit, but suggests that significant s-process nucleosynthesis occurs in low metallicity massive stars
Diagnosis and management of pseudohypoparathyroidism and related disorders: first international Consensus Statement
This Consensus Statement covers recommendations for the diagnosis and management of patients with pseudohypoparathyroidism (PHP) and related disorders, which comprise metabolic disorders characterized by physical findings that variably include short bones, short stature, a stocky build, early-onset obesity and ectopic ossifications, as well as endocrine defects that often include resistance to parathyroid hormone (PTH) and TSH. The presentation and severity of PHP and its related disorders vary between affected individuals with considerable clinical and molecular overlap between the different types. A specific diagnosis is often delayed owing to lack of recognition of the syndrome and associated features. The participants in this Consensus Statement agreed that the diagnosis of PHP should be based on major criteria, including resistance to PTH, ectopic ossifications, brachydactyly and early-onset obesity. The clinical and laboratory diagnosis should be confirmed by a molecular genetic analysis. Patients should be screened at diagnosis and during follow-up for specific features, such as PTH resistance, TSH resistance, growth hormone deficiency, hypogonadism, skeletal deformities, oral health, weight gain, glucose intolerance or type 2 diabetes mellitus, and hypertension, as well as subcutaneous and/or deeper ectopic ossifications and neurocognitive impairment. Overall, a coordinated and multidisciplinary approach from infancy through adulthood, including a transition programme, should help us to improve the care of patients affected by these disorders
- …