201 research outputs found
Migrations et recompositions spatiales en Mauritanie : "Nouadhibou du monde" : ville de transit... et après ?
International audienc
Destination Nouadhibou pour les migrants africains
International audienc
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&
Are some CEMP-s stars the daughters of spinstars?
Carbon-enhanced metal-poor (CEMP)-s stars are long-lived low-mass stars with a very low iron content as well as overabundances of carbon and s-elements. Their peculiar chemical pattern is often explained by pollution from an asymptotic giant branch (AGB) star companion. Recent observations have shown that most CEMP-s stars are in binary systems, providing support to the AGB companion scenario. A few CEMP-s stars, however, appear to be single. We inspect four apparently single CEMP-s stars and discuss the possibility that they formed from the ejecta of a previous-generation massive star, referred to as the “source” star. In order to investigate this scenario, we computed low-metallicity massive-star models with and without rotation and including complete s-process nucleosynthesis. We find that non-rotating source stars cannot explain the observed abundance of any of the four CEMP-s stars. Three out of the four CEMP-s stars can be explained by a 25M⊙ source star with vini ~ 500 km s-1 (spinstar). The fourth CEMP-s star has a high Pb abundance that cannot be explained by any of the models we computed. Since spinstars and AGB predict different ranges of [O/Fe] and [ls/hs], these ratios could be an interesting way to further test these two scenarios. nuclear reactions, nucleosynthesis, abundances / stars: interiors / stars: chemically peculiar / stars: abundances / stars: massiv
A strong neutron burst in jet-like supernovae of spinstars
Some metal-poor stars have abundance patterns which are midway between the
slow (s) and rapid (r) neutron capture processes. We show that the helium shell
of a fast rotating massive star experiencing a jet-like explosion undergoes two
efficient neutron capture processes: one during stellar evolution and one
during the explosion. It eventually provides a material whose chemical
composition is midway between the s- and r-process. A low metallicity
40~ model with an initial rotational velocity of ~km~s was computed from birth to pre-supernova with a nuclear
network following the slow neutron capture process. A 2D hydrodynamic
relativistic code was used to model a ~erg relativistic jet-like
explosion hitting the stellar mantle. The jet-induced nucleosynthesis was
calculated in post-processing with a network of 1812 nuclei. During the star's
life, heavy elements from are produced thanks to an
efficient s-process, which is boosted by rotation. At the end of evolution, the
helium shell is largely enriched in trans-iron elements and in (unburnt)
Ne, whose abundance is times higher than in a non-rotating
model. During the explosion, the jet heats the helium shell up to
GK. It efficiently activates () reactions, such as
Ne(), and leads to a strong n-process with neutron densities
of ~cm during ~second. This has the effect
of shifting the s-process pattern towards heavier elements (e.g. Eu). The
resulting chemical pattern is consistent with the abundances of the
carbon-enhanced metal-poor r/s star CS29528-028, provided the ejecta of the jet
model is not homogeneously mixed. This is a new astrophysical site which can
explain at least some of the metal-poor stars showing abundance patterns midway
between the s- and r-process.Comment: 9 pages, 12 figures, accepted in A&
Nucleosynthesis in the first massive stars
The nucleosynthesis in the first massive stars may be constrained by observing the surface composition of long-lived very iron-poor stars born around 10 billion years ago from material enriched by their ejecta. Many interesting clues on physical processes having occurred in the first stars can be obtained based on nuclear aspects. First, in these first massive stars, mixing must have occurred between the H-burning and the He-burning zone during their nuclear lifetimes; Second, only the outer layers of these massive stars have enriched the material from which the very iron-poor stars, observed today in the halo of the MilkyWay, have formed. These two basic requirements can be obtained by rotating stellar models at very low metallicity. In the present paper, we discuss the arguments supporting this view and illustrate the sensitivity of the results concerning the [Mg/Al] ratio on the rate of the reaction 23 Na(p, γ ) 24 Mg
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
Pratiques du titrement dans les villes en développement : trois cas d'étude (Inde, Ethiopie, Mauritanie) : rapport de recherche présenté dans le cadre de l'appel à projets "La sécurisation du droit de propriété dans les pays en voie de développement"
Les trois cas d’études en Inde, en Ethiopie et en Mauritanie ont permis de voir comment est appréhendé un même cadre référentiel qui pose la sécurisation foncière au coeur de la lutte contre la pauvreté. Plusieurs types de villes (petites, secondaires, capitales) et formes d’urbain (zone centrale, quartiers périurbains, urbain diffus) ont permis de souligner quelques uns des enjeux que représente la titrisation dans les Suds. Le contexte urbain joue fortement. L’impact des politiques de titrement foncier dépend réellement de la taille de la ville et des formes urbaines. L’urbanisation généralisée et rapide dans les trois pays étudiés, longtemps considérés comme étant à dominante rurale, implique l’adaptation et la création de nouveaux régimes fonciers. Dans nos différents cas d’étude, l’accès au sol urbain fait émerger de nouveaux besoins en termes de réglementation foncière, de nouveaux types de reconnaissance, mais aussi et surtout de nouvelles pratiques de sécurisation se tissent entre les habitants et les autorités et entre habitants. La présente étude a fait ressortir l’importance de la circulation à l’échelle internationale de politiques urbaines et « bonnes pratiques », pensées depuis Washington par les Institutions internationales, et réappropriées de façon tout à fait originale localement. La comparaison des trois cas a mis en lumière l’importance des pratiques habitantes, qui, en Ethiopie, en Inde et en Mauritanie, exploitent et construisent des opportunités offertes par les normes juridiques relatives à l’accès à la propriété, plus qu’elles arrivent à en bénéficier pleinement et simplement. Bien souvent, les réformes appuyées par les bailleurs de fond ne font que se surimposer à des juridictions foncières déjà complexes, souvent coûteuses à respecter pour les habitants qui optent pour des pratiques plus informelles. La réforme sur le papier n’est que rarement suivie par la réforme en action
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
The p-process in exploding rotating massive stars
Context. The p-process nucleosynthesis can explain proton-rich isotopes that are heavier than iron, which are observed in the Solar System, but discrepancies still persist (e.g. for the Mo and Ru p-isotopes), and some important questions concerning the astrophysical site(s) of the p-process remain unanswered.
Aims. We investigate how the p-process operates in exploding rotating massive stars that have experienced an enhanced s-process nucleosynthesis during their life through rotational mixing.
Methods. With the Geneva stellar evolution code, we computed 25 M-circle dot stellar models at a metallicity of Z = 10(-3) with different initial rotation velocities and rates for the still largely uncertain O-17(alpha,gamma)Ne-21 reaction. The nucleosynthesis calculation, followed with a network of 737 isotopes, was coupled to stellar evolution, and the p-process nucleosynthesis was calculated in post-processing during both the final evolutionary stages and spherical explosions of various energies. The explosions were modelled with a relativistic hydrodynamical code.
Results. In our models, the p-nuclides are mainly synthesized during the explosion, but not much during the ultimate hydrostatic burning stages. The p-process yields mostly depend on the initial number of trans-iron seeds, which in turn depend on the initial rotation rate. We found that the impact of rotation on the p-process is comparable to the impact of rotation on the s-process. From no to fast rotation, the s-process yields of nuclides with mass number A = 140. The dependence of the p-process yields on the explosion energy is very weak.
Conclusions. Our results suggest that the contribution of core-collapse supernovae from massive stars to the solar (and Galactic) p-nuclei has been underestimated in the past, and more specifically, that the contribution from massive stars with sub-solar metallicities may even dominate. A more detailed study including stellar models with a wide range of masses and metallicities remains to be performed, together with a quantitative analysis that is based on the chemical evolution of the Galaxy
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