1,445 research outputs found
Hope and Inquietudes in Nucleo-cosmochronology
Critical views are presented on some nucleo-cosmochronological questions.
Progress has been made recently in the development of the 187Re-187Os
cosmochronometry. From this, there is good hope for this clock to become of the
highest quality for the nuclear dating of the Universe. The simultaneous
observation of Th and U in ultra-metal-poor stars would also be a most
interesting prospect. In contrast, a serious inquietude is expressed about the
reliability of the chronometric attempts based on the classical 232Th-238U and
235U-238U pairs, as well as on the Th (without U) abundance determinations in
ultra-metal poor stars.Comment: 9 pages, no figures; ASP Conference Series: "Astrophysical Ages and
Time Scales
Non-explosive hydrogen and helium burnings: Abundance predictions from the NACRE reaction rate compilation
The abundances of the isotopes of the elements from C to Al produced by the
non-explosive CNO, NeNa and MgAl modes of hydrogen burning, as well as by
helium burning, are calculated with the thermonuclear rates recommended by the
European compilation of reaction rates for astrophysics (NACRE: details about
NACRE may be found at http://astro.ulb.ac.be. This electronic address provides
many data of nuclear astrophysics interest and also offers the possibility of
generating interactively tables of reaction rates for networks and temperature
grids selected by the user). The impact of nuclear physics uncertainties on the
derived abundances is discussed in the framework of a simple parametric
astrophysical model. These calculations have the virtue of being a guide in the
selection of the nuclear uncertainties that have to be duly analyzed in
detailed model stars, particularly in order to perform meaningful
confrontations between abundance observations and predictions. They are also
hoped to help nuclear astrophysicists pinpointing the rate uncertainties that
have to be reduced most urgently.Comment: 13 pages, 13 figures, Latex, accepted for publication in Astronomy
and Astrophysics main journal. Also available at
http://astro.ulb.ac.be/Htm/iaa0.ht
The p- and r-processes: reviews and other views
A review is presented of the p-process in Type II supernovae, one of its
goals being to enlighten the changes in views on this nucleosynthesis mechanism
since the work of Jean and Jim on the subject in 1975. Specific discussions are
also devoted to cases of particular interest, like the light Mo and Ru stable
isotopes, the rare nuclide 138La or the radionuclide 146Sm. Some comments of
diverse natures are also made on the r-process. These considerations do not aim
at really providing an exhaustive review of the many nuclear physics and
astrophysics intricacies of this process. In contrast, they are hoped to
complement or to put in perspective other views that are often expressed in
relation with this nucleosynthesis mechanismComment: 7 pages, 2 figures. To appear in the conference proceedings of
"Cosmic evolution" (in the honor of J. Audouze and J. Truran), Institut
d'Astrophysique de Paris, November 200
The production of short-lived radionuclides by new non-rotating and rotating Wolf-Rayet model stars
It has been speculated that WR winds may have contaminated the forming solar
system, in particular with short-lived radionuclides (half-lives in the
approximate 10^5 - 10^8 y range) that are responsible for a class of isotopic
anomalies found in some meteoritic materials. We revisit the capability of the
WR winds to eject these radionuclides using new models of single non-exploding
WR stars with metallicity Z = 0.02. The earlier predictions for non-rotating WR
stars are updated, and models for rotating such stars are used for the first
time in this context. We find that (1) rotation has no significant influence on
the short-lived radionuclide production by neutron capture during the core
He-burning phase, and (2) 26Al, 36Cl, 41Ca, and 107Pd can be wind-ejected by a
variety of WR stars at relative levels that are compatible with the meteoritic
analyses for a period of free decay of around 10^5 y between production and
incorporation into the forming solar system solid bodies. We confirm the
previously published conclusions that the winds of WR stars have a radionuclide
composition that can meet the necessary condition for them to be a possible
contaminating agent of the forming solar system. Still, it remains to be
demonstrated from detailed models that this is a sufficient condition for these
winds to have provided a level of pollution that is compatible with the
observations.Comment: 8 pages, 8 figure
Mitochondria Retrograde Signaling and the UPR mt: Where Are We in Mammals?
Mitochondrial unfolded protein response is a form of retrograde signaling that contributes to ensuring the maintenance of quality control of mitochondria, allowing functional integrity of the mitochondrial proteome. When misfolded proteins or unassembled complexes accumulate beyond the folding capacity, it leads to alteration of proteostasis, damages, and organelle/cell dysfunction. Extensively studied for the ER, it was recently reported that this kind of signaling for mitochondrion would also be able to communicate with the nucleus in response to impaired proteostasis. The mitochondrial unfolded protein response (UPR(mt)) is activated in response to different types and levels of stress, especially in conditions where unfolded or misfolded mitochondrial proteins accumulate and aggregate. A specific UPR(mt) could thus be initiated to boost folding and degradation capacity in response to unfolded and aggregated protein accumulation. Although first described in mammals, the UPR(mt) was mainly studied in Caenorhabditis elegans, and accumulating evidence suggests that mechanisms triggered in response to a UPR(mt) might be different in C. elegans and mammals. In this review, we discuss and integrate recent data from the literature to address whether the UPR(mt) is relevant to mitochondrial homeostasis in mammals and to analyze the putative role of integrated stress response (ISR) activation in response to the inhibition of mtDNA expression and/or accumulation of mitochondrial mis/unfolded proteins
The nuclear processes responsible for the CNO synthesis
The abundances of the isotopes of the elements C, N and O are mainly affected
by the cold CNO cycles in non-explosive stellar situations, or by the hot CNO
chains that can develop in certain explosive sites, like classical novae.
Helium burning phases can modify the composition of the ashes of the CNO
transmutations through several -capture reactions, the most famed one
being 12C(a,g)16O. This contribution presents a short review of the purely
nuclear physics limitations imposed on the accuracy of the predicted C, N and O
yields from H-burning in non-explosive stars or novae. This analysis makes
largely use of the NACRE compilation for the rates of the reactions on stable
targets making up the cold CNO cycle. Some more recent rate determinations are
also considered. The analysis of the impact of the rate uncertainties on the
abundance predictions is conducted in the framework of a simple parametric
astrophysical model. These calculations have the virtue of being a guide in the
selection of the nuclear uncertainties that have to be duly analyzed in
detailed model stars, particularly in order to perform meaningful
confrontations between abundance observations and predictions. They are also
hoped to help nuclear astrophysicists pinpointing the rate uncertainties that
have to be reduced most urgently. A limited use of detailed stellar models is
also made for the purpose of some specific illustrations.Comment: 9 pages including 5 figures. Conference proceedings for "CNO in the
Universe", St Luc, Switzerland, 10-14 September 2002 C. Charbonnel, D.
Schaerer & G. Meynet (eds) ASP Conference Seri
Spectroscopy of the heaviest nuclei (theory)
Recent progress in the applications of covariant density functional theory
(CDFT) to the description of the spectroscopy of the heaviest nuclei is
reviewed. The analysis of quasiparticle spectra in actinides and the heaviest A
~ 250 nuclei provides a measure of the accuracy of the description of
single-particle energies in CDFT and an additional constraint for the choice of
effective interactions for the description of superheavy nuclei. The response
of these nuclei to the rotation is rather well described by cranked
relativistic Hartree+Bogoliubov theory and it serves as a supplementary tool in
configuration assignment in odd-mass nuclei. A systematic analysis of the
fission barriers with allowance for triaxial deformation shows that covariant
density functional theory is able to describe fission barriers on a level of
accuracy comparable with the best phenomenological macroscopic+microscopic
approaches.Comment: 10 pages, 7 figures, invited talk of A.V. Afanasjev at the
International Nuclear Physics Conference (INPC 2010), Vancouver, Canada, July
4-9, 2010, to be published in Journal of Physics G: Conference Series (JPCS
The puzzle of the synthesis of the rare nuclide 138La
The calculations of the p-process in the O/Ne layers of Type II supernovae
are quite successful in reproducung the solar system content of p-nuclides.
They predict, however, a significant underproduction of the rare odd-odd
nuclide 138La. A model for the explosion of a 25 Mo star with solar metallicity
is used to suggest that electron neutrino captures on 138Ba may well be its
most efficient production mechanism. The responsibility of an inadequate
prediction of the 138La and 139La photodisintegration rates in the too low
production of 138La is also examined quantitatively. A detailed discussion of
the theoretical uncertainties in these rates suggest that the required rate
changes are probably too high to be fully plausible. Their measurement would be
most welcome. They would help disentangling the relative contributions of
thermonuclear and neutrino processes to the 138La production.Comment: 4 pages to be published by A&A Letter
Improved predictions of nuclear reaction rates with the TALYS reaction code for astrophysical applications
Nuclear reaction rates of astrophysical applications are traditionally
determined on the basis of Hauser-Feshbach reaction codes. These codes adopt a
number of approximations that have never been tested, such as a simplified
width fluctuation correction, the neglect of delayed or multiple-particle
emission during the electromagnetic decay cascade, or the absence of the
pre-equilibrium contribution at increasing incident energies.
The reaction code TALYS has been recently updated to estimate the
Maxwellian-averaged reaction rates that are of astrophysical relevance. These
new developments enable the reaction rates to be calculated with increased
accuracy and reliability and the approximations of previous codes to be
investigated.
The TALYS predictions for the thermonuclear rates of relevance to
astrophysics are detailed and compared with those derived by widely-used codes
for the same nuclear ingredients. It is shown that TALYS predictions may differ
significantly from those of previous codes, in particular for nuclei for which
no or little nuclear data is available. The pre-equilibrium process is shown to
influence the astrophysics rates of exotic neutron-rich nuclei significantly.
For the first time, the Maxwellian-averaged (n,2n) reaction rate is calculated
for all nuclei and its competition with the radiative capture rate is
discussed.
The TALYS code provides a new tool to estimate all nuclear reaction rates of
relevance to astrophysics with improved accuracy and reliability.Comment: 10 pages, 7 figures. Papers accepted for publication in A&A Journa
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