74 research outputs found
The Ubiquity of the Rapid Neutron-Capture Process
To better characterize the abundance patterns produced by the r-process, we
have derived new abundances or upper limits for the heavy elements zinc (Zn),
yttrium (Y), lanthanum (La), europium (Eu), and lead (Pb). Our sample of 161
metal-poor stars includes new measurements from 88 high resolution and high
signal-to-noise spectra obtained with the Tull Spectrograph on the 2.7m Smith
Telescope at McDonald Observatory, and other abundances are adopted from the
literature. We use models of the s-process in AGB stars to characterize the
high Pb/Eu ratios produced in the s-process at low metallicity, and our new
observations then allow us to identify a sample of stars with no detectable
s-process material. In these stars, we find no significant increase in the
Pb/Eu ratios with increasing metallicity. This suggests that s-process material
was not widely dispersed until the overall Galactic metallicity grew
considerably, perhaps even as high as [Fe/H]=-1.4. We identify a dispersion of
at least 0.5 dex in [La/Eu] in metal-poor stars with [Eu/Fe]<+0.6 attributable
to the r-process, suggesting that there is no unique "pure" r-process elemental
ratio among pairs of rare earth elements. We confirm earlier detections of an
anti-correlation between Y/Eu and Eu/Fe bookended by stars strongly enriched in
the r-process (e.g., CS 22892-052) and those with deficiencies of the heavy
elements (e.g., HD 122563). We can reproduce the range of Y/Eu ratios using
simulations of high-entropy neutrino winds of core-collapse supernovae that
include charged-particle and neutron-capture components of r-process
nucleosynthesis. The heavy element abundance patterns in most metal-poor stars
do not resemble that of CS 22892-052, but the presence of heavy elements such
as Ba in nearly all metal-poor stars without s-process enrichment suggests that
the r-process is a common phenomenon.Comment: Accepted for publication in the Astrophysical Journal. 25 pages, 13
figure
The Hamburg/ESO R-process Enhanced Star survey (HERES) IV. Detailed abundance analysis and age dating of the strongly r-process enhanced stars CS 29491-069 and HE 1219-0312
We report on a detailed abundance analysis of two strongly r-process
enhanced, very metal-poor stars newly discovered in the HERES project, CS
29491-069 ([Fe/H]=-2.51, [r/Fe]=+1.1) and HE 1219-0312 ([Fe/H]=-2.96,
[r/Fe]=+1.5). The analysis is based on high-quality VLT/UVES spectra and MARCS
model atmospheres. We detect lines of 15 heavy elements in the spectrum of CS
29491-069, and 18 in HE 1219-0312; in both cases including the Th II 4019 {\AA}
line. The heavy-element abundance patterns of these two stars are mostly
well-matched to scaled solar residual abundances not formed by the s-process.
We also compare the observed pattern with recent high-entropy wind (HEW)
calculations, which assume core-collapse supernovae of massive stars as the
astrophysical environment for the r-process, and find good agreement for most
lanthanides. The abundance ratios of the lighter elements strontium, yttrium,
and zirconium, which are presumably not formed by the main r-process, are
reproduced well by the model. Radioactive dating for CS 29491-069 with the
observed thorium and rare-earth element abundance pairs results in an average
age of 9.5 Gyr, when based on solar r-process residuals, and 17.6 Gyr, when
using HEW model predictions. Chronometry seems to fail in the case of HE
1219-0312, resulting in a negative age due to its high thorium abundance. HE
1219-0312 could therefore exhibit an overabundance of the heaviest elements,
which is sometimes called an "actinide boost"
New attempts to understand nanodiamond stardust
We report on a concerted effort aimed at understanding the origin and history
of the pre-solar nanodiamonds in meteorites including the astrophysical sources
of the observed isotopic abundance signatures. This includes measurement of
light elements by secondary ion mass spectrometry (SIMS), analysis of
additional heavy trace elements by accelerator mass spectrometry (AMS) and
dynamic calculations of r-process nucleosynthesis with updated nuclear
properties. Results obtained indicate: a) there is no evidence for the former
presence of now extinct 26Al and 44Ti in our diamond samples other than what
can be attributed to silicon carbide and other "impurities"; this does not
offer support for a supernova (SN) origin but neither does it negate it; b)
analysis by AMS of platinum in "bulk diamond" yields an overabundance of r-only
198Pt that at face value seems more consistent with the neutron burst than with
the separation model for the origin of heavy trace elements in the diamonds,
although this conclusion is not firm given analytical uncertainties; c) if the
Xe-H pattern was established by an unadulterated r-process, it must have been a
strong variant of the main r-process, which possibly could also account for the
new observations in platinum.Comment: Workshop on Astronomy with Radioactvities VII; Publications of the
Astronomical Society of Australia, accepte
COMPARAISON DE DEUX TECHNIQUES IMMUNOENZYMATIQUES ELISA POUR LA DETECTION DES ANTICORPS IgG SERIQUES ANTIRUBEOLIQUES
OBJECTIF: Il s’agit d’évaluer les caractéristiques de deux techniques immunoenzymatiques ELISA (principe, performances et limites) pour la détection des IgG sériques antirubéolique. MATERIEL ET METHODES: L a recherche des IgG antirubéoliques a été effectuée dans 100 échantillons de sérums recueillis dans le laboratoire d’immunosérologie du CHU Ibn Rochd de Casablanca. Ils proviennent de patients hospitalisés et de consultants externes du CHU. Il s’agit pour la plupart de jeunes femmes (85%) et de nouveaux nés (12%). Les deux méthodes immunoenzymatiques ELISA utilisées sont : 1- kit Vircell®Spain (technique manuelle) 2- AxsymAbbott®USA (technique microparticulaire automatisée). RESULTATS: Les résultats sérologiques pour la détection des anticorps IgG antirubéolique ont montré une concordance entre les deux techniques de 94% (90% des résultats positifs et 4% négatifs et une discordance de 6%. CONCLUSION: Même si la concordance entre les deux techniques est de 94%, les résultats discordants démontrent la nécessité d’intégrer toujours la démarche diagnostique, d’une part les données cliniques associées aux données biologiques, et d‘autres part, de prendre en considération la valeur seuil de détection des marques sérologiques spécifiques de la technique utilisée, ainsi que la qualité des échantillons testés
TOF-Brho Mass Measurements of Very Exotic Nuclides for Astrophysical Calculations at the NSCL
Atomic masses play a crucial role in many nuclear astrophysics calculations.
The lack of experimental values for relevant exotic nuclides triggered a rapid
development of new mass measurement devices around the world. The
Time-of-Flight (TOF) mass measurements offer a complementary technique to the
most precise one, Penning trap measurements, the latter being limited by the
rate and half-lives of the ions of interest. The NSCL facility provides a
well-suited infrastructure for TOF mass measurements of very exotic nuclei. At
this facility, we have recently implemented a TOF-Brho technique and performed
mass measurements of neutron-rich nuclides in the Fe region, important for
r-process calculations and for calculations of processes occurring in the crust
of accreting neutron stars.Comment: 8 pages, 4 figures, submitted to Journal of Physics G, proceedings of
Nuclear Physics in Astrophysics II
Explosive nucleosynthesis in core-collapse supernovae
The specific mechanism and astrophysical site for the production of half of
the elements heavier than iron via rapid neutron capture (r-process) remains to
be found. In order to reproduce the abundances of the solar system and of the
old halo stars, at least two components are required: the heavy r-process
nuclei (A>130) and the weak r-process which correspond to the lighter heavy
nuclei (A<130). In this work, we present nucleosynthesis studies based on
trajectories of hydrodynamical simulations for core-collapse supernovae and
their subsequent neutrino-driven winds. We show that the weak r-process
elements can be produced in neutrino-driven winds and we relate their
abundances to the neutrino emission from the nascent neutron star. Based on the
latest hydrodynamical simulations, heavy r-process elements cannot be
synthesized in the neutrino-driven winds. However, by artificially increasing
the wind entropy, elements up to A=195 can be made. In this way one can mimic
the general behavior of an ejecta where the r-process occurs. We use this to
study the impact of the nuclear physics input (nuclear masses, neutron capture
cross sections, and beta-delayed neutron emission) and of the long-time
dynamical evolution on the final abundances.Comment: 10 pages, 8 figures, invited talk, INPC 2010 Vancouver, Journal of
Physics: Conference Serie
Nucleosynthesis-relevant conditions in neutrino-driven supernova outflows. II. The reverse shock in two-dimensional simulations
After the initiation of the explosion of core-collapse supernovae, neutrinos
emitted from the nascent neutron star drive a supersonic baryonic outflow. This
neutrino-driven wind interacts with the more slowly moving, earlier supernova
ejecta forming a wind termination shock (or reverse shock), which changes the
local wind conditions and their evolution. Important nucleosynthesis processes
(alpha-process, charged-particle reactions, r-process, and vp-process) occur or
might occur in this environment. The nucleosynthesis depends on the long-time
evolution of density, temperature, and expansion velocity. Here we present
two-dimensional hydrodynamical simulations with an approximate description of
neutrino-transport effects, which for the first time follow the post-bounce
accretion, onset of the explosion, wind formation, and the wind expansion
through the collision with the preceding supernova ejecta. Our results
demonstrate that the anisotropic ejecta distribution has a great impact on the
position of the reverse shock, the wind profile, and the long-time evolution.
This suggests that hydrodynamic instabilities after core bounce and the
consequential asymmetries may have important effects on the
nucleosynthesis-relevant conditions in the neutrino-heated baryonic mass flow
from proto-neutron stars.Comment: 13 pages, 8 figures, accepted for publication in A&
The influence of collective neutrino oscillations on a supernova r-process
Recently, it has been demonstrated that neutrinos in a supernova oscillate
collectively. This process occurs much deeper than the conventional
matter-induced MSW effect and hence may have an impact on nucleosynthesis. In
this paper we explore the effects of collective neutrino oscillations on the
r-process, using representative late-time neutrino spectra and outflow models.
We find that accurate modeling of the collective oscillations is essential for
this analysis. As an illustration, the often-used "single-angle" approximation
makes grossly inaccurate predictions for the yields in our setup. With the
proper multiangle treatment, the effect of the oscillations is found to be less
dramatic, but still significant. Since the oscillation patterns are sensitive
to the details of the emitted fluxes and the sign of the neutrino mass
hierarchy, so are the r-process yields. The magnitude of the effect also
depends sensitively on the astrophysical conditions - in particular on the
interplay between the time when nuclei begin to exist in significant numbers
and the time when the collective oscillation begins. A more definitive
understanding of the astrophysical conditions, and accurate modeling of the
collective oscillations for those conditions, is necessary.Comment: 27 pages, 10 figure
The Hamburg/ESO R-process Enhanced Star survey (HERES). V. Detailed abundance analysis of the r-process enhanced star HE 2327-5642
We report on a detailed abundance analysis of the strongly r-process enhanced
giant star, HE 2327-5642 ([Fe/H] = -2.78, [r/Fe] = +0.99). Determination of
stellar parameters and element abundances was based on analysis of high-quality
VLT/UVES spectra. The surface gravity was calculated from the NLTE ionization
balance between Fe I and Fe II, and Ca I and Ca II. Accurate abundances for a
total of 40 elements and for 23 neutron-capture elements beyond Sr and up to Th
were determined. The heavy element abundance pattern of HE 2327-5642 is in
excellent agreement with those previously derived for other strongly r-process
enhanced stars. Elements in the range from Ba to Hf match the scaled Solar
r-process pattern very well. No firm conclusion can be drawn with respect to a
relationship between the fisrt neutron-capture peak elements, Sr to Pd, in HE
2327-5642 and the Solar r-process, due to the uncertainty of the latter. A
clear distinction in Sr/Eu abundance ratios was found between the halo stars
with different europium enhancement. The strongly r-process enhanced stars
reveal a low Sr/Eu abundance ratio at [Sr/Eu] = -0.92+-0.13, while the stars
with 0 < [Eu/Fe] < 1 and [Eu/Fe] < 0 have 0.36 dex and 0.93 dex larger Sr/Eu
values, respectively. Radioactive dating for HE 2327-5642 with the observed
thorium and rare-earth element abundance pairs results in an average age of
13.3 Gyr, when based on the high-entropy wind calculations, and 5.9 Gyr, when
using the Solar r-residuals. HE 2327-5642 is suspected to be radial-velocity
variable based on our high-resolution spectra, covering ~4.3 years.Comment: 16 pages, 12 figures, accepted to A&
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