481 research outputs found
Molecular opacities for low-mass metal-poor AGB stars undergoing the Third Dredge Up
The concomitant overabundances of C, N and s-process elements are commonly
ascribed to the complex interplay of nucleosynthesis, mixing and mass loss
taking place in Asymptotic Giant Branch stars. At low metallicity, the
enhancement of C and/or N may be up to 1000 times larger than the original iron
content and significantly affects the stellar structure and its evolution. For
this reason, the interpretation of the already available and still growing
amount of data concerning C-rich metal-poor stars belonging to our Galaxy as
well as to dwarf spheroidal galaxies would require reliable AGB stellar models
for low and very low metallicities. In this paper we address the question of
calculation and use of appropriate opacity coefficients, which take into
account the C enhancement caused by the third dredge up. A possible N
enhancement, caused by the cool bottom process or by the engulfment of protons
into the convective zone generated by a thermal pulse and the subsequent huge
third dredge up, is also considered. Basing on up-to-date stellar models, we
illustrate the changes induced by the use of these opacity on the physical and
chemical properties expected for these stars.Comment: 23 pages, 8 figures, accepted for publication in Ap
CEMP-s and CEMP-s/r stars: last update
We provide an updated discussion of the sample of CEMP-s and CEMP-s/r stars
collected from the literature. Observations are compared with the theoretical
nucleosynthesis models of asymptotic giant branch (AGB) stars presented by
Bisterzo et al. (2010, 2011, 2012), in the light of the most recent
spectroscopic results.Comment: 10 pages, 2 figures, New advances in stellar physics: from
microscopic to macroscopic processes, May 27-31 2013, Roscoff, France, EDP
Science, EAS Publications Series, in pres
New constraints on the major neutron source in low-mass AGB stars
We compare updated Torino postprocessing asymptotic giant branch (AGB)
nucleosynthesis model calculations with isotopic compositions of mainstream SiC
dust grains from low-mass AGB stars. Based on the data-model comparison, we
provide new constraints on the major neutron source, 13C({\alpha},n)16O in the
He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba
grain data can only be consistently explained by the Torino model calculations
that adopt the recently proposed magnetic-buoyancy-induced 13C-pocket. This
observation provides strong support to the suggestion of deep mixing of H into
the He-intershell at low 13C concentrations as a result of efficient transport
of H through magnetic tubes.Comment: ApJ, accepte
The effects of a revised Be e-capture rate on solar neutrino fluxes
The electron-capture rate on Be is the main production channel for Li
in several astrophysical environments. Theoretical evaluations have to account
for not only the nuclear interaction, but also the processes in the plasma
where Be ions and electrons interact. In the past decades several estimates
were presented, pointing out that the theoretical uncertainty in the rate is in
general of few percents. In the framework of fundamental solar physics, we
consider here a recent evaluation for the Be+e rate, not used up to now
in the estimate of neutrino fluxes. We analysed the effects of the new
assumptions on Standard Solar Models (SSMs) and compared the results obtained
by adopting the revised Be+e rate to those obtained by the one reported
in a widely used compilation of reaction rates (ADE11). We found that new SSMs
yield a maximum difference in the efficiency of the Be channel of about
-4\% with respect to what is obtained with the previously adopted rate. This
fact affects the production of neutrinos from B, increasing the relative
flux up to a maximum of 2.7\%. Negligible variations are found for the physical
and chemical properties of the computed solar models. The agreement with the
SNO measurements of the neutral current component of the B neutrino flux is
improved.Comment: 7 pages, 3 figures, 4 tables. Accepted for the publication on A&
On the Origin of the Early Solar System Radioactivities. Problems with the AGB and Massive Star Scenarios
Recent improvements in stellar models for intermediate-mass and massive stars
are recalled, together with their expectations for the synthesis of radioactive
nuclei of lifetime Myr, in order to re-examine the origins
of now extinct radioactivities, which were alive in the solar nebula. The
Galactic inheritance broadly explains most of them, especially if -process
nuclei are produced by neutron star merging according to recent models.
Instead, Al, Ca, Cs and possibly Fe require
nucleosynthesis events close to the solar formation. We outline the persisting
difficulties to account for these nuclei by Intermediate Mass Stars (2
M/M). Models of their final stages now
predict the ubiquitous formation of a C reservoir as a neutron capture
source; hence, even in presence of Al production from Deep Mixing or Hot
Bottom Burning, the ratio Al/Pd remains incompatible with
measured data, with a large excess in Pd. This is shown for two recent
approaches to Deep Mixing. Even a late contamination by a Massive Star meets
problems. In fact, inhomogeneous addition of Supernova debris predicts
non-measured excesses on stable isotopes. Revisions invoking specific low-mass
supernovae and/or the sequential contamination of the pre-solar molecular cloud
might be affected by similar problems, although our conclusions here are
weakened by our schematic approach to the addition of SN ejecta. The limited
parameter space remaining to be explored for solving this puzzle is discussed.Comment: Accepted for publication on Ap
Chemical evolution with rotating massive star yields II. A new assessment of the solar s- and r- process components
The decomposition of the Solar system abundances of heavy isotopes into their sand r- components plays a key role in our understanding of the corresponding nuclear
processes and the physics and evolution of their astrophysical sites. We present a new
method for determining the s- and r- components of the Solar system abundances,
fully consistent with our current understanding of stellar nucleosynthesis and galactic chemical evolution. The method is based on a study of the evolution of the solar
neighborhood with a state-of-the-art 1-zone model, using recent yields of low and intermediate mass stars as well as of massive rotating stars. We compare our results with
previous studies and we provide tables with the isotopic and elemental contributions
of the s- and r-processes to the Solar system compositionThis article is based upon work partially supported from
the âChETECâ COST Action (CA16117) of COST (European Cooperation in Science and Technology). C.A. acknowledges in part to the Spanish grants AYA2015-63588-P
and PGC2018-095317-B-C21 within the European Founds
for Regional Development (FEDER)
Magnetic-buoyancy-induced mixing in AGB stars: Fluorine nucleosynthesis at different metallicities
DV and SC acknowledge S. Bagnulo for fruitful discussions. DV acknowledges financial support from the German-Israeli Foundation (GIF No. I-1500-303.7/2019). CA acknowledges financial support from the Agencia Estatal de Investigacion of the Spanish Ministerio de Ciencia e Innovacion through the FEDER founds projects PGC2018-095317-B-C2.Asymptotic giant branch (AGB) stars are considered to be among the most significant contributors to the fluorine budget in our Galaxy. While observations and theory agree at close-to-solar metallicity, stellar models at lower metallicities overestimate the fluorine production with respect to that of heavy elements. We present F-19 nucleosynthesis results for a set of AGB models with different masses and metallicities in which magnetic buoyancy acts as the driving process for the formation of the C-13 neutron source (the so-called C-13 pocket). We find that F-19 is mainly produced as a result of nucleosynthesis involving secondary N-14 during convective thermal pulses, with a negligible contribution from the N-14 present in the C-13 pocket region. A large F-19 production is thus prevented, resulting in lower fluorine surface abundances. As a consequence, AGB stellar models with mixing induced by magnetic buoyancy at the base of the convective envelope agree well with available fluorine spectroscopic measurements at low and close-to-solar metallicity.German-Israeli Foundation for Scientific Research and Development I-1500-303.7/2019Agencia Estatal de Investigacion of the Spanish Ministerio de Ciencia e Innovacion through the FEDER founds projects PGC2018-095317-B-C
Evolution, nucleosynthesis and yields of low mass AGB stars at different metallicities (II): the FRUITY database
By using updated stellar low mass stars models, we can systematically
investigate the nucleosynthesis processes occurring in AGB stars, when these
objects experience recurrent thermal pulses and third dredge-up episodes. In
this paper we present the database dedicated to the nucleosynthesis of AGB
stars: the FRUITY (FRANEC Repository of Updated Isotopic Tables & Yields)
database. An interactive web-based interface allows users to freely download
the full (from H to Bi) isotopic composition, as it changes after each third
dredge-up episode and the stellar yields the models produce. A first set of AGB
models, having masses in the range 1.5 < M/Msun < 3.0 and metallicities 1e-3 <
Z < 2e-2, is discussed here. For each model, a detailed description of the
physical and the chemical evolution is provided. In particular, we illustrate
the details of the s-process and we evaluate the theoretical uncertainties due
to the parametrization adopted to model convection and mass loss. The resulting
nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and
[Pb/hs] ratios to those obtained from the available abundance analysis of
s-enhanced stars. On the average, the variation with the metallicity of these
spectroscopic indexes is well reproduced by theoretical models, although the
predicted spread at a given metallicity is substantially smaller than the
observed one. Possible explanations for such a difference are briefly
discussed. An independent check of the third dredge-up efficiency is provided
by the C-stars luminosity function. Consequently, theoretical C-stars
luminosity functions for the Galactic disk and the Magellanic Clouds have been
derived. We generally find a good agreement with observations.Comment: Accepted for Publication on The Astrophysical Journal Supplement
Modeling the Li abundances of RGB and AGB stars with a new estimate for the 7Be half-life time
Stars with
Mâ€2:3Mâ are considered Li depletion sites during their early and late evolutionary
stages. Indeed 7Li, synthesized through electron-captures on
7Be, is burnt into two alpha particles,
when the H-shell burns below convective envelopes of Red Giant Branch (RGB) and Asymptotic
Giant Branch (AGB) stars. Furthermore, Li abundances observed in the spectra of these stars
cover a wide range of values, which is difficult to be explained by stellar models, both Li-rich
and Li-poor objects being observed. Difficulties arise in measuring very low Li abundances in
O-rich AGBs and the lack of knowledge about the physical causes of extra-mixing, but the main
source of uncertainty in investigating Li nucleosynthesis concerns the
7
Be life-time, as the avail-
able estimations actually are valid only for the Sun [1]. Since in RGB and AGB H-shell burning
temperatures and densities might be respectively up to five times higher and five orders of mag-
nitudes lower than in the solar core, using for giants a
7Be life-time, extrapolated from the one
valid in solar condition is at least hazardous. We present the consequence on Li nucleosynthesis
of a new 7Be life-time estimation computed by a theoretical approach inherited from quantum
chemistry. Extra-mixing models integrated with this new datum well reproduce the trends of Li
destruction observed in RGBs and are suitable to account for Li observations in AGBs, explaining
also the amount of nuclei from C to A
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