146 research outputs found
s-Process in Low Metallicity Stars. I. Theoretical Predictions
A large sample of carbon enhanced metal-poor stars enriched in s-process
elements (CEMP-s) have been observed in the Galactic halo. These stars of low
mass (M ~ 0.9 Msun) are located on the main-sequence or the red giant phase,
and do not undergo third dredge-up (TDU) episodes. The s-process enhancement is
most plausibly due to accretion in a binary system from a more massive
companion when on the asymptotic giant branch (AGB) phase (now a white dwarf).
In order to interpret the spectroscopic observations, updated AGB models are
needed to follow in detail the s-process nucleosynthesis. We present
nucleosynthesis calculations based on AGB stellar models obtained with FRANEC
(Frascati Raphson-Newton Evolutionary Code) for low initial stellar masses and
low metallicities. For a given metallicity, a wide spread in the abundances of
the s-process elements is obtained by varying the amount of 13C and its profile
in the pocket, where the 13C(a, n)16O reaction is the major neutron source,
releasing neutrons in radiative conditions during the interpulse phase. We
account also for the second neutron source 22Ne(a, n)25Mg, partially activated
during convective thermal pulses. We discuss the surface abundance of elements
from carbon to bismuth, for AGB models of initial masses M = 1.3 -- 2 Msun, low
metallicities ([Fe/H] from -1 down to -3.6) and for different 13C-pockets
efficiencies. In particular we analyse the relative behaviour of the three
s-process peaks: light-s (ls at magic neutron number N = 50), heavy-s (hs at N
= 82) and lead (N = 126). Two s-process indicators, [hs/ls] and [Pb/hs], are
needed in order to characterise the s-process distribution. In the online
material, we provide a set of data tables with surface predictions. ...Comment: 31 pages, 15 figures + 6 online material, 10 table
Impact of Nuclear Reaction Uncertainties on AGB Nucleosynthesis Models
Asymptotic giant branch (AGB) stars with low initial mass (1 - 3 Msun) are
responsible for the production of neutron-capture elements through the main
s-process (main slow neutron capture process). The major neutron source is
13C(alpha, n)16O, which burns radiatively during the interpulse periods at
about 8 keV and produces a rather low neutron density (10^7 n/cm^3). The second
neutron source 22Ne(alpha, n)25Mg, partially activated during the convective
thermal pulses when the energy reaches about 23 keV, gives rise to a small
neutron exposure but a peaked neutron density (Nn(peak) > 10^11 n/cm^3). At
metallicities close to solar, it does not substantially change the final
s-process abundances, but mainly affects the isotopic ratios near s-path
branchings sensitive to the neutron density. We examine the effect of the
present uncertainties of the two neutron sources operating in AGB stars, as
well as the competition with the 22Ne(alpha, gamma)26Mg reaction. The analysis
is carried out on AGB the main-s process component (reproduced by an average
between M(AGB; ini) = 1.5 and 3 Msun at half solar metallicity, see Arlandini
et al. 1999), using a set of updated nucleosynthesis models. Major effects are
seen close to the branching points. In particular, 13C(alpha, n)16O mainly
affects 86Kr and 87Rb owing to the branching at 85Kr, while small variations
are shown for heavy isotopes by decreasing or increasing our adopted rate by a
factor of 2 - 3. By changing our 22Ne(alpha, n)25Mg rate within a factor of 2,
a plausible reproduction of solar s-only isotopes is still obtained. We provide
a general overview of the major consequences of these variations on the s-path.
A complete description of each branching will be presented in Bisterzo et al.,
in preparation.Comment: Proceedings of Science 108, XII International Symposium on Nuclei in
the Cosmos 2012 (Cairns, Australia); 6 pages, 2 figure
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
Stellar neutron capture cross sections of ⁴¹K and ⁴⁵Sc
The neutron capture cross sections of light nuclei (
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
Stellar (n,γ) cross sections of ²³Na
The cross section of the ²³Na(n,γ)²⁴Na reaction has been measured via the activation method at the Karlsruhe 3.7 MV Van de Graaff accelerator. NaCl samples were exposed to quasistellar neutron spectra at kT = 5.1 and 25 keV produced via the ¹⁸O(p,n)¹⁸F and ⁷Li(p,n)⁷Be reactions, respectively. The derived capture cross sections (σ)kT=5keV = 9.1 ± 0.3mb and (σ)kT=25keV = 2.03 ± 0.05 mb are significantly lower than reported in literature. These results were used to substantially revise the radiative width of the first ²³Na resonance and to establish an improved set of Maxwellian average cross sections. The implications of the lower capture cross section for current models of s-process nucleosynthesis are discussed
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