164 research outputs found
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
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
Re-evaluation of the ¹⁶O(n, γ)¹⁷O cross section at astrophysical energies and its role as a neutron poison in the s-process
The doubly magic nucleus ¹⁶O has a small neutron-capture cross section of just a few tens of microbarns in the astrophysical energy region. Despite this, ¹⁶O plays an important role as a neutron poison in the astrophysical slow neutron capture (s) process due to its high abundance. We present in this paper a re-evaluation of the available experimental data for ¹⁶O(n, γ)¹⁷O and derive a new recommendation for the Maxwellian-averaged cross sections between kT = 5 and 100 keV. Our new recommendations are lower up to kT = 60 keV compared to the previously recommended values but up to 14% higher at kT = 100 keV. We explore the impact of this different energy dependence on the weak s-process during core helium burning (kT = 26 keV) and shell carbon burning (kT = 90 keV) in massive stars where ¹⁶O is the most abundant isotope
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
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