1,042 research outputs found
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. II. Statistical analysis of a sample of 67 CEMP- stars
Many observed CEMP stars are found in binary systems and show enhanced
abundances of -elements. The origin of the chemical abundances of these
CEMP- stars is believed to be accretion in the past of enriched material
from a primary star in the AGB phase. We investigate the mechanism of mass
transfer and the process of nucleosynthesis in low-metallicity AGB stars by
modelling the binary systems in which the observed CEMP- stars were formed.
For this purpose we compare a sample of CEMP- stars with a grid of
binary stars generated by our binary evolution and nucleosynthesis model. We
classify our sample CEMP- stars in three groups based on the observed
abundance of europium. In CEMP stars the europium-to-iron ratio is more
than ten times higher than in the Sun, whereas it is lower than this threshold
in CEMP stars. No measurement of europium is currently available for
CEMP- stars. On average our models reproduce well the abundances observed
in CEMP- stars, whereas in CEMP- stars and CEMP- stars the
abundances of the light- elements are systematically overpredicted by our
models and in CEMP- stars the abundances of the heavy- elements are
underestimated. In all stars our modelled abundances of sodium overestimate the
observations. This discrepancy is reduced only in models that underestimate the
abundances of most of the -elements. Furthermore, the abundance of lead is
underpredicted in most of our model stars. These results point to the
limitations of our AGB nucleosynthesis model, particularly in the predictions
of the element-to-element ratios. Finally, in our models CEMP- stars are
typically formed in wide systems with periods above 10000 days, while most of
the observed CEMP- stars are found in relatively close orbits with periods
below 5000 days.Comment: 23 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. I. Detailed analysis of 15 binary stars with known orbital periods
AGB stars are responsible for producing a variety of elements, including
carbon, nitrogen, and the heavy elements produced in the slow neutron-capture
process (-elements). There are many uncertainties involved in modelling the
evolution and nucleosynthesis of AGB stars, and this is especially the case at
low metallicity, where most of the stars with high enough masses to enter the
AGB have evolved to become white dwarfs and can no longer be observed. The
stellar population in the Galactic halo is of low mass () and only a few observed stars have evolved beyond the first
giant branch. However, we have evidence that low-metallicity AGB stars in
binary systems have interacted with their low-mass secondary companions in the
past. The aim of this work is to investigate AGB nucleosynthesis at low
metallicity by studying the surface abundances of chemically peculiar very
metal-poor stars of the halo observed in binary systems. To this end we select
a sample of 15 carbon- and -element-enhanced metal-poor (CEMP-) halo
stars that are found in binary systems with measured orbital periods. With our
model of binary evolution and AGB nucleosynthesis, we determine the binary
configuration that best reproduces, at the same time, the observed orbital
period and surface abundances of each star of the sample. The observed periods
provide tight constraints on our model of wind mass transfer in binary stars,
while the comparison with the observed abundances tests our model of AGB
nucleosynthesis.Comment: 18 pages, 20 figures, accepted for publication on A&
Reaction rate uncertainties and the operation of the NeNa and MgAl chains during HBB in intermediate-mass AGB stars
We test the effect of proton-capture reaction rate uncertainties on the
abundances of the Ne, Na, Mg and Al isotopes processed by the NeNa and MgAl
chains during hot bottom burning (HBB) in asymptotic giant branch (AGB) stars
of intermediate mass between 4 and 6 solar masses and metallicities between
Z=0.0001 and 0.02. We provide uncertainty ranges for the AGB stellar yields,
for inclusion in galactic chemical evolution models, and indicate which
reaction rates are most important and should be better determined. We use a
fast synthetic algorithm based on detailed AGB models. We run a large number of
stellar models, varying one reaction per time for a very fine grid of values,
as well as all reactions simultaneously. We show that there are uncertainties
in the yields of all the Ne, Na, Mg and Al isotopes due to uncertain
proton-capture reaction rates. The most uncertain yields are those of 26Al and
23Na (variations of two orders of magnitude), 24Mg and 27Al (variations of more
than one order of magnitude), 20Ne and 22Ne (variations between factors 2 and
7). In order to obtain more reliable Ne, Na, Mg and Al yields from IM-AGB stars
the rates that require more accurate determination are: 22Ne(p,g)23Na,
23Na(p,g)24Mg, 25Mg(p,g)26Al, 26Mg(p,g)27Al and 26Al(p,g)27Si. Detailed
galactic chemical evolution models should be constructed to address the impact
of our uncertainty ranges on the observational constraints related to HBB
nucleosynthesis, such as globular cluster chemical anomalies.Comment: accepted for publication on Astronomy & Astrophysic
Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements
The production of the elements heavier than iron via slow neutron captures
(the s process) is a main feature of the contribution of asymptotic giant
branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos.
However, our understanding of the main neutron source, the 13C(alpha,n)16O
reaction, is still incomplete. It is commonly assumed that in AGB stars mixing
beyond convective borders drives the formation of 13C pockets. However, there
is no agreement on the nature of such mixing and free parameters are present.
By means of a parametric model we investigate the impact of different mixing
functions on the final s-process abundances in low-mass AGB models. Typically,
changing the shape of the mixing function or the mass extent of the region
affected by the mixing produce the same results. Variations in the relative
abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are
generally within +/-0.2 dex, similar to the observational error bars. We
conclude that other stellar uncertainties - the effect of rotation and of
overshoot into the C-O core - play a more important role than the details of
the mixing function. The exception is at low metallicity, where the Pb
abundance is significantly affected. In relation to the composition observed in
stardust SiC grains from AGB stars, the models are relatively close to the data
only when assuming the most extreme variation in the mixing profile.Comment: 17 pages, 8 figures, 6 tables, accepted for publications on Monthly
Notices of the Royal Astronomical Societ
The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario
We investigate the suggestion that there are stellar populations in some
globular clusters with enhanced helium (Y from 0.28 to 0.40) compared to the
primordial value. We assume that a previous generation of massive Asymptotic
Giant Branch (AGB) stars have polluted the cluster. Two independent sets of AGB
yields are used to follow the evolution of helium and CNO using a Salpeter
initial mass function (IMF) and two top-heavy IMFs. In no case are we able to
produce the postulated large Y ~ 0.35 without violating the observational
constraint that the CNO content is nearly constant.Comment: accepted for publication in Ap
The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae
Sulfur abundances derived from optical emission line measurements and
ionization correction factors in planetary nebulae are systematically lower
than expected for the objects' metallicities. We have carefully considered a
large range of explanations for this "sulfur anomaly", including: (1)
correlations between the size of the sulfur deficit and numerous nebular and
central star properties; (2) ionization correction factors which under-correct
for unobserved ions; (3) effects of dielectronic recombination on the sulfur
ionization balance; (4) sequestering of S into dust and/or molecules; and (5)
excessive destruction of S or production of O by AGB stars. It appears that all
but the second scenario can be ruled out. However, we find evidence that the
sulfur deficit is generally reduced but not eliminated when S^+3 abundances
determined directly from IR measurements are used in place of the customary
sulfur ionization correction factor. We tentatively conclude that the sulfur
anomaly is caused by the inability of commonly used ICFs to properly correct
for populations of ionization stages higher than S^+2.Comment: 40 pages, 14 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
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