1,537 research outputs found
Heavy Element Nucleosynthesis in the Brightest Galactic Asymptotic Giant Branch stars
We present updated calculations of stellar evolutionary sequences and
detailed nucleosynthesis predictions for the brightest asymptotic giant branch
(AGB) stars in the Galaxy with masses between 5Msun to 9Msun, with an initial
metallicity of Z =0.02 ([Fe/H] = 0.14). In our previous studies we used the
Vassiliadis & Wood mass-loss rate, which stays low until the pulsation period
reaches 500 days after which point a superwind begins. Vassiliadis & Wood noted
that for stars over 2.5Msun the superwind should be delayed until P ~ 750 days
at 5Msun. We calculate evolutionary sequences where we delay the onset of the
superwind to pulsation periods of P ~ 700-800 days in models of M = 5, 6, and
7Msun. Post-processing nucleosynthesis calculations show that the 6 and 7Msun
models produce the most Rb, with [Rb/Fe] ~ 1 dex, close to the average of most
of the Galactic Rb-rich stars ([Rb/Fe] ~ 1.4 plus or minus 0.8 dex). Changing
the rate of the 22Ne + alpha reactions results in variations of [Rb/Fe] as
large as 0.5 dex in models with a delayed superwind. The largest enrichment in
heavy elements is found for models that adopt the NACRE rate of the
22Ne(a,n)25Mg reaction. Using this rate allows us to best match the composition
of most of the Rb-rich stars. A synthetic evolution algorithm is then used to
remove the remaining envelope resulting in final [Rb/Fe] of ~ 1.4 dex although
with C/O ratios > 1. We conclude that delaying the superwind may account for
the large Rb overabundances observed in the brightest metal-rich AGB stars.Comment: 37 pages, accepted for publication in the Astrophysical Journal,
minor modifications to text and Tables 2 and 3, reference adde
Updated stellar yields from Asymptotic Giant Branch models
An updated grid of stellar yields for low to intermediate-mass
thermally-pulsing Asymptotic Giant Branch (AGB) stars are presented. The models
cover a range in metallicity Z = 0.02, 0.008, 0.004, and 0.0001, and masses
between 1Msun to 6Msun. New intermediate-mass Z = 0.0001 AGB models are also
presented, along with a finer mass grid than used in previous studies. The
yields are computed using an updated reaction rate network that includes the
latest NeNa and MgAl proton capture rates, with the main result that between ~6
to 30 times less Na is produced by intermediate-mass models with hot bottom
burning. In low-mass AGB models we investigate the effect on the production of
light elements of including some partial mixing of protons into the intershell
region during the deepest extent of each third dredge-up episode. The protons
are captured by the abundant 12C to form a 13C pocket. The 13C pocket increases
the yields of 19F, 23Na, the neutron-rich Mg and Si isotopes, 60Fe, and 31P.
The increase in 31P is by factors of ~4 to 20, depending on the metallicity.
Any structural changes caused by the addition of the 13C pocket into the
He-intershell are ignored. However, the models considered are of low mass and
any such feedback is likely to be small. Further study is required to test the
accuracy of the yields from the partial-mixing models. For each mass and
metallicity, the yields are presented in a tabular form suitable for use in
galactic chemical evolution studies or for comparison to the composition of
planetary nebulae.Comment: Accepted for publication in MNRAS; 15 page
Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars
A recent survey of a large sample of Galactic intermediate-mass (>3 Msun)
asymptotic giant branch (AGB) stars shows that they exhibit large
overabundances of rubidium (Rb) up to 100--1000 times solar. These observations
set constraints on our theoretical notion of the slow neutron capture process
(s process) that occurs inside intermediate-mass AGB stars. Lithium (Li)
abundances are also reported for these stars. In intermediate-mass AGB stars,
Li can be produced by proton captures occuring at the base of the convective
envelope. For this reason the observations of Rb, Zr, and Li set complementary
constraints on different processes occurring in the same stars. We present
predictions for the abundances of Rb, Zr, and Li as computed for the first time
simultaneously in intermediate-mass AGB star models and compare them to the
current observational constraints. We find that the Rb abundance increases with
increasing stellar mass, as is inferred from observations but we are unable to
match the highest observed [Rb/Fe] abundances. Inclusion of a partial mixing
zone (PMZ) to activate the 13C(a,n)16O reaction as an additional neutron source
yields significant enhancements in the Rb abundance. However this leads to Zr
abundances that exceed the upper limits of the current observational
constraints. If the third dredge-up (TDU) efficiency remains as high during the
final stages of AGB evolution as during the earlier stages, we can match the
lowest values of the observed Rb abundance range. We predict large variations
in the Li abundance, which are observed. Finally, the predicted Rb production
increases with decreasing metallicity, in qualitative agreement with
observations of Magellanic Cloud AGB stars. However stellar models of Z=0.008
and Z=0.004 intermediate-mass AGB stars do not produce enough Rb to match the
observed abundances.Comment: 11 pages, 7 figures, accepted for publication on Astronomy &
Astrophysic
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&
Gas and dust from solar metallicity AGB stars
We study the asymptotic giant branch (AGB) evolution of stars with masses
between . We focus on stars with a solar chemical
composition, which allows us to interpret evolved stars in the Galaxy. We
present a detailed comparison with models of the same chemistry, calculated
with a different evolution code and based on a different set of physical
assumptions. We find that stars of mass experience hot
bottom burning at the base of the envelope. They have AGB lifetimes shorter
than yr and eject into their surroundings gas contaminated
by proton-capture nucleosynthesis, at an extent sensitive to the treatment of
convection. Low mass stars with become
carbon stars. During the final phases the C/O ratio grows to . We find
a remarkable agreement between the two codes for the low-mass models and
conclude that predictions for the physical and chemical properties of these
stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB
phase. The dust produced is also dependent on the mass: low-mass stars produce
mainly solid carbon and silicon carbide dust, whereas higher mass stars produce
silicates and alumina dust. Possible future observations potentially able to
add more robustness to the present results are also discussed.Comment: 27 pages, 24 figures; accepted for publication in MNRA
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