244 research outputs found
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
Isotope Anomalies in the Fe-group Elements in Meteorites and Connection to Nucleosynthesis in AGB Stars
We study the effects of neutron captures in AGB stars on \oq Fe-group\cqb
elements, with an emphasis on Cr, Fe, and Ni. These elements show anomalies in
Cr, Fe, and Ni in solar-system materials, which are
commonly attributed to SNe. However, as large fractions of the interstellar
medium (ISM) were reprocessed in AGB stars, these elements were reprocessed,
too. We calculate the effects of such reprocessing on Cr, Fe, and Ni through
1.5\msb and 3\msb AGB models, adopting solar and 1/3 solar metallicities. All
cases produce excesses of Cr, Fe, and Ni, while the other
isotopes are little altered; hence, the observations may be explained by AGB
processing. The results are robust and not dependent on the detailed initial
isotopic composition. Consequences for other \oq Fe group\cqb elements are then
explored. They include Ti excesses, and some production of
Ti. In many circumstellar condensates, Ti quantitatively reflects
these effects of AGB neutron captures. Scatter in the data results from small
variations (granularity) in the isotopic composition of the local ISM. For Si,
the main effects are instead due to variations in the local ISM from different
SNe sources. The problem of Ca is discussed, particularly with regard to
Ca. The measured data are usually represented assuming terrestrial
values for Ca/Ca. Materials processed in AGB stars or sources
with variable initial Ca/Ca ratios can give apparent Ca
excesses/deficiencies, attributed to SNe. The broader issue of Galactic
Chemical Evolution is also discussed in view of the isotopic granularity in the
ISM. \end{abstract
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
On the need of the Light Elements Primary Process (LEPP)
Extant chemical evolution models underestimate the Galactic production of Sr,
Y and Zr as well as the Solar System abundances of s-only isotopes with
90<A<130. To solve this problem, an additional (unknown) process has been
invoked, the so-called LEPP (Light Element Primary Process). In this paper we
investigate possible alternative solutions. Basing on Full Network Stellar
evolutionary calculations, we investigate the effects on the Solar System
s-only distribution induced by the inclusion of some commonly ignored physical
processes (e.g. rotation) or by the variation of the treatment of convective
overshoot, mass-loss and the efficiency of nuclear processes. Our main findings
are: 1) at the epoch of the formation of the Solar System, our reference model
produces super-solar abundances for the whole s-only distribution, even in the
range 90<A<130; 2) within errors, the s-only distribution relative to 150Sm is
flat; 3) the s-process contribution of the less massive AGB stars (M<1.5 M_SUN)
as well as of the more massive ones (M>4.0 M_SUN) are negligible; 4) the
inclusion of rotation implies a downward shift of the whole distribution with
an higher efficiency for the heavy s-only isotopes, leading to a flatter s-only
distribution; 5) different prescriptions on convection or mass-loss produce
nearly rigid shifts of the whole distribution. In summary, a variation of the
standard paradigm of AGB nucleosynthesis would allow to reconcile models
predictions with Solar System s-only abundances. Nonetheless, the LEPP cannot
be definitely ruled out, because of the uncertainties still affecting stellar
and Galactic chemical evolution models.Comment: Accepted for publication on Ap
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
Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins
We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19)
and Z (18), rare types commonly argued to have formed in lower-than-solar
metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and
Z grain data with data for mainstream grains from AGB stars of close-to-solar
metallicity demonstrates that the three types of grains have indistinguishable
Mo isotopic compositions. We show that the Mo isotope data can be used to
constrain the maximum stellar temperatures (TMAX) during thermal pulses in AGB
stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with
the grain data for Mo isotopes points to an origin from low-mass (~1.5-3 Msun)
rather than intermediate-mass (>3-~9 Msun) AGB stars. Because of the low
efficiency of 22Ne({\alpha},n)25Mg at the low TMAX values attained in low-mass
AGB stars, model calculations cannot explain the large 30Si excesses of Z
grains as arising from neutron capture, so these excesses remain a puzzle at
the moment.Comment: The Astrophysical Journal (Accepted
Magnetic-buoyancy Induced Mixing in AGB Stars: Presolar SiC Grains
Isotope ratios can be measured in presolar SiC grains from ancient Asymptotic
Giant Branch (AGB) stars at permil-level (0.1\%) precision. Such precise grain
data permit derivation of more stringent constraints and calibrations on mixing
efficiency in AGB models than traditional spectroscopic observations. In this
paper we compare SiC heavy-element isotope ratios to a new series of FRUITY
models that include the effects of mixing triggered by magnetic fields. Based
on 2D and 3D simulations available in the literature, we propose a new
formulation, upon which the general features of mixing induced by magnetic
fields can be derived. The efficiency of such a mixing, on the other hand,
relies on physical quantities whose values are poorly constrained. We present
here our calibration by comparing our model results with the heavy-element
isotope data of presolar SiC grains from AGB stars. We demonstrate that the
isotopic compositions of all measured elements (Ni, Sr, Zr, Mo, Ba) can be
simultaneously fitted by adopting a single magnetic field configuration in our
new FRUITY models.Comment: 9 pages, 4 figures, 1 table. Accepted for publication on ApJ
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
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