1,721,141 research outputs found
The puzzle of silicon, titanium, and magnesium anomalies in meteoritic silicon carbide grains
No full textAn astrophysical interpretation of the silicon, titanium, and magnesium
isotope anomalies measured in the mainstream population of single
silicon carbide (SiC) grains extracted from carbonaceous meteorites is
presented. The condensation site of the grains is envisaged in the cool
atmospheres of carbon stars. The Si isotope anomalies show a general
linear correlation between Si-29/Si-28 aand Si-30/Si-28, whose slope
cannot be explained only by s-processing in the He-burning shell and
dilution with material of solar composition from the envelope. We
suggest a multiple star scenario in which the SiC grains form in stellar
envelopes with slightly nonsolar initial Si isotope composition and
metallicities from one-half solar to solar. The initial Si abundances
are inferred from considerations of galactic chemical evolution,
coupling spectroscopic observations of abundances in stars of different
metal content with current predictions of stellar nucleosynthesis. The
isotopes Si-29 and Si-30 are assumed to be entirely produced by
short-lived massive stars exploding as supernovae, which also contribute
approximately 70% of the solar Si-28 abundance, the remaining
approximately 30% coming from long-lived stars, evolving in binary
systems, and leading to supernovae of Type Ia. More detailed
calculations of the Si isotopes yields from stars of various mass and
initial metallicity are, however, required, and a better understanding
of how the nucleosynthetic ejecta by supernovae are well homogenized
with the interstellar matter. Even the Ti isotope anomalies in SiC
grains cannot be explained as only an s-process signature. The linear
correlation shown by Ti and Si anomalies indicates that a similar
approach can be used to interpret the Ti anomalies as a mixture of a
pure s-component and of a variable nonsolar isotopic composition
initially present in the envelope of carbon stars. The question of the
large abundance of extinct Al-26 in many SiC grains is also considered.
We find that the production of Al-26 in the H shell of thermally pulsing
AGB stars, although followed by substantial consumption by neutron
captures during He thermal pulses, can account for the high Al-26/Al-27
ratios. The spread of carbon anomalies is interpreted as a consequence
of an initial spread of C-12/C-13 as observed in M stars and of the
subsequent enrichment in C-12 of the envelope during thermal pulses.
Finally, the nitrogen isotope anomalies are discussed
Understanding n-capture nucleosynthesis - A test for stellar and galactic evolution
No full textTheoretical and experimental data are reviewed as they relate to stellar
n-capture nucleosynthesis and therefore to constraints on advanced
stellar evolution. Attention is given to the mechanisms of n-captures
occuring near the valley of beta-stability or with very strong neutron
fluxes under dynamical conditions. High-resolution spectroscopic data
and advances in nuclear theory are shown to be useful for constraining
the explosive stages of stellar evolution as well as the age of the
universe
S-process nucleosynthesis in massive stars and the weak component. II - Carbon burning and galactic enrichment
No full textThe s-process that occurs in the shell carbon-burning phase of a typical
massive star of 25 solar masses is examined. It is shown that neutron
captures during shell C-burning can significantly change the
s-abundances. The composition of the s-processed material ejected by a
25 solar mass model is evaluated, and the contributions of massive stars
to the chemical enrichment of the Galaxy in s-isotopes are estimated.
The s-contributions to the solar composition from low-mass stars are
then taken into account, and possibility of matching the solar
distribution of the s-isotopes produced by the weak component is
investigated. It is found that the s-process in massive stars can
completely account for the weak component observed in the solar system
and can put important constraints on the scenario of the chemical
evolution of the Galaxy
Nuclei in the Cosmos III Third International Symposium on Nuclear Astrophysics
No full textproceedings book (615 pages) of the Nuclei in the Cosmos n. III symposium held in Assergi, L'Aquila, in 1994. M. Busso is the chairman of the organizing commitee and the first edito
S-process nucleosynthesis in massive stars and the weak component. II - Carbon burning and galactic enrichment
No full textThe s-process that occurs in the shell carbon-burning phase of a typical
massive star of 25 solar masses is examined. It is shown that neutron
captures during shell C-burning can significantly change the
s-abundances. The composition of the s-processed material ejected by a
25 solar mass model is evaluated, and the contributions of massive stars
to the chemical enrichment of the Galaxy in s-isotopes are estimated.
The s-contributions to the solar composition from low-mass stars are
then taken into account, and possibility of matching the solar
distribution of the s-isotopes produced by the weak component is
investigated. It is found that the s-process in massive stars can
completely account for the weak component observed in the solar system
and can put important constraints on the scenario of the chemical
evolution of the Galaxy
S-processing in massive stars as a function of metallicity and interpretation of observational trends
No full textThe s-process in massive stars is analyzed as a function of metallicity.
The nucleosynthesis occurring in both core helium and shell carbon
burning is investigated by numerically modeling nuclear reaction
networks, subject to the conditions provided by stellar models. The
s-process in massive stars is found to be 'secondary-like' in the disk,
the amount of s-process matter ejected being roughly proportional to Fe.
In the halo, the s-efficiency drops at low metallicities, with Fe/H
below about -2. The elemental s-contributions from massive stars to the
solar abundances from iron to zirconium are presented. It is concluded
that the weak component can account for a consistent fraction of the
solar Cu, Ga, Ge, and Se. The evolutionary trends of elements such as
Co, Ni, Cu, Zn, Rb, Sr, Y, and Zr are discussed in comparison with
observations of stars belonging to the halo, where massive stars give
their major imprint
Carbon stars and isotopic BA anomalies in meteoritic SiC grains
No full textNew calculations of s-processing in TP-AGB stars of low mass based on an
updated compilation of neutron capture cross sections are presented. It
is shown how the reproduction of solar abundances for the s-isotopes of
Ba requires a special choice of neutron capture cross sections of Ba-136
and Ba-138. It is further shown how agreement between the predictions of
He shell nucleosynthesis and the Ba isotopic mixture of the G-component
in SiC can be obtained, once C-stars with mean neutron exposures lower
than that required to fit the solar main component are considered.
Problems related to the interpretation of other isotope anomalies in SiC
are also examined
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