252 research outputs found
Uncertainties in AGB Evolution and Nucleosynthesis
We summarise the evolution and nucleosynthesis in AGB and Super-AGB stars. We
then examine the major sources of uncertainty, especially mass-loss.Comment: 8 pages, no figures. Invited review presented at The 11th Pacific Rim
Conference on Stellar Astrophysics "Physics and Chemistry of the Late Stages
of Stellar Evolution
Stellar yields from metal-rich asymptotic giant branch models
We present new theoretical stellar yields and surface abundances for three
grids of metal-rich asymptotic giant branch (AGB) models. Post-processing
nucleosynthesis results are presented for stellar models with initial masses
between 1 and 7.5 for , and 1 and
8 for (solar) and . We include stellar surface
abundances as a function of thermal pulse on the AGB for elements from C to Bi
and for a selection of isotopic ratios for elements up to Fe and Ni (e.g.,
C/C), which can be obtained from observations of molecules in
stars and from the laboratory analysis of meteoritic stardust grains. Ratios of
elemental abundances of He/H, C/O, and N/O are also included, which are useful
for direct comparison to observations of AGB stars and their progeny including
planetary nebulae. The integrated elemental stellar yields are presented for
each model in the grid for hydrogen, helium and all stable elements from C to
Bi. Yields of Li are also included for intermediate-mass models with hot bottom
burning. We present the first neutron-capture (-process) yields for
super-solar metallicity AGB stars with , and the first complete
-process yields for models more massive than 6 at all three
metallicities.Comment: 20 pages, 20 figures, includes supplementary surface abundance and
yield data tables; accepted for publication in Ap
Heavy element abundances in planetary nebulae: A theorist's perspective
The determination of heavy element abundances from planetary nebula (PN)
spectra provides an exciting opportunity to study the nucleosynthesis occurring
in the progenitor asymptotic giant branch (AGB) star. We perform
post-processing calculations on AGB models of a large range of mass and
metallicity to obtain predictions for the production of neutron-capture
elements up to the first s-process peak at strontium. We find that solar
metallicity intermediate-mass AGB models provide a reasonable match to the
heavy element composition of Type I PNe. Likewise, many of the Se and Kr
enriched PNe are well fitted by lower mass models with solar or close-to-solar
metallicities. However the most Kr-enriched objects, and the PN with sub-solar
Se/O ratios are difficult to explain with AGB nucleosynthesis models.
Furthermore, we compute s-process abundance predictions for low-mass AGB models
of very low metallicity ([Fe/H] =-2.3) using both scaled solar and an
alpha-enhanced initial composition. For these models, O is dredged to the
surface, which means that abundance ratios measured relative to this element
(e.g., [X/O]) do not provide a reliable measure of initial abundance ratios, or
of production within the star owing to internal nucleosynthesis.Comment: 5 pages, presentation at the workshop on the Legacies of the
Macquarie/AAO/Strasbourg H-alpha Planetary Nebula project, accepted for
publication in PAS
Reaction Rate Uncertainties: NeNa and MgAl in AGB Stars
We study the effect of uncertainties in the proton-capture reaction rates of
the NeNa and MgAl chains on nucleosynthesis due to the operation of hot bottom
burning (HBB) in intermediate-mass asymptotic giant branch (AGB) stars. HBB
nucleosynthesis is associated with the production of sodium, radioactive Al26
and the heavy magnesium isotopes, and it is possibly responsible for the O, Na,
Mg and Al abundance anomalies observed in globular cluster stars.
We model HBB with an analytic code based on full stellar evolution models so
we can quickly cover a large parameter space. The reaction rates are varied
first individually, then all together. This creates a knock-on effect, where an
increase of one reaction rate affects production of an isotope further down the
reaction chain. We find the yields of Ne22, Na23 and Al26 to be the most
susceptible to current nuclear reaction rate uncertainties.Comment: Presented at NIC-IX, International Symposium on Nuclear Astrophysics
- Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 200
R Coronae Borealis Stars are Viable Factories of Pre-solar Grains
We present a new theoretical estimate for the birthrate of R Coronae Borealis
(RCB) stars that is in agreement with recent observational data. We find the
current Galactic birthrate of RCB stars to be 25% of the Galactic
rate of Type Ia supernovae, assuming that RCB stars are formed through the
merger of carbon-oxygen and helium-rich white dwarfs. Our new RCB birthrate
( yr) is a factor of 10 lower than previous
theoretical estimates. This results in roughly 180--540 RCB stars in the
Galaxy, depending on the RCB lifetime. From the theoretical and observational
estimates, we calculate the total dust production from RCB stars and compare
this rate to dust production from novae and born-again asymptotic giant branch
(AGB) stars. We find that the amount of dust produced by RCB stars is
comparable to the amounts produced by novae or born-again post-AGB stars,
indicating that these merger objects are a viable source of carbonaceous
pre-solar grains in the Galaxy. There are graphite grains with carbon and
oxygen isotopic ratios consistent with the observed composition of RCB stars,
adding weight to the suggestion that these rare objects are a source of
stardust grains.Comment: Accepted for publication in The Astrophysical Journal, 7 page
On Potassium and Other Abundance Anomalies of Red Giants in NGC 2419
Globular clusters are of paramount importance for testing theories of stellar
evolution and early galaxy formation. Strong evidence for multiple populations
of stars in globular clusters derives from observed abundance anomalies. A
puzzling example is the recently detected Mg-K anticorrelation in NGC 2419. We
perform Monte Carlo nuclear reaction network calculations to constrain the
temperature-density conditions that gave rise to the elemental abundances
observed in this elusive cluster. We find a correlation between stellar
temperature and density values that provide a satisfactory match between
simulated and observed abundances in NGC 2419 for all relevant elements (Mg,
Si, K, Ca, Sc, Ti, and V). Except at the highest densities (~g/cm), the acceptable conditions range from ~MK at
~g/cm to ~MK at
~g/cm. This result accounts for uncertainties in nuclear reaction
rates and variations in the assumed initial composition. We review hydrogen
burning sites and find that low-mass stars, AGB stars, massive stars, or
supermassive stars cannot account for the observed abundance anomalies in NGC
2419. Super-AGB stars could be viable candidates for the polluter stars if
stellar model parameters can be fine-tuned to produce higher temperatures.
Novae, either involving CO or ONe white dwarfs, could be interesting polluter
candidates, but a current lack of low-metallicity nova models precludes firmer
conclusions. We also discuss if additional constraints for the first-generation
polluters can be obtained by future measurements of oxygen, or by evolving
models of second-generation low-mass stars with a non-canonical initial
composition.Comment: 24 pages, 9 Figure
The diverse origins of neutron-capture elements in the metal-poor star HD 94028 : possible detection of products of i-process nucleosynthesis
We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = −1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = −0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy
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