Abridged Barium stars are moderately rare, chemically peculiar objects, which are believed to be the result of the pollution of an otherwise normal star by material from an evolved companion on the ...asymptotic giant branch (AGB). We aim to derive carbon, nitrogen, oxygen, and fluorine abundances for the first time from the infrared spectra of the barium red giant star HD 123396 to quantitatively test AGB nucleosynthesis models for producing barium stars via mass accretion. High-resolution and high S/N infrared spectra were obtained using the Phoenix spectrograph mounted at the Gemini South telescope. The abundances were obtained through spectrum synthesis of individual atomic and molecular lines, using the MOOG stellar line analysis program, together with Kurucz's stellar atmosphere models. The analysis was classical, using 1D stellar models and spectral synthesis under the assumption of local thermodynamic equilibrium. We confirm that HD 123396 is a metal-deficient barium star (Fe/H = -1.05), with A(C) = 7.88, A(N) = 6.65, A(O) = 7.93, and A(Na) = 5.28 on a logarithmic scale where A(H) = 12, leading to (C+N)/Fe ~ 0.5. The A(CNO) group, as well as the A(Na) abundances, is in excellent agreement with those previously derived for this star using high resolution optical data. We also found A(F) = 4.16, which implies F/O = 0.39, a value that is substantially higher than the F abundances measured in globular clusters of a similar metallicity, noting that there are no F measurements in field stars of comparable metallicity. The observed abundance pattern of the light elements (CNO, F and Na) recovered here as well as the heavy elements (s-process) studied elsewhere suggest that the surface composition of HD 123396 is well fitted by the predicted abundance pattern of a 1.5Msun AGB model star with Z = 0.001.
Astrophys.J.656:L73-L76,2007 Observations of planetary nebulae (PNe) by Sterling, Dinerstein and Bowers
have revealed abundances in the neutron-capture element Germanium (Ge) from
solar to factors of ...3 -- 10 above solar. The enhanced Ge is an indication that
the slow-neutron capture process (s process) operated in the parent star during
the thermally-pulsing asymptotic giant branch (TP-AGB) phase. We compute the
detailed nucleosynthesis of a series of AGB models to estimate the surface
enrichment of Ge near the end of the AGB. A partial mixing zone of constant
mass is included at the deepest extent of each dredge-up episode, resulting in
the formation of a 13C pocket in the top ~1/10th of the He-rich intershell. All
of the models show surface increases of Ge/Fe less than about 0.5, except the
2.5Msun, Z=0.004 case which produced a factor of 6 enhancement of Ge. Near the
tip of the TP-AGB, a couple of extra TPs could occur to account for the
composition of the most Ge-enriched PNe. Uncertainties in the theoretical
modeling of AGB stellar evolution might account for larger Ge enhancements than
we predict here. Alternatively, a possible solution could be provided by the
occurrence of a late TP during the post-AGB phase. Difficulties related to
spectroscopic abundance estimates also need to be taken into consideration.
Further study is required to better assess how the model uncertainties affect
the predictions and, consequently, if a late TP should be invoked.
Astrophys.J.652:1240-1245,2006 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.
PoSNIC-IX:038,2006 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.
Using a chemical evolution model we investigate the intriguing suggestion
that there are populations of stars in some globular clusters (e.g. NGC 2808,
omega Centauri) with enhanced levels of helium ...(Y from about 0.28 to 0.40)
compared to the majority of the population that presumably have a primordial
helium abundance. We assume that a previous generation of massive
low-metallicity Asymptotic Giant Branch (AGB) stars has polluted the cluster
gas via a slow stellar wind. We use two independent sets of AGB yields computed
from detailed models to follow the evolution of helium, carbon, nitrogen and
oxygen in the cluster gas using a Salpeter initial mass function (IMF) and a
number of top-heavy IMFs. In no case were we able to fit the observational
constraints, Y > 0.30 and C+N+O approximately constant. Depending on the shape
of the IMF and the yields, we either obtained Y approximately greater than 0.30
and large increases in C+N+O or Y < 0.30 and C+N+O approximately constant.
These results suggest that either AGB stars alone are not responsible for the
large helium enrichment or that any dredge-up from this generation of stars was
less than predicted by standard models.
Observations of planetary nebulae (PNe) by Sterling, Dinerstein and Bowers have revealed abundances in the neutron-capture element Germanium (Ge) from solar to factors of 3 -- 10 above solar. The ...enhanced Ge is an indication that the slow-neutron capture process (s process) operated in the parent star during the thermally-pulsing asymptotic giant branch (TP-AGB) phase. We compute the detailed nucleosynthesis of a series of AGB models to estimate the surface enrichment of Ge near the end of the AGB. A partial mixing zone of constant mass is included at the deepest extent of each dredge-up episode, resulting in the formation of a 13C pocket in the top ~1/10th of the He-rich intershell. All of the models show surface increases of Ge/Fe less than about 0.5, except the 2.5Msun, Z=0.004 case which produced a factor of 6 enhancement of Ge. Near the tip of the TP-AGB, a couple of extra TPs could occur to account for the composition of the most Ge-enriched PNe. Uncertainties in the theoretical modeling of AGB stellar evolution might account for larger Ge enhancements than we predict here. Alternatively, a possible solution could be provided by the occurrence of a late TP during the post-AGB phase. Difficulties related to spectroscopic abundance estimates also need to be taken into consideration. Further study is required to better assess how the model uncertainties affect the predictions and, consequently, if a late TP should be invoked.
We present detailed models of low and intermediate-mass asymptotic giant branch (AGB) stars with and without the 18F(a,p)21Ne reaction included in the nuclear network, where the rate for this ...reaction has been recently experimentally evaluated for the first time. The lower and recommended measured rates for this reaction produce negligible changes to the stellar yields, whereas the upper limit of the rate affects the production of 19F and 21Ne. The stellar yields increase by ~50% to up to a factor of 4.5 for 19F, and by factors of ~2 to 9.6 for 21Ne. While the 18}F(a,p)21Ne reaction competes with 18O production, the extra protons released are captured by 18O to facilitate the 18O(p,a)15N(a,g)19F chain. The higher abundances of 19F obtained using the upper limit of the rate helps to match the F/O ratios observed in AGB stars, but only for large C/O ratios. Extra-mixing processes are proposed to help to solve this problem. Some evidence that the 18F(a,p)21Ne rate might be closer to its upper limit is provided by the fact that the higher calculated 21Ne/22Ne ratios in the He intershell provide an explanation for the Ne isotopic composition of silicon-carbide grains from AGB stars. This needs to be confirmed by future experiments of the 18F(a,p)21Ne reaction rate. The availability of accurate fluorine yields from AGB stars will be fundamental for interpreting observations of this element in carbon-enhanced metal-poor stars.
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.
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.
Using a chemical evolution model we investigate the intriguing suggestion that there are populations of stars in some globular clusters (e.g. NGC 2808, omega Centauri) with enhanced levels of helium ...(Y from about 0.28 to 0.40) compared to the majority of the population that presumably have a primordial helium abundance. We assume that a previous generation of massive low-metallicity Asymptotic Giant Branch (AGB) stars has polluted the cluster gas via a slow stellar wind. We use two independent sets of AGB yields computed from detailed models to follow the evolution of helium, carbon, nitrogen and oxygen in the cluster gas using a Salpeter initial mass function (IMF) and a number of top-heavy IMFs. In no case were we able to fit the observational constraints, Y > 0.30 and C+N+O approximately constant. Depending on the shape of the IMF and the yields, we either obtained Y approximately greater than 0.30 and large increases in C+N+O or Y < 0.30 and C+N+O approximately constant. These results suggest that either AGB stars alone are not responsible for the large helium enrichment or that any dredge-up from this generation of stars was less than predicted by standard models.
