1,801 research outputs found
Massive AGB models of low metallicity: the implications for the self-enrichment scenario in metal poor Globular Clusters
Context: We present the physical and chemical properties of intermediate-mass
stars models of low metallicity, evolved along the thermal pulse phase.
Aims: The target of this work is to extend to low metallicities, Z=1,2 and 6
x 10^{-4}, the models previously computed for chemistries typical of Globular
Clusters of an intermediate metallicity (Z=0.001), and for the most metal-rich
clusters found in our Galaxy (Z=0.004); the main goal is to test the
self-enrichment scenario also for metal poor Globular Clusters
Methods: We calculated three grids of intermediate-mass models with
metallicities Z=10^{-4}, 2x10^{-4}, and 6x10^{-4}; the evolutionary sequences
are followed from the pre-main sequence throughout the AGB phase, almost until
the ejection of the whole envelope. We discuss the chemistry of the ejecta, and
in particular the mass fractions of those elements that have been investigated
during the many, deep, spectrocopic surveys of Globular Clusters
Results: Although the data for oxygen and sodium are scarce for low
metallicity Globular Clusters, the few data for the unevolved stars in NGC6397
are compatible with the models. Further, we find good agreement with the C--N
anticorrelation of unevolved stars in the cluster M15. In this cluster,
however, no stars having low oxygen ([O/Fe] = -1) have been detected. The most
massive, very metal poor clusters, should contain such stars, according to the
present models. At the lowest metallicity Z=10^{-4}, the ejecta of the most
massive AGBs have C/O>1, due to the dramatic decrease of the oxygen abundance.
We discuss the possible implications of this prediction.Comment: 15 pages, 9 figure
Full computation of massive AGB evolution. I. The large impact of convection on nucleosynthesis
It is well appreciated that the description of overadiabatic convection
affects the structure of the envelopes of luminous asymptotic giant branch
(AGB) stars in the phase of ``hot bottom burning '' (HBB). We stress that this
important uncertainty in the modeling plays a role which is much more dramatic
than the role which can be ascribed, e.g., to the uncertainty in the nuclear
cross-sections. Due to the role tentatively attributed today to the HBB
nucleosynthesis as the site of self-enrichment of Globular Clusters stars, it
is necessary to explore the difference in nucleosynthesis obtained by different
prescriptions for convection. We present results of detailed evolutionary
calculations of the evolution of stars of intermediate mass during the AGB
phase. We follow carefully the nucleosynthesis at the base of the external
convective region, showing that very different results can be obtained
according to the presciption adopted to find out the temperature gradient
within the instability regions. We discuss the uncertainties in the yields of
the various chemical species and the role which these sources can play as
polluters of the interstellar medium.Comment: 11 pages, 12 figures. Accepted for pubblication on A&
Full computation of massive AGB evolution. II. The role of mass loss and cross-sections
In the course of a systematic exploration of the uncertainties associated to
the input micro- and macro-physics in the modeling of the evolution of
intermediate mass stars during their Asymptotic Giant Branch (AGB) phase, we
focus on the role of the nuclear reactions rates and mass loss. We consider
masses 3<M/Msun<6.5 for a metallicity typical for Globular Cluster, Z=0.001,
and compare the results obtained by computing the full nucleosynthesis with hot
bottom burning (HBB), for a network of 30 elements, using either the NACRE or
the Cameron & Fowler (1988) cross-sections. The results differ in particular
with respect to the Na23 nucleosynthesis (which is more efficient in the NACRE
case) and the magnesium isotopes ratios. For both choices, however, the CNO
nucleosynthesis shows that the C+N+O is constant within a factor of two, in our
models employing a very efficient convection treatment. Different mass loss
rates alter the physical conditions for HBB and the length of the AGB phase,
changing indirectly the chemical yields. These computations show that the
predictive power of our AGB models is undermined by these uncertainties. In
particular, it appears at the moment very difficult to strongly accept or
dismiss that these sources play a key-role in the pollution of Globular
Clusters (GCs)Comment: 27 pages (referee format) + 19 figure
The self-enrichment scenario in intermediate metallicity globular clusters
We present stellar yields computed from detailed models of intermediate mass
asymptotic giant branch stars of low metallicity. In this work, the whole main
microphysics inputs have been updated, and in particular alpha-enhancement is
explicitly taken into account both in the opacities and equation of state. The
target of this work is to provide a basis to test the reliability of the AGB
self-enrichment scenario for Globular Clusters of intermediate metallicity.
These Globular Clusters exhibit well defined abundance patterns, which have
often been interpreted as a consequence of the pollution of the interstellar
medium by the ejecta of massive AGBs. We calculated a grid of intermediate mass
models with metallicity Z=0.001; the evolutionary sequences are followed from
the pre-Main sequence along the whole AGB phase. We focus our attention on
those elements largely studied in the spectroscopic investigations of Globular
Clusters stars, i.e. oxygen, sodium, aluminum, magnesium and fluorine.} The
predictions of our models show an encouraging agreement with the demand of the
self-enrichment scenario for what concerns the abundances of oxygen, aluminum,
fluorine and magnesium. The question of sodium is more tricky, due to the large
uncertainties of the cross-sections of the Ne-Na cycle. The present results
show that only a relatively small range of initial masses (M=5,6 solar masses)
can be responsible for the self enrichment
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