37 research outputs found
Interpretation of CEMP(s) and CEMP(s + r) Stars with AGB Models
Asymptotic Giant Branch (AGB) stars play a fundamental role in the s-process
nucleosynthesis during their thermal pulsing phase. The theoretical predictions
obtained by AGB models at different masses, s-process efficiencies, dilution
factors and initial r-enrichment, are compared with spectroscopic observations
of Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s),
collected from the literature. We discuss here five stars as example, CS
22880-074, CS 22942-019, CS 29526-110, HE 0202-2204, and LP 625-44. All these
objects lie on the main-sequence or on the giant phase, clearly before the
TP-AGB stage: the hypothesis of mass transfer from an AGB companion, would
explain the observed s-process enhancement. CS 29526-110 and LP 625-44 are
CEMP(s+r) objects, and are interpreted assuming that the molecular cloud, from
which the binary system formed, was already enriched in r-process elements by
SNII pollution. In several cases, the observed s-process distribution may be
accounted for AGB models of different initial masses with proper 13C-pocket
efficiency and dilution factor. Na (and Mg), produced via the neutron capture
chain starting from 22Ne, may provide an indicator of the initial AGB mass.Comment: 8 pages, 6 figures, 2 table
Galactic Chemical Evolution of the s Process from AGB Stars
We follow the chemical evolution of the Galaxy for the s elements using a
Galactic chemical evolution (GCE) model, as already discussed by Travaglio et
al. (1999, 2001, 2004), with a full updated network and refined asymptotic
giant branch (AGB) models. Calculations of the s contribution to each isotope
at the epoch of the formation of the solar system is determined by following
the GCE contribution by AGB stars only. Then, using the r-process residual
method we determine for each isotope their solar system r-process fraction, and
recalculate the GCE contribution of heavy elements accounting for both the s
and r process. We compare our results with spectroscopic abundances at various
metallicities of [Sr,Y,Zr/Fe], of [Ba,La/Fe], of [Pb/Fe], typical of the three
s-process peaks, as well as of [Eu/Fe], which in turn is a typical r-process
element. Analysis of the various uncertainties involved in these calculations
are discussed.Comment: 8 pages, 6 figures, 1 tabl
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
Barium Stars: Theoretical Interpretation
Barium stars are extrinsic Asymptotic Giant Branch (AGB) stars. They present
the s-enhancement characteristic for AGB and post-AGB stars, but are in an
earlier evolutionary stage (main sequence dwarfs, subgiants, red giants). They
are believed to form in binary systems, where a more massive companion evolved
faster, produced the s-elements during its AGB phase, polluted the present
barium star through stellar winds and became a white dwarf. The samples of
barium stars of Allen & Barbuy (2006) and of Smiljanic et al. (2007) are
analysed here. Spectra of both samples were obtained at high-resolution and
high S/N. We compare these observations with AGB nucleosynthesis models using
different initial masses and a spread of 13C-pocket efficiencies. Once a
consistent solution is found for the whole elemental distribution of
abundances, a proper dilution factor is applied. This dilution is explained by
the fact that the s-rich material transferred from the AGB to the nowadays
observed stars is mixed with the envelope of the accretor. We also analyse the
mass transfer process, and obtain the wind velocity for giants and subgiants
with known orbital period. We find evidence that thermohaline mixing is acting
inside main sequence dwarfs and we present a method for estimating its depth.Comment: 8 pages, 11 figures, 6 table
The Hobby-Eberly Telescope Chemical Abundances Of Stars In The Halo (CASH) Project. I. The Lithium-, s-, And r-Enhanced Metal-Poor Giant HKII 17435-00532
We present the first detailed abundance analysis of the metal-poor giant HKII 17435-00532. This star was observed as part of the University of Texas long-term project Chemical Abundances of Stars in the Halo ( CASH). A spectrum was obtained with the High Resolution Spectrograph (HRS) on the Hobby-Eberly Telescope with a resolving power of R similar to 15,000. Our analysis reveals that this star may be located on the red giant branch, red horizontal branch, or early asymptotic giant branch. We find that this metal-poor (Fe/H = -2.2) star has an unusually high lithium abundance [log epsilon(Li) +2.1], mild carbon (C/Fe = +0.7) and sodium (]Na/Fe] = +0.6) enhancement, as well as enhancement of both s-process ([Ba/Fe] = +0.8) and r-process ([Eu/Fe] = +0.5) material. The high Li abundance can be explained by self-enrichment through extra mixing that connects the convective envelope with the outer regions of the H-burning shell. If so, HKII 17435-00532 is the most metal-poor star in which this short-lived phase of Li enrichment has been observed. The Na and n-capture enrichment can be explained by mass transfer from a companion that passed through the thermally pulsing AGB phase of evolution with only a small initial enrichment of r-process material present in the birth cloud. Despite the current nondetection of radial velocity variations (over similar to 180 days), it is possible that HKII 17435 - 00532 is in a long-period or highly inclined binary system, similar to other stars with similar n-capture enrichment patterns.NASA AAS Small Research Grant ProgramGALEX GI 05-GALEX05-27Italian MIUR-PRIN06 ProjectNSF AST 06-07708, AST04-06784, AST 07-0776, PHY 02-15783JINA AST 07-07447Astronom
The Weak s-Process in Massive Stars and its Dependence on the Neutron Capture Cross Sections
The slow neutron capture process in massive stars (weak s process) produces most of the s-process isotopes between iron and strontium. Neutrons are provided by the 22Ne(?,n)25Mg reaction, which is activated at the end of the convective He-burning core and in the subsequent convective C-burning shell. The s-process-rich material in the supernova ejecta carries the signature of these two phases. In the past years, new measurements of neutron capture cross sections of isotopes beyond iron significantly changed the predicted weak s-process distribution. The reason is that the variation of the Maxwellian-averaged cross sections (MACS) is propagated to heavier isotopes along the s path. In the light of these results, we present updated nucleosynthesis calculations for a 25 M ? star of Population I (solar metallicity) in convective He-burning core and convective C-burning shell conditions. In comparison with previous simulations based on the Bao et?al. compilation, the new measurement of neutron capture cross sections leads to an increase of s-process yields from nickel up to selenium. The variation of the cross section of one isotope along the s-process path is propagated to heavier isotopes, where the propagation efficiency is higher for low cross sections. New 74Ge, 75As, and 78Se MACS result in a higher production of germanium, arsenic, and selenium, thereby reducing the s-process yields of heavier elements by propagation. Results are reported for the He core and for the C shell. In shell C-burning, the s-process nucleosynthesis is more uncertain than in the He core, due to higher MACS uncertainties at higher temperatures. We also analyze the impact of using the new lower solar abundances for CNO isotopes on the s-process predictions, where CNO is the source of 22Ne, and we show that beyond Zn this is affecting the s-process yields more than nuclear or stellar model uncertainties considered in this paper. In particular, using the new updated initial composition, we obtain a high s-process production (overproduction higher than 16O, ~100) for Cu, Ga, Ge, and As. Using the older abundances by Anders & Grevesse, also Se, Br, Kr, and Rb are efficiently produced. Our results have important implications in explaining the origin of copper in the solar abundance distribution, pointing to a prevailing contribution from the weak s-process in agreement with spectroscopic observations and Galactic chemical evolution calculations. Because of the improvement due to the new MACS for nickel and copper isotopes, the nucleosynthesis of copper is less affected by nuclear uncertainties compared to heavier s-process elements. An experimental determination of the 63Ni MACS is required for a further improvement of the abundance prediction of copper. The available spectroscopic observations of germanium and gallium in stars are also discussed, where most of the cosmic abundances of these elements derives from the s-process in massive stars