The origin of the lead-rich stars in the Galactic halo: Investigation of model parameters for the s-process

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 03/2006; 368(1). DOI: 10.1111/j.1365-2966.2006.10109.x
Source: arXiv


Several stars at the low-metallicity extreme of the Galactic halo show large spreads of lead and associated ‘heavy’ s-process
elements ([Pb/hs]). Theoretically, an s-process pattern should be obtained from an AGB star with a fixed metallicity and initial
mass. For the third dredge-up and the s-process model, several important properties depend primarily on the core mass of AGB
stars. Zijlstra reported that the initial-to-final mass relation steepens at low metallicity, due to low mass-loss efficiency.
This might affect the model parameters of the AGB stars, e.g. the overlap factor and the neutron irradiation time, in particular
at low metallicity. The calculated results do indeed show that the overlap factor and the neutron irradiation time are significantly
small at low metallicities, especially for 3.0 M⊙ AGB stars. The scatter of [Pb/hs] found in low metallicities can therefore be explained naturally when varying the initial
mass of the low-mass AGB stars.

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Available from: W.Y. Cui, Aug 28, 2013
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    ABSTRACT: Thermally pulsating asymptotic giant branch (AGB) stars are the main producers of slow neutron capture (s-) process elements, but there are still large uncertainties associated with the formation of the main neutron source, 13C, and with the physics of these stars in general. Observations of s-process element enhancements in stars can be used as constraints on theoretical models. For the first time we apply stellar population synthesis to the problem of s-process nucleosynthesis in AGB stars, in order to derive constraints on free parameters describing the physics behind the third dredge-up and the properties of the neutron source. We utilize a rapid evolution and nucleosynthesis code to synthesize different populations of s-enhanced stars, and compare them to their observational counterparts to find out for which values of the free parameters in the code the synthetic populations fit best to the observed populations. These free parameters are the amount of third dredge-up, the minimum core mass for third dredge-up, the effectiveness of 13C as a source of neutrons and the size in mass of the 13C pocket. We find that galactic disk objects are reproduced by a spread of a factor of two in the effectiveness of the 13C neutron source. Lower metallicity objects can be reproduced only by lowering by at least a factor of 3 the average value of the effectiveness of the 13C neutron source needed for the galactic disk objects. Using observations of s-process elements in post-AGB stars as constraints we find that dredge-up has to start at a lower core mass than predicted by current theoretical models, that it has to be substantial ($\lambda$ >~ 0.2) in stars with mass M <~ 1.5 M_sun and that the mass of the 13C pocket must be about 1/40 that of the intershell region. Comment: 16 pages, 15 figures, accepted for publication in Astronomy & Astrophysics
    Preview · Article · Mar 2007 · Astronomy and Astrophysics
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    ABSTRACT: The primary nature of the 13C neutron source is very significant for the studies of the s-process nucleosynthesis. In this paper we present an attempt to fit the element abundances observed in 16 s-rich stars using parametric model of the single neutron exposure. The calculated results indicate that almost all s-elements were made in a single neutron exposure for nine sample stars. Although a large spread of neutron exposure is obtained, the maximum value of the neutron exposure will reach about 7.0 mbarn−1, which is close to the theoretical predictions by the asymptotic giant branch (AGB) model. The calculated result is a significant evidence for the primary nature of the neutron source. Combining the result obtained in this work and the neutron exposure–initial mass relations, a large spread of neutron exposure can be explained by the different initial stellar mass and their time evolution. The possibility that the rotationally induced mixing process can lead to a spread of the neutron exposure in AGB stars is also existent.
    Preview · Article · Mar 2007 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The s-enhanced and very metal-poor star CS 30322-023 shows a puzzling abundance pattern of the neutron-capture elements, i.e. several neutron-capture elements such as Ba, Pb etc. show enhancement, but other neutron-capture elements such as Sr, Eu etc. exhibit deficient with respect to iron. The study to this sample star could make people gain a better understanding of s- and r-process nucleosynthesis at low metallicity. Using a parametric model, we find that the abundance pattern of the neutron-capture elements could be best explained by a star that was polluted by an AGB star and the CS 30322-023 binary system formed in a molecular cloud which had never been polluted by r-process material. The lack of r-process material also indicates that the AGB companion cannot have undergone a type-1.5 supernova, and thus must have had an initial mass below 4.0M$_\odot$, while the strong N overabundance and the absence of a strong C overabundance indicate that the companion's initial mass was larger than 2.0M$_\odot$. The smaller s-process component coefficient of this star illustrates that there is less accreted material of this star from the AGB companion, and the sample star should be formed in the binary system with larger initial orbital separation where the accretion-induced collapse (AIC) mechanism can not work.
    Full-text · Article · Mar 2007 · The Astrophysical Journal
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