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162 results on '"Lattanzio, John C."'

151. Reaction Rate Uncertainties and the Production of 19F in Asymptotic Giant Branch Stars

Author

Lugaro, Maria, Ugalde, Claudio, Karakas, Amanda I., Gorres, Joachim, Wiescher, Michael, Lattanzio, John C., and Cannon, Robert C.

Abstract

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. During the asymptotic giant branch (AGB) phase, 19F is produced as a consequence of nucleosynthesis occurring during the convective thermal pulses and also during the interpulse periods if protons from the envelope are partially mixed in the top layers of the He intershell (partial mixing zone). We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge-up, and the extent of the partial mixing zone. Then we perform a detailed analysis of the reaction rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(a, g)18O and 19F(a, p)22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. In both cases the revised rates are lower than previous estimates. The effect of the inclusion of the partial mixing zone on the production of fluorine strongly depends on the very uncertain 14C(a, g)18O reaction rate. The importance of the partial mixing zone is reduced when using our estimate for this rate. Overall, rate uncertainties result in uncertainties in the fluorine production of about 50% in stellar models with mass [?]3 M and of about a factor of 7 in stellar models of mass [?]5 M. This larger effect at high masses is due to the high uncertainties of the 19F(a, p)22Ne reaction rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, as well as in relation to reducing the uncertainties of the 14C(a, g)18O reaction rate. At the same time, the possible effect of cool bottom processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could, in principle, help to match the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

Published
2004

152. Monash Chemical Yields Project (Mon¿ey) - Element production in low- and intermediate-mass stars of metallicities Z = 0 to 0.04

Author

Doherty, Carolyn L., Lattanzio, John C., Angelou, George, Campbell, Simon W, Church, Ross, Constantino, Thomas, Cristallo, Sergio, Gil Pons, Pilar|||0000-0001-5410-3564, Karakas, Amanda, Lugaro, Maria, Stancliffe, Richard, and Universitat Politècnica de Catalunya. Departament de Física

Subjects
AGB-stars, Asymptotic giant branch stars, Reaccions nuclears, Física::Astronomia i astrofísica [Àrees temàtiques de la UPC], Astrophysics::Solar and Stellar Astrophysics, Astrophysics::Earth and Planetary Astrophysics, Astrophysics::Cosmology and Extragalactic Astrophysics, Nuclear reactions, Stellar Evolution, Nucleosynthesis, Astrophysics::Galaxy Astrophysics, Nucleosíntesi
Abstract

The Mon¿ey project will provide a large and homogeneous set of stellar yields for the low- and intermediate- mass stars and has applications particularly to galactic chemical evolution modelling. We describe our detailed grid of stellar evolutionary models and corresponding nucleosynthetic yields for stars of initial mass 0.8 M¿ up to the limit for core collapse supernova (CC-SN) ˜ 10 M¿. Our study covers a broad range of metallicities, ranging from the first, primordial stars (Z = 0) to those of super-solar metallicity (Z = 0.04). The models are evolved from the zero-age main-sequence until the end of the asymptotic giant branch (AGB) and the nucleosynthesis calculations include all elements from H to Bi. A major innovation of our work is the first complete grid of heavy element nucleosynthetic predictions for primordial AGB stars as well as the inclusion of extra-mixing processes (in this case thermohaline) during the red giant branch. We provide a broad overview of our results with implications for galactic chemical evolution as well as highlight interesting results such as heavy element production in dredge-out events of super-AGB stars. We briefly introduce our forthcoming web-based database which provides the evolutionary tracks, structural properties, internal/surface nucleosynthetic compositions and stellar yields. Our web interface includes user- driven plotting capabilities with output available in a range of formats. Our nucleosynthetic results will be available for further use in post processing calculations for dust production yields.

153. Super and massive AGB stars - III. nucleosynthesis in metal-poor and very metal-poor stars - Z = 0.001 and 0.0001

Author

Universitat Politècnica de Catalunya. Departament de Física Aplicada, Universitat Politècnica de Catalunya. DF - Dinàmica No Lineal de Fluids, Doherty, Carolyn L., Gil Pons, Pilar, Lau, Herbert B., Lattanzio, John C., Siess, Lionel, Campbell, Simon W, Universitat Politècnica de Catalunya. Departament de Física Aplicada, Universitat Politècnica de Catalunya. DF - Dinàmica No Lineal de Fluids, Doherty, Carolyn L., Gil Pons, Pilar, Lau, Herbert B., Lattanzio, John C., Siess, Lionel, and Campbell, Simon W

Abstract

We present a new grid of stellar models and nucleosynthetic yields for super-AGB stars with metallicities Z = 0.001 and 0.0001, applicable for use within galactic chemical evolution models. Contrary to more metal-rich stars where hot bottom burning is the main driver of the surface composition, in these lower metallicity models the effect of third dredge-up and corrosive second dredge-up also have a strong impact on the yields. These metal-poor and very metal-poor super-AGB stars create large amounts of 4He, 13C, 14N and 27Al as well as the heavy magnesium isotopes 25Mg and 26Mg. There is a transition in yield trends at metallicity Z ˜ 0.001, below which we find positive yields of 12C, 16O, 15N and 28Si, which is not the case for higher metallicities. We explore the large uncertainties derived from wind prescriptions in super-AGB stars, finding ˜2 orders of magnitude difference in yields of 22Ne, 23Na, 24, 25, 26Mg, 27Al and our s-process proxy isotope g. We find inclusion of variable composition low-temperature molecular opacities is only critical for super-AGB stars of metallicities below Z ˜ 0.001. We analyse our results, and those in the literature, to address the question: Are super-AGB stars the polluters responsible for extreme population in the globular cluster NGC 2808? Our results, as well as those from previous studies, seem unable to satisfactorily match the extreme population in this globular cluster, Peer Reviewed, Postprint (author’s final draft)

161. Deep Mixing of 3He: Reconciling Big Bang and Stellar Nucleosynthesis.

Author

Eggleton, Peter P., Dearborn, David S. P., and Lattanzio, John C.

Subjects
*LOW mass stars, *BIG bang theory, *CHEMICAL elements, *RED giants, *NUCLEOSYNTHESIS, *HYDROGEN, *NOBLE gases, *COSMOCHEMISTRY, *STARS
Abstract

Low mass stars, 1 to 2 solar masses, near the Main Sequence are efficient at producing the helium isotope ³He, which they mix into the convective envelope on the giant branch and should distribute into the Galaxy by way of envelope Loss, This process is so efficient that it is difficult to reconcile the low observed cosmic abundance of ³He with the predictions of both stellar and Big Bang nucleosymthesis. Here We find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell, where a nuclear reaction lowers the mean molecular weight slightly. Thus, we are able to remove the threat that ³He production in low-mass stars poses to the Big Bang nucleosynthesis of ³He. [ABSTRACT FROM AUTHOR]

Published
2006
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162. Stellar origin of the 182Hf cosmochronometer and the presolar history of solar system matter.

Author

Lugaro, Maria, Heger, Alexander, Osrin, Dean, Goriely, Stephane, Zuber, Kai, Karakas, Amanda I., Gibson, Brad K., Doherty, Carolyn L., Lattanzio, John C., and Ott, Ulrich

Subjects
*RADIOACTIVE substances, *METEORITICS, *HALF-life (Nuclear physics), *NEUTRON capture, *ASYMPTOTIC giant branch stars, *SLOW neutrons, *RADIOACTIVITY measurements, ORIGIN of the solar system
Abstract

Among the short-lived radioactive nuclei inferred to be present in the early solar system via meteoritic analyses, there are several heavier than iron whose stellar origin has been poorly understood. In particular, the abundances inferred for 182Hf (half-life = 8.9 million years) and 129I (half-life = 15.7 million years) are in disagreement with each other if both nuclei are produced by the rapid neutron-capture process. Here, we demonstrate that contrary to previous assumption, the slow neutron-capture process in asymptotic giant branch stars produces 182Hf. This has allowed us to date the last rapid and slow neutron-capture events that contaminated the solar system material at ~100 million years and ~30 million years, respectively, before the formation of the Sun. [ABSTRACT FROM AUTHOR]

Published
2014
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