What we do and do not know about the s-process

AGB stars are the source for the main component of the $s$-process. Here we discuss both the properties which are reasonably well known and those which still suffer from substantial uncertainties. In the former case, we are fairly sure that the $s$-process contribution from AGB stars comes from masses between about 1 and 3 \msun, and the dominant neutron source is the $^{13}$C$(\alpha$,n)$^{16}$O reaction. In the latter category remains the formation mechanism for the $^{13}$C-pocket. Attempts at including rotation seem to inhibit neutron capture reactions. Explaining the observations seems to require a spread in the size of the $^{13}$C-pocket so some stochastic process, such as rotation, must be involved.Comment: To be published in Nuclear Physics A; Invited Review for "Nuclei in the Cosmos VIII", Vancouver, July 200

Catching Element Formation In The Act ; The Case for a New MeV Gamma-Ray Mission: Radionuclide Astronomy in the 2020s

High Energy Astrophysic

Abundance Anomalies in Globular Cluster Stars

We present a summary of the extensive problems posed by the curious abundance correlations seen in stars in Galactic globular clusters. We discuss three scenarios, and conclude that only one seems to be consistent with the observations. Even this is still rather unsatisfying from many viewpoints. We determine the kind of nucleosynthesis required and discuss AGB stars as the possible source. Some calculations seem to rule out AGB stars and others seem to favour them. The reasons for the differences are discussed. No satisfying conclusion is reached

Origin of the early-type R stars: a binary-merger solution to a century-old problem?

The early-R stars are carbon-rich K-type giants. They are enhanced in C12, C13 and N14, have approximately solar oxygen, magnesium isotopes, s-process and iron abundances, have the luminosity of core-helium burning stars, are not rapid rotators, are members of the Galactic thick disk and, most peculiarly of all, are all single stars. Conventional single-star stellar evolutionary models cannot explain such stars, but mergers in binary systems have been proposed to explain their origin. We have synthesized binary star populations to calculate the number of merged stars with helium cores which could be early-R stars. We find many possible evolutionary channels. The most common of which is the merger of a helium white dwarf with a hydrogen-burning red giant branch star during a common envelope phase followed by a helium flash in a rotating core which mixes carbon to the surface. All the channels together give ten times more early-R stars than we require to match recent Hipparcos observations - we discuss which channels are likely to be the true early-R stars and which are not. For the first time we have constructed a viable model of the early-R stars with which we can test some of our ideas regarding common envelope evolution in giants, stellar mergers, rotation, the helium flash and the origin of the early-R stars

Super asymptotic giant branch stars. I - Evolution code comparison

We present an extensive set of detailed stellar models in the mass range 7.7-10.5 M⊙ over the metallicity range Z = 10-5-0.02. These models were produced using the Monash University version of the Mount Stromlo Stellar Structure Program (monstar) and follow the evolution from the pre-main sequence to the first thermal pulse of these super asymptotic giant branch stars. A quantitative comparison is made to the study of Siess. Prior to this study, only qualitative comparisons and code validations existed in this critical mass range, and the large variations in the literature were largely unexplained. The comparison presented here is particularly detailed due to the standardization of the input physics, where possible. The minimum initial mass of star which ignites carbon, Mup, was found to agree within 0.2 M⊙ between the codes over the entire metallicity range. We find exceptional agreement in the model results between these two codes for all stages of evolution up to and including carbon burning. For additional comparison, we also present results from the evolve code, a modified version of the iben code as described in Gil-Pons, Gutiérrez & García-Berro for some important variables during the carbon burning phase. Several numerical tests showed that the carbon burning phase is weakly dependent on the spatial resolution but that inadequate temporal resolution alters the behaviour of the convective zones. We also discovered that stars just below Mup may experience a carbon flash that is not followed by the development of the flame. Such aborted carbon burning models thus preserve a CO core surrounding by a 0.2-0.3 M⊙ shell of partially burnt carbon material. We present a simplified algorithm for calculating carbon burning that only relies on tracking two species, 12C and 16O, but which tests show works quite accurately for the a wide range of initial masses and compositions. © 2009 RAS.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Hiding in plain sight - red supergiant imposters? Super-AGB stars

info:eu-repo/semantics/publishe

Super and Massive AGB Stars: II - Nucleosynthesis and Yields - Z=0.02, 0.008 and 0.004

info:eu-repo/semantics/publishe

Production of Short-Lived Radionuclides in Super Asymptotic Giant Branch Stars

info:eu-repo/semantics/publishe

Super and massive AGB stars - III. Nucleosynthesis and yields - Z = 0.001 and 0.0001

info:eu-repo/semantics/publishe