Session K2: Giant Star Evolution and Nucleosynthesis

Nucleosynthesis and stellar modeling of AGB stars

The last brief phase of nuclear burning for low to intermediate mass stars is the Asymptotic Giant Branch (AGB). It is during the AGB that the richest nucleosynthesis occurs, driven by thermal instabilities of the helium-burning shell, the products of which are dredged to the stellar surface by recurrent mixing episodes. Envelope burning occurs in the most massive AGB stars, also altering the surface composition. The AGB phase is terminated when rapid, episodic mass loss expels the envelope into the interstellar medium, making these stars important contributors to the chemical evolution of galaxies and stellar systems. We review current issues and recent developments in the evolution and nucleosynthesis of AGB stars. We begin with a brief introduction to the topic, followed by an outline of the regions and time-scales for the various important nuclear reactions that occur in these stars. Then we focus on the different numerical approaches adopted in modeling this brief evolutionary phase (parametric, detailed models) with a special emphasis on the associated nucleosynthesis. We will discuss recent efforts to understand the effect that convection, mass loss and reaction rate uncertainties have on the nucleosynthesis, and the role that AGB stars plays in the chemical evolution of the Galaxy.   

$^{14}$N(p,$\gamma$)$^{15}$O and the Age of the Galaxy

The carbon-nitrogen (CN) cycle is the energy source of choice at some point in the life of every star and the power generated by the CN-cycle is determined by the rate of its slowest reaction, namely $^{14}$N(p,$\gamma$)$^{15}$O. Recent experiments have shown that the rate of this reaction is about a factor of 2 less than previously thought for main-sequence and red-giant stars. The astrophysical implications of this are still being explored. However, one clear consequence is that the ages of globular clusters have to be revised upwards by about a billion years. This talk will focus on the connection between the $^{14}$N(p,$\gamma$)$^{15}$O reaction, globular clusters and the age of the galaxy.   

Neutron capture measurements for nuclear astrophysics

Almost all of the heavy elements are produced via neutron capture reactions in a multitude of stellar production sites. The predictive power of the underlying stellar models is currently limited because they contain poorly constrained physics components such as convection, rotation or magnetic fields. Neutron captures measurements on heavy radioactive isotopes provide a unique opportunity to largely improve these physics components, and thereby address important questions of nuclear astrophysics. Such species are branch-points in the otherwise uniquely defined path of subsequent n-captures along the s-process path in the valley of stability. These branch points reveal themselves through unmistakable signatures recovered from pre-solar meteoritic grains that originate in individual element producing stars. Measurements on radioactive isotopes for neutron energies in the keV region represent a stringent challenge for further improvements of experimental techniques. This holds true for the neutron sources, the detection systems and the technology to handle radioactive material. Though the activation method or accelerator mass spectroscopy of the reaction products could be applied in a limited number of cases, Experimental facilities like DANCE at LANL, USA and n-TOF at CERN, Switzerland are addressing the need for such measurements on the basis of the more universal method of detecting the prompt capture gamma-rays, which is required for the application of neutron time-of-flight (TOF) techniques. With a strongly optimized neutron facility at the Rare Isotope Accelerator (RIA) isotopes with half-lives down to tens of days could be investigated, while present facilities require half-lives of a few hundred days. Recent neutron capture experiments on radioactive isotopes with relevance for nuclear astrophysics and possibilities for future experimental setups will be discussed during the talk.