Tyl Dermine, Robert Izzard, Ross Church and Alain Jorissen
The barium stars are thought to be formed by accretion from a companion AGB star wind. However, this should also circularise their orbits which are observed to be eccentric. We examine two mechanisms for increasing the eccentricity, white-dwarf kicks and circumbinary disks. Both are promising ways to generate eccentricity, but both also have their problems.
The R Stars are carbon-rich K-giants which should not have their carbon. They are also all single stars so could not have accreted it from a companion. We investigate the number of stars which may form from mergers of white dwarfs and red giants and find that it is compatible with the observed number of R stars.
Galactic chemical evolution models which include sodium from Type II supernovae alone underestimate the abundance of sodium in the interstellar medium, particularly in stars with metallicity up to [Fe/H]=+0.4. We have combined the latest AGB, massive-star and supernova yields in an attempt to resolve the problem.
International Astronomical Union General Assembly in Prague and the Netherlands Astronomy Conference in 2006
Metallicity Dependence of Gamma-Ray Burst Progenitors
Robert Izzard, Enrico Ramirez-Ruiz and Christopher Tout
We investigate the binary channel for collapsars and the dependence of its rate on the initial metallicity. The lower the metallicity the less the mass, and angular momentum, lost from the star before it explodes. We find this effect significantly increases the GRB rate.