Following core-helium burning, a low-mass star expands to become a giant as it consumes its final reserves of nuclear fuel and, subsequently, contracts to become a white dwarf. A final pulse of nuclear burning, or a merger between two white dwarfs can cause the star to expand as a giant. Our research on extreme helium stars and other post-AGB stars is directed towards understanding these processes and towards developing models of stellar evolution that fit the observations.
Working with Pandey, Rao (Indian Institute of Astrophysics) and Lambert (University of Texas), Simon Jeffery is using Hubble Space Telescope observations to make measurements of -process elements in extreme helium stars. These elements are produced while a star is a red giant and when light elements in the region between the hydrogen and helium-burning shells capture neutrons released in other nuclear reactions. These measurements reinforce the phenomenological and probable evolutionary connections between cool RCoronae Borealis stars and the hotter extreme helium stars.
Meanwhile Natalie Behara and Simon Jeffery have completely updated the treatment of continuous and line opacities in the Armagh model atmosphere code STERNE. Continuous opacities are taken from the Opacity Project, whilst up to 10 million atomic transitions are incorporated using an opacity sampling method. The consequences for hydrogen-deficient atmospheres, in particular, are very important because they have very low background opacities. Early results indicate changes of up to 1,000K in the derived effective temperatures of extreme helium stars.
After joining the group in 2004 September, Timur Sahin has commenced his studies by reducing échelle spectra of post-AGB stars obtained with the Anglo-Australian Telescope. These will be analyzed in order to compare the abundances of -process elements in conventional post-AGB stars with the abundances measured in extreme helium stars.