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Post-AGB Stars

Following core helium burning, a low-mass star expands first to become a giant, as it consumes its final reserves of nuclear fuel, and then 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.

Figure 7: A prediction of the future evolution of the blue spectrum of FGSge. Over the next thousand years, its temperature (left-hand axis) will increase from 5500K to over 15000K (bottom to top). The characteristic spectrum of a cool star, rich in metal lines will be replaced by that of a hot star, dominated by hydrogen and helium. Helium, carbon (right-hand axis) and other products of nucleosynthesis will be dredged towards the surface, producing the strong CII line in the top left of the figure. The spectrum is overlaid on a false-colour image of the planetary nebula around FGSge obtained with the Keck 1 10-m telescope on Mauna Kea (see www.aloha.net/$^\sim$joel/keckphot.htm). Light from FGSge itself has been masked out (black bar).

Working with Pandey, Lambert (University of Texas) and Rao (Indian Institute of Astrophysics), Simon Jeffery has used observations with the Hubble Space Telescope to make the first measurements of s-process elements in extreme helium stars. s-process elements are produced while a star is a red giant and when light elements in the region between hydrogen and helium-burning shells capture neutrons released in other nuclear reactions. The importance of these observations is that it reinforces the phenomenological and probable evolutionary connections between the cool and strongly-variable R Coronae Borealis stars and the extreme helium stars.

Jeffery continues to work with Woolf (University of Washington) on long-term evolutionary changes in extreme helium stars. A new study of abundances in the born-again star FGSge (see Figure 7) is reported elsewhere.