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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.
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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.