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Thomas R. Geballe |
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"Oxygen-18
and the Origins of R Coronae Borealis Stars and Hydrogen-deficient
Carbon Stars" |
| Many hydrogen-deficient carbon (HdC) and R Coronae Borealis (RCB) stars have 16O/18O ratios close to and in some cases less than unity, values that are two or more orders of magnitude lower than measured in other stars (the solar value is 500). This discovery is an important clue in determining the origins of these stars. For RCB stars there have been two leading models proposed: double degenerate (white dwarf (WD)) mergers, and the final helium-shell flashes. Thermonuclear reactions during the latter process do not result in large abundances of 18O. Initial simple models of the nucleosynthesis that occurs during the merger of a CO-WD and He WD mergers suggest that large amounts of 18O can be produced, and show that more detailed simulations and modeling are justified. |