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Session 72 - Late Type Stars and Planetary Nebulae.
Display session, Thursday, June 13
The study of the origin and evolution of the elements is one of the cornerstones of modern astrophysics. For any given isotope of an element a crucial step is the observational determination of the abundance of that isotope and how that abundance varies temporally and spatially. We are making precise determinations of the abundance of the light isotope of helium, ^3He, in the interstellar medium of the Milky Way. The ^3He abundance is derived from measurements of the spin-flip line of ^3He^+ with a rest wavelength of 3.46 cm. Potentially observable sources of ionized gas include H\thinspace II regions and planetary nebulae located throughout the Galaxy (e.g., Balser et al. 1994 ApJ, 430, 667). ^3He can serve both as a probe of cosmology and stellar/galactic evolution.
Nuclear fusion reactions inside stars will change the relative amounts of the light elements from the primordial abundances produced by the Big Bang. Theory predicts not only that common solar-type stars are net producers of ^3He but also that the mass lost from winds generated at advanced stages of their evolution and the final planetary nebulae should be substantially enriched in ^3He. Planetary nebula ^3He abundances are therefore important tests of stellar evolution theory since these low-mass, evolved objects are expected to be significant sources of ^3He.
We report observations of the cosmic abundance of ^3He for a sample of six Galactic planetary nebulae. The derived abundances range from ^3He/H = 10^-4 to 10^-3. These abundances are consistent with standard models of stellar evolution of low-mass stars. They are also a factor of ten larger than those found in Galactic H\thinspace II regions.
This research was supported by NSF grant AST\thinspace 91--21169.
Program listing for Thursday