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The 3He Abundance in Planetary Nebulae

We report measurements of the abundance of 3He for a sample of six Galactic planetary nebulae: IC 289, NGC 3242, NGC 6543, NGC 6720, NGC 7009, and NGC 7662. Based on observations of the 8.665 GHz hyperfine transition of 3He+, we derive 3He/H abundances ranging from ~0.1 to 1.0 × 10-3 by number. These abundances are more than an order of magnitude larger than those found in any H II region, the local interstellar medium, or the proto-solar system. If planetary nebulae are surrounded by large, low-density, ionized halos, modeling suggests that these abundances will decrease by a factor of about 2. Our source sample is highly biased in that we selected objects on the basis of several criteria that maximized the likelihood of 3He detections. The abundances are nonetheless consistent with the idea that 3He is produced in significant quantities by stars of 1-2 M☉. We conclude that there is some stellar production of 3He.