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.