The Detection of 3He+ in a Planetary Nebula Using the VLA

We used the VLA to search for 3He+ emission from two Galactic planetary nebulae (PNe): NGC 6572 and J320. Standard stellar models predict that the 3He/H abundance ratios for PNe should be 1-2 orders of magnitude higher than the primordial value (3He/H ~ 10-5 by number) determined from Galactic H II region abundances and confirmed by WMAP cosmic microwave background results. Chemical evolution models suggest that fewer than 5% of all PNe enrich the interstellar medium (ISM) with 3He at the level of standard stellar models. Our target PNe are therefore anomalous in that they were selected from a sample deliberately biased to contain objects with properties that maximized the likelihood of a 3He detection by the VLA. We have detected the 8.665 GHz hyperfine 3He+ transition in J320 at the 4 σ level. The 3He/H abundance ratio is 1.9 × 10-3 with roughly a factor of 2 uncertainty. For NGC 6572 we find an upper limit of 3He/H ≲ 10-3. This detection of 3He in J320 makes it the second PN known to have an anomalously high 3He abundance, confirming that at least some low-mass stars produce significant amounts of 3He that survives to the PN stage and enriches the ISM.