2021ApJ...910...73B

We use the Green Bank Telescope to search for ³He⁺ emission from a sample of four Galactic planetary nebulae: NGC 3242, NGC 6543, NGC 6826, and NGC 7009. During the era of primordial nucleosynthesis, the light elements ²H, ³He, ⁴He, and ⁷Li were produced in significant amounts, and these abundances have since been modified primarily by stars. Observations of ³He⁺ in H II regions located throughout the Milky Way disk reveal very little variation in the ³He/H abundance ratio-the "³He Plateau"-indicating that the net effect of ³He production in stars is negligible. This is in contrast to much higher ³He/H abundance ratios reported for some planetary nebulae. This discrepancy is known as the "³He Problem." We use radio recombination lines observed simultaneously with the ³He⁺ transition to make a robust assessment of the spectral sensitivity that these observations achieve. We detect spectral lines at ∼1-2 mK intensities, but at these levels, instrumental effects compromise our ability to measure accurate spectral line parameters. We do not confirm reports of previous detections of ³He⁺ in NGC 3242 nor do we detect ³He⁺ emission from any of our sources. This result calls into question all reported detections of ³He⁺ emission from any planetary nebula. The ³He/H abundance upper limit we derive here for NGC 3242 is inconsistent with standard stellar production of ³He and thus requires that some type of extra-mixing process operates in low-mass stars.