Green Bank Telescope Observations of ${\bf ^3He^{\bf +}}$: Planetary Nebulae

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 $\sim$ 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.