Green Bank Telescope Observations of 3He+: Planetary Nebulae

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