Green Bank Telescope Observations of ${}^3\mathbf{H}{\mathbf{e}}^{+}$: Planetary Nebulae

Abstract

We use the Green Bank Telescope to search for ${}^{3}H{e}^{+}$ 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 ${}^{2}H$, ${}^{3}He$, ${}^{4}He$, and ${}^{7}Li$ were produced in significant amounts and these abundances have since been modified primarily by stars. Observations of ${}^{3}H{e}^{+}$ in H II regions located throughout the Milky Way disk reveal very little variation in the ${}^{3}He/H$ abundance ratio -- the "${}^{3}He$ Plateau" -- indicating that the net effect of ${}^{3}He$ production in stars is negligible. This is in contrast to much higher ${}^{3}He/H$ abundance ratios reported for some planetary nebulae. This discrepancy is known as the "${}^{3}He$ Problem". We use radio recombination lines observed simultaneously with the ${}^{3}H{e}^{+}$ 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 ${}^{3}H{e}^{+}$ in NGC 3242 nor do we detect ${}^{3}H{e}^{+}$ emission from any of our sources. This result calls into question all reported detections of ${}^{3}H{e}^{+}$ emission from any planetary nebula. The ${}^{3}He/H$ abundance upper limit we derive here for NGC 3242 is inconsistent with standard stellar production of ${}^{3}He$ and thus requires that some type of extra mixing process operates in low-mass stars.

arXiv commentAccepted for publication in The Astrophysical Journal

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