First Detection of ³He+ in the Planetary Nebula IC418

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The ³He isotope is important to many fields of astrophysics, including stellar evolution, chemical evolution, and cosmology. The isotope is produced in low-mass stars which evolve through the planetary nebula (PN) phase. ³He abundances in PNe can help test models of the chemical evolution of the Galaxy. We present the detection of the ³He+ emission line using the single dish Deep Space Station 63, towards the PN IC418. We derived a ³He/H abundance in the range 1.74±0.8×10⁻³ to 5.8±1.7×10⁻³, depending on whether part of the line arises in an outer ionized halo. The lower value for ³He/H ratio approaches values predicted by stellar models which include thermohaline mixing, but requires that large amounts of ³He are produced inside low-mass stars which enrich the interstellar medium (ISM). However, this over-predicts the ³He abundance in HII regions, the ISM, and proto-solar grains, which is known to be of the order of 10⁻⁵. This discrepancy questions our understanding of the evolution of the ³He, from circumstellar environments to the ISM.