2016MNRAS.460L..35G

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 IC 418. We derived a ^3He/H abundance in the range 1.74±0.8 x 10^-3 to 5.8±1.7 x 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 overpredicts the ^3He abundance in H ii regions, the ISM, and protosolar 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.