First Detection of ${}^3$He${}^{+}$ in the Planetary Nebula IC418

Abstract

The ${}^3$He 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. ${}^3$He abundances in PNe can help test models of the chemical evolution of the Galaxy. We present the detection of the ${}^3$He${}^{+}$ emission line using the single dish Deep Space Station 63, towards the PN IC418. We derived a ${}^3$He/H abundance in the range 1.74$\pm$0.8$\times$10${}^{-3}$ to 5.8$\pm$1.7$\times$10${}^{-3}$, depending on whether part of the line arises in an outer ionized halo. The lower value for ${}^3$He/H ratio approaches values predicted by stellar models which include thermohaline mixing, but requires that large amounts of ${}^3$He are produced inside low-mass stars which enrich the interstellar medium (ISM). However, this over-predicts the ${}^3$He 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 ${}^3$He, from circumstellar environments to the ISM.

5 pages, 4 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Society Letters