First Detection of $^3$He$^+$ in the Planetary Nebula IC$\,$418

TL;DR

This paper reports the first detection of the $^3$He$^+$ emission line in the planetary nebula IC 418, providing new data on $^3$He abundance that challenges existing stellar and galactic chemical evolution models.

Contribution

The study presents the first detection of $^3$He$^+$ in a planetary nebula and discusses its implications for stellar evolution and chemical evolution models.

Findings

$^3$He/H abundance range 1.74e-3 to 5.8e-3

Detection supports models with thermohaline mixing

Discrepancy with $^3$He levels in ISM and proto-solar grains

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 IC$\,$418. 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.