Constraints on the Galactic Chemical Evolution of $^3\rm{He}$

TL;DR

This study models the galactic chemical evolution of helium-3, comparing predictions with observations to constrain stellar yields and star formation parameters, highlighting the slow evolution of helium-3 and the importance of outflows.

Contribution

It provides new constraints on helium-3 stellar yields and galactic evolution models, emphasizing the role of star formation history and outflows in matching observed abundances.

Findings

GCE models tend to overpredict helium-3 without slow star formation onset.

Higher oxygen yields imply stronger outflows are needed to match observations.

Future measurements at higher metallicity can distinguish between stellar yield uncertainties.

Abstract

We examine the galactic chemical evolution (GCE) of $^3\rm{He}$ in one-zone and multi-zone models, with particular attention to the stellar yields and GCE parameters that can reproduce both the protosolar $^3\rm{He}$ abundance and recent gas-phase $^3\rm{He}/^4\rm{He}$ measurements in the Orion nebula. Published stellar models indicate negligible net $^3\rm{He}$ production by massive stars, while the predicted yields from asymptotic giant branch (AGB) stars are metallicity-dependent and span a range of $\sim 2.5$ depending on the extra mixing processes incorporated in the stellar models. The dominant contribution to $^3\rm{He}$ production comes from $1-2\ M_\odot$ stars, making $^3\rm{He}$ evolution slow compared to other AGB elements and to Fe enrichment from Type Ia supernovae. We constrain our GCE models to reproduce the observed [O/H] in the interstellar medium, and our fiducial models adopt an empirically motivated IMF-averaged oxygen yield $y_{\rm O} \approx 1.2\ Z_{\rm O, \odot}$. Even with the lowest of the AGB $^3\rm{He}$ yields, based on stellar models with rotational and thermohaline mixing, our GCE models tend to overpredict the protosolar and Orion $^3\rm{He}$ abundances; they require a slow onset of star formation and low star formation efficiency to come close to the observed values. With a higher oxygen yield, calibration to observed [O/H] implies stronger outflows, making it easier to reproduce the observed $^3\rm{He}$. Alternatively, the true $^3\rm{He}$ yield could be lower than that predicted by existing stellar models, suggesting that mixing in red giants is not yet fully captured. Future $^3\rm{He}$ measurements that probe higher metallicity environments could help distinguish these possibilities.