Constraints on the Galactic Chemical Evolution of 3He

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 ~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⊙ 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 yO ≈ 1.2 ZO, ⊙. 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.