Do All Low-Mass Stars Undergo Extra Mixing Processes?

Standard stellar evolution models that only consider convection as a physical process to mix material inside of stars predict the production of significant amounts of ³He in low-mass stars (M < 2 M _⊙), with peak abundances of ³He/H ~ few × 10^-3 by number. Over the lifetime of the Galaxy, this ought to produce ³He/H abundances that diminish with increasing Galactocentric radius. Observations of ³He⁺ in H II regions throughout the Galactic disk, however, reveal very little variation in the ³He abundance with values of ³He/H similar to the primordial abundance, ${\left({}^{3}{\rm{H}}{\rm{e}}/{\rm{H}}\right)}_{p}\sim {10}^{-5}$ . This discrepancy, known as the "³He problem," can be resolved by invoking in stellar evolution models an extra mixing mechanism due to the thermohaline instability. Here we observe ³He⁺ in the planetary nebula (PN) J320 (G190.3-17.7) with the Jansky Very Large Array to confirm a previous ³He⁺ detection made with the Very Large Array that supports standard stellar yields. This measurement alone indicates that not all stars undergo extra mixing. Our more sensitive observations do not detect ³He⁺ emission from J320 with an rms noise of 58.8 μJy beam^-1 after smoothing the data to a velocity resolution of 11.4 km s^-1. We estimate an abundance limit of ³He/H ≤ 2.75 × 10^-3 by number using the numerical radiative transfer code NEBULA. This result nullifies the last significant detection of ³He⁺ in a PN and allows for the possibility that all stars undergo extra mixing processes.