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 3 He in low-mass stars ( M < 2 M ⊙ ), with peak abundances of 3 He/H ∼ few × 10 −3 by number. Over the lifetime of the Galaxy, this ought to produce 3 He/H abundances that diminish with increasing Galactocentric radius. Observations of 3 He + in H ii regions throughout the Galactic disk, however, reveal very little variation in the 3 He abundance with values of 3 He/H similar to the primordial abundance, 3 H e / H p ∼ 10 − 5 . This discrepancy, known as the “ 3 He problem,” can be resolved by invoking in stellar evolution models an extra mixing mechanism due to the thermohaline instability. Here we observe 3 He + in the planetary nebula (PN) J320 (G190.3–17.7) with the Jansky Very Large Array to confirm a previous 3 He + 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 3 He + emission from J320 with an rms noise of 58.8 μ Jy beam −1 after smoothing the data to a velocity resolution of 11.4 km s −1 . We estimate an abundance limit of 3 He/H ≤ 2.75 × 10 −3 by number using the numerical radiative transfer code NEBULA. This result nullifies the last significant detection of 3 He + in a PN and allows for the possibility that all stars undergo extra mixing processes.