Do All Low-Mass Stars Undergo Extra Mixing Processes?

Abstract

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, . 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.