Do All Low-Mass Stars Undergo Extra Mixing Processes?

TL;DR

This study investigates whether all low-mass stars experience extra mixing processes by measuring 3He+ in a planetary nebula, finding no detectable 3He+ emission which suggests that not all stars undergo such mixing.

Contribution

The paper provides observational evidence challenging the assumption that all low-mass stars undergo extra mixing, using sensitive radio observations to set new upper limits on 3He+ abundance.

Findings

No 3He+ emission detected in J320 planetary nebula.

Sets an upper limit of 3He/H <= 2.75 x 10^-3 in J320.

Supports the possibility that some low-mass stars do not undergo extra mixing.

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 Msun), with peak abundances of 3He/H ~ few x 10-3 by number. Over the life-time of the Galaxy, this ought to produce 3He/H abundances that diminish with increasing Galactocentric radius. Observations of 3He+ in HII regions throughout the Galactic disk, however, reveal very little variation in the 3He abundance with values of 3He/H similar to the primoridal abundance, (3He/H)p ~ 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 J320 (PN G190.3-17.7) with the Jansky Very Large Array (JVLA) to confirm a previous 3He+ detection made with the VLA 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 microJy/beam after smoothing the data to a velocity resolution of 11.4 km/s . We estimate an abundance limit of 3He/H <= 2.75 x 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.