Probing O-enrichment in C-rich dust planetary nebulae

TL;DR

This paper demonstrates through AGB nucleosynthesis models that oxygen can be overproduced in low-metallicity, low-mass stars, challenging its use as a reliable metallicity indicator in planetary nebulae.

Contribution

It provides new theoretical predictions showing oxygen enrichment in C-rich dust planetary nebulae, highlighting the need to reconsider metallicity proxies.

Findings

AGB models predict oxygen overproduction in low-metallicity, low-mass stars.

O enrichment explains recent observations in C-rich dust PNe.

Oxygen is not a reliable metallicity indicator in certain PNe cases.

Abstract

The abundance of O in planetary nebulae (PNe) has been historically used as a metallicity indicator of the interstellar medium (ISM) where they originated; e.g., it has been widely used to study metallicity gradients in our Galaxy and beyond. However, clear observational evidence for O self enrichment in low-metallicity Galactic PNe with C-rich dust has been recently reported. Here we report asymptotic giant branch (AGB) nucleosynthesis predictions for the abundances of the CNO elements and helium in the metallicity range Zsun/4 < Z < 2Zsun. Our AGB models, with diffusive overshooting from all the convective borders, predict that O is overproduced in low-Z low-mass (~1-3 Msun) AGB stars and nicely reproduce the recent O overabundances observed in C-rich dust PNe. This confirms that O is not always a good proxy of the original ISM metallicity and another chemical elements such as Cl or Ar should be used instead. The production of oxygen by low-mass stars should be thus considered in galactic-evolution models.