On the Production of He, C and N by Low and Intermediate Mass Stars: A Comparison of Observed and Model-Predicted Planetary Nebula Abundances

TL;DR

This study compares observed and model-predicted abundances of helium, carbon, and nitrogen in planetary nebulae, revealing discrepancies that suggest the need for additional mixing processes in stellar models.

Contribution

It provides a homogeneous analysis of planetary nebulae abundances and highlights the mismatch between observations and models, especially for nitrogen enrichment.

Findings

N/O exceeds solar value, indicating N enrichment.

Hot bottom burning begins around 2 solar masses, lower than predicted.

Models fail to reproduce observed N/O ratios, suggesting extra mixing processes.

Abstract

The primary goal of this paper is to make a direct comparison between the measured and model-predicted abundances of He, C and N in a sample of 35 well-observed Galactic planetary nebulae (PN). All observations, data reductions, and abundance determinations were performed in house to ensure maximum homogeneity. Progenitor star masses (M < 4M_sun) were inferred using two published sets of post-AGB model tracks and L and T_eff values. We conclude the following: 1) the mean values of N/O across the progenitor mass range exceeds the solar value, indicating significant N enrichment in the majority of our objects; 2) the onset of hot bottom burning appears to begin around 2 solar masses, i.e., lower than ~5 M_sun implied by theory; 3) most of our objects show a clear He enrichment, as expected from dredge-up episodes; 4) the average sample C/O value is 1.23, consistent with the effects of third dredge-up; and 5) model grids used to compare to observations successfully span the distribution over metallicity space of all C/O and many He/H data points but mostly fail to do so in the case of N/O. The evident enrichment of N in PN and the general discrepancy between the observed and model-predicted N/O abundance ratios signal the need for extra-mixing as an effect of rotation and/or thermohaline mixing in the models. The unexpectedly high N enrichment that is implied here for low mass stars, if confirmed, will likely impact our conclusions about the source of N in the Universe.