C/O ratios in planetary nebulae with dual-dust chemistry from faint optical recombination lines

TL;DR

This study analyzes the chemical compositions of nine planetary nebulae with dual-dust chemistry using high-resolution optical spectroscopy, revealing insights into their progenitor stars and dust formation processes.

Contribution

It provides detailed ionic and elemental abundance measurements, re-analyzes additional spectra, and discusses the origins of dust and chemical enrichment in planetary nebulae.

Findings

Approximately half are consistent with low-mass progenitors (<1.5 Msun).

Most nebulae likely formed PAHs through CO dissociation.

C/O ratios suggest limited contribution from very massive progenitors.

Abstract

We present deep high-resolution (R~15,000) and high-quality UVES optical spectrophotometry of nine planetary nebulae with dual-dust chemistry. We compute physical conditions from several diagnostics. Ionic abundances for a large number of ions of N, O, Ne, S, Cl, Ar, K, Fe and Kr are derived from collisionally excited lines. Elemental abundances are computed using state-of-the-art ionization correction factors. We derive accurate C/O ratios from optical recombination lines. We have re-analyzed additional high-quality spectra of 14 PNe from the literature following the same methodology. Comparison with asymptotic giant branch models reveals that about half of the total sample objects are consistent with being descendants of low-mass progenitor stars (M < 1.5 Msun). Given the observed N/O, C/O, and He/H ratios, we cannot discard that some of the objects come from more massive progenitor stars (M > 3--4 Msun) that have suffered a mild HBB. None of the objects seem to be a descendant of very massive progenitors. We propose that in most of the planetary nebulae studied here, the PAHs have been formed through the dissociation of the CO molecule. The hypothesis of a last thermal pulse that turns O-rich PNe into C-rich PNe is discarded, except in three objects, that show C/O > 1. We also discuss the possibility of a He pre-enrichment to explain the most He-enriched objects. We cannot discard other scenarios like extra mixing, stellar rotation or binary interactions to explain the chemical abundances behaviour observed in our sample.