Confirmation of the link between central star binarity and extreme abundance discrepancy factors in planetary nebulae

TL;DR

This study confirms a strong correlation between close binary central stars in planetary nebulae and extreme abundance discrepancy factors, suggesting nova-like eruptions post-common-envelope phase cause these anomalies.

Contribution

It provides new deep spectra of seven binary central star nebulae, nearly doubles known cases, and statistically links binarity with extreme abundance discrepancies and electron density variations.

Findings

Binary central stars are strongly linked to high abundance discrepancy factors.

Nebulae with short-period binaries have lower electron densities and higher abundance discrepancies.

Recombination excitation can enhance [O II] density diagnostic lines.

Abstract

It has recently been noted that there seems to be a strong correlation between planetary nebulae with close binary central stars, and highly enhanced recombination line abundances. We present new deep spectra of seven objects known to have close binary central stars, and find that the heavy element abundances derived from recombination lines exceed those from collisionally excited lines by factors of 5-95, placing several of these nebulae among the most extreme known abundance discrepancies. This study nearly doubles the number of nebulae known to have a binary central star and an extreme abundance discrepancy. A statistical analysis of all nebulae with measured recombination line abundances reveals no link between central star surface chemistry and nebular abundance discrepancy, but a clear link between binarity and the abundance discrepancy, as well as an anticorrelation between abundance discrepancies and nebular electron densities: all nebulae with a binary central star with a period of less than 1.15 days have an abundance discrepancy factor exceeding 10, and an electron density less than $\sim$1000 cm$^{-3}$; those with longer period binaries have abundance discrepancy factors less than 10 and much higher electron densities. We find that [O~{\sc ii}] density diagnostic lines can be strongly enhanced by recombination excitation, while [S~{\sc ii}] lines are not. These findings give weight to the idea that extreme abundance discrepancies are caused by a nova-like eruption from the central star system, occurring soon after the common-envelope phase, which ejects material depleted in hydrogen, and enhanced in CNONe but not in third-row elements.