NGC 6778: Strengthening the link between extreme abundance discrepancy factors and central star binarity in planetary nebulae

TL;DR

This study of NGC 6778 reveals extremely high abundance discrepancy factors in the nebula, especially near the central star, and confirms the strong link between such discrepancies and the binarity of the central star.

Contribution

First detailed abundance analysis of NGC 6778, demonstrating the correlation between high abundance discrepancy factors and central star binarity.

Findings

Abundance discrepancy factor reaches ~40 near the central star.

Heavy element abundances from recombination lines exceed collisionally excited lines by ~20 times.

Central star is a short period binary, linking binarity to high abundance discrepancies.

Abstract

We present new optical spectra of the nearby, bright, planetary nebula NGC 6778. The nebula has been known to emit strong recombination lines for more than 40 years but this is the first detailed study of its abundances. Heavy element abundances derived from recombination lines are found to exceed those from collisionally excited lines by a factor of ~20 in an integrated spectrum of the nebula, which is among the largest known abundance discrepancy factors. Spatial analysis of the spectra shows that the abundance discrepancy factor is strongly, centrally peaked, reaching ~40 close to the central star. The central star of NGC 6778 is known to be a short period binary, further strengthening the link between high nebular abundance discrepancy factors and central star binarity.