Neon, sulphur and argon abundances of planetary nebulae in the sub-solar metallicity Galactic anti-centre

TL;DR

This study measures neon, sulphur, and argon abundances in planetary nebulae towards the Galactic anti-centre to investigate the metallicity gradient of the Milky Way beyond 10 kpc, using infrared and optical data.

Contribution

It provides new infrared-based abundance measurements of planetary nebulae in the outer Galaxy, extending the understanding of the metallicity gradient beyond 10 kpc.

Findings

Abundances are lower than in the solar neighbourhood.

The metallicity gradient extends up to ~20 kpc from the Galactic centre.

Data show some dispersion but generally support a gradient continuation.

Abstract

The abundances derived from planetary nebula emission show the presence of a metallicity gradient within the disk of the Milky Way up to Galactocentric distances of ~10 kpc, which are consistent with findings from studies of different types of sources, including H II regions and young B-type stars. The radial dependence of these abundances further from the Galactic centre is in dispute. We aim to derive the abundances of neon, sulphur and argon from a sample of planetary nebulae towards the Galactic anti-centre, which represent the abundances of the clouds from which they were formed, as they remain unchanged throughout the course of stellar evolution. We then aim to compare these values with similarly analysed data from elsewhere in the Milky Way in order to observe whether the abundance gradient continues in the outskirts of our Galaxy. We have observed 23 planetary nebulae at Galactocentric distances of 8-21 kpc with Spitzer IRS. The abundances were calculated from infrared emission lines, for which we observed the main ionisation states of neon, sulphur, and argon, which are little affected by extinction and uncertainties in temperature measurements or fluctuations within the planetary nebula. We have complemented these observations with others from optical studies in the literature, in order to reduce or avoid the need for ionisation correction factors in abundance calculations. The overall abundances of our sample of planetary nebulae in the Galactic anti-centre are lower than those in the solar neighbourhood. The abundances of neon, sulphur, and argon from these stars are consistent with a metallicity gradient from the solar neighbourhood up to Galactocentric distances of ~20 kpc, albeit with varying degrees of dispersion within the data.