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