Abundance scaling in stars, nebulae and galaxies
  We present a new basis for scaling abundances with total metallicity in
nebular photoionisation models, based on extensive Milky Way stellar abundance
data, to replace the uniform scaling normally used in the analysis of HII
regions. Our goal is to provide a single scaling method and local abundance
reference standard for use in nebular modelling and its key inputs, the stellar
atmosphere and evolutionary track models. We introduce a parametric enrichment
factor, zeta, to describe how atomic abundances scale with total abundance, and
which allows for a simple conversion between scales based on different
reference elements (usually oxygen or iron) . The models and parametric
description provide a more physically realistic approach than simple uniform
abundance scaling. With appropriate parameters, the methods described here may
be applied to HII regions in the Milky Way, large and dwarf galaxies in the
local universe, Active Galactic Nuclei (AGNs), and to star forming regions at
high redshift.
