Abundance scaling in stars, nebulae and galaxies

We present a new basis for scaling abundances with totalmetallicity in nebular photoionizationmodels, based on extensive Milky Way stellar abundance data, to replace the uniform scalingnormally used in the analysis of HII regions. Our goal is to provide a single scalingmethod andlocal abundance reference standard for use in nebular modelling and its key inputs, the stellaratmosphere and evolutionary track models. We introduce a parametric enrichment factor, ζ, to describe how atomic abundances scale with total abundance, which allows for a simpleconversion between scales based on different reference elements (usually oxygen or iron). The models and parametric description provide a more physically realistic approach thansimple uniform abundance scaling. With appropriate parameters, the methods described heremay be applied toHII regions in theMilkyWay, large and dwarf galaxies in the local Universe, active galactic nuclei, and to star-forming regions at high redshift.