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. 
David C. Nicholls, Ralph S. Sutherland, Michael A. Dopita, Lisa J.   Kewley, and Brent A. Groves
astro-ph.GA