  Solar photospheric abundances of refractory elements mirror the Earth's to
within ~10 mol% when normalized to the dominant terrestrial planet-forming
elements Mg, Si and Fe. This allows for the adoption of Solar composition as an
order-of-magnitude proxy for Earth's. It is not known, however, the degree to
which this mirroring of stellar and terrestrial planet abundances holds true
for other star-planet systems without determination of the composition of
initial planetesimals via condensation sequence calculations and post
condensation processes. We present the open-source Arbitrary Composition
Condensation Sequence calculator (ArCCoS) to assess how the elemental
composition of a parent star affects that of the planet-building material,
including the extent of oxidation within the planetesimals. We demonstrate the
utility of ArCCoS by showing how variations in the abundance of the stellar
refractory elements Mg and Si affect the condensation of oxygen, a controlling
factor in the relative proportions of planetary core and silicate mantle
material. This, thereby, removes significant degeneracy in the interpretation
of the structures of exoplanets as well as providing observational tests for
the validity of this model.
