  Between 30 - 50% of white dwarfs (WDs) show heavy elements in their
atmospheres. This "pollution" is thought to arise from the accretion of
planetesimals perturbed by outer planet(s) to within the WD's tidal disruption
radius. A small fraction of these WDs show either emission or absorption from
circumstellar (C-S) gas. The abundances of metals in the photospheres of WDs
with C-S gas are mostly similar to the bulk composition of the Earth. The C-S
component arises from gas produced through collisions and/or the sublimation of
disintegrating planetesimals. High resolution spectroscopic observations of WD
1124-293 reveal photospheric and C-S absorption of Ca in multiple transitions.
Here, we present high signal-to-noise ratio spectra, an updated WD atmosphere
analysis, and a self-consistent model of its C-S gas. We constrain the
abundances of Ca, Mg, and Fe in the photosphere of WD 1124-293, and find
agreement with the abundances of these 3 species in the C-S gas. We find the
location of the C-S gas is about a hundred white dwarf radii, the C-S and
photospheric compositions are thus far consistent, the gas is not isothermal,
and the amount of C-S Ca has not changed in two decades. We also demonstrate
how to use Cloudy to model C-S gas viewed in absorption around polluted WDs.
Modeling the abundances of C-S gas around polluted WDs with Cloudy provides a
new method to measure the composition of exo-planetesimals and will allow a
direct comparison to the composition of rocky bodies in the Solar System.
