The Effect of Mixing on the Observed Metallicity of the Smith Cloud
  Measurements of high-velocity clouds' metallicities provide important clues
about their origins, and hence on whether they play a role in fueling ongoing
star formation in the Galaxy. However, accurate interpretation of these
measurements requires compensating for the galactic material that has been
mixed into the clouds. In order to determine how much the metallicity changes
as a result of this mixing, we have carried out three-dimensional
wind-tunnel-like hydrodynamical simulations of an example cloud. Our model
cloud is patterned after the Smith Cloud, a particularly well-studied cloud of
mass $\sim 5 \times 10^6~M_\odot$. We calculated the fraction of the
high-velocity material that had originated in the galactic halo,
$F_\mathrm{h}$, for various sight lines passing through our model cloud. We
find that $F_\mathrm{h}$ generally increases with distance from the head of the
cloud, reaching $\sim$0.5 in the tail of the cloud. Models in which the
metallicities (relative to solar) of the original cloud, $Z_\mathrm{cl}$, and
of the halo, $Z_\mathrm{h}$, are in the approximate ranges $0.1 \lesssim
Z_\mathrm{cl} \lesssim 0.3$ and $0.7 \lesssim Z_\mathrm{h} \lesssim 1.0$,
respectively, are in rough agreement with the observations. Models with
$Z_\mathrm{h} \sim 0.1$ and $Z_\mathrm{cl} \gtrsim 0.5$ are also in rough
agreement with the observations, but such a low halo metallicity is
inconsistent with recent independent measurements. We conclude that the Smith
Cloud's observed metallicity may not be a true reflection of its original
metallicity and that the cloud's ultimate origin remains uncertain.
