Numerical Study of Turbulent Mixing Layers with Non-Equilibrium
  Ionization Calculations
  Highly ionized species such as C IV, N V, and O VI, are commonly observed in
diffuse gas in various places in the universe, such as in our Galaxy's disk and
halo, high velocity clouds (HVCs), external galaxies, and the intergalactic
medium. One possible mechanism for producing high ions is turbulent mixing of
cool gas with hotter gas in locations where these gases slide past each other.
By using hydrodynamic simulations with radiative cooling and non-equilibrium
ionization (NEI) calculations, we investigate the physical properties of
turbulent mixing layers and the production of high ions. We find that most of
the mixing occurs on the hot side of the hot/cool interface and that the mixed
region separates into a tepid zone containing radiatively cooled, C IV-rich gas
and a hotter zone which is rich in C IV, N V, and O VI. Mixing occurs faster
than ionization or recombination, making the mixed gas a better source of C IV,
N V, and O VI in our NEI simulations than in our collisional ionization
equilibrium (CIE) simulations. In addition, the gas radiatively cools faster
than the ions recombine, which also allows large numbers of high ions to linger
in the NEI simulations. For these reasons, our NEI calculations predict more
high ions than our CIE calculations predict. We also simulate various initial
configurations of turbulent mixing layers and discuss their results. We compare
the results of our simulations with observations and other models, including
other turbulent mixing calculations. The ratios of C IV to N V and N V to O VI
are in reasonable agreement with the averages calculated from observations of
the halo. There is a great deal of variation from sightline to sightline and
with time in our simulations. Such spatial and temporal variation may explain
some of the variation seen among observations.
