Plasma code for astrophysical charge exchange emission at X-ray
  wavelengths
  Charge exchange X-ray emission provides unique insights into the interactions
between cold and hot astrophysical plasmas. Besides its own profound science,
this emission is also technically crucial to all observations in the X-ray
band, since charge exchange with the solar wind often contributes a significant
foreground component that contaminates the signal of interest. By approximating
the cross sections resolved to $n$ and $l$ atomic subshells, and carrying out
complete radiative cascade calculation, we create a new spectral code to
evaluate the charge exchange emission in the X-ray band. Comparing to
collisional thermal emission, charge exchange radiation exhibits enhanced lines
from large-$n$ shells to the ground, as well as large forbidden-to-resonance
ratios of triplet transitions. Our new model successfully reproduces an
observed high-quality spectrum of comet C/2000 WM1 (LINEAR), which emits purely
by charge exchange between solar wind ions and cometary neutrals. It
demonstrates that a proper charge exchange model will allow us to probe
remotely the ion properties, including charge state, dynamics, and composition,
at the interface between the cold and hot plasmas.
