The Excitation Conditions of CN in TW Hya
  We report observations of the cyanide anion, CN, in the disk around TW~Hya
covering the $N=1-0$, $N=2-1$ and $N=3-2$ transitions. Using line stacking
techniques, 24 hyperfine transitions are detected out of the 30 within the
observed frequency ranges. Exploiting the super-spectral resolution from the
line stacking method reveals the splitting of hyperfine components previously
unresolved by laboratory spectroscopy. All transitions display a similar
emission morphology, characterized by an azimuthally symmetric ring, peaking at
$\approx 45$~au (0.75"), and a diffuse outer tail extending out to the disk
edge at $\approx 200$~au. Excitation analyses assuming local thermodynamic
equilibrium (LTE) yield excitation temperatures in excess of the derived
kinetic temperatures based on the local line widths for all fine structure
groups, suggesting assumptions of LTE are invalid. Using the 0D radiative
transfer code RADEX, we demonstrate that such non-LTE effects may be present
when the local H$_2$ density drops to $10^{7}~{\rm cm^{-3}}$ and below.
Comparison with models of TW~Hya find similar densities at elevated regions in
the disk, typically $z \, / \, r \gtrsim 0.2$, consistent with model
predictions where CN is formed via vibrationally excited H$_2$ in the disk
atmospheric layers where UV irradiation is less attenuated.
