The Excitation Conditions of CN in TW Hya

Abstract: 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.