The Excitation Conditions of CN in TW Hya

TL;DR

This study observes CN in the TW Hya disk, revealing hyperfine splitting, emission morphology, and non-LTE excitation conditions, providing insights into disk chemistry and structure influenced by UV irradiation.

Contribution

It presents high-resolution observations of CN hyperfine structure and analyzes excitation conditions, highlighting non-LTE effects and disk atmospheric chemistry.

Findings

Hyperfine splitting resolved with line stacking techniques.

CN emission peaks at ~45 au with an outer tail to 200 au.

Non-LTE excitation conditions are indicated by temperature and density analyses.

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.