NEW DETECTIONS of HNC in PLANETARY NEBULAE: EVOLUTION of the [HCN]/[HNC] RATIO

New detections of HNC have been made toward 11 planetary nebulae (PNe), including K4-47, K3-58, K3-17, M3- 28, and M4-14. These sources, which represent a wide range of ages and morphologies, had previously been observed in HCN by Schmidt & Ziurys. Measurements of the J=10 and J = 3 → 2 transitions of HNC near 90 and 271 GHz were conducted using the new 12 m and the Sub-Millimeter Telescope of the Arizona Radio Observatory. HCN and HNC were also identified via their J = 1 → 0 lines toward eight positions across the Helix Nebula (NGC 7293). Column densities for HNC, determined from radiative transfer modeling, were N_tot(HNC) ∼ (0.06-4.0) ×10¹³ cm^-2, corresponding to fractional abundances with respect to H₂ of f ∼ (0.02-1.4) × 10^-7. The HCN and HNC column densities across the Helix were found to be Ntot (HCN)∼(0.2-2.4)×10¹²cm^-2 and N (HNC) tot ∼ (0.07-1.6)×10¹²cm^-2, with fractional abundances of (0.2-3.2)×10^-7 and (0.09-2.2) × 10^-7. The [HCN]/[HNC] ratio varied between sim; 1-8 for all PNe, with [HCN]/[HNC] ∼1-4 across the Helix. These values are greatly reduced from what has been found in asymptotic giant branch stars, where the ratio is typically >100. Both the abundance of HNC and the [HCN]/[HNC] ratio do not appear to vary significantly with nebular age across a time span of ∼10,000 years, in contrast to predictions of chemical models. The increase in HNC appears to arise in the proto-planetary stage, but becomes "frozen" once the PN phase is reached.