NEW DETECTIONS OF HNC IN PLANETARY NEBULAE: EVOLUTION OF THE [HCN]/[HNC] RATIO

Abstract

ABSTRACT 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 = 1 → 0 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 N tot ( HCN ) ∼ (0.2–2.4) × 1012 cm−2 and N tot ( HNC ) ∼ (0.07–1.6) × 1012 cm−2, with fractional abundances of (0.2–3.2) × 10−7 and (0.09–2.2) × 10−7. The [HCN]/[HNC] ratio varied between ∼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.