The Helix Nebula viewed in HCO⁺: Large-scale mapping of the J = 1 → 0 transition

The J = 1 → 0 transition of HCO⁺ at 89 GHz has been mapped across the Helix Nebula (NGC 7293) with 70″ spatial resolution (1.68 km s^-1 velocity resolution) using the Arizona Radio Observatory 12 m telescope. This work is the first large-scale mapping project of a dense gas tracer (n(H₂) ∼ 10⁵ cm^-3) in old planetary nebulae. Observations of over 200 positions encompassing the classical optical image were conducted with a 3σ noise level of ∼20 mK. HCO⁺ was detected at most positions, often exhibiting multiple velocity components indicative of complex kinematic structures in dense gas. The HCO⁺ spectra suggest that the Helix is composed of a bipolar, barrel-like structure with red- and blue-shifted halves, symmetric with respect to the central star and oriented ∼10° east from the line of sight. A second bipolar, higher velocity outflow exists as well, situated along the direction of the Helix "plumes." The column density of HCO⁺ across the Helix is N _tot ∼ 1.5 × 10¹⁰-5.0 × 10¹¹ cm^-2, with an average value N _ave ∼ 1 × 10 ¹¹ cm^-2, corresponding to an abundance, relative to H ₂, of f ∼ 1.4 × 10^-8. This value is similar to that observed in young PN, and contradicts chemical models, which predict that the abundance of HCO⁺ decreases with nebular age. This study indicates that polyatomic molecules readily survive the ultraviolet field of the central white dwarf, and can be useful in tracing nebular morphology in the very late stages of stellar evolution.