  Observations of molecular clouds, prestellar cores, and protoplanetary disks
have established that the HNC/HCN ratio may be a potent diagnostic of molecular
gas physical conditions. The processes that govern the relative abundances of
these molecules nevertheless remain poorly understood. We seek to exploit the
wide range of UV irradiation strengths within the 1 pc diameter Helix planetary
nebula to explore the potential role of UV radiation in driving HNC/HCN. We
performed IRAM 30 m and APEX 12 m radio line observations across six positions
within the Helix Nebula, making use of radiative transfer and photodissociation
modeling codes to interpret the results for line intensities and line ratios in
terms of the molecular gas properties. We have obtained the first detections of
the plasma-embedded Helix molecular knots (globules) in HCN, HNC, HCO+, and
other trace molecules. Analysis of the HNC/HCN integrated line intensity ratio
reveals an increase with radial distance from the Helix central star. In the
context of molecular line ratios of other planetary nebulae from the
literature, the HNC/HCN ratio appears to be anticorrelated with UV emission
over four orders of magnitude in incident flux. Models of the photodissociation
regions within the Helix using the RADEX and Meudon codes reveal strong
constraints on column density of the molecular gas, as well as pressure and
temperature. Analysis of the molecular ion HCO+ across the Helix indicates that
X-ray irradiation is likely driving HCO+ production in the outer regions of
planetary nebulae, where photodissociation is limited, yet cold gas and ionized
molecules are abundant. Although the observational results clearly indicate
that UV irradiation is important in determining the HNC/HCN ratio, our PDR
modeling indicates that the UV flux gradient alone cannot reproduce the
observed variation of HNC/HCN across the Helix.
