A New Radio Molecular Line Survey of Planetary Nebulae: HNC/HCN as a
  Diagnostic of Ultraviolet Irradiation
  Certain planetary nebulae contain shells, filaments, or globules of cold gas
and dust whose heating and chemistry are likely driven by UV and X-ray emission
from their central stars and from wind-collision-generated shocks. We present
the results of a survey of molecular line emission in the 88-236 GHz range from
nine nearby (<1.5 kpc) planetary nebulae spanning a range of UV and X-ray
luminosities, using the 30 m telescope of the Institut de Radioastronomie
Millimetrique. Rotational transitions of thirteen molecules, including CO
isotopologues and chemically important trace species, were observed and the
results compared with and augmented by previous studies of molecular gas in
PNe. Lines of the molecules HCO+, HNC, HCN, and CN, which were detected in most
objects, represent new detections for five planetary nebulae in our study.
Specifically, we present the first detections of 13CO (1-0, 2-1), HCO+, CN,
HCN, and HNC in NGC 6445; HCO+ in BD+303639; 13CO (2-1), CN, HCN, and HNC in
NGC 6853; and 13CO (2-1) and CN in NGC 6772. Flux ratios were analyzed to
identify correlations between the central star and/or nebular UV and X-ray
luminosities and the molecular chemistries of the nebulae. This analysis
reveals a surprisingly robust dependence of the HNC/HCN line ratio on PN
central star UV luminosity. There exists no such clear correlation between PN
X-rays and various diagnostics of PN molecular chemistry. The correlation
between HNC/HCN ratio and central star UV luminosity demonstrates the potential
of molecular emission line studies of PNe for improving our understanding of
the role that high-energy radiation plays in the heating and chemistry of
photodissociation regions.
