First survey of HCNH$^+$ in high-mass star-forming cloud cores
  Most stars in the Galaxy, including the Sun, were born in high-mass
star-forming regions. It is hence important to study the chemical processes in
these regions to better understand the chemical heritage of both the Solar
System and most stellar systems in the Galaxy. The molecular ion HCNH+ is
thought to be a crucial species in ion-neutral astrochemical reactions, but so
far it has been detected only in a handful of star-forming regions, and hence
its chemistry is poorly known. We have observed with the IRAM-30m Telescope 26
high-mass star-forming cores in different evolutionary stages in the J=3-2
rotational transition of HCNH+. We report the detection of HCNH+ in 16 out of
26 targets. This represents the largest sample of sources detected in this
molecular ion so far. The fractional abundances of HCNH+, [HCNH+], w.r.t. H2,
are in the range 0.9 - 14 X $10^{-11}$, and the highest values are found
towards cold starless cores. The abundance ratios [HCNH+]/[HCN] and
[HCNH+]/[HCO+] are both < 0.01 for all objects except for four starless cores,
for which they are well above this threshold. These sources have the lowest gas
temperature in the sample. We run two chemical models, a "cold" one and a
"warm" one, which attempt to match as much as possible the average physical
properties of the cold(er) starless cores and of the warm(er) targets. The
reactions occurring in the latter case are investigated in this work for the
first time. Our predictions indicate that in the warm model HCNH+ is mainly
produced by reactions with HCN and HCO+, while in the cold one the main
progenitor species of HCNH+ are HCN+ and HNC+. The results indicate that the
chemistry of HCNH+ is different in cold/early and warm/evolved cores, and the
abundance ratios [HCNH+]/[HCN] and [HCNH+]/[HCO+] is a useful astrochemical
tool to discriminate between different evolutionary phases in the process of
star formation.
