Protonated hydrogen cyanide as a tracer of pristine molecular gas

Context. Protonated hydrogen cyanide, HCNH+, plays a fundamental role in astrochemistry because it is an intermediary in gas-phase ion-neutral reactions within cold molecular clouds. However, the impact of the environment on the chemistry of HCNH+ remains poorly understood. Aims. We aim to study HCNH+, HCN, and HNC, as well as two other chemically related ions, HCO+ and N2H+, in different star formation regions in order to investigate how the environment influences the chemistry of HCNH+. Methods. With the IRAM 30 m and APEX 12 m telescopes, we carried out HCNH+, H13CN, HN13C, H13CO+, and N2H+ imaging observations toward two dark clouds, the Serpens filament and Serpens South, both of which harbor sites of star formation that include protostellar objects and regions that are quiescent. Results. We report the first robust distribution of HCNH+ in the Serpens filament and in Serpens South. Our data suggest that HCNH+ is abundant in cold and quiescent regions but is deficient in active star-forming regions. The observed HCNH+ fractional abundances relative to H2 range from 3.1 × 10-11 in protostellar cores to 5.9 × 10-10 in prestellar cores, and the HCNH+ abundance generally decreases with increasing H2 column density, which suggests that HCNH+ coevolves with cloud cores. Our observations and modeling results suggest that the abundance of HCNH+ in cold molecular clouds is strongly dependent on the H2 number density. The decrease in the abundance of HCNH+ is caused by the fact that its main precursors (e.g., HCN and HNC) undergo freeze-out as the number density of H2 increases. However, current chemical models cannot explain other observed trends, such as the fact that the abundance of HCNH+ shows an anticorrelation with that of HCN and HNC but a positive correlation with that of N2H+ in the southern part of Serpens South's northern clump. This indicates that additional chemical pathways have to be invoked for the formation of HCNH+ via molecules such as N2 in regions in which HCN and HNC freeze out. Conclusions. Both the fact that HCNH+ is most abundant in molecular cores prior to gravitational collapse and the fact that low-J HCNH+ transitions have very low H2 critical densities make this molecular ion an excellent probe of pristine molecular gas.