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.