Protonated hydrogen cyanide as a tracer of pristine molecular gas

Abstract

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. With the IRAM-30 m and APEX-12 m observations, 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 deficit in active star-forming regions. The observed HCNH${}^{+}$ fractional abundances relative to H${}_2$ range from $3.1\times 10^{-11}$ in protostellar cores to $5.9\times 10^{-10}$ in prestellar cores, and the HCNH${}^{+}$ abundance generally decreases with increasing H${}_2$ 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 H${}_2$ 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 H${}_2$ increases. However, current chemical models cannot explain other observed trends, such as the fact that the abundance of HCNH${}^{+}$ shows an anti-correlation with that of HCN and HNC, but a positive correlation with that of N${}_2$H${}^{+}$ in the southern part of the Serpens South northern clump. This indicates that additional chemical pathways have to be invoked for the formation of HCNH${}^{+}$ via molecules like N${}_2$ in regions in which HCN and HNC freeze out. 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 H${}_2$ critical densities make this molecular ion an excellent probe of pristine molecular gas.

arXiv comment25 pages, 26 figures, accepted for publication in A&A

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