Bibliographic Details
| Title: |
Protonated hydrogen cyanide as a tracer of pristine molecular gas |
| Authors: |
Gong, Y., Du, F. J., Henkel, C., Jacob, A. M., Belloche, A., Wang, J. Z., Menten, K. M., Yang, W., Quan, D. H., Bop, C. T., Ortiz-León, G. N., Tang, X. D., Rugel, M. R., Liu, S. |
| Source: |
A&A 679, A39 (2023) |
| Publication Year: |
2023 |
| Collection: |
Astrophysics |
| Subject Terms: |
Astrophysics - Astrophysics of Galaxies |
| More Details: |
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.
Comment: 25 pages, 26 figures, accepted for publication in A&A |
| Document Type: |
Working Paper |
| DOI: |
10.1051/0004-6361/202347409 |
| Access URL: |
http://arxiv.org/abs/2308.15521 |
| Accession Number: |
edsarx.2308.15521 |
| Database: |
arXiv |
