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.

25 pages, 26 figures, accepted for publication in A&A

References in corpus (28)

• The NumPy array: a structure for efficient numerical computation
• Cold Dark Clouds: The Initial Conditions for Star Formation
• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• A Large Catalog of Accurate Distances to Local Molecular Clouds: The Gaia DR2 Edition
• Comparing Star Formation on Large Scales in the c2d Legacy Clouds: Bolocam 1.1 mm Dust Continuum Surveys of Serpens, Perseus, and Ophiuchus
• The interstellar gas-phase chemistry of HCN and HNC
• The Spitzer Gould Belt Survey of Large Nearby Interstellar Clouds: Discovery of a Dense Embedded Cluster in the Serpens-Aquila Rift
• The Gould's Belt Distances Survey (GOBELINS) III. The distance to the Serpens/Aquila Molecular Complex
• The HNC/HCN Ratio in Star-Forming Regions
• HCN/HNC intensity ratio: a new chemical thermometer for the molecular ISM
• Outflows, infall and evolution of a sample of embedded low-mass protostars. The William Herschel Line Legacy (WILL) survey
• Cluster Formation Triggered by Filament Collisions in Serpens South
• A spectral line survey in the 2 mm and 1.3 mm windows toward the carbon rich envelope of IRC +10216
• Electron Excitation of High Dipole Moment Molecules Reexamined
• Electron-impact rotational and hyperfine excitation of HCN, HNC, DCN and DNC
• APECS - The Atacama Pathfinder Experiment Control System
• Detection of the HC${}_3$NH${}^{+}$ and HCNH${}^{+}$ ions in the L1544 pre-stellar core
• Assessing molecular outflows and turbulence in the protostellar cluster Serpens South
• The fragmentation and stability of hierarchical structure in Serpens South
• Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)
• Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253
• Physical and chemical structure of the Serpens filament -- fast formation and gravity-driven accretion
• Chemical composition of the circumstellar disk around AB Aurigae
• First survey of HCNH${}^{+}$ in high-mass star-forming cloud cores
• Can we observe the ion-neutral drift velocity in prestellar cores?
• The distance to the Serpens South Cluster from H2O masers
• Star-forming regions of the Aquila rift cloud complex. II. Turbulence in molecular cores probed by NH3 emission
• Deep Near-Infrared Survey Toward the W40 and Serpens South Region in Aquila Rift: A Comprehensive Catalog of Young Stellar Objects