Protonated hydrogen cyanide as a tracer of pristine molecular gas^★

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: ygong@mpifr-bonn.mpg.de
² Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Qixia District, Nanjing 210023, PR China
³ School of Astronomy and Space Science, University of Science and Technology of China, 96 JinZhai Road, Baohe District, Hefei 230026, PR China
⁴ Astronomy Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
⁵ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi, Xinjiang 830011, PR China
⁶ William H. Miller III Department of Physics & Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁷ Guangxi Key Laboratory for Relativistic Astrophysics, Department of Physics, Guangxi University, 100 Daxuedong Road, Nanning 530004, PR China
⁸ Research Center for Intelligent Computing Platforms, Zhejiang Laboratory, Hangzhou 311100, PR China
⁹ Univ. Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
¹⁰ Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51 y 216, Puebla 72000, Mexico
¹¹ Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi 830011, PR China
¹² University of Chinese Academy of Sciences, Beijing 100049, PR China
¹³ Center for Astrophysics – Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁴ National Radio Astronomy Observatory, 1003 Lopezville RD, Socorro, NM 87801, USA
¹⁵ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, PR China

Received: 9 July 2023
Accepted: 29 August 2023

Abstract

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.