The role of polycyclic aromatic hydrocarbons in astrochemical modeling at different evolutionary stages of cold dense cloud cores

¹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, China
² Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Urumqi 830011, China
³ Departamento de Astronomía, Universidad de Chile, Camino el Observatorio 1515, Las Condes, Santiago, Chile
⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
⁵ Universidad Autónoma de Chile, Facultad de Ingeniería, Núcleo de Astroquímica & Astrofísica, Av. Pedro de Valdivia 425, Providencia, Chile
⁶ Department of Physics, Faculty of Science, Kunming University of Science and Technology, Kunming 650500, PR China
⁷ Yunnan Observatories, Chinese Academy of Sciences, 396 Yangfangwang, Guandu District, Kunming 650216, Yunnan, PR China
⁸ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, PR China

^★ Corresponding author: gejixing@xao.ac.cn

Received: 12 October 2024
Accepted: 6 August 2025

Abstract

Context. Although the role of PAHs have been highlighted in recent years, it is crucial to verify their functions at different evolutionary stages of dark cloud cores.

Aims. Fitting molecular abundances with gas-grain chemical models has revealed shorter chemical ages for denser cold dark cloud cores. To deepen the understanding of chemical ages and the functions of PAHs, we investigated the influence of PAHs in our chemical models by fitting observational data.

Methods. We collected observed molecular abundances (at least five species per core) from the literature for 39 cold dense cloud cores (T < 25 K and 10⁴ ≤ n_H ≤ 10⁷ cm⁻³). Using a gas-grain chemical model, we fit the molecular abundances of the 39 samples to determine the best-fit chemical ages and discuss the role of PAHs in an evolutionary view.

Results. We find an anticorrelation between the gas density and the fit chemical age through molecular abundance fitting. The best-fit chemical ages of the low-density cores (<7 × 10⁴ cm⁻³) are comparable to the dynamical ages, which can be a good alternative indicator of physical age. However, for high-density cores (>7 × 10⁴ cm⁻³), the chemical age is far from the physical age and therefore useless. The inclusion of PAHs do not change the estimation of the chemical ages much, which indicates the small effect of PAHs on average. However, we identified molecular tracers that are sensitive to PAHs at different evolutionary stages, thus revealing their diagnostic potential for chemical evolution.