Rotational Excitation of HC${}_5$NH${}^{+}$ by He: Collision Dynamics and Astrophysical Applications

Abstract

The protonated cyanopolyyne HC${}_5$NH${}^{+}$ is an interstellar molecular ion of astrochemical interest, and understanding its rotational excitation is essential for interpreting its emission in molecular clouds. In this work, we construct a two-dimensional potential energy surface (PES) for the HC${}_5$NH${}^{+}$--He system using the CCSD(T)-F12B/cc-pVTZ-F12 method. Rotationally inelastic cross sections are calculated using the quantum close-coupling (CC) method, and the corresponding collisional rate coefficients are obtained by thermal averaging over a Maxwell--Boltzmann distribution. These rate coefficients are incorporated into a non-local thermodynamic equilibrium (non-LTE) radiative transfer model to investigate the excitation of HC${}_5$NH${}^{+}$ in the cold dark cloud TMC-1. A global ${\chi }^2$ minimization analysis of seven observed transitions yields best-fitting values of $n({\mathrm{H}}_2)=1.63\times 10^5$ cm${}^{-3}$ and $N({\mathrm{HC}}_5{\mathrm{NH}}^{+})=5.09\times 10^{11}$ cm${}^{-2}$, with a minimum reduced ${\chi }^2$ of 0.87. The modelled line intensities are in good agreement with the observations. This study provides the first-principles collisional data needed to model the rotational excitation of HC${}_5$NH${}^{+}$ and offers new constraints on its physical and spectroscopic properties in TMC-1.

arXiv comment7 pages, 7 figures