Collisional excitation of HMgC${}_3$N by He: rate coefficients and astrophysical implications

Abstract

Reliable collisional data are essential for interpreting the rotational emission of HMgC${}_3$N under the non-equilibrium conditions of molecular clouds. We investigate the rotational excitation and de-excitation of HMgC${}_3$N induced by collisions with He, combining \textit{ab initio} calculations with quantum scattering theory. A two-dimensional potential energy surface (PES) for HMgC${}_3$N--He was constructed using the CCSD(T)-F12A method in conjunction with the aug-cc-pVTZ basis set. Quantum close-coupling (CC) calculation were then performed to obtain state-to-state integral cross sections and temperature-dependent rate coefficients for rotational transitions with $\Delta j=\pm 1$, $\pm 2$, and $\pm 3$, covering rotational levels up to $j=20$ and temperatures from $3$ to $50$~K. The rate coefficients exhibit a clear propensity for $\Delta j=\pm 2$ transitions. The resulting collisional data were incorporated into radiative transfer modelling to constrain the physical conditions and column density of HMgC${}_3$N from its observed spectral line emission. A ${\chi }^2$ minimization yields a best-fitting H${}_2$ volume density of $n({\mathrm{H}}_2)=1.233\times 10^5{\mathrm{cm}}^{-3}$ and column density of $N({\mathrm{HMgC}}_3\mathrm{N})=1.831\times 10^{12}{\mathrm{cm}}^{-2}$. The derived column density is in approximate agreement with the observational estimate of $(3.0\pm 0.6)\times 10^{12}{\mathrm{cm}}^{-2}$. These results provide new collisional data for non-LTE studies of HMgC${}_3$N and support a more realistic treatment of its rotational excitation and radiative transfer in molecular clouds.

arXiv comment8 pages,9 figures