Collisional excitation of HCN by H2O

ABSTRACT Hydrogen cyanide (HCN) is one of the commonly observed molecules in the atmosphere of comets. As it may serve as a crucial building block in the reaction chain leading to the formation of amino acids, measuring its abundance, and comparing it to that observed in the interstellar medium can provide clues about star-forming processes and the emergence of life. However, to model HCN observations accurately, a non-local thermodynamic equilibrium analysis is necessary. This requires information about the radiative properties of HCN and its collisional excitation due to collisions with water (H$_{2}$O) since water is the most abundant projectile in comae. Here, we present a new set of rate coefficients for the rotational excitation of HCN colliding with both para- and ortho-H$_{2}$O. The calculations were performed using the statistical adiabatic channel model method and a recently published HCN–H$_{2}$O interaction potential energy surface. We provide rate coefficients for transitions between the rotational energy levels of HCN up to $j = 10$ for temperatures up to 100 K. We present a comparison of the excitation of HCN due to collisions with para- and ortho-H$_{2}$O and discuss its potential impact on astrophysical modelling. Finally, we compared our new data with those available in the literature and found significant differences, especially at low temperatures.