The rotational excitation of HF by H

The HF molecule is a key tracer of molecular hydrogen in diffuse interstellar medium (ISM). Accurate modelling of the HF abundance in such media requires one to model its excitation by both radiation and collisions. In diffuse ISM, the dominant collisional partners are atomic and molecular hydrogen. We report quantum time-independent calculations of collisional cross-sections and rate coefficients for the rotational excitation of HF by H. The reactive hydrogen exchange channels are taken into account in the scattering calculations. For the first time, HF-H rate coefficients are provided for temperature ranging from 10 to 500 K. The strongest collision-induced rotational HF transitions are those with Delta j = 1, and the order of magnitude of the new HF-H rate coefficients is similar to that of the HF-H-2 ones previously computed. As a first application, we simulate the excitation of HF by both H and H-2 in typical diffuse ISM. We show that, depending on the rotational transition, hydrogen atoms increase or decrease the simulated excitation temperatures compared to collisional excitation only due to H-2 molecules. Such results suggest that the new HF-H collisional data have to be used for properly modelling the abundance of HF in diffuse ISM.