2019MNRAS.490.2178B

Accurate estimation of the abundance of the NH₂ radical in the interstellar medium requires accurate radiative and collisional rate coefficients. The calculation of hyperfine-resolved rate coefficients for the collisional (de-)excitation of NH₂ by both ortho- and para-H₂ is presented in this work. Hyperfine-resolved rate coefficients are calculated from pure rotational close-coupling rate coefficients using the M_j randomizing approximation. Rate coefficients for temperatures ranging from 5 to 150 K were computed for all hyperfine transitions among the first 15 rotational energy levels of both ortho- and para-NH₂ in collisions with ortho- and para-H₂. The new data were then employed in radiative transfer calculations to simulate the excitation of NH₂ in typical star-forming regions such as W31C, where NH₂ is seen in emission. We compared the excitation and brightness temperatures for different NH₂ transitions obtained using the new and the previously available collisional data. It is found that the new rate coefficients increase the simulated line intensities by a factor ∼10-30. As a consequence, NH₂ abundance derived from the observations will be significantly reduced by the use of the present rate coefficients.