Title: Collisional excitation of NH3 by atomic and molecular hydrogen,
Abstract: We report extensive theoretical calculations on the rotation-inversion
excitation of interstellar ammonia (NH3) due to collisions with atomic and
molecular hydrogen (both para- and ortho-H2). Close-coupling calculations are
performed for total energies in the range 1-2000 cm-1 and rotational cross
sections are obtained for all transitions among the lowest 17 and 34
rotation-inversion levels of ortho- and para-NH3, respectively. Rate
coefficients are deduced for kinetic temperatures up to 200 K. Propensity rules
for the three colliding partners are discussed and we also compare the new
results to previous calculations for the spherically symmetrical He and para-H2
projectiles. Significant differences are found between the different sets of
calculations. Finally, we test the impact of the new rate coefficients on the
calibration of the ammonia thermometer. We find that the calibration curve is
only weakly sensitive to the colliding partner and we confirm that the ammonia
thermometer is robust.