On the robustness of the ammonia thermometer
  Ammonia inversion lines are often used as probes of the physical conditions
in the dense ISM. The excitation temperature between the first two para
metastable (rotational) levels is an excellent probe of the gas kinetic
temperature. However, the calibration of this ammonia thermometer depends on
the accuracy of the collisional rates with H2. Here we present new collisional
rates for ortho-NH3 and para-NH3 colliding with para-H2 (J=0) and we
investigate the effects of these new rates on the excitation of ammonia.
Scattering calculations employ a new, high accuracy, potential energy surface
computed at the coupled-cluster CCSD(T) level with a basis set extrapolation
procedure. Rates are obtained for all transitions involving ammonia levels with
J <= 3 and for kinetic temperatures in the range 5-100 K. We find that the
calibration curve of the ammonia thermometer -- which relates the observed
excitation temperature between the first two para metastable levels to the gas
kinetic temperature -- does not change significantly when these new rates are
used. Thus, the calibration of ammonia thermometer appears to be robust.
Effects of the new rates on the excitation temperature of inversion and
rotation-inversion transitions are also found to be small.
