A recalibration of the interstellar ammonia thermometer

Rate coefficients for the rotational excitation of para- and ortho-NH₃ by para-H₂ (J=0) are presented for kinetic temperatures in the $$15\leqslant T\leqslant300\enspace\text K$$. The rate coefficients were derived from quantal calculations of the cross-sections on a fine grid of collision energies, extending and improving the earlier work of Danby et al. These results, together with extrapolations for levels (J, K) of ammonia with J>6, have been used to recalibrate the ammonia thermometer, discussed by Walmsley & Ungerechts, which may be used to determine the kinetic temperatures of interstellar molecular clouds. We find that the kinetic temperatures which are deduced after recalibration are significantly lower than were predicted with the previous calibration, based on NH₃–He rate coefficients computed by Green. Furthermore, the recalibrated NH₃ temperatures are in better agreement with CO temperatures for a sample of sources.