On the robustness of the ammonia thermometer

TL;DR

This study provides new collisional rates for ammonia molecules with H2, demonstrating that the ammonia thermometer's calibration remains robust, with minimal impact from the updated data on excitation temperatures.

Contribution

The paper introduces high-accuracy collisional rates for ammonia with H2 and assesses their impact on the ammonia thermometer calibration, confirming its robustness.

Findings

New collisional rates for NH3-H2 interactions were computed.

Calibration curve of the ammonia thermometer remains stable with new rates.

Effects on excitation temperatures are minimal.

Abstract

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.