Collisional excitation of NH3 by atomic and molecular hydrogen

Abstract

We report extensive theoretical calculations on the rotation-inversion excitation of interstellar ammonia (NH 3 ) due to collisions with atomic and molecular hydrogen (both para-and ortho-H 2 ). 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-NH 3 , 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-H 2 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.

NH3(10–00) in the pre-stellar core L1544

“…We took into account the hyperfine structure, produced by the electric quadrupole coupling of the N nucleus with the electric field of the electrons, as well as the magnetic hyperfine structure that is due to the three protons (Cazzoli et al 2009). The hyperfine collision rate coefficients were taken from the recent calculations of Bouhafs et al (2017), which, for the first time, include the non-spherical structure of p-H 2 (the main form of H 2 in cold molecular gas, Flower et al 2006;Brünken et al 2014), so that they differ (by up to a factor 2) from those of Maret et al (2009). The calculations were restricted to the lowest nine hyperfine levels of o-NH 3 , corresponding to the first two rotational levels (0 0 and 1 0 ), within a temperature range 5−30 K. Since only the ground-state o-NH 3 transitions were considered, no scaling of the rotational rates was necessary: all hyperfine rates are equal to the pure rotational rates, and intra-multiplet rates are set to zero, as in the standard statistical approach.…”

Circumstellar ammonia in oxygen-rich evolved stars

“…Similar to the analysis of Schmidt et al (2016, their Sect. 3), we compare the synthesised spectra of the transitions discussed in this article using the above-mentioned extended coefficients with those using only the coefficients of either Danby et al (1988) or Bouhafs et al (2017). Neglecting collisional transitions of high-J and 2 -excited levels, the coefficients of Danby et al (1988) and Bouhafs et al (2017) do not produce qualitatively different spectra. While the spectra of IK Tau are insensitive to the presence of the extended rate coefficients, those of the other targets are noticeably affected, especially in the rotational transitions (<50% in the peak intensity).…”

“…By adopting some constant rate coefficients for other transitions, Schmidt et al (2016) found that the collisional transitions involving higher ground-state levels or vibrationally excited levels were not important. Bouhafs et al (2017) reported new calculations on the rate coefficients of the collisions between NH 3 and atomic hydrogen (H), ortho-H 2 , and para-H 2 at temperatures up to 200 K and in transitions involving energy levels up to E up /k = 600 K (J ortho ≤ 6 and J para ≤ 7). Their calculations were based on the full-dimensional (9-D) NH 3 -H potential energy surface (PES) of Li & Guo (2014) and the five-dimensional NH 3 -H 2 PES of Maret et al (2009) with an ad hoc approximation of the inversion wave functions using the method of Green (1976).…”

“…Fig. 2 of Bouhafs et al 2017), we apply constant extrapolation of the coefficients from 200 K to higher temperatures. Vibrations were not included in the 5-D PES of NH 3 -H 2 .…”

“…We thank M. R. Schmidt for kindly providing the molecular data files of orthoand para-NH 3 . We are grateful to A. Faure for the collisional rate coefficients of ortho-and para-NH 3 with ortho-H 2 , para-H 2 , and H (Bouhafs et al 2017); and for the useful comments of the extrapolation methods of the rate coefficients. We are indebted to D. Gobrecht for the insightful discussions of the chemical models of IK Tau (Gobrecht et al 2016).…”