Collisional excitation of singly deuterated ammonia NH2D by H2

F Daniel,E Roueff,L Wiesenfeld,D C Lis,P Hily-Blant,A Faure

doi:10.1093/mnras/stu1670

Abstract

The availability of collisional rate coefficients with H$_2$ is a pre-requisite for interpretation of observations of molecules whose energy levels are populated under non local thermodynamical equilibrium conditions. In the current study, we present collisional rate coefficients for the NH$_2$D / para--H$_2$($J_2 = 0,2$) collisional system, for energy levels up to $J_\tau = 7_7$ ($E_u$$\sim$735 K) and for gas temperatures in the range $T = 5-300$K. The cross sections are obtained using the essentially exact close--coupling (CC) formalism at low energy and at the highest energies, we used the coupled--states (CS) approximation. For the energy levels up to $J_\tau = 4_2$ ($E_u$$\sim$215 K), the cross sections obtained through the CS formalism are scaled according to a few CC reference points. These reference points are subsequently used to estimate the accuracy of the rate coefficients for higher levels, which is mainly limited by the use of the CS formalism. Considering the current potential energy surface, the rate coefficients are thus expected to be accurate to within 5\% for the levels below $J_\tau = 4_2$, while we estimate an accuracy of 30\% for higher levels.

Highlights

Deuterated ammonia, NH2D, was first tentatively detected towards the Kleinmann-Low (KL) nebula in Orion (Rodriguez Kuiper, Kuiper & Zuckerman 1978) and towards Sgr B2 (Turner et al 1978)
We reported the calculation of collisional rate coefficients for the NH2D/H2 system for NH2D rotational energy levels up to Jτ = 77 (E ∼ 735 K) and for temperatures in the range T = 5–300 K
The dynamical calculations were performed at the CC level at low total energy (E < 110 cm−1) and with the CS formalism at higher energies

Summary

Introduction

NH2D, was first tentatively detected towards the Kleinmann-Low (KL) nebula in Orion (Rodriguez Kuiper, Kuiper & Zuckerman 1978) and towards Sgr B2 (Turner et al 1978). The first unambiguous detection of NH2D was subsequently performed by Olberg et al (1985) in the cold environment of L183, S140 and DR21(OH) at 85 and 110 GHz, thanks to the high-spectral resolution of the observations which enabled them to resolve the hyperfine structure of the lines. The deuterium fractionation was derived to be several 10−2. Such high fractionation ratios are within the values predicted by gas-phase models at low temperatures (Roueff et al 2005). In warm environments such as Orion KL, the fractionation ratio was derived to be 0.003 by Walmsley et al (1987) and interpreted as

Objectives

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Discussion

Conclusion