Contribution Contributed Talk
Spin-state chemistry of deuterated ammonia
- Dr. Olli SIPILÄ
- Dr. Olli SIPILÄ (Max Planck Institute for Extraterrestrial Physics)
We report on a new gas-grain chemistry model (Sipilä et al. 2015b) that contains multiply deuterated species and separates out the different nuclear spin isomers of molecules with several H and/or D nuclei. The nuclear spin branching ratios are calculated using symmetry rules under the assumption of complete scrambling. The method has been previously used for systems consisting of five H and/or D nuclei by Hugo et al. (2009). The present model is an evolved version of our previous spin-state chemical model (Sipilä et al. 2015a) which included the spin chemistry of hydrogenated species calculated with the formalism of Oka (2004), based on angular momentum algebra applied to nuclear spin statistics.
We apply the model to studying the particular case of (deuterated) ammonia, for which the model predicts that the relative abundances of $\rm NH_3$, $\rm NH_2D$, $\rm NHD_2$, and $\rm ND_3$, as well as their nuclear spin ratios, depend strongly on the kinetic temperature and the gas density. This is illustrated by the attached Figure, where we plot the spin-state ratios of the deuterated forms of ammonia as functions of time at different densities (upper panels) or temperatures (lower panels). According to the present model, the deuteration of ammonia occurs primarily in reactions between $\rm NH_3$ and the deuterated forms of $\rm H_3^+$, i.e., $\rm H_2D^+$, $\rm D_2H^+$, and $\rm D_3^+$.
In cold, dense interstellar cores, the deuterated isotopologs of ammonia are expected to be most abundant toward the center where the density is highest. The spectral lines of the mentioned species are therefore likely to be extremely useful probes of the deep interiors of prestellar cores. The ground-state rotational lines of $\rm NH_2D$, $\rm NHD_2$, and $\rm ND_3$ are observable from the ground (e.g., Roueff et al. 2005). We have recently observed ortho and para $\rm NH_2D$ and $\rm NHD_2$, and meta and para $\rm ND_3$ (no detection for the latter) toward the starless core H-MM1 with APEX (Harju et al. 2015, in prep.), and we find that our new spin-state model reproduces the various abundance ratios well.
Hugo, Asvany & Schlemmer 2009, J. Chem.Phys. 130, 164302
Oka 2004, J.Mol.Spec. 228, 635
Roueff et al. 2005, A&A 438, 585
Sipilä, Caselli & Harju 2015a, A&A 578, A55
Sipilä, Harju, Caselli & Schlemmer 2015b, submitted to A&A