Chemistry of TMC-1 with multiply deuterated species and spin chemistry of H ₂ , H ₂ ⁺ , H ₃ ⁺ and their isotopologues

Abstract

Deuterated species are unique and powerful tools in astronomy since they can probe the physical conditions, chemistry, and ionization level of various astrophysical media. Recent observations of several deuterated species along with some of their spin isomeric forms have rekindled the interest for more accurate studies on deuterium fractionation. This paper presents the first publicly available chemical network of multiply deuterated species along with spin chemistry implemented on the latest state-of-the-art gas-grain chemical code 'NAUTILUS'. D/H ratios for all deuterated species observed at different positions of TMC-1 are compared with the results of our model, which considers multiply deuterated species along with the spin chemistry of light hydrogen bearing species H 2 , H 2 + , H 3 + and their isotopologues. We also show the differences in the modeled abundances of non-deuterated species after the inclusion of deuteration and spin chemistry in the model. Finally, we present a list of potentially observable deuterated species in TMC-1 awaiting detection.

H 2 formation on interstellar dust grains: The viewpoints of theory, experiments, models and observations

“…Ortho to para conversion of H 2 on ice. The ortho-to-para ratio (OPR) of H 2 released into the gas phase affects the chemical evolution of a cloud, including deuterium fractionation, (Pagani et al, 2011;Bron et al, 2016;Majumdar et al, 2017). As noted above this affect of the OPR arises because the energy difference between the ortho and para rotational ground states is ∼14.7 meV; this energy corresponds to a temperature of 170 K, which is significantly higher than the local temperature of molecular clouds.…”

Experimental and Theoretical Studies of the Isotope Exchange Reaction

“…This is an insignificant difference with respect to the reaction barrier energy. Deuterated astrochemical models currently assume a rate coefficient for Reaction(1) of ∼1×10 −9 cm 3 s −1 (Roberts & Millar 2000;Walmsley et al 2004;Albertsson et al 2013;Majumdar et al 2017). These should be updated to use our results presented here, but we expect that the result will be to essentially turn off this channel for deuterating + H 3 at prestellar core temperatures.…”

“…This fractionation process explains why, in cold environments, the observed abundance ratios of deuterated molecules relative to their H-bearing analogs are orders of magnitude larger than the galactic D/H ratio. Since these findings became apparent, numerous astrochemical models have been developed to explain the observations (an incomplete list of models includes Millar et al 1989;Rodgers & Millar 1996;Roberts & Millar 2000;Walmsley et al 2004;Flower et al 2006;Aikawa et al 2012;Albertsson et al 2013;McElroy et al 2013;Sipilä et al 2013;Kong et al 2015;Lee & Bergin 2015;Majumdar et al 2017).…”

Deuterium chemodynamics of massive pre-stellar cores

“…As a consequence, the abundances of electrons, water, HCO + , DCO + , N 2 H + , N 2 D + are improved and have a good agreement with the even more extensive network of Sipilä et al (2013). More recently, Majumdar et al (2016), based on the work of Wakelam et al (2015), presented a complete network including spin state chemistry with 13 elements (H, He, C, N, O, Si, So, Fe, Na, Mg, Cl, Ph, F).…”