Modelling of Deuterium Chemistry in Star-Forming Regions

Abstract

Several new multiply deuterated species have been detected over the past three years, including ND₃ (van der Tak et al. 2002; Lis et al. 2002), CHD₂OH, CD₃OH (Parise et al. 2002, 2004), D₂S (Vastel et al. 2003), HD₂⁺ (Vastel et al. 2004) and D₂CS (Marcelino et al. 2005). In addition, mono-deuterated species have been observed with abundances >10% of their un-deuterated analogues (e.g. CH₂DOH observed by Parise et al. 2002; NH₂D observed by Saito et al. 2000 and Hatchell 2003). These are remarkable results, given that the underlying abundance of deuterium in the local interstellar medium (ISM) is ∼10⁻⁵ times lower than that of hydrogen (Linsky 1998; Sonneborn et al. 2000).

Such large enhancements in the abundances of deuterium-bearing molecules can either be due to gas-phase or to grain-surface fractionation. Grain-surface reactions are undoubtedly important in producing saturated species such as methanol, water, ammonia, and hydrogen sulphide. Water ice is observed to be abundant and ubiquitous throughout the ISM, and enhanced abundances of gas-phase NH₃, CH₃OH, H₂CO and H₂S (among others) are observed in warmer regions around protostars where grain mantles have evaporated.

Recent observational and theoretical evidence suggests that the deuterium fractionation in star-forming regions is set by gas-phase and grain-surface reactions during the cold, dense pre-protostellar phase. For species which form on grain surfaces via H atom addition to CO, N, O and S, the deuterium fractionation on grains comes from the relative amounts of atomic D and H which are accreting from the gas. The observations of deuterated methanol and D₂S require that the gas-phase atomic D/H ratio at the time the molecules formed was ≥ 0.1.

This paper presents results from chemical models of the prestellar core phase of star formation, showing how this high atomic D/H ratio can be produced, and discusses how models can also be used to look at deuterium fractionation in the protostellar stages of star formation.