Reconstructing the history of water ice formation from HDO/H₂O and D₂O/HDO ratios in protostellar cores

Abstract

Recent interferometer observations have found that the D 2 O/HDO abundance ratio is higher than that of HDO/H 2 O by about one order of magnitude in the vicinity of low-mass protostar NGC 1333-IRAS 2A, where water ice has sublimated. Previous laboratory and theoretical studies show that the D 2 O/HDO ice ratio should be lower than the HDO/H 2 O ice ratio, if HDO and D 2 O ices are formed simultaneously with H 2 O ice. In this work, we propose that the observed feature, D 2 O/HDO > HDO/H 2 O, is a natural consequence of chemical evolution in the early cold stages of low-mass star formation as follows: 1) majority of oxygen is locked up in water ice and other molecules in molecular clouds, where water deuteration is not efficient; and 2) water ice formation continues with much reduced efficiency in cold prestellar/protostellar cores, where deuteration processes are highly enhanced as a result of the drop of the ortho-para ratio of H 2 , the weaker UV radiation field, etc. Using a simple analytical model and gas-ice astrochemical simulations, which traces the evolution from the formation of molecular clouds to protostellar cores, we show that the proposed scenario can quantitatively explain the observed HDO/H 2 O and D 2 O/HDO ratios. We also find that the majority of HDO and D 2 O ices are likely formed in cold prestellar/protostellar cores rather than in molecular clouds, where the majority of H 2 O ice is formed. This work demonstrates the power of the combination of the HDO/H 2 O and D 2 O/HDO ratios as a tool to reveal the past history of water ice formation in the early cold stages of star formation, and when the enrichment of deuterium in the bulk of water occurred. Further observations are needed to explore if the relation, D 2 O/HDO > HDO/H 2 O, is common in low-mass protostellar sources.

Recommended Ideal-Gas Thermochemical Functions for Heavy Water and its Substituent Isotopologues

“…Temperature-dependent ideal-gas internal partition functions, Q int (T ), have been determined, in the range of 0 to 6000 K, for the following molecules: ortho-and para-D 2 16 O, set of first-principles energy levels, over three million for each isotopologue, have been utilized during the explicit summation. These ideal-gas internal partition functions and their first two moments were then employed to obtain the following thermochemical functions in the same temperature range: Gibbs energy function, enthalpy, entropy, and isobaric heat capacity.…”

“…This procedure does not ensure that we have the complete set of rovibrational states very close to the dissociation limit. This minor shortcoming of the present treatment is reflected in the somewhat increased uncertainties at the highest temperatures for D 2 17 O and D 2 18 O (note the large uncertainties employed for energy levels above 35 000 cm −1 , Table 1). …”

“…Table 3. The temperature-dependent nuclear-spin-equilibrated internal partition functions, Q int (T ), of D 2 16 O and its first two moments, Q int (T ) and Q int (T ). The standard uncertainties associated with the data are given in parentheses.…”

“…Despite the low cosmological abundance of deuterium, D 2 16 O has been detected in the interstellar medium. 1 D 2 16 O is particularly prevalent in the environments of low-mass starforming regions, where its gas-phase abundance can be surprisingly large 2 and where analysis of ratios of ortho and para species point to formation at very low temperatures.…”

Molecular hydrogen production from amorphous solid water during low energy electron irradiation

“…Indeed the D/H ratio in cold dense objects can reach of the order of 0.1 -0.2 compared to the average of 1.5×10 -5 in the galactic interstellar medium 28,29 . Recent studies have reported the degree of deuterium enrichment and fractionation in many stellar objects at different evolutionary stages [30][31][32][33][34][35][36][37][38][39][40][41] . Fontani et al 42 show that surface chemistry has an important role in determining the fractional deuteration of some molecules (e.g.…”