Oxygen fractionation in dense molecular clouds

Abstract

We have developed the first gas-grain chemical model for oxygen fractionation (also including sulphur fractionation) in dense molecular clouds, demonstrating that gas-phase chemistry generates variable oxygen fractionation levels, with a particularly strong effect for NO, SO, O₂, and SO₂. This large effect is due to the efficiency of the neutral ¹⁸O + NO, ¹⁸O + SO, and ¹⁸O + O₂ exchange reactions. The modeling results were compared to new and existing observed isotopic ratios in a selection of cold cores. The good agreement between model and observations requires that the gas-phase abundance of neutral oxygen atoms is large in the observed regions. The S¹⁶O/S¹⁸O ratio is predicted to vary substantially over time showing that it can be used as a sensitive chemical proxy for matter evolution in dense molecular clouds.

Keywords: ISM: abundances; ISM: clouds; Physical Data and Processes: astrochemistry.

Figure 1

Calculated gas-phase abundance ratios, relative to H₂, of CO, HCO⁺, OH, O₂, OCS, H₂CO, CH₃OH, NO, SO and SO₂ studied in this work as a function of time predicted by our model for N(H₂) = 2×10⁴ cm^-3 and T = 10K. The horizontal grey rectangles represent the abundances observed in the cold core TMC-1 (CP),(Gratier et al. 2016, Lique et al. 2006) including an arbitrary factor 3 for the uncertainties. In this figure, sulphur is depleted by a factor of 10, the S/H₂ ratio being equal to 1.5×10^-6.

Figure 2

Calculated ¹⁶O/¹⁸O ratio for the main oxygen species in the gas-phase as a function of time predicted by our model (N(H₂) = 2×10⁴ cm^-3, T = 10K). The ¹⁶O/¹⁸O elemental ratio is taken equal to 500.

Figure 3

Calculated SO/S¹⁸O, ³²SO/³⁴SO and ³⁴S¹⁶O/³²S¹⁸O ratios as a function of time predicted by our model for n(H) = 2×10⁴ cm^-3, T = 10K. The horizontal grey rectangle represents the average of dense core observations listed in Table 4.