Prestellar grain-surface origins of deuterated methanol in comet 67P/Churyumov-Gerasimenko

Deuterated methanol is one of the most robust windows astrochemists have on the individual chemical reactions forming deuterium-bearing molecules and the physicochemical history of the regions where they reside. The first-time detection of mono- and di-deuterated methanol in a cometary coma is presented for comet 67P/Churyumov-Gerasimenko using Rosetta-ROSINA data. D-methanol (CH3OD and CH2DOH combined) and D2-methanol (CH2DOD and CHD2OH combined) have an abundance of 5.5+/-0.46 and 0.00069+/-0.00014 per cent relative to normal methanol. The data span a methanol deuteration fraction (D/H ratio) in the 0.71-6.6 per cent range, accounting for statistical corrections for the location of D in the molecule and including statistical error propagation in the ROSINA measurements. It is argued that cometary CH2DOH forms from CO hydrogenation to CH3OH and subsequent H-D substitution reactions in CH3-R. CHD2OH is likely produced from deuterated formaldehyde. Meanwhile, CH3OD and CH2DOD, could form via H-D exchange reactions in OH-R in the presence of deuterated water ice. Methanol formation and deuteration is argued to occur at the same epoch as D2O formation from HDO, with formation of mono-deuterated water, hydrogen sulfide, and ammonia occurring prior to that. The cometary D-methanol/methanol ratio is demonstrated to agree most closely with that in prestellar cores and low-mass protostellar regions. The results suggest that cometary methanol stems from the innate cold (10-20 K) prestellar core that birthed our Solar System. Cometary volatiles individually reflect the evolutionary phases of star formation from cloud to core to protostar.Comment: Accepted for publication in MNRAS; 29 pages, 8 figures, 4 table

Physicochemical models: source-tailored or generic?

Physicochemical models can be powerful tools to trace the chemical evolution of a protostellar system and allow to constrain its physical conditions at formation. The aim of this work is to assess whether source-tailored modelling is needed to explain the observed molecular abundances around young, low-mass protostars or if, and to what extent, generic models can improve our understanding of the chemistry in the earliest stages of star formation. The physical conditions and the abundances of simple, most abundant molecules based on three models are compared. After establishing the discrepancies between the calculated chemical output, the calculations are redone with the same chemical model for all three sets of physical input parameters. With the differences arising from the chemical models eliminated, the output is compared based on the influence of the physical model. Results suggest that the impact of the chemical model is small compared to the influence of the physical conditions, with considered time-scales having the most drastic effect. Source-tailored models may be simpler by design; however, likely do not sufficiently constrain the physical and chemical parameters within the global picture of star-forming regions. Generic models with more comprehensive physics may not provide the optimal match to observations of a particular protostellar system, but allow a source to be studied in perspective of other star-forming regions

Fevering Interstellar Ices Have More CH3OD

Monodeuterated methanol is thought to form during the prestellar core stage of star formation. Observed variations in the CH2DOH/CH3OD ratio suggest that its formation is strongly dependent on the surrounding cloud conditions. Thus, it is a potential tracer of the physical conditions before the onset of star formation. A single-point physical model representative of a typical prestellar core is coupled to chemical models to investigate potential formation pathways toward deuterated methanol at the prestellar stage. Simple addition reactions of H and D are not able to reproduce observed abundances. The implementation of an experimentally verified abstraction scheme leads to the efficient formation of methyl-deuterated methanol, but lacks sufficient formation of hydroxy-deuterated methanol. CH3OD is most likely formed at a later evolutionary stage, potentially from H–D exchange reactions in warm ices between HDO (and D2O) and CH3OH. The CH2DOH/CH3OD ratio is not an appropriate tracer of the physical conditions during the prestellar stage, but might be better suited as a tracer of ice heating