SIMBAD references

We report the first detection in space of the two doubly deuterated isotopologues of methyl acetylene. The species CHD₂CCH and CH₂DCCD were identified in the dense core L483 through, respectively, nine and eight rotational lines in the 72-116 GHz range using the 30 m telescope of the Institut de Radioastronomie Millimetrique (IRAM). We combined the astronomical frequencies observed here with laboratory frequencies from the literature measured in the 29-47 GHz range to derive more accurate spectroscopic parameters for the two isotopologues. We derive beam-averaged column densities of (2.7±0.5)x10¹²cm^–2 for CHD₂CCH and (2.2±0.4)x10¹²cm^–2 for CH₂DCCD, which translate to abundance ratios of CH₃CCH/CHD₂CCH=34±10 and CH₃CCH/CH₂DCCD=42±13. The doubly deuterated isotopologues of methyl acetylene are only a few times less abundant than the singly deuterated ones, specifically around 2.4 times less abundant than CH₃CCD. The abundances of the different deuterated isotopologues with respect to CH₃CCH are reasonably accounted for by a gas-phase chemical model in which deuteration occurs from the precursor ions C₃H₆D⁺ and C₃H₅D⁺, when the ortho-to-para ratio of molecular hydrogen is sufficiently low. This points to gas-phase chemical reactions, rather than grain-surface processes, as being responsible for the formation and deuterium fractionation of CH₃CCH in L483. The abundance ratios CH₂DCCH/CH₃CCD=3.0±0.9 and CHD₂CCH/CH₂DCCD=1.25±0.37 observed in L483 are consistent with the statistically expected values of three and one, respectively, with the slight overabundance of CHD₂CCH compared to CH₂DCCD being well explained by the chemical model.